Atomic Radiation and Polymers

TO F . A . F . AND THE H . C . IN APPRECIATION

ATOMIC RADIATION AND POLYMERS

A. CHARLESBY Professor of Physics

Royal Military College of Science Shrivenham

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PREFACE

ALTHOUGH research into the effect of radiation on materials has been in progress for many years, interest in the subject has been greatly stimulated recently by a number of factors both technical and scientific. In the development of power from nuclear energy there is a constant search for radiation-resistant materials, capable of use in the intense radiation field present in reactors and associated plants. In the chemical industry there has arisen the possibility of inducing useful changes in structure by the use of such radiation fields, and here the search is directed towards materials which are very sensitive to radiation. Exposure to high energy radiation can promote drastic changes in the physical and chemical properties of solids and this in the quantitative manner which can be readily studied. Interesting analogies have been observed between many of the radiation effects in simple chemical systems and in the more com-plex biological structures, so that the former can often act as a guide to the reactions occurring in radiobiology. The greater availability of powerful radiation sources, either in the form of radioactive isotopes or of high voltage electrical accelerators, has also increased the tempo of research.

The rapid growth of scientific interest in radiation effects can be readily traced in the increasing number of papers, scientific and technical, published on the subject, in the formation of radiation research societies and in the appearance of specialist scientific journals. Industrial appli-cations have also emerged, and this in a relatively few years after the initial fundamental discoveries were made. The scope of these appli-cations is as yet largely determined by the cost of radiation, and this is being rapidly reduced. Here we may expect to see keen competition between the use of radioactive isotopes, obtained as a by-product from the nuclear power industry, and electrical equipment produced by the electrical industry. The present series of volumes on radiation effects in materials, of which this is the first, is intended to cater for these very varied aspects of the subject.

One often finds that the most rapid advances occur when two apparently diverse branches of science first converge, and this is certainly true in the case of irradiated polymers. Polymer science has only recently become recognized as a distinct branch of science with its own methods, opinions and outlook. It occupies a unique position, intermediate between the fields of solid state physics, physical and organic chemistry, and has close connections with certain branches of mathematics and biology. The chemical changes produced when polymers are exposed to radiation are not essentially different from those observed in low molecular weight compounds, but even very small chemical changes, of the character

xi

xn PREFACE

produced by irradiation, cause profound modifications in the physical properties which can be readily interpreted. The use of radiation in polymer science offers not only a new method of promoting useful changes, but also constitutes a powerful tool for the quantitative study of macro-molecules. The ability to induce reactions over an extremely wide range of temperatures, and in the solid state, is a valuable feature of this technique.

In writing this book, the author was confronted with the difficulty of presenting the material in a form suitable for a very diverse audience. Potential readers include nuclear and solid state physicists, organic and polymer chemists, nuclear engineers and radiobiologists, for each of whom a different approach and emphasis would be desirable. Furthermore, understanding of the basic reactions involved has not kept pace with the output of published data; numerous revisions were made during the course of writing the book to deal with this new material. The method finally adopted was to review and summarize much of the published experimental material, and subsequently to discuss the underlying theories, often in the somewhat tentative form. It is to be hoped that in future books on the subject a more selective treatment will be possible and more definite conclusions may be derived.

The method of presentation adopted is reflected in the arrangement of the chapter headings. After a general introduction, chapters 2-6 deal with the interaction of radiation and matter, with radiation sources and dosimetry. There follows in chapters 7-11 a discussion on the general properties of long chain polymers, particularly those quantitative aspects which will be frequently required in discussing radiation-induced changes in individual polymers. After an introductory chapter on radiation-induced changes in some simple organic molecules (chapter 12) there follows a series of chapters (13-21) which summarize existing data on irradiated polymers, both those which crosslink and those which degrade under radiation. This information, which is largely of an experimental character, is classified under individual types of polymer. Chapters 22-24 deal with radiation-induced changes in which a chain reaction is in-volved—polymerization, grafting and polyester cure. A separate chapter (25) is concerned with the irradiation of polymers in solution where both direct and indirect effects occur. The next chapters (26-29) attempt to trace in a more theoretical manner the various reactions which may occur between the initial acts of ionization or excitation (discussed in chapter 3) and the final chemical changes which have been measured directly and are described in the earlier chapters. The wide range of theories discussed in this section is a measure of our uncertainty as to the precise mechanisms involved. Information on the conductivity changes at low radiation intensities (where no significant permanent changes occur) and some relevant technical data on radiation damage at very high intensities are presented in chapters 30 and 31.

The author first became interested in the study of radiation effects in polymers when, as a student at the Imperial College of Science and Techno-logy, London, he observed some unusual phenomena in the melting

PREFACE Xl l l

behaviour of polyethylene when exposed to electron radiation. Subse-quently he was able to continue this work, making use of the extensive facilities at Harwell, and later to extend and apply the knowledge thus gained at the Tube Investments Research Laboratories at Hinxton, Cambridge. For the stimulating experience of working in these three laboratories—university, government and industrial—the author would like to express his deep appreciation to Professor G. I. Finch, M.B.E., F.R.S., Sir John Cockcroft, O.M., K.C.B., C.B.E., F.R.S., and to Dr. F. P. Bowden, C.B.E., F.R.S., and to his colleagues there. He would also like to thank his associates and friends who have constantly encouraged him by their enthusiasm and friendship and by their discussions and criticisms, sometimes severe, often well deserved, but always helpful. Many chapters of the present volume have been submitted to their ruthless attention but the final choice of emphasis was of course entirely the responsibility of the author. It would be invidious to select names out of a long list, but the author would like to add that this collaboration has been one of the most enjoyable aspects of his research work, and has served as a starting point for some long friendships.

A. CHARLESBY

CHAPTER 1

INTRODUCTION THE extremely rapid industrial growth of nuclear energy has greatly stimulated interest in the effects observed when materials are exposed to high energy radiation. Nevertheless the scientific study of these changes must be considered to fall in a different field to the problems considered in nuclear physics. In nuclear physics one is largely concerned with the arrangement and interaction of the particles constituting the nucleus of the atom; its association with orbital electrons or other atoms is of secondary importance. Most of the radiation-induced changes considered here do not involve changes in the individual nuclei ; it is the study of the rearrangements of nuclei and electrons relative to each other and the resultant effect on physical and chemical properties which constitutes the major objective. Many aspects of the subject are closely related to solid state physics, others to radiation chemistry, while certain problems are analogous to those studied in radiobiology.

Early investigations of the effect of high energy radiation on materials preceded by many years the discovery of the nature of these radiations, and of the forces binding atoms together.

As early as 1815, Wollaston and Berzelius investigated thermal luminescence in materials containing radioactive elements. In the middle of the nineteenth century a study was being made of minerals whose structure had been disordered by α-radiation from naturally occurring sources. Investigations into the effect of electrical discharges on a number of gases were reported by Andrews and Tait in 1860 and by Brodie in 1873. In 1874 Thenard converted gaseous acetylene to a solid or a liquid under the influence of the silent electrical discharge. In the last quarter of the century, Berthelot extended this work to a large number of gases and mixtures of gases and concluded that the electrical discharge can cause both decomposition and aggregation of compounds, a striking deduction in view of our present knowledge on the behaviour of irradiated polymers.

The discovery of the nature of high energy radiations (α, β and γ) emitted by radioactive elements immediately led to a number of observations of their effect on materials. In 1899 the Curies noted the coloration of glass and porcelain and in 1900 Giesel observed decomposition of water and the coloration of alkaline halides. Becquerel (1901) compared the effects of β- and γ-radiation with those produced by light. A number of investi-gators, including Cameron and Ramsay (1907-8), Usher (1910), Lind (1911) and Wourtzel (1913-1919) used radon as a convenient source of a radiation and studied its effect on some simple gases. Kailan (1917— 1919) investigated the effect of the more penetrating ß- and γ-rays ön

J

2 ATOMIC RADIATION AND POLYMERS

liquids such as chloroform, carbon tetrachloride, toluene. Much of this early work has been reviewed in the classic book by Lind (1921).

Between 1924 and 1926 there appeared a number of papers concerned with the quantitative chemical effects produced in some simple hydro-carbon gases (methane, ethane, propane and butane, ethylene, acety-lene, etc.). This work, carried out independently by Lind and Bardwell in the United States, and by Mund and colleagues in Belgium, made use of α-radiation from radon. More qualitative results on acetylene and several liquids and solids were obtained by Coolidge (1925) using cathode rays from a high voltage source of up to 250 kV, and therefore not involving any nuclear transformations.

In the late 1930's interest in long chain polymers began to develop and x-rays, γ-rays and neutron radiations were found capable of inducing polymerization of some simple monomers. Studies on some of the newly discovered polymers by electron diffraction indicated that electron bombardment produced changes in their melting properties. A major limitation to the extension of this work arose from the limited power output or penetration of the radiation sources available at the time.

In the last decade, the position has experienced a fundamental change. Adequate sources of radiation have become readily available and there has been a rapidly increasing interest in the effect of radiation on a variety of materials. At the same time, the study of polymers has become recog-nized as a distinctive branch of science with close connexions with certain branches of organic chemistry, physical chemistry and solid state physics. These favourable conditions have encouraged a rapid development of the earlier discoveries.

The effect of radiation on plastics may be considered from two aspects ; certain of the permanent changes produced can be deleterious while others may be beneficial in character. In nuclear reactors, intense fields of high energy radiation are present and information on radiation damage to structural materials is of great importance in their design, since high power reactors must be capable of functioning for very long periods without replacement of vital components. For many applications in the nuclear energy field, plastics have an important part to play and a vast range of practical data has been accumulated on their expected lifetime under various radiation conditions. Efforts are being made to discover plastic materials capable of resisting high radiation doses with a minimum change in their physical properties. However, some of the plastics which show considerable changes under radiation are also found to acquire improved properties, so that the possibility has arisen of using atomic radiations as a means of modifying and improving plastic materials. At present, there are two quite distinct objectives, one being the discovery of polymers or similar materials with high resistance to radiation, the other the search for materials with high radiation sensitivity, but in which the changes pro-duced are beneficial. Both aspects have one basic factor in common—the need to study the mechanism by which radiation affects materials, and to discover means of modifying the reaction.

Although the radiation treatment of polymers has received more

I N T R O D U C T I O N 3

industrial attention than has the irradiation of low molecular weight organic molecules, this is not due to any inherent difference in their reaction to radiation. The chemical changes suffered by long chain poly-mers do not differ fundamentally from the effects produced by similar radiation doses on simple organic compounds. The distinctive interest in polymer work depends on the fact that small chemical changes induced by radiation may produce very large changes in the subsequent physical behaviour of the irradiated material.

NUCLEAR AND ELECTRONIC SOURCES Already in the early work radiation was obtained from two distinct

types of sources: α-, β-, and γ-radiation derived from naturally occurring radioactive materials; and x-rays and fast electrons produced directly or indirectly by high voltage machines, not involving any nuclear reactions. The effects produced by radiation from nuclear and electronic sources are not basically different and the choice of a suitable source for radiation work depends on experimental considerations such as the beam penetration and intensity, cost and availability (Fig. 1.1).

Naturally occurring radioactive materials are far too expensive and weak to be of value for any large scale radiation project. Nuclear reactors designed for experimental work or for the production of power or plutonium constitute powerful sources of mixed radiation and much experimental work has been carried out making use of their radiation facilities. Radioactive isotopes produced during the functioning of these reactors also provide useful sources of high energy radiation which can conveniently be installed in a laboratory away from the reactor. These radioactive sources can be far more powerful than naturally occurring radioactive elements such as radium and have almost completely displaced them for radiation research. The provision of more powerful radioisotope sources is directly dependent on the development of high-powered nuclear reactors designed for power production and of low cost methods of separating the fission products.

Electrical machines capable of accelerating particles to very high energies of the order of millions of volts can provide extremely powerful sources of great flexibility. Since these machines do not rely on any nuclear transformation to obtain high energy particles, the danger of radioactivity can be entirely eliminated. Most of these accelerators pro-duce high energy electrons, which can either be used directly for radiation research or, by allowing them to impinge on targets of high atomic number, give rise to high voltage x-rays which achieve the same objective. x-Ray machines of lower voltage have been available for many years and these can produce the changes described in this book, but the lower efficiency of conversion of electron energy to x-rays results in low energy output, often leading to an impracticably long exposure time.

FUNDAMENTAL REACTIONS The term "high energy radiation" is generally taken to include beams

of fast electrons or ß-particles; heavier particles of high energy such as fast protons, fast neutrons, a-particles and charged particles of higher

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mass; and also electro-magnetic radiation such as γ-rays or x-rays. The effects produced by γ- or x-rays may best be understood as due to discrete high energy photons which may therefore also be considered as particles in this context. The common property of these varied forms of radiation is the large amount of energy carried by each particle or photon, an amount which is considerably greater than that binding an orbital electron to its nucleus or an atom to its neighbour. However, these energies will usually be less than that required to affect the binding force within the atomic nucleus.

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The large amount of energy carried by each particle allows it to pene-trate within a specimen and disrupt the binding forces between atoms. One of the most frequent processes is that of ionization whereby the incident high energy particle removes an electron from its parent atom or

INTRODUCTION 5

molecule and leaves a charged species termed an ion. This ability to produce ions throughout a specimen is a distinctive feature of high energy radiation, and for this reason the radiations considered here are often referred to as ionizing radiations. However, ionization is not the only method by which high energy radiation interacts with materials and is not necessarily the most important one. For this reason, we prefer to use the alternative terms high energy radiation, or atomic radiation, which implies that the radiation concerned may affect the structure of individual atoms but not of the nucleus.

Because of their high velocity, the distance between successive particles in a radiation beam is large compared with the distance over which each particle can exercise its influence during its passage through a specimen. It is therefore possible to consider a beam of high energy radiation as a series of independent particles each reacting separately with the atoms of the medium through which it passes. After their passage, secondary reactions may occur between the different atoms affected by the same or by different incident particles.

In its passage through the medium, which may be solid, liquid or gaseous, each high energy particle loses energy by interaction with the electrons and nuclei of the medium which is thereby affected. The basic mechanisms of any energy interchange with which we are mainly concerned comprise:

(i) Ionization—a process in which an orbital electron is removed from its parent nucleus giving rise to a free electron and a positively charged (ionized) atom or molecule.

(ii) Excitation—in which an electron is raised to a high energy level but remains bound to its parent nucleus. In this case, the atom or molecule remains neutral.

(iii) Displacement of a nucleus with or without its attendant electrons. (iv) Capture by an atomic nucleus and transformation of the nuclear

structure. (v) Scattering of the incident particle or photon and emission of

secondary radiation. These basic processes may in their turn give rise to secondary changes.

Thus, an unstable nucleus may be formed in process (iv) which on dis-integration emits a further high energy particle capable of inciting further ionization, excitation 01 nuclear displacement. The electron emitted during ionization may also have sufficient energy to cause secondary ionization and excitation in neighbouring atoms, until it loses most of its energy and is then captured by the same or another atomic nucleus. Much of the energy absorbed within the specimen will be degraded into thermal vibrations and eventually appear in the form of heat.

In practice, radiation in the range of energies considered here loses most of this energy by interaction with orbital electrons to produce ions, free electrons and excited atoms or molecules. Seitz and Koehler (1955) have quoted a value of only 1 per cent for the amount of energy lost in other ways; obviously this proportion will vary with the type and energy of the incident radiation and with the structure of the irradiated material.


Ultraviolet light of sufficient energy may also cause excitation ; the study of its effect falls in the field of photochemistry. Ultraviolet light is not considered as a form of ionizing radiation as the energy available per photon is usually insufficient to cause ionization of a molecule. Moreover, the energy required to cause excitation is often quite close to the total energy of an ultraviolet photon, so that marked resonance effects occur which are not observed when high energy particles or photons are used. Nevertheless, there exist many similarities between the effects pro-duced in photochemistry and in radiation chemistry and a detailed com-parison of these effects may provide a means of distinguishing between reactions resulting from excitation and those involving ionization.

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FIG. 1.2. Crosslinking by side chain fracture (S is a side group, e.g. hydrogen). Degradation by main chain fracture.

In the former reaction two long chain molecules are linked together, in the latter a single long chain molecule gives rise to two smaller ones.

(The figure is intended to show only the general features of these two processes, not the detailed chemical changes.)

The high energy radiation present in a nuclear reactor consists of several types of high energy radiation and the effects produced by each may

INTRODUCTION 7

sometimes be difficult to separate. Fortunately, for most of the processes to be discussed here the changes produced depend mainly on the total energy absorbed and very little on the type of radiation or its energy. To some extent, this simplification also arises in the ionization of gases where the energy absorbed per ion pair produced varies remarkably little. In other types of reaction, the characteristics of the incident radiation can be of great importance.

Included in the radiations present in a nuclear reactor are thermal neutrons, i.e. neutrons with very low kinetic energies, comparable to those of hydrogen atoms. Due to their low energy, such neutrons cannot react directly with electrons or nuclei to cause ionization, excitation or nuclear displacement. They are therefore not considered as a form of high energy radiation. Thermal neutrons may, however, be captured by an atomic nucleus (process (iv) above) and produce a radioactive isotope, which eventually emits a particle or photon capable of causing ionization, excita-tion or nuclear displacements. The probability of capture of a slow neutron by a nucleus and the emission of a high energy particle depends on the chemical species of a nucleus involved. The radiation effect of the slow neutron flux in a nuclear reactor will therefore depend primarily on the types of atoms present in or near the irradiated material.

Ionization In their passage through air, x-rays cause ionization of the gas mole-

cules and render the air conducting. In the ionization process, the electron which is removed from one molecule is captured by another resulting in the production of an ion pair, i.e. the positively charged mole-cule which has lost the electron and the negatively charged molecule which has captured it. In air, the average energy absorbed per ion pair formed (represented by W) is about 34 eV (electron-volts, see page 18). This energy is considerably greater than the minimum energy required to ionize either oxygen or nitrogen molecules (see page 17), the remaining energy absorbed being dissipated in the form of electronic excitation and kinetic energy or heat. The number of excited molecules produced simultaneously cannot be measured but it is estimated that in many cases the total absorbed energy is shared between ionization and excitation in very roughly equal amounts.

The energy absorbed per ion pair formed in air ( W) varies remarkably little with the type of radiation—heavy particles, fast electrons, x- or γ-rays—or with their energy over a wide range. In many other gases, somewhat similar values are found for the energy absorbed per ion pair formed.

Much of the early experimental work on the a bombardment of gases was concerned with the number of molecules reacting for a given energy input. Lind proposed a cluster theory according to which each ion formed by radiation surrounds itself by a number of uncharged gaseous mole-cules. The yield is then expressed in terms of a ratio M/N, the number of molecules reacting in the cluster (M) to the number of ion pairs formed (N) by a given radiation dose.


In liquids and solids, the measurement of ionization is greatly compli-cated by the close proximity of neighbouring molecules or atoms which allows reactions between ions, free electrons and excited atoms or mole-cules to proceed very rapidly. The primary yield of ions cannot be measured directly and is either assumed to be the same as in a gas or alternatively it is deduced from the finally measured products by assuming appropriate reaction mechanisms.

Nuclear Changes and Displacements

In the changes to be considered in this book nuclear transformations do not play an important part. When slow neutrons are present, radioactive elements may be produced, and these will merely serve as internal sources of high energy radiation. Other forms of radiation such as fast electrons or γ-rays are only capable of causing nuclear transformations if their energies are high, usually much higher than are customary in studying radiation effects on materials.

In solids such as metals, where no permanent ionization or excitation effects are possible due to the presence of free electrons, the displacement of nuclei by collision with particles of high energy radiation is the major source of radiation damage. The energy required to produce such a dis-placement depends on the structure and nature of the binding forces within the solid. The average energy actually imparted to the atomic nucleus in a collision depends on the type of radiation, on its energy and on the ratio of the masses of the two colliding particles.

Conservation of momentum severely limits the amount of energy which can be transferred from a fast electron to a nucleus. In copper, for example, minimum electron energies of about 500,000 eV are required to produce measurable resistivity changes at liquid nitrogen temperatures, although the threshold energy for displacement of a copper atom in the metallic lattice is only about 25 ± 1 eV. In other types of structure such as non-conductors in which ionization and excitation can play a consider-able role, very marked radiation effects can, however, be obtained at much lower electron energies. In studying the different sensitivity of solids to radiation it is therefore necessary to classify materials on the basis of their structure and the character of the binding forces between atoms, in order to determine the nature of the changes—electronic or nuclear—most likely to alter their properties.

Four main types of structure may be considered : (1) metallic lattices (2) covalent lattices (3) ionic lattices (4) molecular structures.

METALS The only effect of exposure of metals to high energy radiation arises

from the displacement of atoms, giving rise to interstitial atoms and lattice vacancies. Conduction electrons already present in the metal react

INTRODUCTION 9

with any ions formed and prevent radiation induced ionization or excita-tion from producing any permanent change in properties. Changes in the atomic arrangements will lead to changes in electrical resistance, elastic modulus and elastic damping, creep, yield strength, stored energy due to the distorted lattice, hardness and ductility. For many metals these effects anneal out at room temperature, and investigations on the rate of anneal-ing at various temperatures provide information on the binding energies involved. The stability of metals to radiation is of particular importance in the design of high powered nuclear reactors, where enormous radiation fluxes must be absorbed without serious changes in mechanical properties. That metals are very resistant to many radiation effects can be readily seen in the case of x-ray tubes, where the heavy metal anodes suffer no appreciable change after years of bombardment by high energy electrons. Again the structural members of nuclear reactors show no deterioration after considerable periods of irradiation.

Special considerations arise in the case of single crystals, of ordered alloys and of anistropic metals, where the rearrangement of atoms dis-placed from their initial position by radiation may produce a different type of crystalline structure. Information obtained from such studies can be of considerable scientific value in the study of the metallic state. No further attention will be paid in this book to the irradiation of metals, but references to summary articles are appended.

COVALENT AND IONIC STRUCTURES Here the major effect of radiation occurs both through displacement of

nuclei and the production of free electrons. The same considerations of conservation of momentum apply to the amount of energy which can be transferred to an atom by collision, and this therefore sets a lower limit to the energy of each incident particle, below which no atomic displace-ments are possible and only ionization and excitation effects can be obtained. Varley (1954) has however suggested an alternative mechanism of lattice displacement applicable to ionic lattices. If an atom in such a lattice suffers multiple ionization, e.g. by low energy x-rays, incapable of causing displacement directly, a situation can arise whereby a negative ion surrounded by positive ions may become positively charged, and then be displaced because of electrostatic forces acting on it. This theory would account for displacements of the atomic nuclei in crystalline lattices at radiation energies too low to allow displacement by collision.

Among the changes observed as a result of nuclear displacements, there may be mentioned dilatation of the crystal lattice, increases in electrical resistance and changes in the elastic constants, in energy content and thermal conductivity. These are often recoverable by subsequent thermal treatment, although not all the different properties may become annealed out simultaneously. The materials studied so far include diamond, graphite, quartz and beryl, the alkali halides, germanium, silicon, zircon, boron nitride and silicon carbide.

In addition to the above changes which arise primarily from nuclear displacements, many materials change colour due to the liberation of


electrons, which become trapped at lattice sites to give F centres. Charac-teristic absorption bands are produced, which can be bleached either optically or thermally. The trapping site may be an impurity centre, or possibly a strained part of the structure. These traps may interact with sites of radiation damage, resulting in a transformation of the absorption bands with increasing radiation doses.

The two major causes of radiation damage—displaced atomic nuclei and trapped electrons—may be distinguished by a low temperature ther-mal anneal or optical bleach which removes the F centres without neces-sarily affecting the displaced atoms or lattice vacancies.

Much smaller radiation doses suffice to produce noticeable changes in these structures than in polycrystalline metals. At room temperature an average expenditure of 60 eV suffices to produce an F centre in KC1. The conductivity of germanium is profoundly affected by very small radiation doses. In n type germanium, the number of negative carriers is decreased, and it can be converted to a p type semiconductor. On the other hand p type germanium becomes increasingly so after irradiation. This opens up the possibility of forming a p-n boundary by bombarding a specimen with low penetrating radiation, thereby producing a rectifier. In silicon or other semiconductors on the other hand the resistance always increases on irradiation whether the initial material be n or p type.

MOLECULAR STRUCTURES In such systems, where atoms are bound together by shared electrons

to form molecules, excitation and ionization are by far the most important effects of exposure to high energy radiation. The removal of an electron, or even its excitation to a higher energy level, may render such molecules unstable and decomposition ensues. The active fragments produced can then react with each other or with neutral molecules to yield chemically very different molecules or structures, which may themselves be unstable and cause further reactions. In this way, one ionization or excitation can lead to a number of reactions and, in suitable systems, the number of molecules modified per ion pair produced by radiation may be high.

The nature of the processes taking place following the initial ionization or excitation depend on the chemical nature of the system and may be very complex; they are often found to result in radical reactions, i.e. reactions between molecules with an unpaired electron. Of the many systems studied water has received by far the greatest attention. The products of the reaction depend on the presence of oxygen or other solute and on the type of radiation, which determines the concentration of the ionization about the track of each incident high energy particle or photon. In spite of the considerable amount of scientific effort devoted to this problem, full agreement has not yet been reached as to the precise mechanism involved. The irradiation of water, despite its considerable theoretical importance, will not be discussed here and, where necessary, attention will be largely devoted to the effect of the changes produced in it rather than to the mechanism of the reaction.

Irradiation of organic compounds may result in a variety of chemical

INTRODUCTION 11

changes, such as oxidation, halogenation, nitration, decarboxylation, and in changes in isomerism, dimerization and degradation, depending on the compound irradiated. A remarkable feature of these reactions is that, while the incident energy is absorbed at random, i.e. all electrons in a compound are approximately equally likely to be affected by ionization or excitation, the final reactions show strong evidence of selective effects. Thus in long chain fatty acids or alcohols, where the end group contains only a small proportion of the total electrons in the molecules, it suffers a disproportionate amount of damage and decomposition. The reactions do not always follow the lines which would be predicted on thermo-chemical grounds, and this renders the field of radiation chemistry especially interesting. From the experimental point of view, novel types of reaction may be expected to occur. The elucidation of the various stages of such reactions might be expected to provide valuable information on the processes which occur not only in radiation chemistry, but also in photo- and thermal chemistry.

The sequence of events from initial ionization or excitation to final chemical product must occur in a series of stages, which are only incom-pletely understood. It is as yet impossible to predict, on the basis of conventional chemical information, the final products obtained by the irradiation of a simple organic molecule and it is often necessary to proceed by analogy with the effects of radiation on other comparable materials. The same problem arises in mass spectrometry where a variety of ionized groups may be obtained from the decomposition of a simple organic structure, but the proportions cannot be calculated directly from a know-ledge of the compound.

Although the average energy needed to abstract an electron by means of high energy radiation does not vary very widely from one organic compound to another there are considerable differences in the yield of modified products. From a comparative study of the nature and amounts of the products obtained certain general concepts emerge. One of these is a so-called protection or sponge effect and is best demonstrated in the case of benzene. As compared with most organic compounds, benzene is remarkably resistant to radiation and it is necessary to assume that, because of the resonant nature of the molecule, the abstraction of an electron or its excitation still leaves a relatively stable system. This stability can be extended to neighbouring molecules, the benzene molecule acting as a "sponge" to soak up the excess energy. Molecules containing such resonant structures will tend to be more radiation-resistant.

A second concept is often referred to as energy transfer although it is not implied that energy as such is necessarily transferred. The transfer may occur through electronic excitation, movement of electrons or through other mechanisms. The concept is introduced to denote the possibility that the result of a primary reaction (such as ionization) occur-ring at one point in a molecule is transferred in some way to cause changes in other parts of a molecule or even in other neighbouring molecules. Protection of one molecule by another may often be considered as an


example of energy transfer, although in other cases it may be simply due to a chemical stabilization of an otherwise reactive species.

FIELDS OF APPLICATION The use of radiation as a means of initiating chemical changes offers a

number of unique advantages over conventional chemical techniques. Certain reactions can be induced which are not possible or convenient by other means. Moreover, it extends the control over the reaction conditions ; the temperature is no longer a limitation since radiation-induced changes are little if at all affected, and reactions can even be promoted in the solid state.

The industrial uses of radiation are at present restricted by the cost of high energy radiation which renders large-scale radiation treatment of many materials uneconomic. As the cost of radiation decreases, the fields of potential uses of radiation will be greatly extended. At present attention is being directed towards those reactions in which a relatively small amount of radiation can produce large effects or valuable products not readily obtainable by other means. Such processes include medical and biological applications, sterilization, chemical chain reactions and the modification of long chain polymers. The medical and biological applica-tions depend on the considerable effect of very small amounts of high energy radiation on biological systems which, being complex, can be radically altered by minute chemical modifications.

In chain reactions in chemical systems, the initiation of a single chemical reaction by radiation may result in the modification of a large number of chemical bonds and the efficiency of the process measured in terms of the number of molecular changes produced per ion pair may be very high. An example of such a chain reaction is the radiation-induced polymeri-zation of vinyl monomers. The products obtained are generally com-parable with those using conventional catalysts but have the advantages of higher purity, better control of initiation conditions and of molecular weight.

The modification of long chain polymers by radiation has attracted considerable interest in the last few years. The marked changes produced in such polymers by radiation do not rely on any special chemical effect different from those obtained in smaller organic molecules but result from the fact that the physical properties of such polymers are readily modified by small changes in molecular arrangement. Many organic molecules can be dimerized by radiation, i.e. linked together in pairs. When small mole-cules containing, say, 10 bonds each are dimerized, this requires the modification of at least two bonds in 20, i.e. a 10 per cent chemical change. If no chain reaction is involved, the energy input required is correspond-ingly high. In long chain polymers containing perhaps 10,000 chemical bonds each, dimerization will require the modification of only two bonds in 20,000 and the energy input is therefore very much less, although the relative change in molecular weight is equally great. Many of the physical properties of polymers are determined by the size and shape of their molecules and a change such as dimerization would often have a profound

INTRODUCTION 13

effect. We will therefore be largely concerned with the relationship between physical properties and molecular arrangements of polymeric materials, and with the effect of small chemical changes on such arrangements.*

Two cases may be considered; in the first, which corresponds to dimeri-zation, there is an increased molecular weight and eventually the formation of a closed network system. In the second, the molecular weight is decreased. The former effect, termed crosslinking, is generally ascribed to the fracture of side chains followed by linking between adjacent molecules. Other processes may, however, be adduced to describe the observed effects. The alternative effect is ascribed to main chain fracture and is termed degradation. These two patterns of behaviour under radiation produce very different polymer properties which may be traced both theoretically and experimentally.

REFERENCES Early Work LIND, S. C , Chemical Effects of Alpha Particles and Electrons, 1st edition 1921,

2nd edition 1928, Chemical Catalogue Co. M UND, W., U action chimique des rayons alpha en phase gazeuse, Hermann, Paris,

1935.

Metals BILLINGTON, D. S. and SIEGEL S., Metal Progr. 58, 847, 1950. BILLINGTON, D. S., International Conference on Peaceful Uses of Atomic Energy,

Geneva, 7, 421, 1955. COTTRELL, A. H., Metallurg. Rev. 1, 479, 1956. DIENES, J. C, Ann. Rev. Nucl. Sei. 2, 187, 1953; / . Appl. Phys. 24, 666, 1953;

International Conference on Peaceful Uses of Atomic Energy, Geneva, 7, 634, 1955.

DUGDALE, R. A., Conference on Defects in Crystalline Solids, p. 246, Physical Society, London, 1955.

GLEN, J. W., Advanc. Phys. 4, 381, 1955. KINCHIN, G. H. and PEASE, R. S., Rep. Prog. Phys. 18, 1, 1955. KONOBEEVSKY, S. T., PRAVDYUK, N. F. and KUTAITSEV, V. I., International

Conference on Peaceful Uses of Atomic Energy, Geneva, 7, 433, 1955. KONOBEEVSKY, S. T., Atomnaya Energiya 2, 208, 1956. SEITZ, F. and KOEHLER, J. S., International Conference on Peaceful Uses of

Atomic Energy, Geneva, 7, 615, 1955. SHOCKLEY, W., et al., Imperfections in Nearly Perfect Crystals, Wiley, New York,

1951; Conference on Defects in Crystalline Solids p. 238, Physical Society, London, 1955.

SLATER, J. C , / . Appl. Phys. 22, 237, 1951. SUTTON, C. R. and LEESER, D. O., Chem. Eng., Prog. Symposium 50(12), 208,

1954. VARLEY, J. H. O., International Conference on Peaceful Uses of Atomic Energy,

Geneva, 7, 642, 1955. WOODWARD, A. S. and LONG, F. R., Bibliography on radiation effects, A.E.C.

Report NAA-SRA420, 1955.

* In this respect the subject differs from radiation chemistry, where the chemical changes are of prime importance.


Co va lent and Ionic Lattices

BROOKS, H., Ann. Rev. Nucl. Sei. 6, 215, 1956. CLELAND, J. W., et al., Phys. Rev. 83, 312, 1951; 95, 1177, 1954. CRAWFORD, J. H. and WITTELS, M. C , International Conference on Peaceful

Uses of Atomic Energy, Geneva, 7, 654, 1955. CRAWFORD, J. H., Ceram. Soc. Bull. 36, 95, 1957. DAINTON, F. S. and ROWBOTTOM, J., Trans. Faraday Soc. 50, 480, 1954. DIENES, C. J. and KLEINMAN, D. A., Phys. Rev. 91,238, 1953. FUTAGAMI, T., Proc. Phys.-Math. Soc. Japan 20, 458, 1938. HURLEY, P. M. and FAIRBAIRN, H. W., Bull. Geol. Soc. Amer. 64, 659, 1953. LARK-HOROVITZ, K., Semiconducting Materials, Butterworths, London, 1951. LEVY, P. W., J. Chem. Phys. 23, 764, 1955. LEVY, P. W. and DIENES, G. J., O.N.R. Symposium Report ACR2, p. 39,

December 1954. MAYER, G. and GUERON, J., / . Chim. Phys. 49, 204, 1952. MAYER, G., et al., International Conference on Peaceful Uses of Atomic Energy,

Geneva, 7, 647, 1955. Μοττ, N. F. and GURNEY, R. W., Electronic Processes in Ionic Crystals,

Clarendon Press, Oxford, 1948. PEARLSTEIN, E. A., Phys. Rev. 92, 881, 1953. PRIMAK, W., FUCHS, L. H. and DAY, P., Phys. Rev. 92, 1064, 1953. ROZMAN, I. M. and TSIMMER, K. G., Zh. Tekh. Fiz. 1681, 1956. SEITZ., F., Disc. Faraday Soc. 5, 271, 1949. SEITZ, F., Rev. Mod. Phys. 18, 384, 1946; 26, 7, 1954. SLATER, J. C , / . Appl. Phys. 22, 237, 1951. SMOLUCHOWSKI, R., International Conference on Peaceful Uses of Atomic Energy,

Geneva, 7, 676, 1955. SUN, K. and KREIDL, N. J., Glass Ind. 33, 511, 1952. VARLEY, J. H. O., / . Inst. Metals 84, 103, 1955. VARLEY, J. H. O., Progr. Nucl. Energy 1, 672, 1956. VARLEY, J. H. O., J. Nucl. Energy 1, 130, 1954. VEDENEEVA, N. E., Dokl., Akad. Nauk SSSR 60, 649, 865, 1949. WITTELS, M., Phys. Rev. 89, 656, 1953. WITTELS, M. and SHERRILL, F. A., Phys. Rev. 93, 117, 1954.

General Reviews—Organic Systems

ALLEN, A. O., Ann. Rev. Phys. Chem. 3, 57, 1952. BURTON, M., Ann. Rev. Phys. Chem. 1, 113, 1950. COLLINSON, E. and SWALLOW, A. J., Chem. Rev. 56, 471, 1956. DAINTON, F. S., Ann. Repts. Progr. Chem. (Chem. Soc. London) 45, 5, 1948. DAINTON, F. S. and COLLINSON, E., Ann. Rev. Phys. Chem. 2, 99, 1951. DAINTON, F. S., Ann. Rev. Nucl. Sei. 5, 213, 1955. GARRISON, W. M., Ann. Rev. Phys. Chem. 8, 129, 1957. HART, E. J., Ann. Rev. Phys. Chem. 5, 139, 1954. HOCHANADEL, C. J., Ann. Rev. Phys. Chem. 7, 83, 1956. MAGEE, J. L., Ann. Rev. Nucl. Sei. 3, 171, 1953. NERMEYANOV, A. N., SAZONOV, L. A. and SAZONOVA, I. S., Russ. Progr. Chem.

22, 2, 1953. PLATZMANN, R. L., Symposium on Radiobiology, Wiley, London, 1952. WEISS, J., Ann. Rev. Phys. Chem. 4, 143, 1953. WILLARD, J. E., Ann. Rev. Phys. Chem. 6, 141, 1955. Radiation Chemistry Symposium, Leeds, Disc. Faraday Soc. 12, 1952.

INTRODUCTION 15

International Conference on Peaceful Uses of Atomic Energy, Geneva, 7, 14, 15, 1955.

/ . Chim. Phys. September 1955. Collection of papers on radiation chemistry, Academy of Science, U.S.S.R.,

1955. Amer. Chem. Soc. Meeting, Cincinnati Ohio, 1955. Nuclear Engineering and Science Congress, Cleveland, Ohio, 1955.

Biological Effects ALEXANDER, P., Atomic Radiation and Life, Penguin Books, London, 1957. ALLSOP, C. B., Brit. J. Radiol. 24, 413, 1951. BACQ, Z. M. and ALEXANDER, P., Fundamentals of Radiobiology, Butterworths,

London, 1955. HAISSINSKY, M. (Editor), Actions Chimiques et Biologiques des Radiations,

Masson, Paris, 1955 and later. HANNAN, R. S., Scientific and Technological Problems Involved in Using Ionizing

Radiations for the Preservation of Food, H.M. Stationery Office, London, 1955. HOLLAENDER, A. (Editor), Radiation Biology, McGraw-Hill, New York, 1954. LEA, D. E., Actions of Radiations on Living Cells, Cambridge University Press,

1955. SPEAR, F. G., Radiations and Living Cells, Chapman and Hall, London, 1953.

CHAPTER 2

RADIATION UNITS ENERGY PER PARTICLE

IN RADIATION work, the most convenient unit of energy is the electron volt (eV), defined as the energy acquired by a single electron (charge l-602xl0~19 coulombs) falling through a potential difference of I V . This very small unit of energy amounting to 1-602 x 10"12 ergs or l-602x 10~19 joules is most frequently used in relation to a single atom, molecule or chemical bond. One mole of molecules or chemical bonds of a specific type comprises 6Ό2χ102 3 such molecules or bonds, so that energy equivalents are

1 eV per molecule or bond=6O2 x 1023 eV/mole = 1·602χ 10- 1 9 x602x 1023 or 9-6x 104 joules/mole = 23-05 x 103 cal/mole = 23-05 kcal/mole.

In single atoms the energies binding an electron to the nucleus range from some ten electron volts for electrons in the outer orbit to many thousand electron volts for electrons in the inner orbits of the heavier atoms. Table 2.1a shows the minimum energy needed to remove a single

Table 2.1a. lonization Potential of Atoms Minimum energy required to remove an electron from a free unexcited atom

(in eV)

H 13 6 He 24-6 Li 5-39 Be 9-32 B 8-3 C 11-27 N 14-54 O 13-62 F 17-42 Ne 21-56

Na 5-14 Mg 7-64 Al 5-98 Si 815 P 10-6 S 10-36 Cl 130 A 15-76 K 4-34 Ca 611

Ti 6-83 Mn 7-43 Fe 7-90 Co 7-86 Ni 7-63 Cu 7-72 Br 11-84 Kr 14-0 Sr 5-69 Ag 7-57

I 10-44 Xe 1213 Cs 3-89 Ba 5-21 W 7-98 Pt 8-96 Au 9-22 Pb 7-42 Ra 5-28

Table 2.1b. lonization Potential of Ionized Atoms (in eV)

He+ Li+ C+ C++

54-4 75-6 24-38 47-87

N+ N++ o+ o++

29-61 47-63 3515 54-94

Ca+ Ti+ Fe+ Fe++

11-87 13-58 16-18 30-65

Ba+ 9-96

16

RADIATION UNITS 17

Table 2.1c. First Ionization Potential of Some Simple Molecules and Radicals

Minimum energy required to ionize a neutral molecule (in eV)

Simple molecules

H2 N?, o2 F2 Cl2 Br2

I2

s2 H 2 0 H2S H F H C 1 H B r H I

15-4 15-6 12-2 17-8 13-2 12-8 9-7

10-7 12-6 10-42 17-7 13-8 13-2 12-8

NO N 0 2 N 2 0 CO

co2 CN

cs cs2 so2 CH3C1 CH3Br CH3I

9-5 110 12-9 141 13-8 140 10-6 10-4 131 10-7 100 9 1

Saturated hydrocarbons

Methane 13-1 Ethane 11-6 Propane 11-3 «-Butane 10-3

Unsaturated hydrocarbons

Ethylene 10-5 Propylene 9-7 iso Butylène 9-65 Butadiene 9-1 Acetylene 11 -4 Benzene 9-2 Toluene 8-9

Radicals

Methyl 101 Ethyl 8-7 H-Propyl 7-8 /soPropyl 7-8 fm.-Butyl 7-2

For references and other values see:

PRICE, W. C, Chem. Rev. 41 , 257, 1947. Handbook of Chemistry and Physics, Chemical Rubber Publishing Co. , Cleveland. H O N I G , R. E.,J. Chem. Phys. 16, 105, 1948. HERZBERG, G., Atomic Spectra and Atomic Structure, Prentice Hall, New York,

1937; Spectra of Diatomic Molecules, Van Nostrand, New York, 1950. MAGEE, J. L.,J. Phys. Chem. 56, 555, 1952. LANGER, A . , / . Phys. Chem. 54, 618, 1950.

electron from various free atoms. Table 2.1b shows the minimum energy needed to extract a further electron from a singly or doubly ionized atom. Table 2.1c gives the lowest ionization potential of some simple molecules and radicals, ranging from about 7 to 15 eV. Owing to the other causes of energy loss, the energy absorbed per ion pair produced in air or many

Table

2.2

. E

ner

gy

Ab

sorp

tio

n f

or

Ion

Pa

ir

Fo

rma

tio

n

(Win

eW)

Rad

iati

on t

ype

ß fro

m H

3

Fast

ele

ctro

ns

Fast

ele

ctro

ns

Fast

pro

tons

a

part

icle

s fr

om P

o

a pa

rtic

les

from

Pu

Ener

gy

Ave

rage

18

keV

1

MeV

1

MeV

17

-5 M

eV

340

MeV

Air

35-0

34

0 33

-9

34-3

33

-3

35-5

35

-2

35-6

—

He

32-5

42

-3

—

—

29-9

42

-7

31-7

—

(4

20)

H2

38-0

36

-3

—

—

35-3

36

-3

370

—

—

N2

35-8

34

-9

34-8

—

33

-6

36-6

36

0 36

-4

36-3

o 2

32-2

30

9 30

-9

—

31-5

32

-5

32-2

32

-9

—

CH

4

30-2

27

-3

26-8

—

—

29

-2

290

291

29-6

co2

—

33-0

32

-6

—

—

34-5

—

34

-2

—

C2H

6

—

24-7

—

. ■

—

■—

26-6

—

—

—

C2H

4

—

26-2

26

-3

—

—

28-0

—

28

0 —

(Fro

m M

arin

elli,

L. D

., A

nn

. Rev

. Nuc

l. S

ei. 3

;249

,195

3.

Ta

ble

2.3

. E

ner

gy

Eq

uiv

ale

nts

eV

erg

joul

e ca

lori

e gr

am-m

egar

ad

kWb

eV

1 6-

242x

1011

6-

242

xlO

i8

2-61

3 X

l0i9

6-24

2 xl

Oi9

2-24

7 χ1

02δ

Erg

1-60

2 xl

O-i

2

1 107

4-18

6 xl

O7

108

3-6x

l0i3

Joul

e

1-60

2 xl

O-1

9

IO-7

1 41

86

10

3-6

XlO

6

Cal

orie

3-82

8 xl

O-20

2-38

9 xl

O-8

2-38

9X10

-1

1 2-

389

8-60

1 xl

O5

Gra

m-m

egar

ad

1-60

2 xl

O-2

0

10-8

lO-i

0-41

86

1 3-

6xl0

5

kWh

4-45

0 xl

O-26

2-77

8 xl

O-1

4

2-77

8 X

lO-7

1163

xlO

-6

2-77

8 X

lO-6

1

1 kW

h ab

sorb

ed g

ives

1 m

egar

ad t

o 36

0 kg

A

voga

dro'

s nu

mbe

r =

602x

1023

1 m

ole

of c

hang

e re

quir

es 6

-02

x 10

25/G

eV

= 2

-68/

G k

Wh

1 co

ulom

b =

lO-i

ern

>lL

=

2-99

8 x

109 e

.s.u

. =

6-24

2 x

10i8

ele

ctro

n ch

arge

s 1

curie

=

3-70

x 1

010 d

isin

tegr

atio

ns p

er s

econ

d

18 ATOMIC R A D I A T I O N AND POLYMERS

RADIATION UNITS 19

Table 2.4. Bond Energies and Internuclear Distances

Bond

H - F T C-FT~

c—cT c=c c=c C E C

c—o c=o C E O

C—N C E E N

O—H

O—O 0 = 0

S—S S—S S—H s=o N - H " Ν Ξ Ξ Ν N—N N—O N = 0 Ν Ξ Ξ Ο

F—F Cl—Cl Br—Br I—I H—F H—Cl H—Br H—I C—F C—Cl

C—Br C—I

Molecule

H2

Paraffins Olefins Acetylene, HCN, CHC13 Benzene

Paraffins, (CN)2 R.CHO; RxR2CO Benzene Olefins Acetylenes

Alcohols, ethers R.CHO; RiR2CO co2 CO Amines, nitroparaffins HCN, (CN)2

H 2 0 Alcohols H 2 0 2

oa s2 >j2v^l2

H8S so2 NH3, amines N2 N 2 0 4 NO Nitroparaffins NO

F . Cl2 Br2

I t H F HC1 HBr HI CF4 Alkyl chlorides CC14, CHCI3 COCl2 Alkyl bromides Alkyl iodides

Energy kcal/mole

ΤΟΦ2 "~98~7

99-4 96-3

100-7 79-3 83-8

116-4 140-5 196-7

~_ 79-6 168-7 1910 255-8

65-9 207-9 109-4 104-7 33-3

117-2

102-6 690 86-8

125-9

~~92:0 224-9

42-5 149-4 103-9 149-4

370 57-9 46-1 361

134-1 1021 860 71-3

1020 760 75-8 74-4 63-3 47-2

eV

"4 7 52 _

"4"·28" 4-31 4-18 4-37

3-44 3-64 505 609 8-53

~3:45" 7-32 8-29

1110 2-86 902 4-75 4-54 1-44 508

4^5 2-99 3-77 5-46 3-99 9-76 1-84 6-48 4-51 6-48 1-61 2-51 200 1-57 5-82 4-43 3-73 3 09 4-42 3-30 3-29 3-23 2-75 205

Internuclear spacing (A)

0-75

1Ό8

f.54

1-35 1-21

1-47 116

1-21

1-43

144 200 2-29 2-67 0-926 1-284 1-423 1-615 1-36

1-76

References STEARIE, E. W. R. Atomic and Free Radical Reactions, Reinhold, 1954. COTTRELL, T. L. Strength of Chemical Bonds, Butterworths, London, 1954. ROSSINI, F. D. Selected values of chemical thermodynamic properties, Vol. 1,

Nat. Bur. Stand., Wash., 1947. MOELWYN HUGHES, E. A. Physical Chemistry, Pergamon Press, 1957.


other gases (Table 2.2) is approximately three times the ionization potential. In organic molecules, bond energies are lower than ionization energies (Table 2.4) whereas the binding energies of neutrons and protons in atomic nuclei are greater by at least five orders of magnitude, and usually exceed the energy of the radiation beams used in this work. For example, the energy liberated in various forms by the fission of a single uranium nucleus totals 196 million electron volts (MeV).

The energy of thermal vibration (kT) is about 0Ό25 eV at room tem-perature. A photon of wavelength λ Â has an energy of 12395/λ eV. Infra-red photons have energies of up to l-5eV, visible light (8000-4000Â) between 1-5 and 3 eV and ultraviolet (above about 2000 Â) up to 6 eV. Far ultraviolet photons carry larger energies, but are readily absorbed in air. A comparison of these figures with the ionization values in Tables 2. la,b,c, show that while the energy in ultraviolet radiation is sufficient to cause excitation in many organic structures, it is insufficient to cause ionization (except for very far ultraviolet radiation).

The energy carried in an ultraviolet photon is very similar to the difference in electron energy levels of many compounds and resonance absorption may occur, resulting in the specific chemical changes observed in photo-chemistry. For high energy radiations the energy carried per primary particle or photon (usually above 105 eV) is considerably greater than the ionization potential of most atoms, so that resonance absorption no longer appears to occur.* Electrons in an organic structure are therefore removed at random from their parent atoms or molecules by such radiation. When the energies involved in high energy radiation exceed several million electron volts they may affect the nucleus itself, leading to nuclear changes and possible radioactivity. The effects we are considering arise primarily from changes in the arrangement of orbital electrons, and nuclear reactions when they occur in such work are purely incidental. They may indeed be disadvantageous as the radioactivity produced intro-duces health hazards and handling difficulties. It is therefore preferable to work with radiation at energy levels too low to cause nuclear changes in the material considered. Fortunately the elements commonly occurring in polymers do not readily become radioactive. For carbon 12, for example, electron or γ-energies of at least 18-7 MeV are required to cause any induced radioactivity.

RADIATION YIELD—G VALUES Although the individual reactions may not be known, the sensitivity of

a system to radiation can be expressed in terms of the number of changes produced by a given radiation dose. For example, in the case of air one can refer to the W value (about 34 eV), the energy absorbed per ion pair produced. In the early literature on radiation reaction in gases, the yield was represented by the ratio A///V, defined as the ratio of molecules reacting (M) divided by the number of ion pairs formed (TV). In more

* Exceptions occur with low or medium energy x-rays which give photoelectrons (see Chapter 3).

RADIATION UNITS 21

recent work, the yield is usually expressed in terms of a G value, this being defined as the number of chemical changes of a given kind produced per 100 eV absorbed. In using G values it is not implied that all the energy absorbed is used to produce the relevant entities. This definition has the advantage that no assumptions are made as to the mechanisms of the reaction, and for any system different G values can be given to denote each of a series of products formed in the course of a single irradiation. Thus the ionization of air has a G value of 2-9 if the energy absorbed per ion pair formed is 34 eV. To convert the earlier results in the literature, it is often sufficient to take G = 3M/N to within experimental error. For many chemical systems G values of the order of 1 to 10 are common.

INTENSITY OF RADIOACTIVE SOURCES—THE CURIE Radioisotopes emit high energy radiation by a rearrangement of an

unstable nucleus. The intensities of such sources of radiation are expressed in curies. The early définition of the curie was based on the equilibrium between radium and radon but the present definition, which is applicable to all isotopes, defines the curie as the amount of a radioactive element in which there are 3-7 x 1010 disintegrations per second. Thus, a source of c curies emitting gamma radiation of E MeV, will emit

3-7 x 1010x 106 Ec eV/second or 5-92 x 104 Ec ergs/second or 5-92 Ec milliwatts.

In many cases, the disintegration of a radioactive nucleus gives rise to other unstable nuclei which disintegrate in their turn and contribute to the energy emitted. To obtain the total energy produced by such isotopes, it is therefore necessary to add together the energy of each of these successive radiations. This is notably the case for Co60 where, in the course of dis-integration, two γ-photons of energies 1-33 and 1-17 MeV are emitted. The energy output per curie of cobalt 60 is therefore 5-92 x (1 -33 + 1 -17) = 14-8 milliwatts/curie. In other cases, the radiations emitted are of such different energies, penetrations or half lives that only one of the resultant radiations can be utilized at any one time. Thus, caesium 137, with a half life of 33 years, in the course of decay to an excited barium 137 nucleus emits beta radiation of 0-5 MeV and 1-19 MeV; the excited barium nucleus then decays further with a half life of 2-6 minutes to barium 137 emitting a 0*66 MeV γ-photon. In practice caesium 137 is used for radiation purposes only for its γ-photon, the ß-particles generated being absorbed in the container. Thus, the potentially useful radiation from caesium amounts to only 5-92 x 0-66 or 3-92 milliwatts/curie. The power output from a curie of cobalt is nearly four times as great as that of a curie of caesium due both to the higher energy of each γ-photon emitted and to the fact that the energies of the two γ-photons are sufficiently close for them both to be used in the same process. On the other hand, radioactive caesium has a much longer half life, so that the total energy emitted by a curie over a period of many years may be higher. For ß-emitting isotopes the energy of the ß-particles varies over a wide


range; the useful penetration may have to be restricted to a narrow range, with a resultant loss of effective output.

The total power output from mixed fission products is often expressed in MeV-curies obtained by multiplying the number of curies of each con-stituent by the energies of the radiations emitted. 1 MeV-curie corresponds to an output of 5-92 milliwatts. It has been estimated that by 1970 some 2 x l 0 1 0 MeV-curies or 118 megawatts of fission product power output will be available in Britain as a by-product of the use of nuclear fission for power production.

The number of curies in a source multiplied by the energies of the useful radiations emitted represents the total output of power from a radioactive source and may usefully be expressed in watts. In this way, the output may be directly compared with that of alternative sources such as electrical machines. For example, the output of γ-radiation from a 1000-curie cobalt source is 14-8 watts, but the useful power will be smaller by a factor which depends on the fraction usefully absorbed.

UNIT OF RADIATION FLUX—THE ROENTGEN AND REP From the point of view of utilization of atomic radiation to achieve

chemical changes it is necessary to consider not only the total power output of a source but the actual beam intensity or flux at any point. Until very recently, this was generally expressed in terms of the roentgen, a unit of radiation flux based on the ionization produced in air.

The roentgen (r) is defined as the quantity of x-ray or γ-radiation such that the associated corpuscular emission per 0-001293 g of air (i.e. 1 cm3 of air at s.t.p.) produces, in air, ions carrying one e.s.u. of electricity of either sign. The charge on the electron is 4-8025 x 10~10 e.s.u., so that 1 e.s.u. corresponds to the formation of 2-082 x 109 ion pairs. If the energy absorbed per ion pair formed in air is W the energy absorbed per roentgen is 2-082 x 109 WeV per 0001293 g of air, or taking Was 32-5 eV 5*23 x 1014eV/g or 83-8 ergs/g of air. This definition permits radiation doses in air to be measured by electrical means without any need to deter-mine the energy absorbed. Difficulties arise however when considering the energy absorbed in other media or in measuring the doses of other kinds of ionizing radiation. Moreover for a given dose (measured in roentgens) the energy absorption in air will depend on the value of W, which must be determined experimentally; recent figures give a value of about 34-5 eV per ion pair formed,* corresponding to 89 ergs/g. In water the energy absorbed per roentgen is greater than in the same mass of air, due largely to the different concentration of orbital electrons, and the same radiation dose of one roentgen liberates 93 ergs/g of water. To over-come these difficulties it has become common to work with alternative units which are equivalent to the roentgen in some respect. The rep (roentgen equivalent physical) is often used for electron radiation and is taken to refer to a dose which deposits 84 ergs/cm3 in tissue, later changed

* The International Commission on Radiological Units has recently (1956) recommended a value for W of 34 eV.

RADIATION UNITS 23

to 93 ergs/cm3 to correspond to the energy deposited in tissue by 1 r of x-rays or γ-rays. It does not follow that 1 rep of electron radiation will deposit the same amount of energy in other media as does 1 r of x-rays, and an alternative definition may be in terms of equivalent energy deposition in water (93 ergs/g). Another unit primarily of biological interest is the rem (roentgen equivalent man), the dose of any radiation which produces the same biological effect as 1 roentgen of x-rays or γ-rays. Obviously the rem will depend on the particular biological system chosen; it is conveniently related to the roentgen or the rad by a factor, termed the relative biological effectiveness (R.B.E.).

ENERGY DEPOSITED—THE RAD To obviate these difficulties a further unit, the rad, is now coming into

use. One rad corresponds to an energy absorption of 100 ergs/g in the particular medium being studied. Thus an absorbed dose of one megarad corresponds to an energy absorption input of 108 ergs, 10 joules or 2-4 calories/g. The rad differs from the roentgen and related units in that it is not a measure of the total radiation flux or dose of x-or γ-rays but of energy deposited in 1 g of any medium by ionizing radiation. Thus 1 g-megarad merely represents 10 joules of absorbed high energy radiation. The same radiation beam will therefore give rise to different doses expressed in rads, depending on the material encountered, and a beam producing 1 rad in water will only produce about 0-9 rads in air. Many organic materials are sufficiently close to water in their absorption capacity for this difference to be ignored.

Most published work uses doses expressed in terms of roentgen or rep. Wherever possible these results have been converted to rads, which is a more convenient unit to use in relating energy output from a source to chemical changes produced. Many chemical changes require doses of the order of megarads, often written as Mrad. Thus the energy which must be absorbed to produce a chemical change requiring r megarads in m grams is

(r) m x 108 ergs or 10 r m joules.

It should perhaps be emphasized that the roentgen and the rad are units of integrated radiation dose and energy absorption. The radiation intensity is often expressed in units of roentgen/min (for cobalt radiation), or megarads (r)/second (for intense electron beams).

One kilowatt-hour of high energy radiation is equal to 3-6 x 106 joules or 3-6 x 1013 ergs. If fully absorbed it can therefore give a dose of r megarads to

3-6 x 1013/108r g or 360/r kg of material,

or approximately 800/r lb. Thus if the radiation dose required for a given chemical change

(expressed in megarads) and the cost of high energy radiation (expressed in cost per kilowatt-hour including overheads) are known, the cost of


radiation treatment can be computed. In chain reactions such as poly-merization attention must also be paid to the radiation intensity which affects the dose required.

The yield of a process can also be expressed in terms of the G value if this is known.

1 kWh = 3-6 x 1013 ergs = 2-25 x 1025 eV, and if this energy is fully absorbed in a system it will produce 2-25 xlO2 5 G/102 or 2-25xl023G changes of the type to which the G value refers. Since 1 mole refers to 6*02 x 1023 molecules or bonds.

1 kWh gives 2-25 x 1023G/602x 1023 - 0-374G moles of product.

For many reactions which are not of the chain type, G is of the order of 3, and 1 kWh of high energy radiation, if fully utilized, will therefore produce about 1 mole of irradiated product.

REFERENCES (Radiation Units)

Stockholm Congress of Radiology, 1928. Chicago Congress of Radiology, 1937. International Commission on Radiological Protection, Brit. J. Radiol. Supple-

ment 6, 1955. International Commission on Radiological Units, Copenhagen, 1953, Brit. J.

Radiol. 27, 243, 1954. Brit. J. Radiol. 29, 355, 1956.

CHAPTER 3

INTERACTION OF RADIATION WITH MATTER THE TERM high energy radiation is applied both to particles moving with high velocity—fast electrons or ß-particles, fast protons, neutrons and a-particles—and to electromagnetic radiation of short wavelength—x-rays and γ-rays. In the latter the radiation can best be considered as a series of individual particles (photons) each of high energy. The processes by which these different forms of radiation react with the atoms of a specimen through which they pass may be very different, the common feature being the high energy carried by each particle or photon, this energy being very much greater than that binding any orbital electron to an atomic nucleus. In this respect they differ from slow or thermal neutrons, and from ultraviolet light, in which the energy carried per particle or photon is usually smaller than the ionization energy of an atom or molecule.

In passing through matter, all these forms of high energy radiation lose energy by reacting with the electrons and nuclei of the medium, and may

FIG. 3.1. The range or half-value thickness of various forms of high energy radiation in materials similar to polyethylene.

(From K. H. Sun, 1954) 25

26 ATOMIC R A D I A T I O N AND P O L Y M E R S

give rise to displaced nuclei, free electrons, ionized atoms or molecules (which have lost these electrons) and excited atoms or molecules (in which an electron is raised to a higher energy level). Changes in the nuclear structure only occur with sufficiently energetic particles or photons, but such changes are not of direct importance here. The entities produced may subsequently react with each other, and with other atoms or mole-cules to give rise to new chemical structures.

From the point of view of chemical effects, the most important differences between these various forms of high energy radiation depend on the rate of energy loss per unit of path travelled. This determines the penetration of the incident beam, and the density and distribution of the ions and excited molecules about the path of each incident particle. Where the ion density is low, as in electron or γ-ray irradiation, reactions will be largely between an ionized or excited molecule and the surrounding neutral molecules. Where local ion densities are high, as in a-particle bombardment, reactions between two adjacent ions may complicate the picture.

1000

E 100kr

10keV 10MeV 100 MeV

FIG,

100 keV IMeV Energy of incident particle

3.2. Specific ionization of electrons, protons and α-particles in air. The number o f ion pairs produced for a given path is a measure of the rate of energy loss, since the energy absorbed, per ion pair produced, varies little with particle energy. The unit chosen (mg/cm2) corresponds to a thickness (10-3cm) multiplied by a density (g/cm3).

The distinction between fast electron or γ-radiation on the one hand, and fast protons or a-particles on the other, as producing sparse and dense ionization respectively is not completely clear cut. In both cases secondary electrons are produced, of low energy and high ionizing power. These secondary electrons produce in the immediate neighbour-hood of the track of the primary particle other regions of dense ionization, and therefore reduce the distinction between the two groups of radiation.

Information on the ionization produced in gases can be readily obtained from ionization chambers, and from Wilson cloud chambers. Fig. 3.3a-d

INTERACTION OF RADIATION WITH MATTER 27

show for example the ionization along the tracks of fast and slow electrons, γ-photons and a-particles, and indicate clearly the dependence of ioniza-tion density on particle mass and energy. In the solid or liquid state (with which we mainly are concerned) no such direct evidence is available, and it is necessary to proceed largely by deductions from theory and from experimental observations made in the gas phase. Evidence on excitation is even more scarce and a detailed comparison of radiation effects with those observed in photochemistry is highly desirable. While these limita-tions are not a very serious handicap in the more practical applications of radiation, they do render more difficult a full understanding of the basic processes underlying the observed radiation-induced changes. One can expect the study of such changes in selected systems to lead to information on these basic processes, and this aspect is discussed in Chapter 26, after the experimental data are presented. The present chapter is mainly concerned with the mechanisms by which energy is transferred from the incident radiation to the specimen.

ELECTRONS High energy electron beams used in radiation work may either be sub-

stantially monoenergetic, as in certain electronic sources, or they may cover a wide spectrum of energies, as in radioactive ß-emitters. In passing through a specimen each electron loses energy by reaction with the orbital electrons or the nuclei, and will also be scattered. The effects produced in the irradiated specimen will therefore vary with its thickness. Energy can be transferred to the orbital electrons to produce ionization and excitation, and to the nuclei causing displacements. At very high energy a considerable proportion of the energy loss arises from the deceleration of the electron by the nuclear field, giving rise to x-rays {Bremsstrahlung) which in turn induces further radiation changes in the specimen (see page 28).

Electron-Nucleus Reactions

Electrostatic attraction will cause a sudden change in the trajectory of an electron if it passes close to an atomic nucleus. Owing to the consider-able difference in the masses involved, very little energy is transmitted to the nucleus, however, and the collision may be referred to as elastic. The maximum energy which can be transferred to a nucleus of mass M by an electron of energy E (in MeV) is approximately

Emax = 2200 E(l+E)/M (3.1)

where Emax is expressed in eV. Thus a collision of a 100 keV electron with a hydrogen atom can only impart to it a maximum of 240 eV, while for a carbon atom the maximum energy transfer is only about 20 eV, barely sufficient to break a chemical bond. Thus only electrons of high energy are capable of causing chemical change by direct displacement of the atomic nucleus, or by the subsequent ionization or excitation resulting from the motion of the ejected nucleus through the specimen. The


number of such close collisions is in any case small, and the main effect of electron-nucleus interaction is the scattering it causes in the incident beam. In passing through a specimen, an electron may suffer a number of such close collisions, be reversed in direction and emerge on the incident face of the specimen. The deflection of an electron depends on the square of the nuclear charge (i.e. Z2) and will therefore be most serious for specimens with atoms of high atomic number, where a con-siderable proportion of the energy in the incident beam may be lost by back-scatter. In specimens with atoms of low Z number the effect is far less important since the proportion of incident electrons backscattered is relatively small; moreover the residual amount of energy carried by these back scattered electrons is also lower. For most plastic materials, energy loss due to backscatter need only be taken into account in the most accurate measurements. Owing to multiple scattering caused by electron-nuclear reaction the depth of penetration of an electron into a specimen differs from the true range (or track length measured along its path). An expression for the true range of electrons of energy 10 to 200 keV, in materials of low atomic weight, is

R = V25E2A/pZ (3.2)

where E is the electron energy (in MeV), A and Z are the atomic weight and number, and p is the density of the material. For higher energies the range is more nearly proportional to its energy. Fig. 3.4 shows the increase in track length in a medium (tissue) of unit density which approxi-mates in stopping power to many polymers.

At very high energies, an appreciable amount of the incident electron energy is lost by the production of x-rays bremsstrahlung. The extent of this loss depends both on the energy E of the incident electron, and on the atomic number Z. The ratio of energy loss by bremsstrahlung, to that by ionization and excitation, is £Z/800 {Em MeV). Thus for a heavy element such as gold (Z = 79) used as a target for electron bombardment by 2 MeV electrons, this ratio is 0-2, but decreases rapidly as the electron energy is reduced. For plastic specimens in which Z rarely exceeds 6 or 8, subjected to 3 MeV electron bombardment, the energy loss due to brems-strahlung only averages about 1 per cent, well within the usual experi-mental accuracy of dose measurements.

The rapid increase in energy loss due to bremsstrahlung emission limits the penetration of very high voltage electrons. The bremsstrahlungen themselves cause further ionization and pair production.

Ionization and Excitation

Electron beams of the energies used in radiation work lose most of their energy by reacting with orbital electrons; the primary electron is deviated, and the bound electron may either be given sufficient energy to leave its parent atom completely (ionization) or move to an orbit of higher energy (excitation). In the former case a positively charged atom or molecule (ion) and a free electron are left. The positive ion is in an

FIG

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FIG. 3.3 (c). High ionization density and irregular path of slow electrons, as compared with low ionization density and more linear path of high energy electrons (in mercury vapour).

FIG

. 3.3

(d)

. H

igh

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unstable state, and may then undergo decomposition, or react with neighbouring molecules or other ions. The free electron may either return to its parent molecule to give a highly excited molecule, or it may be captured elsewhere, giving a negative ion.

The observed chemical effects may therefore be due to the primary positive ion, the free electron, the excited molecule, or to ions or radicals (uncharged molecules with an unpaired electron) which these may subsequently produce.

The ratio of energies lost in excitation and in ionization is not known accurately, and it is generally assumed that they are comparable in amount.

1-OF 1 1 ! 1 c m E I I

0-1hr cmF

100/4-= 10"2cmE

<υ 10/zL c t-

-* Γ υ σ

=io4Âf

lOOOAk

100ÀL

ΊΟΑΐ I I I I ! I ! M I l ι Mini ι l ι Mini ι ι ι ι ι ι ι ι Ι 102 103 104 105 10 6

Energy of electrons, eV

FIG. 3.4. Track range of electrons in tissue of unit density. Track range is about 40 per cent more than penetration.

(From Lea, 1955.)

The probability of ionization or excitation depends on the velocity of the electron, and increases rapidly as the electron slows down towards the end of its path. On the other hand, the energy lost during such a collision is greatest early on in the path, when the primary electron has its maximum energy (as shown in Fig. 3.5 based on data given by Lea). The electrons


liberated by ionization may therefore have sufficient energy to cause further ionization and excitation. Such secondary electrons are termed δ-electrons.

^-electrons

The average energy of δ-electrons is small, only about 5 per cent of the ejected electrons having energies in excess of 100 eV, and 1 per cent in excess of 500 eV. Due to their low velocity these electrons have a short track, and each produces a small cluster of ions in the neighbourhood of the track of the primary ion, mainly within a cylinder of 100 Â radius (Williams, 1933); the average distance between ions in the same cluster is of course much smaller. According to Lea (1955) the integrated track length of all the δ-electrons is only about 2-5 per cent of that of a primary

<™ 2000

1000

500

200 c

100 g

so ;Ë

20 .o

io £ 5 |

Q_

2 100 103 104 105 10° 107

Energy of incident electron ^ eV

FIG. 3.5. Rate of energy loss from electron beam and formation of primary ions (in tissue of unit density).

electron between 10 and 400 keV, but the number of ions produced is nearly equal. This is of course due to the higher rate of energy loss per unit path for lower energy electrons (equation 3-3). The number of δ-electrons is considerably greater than the number of electrons in the primary beam; if the average energy of δ-electrons is about 50 eV, and they account for half the incident energy, then in a 1 MeV beam each incident electron produces about 10; δ-electrons.

After the electron has lost most of its energy it will be incapable of causing further ionization or excitation. The subsequent fate of the electron is a subject still under active discussion. In due course it may be captured by another atom—often oxygen—to form a negative ion. This is certainly the case in gases where the separation of the positive and negative ion pairs formed may be judged from Wilson Chamber photo-graphs and is usually about 20μ (2χ105Α); the corresponding figure for water or solids cannot be obtained directly, but certain theories suggest it is about 200 A. According to the theory of Burton, Magee and Samuel (1950), the free electron is unable to escape the field of the positive ion, and rapidly returns to it to give a neutral but excited atom. If this

\J\J

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1

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100 - £ o 60 40 30 20

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theory is accepted, it is necessary to explain the chemical changes produced by radiation purely in terms of the behaviour of excited mole-cules. According to views expressed by Platzman, however, the electron may, in suitable media, move out of the sphere of influence of its parent molecule or ion, losing energy until it becomes a subexcitation electron, i.e. an electron with insufficient energy to cause further excitation. In this state it may still react with impurity atoms having a lower excitation level.

Stopping Power

The rate of energy loss (or stopping power of the medium) due to inelastic collisions by a charged particle per unit length of path traversed can be written

άΕ dx

_2πβ*ζ2ΝΖ mV2 B

(3.3)

where TV is the number of atoms of atomic number Z, per cm3, z and V are the charge (1 for an electron) and velocity of the incident particle, and e and m are the charge and mass of the electron. B is the stopping number of the medium, which can be calculated on quantum theoretical considera-tions (Bethe, 1933). B rises only slowly with particle energy, hence dE/dx falls at first, reaches an approximately constant value as V approaches the speed of light, then rises again slowly for very high energy particles. There exists therefore a broad minimum rate of energy loss at energies of the order of 1 MeV for electrons (Fig. 3.6). The stopping number B depends on the excitation potentials of the atoms of the medium trav-ersed; at higher electron energies when the energies available are con-siderably greater than these potentials, the rate of energy loss depends only on the electron density NZ of the medium and not on its chemical structure. For very low energy electrons (e.g. the secondary δ-radiation) more specific interactions may however be expected.

2-2

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1 1 i Graphite

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0

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il 100

Energy of incident e lec t ron , MeV

FIG. 3.6. Stopping power of various materials for fast electrons. (Energy loss per mg/cm2.)


The variation in the rate of loss of energy for electrons of different energies is shown in Fig. 3.5 which also gives the number of primary ionizations produced for this loss of energy.* The ratio of these two gives the average loss per ionizing collision, but part of this energy loss is, of course, taken up by excitation. At higher energies of the incident electron, the average loss per ionization is about 80 or 90 eV and part of this energy will appear in a δ-electron, capable of producing further ionization.

Electron Penetration With an incident parallel beam of high energy electrons, the net effect

of scattering of the primary electrons, the decrease in their energy, and the production of δ-rays is to cause very non-uniform ionization up to the maximum penetration of the primary electrons. For incident electrons of 1 MeV energy or above, the maximum penetration in water amounts to approximately 0-5-0-6 cm per MeV. For materials of different density the penetration can best be expressed in g/cm2, since for most atoms the number of electrons (which determines the ionization) is approximately proportional to the atomic weight (more accurately the atomic number). The Feather formula gives the penetration R (in g/cm2) in aluminium of ß-rays of maximum energy E (in MeV) between 0-7 and 15 MeV:

R = 0-543£-0160. (3.4)

Variation of Ionization Density with Penetration Because of the various factors discussed above, the ionization produced

by an initially uniform electron beam as it penetrates a specimen varies 100

80

c .9

8 60 'c ,o

E | 4 0

'x σ E

** 20

u 0-1 0-2 0-3 0-4 0-5 Penetration g/cm2 MeV

FIG. 3.7. Ionization density vs. penetration for single irradiation from one side.

* These figures only apply to a short range of track along which the incident electron still conserves its initial energy almost intact. The total number of primary ions produced must, of course, be derived by integrating these curves over an incident electron energy from its initial value to zero.

\\+—o 1

33 gm/c m/Mev-


with depth. Fig. 3.7 shows the change in the ionization density which reaches a maximum value at a depth of about 0-16 g/cm2, per MeV of initial electron energy, subsequently dropping to very small values at about 0-5 g/cm2. For accurate radiation measurements it is highly desirable to work with a uniform field of ionization. This can be achieved by using thin specimens, located between similar material, of thickness such that the specimen to be studied occupies an approximately flat portion of the curve, i.e. near the maximum.

For many applications this procedure would cause an excessive waste in radiation energy. It is therefore judged sufficient to use the dotted area under the curve, so that the ionization varies from 60 per cent of the maximum value to the maximum, i.e. a variation of ±25 per cent. Approximately one-third of the maximum penetration is lost in this pro-cedure, but the energy loss is of course very much smaller. The useful range, defined as the maximum distance in which the ionization does not fall below 60 per cent of the peak value, is given in the following table.

Table 3.1. Electron Beam Penetration for 60 per cent of Maximum Ionization

Incident beam (MeV) 2 3 10-6 21-6 40 55 Penetration (g/cm2) | 0-68 106 3-9 8 12-1 15-2 Penetration/MeV (g/cm2) 0-34 0-35 0-37 0-37 0-33 0-36

A greater utilization of the energy in the beam can be achieved in certain cases by irradiating a specimen from both sides (Fig. 3.8). In this manner full use is made of the energy in the tail of the beam. An effective figure of

100 c o

& 80 c .o Έ | 60

'x Ό E 40

0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 Depth of penetration, gm/cm2/M^'

FIG. 3.8. Ionization intensity vs. penetration for double irradiation.

0-8 g/cm2 penetration per MeV can be achieved by this double treatment, again with a variation of ionization from 100 per cent to 60 per cent of maximum.

Taking a figure of 0-35ΖΓ g/cm2 as the effective penetration of an incident electron of energy E MeV, the depth of penetration in a material of density p is 0-35 £p- 1 cm or 0-35 x 104 £p - 1 microns. The average rate of energy loss is therefore Ex 106/ 0-35 x 104 up - 1 or 286p eV per micron. An

0 81 gm/cm2/Mev


electron beam current of / μΑ/cm2 is absorbed in 0-35E g of material, where it liberates Ei watts, corresponding to an average energy absorption of Ei/0-35E or 2-86/watts/g. This is equivalent to an average dose rate of 0-286/ megarads/sec.

When ß-radiation is used from a radioactive source, the maximum penetration is still given as about 0-35E g/cm2, if Eis taken as the maximum energy of the ß-particles. The shape of the ionization curve is, however, very different from that shown in Fig. 3.7, due to presence of lower energy particles, with shorter maximum range. It is often sufficiently accurate to consider the ionization of ß-radiation as falling off exponentially. If ß-ray ionization falls to half its initial value in a thickness d of a material, a variation of ±25 per cent in ionization density will only be achieved if the specimen thickness is restricted to 0-1 Ad, half the incident energy being lost.

The change of ionization with medium, and the effect of scatter may cause serious errors if there is a sudden change in medium. Fig. 3.9 shows that with 100 keV x-rays, the ionization near the surface is greatly increased by the presence of glass. With a paraffin-wax tissue interface there is little discontinuity in stopping power, and no important surface effects occur.

s

b c

• ^ _=___ __

0-02 0-04 0-06 0-08 0-1 mm from interface

FIG. 3.9. Effect on ionization of change of medium near an interface. Electrons emitted from wall may cause ionization in medium.

(a) Glass/tissue interface. (b) Extrapolated uniform dose. (c) Paraffin wax/tissue interface.

(100 keV x-rays.) (From Spiers, 1949.)

X - R A Y S A N D γ - R A Y S

In passing through matter, x-rays and γ-rays may lose energy by colliding with the orbital electrons (Compton effect), by photoelectric absorption, by reacting with the nucleus to form a radioactive isotope, or by pair production. In water the photoelectric effect is the major process below about 60 keV, and Compton scattering predominates at energies between 60 keV and 25 MeV. Pair production can only occur at energies above 1-02 MeV, and in water is the major effect only above 25 MeV (Fig. 3.10).


Nuclear changes Nuclear reactions occur at energies depending on the particular nucleus

involved, but usually well above 8 MeV. Table 3.2 shows a number of such reactions resulting from the capture of a γ-particle by the nucleus. Nuclear reactions are in general to be avoided in radiation work, since the radioactive materials produced, unless very shortlived, may render

0-16r

0-1 0-2 0-5 1-0 2 5 10 20 50 100 200 500 1000 Photon energy, MeV

FIG. 3.10. Absorption of x-rays due to the photoelectric effect, Compton scattering, and pair production in water.

(From Bacq and Alexander, Fundamentals of Radiobiology.)

handling of the irradiated material dangerous, while the radiations emitted by the nucleus can cause continuing radiation induced changes in the specimen.

The passage of an x-ray or a γ-photon through a specimen produces one or more fast electrons, and an x-ray photon of lower energy. It is these fast electrons which, by their subsequent ionization or excitation as described in the previous section, cause the major radiation induced changes in organic materials. x-Rays and γ-rays are therefore best considered as internal sources of electron radiation.

Table 3.2. Radioactivity Induced by y-Radiation

Element

Be8

H2

J127

Li7

Br81

N 1 4

p 3 1

K 3 9

S 3 2

0 1 6

Si28

C12

Type of nuclear reaction

(γ,Λ) (γ,Ό (γ,Ό (γ,/>) (γ,Ό (γ,/ι) (Ύ,η)

(γ,ι) (γ,Ό ( Ϊ ,Λ) (γ,Ό (Υ,Λ)

Threshold energy (MeV)

1-67 2-2 9-3 9-8

10-7 10-65 12-35 13-2 14-8 16-3 16-8 18-7

Half-life of product

13 days 0-85 sec 6-4 min 10 min 25 min 7-5 sec 3-2 sec 2-1 min 5 sec 21 min

D


Photoelectric Absorption

x-Ray photons of low energies, of the order of a few keV for atoms of low molecular weight, may be absorbed in the inner electronic shell of an atom and emit an electron. The energy of the electron ejected will equal that of the incident photon, less the binding energy of the electron which, for carbon, nitrogen or oxygen atoms is a few hundred volts. An internal rearrangement of the ionized atom may then take place and eventually result in the ejection of one or more electrons of low energy (Auger effect) or of a low energy x-ray photon; the latter is rare for elements of low atomic number.

Since the incident photon is fully absorbed, the beam intensity falls off exponentially. The photoelectric absorption coefficient τ in the formula / = /0exp(—xr) determines the number / of incident photons I0 which will remain unabsorbed by the photoelectric process in passing through a layer of thickness x.

An empirical value of τ for the elements C, N or O can be obtained from the formula τ = 270*0089 λ 3*05Z4,1p/^ where Z is the atomic number, A the atomic weight, λ the wavelength and p the partial density of each element in the specimen.* For water, this becomes τ = 2-50 λ3'05, so that for an incident x-ray beam of wavelength 1Â (12-4 keV) the intensity will be reduced to 37 per cent (e_1) in a thickness of 1/2-5 or 0-4 cm due to photoelectric absorption only. For a beam of 0-5Â (24-8 keV) the corresponding distance will be about 3-2 cm, while for a 01Â x-ray (124 keV) it amounts to 400 cm. For higher energy photons, however, most of the absorption arises from the Compton scattering, and this absorption is additional to that due to the photoelectric effect which for these energies is often negligible.

For plastics consisting mainly of C, H, N and O atoms the formula for water can be used to provide an estimate of the rate of energy absorption for low energy photons up to about 50 keV. For materials containing appreciable proportions of heavier elements such as Cl or Si, its use is somewhat less satisfactory for energies up to 100 keV.

The electrons ejected by the photoelectric effect lose their energy by ionization and excitation of neighbouring molecules (see above) and are absorbed in a distance which is small compared with the path of the incident x-ray photon. For monochromatic x-rays of energies below about 50 keV, the dose distribution of ionization and excitation in an irradiated material is therefore approximately exponential in character. In practice, x-ray machines give rise to a spectrum of wavelenths, ranging down from that corresponding to the applied voltage with a peak intensity at about half this voltage. As a result the drop in ionization density with depth of penetration will be greater than that calculated from the applied peak voltage.

* A derivation gives for the approximate cross-section σ9 =10- 8 3 Ζ β E-*-& ~ 0 ·5χ10- 2 β Ζ β λ ί β

for photoelectric absorption by K electrons. The L and M electrons contribute only a small amount.


Compton Scattering The loss of energy of x-ray photons by Compton scattering arises from

elastic collisions between the photon and an electron of the medium through which it passes. The interaction can be considered as a billiard ball collision between the two particles (photon and electron), energy and momentum being shared between them. The binding energy of the electron can generally be ignored above a few keV; the electron is ejected from its parent atom while the photon continues in a different direction, and at a lower energy. Due to the Compton effect, an incident mono-chromatic x-ray or γ-ray beam will therefore become scattered, the scattered photons having different (and lower) energies. Although the intensity of the primary beam falls off exponentially, for thick specimens further energy absorption from the ill-defined secondary beams may also occur to an extent which depends on the geometry, but will often be difficult to calculate (Fano, 1953).

The Compton recoil electrons may have energies ranging from 0 to a

fraction —,--r~>- of that of the incident photon, where h\> is the energy of

the photon and mc2 is the rest energy of the electron. The maximum energy of these recoil electrons and their average value are shown in Fig. 3.11 for a range of energies of the incident photon. The average energy

11 i I I N / i V I M i i M 1 I 1 I ! I 1 i 11 ! I 5 10 20 50 100 200 500 1000 2000

Energy of incident gamma photon, keV

FIG. 3.11. Energy of photoelectrons and Compton recoil electrons from x-rays or γ-photons.

of the recoil electron is approximately half this value. The energy of a photoelectron (shown in the same figure) would on the other hand be equal


to that of the incident photon, less a very small amount corresponding to its binding energy.

To calculate the rate of energy loss it is necessary to know not only the average loss per collision but also the number of such collisions. The probability of a collision depends only on the electron density traversed and not on the chemical constitution or structure of the medium. It is therefore possible to obtain a scattering probability per electron, and multiply by the number of electrons per gram or cm3 to deduce the number of collisions. The probability άΡ of an incident photon being scattered by a Compton collision in traversing a thickness ax can be written dP = ean ax where n is the number of electrons per cm3 and ea is the Compton scattering coefficient per electron which depends only on the energy of the incident particle. For energies in excess of 0-5 MeV ea is usually inversely proportional to the photon energy. The average energy transferred per collision may be written Ë where Ë lies between 0 and

2/zv hv — 2 T2 /T~ a n d hv is the energy of the photon. Since mc2 equals

0-51 MeV, if the energy hv of the incident photon is written as E0 when measured in MeV, this maximum value is about 4 £0

2/(4 £ 0 + l ) MeV. The energy transferred to the electrons is therefore άΕ = ΝβσηΕάχ where there are TV incident photons per cm2.

Values of e<5 for different energies E0 of the incident primary photon can be deduced from the formula derived by Klein and Nishina (1929). At E0 = 0-1 MeV, e<5 = 5 x 10~25; at E0 = 1 MeV ea = 2-1 x 1 0 2 5 while for E0 = 10 MeV, σ = 0-4 x 10~25. The average fraction of energy transferred Ë/Eo increases from 14 per cent at E9 =0 -1 MeV, and 49 per cent at E0 = 1 MeV to 68 per cent at 10 MeV.

It is often convenient to combine both the probability of a Compton collision and the proportion of energy transferred Ë/E0 into a single coefficient, termed the energy absorption coefficient eaa, defined by the equation

eaa = ea Ë/Eo (3.5)

where e<sa refers to the fraction of incident energy absorption per electron. The total energy transferred when N γ-photons of energy E0 pass through a specimen of thickness ax containing n electrons per cm3

άΕ = Ne<saE0ndx. (3.6)

Fig. 3.12 shows the fraction of the incident energy absorbed per electron due to Compton scattering of the primary beam, for a range of energies of the incident photon. Multiplying these values by the number of electrons/cm3 gives the total energy transferred per cm path. The figures on the right of this figure give the appropriate scale for water (3-34 x 1023

electrons/cm3) and for long chain paraffins (polyethylene) approximating to (CH2)n (3-53 x 1023 electron«/cm3). For thin specimens, in which energy lost from secondary (recoil) photons may be ignored γ-photons of between 0-3 and 1 MeV will transfer about 3 per cent of their energy per


g/cm2 to electrons, which will then cause further ionization and excita-tion. For thicker specimens the contribution of the scattered x-ray beam in causing further ionization must be taken into account; this will depend largely on the geometry of the system. From Fig. 3.12, it will be seen that the energy transferred to each electron can be considerably greater than that needed for ionization. Each of the recoil electrons will therefore be capable of causing a large number of secondary ionizations

Vvavdenych, A -24 0-5 02 0-1 0-05 0-02 0-01

a;

υ

^

-*J c to

T> υ c

<*- -C O -fc?

σ £ o-o

'U .*- Ό * - to -ö-p c ^

°H t O a Φ >>

JO en >> a» c- c σ a; fc c_ CL

O

cn c £_ to

Ü o υ

in

0-005 0O2 0-05 0-1 0-2 0-5 1 2

Energy of incident X-ray or gamma photon, MeV

FIG. 3.12. Scattering coefficient and energy absorption due to Compton scattering. Left-hand scale: scattering coefficient ea and energy absorption coefficient eaa for

Compton scattering by a single electron. Right-hand scale: fraction of energy of incident beam captured by electrons per 1 cm

path in water or polyethylene.

and excitations. Since chemical changes arise from all such ionizations and excitation, however caused, it will be seen that the major chemical effect arises from these recoil electrons, rather than from the primary ionization due to collision with the x-ray or γ-photon.

Pair Production Production of an electron and a positron by a photon can only occur at

photon energies exceeding 1 -02 MeV, equivalent to twice the rest mass of an electron. The presence of an atomic nucleus is also necessary to carry off excess momentum. Above this value, the probability of pair production increases slowly with energy, and above 4 MeV is propor-tional to logE and Z2. It is followed by annihilation of the positron, with the simultaneous emission of two 0-51 MeV photons. In radiation work, the effect of pair production is usually negligible in so far as the


Compton effect predominates at the energies used, and in any case the net effect of pair production and annihilation is merely the transformation of a high energy photon to lower energy ones. Absorption and Energy Deposition

As a beam of x-rays or γ-rays of a given energy passes through a speci-men, the number of primary photons decreases exponentially due to photoelectric absorption, Compton scattering and pair production. The fraction remaining after a depth x (measured in g/cm2 to allow for speci-men density) is exp (—μχ), where the mass absorption coefficient μ depends on the energy of the incident photons, and on the material traversed. In the region of about 1 MeV, the mass absorption coefficient varies little with the elements present, since it is due primarily to Compton scattering (Fig. 3.13). At much lower energies differences become more appreciable

Energy of incident photon , MeV

FIG. 3.13. Mass absorption coefficient of x-ray and γ-photons in water, aluminium, and iron and lead. (Note relatively small dependence on chemical structure except at low energies, due to photoelectric effect, and at high energies, due to pair production.)

due to the photoelectric effect, while at higher energies pair production is favoured in heavier elements such as lead. Thus for 1 MeV γ-photons, about 6-7 per cent of the incident photons are scattered in passing through a thickness of 1/p cm of most materials (p being the density).

The incident beam falls to half its initial intensity in a thickness x± (in g/cm2) where exp (—μχ$)=0·5 or ^=0·69/μ. Thus for Cs137 γ-radiation of 0-66 MeV, μ = 8-6 x 10~2 in water, and ** = 0-69/8-6x 10~2 = 8 cm; in lead the corresponding figures are μ=0·106, χ^=6·5 g/cm2. For Co60

γ-radiation of 1-33 MeV, the values are much closer: μ = 0-06, x± =

I N T E R A C T I O N OF R A D I A T I O N W I T H MATTER 4 1

11 ·5 cm in water, μ = 0Ό56, xj = 12-3 g/cm2 in lead. Since the density of lead is 11-34 g/cm2, the thickness needed to halve the incident photon flux is 0-57 cm for 0-66 MeV photons, and 1 -085 cm for 1 -33 MeV photons. To reduce the flux by a thousandfold requires about 10 cm of lead. This illustrates the considerable thickness of protection needed for shielding purposes when using high energy photons. Accurate calculations of shielding are rendered more difficult by the presence of scattered radiation, where intensity depends on the geometry.

The energy deposited in the specimen traversed is less than the energy lost from the primary beam, the remainder being diverted to secondary (scattered) radiation, which is not considered here. The proportion of the incident energy absorbed can be deduced from the total energy absorption coefficient (which includes energy absorbed due to the photoelectric effect, Compton scattering and pair production). The total energy absorption coefficient μβ (which is less than the mass absorption coefficient μ described above) is shown in Fig. 3.14 for water and for aluminium. At low energies,

Energy of incident photon, MeV F I G . 3.14. Total energy absorption due to photoelectric effect, Compton scattering

and pair production.

μβ is greater for aluminium due to photoelectric absorption, at inter-mediate energies it is greater in water due to its higher electron density. Only at the highest energies shown does pair production become a signi-ficant factor.

From these data, the dose rate obtained from a given primary beam can be readily evaluated. For example for an incident flux of 2 MeV γ-photons of intensity 1 watt/cm2, 2-6 per cent of this energy is absorbed per 1 cm path in water. Since 1 megarad is defined as an energy absorption of 108ergs/g, or 10 watt sec/g, the dose rate is 0Ό26χ 1 0 1 or 2-6 x 10~3


megarads/sec. For the 0-66 MeV γ-photons obtained from caesium 137, the energy absorption coefficient is 0-0327, and the dose rate is 3-27 x 10-3

megarads/sec, per incident flux of 1 watt/cm2. These calculations only apply to the energy deposited from the primary

photon beam. A further energy contribution is provided by the scattered photons of lower energy and this increases with specimen thickness. An accurate calculation is rendered difficult by their range in energies and directions, which are greatly affected by the geometrical arrangement of the irradiated system.

In the range where absorption is primarily due to Compton scattering, the effect of chemical structure is slight. For many plastics the electron density lies between 2-88xl023/g for polytetrafluorethylene and 3-43xl023/g for polyethylene and the energy deposition is close to that in the same mass of water where the electron density is 3-33 x 1023/g. For materials such as polyethylene (CH2)n, the photoelectric absorption will also be close to that for water, although for polyvinyl chloride (CH2 CHCl)n, it is higher. In the range from 0-2 MeV to 1 -3 MeV, the energy absorption from γ-photons may usually be taken as roughly 3 per cent of the incident flux, deposited per gramme of material, provided that this is sufficiently thin for absorp-tion from the secondary radiation to be negligible.

PROTONS AND oc-PARTICLES Although oc-particles, and protons of high energy, can cause displace-

ment of nuclei by collision, their main effect is to produce ionization and excitation. According to equation (3.3), the rate of energy loss depends only on the speed and charge of an ionizing particle, and not on its mass. Protons and a-particles will therefore behave in the same manner as electrons of the same velocity and charge. The considerable difference in mass however requires a 20 MeV proton to be compared with an electron of only 10 keV as far as ionization density is concerned. Due to its higher mass, the path of the proton will be much more nearly linear. However the relatively low velocity of a proton or α-particle of a few MeV results in intense ionization, and the range is correspondingly short. Protons and deuterons of much higher energies can be obtained in electrical accelerators, and have been used for radiation studies, but the output of such machines, measured in kilowatts of high energy radiation, is low in terms of equipment cost. The use of such densely ionizing particles is therefore largely confined to research, although they may possibly be used for the treatment of surfaces and thin foils, or for initiating reactions in gases.

The range in any medium is a complex function of its atomic number and weight, and of the energy of the incident particle. The range Rz in a medium of atomic number Z ( 2 < Z < 10) as compared with the range i?air in air can be obtained approximately from the formula

R E R

z = l + ( 0 - 0 6 - 0 - 0 0 8 6 Z ) l o g ^ (3-7) where E is the energy of the particle in MeV, the mass number M = 1


for high energy protons, and 4 for a-particles. For hydrogen atoms in the irradiated material

^L = 0-30+0051 log ξ-. (3-8)

The range R in a compound consisting of elements 1, 2, 3, . . . with respective weight fractions Wl9 W2, Wz . . . can be deduced from the ranges in each element Rl9 R2, R3 . . . computed as above

1 __Wy W2 , ^ 3 R Ri R2 R3

As the proton or α-particle is slowed down the ionization density increases rapidly. Approximately uniform radiation conditions can there-fore only be obtained by the use of specimens considerably thinner than the range of the incident particle.

Table 3.3. Comparison of Ranges of Protons, en-Particles, Electrons in Aluminium

Energy Proton a-Particle Electron

Energy Proton a-Particle

01 0-2 —

13

20 0-480 004

0-2 0-36 —

40

50 2-6 0-2

0-5 1

— 170

100 8-8 0-7

1 2-8 0-7

400

200 29 2-5

2 8-6 1-3

900

500 130 13

5 42

4-2 2600

1000 370 45

10 140 13

5500

2000 —

130

MeV mg/cm2

mg/cm2

mg/cm2

MeV g/cm2

g/cm2

(From Frbdlander and Kennedy, 1955.)

The range of an a-particle is approximately the same as a proton of the same velocity (i.e. of one quarter the energy).

Fission products, produced by the disintegration of a uranium nucleus in a nuclear reactor, have a much higher mass than protons and a-particles and their initial energies are of the order of 100 MeV. The charge carried by each fission atom is also much greater, except towards the end of its path, when it can collect electrons. The range of such fission pro-ducts is about 2 cm in air, but the density of ionization produced is considerably greater than for protons of the same energy.

In producing ionization, protons and heavier particles give rise to δ-electrons in the same way as fast electrons or ß-particles. The maximum velocity of the δ-electrons may be twice that of the proton and the total amount of energy carried by these secondary electrons may account for two-thirds of the total energy loss. Chemical effects due to heavy particles will therefore be centred along the densely ionized main track of the primary particle, and along a series of spurs, where ionization results from fast δ-electrons.


FAST NEUTRONS Neutrons with kinetic energies of the order of 1 MeV form one of the

major constituents of the radiation present in the nuclear reactor, and they are responsible for a considerable proportion of the effects observed when plastics are irradiated in nuclear reactors. Being uncharged, they do not interact with the orbital electrons, but lose their energy primarily by elastic collisions with the atomic nuclei. Nuclear reactions, particularly (n9 n) reactions, whereby the neutron is captured, and a neutron emitted, possibly with an additional γ-photon, occur in this range of energies, but not to any extent with the elements usually present in polymers.

In an elastic collision with a nucleus of mass M, the maximum amount of energy which can be transferred amounts to 4 Μ/(Μ+\)2. For hydrogen M = 1, and in a fast neutron collision all the neutron energy may be transferred to the hydrogen atom, producing a high energy proton. (The binding energy of the hydrogen atom in a chemical system amounts to only a few eV and may be neglected.) For a carbon atom the maximum amount of energy which can be transferred is only 48/169 or 28 percent, while for oxygen it is 64/289 or 22 per cent. To obtain the total energy transfer, when a neutron traverses a specimen containing these elements, these values must be multiplied by the corresponding collision cross-section, and by the number of atoms of each type in the specimen. The collision cross-section for fast neutrons is low, and their penetration may amount to several centimetres. In most polymers hydrogen constitutes the largest number of atoms present, and the main effect of fast neutron irradiation is the production of fast protons within the specimen. These protons have a very short range, but are responsible for intense local ionization and excitation. For the same energy absorption, the changes resulting from neutron irradiation may therefore be different from those due to electron or γ-radiation, ionization and excitation being confined to a number of discrete regions in the former case. Unfortunately, in most powerful sources of fast neutrons, such as nuclear reactors, any difference is to some extent masked by the accompanying γ-radiation. The dosimetry of fast neutrons is also rendered more difficult under these circumstances. As a result no quantitative comparison of the effects of neutron radiation and fast electron or γ-radiation of polymers has been made.

In polyethylene the energy absorbed from fast neutrons arises primarily from collisions with hydrogen atoms. The collision cross-section σ for various values of the incident energy of the fast neutron is given in Table 3.4. From the known concentration of hydrogen atoms, and the average proportion of energy transferred, the energy absorbed can be calculated.

For example, a fast neutron flux of 1016 fast neutrons/cm2, of average energy 1 MeV, will lose about 1-96 x 105x 1016 eV in passing through 1 g/cm2 of polyethylene. The corresponding radiation dose is 32 mega-rads. In other polymers such as PTFE it is considerably less. An assessment of the fast neutron contribution to the radiation effects observed when polymers are subjected to pile radiation will therefore depend on a


prior knowledge of the fast neutron flux constituent in pile radiation. This varies within the reactor, and from reactor to reactor, being generally greater in heavy water moderated reactors.

Table 3.4. Energy Absorption by Polyethylene Due to Fast Neutron Irradiation

Energy of incident neutron (MeV) Collision cross-section σ (barns) Proportion of neutrons colliding

per g/cm2

Energy transferred per incident neutron, per g/cm2 in keV

2 3

0-23

290

1 4-3

0-31

196

0-4 7

0-45

114

01 13

0-67

42

001 19-5

081

5

Two further forms of radiation may be considered which, while not included under the heading of ionizing or high energy radiation, may under suitable conditions produce similar chemical changes.

SLOW NEUTRONS Neutrons with low kinetic energy of the order of thermal energies

(0Ό25 eV) have insufficient energy to cause ionization and excitation themselves. Such neutrons are however present in nuclear reactors, and may indirectly be responsible for some of the chemical changes when materials are irradiated in these reactors.

Many elements have large capture cross-sections for slow neutrons, and the composite nucleus formed may be unstable, suffering a nuclear transformation in the course of which a γ-photon, an electron or a proton is emitted. These secondary radiations will then give rise to ionization and excitation within the material. The probability of this process depends on the elements present in the specimen. One such reaction occurs with hydrogen, though with low probability,

Hin, Ύ)Η2

where the hydrogen atom captures a slow neutron, and is transformed to deuterium, emitting a 2-17 MeV γ-photon in the process. The additional ionization and excitation arising from this photon depends on the size of the specimen, and due to its high penetration is usually very small.

In polyethylene, irradiated in a nuclear reactor, most of the incident slow neutrons are scattered and emerge again from the specimen. The cross-section for capture is low (4-5 x 10~3 barns as against 80 barns for scatter-ing). For very small specimens of thickness / only a small proportion (8-6 x 1022x4-5x 10-3x 10~24/ or 3-9 x 10~4/) will be captured. The recoil energy of the deuteron formed (1400 eV) will be completely absorbed, but only a small fraction (about 30xl0 3 /eV) of the energy of the γ-ray emitted giving a total of 30 χ 103f+1400 eV per neutron absorbed. For a total incident slow neutron flux of 1017 neutrons/cm2 (the unit often


referred to in pile radiation work) the energy deposited will be about 3-9 x 10-4/ (1400+3 x 104/)x 1017 eV. If / — 1 mm, this amounts to only 2 x 1016 eV, which is quite negligible compared with the energy deposition of about 3x 1029 eV from fast neutrons and γ-radiation.

In polymers containing chlorine (for example polyvinyl chloride, poly-vinylidene chloride), the effect of induced radioactivity may be consider-able, and the same exposure time will result in such polymers being subjected to a much higher radiation dose than polyethylene or polytetra-fluorethylene. Among the reactions which may be considered are

Cl35(«, Y)C136; Cl37(/7, Y)C138; Cl35(«, /?)S35

Nitrogen-containing polymers will also receive an additional radiation component due to the reaction

N14(/7,/>)C14

in which the proton will liberate all its kinetic energy within the specimen. During the emission of these particles or photons the atom itself may

recoil with sufficient energy to break the chemical bond linking it to other atoms. This phenomenon is known as the Szilard-Chalmers process. Moreover the recoil atom can have sufficient kinetic energy to cause further ionization and excitation in its immediate vicinity. In nuclear reactors however this effect is completely overshadowed by the ionization and excitation produced by the fast neutrons and γ-photons which are also present.

ULTRAVIOLET LIGHT

Ultraviolet light is not considered as a form of high-energy or ionizing radiation, as the energy available per photon is generally smaller than that required to remove an electron from its parent atom or molecule. It can however cause excitation. A photon of wavelenth λ Â has an energy of

12395 eV, so that an ultraviolet photon of 2500 Â carries an energy À

of 4-96 eV. The lowest ionization level for most elements and organic compounds is about 8 to 15 eV but many excitation levels may lie close to 5 eV.

The main effect of ultraviolet light in photochemistry results from excitation, and since the energies involved are often close together, marked resonance phenomena occur. As compared with ionizing radiation therefore photochemistry will be highly selective in its reactions. Experi-mental difficulties limit the study of these effects in the far ultraviolet, where energies adequate to cause ionization may be obtained. A similar objection arises to the use of very soft x-rays where the output is very small, and the range of penetration extremely limited.

During the exposure of polymers to high radiation strong fluorescence is often observed. In the degradation of the energy to lower values and


eventually to visible light, far ultraviolet light will be produced and this may account for some of the selective reactions observed.

DISTRIBUTION OF IONIZATION Although marked differences may be observed in the products

obtained with different types of high energy radiation (as in the irradiation of pure water) no such differences have yet been reported when long chain polymers are irradiated. The distribution of ionized and excited molecules left around the track of an incident high energy particle may be expected to have an effect on the resultant reactions ; this distribution has been closely studied, particularly in the field of radiobiology, and much of the data and terminology derives from workers in that field. The most useful quantitative description is in terms of average energy loss per unit path (linear energy transfer, LET) or of the ion density on the assumption of constant energy absorption per ion produced. Both these quantities are measured along the track of the incident particle. A full description would also involve the distribution of ions about the track.

Consider a source of high energy radiation in which each individual particle of a given energy loses an average of E keV per micron of track length in material of unit density. If the energy absorbed per ion pair formed is 100/G eV, the average number of ions formed per micron of track length is 10 GE and the average distance between successive ions measured along the track is \03/GE A. The use of an average distance is to some extent misleading since the density along the track will differ widely depending on whether the ions considered arise directly from collisions with the incident particle or are due to secondary particles (δ-elcctrons). Moreover, the ions formed will not be located along the track but may be situated some distance from it due to the spread of the δ-electrons. Thus, the distribution of these distances is non-random, a radiation of low average ion density still containing local regions of high ion density. In addition, the ion density will rise as the incident particle penetrates within a specimen and loses energy. One can therefore speak of an ion density or LET for any given energy; alternatively one can refer to the average value for the particle from its initial maximum energy to full range of penetration. Some typical values of the rate of energy loss (LET) and the corresponding ion density for different types of radiation are shown in Table 3.5.

For fast electrons of about 1 MeV, the average ion density is about 6 ions/micron and the corresponding L.E.T. 0-2 keV/micron. This figure varies but slowly as the energy of the incident electron is increased. At low electron energies, however, the ion density increases and for an incident electron of 10 keV it amounts to about 150/micron. Gamma rays produce Compton electrons of varying energy each with its own average ion density; for high energy gamma radiation (e.g. cobalt 60 radiation) most of these Compton electrons have ion densities of about 6 per micron and the distribution of these ions is similar to that for high energy electrons. When x-rays are produced by electron bombardment of a target, the resultant radiation is polychromatic, the average energy being less than


half that of the peak voltage applied. The energy of the electrons liberated when these x-ray photons enter a specimen have a considerable spread, many of the electrons being produced by the photoelectric effect. As a result, there is a much wider dispersion of the ion density than is found for fast electrons or γ-rays. The same effect may be observed with low energy ß-radiation emitted by some radioactive isotopes since the primary electrons emitted have a wide range of energies. For a 50 kV peak x-ray beam producing both photoelectrons and Compton recoil electrons in water, the average ion density due to photoelectrons is about 72 ion pairs per micron whereas the Compton electrons which are fewer in number have a corresponding ion density of about 629 ion pairs per micron. For such radiation, the overall average is still only 74 ion pairs per micron and the average distance between successive ionizations, measured along the primary track, is about 135 Â. However, the fluctua-tions in this density will be greater than those observed with high energy electron radiation.

For the ions produced by heavy particles such as fast protons, deuterons, a-particles or fast neutrons (which impart most of their energy by collision with hydrogen atoms to give fast protons), the ion density is considerably greater; even when the energy of the incident particle amounts to several MeV, its speed is relatively low and its ionizing power correspondingly high. A typical figure is about 1000 ion pairs per micron and the average distance between successive ion pairs along the track is only 10 A. Under these conditions, the probability of ion-ion interactions becomes quite considerable. When heavier particles are used, such as C6+ or the fission products arising from the splitting of the uranium nucleus, the density of ions along the track will be even greater, sufficiently to render interaction between adjacent ions an extremely probable event. Qualitative differences in the types of reaction produced may be expected when such radiations are used.

In the radiation treatment of materials, an important consideration is the distribution of the chemical changes produced by radiation within the material. For many calculations of the changes produced by radiation (e.g. elastic modulus, solubility and swelling of polymers), it is necessary to assume that such changes occur at random throughout the specimen, allowance only being made for the major variation in the average ionizing intensity with depth of penetration. Whether this assumption is a valid one may therefore depend on the variation of the ion distribution both along the track and between ions produced in neighbouring tracks. The first consideration, i.e. the density variations along the track of a single ionizing particle, must be considered as determined by the type of radiation. The distribution of ions formed by different particles relative to their spacing within each particle track depends on the radiation intensity. If an incident particle produces x ions per micron of track in a medium of unit density, the average distance between ions (measured along the track) is ÎO4/* Â. If the distribution of ions is to be considered as random through the specimen (and not largely confined to the tracks of the incident par-ticles) the distance between tracks of the incident particles must be closer

Tabl

e 3.

5.

App

roxi

mat

e V

alue

s of

Lin

ear

Ene

rgy

Tran

sfer

(L

.E.T

. in

keV

/μ)

and

Ion

Den

sity

in

W

ater

Fast

ele

ctro

ns

and

ß's

y's

and

x-ra

ys

Fast

Neu

tron

s

Prot

ons

Deu

tero

ns

α-pa

rtic

les

Fiss

ion

prod

ucts

1 M

eV

and

abov

e 1

MeV

i 10

103 l_

For

X-r

ays,

100

KeV

10

KeV

1

KeV

C

14

100

keV

p (x

-ray

s)

H3

Soft

x-ra

ys

(mea

sure

d al

ong

trac

k of

pho

toe!

ectr

on o

r C

ompt

on e

lect

ron)

10

MeV

Li

(d, n

) D

(S,n

) 10

0 M

eV

10 M

eV

1 M

eV

etc.

100

MeV

Γ

102

102 L

1

_J

10 M

eV

1 M

eV

100

MeV

10

MeV

i 10

3 10 1

10 1

ther

e is

a co

nsid

erab

le s

prea

d.

See,

for

ex

Rad

iatio

ns (

Hai

ssin

sky

ed. 1

, 58,

195

5).

(Po

α,)

(Rno

c)

100 1

ampl

e:

GRA

Y B

OA

G,

J. W

.,

(for

x-r

ays

ion

dens

ity a

nd

L.E

.T.

depe

nd o

n fil

ters

an

d an

ode)

1 M

eV

i 10

4 1

_J

L. H

., A

i R

ad.

Res

C6+

(50

MeV

) H

eavy

ion

s fr

om u

rani

um

fissi

on

i 10°

Ion

dens

ity p

er

mic

ron

of t

rack

10

-1

Ave

rage

dis

tanc

e |

(Â)

betw

een

ions

al

ong

trac

k

1000

Li

near

Ene

rgy

Tran

sfer

|

in k

eV/μ

étio

ns C

him

ique

s et

Bio

logi

ques

des

1,

323,

195

4.

INTERACTION OF RADIATION WITH MA TTER 49


than this average distance 104/*· The energy absorbed per ion pair being 100/G eV, the energy dissipation in the medium must be at least 100/G eV per volume of 1012^_3Â3or 1014x3/GeVpercm3. Since 1 radis by definition an absorption of 0-625 x 1014 eV/g and the medium is of unit density, the minimum dose for random distribution of ions is l-6x3/G or 0-5x3 rads if G is about 3. For fast electrons, gamma rays and hard x-rays with ion densities of the order of 10 to 50 ions/micron, this minimum dose lies in the range 500 to 62,500 rads; in most polymer work doses of the order of megarads are required to produce noteworthy changes so that the assumption of randomness in the ion distribution is justified by the overlap of the individual tracks. For heavier particles or for irradiation with slow electrons or soft x-rays, the ion density may amount to 1000 ions/micron and the minimum dose for their random distribution then amounts to about 500 megarads, a dose only used when very profound chemical changes are required. For such irradiations the effect of non-random distribution in the changes produced by radiation is worthy of examina-tion. In all forms of radiation there will, of course, be local variations due to the high ionization in the δ-spurs.

Even when the total dose is sufficiently high to ensure an appreciably random distribution of ions in space, there still remains the time factor to consider; ions in the same track are produced simultaneously and may react with each other preferentially before a neighbouring ion track is formed in its vicinity. In this case, a dose rate dependence may be observed although the average distribution of ions considered over the entire radiation period is in fact random. However, in the work published to date on the effects of radiation on polymers, no firm evidence has been found of any discrepancy which can definitely be ascribed to the non-random distribution of these ions. If such an effect can be proved to exist, valuable information could be derived on the distribution of ions, and excited molecules, and of the time scale for the ensuing reactions.

REFERENCES

BETHE, H. A., Handb. Phys. 24(1), 273, 1933. BURTON, M, MAGEE, J. L. and SAMUEL, A. H., J. Chem. Phys. 20, 760, 1950. COMPTON, A. H. and ALLISON, S. K., X-rays in Theory and Experiment, Van

Nostrand, New York, 1935. EBERT, M. and BOAG, J. W., Disc. Faraday Soc. 12, 189, 1952. FAILLA, G., Amer. J. Roentgenol. 54, 553, 1945. FANO, U., Nucleonics 11(8) 8, (9), 55, 1953. FANO, U., Biological Effects of Radiation, 1, Editor, A. HOLLAENDER, McGraw-

Hill, New York, 1953. FRIEDLANDER, G. and KENNEDY, J. W., Nuclear and Radio chemistry, Wiley,

Chapman and Hall, 1955. GRAY, L. H., Actions Chimiques et Biologiques des Radiations, 1, 1, Editor:

HAISSINSKY, Masson, Paris, 1955. GRAY, L. H., Brit. J. Radiol. 22, 677, 1949. GRAY, L. H., Proc. Camb. Phil Soc. 40, 72, 1944. HEITLER, W., Quantum Theory of Radiation, Oxford University Press, 1944. KLEIN, O. and NISHINA, Y., Z. Phys. SI, 852, 1929.

INTERACTION ÔF RADIATION WITH MATTER 51

LEA, D. E., Action of Radiation on Living Cells, Cambridge University Press, 1955.

MAYNEORD, W. V., Brit. S. Radiol. Suppl. 2, 138, 1950. NELMS, A. T., National Bureau Standards Circular, 542, August 1953. RASETTI, F., Elements of Nuclear Physics, Prentice-Hall, New York, 1947. RUTHERFORD, E., CHAD WICK, J. and ELLIS, C. D., Radiations from Radioactive

Substances, Cambridge, University Press, 1930. SPIERS, F. W., Brit. J. Radiol. 22, 521, 1949. VICTOREEN, J. A., J. Appl. Phys. 14, 95, 1943. WILLIAMS, Ε. J., Proc. Roy, Soc, Λ130, 310, 1930; Λ139, 163, 1933.

E

CHAPTER 4

NUCLEAR SOURCES OF RADIATION THE VARIOUS sources of high energy radiation fall into two distinct groups. The first group makes use of the radiations (α, β, γ, proton, neutron) emitted when unstable atomic nuclei disintegrate; such nuclei may be either naturally occurring or produced artificially. The second group is not in any way related to nuclear changes and relies directly on the acceleration of particles to high energies by the use of high voltage tech-niques. Sources in which a nuclear change is produced by bombarding a target with a high voltage particle accelerated electrically (e.g. neutrons emitted from Be subjected to proton bombardment) fall in the first cate-gory. Even in the early radiation work, both nuclear and electronic sources were used, the particular radiation source being to a large extent dictated by its availability. At present, sources can be more readily selected in terms of the type and energy of the radiation required for a particular study. The high cost of the equipment no longer appears to be the limiting factor.

Table 4.1. Radiation Sources

Nuclear Sources

Nuclear reactor (interior) Nuclear reactor (shield) Circulating radioactive fluids (Na, K) Gaseous fission products (Kr, Xe) Spent fuel rods Liquid reactor fuel Fission products

solutions separated (Cs 137, Sr 90)

Natural radioactive elements (e.g. Ra, Rn) Radioactive isotopes (Co 60)

Electronic Sources

High voltage transformer Voltage doubler circuit Impulse generator (capacitron) Van de Graaff Resonant transformer Linear accelerator Betatron Cyclotron, synchrocyclotron, etc.

Table 4.2. shows some typical radiation sources, and the range of radiation intensity they furnish.

Nuclear reactors designed for power production give rise to considerable amounts of radiation which can be utilized directly for research purposes. Fission products arising from uranium fission in reactors may be removed and either used directly for radiation purposes or, if long lived, separated to give sources of higher purity with a well defined type of radiation. Elements may also be introduced into the reactor and rendered radioactive;

52

Tabl

e 4.

2.

Typi

cal

Rad

iatio

n So

urce

s

Rad

iatio

n

γ-R

ays

Elec

trons

Neu

trons

and

γ-r

ays

x-ra

ys

Char

ged

posit

ive

parti

cles

Sour

ces

(1) C

o60, C

s137 , e

tc.

(2) U

-Fue

l Slu

gs

(3) F

issio

n by

-pro

duct

(u

nsep

arat

ed)

(1) A

ccel

erat

ors:

V

an d

e G

raaf

f Li

near

acc

eler

ator

C

apac

itron

Re

sona

nce

trans

form

er

(2) A

u198 , S

r90, e

tc.

(1) R

eact

ors

(2) R

a-B

e, a

nd

Phot

o-ne

utro

n (1

) So

ft (2

) Har

d (2

MeV

pea

k)

(1) A

ccel

erat

ors

Cycl

otro

n Li

near

acc

eler

ator

(2

) Rea

ctor

s fo

r fis

sion

fra

gmen

ts

Stre

ngth

Max

.

10,00

0 cu

ries

2000

cur

ies

0-5-

10 M

eV

and

abou

t 10

0-40

00 μ

amps

1 cu

rie/c

m2

10n -4

xl014

neut

rons

/cm

2 sec

16 M

eV, 2

00μa

mp.

5 x

1014

neu

trons

/ cm

2 sec

Rad

iatio

n Fi

eld

Ord

er o

f M

agni

tude

lO

M^T

/hr

5 X

103 -5

x 1

07 r/hr

To

4 r/hr

1010

rep

/hr

(or

mor

e)

106 r

ep/h

r 10

5 -109 r

/hr

Neg

ligib

ly s

mal

l

108 r

/hr

5xl0

6 r/hr

10

12 r

ep/h

r

~10u r

ep/h

r

Pene

tratio

n

Pene

tratin

g

Pene

tratio

n lim

ited

from

i

cm t

o a

few c

entim

eter

s

Frac

tion

of c

m

Pene

tratin

g

1 m

m

Pene

tratin

g V

ery

limite

d th

ickn

ess

Extre

mel

y th

in s

ectio

n

Fro

m:

Sun,

195

4

NUCLEAR SOURCES OF RADIATION 53


cobalt is the most frequently used source of this character when appreciable energy outputs of radiation are required.

High energy radiation can also be produced by direct acceleration of electrons or heavier particles. In this case, no nuclear transformations are involved and the subject no longer falls in the field of nuclear energy; rather should the provision of such sources be considered as a branch of electronic engineering. The particles accelerated in this way may either be used directly to induce radiation changes or, by allowing them to impringe on suitable targets, x-rays may be produced which are sometimes more useful when high penetration is required.

NUCLEAR REACTORS Most nuclear reactors contain uranium as the fissile material and

graphite or heavy water to act as the moderator needed to slow down the fast neutrons to thermal velocities. Much of the published work on radiation effects on polymers is based on experiments carried out in the BEPO reactor at Harwell, the Brookhaven reactor near New York and the Oak Ridge reactor at Tennessee, all of which are slow neutron reactors using graphite.

Specimens placed within them are subjected to a mixture of radiations; primarily fast and slow neutrons and γ-radiation. The total energy flux through a specimen depends not only on the design of the reactor, but on its mode of operation, the irradiation position chosen within it, and some-times even on the presence of neighbouring specimens, which may absorb neutron energy preferentially. Since there is no ready method of measuring the energy deposited in individual specimens, different methods of defining the radiation dose are given in the literature. Methods of converting these somewhat arbitrary but convenient units to standard radiation doses are only approximate in character, and consequently apparent discrepancies arise as between the earlier publications. More accurate and direct methods are now available and fairly reliable conversion factors can be applied to these early data.

Units of Pile Radiation The direct calculation of energy deposition in a specimen irradiated in

a nuclear reactor requires a knowledge of the number of neutrons and gammas of different energies, and of the cross-section of the elements in the specimens at each of these energies. The spectrum of energy distri-bution in a reactor varies from one position to another, and is affected by the presence of neighbouring absorbers. Only approximate estimates are available, e.g. the flux spectrum for the Oak Ridge reactor has recently been given as follows (Collins and Calkins, 1956):

Thermal neutrons Neutrons (>01 MeV) Neutrons (>0·5 MeV) Fast neutrons (> 1 MeV) γ-photons (average energy 1 MeV)

HxlO^/cir^sec l-4xl011/cm2sec 6-7xl010/cm2sec 4-2xl010/cm2sec. About 5 x lO^/cm2 sec


Other values for the neutron and γ-flux have also been quoted depending on a number of factors such as position in the reactor and operating condition.* These values do not provide adequate data for accurate estimates of radiation doses. Calorimetric measurements of energy deposition have also been made, but as these involve considerable experimental difficulties, they are not suitable for routine dose measure-ments. In practice it has proved more convenient to irradiate specimens under standard conditions of reactor operation, and quote relative doses in terms of exposure time.

To express these relative doses in absolute units the dose rate may be calibrated by an accurate measurement of energy deposition under these standard conditions. Alternatively the conversion factor may be derived from a comparison of the changes produced in the pile with those obtained when the specimen is subjected to a known flux of γ- or electron radiation.

In practice, it is rarely possible to measure the γ or fast neutron flux with-in the reactor, and reactor doses are usually quoted in terms of the slow neutron flux, which can then be converted to absolute doses (e.g. mega-rads) when the conversion factors are known. The pile unit employed in many Oak Ridge publications is a neutron flux of 1018 thermal neutrons per cm2. It is written 1018 nvt, where n represents the density of these neutrons, v their velocity, (nv is the flux per second) and / the exposure time. When the reactor operates at a power output of 3*5 megawatts, the neutron flux amounts to 0-74 x 1012/sec (Sisman and Bopp 1951) and the time to absorb the unit dose of 1018«v/ is 1-35 xlO6 sec or about 16 days. With an increased power output, the time required per 1018 nvt is reduced.

At Harwell early exposures of polymers to reactor radiation were measured in terms of total reactor power output, but were later converted to a slow neutron flux unit of 1017/cm2 sec, which corresponds nominally to one-tenth of the Oak Ridge unit of 1018 nvt. This BEPO unit, sometimes referred to as the "C" unit, is given by an exposure of some 16 hr in a standard position in the reactor, under standard operating conditions of 6 megawatts total output.

Although the Oak Ridge and BEPO reactors are both graphite moderated piles of somewhat similar size, the units chosen are only roughly comparable as far as energy deposition is concerned. The energy deposited in most polymers arises mainly from fast neutrons and γ3 and the ratio of these to slow neutron flux varies from reactor to reactor, and even within each reactor. The effect of exposure to the Oak Ridge unit of 1018 nvt is therefore only approximately ten times that of the BEPO unit of 1017 slow neutrons/cm2. The precise ratio depends on the relative flux distribution and on the chemical nature of the specimens being examined.

* Another estimate gives ratios of: Thermal neutrons (up to 0 03 eV) . . . . 1 Epithermal neutrons (003 eV to 1 MeV) . . 0-6 Fast neutrons ( > 1 MeV) 004 Gamma 0-5


The slow neutron flux can be monitored by including with the specimen a very small piece of cobalt wire. By measuring the radioactivity in this wire before and after exposure, an accurate check can be maintained on the total slow neutron flux in the neighbourhood of the specimen. As a further check the induced radioactivity of the wire may be compared with the total power output of the reactor; under standard operating conditions this ratio must be constant. By using this cobalt wire monitor, and measuring the changes produced in some standard polymer specimen, in different reactor positions, a correction factor can be deduced for different radiation positions in the reactor. In certain positions in the BEPO reactor, this position correction factor varied by a factor of about two.

Conversion Factors

The conversion factor from pile units (1018 nvt for Oak Ridge, 1017

slow neutrons/cm2 for Harwell) to megarads, depends primarily on the elements present in the specimen being irradiated and, to a lesser extent, on its size.

For polymers consisting of H, C, O and N, the energy deposition arises primarily from fast neutrons and from γ3. Several estimates of this energy deposition have been published. From them a conversion factor can be calculated.

Sisman and Bopp (1951) made use of data on γ and neutron distribution in the Oak Ridge reactor (Richardson 1948) to calculate the rate of energy deposition in polyethylene at a given position in the reactor.

Table 4.3. Energy Deposition in Paraffins and Polyethylene

Oak Ridge reactor operating conditions: 3-5 megawatts total power output

Element

C 2xH CH2

Gamma cal/mole sec

000444 000148 000592

Fast neutron scattering

cal/mole sec

000101 001345 001446

Total cal/mole sec

000545 001493 002038

Most of the absorbed energy therefore arises from the scattering of fast neutrons by H atoms. The energy deposition from thermal neutrons is neglected; this is justifiable for small specimens. Expressing the absorbed energy in terms of the roentgen gives an approximate conversion factor for polyethylene :

1018 nvt equivalent to 109 roentgen.

For other polymers with different proportions of H or other atoms such as chlorine, the conversion factors may be very different.


For an unspecified reactor Sworski and Burton (1951) reported the following energy deposition in hydrocarbons :

Carbon 4*6 x 10* cal/g sec Hydrogen 75 x 10"* cal/g sec.

The high contribution of hydrogen would again point to the importance of fast neutrons, which react mainly with the lighter atoms. For polyethy-lene, the energy deposited would be 14-7 x 10~* cal/g sec or 61 -5 x 103 ergs/g sec or 615 rads/sec. An exposure of one day would provide a dose of 53-2 megarads.

For the BEPO pile at Harwell, David and Irving (1957) quote data obtained by Anderson, for calorimetric measurements of energy absorption.

Table 4.4. Energy Deposition in BEPO reactor

Element

H C O S

Energy absorption eV/g per neutron/cm2

29-7x10* 1-9x10* 1-8x10* 21x10*

megarads per 1017 neutrons/cm2

475 30 29 34

For polyethylene, these values lead to a pile equivalent of 94 megarads, which is twice the figure derived (Table 4.5) by a direct comparison between the changes produced by exposure to pile and to γ-radiation.

For a watercooled reactor Calkins (1954) calculated the following energy deposition in two liquids by a radiation flux which for every 1018 fast neutron/cm2 > 0-5 MeV, comprises 2-1 xlO1 8 thermal neutrons and 2-3 x 1018 γ-photons of 2 MeV. The higher deposition in alkyl benzene is due to its greater hydrogen content.

Alkyl benzene Dowtherm M.W. 250 A*

Thermal neutrons 1-68 x 109 rads 0-84 X109 rads Fast neutrons 2-2 x 109 1 ■ 1 x 109

Gammas 1-9 XlO9 1-9 xlO9

Total 5780 megarads 3840 megarads Megarads/1018 thermal neutrons 2750 1780

The contribution from the thermal neutrons arises from the reaction H(AZ,Y) D, where the γ emitted has an energy of 2-17 MeV. Since it has a high penetration its contribution to the energy absorbed will only be appreciable for large specimens containing hydrogenous matter. For small specimens in non-hydrogenous surroundings the total energy depo-sition is about 30 per cent less. The high energy deposition per 1018 thermal neutrons (as compared with the Oak Ridge and Harwell reactors) is due

* Mixture of diphenyl oxide and diphenyl.


to the low ratio of thermal neutrons to fast neutrons and gammas in the reactor considered by Calkins.

The energy deposition in various polymeric specimens exposed to radiation in the BEPO reactor was calibrated by comparing the changes produced in them with those occurring when the same polymers were subjected to gamma or electron radiation. The equivalent dose (in megarads) to 1 pile unit of 1017 thermal neutrons/cm2 is shown in Table 4.5. Differences between polyethylene, polymethyl methacrylate and polyiso-butylene may be expected to arise from differences in hydrogen content. In the case of PTFE, where no hydrogen is present, the energy deposition from the reactor is considerably less.

Direct measurements of the chemical changes in water, and of the heat absorption in PTFE, lead to somewhat similar figures.

In a heavy water moderated reactor the ratio of slow and fast neutrons

Table 4.5. Reactor Equivalent (1017 neutrons/cm2)

Pile

BEPO

BEPO

BEPO

BEPO BEPO BEPO ZOE (10 KW) ZOE ZOE

ZOE

Equivalent in megaroentgen

50

46

43

32 35 32

8 10, 11

7-5

11

Author and method

Black and Charlesby; unsaturation of poly-ethylene

Alexander, Charlesby, Ross; degradation of polymethyl methacrylate

Alexander, Black, Charlesby; degradation of poly/sobutylene

Worrall; fluorine evolution from PTFE Wright ; oxidation of ferrous sulphate Anderson ; energy absorption in PTFE Landler ; polymerization of styrene Chapiro, etc. ; radiolysis of solutions of DPPH Pucheault and Lefort; reduction of ferrous

or///öphenantroline Bouby and Draganic; oxidation of ferrous

sulphate

and y's is very different from that in graphite moderated reactors. The data shown in Table 4.5 for the ZOE reactor in Paris show a much lower γ-equivalent, when dose measurements are expressed in terms of the slow neutron flux.

An important factor to consider in exposing polymers to pile radiation is the temperature effect. It is known that the extent of many reactions induced in polymers depends on their temperature during irradiation. In measuring the reactor equivalent in the BEPO reactor, the polymers were exposed at a temperature of about 80°C, whereas the comparable γ or electron irradiations were carried out at room temperature. The reactor equivalent figure as used by Charlesby in relation to BEPO radiation


should therefore be construed to mean that an exposure to one pile unit in the BEPO reactor under its operating conditions has the same effect as an exposure to about 45 megarads of γ or electron radiation at 20°C, for polymers such as polyethylene and poly/stfbutylene. It does not imply that the two energy depositions are equal; this will only be true for reactions which are not temperature dependent. This qualification does not apply to most of the Oak Ridge data, where specimens were irradiated in a water-cooled hole, and no temperature effect would be involved.

Effect of Chemical Structure Fast neutrons lose their energy primarily by collision with hydrogen

atoms since the masses of the two particles are similar and the maximum amount of energy can be transferred at each collision. For specimens containing large amounts of hydrogen, collisions with fast neutrons knock out hydrogen nuclei and hence give rise to high energy protons within the specimen which produce dense ionization. The contribution to the fast neutron constituent depends mainly on the proportion of hydrogen in the irradiated specimens. For polyethylene (CH2)M irradiated in the BEPO reactor, unit reactor dose is equivalent in its effect to a pure γ-dose of approximately 45 megarads whereas with PTFE (CF2)n when no hydrogen atoms are present, the effect is smaller, the same exposure in the reactor being equivalent to only about 30 megarads of γ-radiation.

Only γ-rays of very high energy are able to react directly with the atomic nucleus and in the nuclear reactor the major effect of the γ-constituent is the ejection of fast electrons by Compton scattering and, to a much lesser extent, by photoelectric absorption and pair production. The number of electrons ejected by Compton scattering depends only on the number of electrons/g; for any element this is proportional to the ratio of atomic number to atomic weight (Z/A), which is close to 0-5 for most elements present in polymers with the exception of hydrogen. Minor differences in energy deposition will arise from differences in hydrogen content, which varies the electron density from 3-4 x 1023/g in polyethylene and poly/^butylene to 2-89 x 1023/g in PTFE.

Slow neutrons can only impart energy to a specimen by reacting with the nucleus to give radioactive elements. In hydrogen, for example, slow neutrons may be captured to form deuterium, a penetrating γ of 2-17 MeV being emitted in the process. The cross-section for this process is low and the absorption of energy from the ensuing γ is also small so that for small specimens containing only hydrogen, oxygen and carbon the contribution of a slow neutron flux may be ignored. For very large specimens it may, however, have to be considered.

With elements such as chlorine which have a large capture cross-section, the contribution of slow neutrons to the total energy deposition may be considerable due to reactions such as C135(«,/>)S35, where the Cl35 isotope, constituting 75 per cent of the total number of chlorine atoms present, captures a slow neutron, emits a proton and is itself trans-formed to S35. The proton emitted produces intense local ionization which greatly increases the total energy deposition within such specimens.


Table 4.6 obtained from data published by Bopp and Sisman using the Oak Ridge reactor shows the equivalent radiation dose for polymers of various structures for the same pile exposure.

Table 4.6. Dose Equivalent to 1018 nvt

Natural rubber, butyl rubber, GR-S 600 megarads Silastic, polyvinyl carbazole, polystyrene, polymethyl methacry-

late, cellulose acetate 700 megarads Polybutadiene, L.P. oil 800 megarads Polyethylene, Teflon, Terylene, nylon, casein 1000 megarads Neoprene,* Hypalon,* polyvinyl chloride,* polyvinyl chloride

acetate* 2500 megarads

(Bopp and Sisman, 1955)

* Chlorine-containing compounds.

Collins and Calkins (1956) have given somewhat lower values for the same reactor; 400 megarads/1018 nvt for polystyrene and polymethyl methacrylate, 500 for rubber, 550 for nylon and 600 for polyethylene.

Changes in the effective dose in nuclear reactors may be caused by slow neutrons which react with neighbouring specimens to give rise to local changes in the radiation pattern. In carrying out dose measurements it may therefore be necessary not only to consider the composition of the material being studied and its position within the reactor, but also the presence of any neighbouring material likely to capture slow neutrons and produce radioactive elements.

Nuclear Reactors for Research

Although the behaviour of a considerable number of materials subjected to pile radiation has been studied, there are a number of serious limitations to its use for basic radiation research. The intensity of radiation as distinct from the total dose accumulated cannot be easily altered since most reactors are run at constant power. The size and shape of the specimen which can be handled is often restricted, while to alter the temperature conditions of radiation involves considerable experimental complications. Perhaps the greatest difficulty lies in determining the effective intensity which varies not only from reactor to reactor and within each reactor, but also depends on the chemical structure of the specimen. While these difficulties limit the effectiveness of a nuclear reactor for basic research, which is preferably carried out with sources of pure radiation, they do permit very high doses to be accumulated over a period of months; pro-vided irradiations are carried out under identical conditions, the effect of the total dose on physical and chemical properties can therefore be conveniently studied over an extremely wide range.

Industrial Uses of Nuclear Reactors for Radiation Treatment

Nuclear reactors have been considered for irradiation treatment of materials on an industrial scale, either as a by-product of their use


for power production or specifically designed for this purpose. The main objections to the former arise from the engineering problems involved in the frequent loading of specimens into and out of the reactor without interfering with its primary function and without exposing operators to excessive radiation doses during the loading process. With radiation energy at its present high cost, the use of an appreciable amount of the radiation energy present in nuclear reactors would appear to offer considerable economic advantages. Provided the material does not con-tain elements with large cross-sections for slow neutrons, the presence of materials within the nuclear reactor should not in theory greatly interfere with its functioning. The useful output from such a reactor will be deter-mined primarily by the amount of space available within the reactor for radiation purposes. In the BEPO graphite moderated reactor, for example, a dose of about 70 megarads can be accumulated in the course of a day. If one assumes a reactor design capable of accommodating one cubic metre of material to be irradiated at one time, the total utilizable energy per day available, e.g. for the irradiation of hydrocarbons of unit density, might be 106 x 70 x 106 g rads or about 200 kWh. This is a very small proportion of the total power output of well over 100 MWh. in the same period, and is essentially limited by the engineering designs involved in inserting and removing large masses of material within a reactor without modifying its design and reducing its efficiency. The provision of adequate shielding would in itself constitute a difficulty. If only hydrocarbons are involved, theoretically there would be no serious interference with the functioning of the reactor, the material merely replacing part of the moderator; it

Dose range, rads x 10

8 400

· - 200

0-Ô4 0-07

0-4 0-7

• -*. ·· ~f

4 0 7Ό

• ·

*.£«

•

•

' - I •

·>

•

• • 10' 10IJ 10'°

Dose, fast neutrons/cm 10"

FIG. 4.1. Increase in viscosity of organic liquids in a nuclear reactor. (From G. Pomeroy, ACR-2, 133, 1954.)


might even be used as a coolant. A number of practical difficulties might however arise, such as the change in viscosity of the hydrocarbons which rises rapidly near the gel point; only a small overdose would cause gelation. A further objection is the possible introduction of impurities capable of capturing neutrons, thereby reducing the efficiency of the pile and themselves rendering the irradiated material radioactive.

An alternative proposal is to insert the material to be irradiated in the reactor shield, where the radiation would otherwise be absorbed or lost. The radiation level would be much lower, but there would be less inter-ference with the functioning of the pile. Moreover, the slow neutron flux could be utilized by the use of cadmium shields, which absorb neutrons and emit γ-radiation. This would not only increase the effective flux, but reduce the danger of induced radioactivity in the sample. For this purpose suitably modified shielding arrangements and automatic conveyor systems would be needed.

COOLING CIRCUITS To extract heat from nuclear reactors, cooling circuits are incorporated

in their design. These may include a recirculating system using a heat transfer element which itself becomes radioactive for a short period. A heat transfer system using liquid sodium might be a convenient system for effec-tively transferring radioactivity from within the nuclear reactor to a more convenient location for radiation treatment. Radioactive Na24 has a half-life of 14-8 hr and emits 2-76 and 1-37 MeV γ-radiation, while K42 with a half-life of 12-5 hr emits 1-51 MeVy's. Either would be suitable for the irradiation of thick specimens.

A reactor has been designed for the sterilization of foodstuffs, in which indium sulphate, of short half-life, is circulated through the reactor, and then through a radiation chamber in which specimens are irradiated. By analogy, such an arrangement may be termed a radiation exchanger.

SHORT-LIVED GASEOUS FISSION PRODUCTS During fission each uranium atom produces two fission fragments

(rarely more) with masses between 71 and 162 and releases 195 MeV of energy, most of which appears as kinetic energy of these fragments (Table 4.7). Only about 11 per cent (21 MeV) of the total energy liberated Table 4.7. Distribution of Energy Released by Fission of a Uranium Atom

Kinetic energy of fission fragments .. . . 162 MeV Beta decay energy 5 Gamma decay energy .. . . . . . . 5 Neutrino 11 Fission neutron energy .. .. . . 6 Instantaneous γ-ray energy . . . . . . 6

Total . . 195

is available in the radioactive products ; of this half is lost when neutrinos are emitted during beta decay so that only about 5 per cent of the energy of fission is available in the form of radioactive fission products.


Most fission products decay very rapidly (Fig. 4.2). Table 4.8 shows the fission products remaining 10 sec after a fission; two of these are

1min Ihr 1 day 10 days 1year Time after withdrawal

FIG. 4.2. Drop in radioactivity of a uranium fuel rod (1 per cent burn-up).

gaseous (xenon and krypton) and constitute one-third of the total. If these gases can be removed rapidly and circulated in thin-walled tubes

Table 4.8. Distribution of Fission Products 10 sec after a fission

Fission product

Rb+Cs Sr+Ba Y + La + Ce Sb Mo+Te Br+I+Ta Xe+Kr

Per cent

20 2-6 4-5 4-8 5-2

27-7 32-4

surrounding the specimens to be irradiated a very useful proportion of the fission energy in the form of ß-rays could be utilized. When canned uranium rods are used as fuel elements this is not possible, but in reactors in which the fuel elements are in the form of liquids, molten metals, salts or slurries, the removal of these gaseous products may be practicable. This procedure has the additional advantage of extending the period over


which a fuel element can be used before its removal for chemical pro-cessing. The radiation power thus made available could amount to nearly 2 per cent of the total energy of the reaction. If used efficiently this radiation might offer a valuable financial contribution to the running cost of a reactor. Brownell et al (1956) have studied the economics of a multi-purpose reactor for power, steam and radiation, radioactive krypton and xenon being used for the vulcanization of thick rubber tyres.

Manowitz (1954) has studied the economic feasibility of a reactor designed specifically for radiation treatment. Such a reactor could be associated directly with a factory and used for such purposes as steriliza-tion, pasteurization, polymerization or disinfestation. A tentative design for a boxcar reactor is shown in Fig. 4.3. This would obviate the need for transport of products to and from the reactor, while permitting the fuel elements to be periodically treated and replaced after use at some central organization without serious transport difficulties.

FIG. 4.3. Proposed boxcar reactor. (From B. Manowitz, 1954.)

FUEL RODS After removal from a nuclear reactor, the spent fuel rods, which are

highly active, are allowed to lose some of their radioactivity before chemical processing, the energy emitted during this cooling process being about half the total gamma energy in the radioactive isotopes.

Wild and Wright (1953) have studied the possibility of using such rods for radiation purposes. By suitable arrays dose levels of a few hundred


roentgens per minute have been obtained. Assuming existing operating schedules for cooling, a reactor running at 100 megawatts power output could provide 30 megarads to 1 ton of material (or nearly 100 kWh of γ-radiation) per day, i.e. 4x 10-5 of the total energy released in the nuclear reaction. By improved design and arrangement of fuel elements a consider-able increase (up to 500 ton-megarads per day) would be possible. A future reactor producing 240 megawatts of useful electric power, and requiring possibly 960 megawatts of nuclear energy, could be expected to provide 2200 ton-megarads of γ-radiation per day, or 6000 kWh. The dose rate might be 2 x 104 r/min.

0-5 1 2 3 Time, year

FIG. 4.4. Decay in γ-activity with age. (Vertical scale is total power output = number of photons x their energy.) Cb (Nb95), Zr9\ Ce144-^ Pr144, Ru103, Ruloe -> Rh10e,

Cs187-> Ba137. (Stanford Research Institute, 1954.)

The major objections to the use of spent fuel rods as a source of radiation are the variation in their intensity (see Fig. 4.4) and the difficulties of transport away from the reactor. If these can be overcome by regular supplies of fresh rods, and by automatic spacing devices, the potential scale of such radiation facilities might help the economics of nuclear power production to a very appreciable extent.

RADIOACTIVE WASTE The spent fuel rods are subjected to chemical treatment for the removal

of plutonium, and the fission products are then available as a radioactive liquid of low intrinsic activity, amounting to perhaps 50 curies/1. The cost of using such dilute sources resides in the high cost of transport and of shielding large volumes, and the frequent renewals which would be necessary. The waste products could be concentrated by evaporation and


much higher powered solid sources could be readily transported. Unless a long cooling-off period is allowed there will be a rapid drop in radiation intensity in use; while if such a long period elapses prior to use, the available radiation intensity is much smaller.

The cost of concentration is not known but figures not exceeding 40 cents per curie have been quoted. At this figure the use of separated fission products might appear a more practicable proposition in spite of

FIG. 4.5. Mobile γ-irradiator using processed fission products (F.P.) can be transported by rail.

(From B. Manowitz, 1953.)

their higher cost. For convenience in transport and ease of installation, radiation products cast in concrete moulds have been suggested. The Argonne National Laboratory has formed such a source by mixing 2 kilo-curies of one-year-old fission waste solution with cement.

SEPARATED FISSION PRODUCTS The fission products remaining after several months of cooling period

consists mainly of the following elements : strontium, yttrium, zirconium, ruthenium, tellurium, caesium, cerium and promethium. The two fission products separable in quantity and with long half-lives are strontium 90 and caesium 137. Together they constitute 10 per cent of the fission products remaining after about two months' decay. The reaction chains by which these radioisotopes yield high energy radiation are as follows:

Sr90 > Y 9 0 ^ Z f 9 0

β (0-61 MeV) β (2-2 MeV) half-life 20 years 61 hr

Cs137 > Ba137* —> Ba137

β (0-5, 1 -2 MeV) γ (0-66 MeV) half-life 33 years 2-6 min

Strontium has the advantage that no gamma is emitted, and the shielding problems are therefore greatly simplified. On the other hand penetration is low and the release of even minute amounts would be biologically very dangerous.

*î

Tab

le 4

.9.

Impo

rtan

t F

issi

on P

rodu

cts

of L

ong

Hal

f L

ife

Isot

ope

Stro

ntiu

m 9

0 Y

ttriu

m 9

1 Zi

rcon

ium

95

Tech

netiu

m 9

9 R

uthe

nium

106

Caes

ium

137

Ceriu

m 1

44

Prom

ethi

um 1

47

Yie

ld

%

5-3

5-4

6-4

6-2

0-5

6-2

5-3

2-6

Hal

f lif

e

25 y

ears

57

day

s 65

day

s

2xl0

5 yea

rs

1 ye

ar

33 y

ears

290

days

2-

6 ye

ars

Rad

iatio

n (M

eV)

Beta

0-61

1

53

0-39

(98%

) 1-

0(2%

)

0-3

003

0-5(

95%

) 1-

19(5

%)

0-35

0-

22

Gam

ma

—

0-73

(93%

) 0-

23(9

3%)

0-92

(7%

) —

—

—

—

Dau

ghte

r

Yttr

ium

90

—

Nio

bium

95

—

Rho

dium

106

Bariu

m 1

37

Pras

eody

miu

m 1

44

—

Hal

f lif

e

62 h

r —

35

day

s

—

30 se

c

2-6

min

17-5

min

—

Rad

iatio

n (M

eV)

Beta

2-3 —

015

—

3-5(

82%

) 2-

3(18

%)

—

3 —

Gam

ma

—

0-76

—

0-51

(17%

) 0-

73(1

7%)

1-2(

1%)

0-66

0-2,

1-2

—

Wat

t/100

0 cu

ries

8-3

ß —

2-

8 γ

—

0-65

γ

(3-9

1 γ)

1-8

4 γ

0-95

γ

1-3 β

Sour

ce: I

ndus

trial

use

s of

Rad

ioac

tive

Fiss

ion

Prod

ucts

, SR

I St

anfo

rd,

1951

Se

e al

so F

letc

her,

J. M

., 19

53



In caesium the 0-66 MeV γ radiation is of interest and as compared with the Co60 1-3 MeV radiation has the advantage of less stringent shielding requirements. The γ-radiation emitted by radioactive caesium corresponds to a power output of 3-91 x 10~3 watts/curie. Even if fully utilized, the radiation from one curie of caesium can only furnish a dose of 1 megarad to 33-8 g of material per day. A megacurie of caesium emits 3-91 kilowatts of radiation power but careful engineering design of the radiation chamber is needed to ensure that an adequate proportion can be utilized. Such powerful sources are not at present obtainable, but sources of up to 105

curies are likely to be available for industrial purposes in the next few years. The cost of such sources is still unspecified; at a figure of 1 dollar per curie, the cost of a megacurie source would be one million dollars to which must be added the cost of the installation, and an annual replace-ment cost for source depreciation of 2 per cent of initial cost. The maximum possible output would be 1-4 ton-megarads/hr.*

A number of other fission products have been suggested, but these are restricted by their relatively short half-lives. Table 4.9 shows the fission chain for these, and the potential power output of β- or γ-radiation/lOOO curies.

RADIOACTIVE COBALT AND TANTALUM Much of the recent research into radiation effects on materials has

made use of cobalt 60, the radioactive isotope obtained when cobalt is subjected to neutron bombardment in the nuclear pile. The decay scheme for Co60 is as follows: Co60 —> Co60 —> Ni60 —> Ni60 —► Ni60

γ(0·059 MeV) β(0·31 MeV) γ(1 · 17 MeV) γ(1 -33 MeV) Each disintegration of a radioactive cobalt nucleus therefore gives rise to two γ-photons, of energy 1*33 and 1-17 MeV, as well as some β and ys which are mainly absorbed in the source.

Each radioactive cobalt 60 nucleus formed in a nuclear reactor involves the capture of a neutron which would otherwise be available to cause uranium scission. The production of radioactive cobalt therefore depresses the activity of the pile, and the amount produced is in practice limited by the availability of excess neutrons. Sources at present in use rarely exceed a few thousand curies, and at a power output of 14-8 x 10~3 watts per curie, a kilocurie source provides a total γ-power of 14*8 watts. The penetration of the beam is high, generally in excess of that required for most research purposes. The fraction of the total power which can be utilized is therefore reduced. The maximum possible radiation output from a kilocurie source amounts to l-28x 105 g megarads (or l-28x 106

joules) per day, and this must be spread over the large amount of material (of the order of 10-100 kg) to make adequate use of the highly pene-trating beam. From a kilocurie source doses of the order of 10 megarads can in practice be accumulated in the course of one day's irradiation.

* 1 ton-megarad corresponds to an irradiation dose of 1 megarad delivered to 1 ton of material or any appropriate combination, e.g. 20 megarads to 1 cwt.


Installation of a Cobalt Source The radiation installations based on the use of Co60 at present used

for experimental work fall into several categories; those involving large chambers for radiation work, where the large volumes of shielding

FIG. 4.6. Outside view of shield. (From Ghormley and Hochanadel, 1951.)

necessitate the use of a cheap material such as concrete; and those using small radiation chambers, where lead can be utilized, although water or earth may be used for additional protection or to reduce the cost. The first type of installation is suitable for chemical reactions requiring large


volumes and auxiliary equipment, but using similar cobalt sources their radiation intensities are generally lower than for the smaller chamber.

A source described by Ghormley and Hochanadel (1951) uses two alter-native cavities 1 § in. in diameter and 4 | in. long, in which specimens can be placed. The cobalt source is held in an upper chamber which can be moved over either hole, and the cobalt lowered in position. With 300 curies of cobalt, the radiation intensity averages about 5000 r/min. The arrangement of the 76 pellets forming the source is designed to give a uniform intensity distribution (to within a few per cent) along the vertical axis. Means are available to introduce services such as electricity for heating; the specimens themselves can be rotated to provide uniform radiation levels. The entire shield contains 6 tons of lead and provides 10 in. of lead shielding around the source. Radiation leakage through 9\ in. of lead amounts to 2-4 mr/hour.

Milton Burton, Ghormley and Hochanadel (1955) have described an in-expensive source in which the cobalt sources are housed in a 7 ft hole below the basement of the laboratory, and specimens are lowered along telescopic brass tubes. Charged with 125 curies, the source is stated to give a maximum level of 1100 r/min.

In the French installation at the Laboratoire de Chimie Physique and lTnstitut de Radium de Paris use is made of a previously existing cave 15 m below the surface, to which access is gained by a lift. This excellent location also helps in maintaining good temperature control of the specimens during radiation. Specimens to be irradiated are located in marked positions on a table surrounding a central tube in which the source can be raised vertically from a shielded container to the operating position. The endless cable which raises the source also operates lamps which indicate its position to operators at ground level.

A simple source at Harwell makes use of a hole some 15 ft deep and 1 ft. in diameter in a large concrete block. At the bottom of this hole there is located a hollow cylinder of lead. The source consists of some 10 cobalt pellets, held in a vertical tube centred in this lead cylinder, and specimens are irradiated in the space between the source tube and the cylindrical wall. The radioactivity of the individual cobalt pellets varies ; by arranging these pellets with maximum intensity at the ends (and a subsidiary maximum in the centre of the tube) the radiation intensity is kept approximately uniform in a direction parallel to the source. Speci-mens to be irradiated are strapped below the lid of the source which is lowered into the hole by a small hand winch into a self-centring position above the lead container. By fixing specimens in a vertical position, the radiation dose is approximately uniform along their length, while by clamping them horizontally the intensity falls off along their length as the radial distance.

The 2300 cubic cobalt source located at the Naval Research Laboratory is located at the bottom of a pool under 12 ft of water. Specimens can be irradiated at the centre of the source, or in its vicinity, by means of a hollow tube from above ground level. Temperature controlling devices


FIG. 4.7. Radiation source at Hinxton, showing radiation chamber and demountable shielding. The cobalt rods are held in a cylindrical container with a shield above them,

and move down before the chamber can be opened for loading.


can also be installed. The use of a swimming pool offers considerable advantages in the way of flexibility in the arrangement of sources and samples.

In the last few years the number and variety of cobalt sources has increased very rapidly. Fig. 4.7 shows a source installed near Cambridge in which six sectors are available for specimens, to allow experiments to proceed independently. The cobalt rods, in the form of long thin pencils, form a hollow cylinder, and a high intensity position is available within the cylinder. Devices for temperature control of each sector, and for introducing gases and liquids during radiation, are also provided. Fig. 4.8 shows an installation at Brecksville in which the sources consist of four flat plates, with provision for irradiation in the central gap and behind each plate.

FIG. 4.8. Isometric view of Goodrich pig shows positions of four cobalt plates and five irradiation volumes. (From Bauman, 1957.)

At Brookhaven, hollow cylindrical sources of radioactive Co60 are in use, usually of 2-3 in. o.d., 1-7 in. i.d. and 13 in. in length. These are sheathed in aluminium to protect against oxidation and corrosion and to prevent any loss, e.g. in the form of radioactive powder. Specimens are usually placed within the core, where intensities of up to 5000 r/min have been obtained. These sources can be loaded in lead containers under water, and then used in air, although for large-scale experimentation irradiation under water is preferable.


Tanta lum sources have also been used at Brookhaven. Ta 1 8 2 has a much shorter half-life (111 days) than cobalt , but is more rapidly activated in a pile. The gammas emitted cover a range of energies, bu t are less energetic than those from cobalt , thus facilitating the shielding problem.

Descriptions of a number of British industrial radiat ion sources have recently been described in Nuclear Power (Dec. 1957).

R E F E R E N C E S

ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. A232, 31,

1955. AMPHLETT, C. B., Symposium on Utilization of Radiation from Fission Products

1953, p. 15. BLACK, R. M. and CHARLESBY, A., unpublished work. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955. BOPP, C. D. and SISMAN, O., O.R.N.L., 928, 1951. BROWNELL, L. E., PUROHIT, S. N., WEECH, M , BALZHISER, R. E. and LOBO,

A. H., 1943:7-77P, University of Michigan, August 1956. CALKINS, V. P., APEX 167, August 1954. CHAPIRO, A. and WAHL, P., C.R. Acad. Sei., Paris 238, 1803, 1954. CHAPIRO, A., / . Chim. Phys. 51, 165, 1954. COLLINS, C. G. and CALKINS, V. P., APEX 261, 1956. CORVAL, M., CHAPIRO, A. and COUSIN, C , C.R. Acad. Sei., Paris 235, 799,

1952. DAVID, V. W. and IRVING, R., Inst. Mech. Eng. Conf. London, October 1957. DUFFEY, D., Nucleonics 11(10), 8, 1953. LANDLER, Y., Thesis, Paris 1952. LEFORT, M. and PUCHEAULT, J., / . Chim. Phys. 50, 580, 1953. MANOWITZ, B., Chem. Eng. Progr. Symp. 50(12), 201, 1954. PREVOT-BERNAS, A., CHAPIRO, A., COUSIN, C , LANDLER, Y. and MAGAT, M.,

Farad. Soc. Disc. 12, 98, 1952. PUCHEAULT, T., C.R. Acad. Sei., Paris 240, 772, 1955. RICHARDSON, D. M., ORNL 129, 1948. SISMAN, O. and BOPP, C. D., ORNL 928, 1951. Stanford Research Institute, Industrial Uses of Radioactive Fission Products,

Stanford, California, 1951. SWORSKI, T. J. and BURTON, M., / . Amer. Chem. Soc. 73, 3790, 1951. WALTON, G. N. and WRIGHT, J., Nature, Lond. 172, 147, 1953. WALTON, G. N., Symposium on Utilization of Radiation from Fission Products

1953, p. 29. WILD, W., Chem. & Ind. 390, 1954. WILD, W. and WRIGHT, J., Symposium on Utilization of Radiation from Fission

Products 1953, p. 3. WRIGHT, J., Disc. Faraday Soc. 12, 60, 1952. WORRALL, R., A.E.R.E., M/R 2159, 1957. ZEITLIN, H. R., ARNOLD, E. D. and ULLMANN, J. W., Nucleonics 15(1), 58, 1957.

Miscellaneous

ANON., Nucleonics 15(11), 170, 1957. TRESISE, H. C , Nuclear Power 2, 492, December 1957. ZEITLIN, H. R., ARNOLD, E. D. and ULLMANN, J. W., Nucleonics 15(1), 58, 1957.


Cobalt Sources AITKEN, P. B., DYNE, P. J. and TRAPP, E. C., Nucleonics 15(1), 100, 1957. ANON., Nuclear Power 2(20), 508, 1957. BAUMAN, R. G., Nucleonics 15(1), 96, 1957. BLOMGREN, R. A., HART, E. J. and MARKHEIM, L. S., Rev. Sei. Instrum. 24, 298,

1953. BURTON, M., GHORMLEY, J. A. and HOCHANADEL, C. J., Nucleonics 13(10), 74,

1955. CALLINAN, T. D., Elect. Eng. June, 1955. CHAPIRO, A., COTTIN, M., HAISSINSKY, M., MAGAT, M. and VERMEIL, C.,

/ . Phys. Radium 14, 687, 1953. CHARLESBY, A. and FLINT, O., Atomics 6(4), 100, April 1955. DAVID, V. W. and IRVING, R., Inst. Mech. Eng. Conf. October 1957. GHORMLEY, J. A. and HOCHANADEL, C. J., Rev. Sei. Instrum. 22, 473, 1951. GOMBERG, H. J., GOULD, S. E., NEHEMIAS, J. V. and BROWNELL, L. E., Food

Engng. 26(9), 78, 1954. GREENFIELD, M. A., SILVERMAN, L. B. and DICKINSON, R. W., Nucleonics 10(12),

65, 1952. HUMMEL, R. W., FREEMAN, G. R., VAN CLEAVE, A. B. and SPINKS, J. W. T.,

Science 119, 159, 1954. JEFFERSON, S., Nuclear Power 2(20), 506, 1957. KUHL, O. A., SINGLETON, W. R. and MANOWITZ, B., Nucleonics 13(7), 42, 1955. KUHL, O. A., SPARROW, A. H. and MANOWITZ, B., Nucleonics 13(11), 128, 1955. MANOWITZ, B., Nucleonics 11(3), 18, 1953. OBRYOKI, R. F., BALL, R. M. and DAVIDSON, W. C , Nucleonics 11(7), 52, 1953. SAUNDERS, D. F. , MOREHEAD, E. F. and DANIELS, F., / . Amer. Chem. Soc. 75,

3096, 1953. SCHWARZ, H. A. and ALLEN, A. O., Nucleonics 12(2), 58, 1954. STEIN, S. and BEARD, D. S., Nuclear Power 2(20), 501, 1957. TAYLOR, D., Nuclear Power 2(20), 503, 1957.

CHAPTER 5

ELECTRICAL SOURCES OF RADIATION

ELECTRONIC equipment can provide a variety of sources for the irradiation of materials. To obtain useful penetration of at least a few millimetres, electrons must be accelerated to high potentials, of the order of 1 MeV or more. Heavier particles—protons or deuterons—even of higher energy usually achieve only a fraction of this penetration. With x-rays lower potentials are adequate for good penetration, but difficulties then arise in obtaining high dose rates.

Various methods are available for producing these high voltages—the choice depends not only on the type of particle to be accelerated and the required voltage, but also on such factors as the beam power, the per-missible variation in the applied voltage, and on whether a continuous or intermittent beam is required. Generally speaking, particles can be Con-veniently accelerated to energies of a few million volts by direct methods, but beyond this figure the insulation problems become very severe, and it is more convenient to impart the energy in a series of steps, each of lower amount. With the former technique a discharge tube is required for accelerating the particles, the design of which can be a crucial factor.

x-Ray beams of suitable penetration can be obtained by accelerating electrons to voltages of the order of 0-1-1 MeV, and allowing the electron beam to impinge on a heavy metal target. One of the main objections to this procedure is the low energy conversion rate of the process.

Table 5.1 shows the efficiency of conversion of electron energy to x-rays for various target materials and electron energies. Thus, for a 2 MeV electron beam impinging on a gold target, only 7-3 per cent of the electron energy is converted to x-rays and of this only a fraction can be utilized owing to the spread of the x-ray beam and its high penetration. The higher the Z number of the target, the greater the efficiency of con-version, but at the same time the beam spread is increased so the useful intensity over small targets is approximately independent of target material. Electron accelerators operating at a few MeV are currently used for radiotherapy purposes and provide x-ray beams of several hundred roentgens per minute at 1 m from the target. When, in addition to high penetration, high energy input into the specimen is required, the route via x-rays appears rather less promising and the use of radioactive isotopes such as caesium 137 may well provide a feasible alternative.

At much lower energies, the efficiency of conveision is considerably reduced and varies approximately as l-4x 10~9 ZVfor electrons of about 0-1 MeV. Here, Z is the atomic number of the target and V the energy of the electrons in volts. Thus, for a 100 KeV electron incident on a tungsten

75


target (Z = 74) the conversion efficiency is only about 1 per cent. To obtain an x-ray power output of 1 kW would require 100 kW of electron energy input, almost all of which appears as heat in the target. Apart from the cost of producing the energy wasted in this way, the problem of removing the heat liberated at the target gives rise to serious design problems.

Table 5.1. Efficiency of x-ray production from fast electrons

Target material

Aluminium (Z = 13) Copper (Z = 29) Gold (Z - 79)

Efficiency per cent

2MeV

11 2-6 7-3

4MeV

2-2 51

13-5

8 MeV

4-5 10 24

20 MeV

11 22 43

From: Miller, 1955

VOLTAGE MULTIPLIER CIRCUITS The simplest form of particle accelerator requires a high voltage trans-

former, and a discharge tube. In practice there is a limit to the potential which can be achieved in this direct manner, and various methods are used to increase the overall potential of the system, e.g. by connecting several sources in series.

Fig. 5.1 shows a cascade rectifier set comprising ten independent rectifier and condenser units connected in series, to provide 1 *4 MeV. The potentials are applied across an accelerator tube likewise split into ten sections, to avoid an excessive voltage drop across any part of the tube. The electron beam travelling down the tube impinges on a tungsten target to produce high voltage x-rays.

In their pioneer work on nuclear disintegration, Cockcroft and Walton used a voltage doubler circuit, in which the final voltage is obtained from a bank of condensers in series, only the first of which is charged from a high voltage d.c. source. By a switching arrangement controlled by rectifier valves, the charge is alternately transferred in part to other con-densers, and back to condensers higher in the bank, so that eventually a much higher voltage is built up than is provided to the first condenser. As used by Cockcroft and Walton, the potential was used to accelerate the protons down a tube and cause disintegration of the lithium nucleus. Somewhat similar circuits are used at present for ion acceleration, for voltages of about 1 MeV. They can produce relatively large currents with only a small amount of voltage ripple.

The replacement of valve rectifiers by metal rectifiers has greatly simpli-fied the circuit arrangements, no highly insulated transformers being needed to supply the filament heaters. By pressurizing and operating at a high recurrence frequency, these high voltage installations can be miniaturized. The maximum voltage appears in practice to be limited to about 2 MeV.

ELECTRICAL SOURCES OF RADIATION 77

Impulse generators do not provide a steady source of high potential. Condensers charged in parallel from a relatively low voltage source are subsequently discharged in series. Switching is usually achieved by means of spark-gaps. One such generator, termed the Capacitron, was designed for irradiation work and has been used in connection with early studies on radiation effects. The average power output from such an accelerator can be high, but it occurs as a series of very intense pulses, and is not therefore suitable for a number of radiation applications where beam intensity, as well as total dose, is relevant. Fluctuations in the beam voltage, and hence penetration, may also be important.

FIG. 5.1. 1-4 MeV constant potential x-ray equipment. 1. Ten-section cascade generator. 3, top corona shield; 16, plate transformer;

17, capacitors ; 20, filament transformer. 2. Ten-section x-ray tube. 4, top corona shield; 6, cathode assembly; 7, first inter-mediate electrode; 13, intermediate corona shields; 30, magnetic focusing coil;

33, tungsten target. (From Craggs and Meek, 19540


^Working chamber

FIG. 5.2. High voltage ion accelerator. 13, high voltage generator 1, ion source 15, shunt condensers 2, accelerator tube 16, resistance for voltage measurement 3, ion capture chamber

(From Craggs and Meek, 1954.)

E L E C T R O S T A T I C G E N E R A T O R S

Electrostatic methods of obtaining high voltages are widely used in radiation work. They have the advantage that the potential supplied is very steady, and can be varied over a wide range. Furthermore, it is relatively easy to convert them for the acceleration of either electrons or positively charged particles. Although considerably higher voltages have been produced for use in research laboratories (such as the 8 MeV machine at M.I.T.) the most useful range lies between 1 and 3 MeV, somewhat higher than is customary for voltage multiplier circuits.

The design in common use is based on that of Van de Graaff, and utilizes a rotating belt on to which charge from a high voltage d.c. source is sprayed from corona points. The belt travels into a high potential terminal where the charge is drawn off by discharge points and trans-ferred to the surface of a sphere, acting as a Faraday cage. A second set of discharge points can serve to recharge the belt with opposite polarity before it leaves the high potential terminal.

To obtain good insulation and moderate size, a commercial Van de Graaff machine operates in a tank with a C02-N2 gas mixture at about 400 lb/in2 pressure to suppress corona discharges. The output from the machine amounts to 2 MeV at a beam current of 250μΑ (an output of 500 watts of electrons). Later machines provide higher potentials and currents (3 MeV, 1 mA) and are very suitable for large-scale irradiation


treatment. In these installations the potential is applied to the filament in the accelerator tube, maintained under a high vacuum, and serves to accelerate electrons down the table to earth potential. At the end of the tube they emerge through a thin aluminium window, and strike the specimen to be irradiated. The electron beam is focused, and can be scanned across the window by electromagnetic deflection coils, providing a long narrow strip of radiation, so that any part of a specimen is irradiated only intermittently.

High voltage terminal

Upper pul ley-

Wire comb' Accelerating tube

Thin window

Specimen

0 1 2 3 f t

FIG. 5.3. Van de Graaff 2 MeV electron accelerator.


FIG. 5.4. Model of industrial installation for irradiating polyethylene-insulated cable with high voltage electrons.

{Courtesy of British Insulated Calender's Cables, Ltd.)


FIG. 5.5. High voltage section of Van de Graaif, showing high pressure tank, and stabilizer system of equipotential plates.


FIG. 5.6. Lower end of a Van de Graaff installation. Electron drift tube, electron scanner, Faraday cage and conveyer system for samples.

With the commercial 2 MeV equipment a focused but unscanned beam having an electron current of ΙΟΟμΑ can be absorbed over a specimen size of perhaps 1 cm2, giving an average dose rate of about 30 megarads/ sec. At some distance from the aluminium window the beam diverges because of scatter during its passage through the window and the air, resulting in a lower radiation intensity over a wider area. The dose distri-bution over the scattered beam can be monitored, but it is often more convenient to use the deflecting coils to obtain a band of radiation, and pass specimens through this beam at a known speed. In this case the average intensity will be much lower—1 megarad per passage through the beam is a convenient figure. The intensity of irradiation then takes on a rather complex pattern, with short bursts lasting perhaps 0-2 msec at 12-5 megarads/sec, such bursts recurring 400 times/sec (the frequency of sweep being 200 c/s). At these very high dose rates, temperature rise may become a problem. At a dose rate of 1 megarad/sec, the energy absorption is 10 watts/g or 2-4 cals/g per sec. For many plastics this gives


a temperature increase of about 6°C/sec, so that temperature control may be necessary. Ozone produced in the radiation chamber by the electrons may cause secondary chemical reactions in the specimen being irradiated.

The Van de Graaff generator is a very flexible research instrument since voltage and current can be immediately altered by the operator and the installation as a whole can be modified to produce not only electron radiation but also x-rays (by bombarding any suitable target of high

100i

Beam center

FIG. 5.7.

Surface radiation dose with an unscanned beam.

100i

Beam center Surface radiation dose with a scanned beam.

(The shaded areas represent the proportion of the incident radiation which can be used to provide a minimum stated dose.)

atomic number with the electron beam). Proton beams can be obtained by reversing the polarity of the equipment and installing a source of hydrogen in the terminal which is ionized to produce protons. These are also accelerated down the tube, but cannot be extracted through a thin window owing to their low penetration. Neutrons can be obtained with the equipment by bombarding a beryllium source with high voltage protons or deuterons.

Electrostatic generators of the Van de Graaff type have been in common use for a number of years in hospitals where they are used as sources of x-rays for cancer therapy. In this application high accuracy in beam geometry rather than high power beam output is necessary. Hospital

G

84 ATOMIC RADIATION A N D POLYMERS

machines have also been used as sources of neutrons for radiobiological work.

A number of alternative forms of electrostatic generator for high power output have been devised; for example, Felici has studied the use of a rotating drum instead of the belt used by the Van de Graaff. For high power electrostatic sources of radiation, the provision of the high voltage supply constitutes only part of the solution, a major limitation being the capacity of the accelerator tube to withstand the very severe operating conditions.

RESONANT TRANSFORMER A resonant transformer has been developed giving high a.c. voltages

suitable for the production of x-rays. By replacing the metal target by a suitable window the equipment can be used as a source of high voltage electrons.

The transformer and discharge tube are placed in a steel pressure tank for insulation. A number of secondary windings in series are used, spaced

Slotted brass shield

Tie-rod spring

Cathode assembly

Accelerating electrodes

Shields Glass tie rod

Primary winding

Focusing coil

Insulating gas cooler

Steel tank

Variable reactor

Pressure plate

Secondary coils

Laminated shield

Reactor drive

Lead top

J ^ j j J J h — Reactor drive motor

Tube window

FIG. 5.8. Cut-away view of a 1 MeV resonant transformer. (From Knowlton, Mahn and Ranftl, 1953.)

so that the voltage gradient along them is reasonably uniform. These windings are tuned to a harmonic of the frequency of a.c. used to feed the


primary and no iron core is required. The sinusoidal potential obtained from a separate winding is applied to the filament in a sealed-off accelerator tube. The system therefore provides an electron beam during half the cycle only, but the voltage varies from 0 to the peak value. To avoid the resultant variation in penetration, the tube is biased to allow current to pass only near the peak, which gives an approximately uniform voltage beam during the working part of the cycle. It has been claimed that the residual change in voltage of the electron beam serves to correct for the variation with depth of the ionization produced by a uniform voltage beam.

After focusing, the electrons emerge via a thin steel window in the accelerator tube, the window being cooled by a strong air blast.

Most of the published work on the radiation effects using a resonant transformer as a source of electron radiation was carried out with a 800 kvp equipment which only gives a penetration of less than 0-2 g/cm2. Larger transformers giving voltages of 2 MeV and several kilowatts are now commercially available.

LINEAR ELECTRON ACCELERATORS

Linear accelerators have been used for some time to provide high voltage beams for nuclear research, and for the production of very high voltage x-rays used in radiotherapy. Their use for radiation treatment is increasing rapidly, and they provide powerful beams of electrons at voltages which because of insulation difficulties cannot be obtained by electrostatic or transformer generators, in which the voltage is built up in a single step. Electrons with energies equivalent to several hundred MeV have been obtained, and machines providing even higher voltages are being built for nuclear research purposes. At much lower voltages of the order of 10 MeV accelerators with power outputs of several kilowatts are available for the radiation treatment of materials, and provide beam penetration of several centimetres.

In the linear accelerator, the electrons are injected at one end of a wave-guide, and are accelerated under the influence of a radiofrequency field (often of 3000 Mc/s) travelling along the guide. In a smooth-walled wave-guide, the phase velocity of the field exceeds that of light, and to match this velocity to that of the electrons being accelerated, the guide is "loaded" by introducing a stries of iris diaphragms of the appropriate spacing. The injection energy of the electrons, obtained from an electron gun, can be quite low, of the order of 50 kV, and by suitable loading of the first stage of the guide the electrons are bunched and accelerated. In the latter stages, the changes in wave velocity need only be slight, since the electrons are already moving at speeds close to that of light. By increasing the number of waveguides, and making other relatively minor alterations, the output voltages of a linear accelerator can be increased over very wide limits. Most of the high powered accelerators installed for radiotherapy or radiation treatment of materials operate at about 4-8 MeV, and give electron penetration of up to one inch in water, but higher potentials can be readily obtained if required. It would, however, appear desirable to


FIG. 5.9. The 8 MeV linear accelerator equipment during erection at Hammersmith Hospital.

This equipment, supported partly by the pumping plant on the right and partly from the ceiling, projects into the treatment room where the rotatable x-ray head can be seen on the left of the picture.

(From Miller, 1953.)

FIG. 5.10. A partly sectioned length of disk-loaded corrugated waveguide.


restrict such potentials to below about 20 MeV to prevent significant amounts of radioactive elements being produced in the target material. To reduce the size of the waveguide to convenient dimensions, the instantaneous radio power flux must be high, and this enforces pulse operation of the equipment. The radiofrequency power is obtained from a pulsed magnetron or a klystron, as used in standard microwave engineer-ing, and considerable increases in power output can be expected from future improvements of these radar devices.

The high energy electron beam produced from the linear accelerator occurs as a series of pulses of very short duration of several microseconds, the power in the beam during this short period possibly amounting to

FIG. 5.11. The accelerator unit for 4 MeV equipment.

megawatts. The average power output depends on the frequency of repetition of these pulses which may amount to 500 pulses/second.

The design of the accelerator tends to bunch the electrons together but some variation in the energy of the beam still remains. Fig. 5.13 shows that for an 8 MeV machine the effective energy varies somewhat with


FIG. 5.12. Cross-section of 4 MeV accelerator.

beam current, and that the spread of energies at a given loading is about 0-5 MeV.

At the end of the waveguide tube the electrons emerge through a thin metallic foil. Scanning of the beam is less commonly used than with a continuous beam, and it may be advisable to scatter it by passing it through a suitable thin target. Since the beam is only being emitted for about 0 1 per cent of the time there will be considerable variations in radiation intensity: if the average intensity is 1 megarad/sec, it can exceed 1000 megarad/sec during the pulses. At the high ion densities associated with these radiation intensities, ion-ion interaction may modify the overall reaction. As for other sources of high-energy electrons, the linear accele-rator can be readily modified to serve as a source of penetrating x-rays. It has the advantage that the energy conversion factor is better for higher voltage electrons (Fig. 5.16). On the other hand the high penetration of the x-rays, produced increases the difficulty of absorbing their energy usefully in a specimen: the shielding problems are also increased.

88

KiY 1. Magnetron. 2. Perminent magnet. 3. Probe and phase »h.fter unit for magnetron frequency pulling. 4. Pumping port. 5. RF absorbing load. t . Rf. wattmeter. 7. R.F. bridge (rat-race). 8. Phase ihifter in feedback loop. 9. Vacuum tank.

10. Focusing co.U. 11. Corrugated waveguide. 12. End feed (doorknob tran»former). 1J. Electron tun. 14. Οπνβ (or feedback phase shifter. 15. Cooling water pipei. 1*. X-rar bead. 17. Lead for absorption of scattered radiation.


/

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? 5 2

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O

£ 2 is

ω 1

U ^

^ ^

r \ 1 \ 1

1 1 1 1 1 1

li li

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i

\

-1 K 1 8

Electron energy, MeV

FIG. 5.13. Electron spectra of linear accelerator. Final arrangement using 15° delay section and operating at design frequency of 2999-0 Mc/s.

High beam current. Medium beam current.

— . — . — . Low beam current. (From Miller, 1954.)

10 20 30 40 Mean beam current t μΑ

FIG. 5.14. Typical performance curves of linear accelerator at 500 pulses/sec operation Electron energy is that at the peak of the spectrum. x-Ray output is measured without

the clinical head. 15° delay section is used and frequency is 2999-0 Mc/s. x-Ray output.

• Electron energy. (From Miller, 1954.)


VERY HIGH VOLTAGE PARTICLE ACCELERATORS

A number of accelerators are installed in research laboratories designed to accelerate electrons or heavier particles to very high energies. These accelerators are intended primarily for nuclear research, and their current output is usually low. Included in these accelerators are betatrons and electron synchrotrons for the acceleration of electrons, and cyclotrons, synchrocyclotrons, proton synchrotrons and linear proton accelerators for heavier particles.

In the betatron the electrons circulate in a circular orbit in an evacuated doughnut-shaped vessel. At right angles to their plane of motion is a magnetic field, whose magnitude increases in step with the electron velocity, to keep the electrons in their orbit, and at the same time to accelerate them. At a suitable step in the cycle an auxiliary field extracts the electron beam, which is often used to provide high-energy x-rays. Very high voltages can be obtained in this way: for example, in a 100 MeV betatron the electron circulates 250,000 times, picking up 400 eV per turn before being extracted. The electrons emerge in a series of pulses, usually about 100/sec. The currents available are generally very low, a fraction of a microamp, and are therefore not usually suitable for radiation work.

The electron synchrotron overcomes the voltage limitation set in the betatron by radiation losses. It applies a very high frequency potential difference to the cavity in which the electrons circulate as in the betatron, and further increases their energy.

In the cyclotron, protons circulate in a horizontal orbit, inside two dee-shaped cavities, a vertical magnetic field maintaining them in a circular orbit. A high frequency alternating field is applied between the two dees and periodically accelerates the protons. Relativistic changes in mass limit the maximum energies obtainable. These are overcome in the synchro cyclotron by appropriate changes in the magnitude of the magnetic field or more usually in the frequency of the electric field. The protons therefore emerge in pulses.

Other particle accelerators designed for very high energies include the proton synchroton and the proton linear accelerator.

For the study of radiation effects these accelerators may be of value in comparing highly ionizing radiation (such as fast protons) with that produced by sparsely ionizing radiation (such as fast electrons). Apart from such specialized purposes however, very high voltage equipment is not desirable ; the lower voltage sources of electron or γ-radiation are far simpler, and have a much higher useful power output at lower cost.

X-RAY EQUIPMENT

Unlike the γ-radiation obtained from radioactive isotopes, which is mono-energetic, the x-ray spectrum of energies is continuous, ranging from that corresponding to the peak electron voltage applied, to very low energies. An approximate expression for the x-ray intensity emitted at a wavelength corresponding to an energy E can be written in the form


IOLE2(ES-E)

where Es is the electron energy applied to the tube. The actual shape of the curve depends on the target thickness and the expression represents the general pattern for thick targets.

x-Rays can be produced from sources fed by a constant or a fluctuating electron voltage; in the latter case the spread of energies will be even wider, with a greater proportion of the energy extending into the low voltage end of the spectrum. This variation in x-ray energy is accompanied by a corresponding variation in penetrating power. In certain applications

FIG. 5.15. Spectral dose-rate distributions, at five constant potential settings, using 1 mm Be inherent filtration :

(a), (b), (c) Zero external filtration, 10, 30, 50 cm focus distance. (d) 0025 mm Al external filtration, 10 cm focus distance. The arbitrary air dose rate units are relative throughout.

(From Jennings, 1953.)

this is not a very serious consideration; in the examination of metallurgical structure, for example, the object being examined will to some extent act as its own filter. In therapeutic work, however, the dose-depth relation-ship is of considerable importance, yet it is difficult to give any general


rules; the ionization depends on the target material, the voltage applied, the elements present in the material being irradiated, etc. For low energy x-rays photoelectric absorption becomes important. The considerable literature which has grown up on the dosimetry of x-rays used in medical applications is indicative of the complexity of the subject.

i f 30.

2 3 4 5 Electron energy,

8910

FIG. 5.16. The relation between x-ray output and electron current as a function of energy. (Intensity is measured in the forward direction and a thick, gold target is

used.) (See Table 5.1.)

A more uniform beam can be obtained by the use of filters, which selectively remove the lower-energy part of the x-ray spectrum, at the same time reducing the overall intensity of the beam. It is often con-venient to express the beam intensity in terms of a half value layer (H.V.L.), usually in aluminium.

The x-ray equipment in widespread use in industry and medicine is not usually suitable for radiation work because of the low beam intensities available. The conversion of electron energy to x-rays is a very inefficient process, most of the energy appearing as heat. To obtain high conversion efficiency of x-rays requires very high voltages and a target of high atomic number.

Low Energy, High Intensity x-Ray Equipment

Low energy x-rays may be advantageous for the tieatment of very thin specimens where all the energy can be absorbed near the surface. Shielding problems are relatively simple and no serious difficulties arise


in insulation or power production because of the relatively low voltages used. To obtain useful amounts of x-ray output considerable energy inputs are required, most of this energy being lost in heating the target. To minimize the further loss of intensity from the x-ray beam, the window through which it emerges must be made of a low molecular weight

Beryllium window

FIG. 5.17. Low voltage, high intensity x-ray tube with beryllium window. (From Radiology, 1947.)

material. In commercially available equipment (such as the Machlett Tube), the window is made of beryllium metal, which permits the heat absorbed from the x-radiation to be readily conducted away. These tubes may be operated at a current of up to 50 milliamps and a voltage of 50 kV. This energy, which appears almost entirely as heat in the target, is removed by water cooling. The radiation produced by the equipment is polychromatic and at 50 kvp a machine of this type has a maximum intensity at a wavelength of 1-6Â. Filters can be used to decrease the spread in wavelength at a cost of reducing the total intensity available. A 1 mm beryllium filter placed in the beam absorbs only 50 per cent of the x-ray energy as against 96-5 per cent for an aluminium window of equal thickness. In the absence of a filter, the depth of penetration in aluminium at which the intensity is halved is only 0Ό7 mm, but the surface radiation intensity can be very high. Close to the window, surface radiation doses of the order of 106 r/min may be obtained.

Fig. 5.18 shows the effect of aluminium filters of varying thickness on the half value depth in tissue (this will be comparable with many plastics). Fig. 5.19 shows the corresponding reduction in surface intensity.

1br

14

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n.

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curv

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how

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9. E

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.)

94 ATOMIC RADIATION AND P O L Y M E R S


R E F E R E N C E S

ANON, Nucleonics 15(10), 126, 1957. BLACK, R. M., Nature, Lond., 178, 1370, 1956. BRASCH, A., U.S. Patent 2429217, October 21, 1947. CHARLESBY, A., Nature, Lond., 178, 60, 1956. CHARLTON, E. E., WESTENDORP, W. F., DEMPSTER, L. E. and HOTALING, C ,

J. Appl. Phys. 10, 374, 1939. CHARLTON, E. E. and WESTENDORP, W. F., Gen. Elect. Rev. 44, 44, 1941. CHARLTON, E. E. and HUBBARD, M. S., Gen. Elect. Rev. 43, 272, 1940. CHICK, D. R. and MILLER, C. W., Commun, and Electronics, October-November

1956. COCKCROFT, J. D. and WALTON, E. T. S., Proc. Roy. Soc. 136, 619, 1932. CRAGGS, J. D. and MEEK, M., High Voltage Laboratory Technique, Butterworths,

1954. DEWEY, D. R., NYGARD, J. C. and KELLIHER, M. G., Nucleonics 12(12), 40, 1954. FORTESCUE, R. L., Progr. Nucl. Phys. 1, 21, 1950. FOSTER, F. L., DEWEY, D. R. and GALE, A. J., Nucleonics 11(10), 14, 1953. FRY, D. W. and WALKINSHAW, W., Rep. Progr. Phys. 12, 102, 1949. GLASSER, O., QUIMBY, E. H., TAYLOR, L. S. and WEATHERWAX, J. L., Physical

Foundations of Radiology, Cassell, 1952. HONIG, R. E., Rev. Sei. Instrum., 18(6), 389, 1947. HOWARTH, J. L., JONES, J. C. and MILLER, H., Brit. J. Radiol. 24, 665, 1951. JENNINGS, W. A., Acta Radiol. 33, 435, 1950; 36, 477, 1951. JENNINGS, W. A., Brit. J. Radiol. 26(304), 193, 1953; Cathode Ray Press 8(3), 4,

1951. KNOWLTON, J. A., MAHN, G. R. and RANFTL, J. W., Nucleonics 11(11), 64,

1953. MANN, W. B., The Cyclotron, John Wiley, New York, 1950. MILLER, C. W., Metrop. Vick. Gaz. 25(413), 119, 1953; / . Brit. Radio Eng.

14(8), 361, 1954. MILLER, C. W., Engineering, September 9, 16, 1955. MILLER, C. W., Proc. Instn. Elect. Engrs. 101(1), 207, 1954. RAPPOPORT, P. and UNDER, E. G., J. Appl. Phys. 24, 1110, 1953. SLATER, J. C , Rev. Mod. Phys. 20, 473, 1948; Ann. Rev. Nucl. Sei. 1, 199, 1952. STOCKMAN, C. H. and BAUMAN, R. G., Chem. Engng. News 35, 16, 1957. TRUMP, J. G. and VAN DE GRAAFF, R. J., Phys. Rev. 55, 1160, 1939. VAN DE GRAAFF, R. J., TRUMP, J. G. and BUECHNER, W. W., Rep. Progr. Phys.

11, 1, 1948.

CHAPTER 6

DOSIMETRY

A CONSIDERABLE literature has grown up, devoted to the design and cali-bration of equipment for the measurement of radiation dose. Applications in the fields of health physics, radiology, radiation chemistry and reactor technology cover an immense range of doses from a very small fraction of a roentgen in tracer applications, to many millions when radiation damage to materials is considered. Further factors to consider include the type or quality of radiation (such as low energy x-rays, a-particles or high energy electrons) and the radiation intensity. A number of methods of dosimetry have been evolved, depending on direct measurement of energy absorption or of ionization, or on some other physical or chemical change which after calibration with known doses can be more readily utilized under given circumstances. For any specific application the most suitable technique depends not only on the type of radiation, the dose range and radiation intensity to be covered, but also on the accuracy, simplicity and speed of measurement required.

Many of the methods developed for use in radiobiology are not suitable for radiation chemistry or polymer work, when the doses and intensities used may be greater by at least several orders of magnitude. Moreover the response of many dosimeters, particularly those based on chemical changes, depends on the density of ionization as well as on the total number of ions produced; the ion density being itself determined by the quality of the radiation and by its energy. The ferrous sulphate dosimeter, for example, shows very different chemical changes for the same energy input when a-particles or fast electrons are used. Even in the case of a nearly monochromatic source such as radioactive cobalt or a high voltage electron beam the radiation to which a specimen is subjected will cover a range of energies with a corresponding variation in ion density.* Many of the effects observed in the treatment of polymers depend mainly on the total dose absorbed, and little, if at all, on the radiation intensity or quality. For measuring the radiation doses to which such materials are exposed it is therefore highly desirable for the dosimeter to be independent of all these factors.

Many of the well-tried techniques suitable for low intensity, low dose work, will not be considered here in any detail. With the increased impor-tance attached to high intensity, high dose irradiation, new methods of dosimetry suitable for doses of a few megarads have been suggested, but

* Bernstein and Schüler (1955) have found cobalt 60 that for about 12 per cent of the quanta can be of low energy, primarily due to self-absorption by cobalt.

96

DOSIMETRY 97

most of these have not yet received the same detailed attention which has been devoted to the earlier low intensity or low dose methods.

ABSOLUTE MEASUREMENTS Two alternative methods of defining a dose are available; the roentgen

(or rep) and the rad. Most of the earlier measurements were expressed in terms of the roentgen, and are therefore related to the ionization in air. To convert these measurements into energy absorption in a specimen, know-ledge of the average energy W expended per ion pair produced in air is needed, as well as of the stopping power of the specimen and of the wall material of the ionization chamber. As experimental determinations of W have varied so has the figure for energy absorption. Where the basic measurements are carried out in terms of the rad, this factor no longer intervenes, and the chemical change produced by a given radiation dose can be expressed directly in terms of a G value for the reaction.

A general summary of the problems involved in the design of ionization chambers is given by Boag (1956). An ionization chamber in frequent use is the Victoreen dosimeter with interchangeable chambers over the range 0-250 r, which is generally too low for the type of work envisaged here. Ionization chambers designed for use with fast electron beams are described by Boag, et al. (1951), and by Laughlin et al. (1953).

A number of calorimeters have been designed to measure on an absolute scale the energy absorbed by an irradiated specimen. One apparatus designed by McElhinney et al. (1956) is intended to measure the radiation in an x-ray beam over the range from 1 to 100 MeV peak. The heat from the radiation is absorbed in a lead cylinder and the temperature rise com-pared by means of thermistors with that in a second cylinder heated electrically. It is claimed that this installation has a probable error of 1 per cent. Such a device cannot be used for routine measurements, particularly as the cylinders must be large enough to be capable of absorb-ing all the incident energy. Other instruments designed to measure energy absorption from radiation have been described by Hochanadel and Ghromley (1953), by Laughlin et al. (1953) and by Lazo et al. (1954).

A simple technique which may be used with high intensity beams con-sists in absorbing the beam in water and measuring the temperature rise. A dose of 1 megarad will by definition raise the temperature of the water by 10/4-184 or 2-4°C. In such measurements it is necessary to correct for the heat loss, for energy absorbed by the container walls, and for radiation escaping from the system by backscatter, bremsstrahlung, etc.

An alternative method of measuring on an absolute scale power absorp-tion from a high energy electron beam depends on knowledge of the incident electron voltage and the charge absorbed in the sample. The voltage of the electron beam can be calibrated accurately from the activa-tion energies of certain nuclear reactions while the total current can be measured by surrounding the specimen with a Faraday cage. Losses occur due to scattered electrons and to bremsstrahlung production of x-rays. These correction factors are generally low for the conditions used in polymer irradiation when the specimens consist of atoms of low atomic


number. Schüler and Allen (1956), for example, found that in the irradia-tion of ferrous sulphate solutions, the energy lost by backscatter was only 0-23 per cent at 2 MeV and 0-41 per cent at 1 MeV, while bremsstrahlung accounted for a further loss of 0-44 per cent and 013 per cent at the same voltages. For many applications a simple collection device consisting of a thin metallic foil connected to earth via a galvanometer may be used as a means of monitoring beam current.

FERROUS SULPHATE AS A SECONDARY STANDARD

There are often serious practical difficulties in the use of absolute physical methods for determining radiation dose and for many purposes a secondary standard has been adopted based on a chemical change, namely the oxidation of an aqueous solution of ferrous sulphate to ferric sulphate. Details of this dosimeter have been given by a number of authors (Miller, Weiss, Hardwick). The standard solution consists of 1 mM of the salt in an air-saturated 0·8Ν sulphuric acid solution. The ferric ion concentration is normally determined by spectrophotometric measurement of absorption at 300-305 ιημ.

This chemical dosimeter has been calibrated absolutely against a number of sources of radiation of different ionizing power, the intensity of the radiation being measured by physical means. For example, irradiations were carried out with radioactive sources of known activity or the energy absorption was measured by calorimetry or by direct calculation of the energy input. Comparative measurements of heat dissipation in water and in the dosimeter from Cobalt 60 gamma radiation gave G values for the oxidation of ferrous ions of 15-6±0-3 (Hochanadel and Ghormley, 1953) and 15-8 ±0-3 (Lazo et al, 1954). Using a known power input from 2 MeV electrons Saldick and Allen (1954) obtained a G value of 15·6±0·5. Ioniza-tion methods which assume an energy absorption-of 34-3 eV per ion pair in air led to a value of 15-9 ±0-3 (Weiss et al, 1954). A recent measurement by Schüler and Allen (1956), again using 2 MeV electrons with a known current density, gave a G value of 15-45 ±011. Haybittle et al. (1956) investigated the variation in G values for various qualities of x- and γ-rays from 21 keV to 7-6 MeV, and derived G values from 13-1 to 16-3.

Radiation Quality and Intensity The yield of the ferrous sulphate dosimeter depends on the distribution

of the ions produced by radiation, i.e. on the quality of the radiation. The range of G values for radiation of different quality and energy is shown in Table 6.1. Previously, some measure of disagreement had arisen in the G values for electrons and γ-radiation and some doses in the literature are based on a G value of 20-5. Such dosages should be corrected to the recent generally accepted value of about 15-5. It will be seen from the table that these G values vary considerably, depending on the quality of the radiation, i.e. on the ion density or L.E.T. (see page 47). On the other hand, the variation with beam intensity is far more satisfactory and the yield has been found to be the same over a range of intensities from

DOSIMETRY 99

0-1 r/sec to about 2x 106 r/sec. Details of the preparation and use of a ferrous sulphate dosimeter are given by Miller (1956).

Sutton and Rotblat (1957) have studied the response of ferrous sulphate

Table 6.1. Effect of Radiation Quality on G value for Oxidation of Ferrous Sulphate

Radiation

a

H3

H2

P

Electrons

x-rays

Ύ

Energy and source

B10(/i, a) Li7

(1-49 MeV)

p 0 210

Li\n, a)H3

(205 MeV)

Cyclotron 5-35 MeV

Li6(/i, a)H3

(2-7 MeV) 6-19 MeV H3

p32

lMeV 1-1-5 MeV 2 MeV 2 MeV lOkVp 50kVp 60kVp 100 kVp 200 kVp 220 kVp 220 kVp 250 kVp 1 MeV 2MeVp 4MeVp 30 MeVp Cobalt 60 1 17,

1 33 MeV

Mean energy

5-6 keV

0-7 MeV

21keV 33keV

56keV

7-6 keV

G

4-2 + 0-4 4-22+008 41+0-4 5-8+0-6 6-0 + 0-4 6-2 + 0-2

60 5-4 + 0-3

5-69 + 0-12 5-2 + 0-5 5-0 + 0-6

6-9-5

6-7

9-4-11-9 12-9 + 0-2

12-7 15-4

15-9+0-4 15-8+0-5 15-6+0-5 15-45+0-3 14-15 + 0-6 14-0 + 0-8 13-1+0-5 14-7 + 0-5 16-1+0-8 14-6 + 0-8 15-0 + 0-5 14-9 + 0-8

15 9 161 15-7

16-3+0-6 15-6 + 0-3

16-9 16-1+0-6 15-8+0-3 15-5+0-5

Method

Effective cross-section Effective cross-section Effective cross-section lonization lonization Counting lonization Effective cross-section Effective cross-section Effective cross-section Effective cross-section Collection of charge

Effective cross-section

Collection of charge Counting Counting Counting Charge collection Charge collection Charge collection Charge collection lonization lonization lonization lonization lonization lonization lonization lonization lonization lonization lonization lonization Calorimetry lonization lonization Calorimetry lonization

For references see Miller (1956), Day (1956) and Haybittle et al (1956). H


and eerie ammonium sulphate dosimeters at the very high instantaneous intensities obtainable from the 15 MeV linear accelerator installed at St. Bartholomew's Hospital. This can provide doses of up to 20,000 rads per pulse lasting only 1·3μ sec. In their experiments Sutton and Rotblat observed that the ratio of the G values for these two dosimeters remained approximately constant (at about 7) for intensities up to about 107 rads/sec, and then decreased to about 4 at intensities of about 1010 rads/sec. Although it was not possible to measure the G values for the individual dosimeters, most of the reduction was ascribed to the variation in the eerie system. Such an intensity dépendance can be due to the increased proba-bility that reaction zones of the separate radical clusters overlap as the time interval between individual ionization tracks is decreased. The changes in G value may therefore serve to measure the radical lifetimes.

Limitations of the Ferrous Sulphate Dosimeter The oxidation of ferrous sulphate to ferric sulphate depends on the

presence of oxygen. In using this dosimeter, it is essential to work at doses such that sufficient oxygen is present for the reaction to proceed at a standard rate. Moreover, the radiation measured should be such that regions of high intensity are avoided to eliminate the danger of local lack of oxygen. In practice, this limits the maximum dose to be measured to about 42,000 rads for γ and electron radiation and to about 200,000 rads for α-radiation. The G value obtained is independent of the initial concentration of ferrous ions between 1 mM and 01 mM and does not depend on the concentration of sulphuric acid within fairly wide limits.* There is a small spectrophotometric temperature coefficient, about 0-8 per cent between 20°C and 30°C, and the system is very sensitive to impurities (Dewhurst, 1952; Vermeil, 1955).

The limitations on the maximum dose which can be measured using this dosimeter are a serious objection against its use in polymer research where most doses required are of the order of megarads. Unless a low intensity source is used with correspondingly long exposure times consider-able inaccuracies in dosimetry may result.

OTHER CHEMICAL DOSIMETERS Ceric sulphate—An alternative method of chemical dosimetry depends

on the reduction of an aqueous solution of ceric sulphate, the disappearance of the ceric ion being measured by a spectrophotometer at a wavelength of 320-330 ιημ. The ceric sulphate dosimeter has received less attention than the ferrous sulphate dosimeter, and the G value for the reaction is less well known (Hochanadel, 1950; Weiss, 1952; Hardwick, 1952). It is also more affected by the presence of impurities. On the other hand it has the advantage that the maximum dose is not limited by the amount of oxygen present, so that the range extends to much higher doses than does the ferrous sulphate system.

* For soft x-rays photo-electric absorption by S atoms may be important, so that the lower acid concentration is to be preferred.

DOSIMETRY 101

Benzene—Solutions of benzene or sodium benzoate in water have been used as dosimeters. The products obtained are complex and only the phenol is used as a measure of the radiation dose. It has not been estab-lished that the yield is independent of the radiation intensity and after-effects may be present to complicate the issue (Stein and Weiss, 1949; Day and Stein, 1949).

Solutions of méthylène blue—The use of solutions of méthylène blue (Shekhtmann et al, 1950; Goldblith et al, 1952) has been suggested but the variation of optical density is not linear with radiation dose and the reproducibility may not be sufficient for accurate determinations of radiation dose.

Hydrogen evolution—Measurement of gas evolution, particularly hydro-gen from irradiated paraffins or other simple organic systems, may be useful as a method of radiation dosimetry for use in polymer work. One of the advantages of such systems would be the high doses which could be measured. It cannot be claimed that such systems would be convenient or rapid. On the other hand the yield is not greatly dependent on radiation quality, intensity, or on temperature (Charlesby and Davison, 1957). Schüler and Allen (1955) obtained G values for the hydrogen yield from cyc/öhexane of about 5-6 for 30 MeV alpha irradiation, and 5-1 for 19 MeV deuterons, as compared with 5-25 for 2 MeV electron beams. The ferrous sulphate dosimeter used under similar conditions gave G values of about 9, 11-6 and 15-5.

Nitrous oxide—Harteck and Dondes (1956) have suggested a dosimeter, based on the decomposition of nitrous oxide, which can be used in the range 5 x 104 to about 108 rads. Under irradiation nitrous oxide decomposes into oxygen, nitrogen and nitrogen dioxide. The doses can be measured either from the nitrogen plus oxygen evolved, or from the nitrogen dioxide. The nitrogen and oxygen are measured with a vacuum gauge after con-densing the remaining nitrous oxide and the nitrogen dioxide formed. The amount of nitrogen dioxide can be determined by raising the tem-perature of the condensate to dry-ice temperature, when only the nitrous oxide is evolved. At doses above about 50 megarads the nitrogen dioxide produced can be measured spectroscopically. The yield is stated to be independent of both intensity and temperature, within limits set by vapour pressure and decomposition temperature.

A number of other chemical systems have been studied, but not yet used in practice; they include the decoloration of DPPH in organic solvents (Chapiro et ai, 1953), the production of iodine from alkyl iodide, and the modification of chlorine compounds (Andrews and Shore, 1950).

COLORIMETRIC METHODS Dyed Plastic

Day and Stein (1951) have suggested the use of a commercial grade (Perspex 400) of polymethyl methacrylate incorporating a red dye. Irradiation produces a considerable change in the height of the absorption

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DOSIMETRY 103

peak of the dye (500 πιμ) and also a further absorption band at 600 ηιμ. This second band, however, is subject to fading after irradiation as the oxygen diffuses in. Very little published information is available on this suggested method of dosimetry, although it would in any case appear to be restricted to doses of less than 2 megarads. At high electron dose rates spark discharges offer a further difficulty. Optical Absorption in Polystyrene and Polymethyl Methacrylate

Fowler and Day (1955) have recently suggested that the change in colour of solid polystyrene and polymethyl methacrylate can be used as a

Totcl dose, x10 roentgens

FIG. 6.1. Optical absorption of polystyrene and polymethyl methacrylate at doses of up to 15 megarads and 200 megarads.

measure of radiation dose. On irradiation, the spectrum of these materials changes, the usual ultraviolet absorption band which cuts off at 280 ιημ


moving over to higher wavelengths. In the case of polystyrene, there is also a small absorption band at 330 ηιμ which may however become submerged in the tail of the ultraviolet absorption. By measuring the optical transmission of irradiated polystyrene, Fowler and Day found a linear response for radiation doses up to about 20 megarads and above this a further linear response of lower magnitude. No sign of saturation in the absorption curve appeared up to at least 200 megarads. Polymethyl methacrylate showed a slightly higher sensitivity but the absorption begins to saturate below 100 megarads. Mechanical properties also tend to fail at these doses.

There is a slow decay in the absorption intensity due to the diffusion of oxygen into the system and to thermal decay. In the case of polymethyl methacrylate, this diffusion of oxygen gives rise to the clear surface layers previously reported by Ross and Charlesby (1953). In polystyrene the decay is largely due to thermal bleaching, absorption being approximately halved in four days. These effects may be very temperature dependent, and the decay would be a serious handicap in comparing short-term with long-term irradiation. Measurements on cobalt 60 γ-radiation and on x-rays of 220 kVp confirmed that there is no major energy dependence for x-rays. No comparable results have yet been reported for electron radiation.

Coloration in Glass

The coloration of glasses and of ionic solids by radiation has received a considerable amount of attention, and their measurement promises a simple method of comparing radiation doses. Difficulties may arise due to variation in composition, to intensity and temperature dependence and to fading of the centres which produce coloration. To obviate these difficulties conditions of use must be carefully controlled.

Schulman and his colleagues designed photoluminescent glasses for measurement of small radiation doses for personnel monitors ; subsequently they extended this work (1955) to systems capable of dealing with much higher doses. They suggested the use of a silver activated phosphate glass. The response was found to be linear for cobalt 60 γ-radiation up to about 100,000 rads, but at higher doses the sensitivity dropped and saturation effects appeared, so that the system is not suitable for doses exceeding 2 megarads. The dose-rate dependence is good, there being no sign of an intensity effect when the radiation intensities range from about 104 to above 107 r/hr. Moreover, the response is approximately indepen-dent of energy from 1-33 MeV to 200 keV, though not below it. The optical changes produced are, however, subject to fading, this being most rapid during the first two days. The time required to reach a fairly steady state can be reduced by heating the specimens for 13 min at 130°C.

While flint glass colours readily under radiation, its rapid fading renders it useless as a radiation dosimeter. Kreidl and Blair (1956) suggested the use of cobalt or silver in a conventional silicate glass. The sensitivity is increased and tendency to fade is decreased by increasing the concentration of cobalt. For a certain type of cobalt glass they find a linear relationship

DOSIMETRY 105

between absorption and dose up to about 4 x 105 rads (depending on the wavelength chosen), although above this dose the change becomes smaller and non-linear. The system is energy independent above 200 keV, but does so depend below this energy, owing to the photoelectric effect. Fading in the course of a week would amount to about one-fifth of the change produced by radiation. Fig. 6-2 shows the relationship for both

10" Dose, rep

FIG. 6.2. Change in optical absorption of cobalt glass after irradiation with Cobalt 60 y's and 2 MeV electrons. Insert shows changes at low doses.

(From: Kreidl and Blair 1956.)

4 0 0 ^ 105 106 107

Wavelength, ηημ

FIG. 6.3. Optical density of irradiated microscope slides. (From: Bauman 1956.)

10°

electrons (high dose rate) and y's (low dose rate) fo: a cobalt glass which could be used to measure doses up to about 1 megarad at a wave-length of 350 ιημ (fading after one week by 7 per cent) and up to about


10 megarads at 500 ιτιμ (fading about 8 per cent in one day, 14 per cent in one week). For doses above about 1 megarad a non-linear calibration curve would be needed.

Bauman (1956) has used standard microscope slides to measure γ-doses in the range 0-1-1 megarad. Above 1 megarad the optical density at 520 ιημ is no longer linear with dose, and saturates at about 10 mega-rads. The density also fades with time, the decrease being 10 per cent or less up to 10 hr. Such slides may offer a simple method of acceptable accuracy for many purposes provided that the calibration is carried out with standard doses of comparable intensity and exposure times.

Further work on the use of cobalt loaded glasses as dosimeters has been reported by Balestio et al. (1957). The effects of both radiation intensity and temperature during irradiation were studied, and correspond-ing calibration curves obtained covering the range 104—106 reps.

Cellophane—Dye Systems Henley (1954) suggested the use of dyed Cellophane making use of

commercially available sheet which has the advantage for large-scale use of being extremely cheap and simple to use. Owing to the variations in the optical density of the original film, it is necessary to measure the optical density (at 655 ηιμ for one type of film) prior to irradiation on the same part of the film as is subsequently to be exposed. The change in percentage transmission is linear with dose up to about 3 megarads. The probable error is estimated to be about 60 per cent at 200,000 r, dropping to 7 per cent at 3 megarads.

In a subsequent paper, Henley and Richman (1956) calibrated these dyed Cellophane films against both γ- and electron radiation sources. For cobalt irradiation there is a linear relation between change in trans-mission and dose up to at least 10 megarads, although at much higher doses it is convenient to relate log absorbance to the dose. The error at 3 megarads is only about 0-2 megarads, and at 0-2 megarad it is propor-tionately higher (0-12 megarad). Over the range 0Ό5-0-2 megarad per hour the effect of intensity of the γ-beam is small or negligible. For electron beam radiation at a much higher dose (1-96 megarads) beam intensity likewise has no significant effect, the increased transmission being proportional to total dose only. However, the difference in transmission produced by a given total electron dose is 2-2 times as great as that pro-duced by an equal dose of γ-radiation. This difference, which has been confirmed by Lloyd (private communication) is difficult to explain.

The cheapness, ease of use and stability of colour of the film appears to render it very useful as a simple method of monitoring radiation treatment on a large scale, in a region of radiation doses where most other simple (if approximate) methods fail. The apparent dependence on radiation quality and the need for prior calibration may complicate its use.

Lloyd has used the Henley type of film to calibrate the dose distribution around a cobalt γ-source, and in the electron beam from a 2 MeV accelerator. A linear relation between transmission and dose was observed up to at least 6-10 megarads, and the difference between low intensity

DOSIMETRY 107

γ- and high intensity electron radiation was confirmed, although the constants found were somewhat different from those given by Henley and Richman. By exposing the film to 2 MeV electrons (Fig. 6.4) Lloyd was

u u

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able to compare the dose-depth relationship with that derived by Trump and Van der Graaff (1950) using ionization techniques. This emphasizes a further advantage of this simple film dosimeter, in that it measures the dose in a very thin layer, and can therefore be used in conjunction with a specimen being irradiated without interfering with the dose being delivered to the latter. Variations in dosimetry at various points of the specimen can also be studied.

CHANGES IN PLASTICS Intrinsic Viscosity

The effect of radiation in producing crosslinking or degradation has been shown to be independent of the radiation intensity over a wide range from at least 10 r/sec to 106 r/sec. It has therefore been suggested that intrinsic viscosity measurements of the degradation of polymethyl metha-crylate (Alexander et ai, 1954) or polywöbutylene (Alexander et al, 1955) offer a convenient method of dosimetry. A small correction may be made for the finite initial molecular weight and possibly for temperature, although both these can be readily calculated. The range of doses available is extremely wide, from a fraction of a megarad up to at least 100 megarads. The results obtained are not oxygen dependent to any marked extent, and


the shape of the specimen can be chosen at will. The main disadvantages of this technique are the time delay and the equipment needed to measure the intrinsic viscosity of the material before and after radiation. However, it may be convenient to include small blocks or films of polymethyl methacrylate with specimens to be irradiated and to keep other blocks for standardization, measurements only being carried out if and when required. For polymer research the method has the great advantage that it may be expected to depend on radiation quality and intensity in very much the same way as most other radiation-induced reactions in polymers. Unsaturation

A somewhat similar method of dosimetry consists in measuring the degree of main chain /raws-unsaturation produced in long chain paraffins such as polyethylene. This is proportional to the radiation dose up to about 100 megarads, and can be determined by infra-red spectroscopy. The degree of unsaturation produced is independent of intensity, and of temperature (Charlesby and Davison, 1957). Polymerization

Prevot (1950) has suggested the use of polymerization as a technique of measuring radiation doses or intensities. Since such systems are very dependent on radiation intensity and on impurities, and are in any case difficult to measure, they do not offer a practical method of routine dosimetry.

PHOTOGRAPHIC METHODS

x-Ray films are used as a routine method of health monitoring, or for the measurement of low doses (Dudley, 1956). Hine (1954) has compared a number of x-ray films from the point of view of the dose needed to pro-duce unit optical density. The figure ranges from 0-25 r for Kodak type K to 300 r for Dupont Adlux type 1290. These doses are obviously far too low for use in the irradiation of plastics, except possibly for very low intensity work.

A material which is useful for rough determination of beam size and distribution is the photographic printing material (Ozalid) used in dupli-cating engineering drawings. This material is not very sensitive to daylight and can be handled without elaborate dark-room facilities. It is of little value in determining total dose received but is used as a qualitative check on dose distribution in a scanned electron beam in the range of about 10 megarads. Coloured plastics may be used for the same purpose or as a check on irradiation conditions, marked colour changes being produced by doses of a few megarads (Pinner and Swallow, 1957).

The use of print-out proof paper has been suggested by Nitka and Jones (1957) as a rapid method of photographic dosimetry in the mega-roentgen range. This paper, which is used for the selection of optimum printing conditions in photography contains, in addition to the silver halide, silver citrate which acts as a physical developer, and therefore does not require wet chemical processing.

The change in optical density can be readily followed by a simple

DOSIMETRY 109

densitometer, or may even be compared visually with standards, to give an estimation of dose to within 30 per cent. The range covered ranges from less than 0-1 to several megarads, and is therefore useful as a check on dosimetry for such applications as food sterilization and polymer treatment. There is a marked energy dependence, so that calibration of the paper must be made with the radiation to be used. On the other hand the intensity dépendance is relatively small, an increase from 10 to 5000 r/min only producing a change in optical density equivalent to a 35 per cent decrease in dose. This intensity dépendance may be ascribed to a failure of the reciprocity law in conventional photography. Nitka and Jones envisage the use of such paper, combined with a standard scale, in an orange-dyed envelope, for rapid visual checking of dose in routine radiation treatment.

DOSIMETRY IN REACTORS Measurement of the doses to which specimens are subjected in nuclear

reactors is complicated by the simultaneous presence of various forms of radiation, particularly of slow neutrons whose effect depends not only on the elements in the irradiated specimen, but also on neighbouring material of high neutron absorbing power. In many cases, it has proved necessary to confine radiation treatment to a given position in the nuclear pile and to calibrate the effects observed by comparison with pure electron or gamma radiation carried out elsewhere on similar materials. Direct measurements have been carried out by Wright (1952), using ferrous sulphate in the BEPO reactor at Harwell. His results give an equivalent radiation dose per reactor unit of 35 megarads, somewhat lower than the value obtained by Charlesby et al. using comparative methods of crosslinking unsaturation or degradation of polymers. Corval et al. (1952) used DPPH as a radical trap to measure the radicals produced by γ-radiation in the presence of slow neutrons. Pucheault (1955) used a chemical reduction technique to measure the intensity of the Chatillon reactor and Worrall (1957) cali-brated the radiation intensity observed in the BEPO reactor at Harwell by comparing the fluorine emitted from irradiated PTFE both in the reactor and in a Cobalt 60 source. Richardson (1948) measured the relative con-stituents of slow and fast neutron radiation and γ-radiation in the Oak Ridge reactor. Bopp and Sisman (1956) have given formulae for the energy absorbed in various polymers in terms of their structure for a given slow neutron flux. It should perhaps be emphasized that not only is the accurate measurement of radiation dose in a nuclear reactor far more difficult than when pure sources of radiation are used, but the interpretation of the results obtained is complicated by the simultaneous presence of radiation of low and high ionizing density. Only in reactions in which the changes depend on the total energy absorbed (and not on its character) can this complication be ignored.

REFERENCES ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. 223, 392, 1954. ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. 232, 31, 1955. ANDREWS, H. L. and SHORE, P. A., / . Chem. Phys. 18, 1165, 1950.


BACK, M. H. and MILLER, N., Nature, Lond. 179, 322, 1957. BALESTIO, F., LECLERC, P. and BONNAUD, M., / . Appl. Rad. Isotopes. 2, 252,

1957. BAUMAN, R. G., Nucleonics 14(6), 90, 1956. BERNSTEIN, W. and SCHÜLER, R. H., Nucleonics 13(11), 110, 1955. BOAG, J. W., Brit. J. Radiol. 25, 649,1952; Radiation Dosimetry, p. 153, Academic

Press, 1956. BOAG, J. W., PILLING, F . D. and WILSON, T., Brit. J. Radiol. 24, 341, 1951. BOAG, J.* W. and WILSON, T., Brit. J. Appl. Phys. 3, 222, 1952. BOPP, C. D. and SISMAN, O., Nucleonics 14(1), 46, 1956. CHAPIRO, A., BOAG, J. W., EBERT, M. and GRAY, L. H., / . Chim. Phys. 50, 468,

1953. CHARLESBY, A. and DAVISON, W. H. T., Chem. & Ind. (Rev.) 232, 1957. CORMACK, D. V., HUMMEL, R. W., JOHNS, H. E. and SPINKS, J. W. T., / . Chem.

Phys. 22, 6, 1954. CORMACK, D. V. and JOHNS, H. E., Rad. Res. 1, 133, 1954. CORVAL, M., CHAPIRO, A. and COUSIN, C , C.R. Acad. Sei., Paris 235, 799, 1952. DAVID, V. W. and IRVING, R., Conference on Lubrication and Wear, Lond,

2 October 1957. DAVISON, S., GOLDBLITH, S. A. and PROCTOR, B. E., Nucleonics 14, 34, 1956. DAY, M. J. and STEIN, G., Nature, Lond. 164, 671, 1949; 166, 146, 1950; 168,

644, 1951; Nucleonics 8(2), 34, 1951. DAY, M. J., STEIN, G. and SCHNEIDER, E. C , Nature, Lond. 168, 644, 1951. DAY, M. J., Brit. J. Radiol. 29, 358, 1956. DEWHURST, H. A., J. Chem. Phys. 19, 1329, 1951; Trans. Faraday Soc. 48, 905,

1952; 49, 1174, 1953. DUDLEY, R. A., Radiation Dosimetry, p. 299, Academic Press, 1956. EHRENBERG, N., and SALLAND, E., JENER Report 8, 1954. EMERY, E. W., Brit. J. Radiol. 26, 370, 1956. FARMER, F. T., Brit. J. Radiol. 28, 304, 1955. FLOWERS, B. H., LAWSON, J. D. and FOSSEY, E. B., Proc. Phys. Soc. B65, 286,

1952. FOWLER, J. F., and DAY, M. J., Nucleonics 13(12), 52, 1955. FRICKE, H., and MORSE, S., Amer. J. Roentgenol. 18, 430, 1927. GLOCKER, R., MESSNER, D. and ROSINGER S., Zeits f Physik Chemie 13, 3/4, 9,

1957. GOLDBLITH, S. A., PROCTOR, B. E., and HAMMERLE, O. A., Industr. Engng. Chem.

(Industr.) 44, 310, 1952. HANNAN, R. S., Scientific and Technological Problems—Preservation of Food,

H.M. Stationery Office, 1955. HARDWICK, T. J., Canad. J. Chem. 30, 17, 1952. HARTECK, P., and DONDES, S., Nucleonics 14(3), 66, 1956. HAYBITTLE, J. L., SAUNDERS, R. D. and SWALLOW, A. J., / . Chem. Phys. 25,

1213, 1956. HENLEY, E. J., Nucleonics 12(9), 62, 1954. HENLEY, E. J. and MILLER, A., Nucleonics 9(6), 62, 1951. HENLEY, E. J. and RICHMAN, R., Analyt. Chem. 28, 1580, 1956. HINE, G. J., Amer. J. Roentgenol. 72, 293, 1954. HINE, G. J. and BROWNELL, G. L., Radiation Dosimetry, Academic Press, 1956. HOCHANADEL, C. J., ORNL 87, 1950. HOCHANADEL, C. J. and GHORMLEY, J. A., / . Chem. Phys. 21, 880, 1953. JOHNSON, G. R. A. and WEISS, J., Proc. Roy. Soc. A240, 189, 1957. KREIDL, N. J. and BLAIR, G. E., Nucleonics 14(1), 56, 1956; 14(3), 82, 1956.

DOSIMETRY 111

LAUGHLIN, J. S., BEATTIE, J. W., HENDERSON, W. J. and HARVEY, R. A., Amer.

J. Roentgenol. 70(2), 1953. LAUGHLIN, J. S., and GENNA, S., Radiation Dosimetry, p. 411, Academic Press,

1956. LAUGHLIN, J. S., OVADIA, J., BEATTIE, J. W., HENDERSON, W. J., HARVEY, R. A.

and HAAS, L. L., Radiology 60, 165, 1953. LAZO, R. M., DEWHURST, H. A. and BURTON, M., / . Chem. Phys. 11,1370,1954. MARINELLI, L. D., Ann. Rev. Nucl. Sei. 3, 249, 1953. MCDONELL, W. R. and HART, E. J., / . Amer. Chem. Soc. 76, 2121, 1954. MCELHINNEY, J., ZENDLE, B. and DOMEN, S., J. Res. Nat. Bur. Stand. 56(1), 9,

1956. MILLER, N., / . Chem. Phys. 18, 79, 1950; Nature, Lond. Ill, 688, 1953; Actions

Chimiques et Biologiques des Radiations, Vol. 2, p. 149 (éd. HAISSINSKY), Masson, Paris 1956; Nature, Lond. 171, 685, 1953.

MILLER, W. and WILKINSON, J., Disc. Faraday Soc. 12, 50, 1952. NITKA, H. F. and JONES, D. P., Nucleonics 15(10), 128, 1957. PINNER, S. H. and SWALLOW, S. J., Plastics 11, 194, 268, 1957.

PREVOT, A., C.R. Acad. Sei. Paris 230, 288, 1950. PROCTOR, B. E. and GOLDBLITH, S. M., Nucleonics 7(2), 83, 1950.

PUCHEAULT, J., C.R. Acad. Sei., Paris 240, 772, 1955. RICHARDSON, D. M., ORNL, 129, 1948. Ross, M. and CHARLESBY, A., Atomics 4, 1953. ROTHSCHILD, W. G. and ALLEN, A. O., Rad. Res. 8, 101, 1958.

SALDICK, J. and ALLEN, A. O., / . Chem. Phys. 11, 438, 1954. SAUNDERS, J. H., J. Sei. Instrum. 26, 36, 1949. SCHULER, R. H. and ALLEN, A. O., Rev. Sei. Instrum. 26, 1128, 1955; / . Chem.

Phys. 24, 56, 1956; J. Amer. Chem. Soc. 11, 507, 1955; 79, 1565, 1957. SCHULER, R. H. and BARR, N. F., J. Amer. Chem. Soc. 78, 5756, 1956. SCHULMAN, J. H., et ai, J. Appl. Phys. 11, 1479, 1951; Nucleonics 11(10), 52,

1953; 13(2), 30, 1955; NRL Memor. 266, 1954. SCHULMAN, J. H., KLICK, C. C. and RABIN, H., Nucleonics 13(2), 30, 1955.

SHEKHTMAN, Y. L., et ai, Dokl. Akad. Nauk. SSSR 74, 767, 1950. SUTTON, H. C. and ROTBLATT, J., Nature, Lond. 180, 1332, 1957. STEIN, G. and WEISS, J., J. Chem. Soc. 3245, 1949. TAYLOR, L., Brit. J. Radiol. 24, 67, 1951. TRUMP, J. G., WRIGHT, K. A. and CLARKE, A. M., / . Appl. Phys. 21(4), 345,1950. VERMEIL, C , / . Chim. Phys. 52, 587, 1955. WEISS, JEROME, Nucleonics 10(7), 28, 1952. WEISS, J., BERNSTEIN, W. and KUPER, J. B. H., / . Chem. Phys. 11, 1593, 1954.

WORRALL, R., AERE, M/R 2159, 1957. WRIGHT, J., Disc. Faraday Soc. 12, 60, 1952. ZSULA, J., LIUZZI, A. and LAUGHLIN, J. S., Rad. Res. 6, 661, 1957.

CHAPTER 7

LONG CHAIN POLYMERS

HIGH polymers may be defined as molecules of very high molecular weight, formed by joining together a number of molecules (monomers) of low molecular weight. These molecular units may all be identical, in which case one refers to a homopolymer, or they may consist of two or more types, when one speaks of a copolymer.

High polymers may be considered to fall into one of three groups : linear or branched polymers, when polymerization occurs primarily in one dimension, and the individual molecules form long chains (possibly with branches); two dimensional polymers in which the molecules form large sheets; and three-dimensional (network) polymers. Examples of these are cellulose and polyethylene; graphite; and diamond. Much of the interest in radiation effects is due to the fact that in suitable systems radiation can be used either to link the individual units together to form polymer chains, or by linking polymer molecules together to convert a one-dimensional long-chain polymer to a three-dimensional network poly-mer. These two processes, polymer formation and polymer modification, are essentially different in character and are discussed separately in this book.

In long chain polymers, the cohesion between separate molecules is assured by secondary forces, by chain entanglement and possibly by hydrogen bonding. Regions of crystallinity, for example, serve to give considerable strength to such polymers. Other factors of importance are the molecular length, its degree of branching and the arrangement of units within the molecule. Chemical structure directly affects the mecha-nical behaviour of such polymers by determining the crystal size and stability, chain stiffness and length, and the ease with which links can be introduced between molecules to convert them to three-dimensional struc-tures. High energy radiation is found to affect all these properties, but changes in molecular dimensions are most readily produced.

The present chapter is largely devoted to a qualitative description of the possible arrangements of units within a long chain molecule; the following chapter is concerned with the quantitative expression of molecular dimen-sions in a form which can be used to trace the changes produced by exposure to radiation. This notation is used in subsequent chapters to analyse quantitatively the changes produced by random crosslinking (Chapter 9), by random main chain fracture (Chapter 10) and by certain other forms of reaction (Chapter 11). In further chapters these theoretical calculations are used to explain many of the experimental results observed when long chain polymers are subjected to high energy radiation.

112

LONG CHAIN POLYMERS 113

Typical Vinyl Polymers formed by Addition

Polymer formed by monomeric units H H H H H H

— C — C — C — C — C — C — H H H H H H

polyethylene

Monomer unit H H C = C H H

ethylene

H H H H Cl H C — C — C — C — C — C — H Cl H Cl H H

H C -H

H C H

polyvinyl chloride

H „ H „ 0 - C — C — C — C — C —

0 H 0 H H polystyrene

CH3 O

CH3 H CO H — C — C — C — C —

CO H CH3 H O CH3

CH3 C — C O CH3

H Cl C - C H H

vinyl chloride

H C = C H H

styrène

H C : H

CH3 : C CO o CH3

polymethyl methacrylate

Typical Condensation Polymers

Polyester

Formed by condensation of a difunctional acid and alcohol, e.g. ethylene glycol + phthalic acid.

HO-(CH2)2-OH+HOOC-C6H4-COOH -> - [ 0 - ( C H 2 ) 2 - 0 - C O - C 6 H 4 - C O ] n - +2n H20.

Polyamide

Formed by condensation of dicarboxylic acid and a diamine, e.g. adipic acid plus hexamethylene diamine (nylon 66).

H 2 N - (CH2)6-NH2+HOOC- (CH2)4- COOH -» - [ N H - ( C H 2 ) 6 - N H - C O - ( C H 2 ) 4 - C O ] „ - +2n H20.

FIG. 7.1. Structure of some typical long chain polymers.


P O L Y M E R I Z A T I O N

Long chain polymers may be synthesized by the poly-condensation of suitable mixtures of saturated poly-functional compounds or by the addi-tion polymerization of suitable olefinic compounds known as monomers.

In the case of addition polymerization, the monomeric units have the same chemical constitution as the monomers from which the polymer has been prepared. In the case of poly-condensation, the monomeric units differ slightly in structure from the reactants insofar as water (usually) has been eliminated during the step-wise reactions leading to the formation of the polymer.

Addition Polymerization A typical olefinic compound that can be linked into a long chain mole-

cule by the successive opening of double bonds is CH2=CHC1, vinyl chloride. Generally, compounds of the type CH2 = CRiR2, where Rx

and/or R2 may be hydrogen or a radical, are known as vinyl compounds or vinyl monomers and the polymerization products as vinyl polymers. Chain growth of such vinyl monomers can be initiated by thermal or photo-chemical treatment, by the addition of a free radical or ionic catalyst or by irradiation. Once an active centre has been formed by these means, chain propagation proceeds extremely rapidly till termination occurs by mutual combination, disproportionation or chain transfer. Chain initiation, pro-pagation and termination occur at random and there results a polymer which consists of a mixture of homologous molecules, whose sizes can best be described in terms of an average molecular weight and a molecular weight distribution.

Ό

S -

o t/) 0»

D 0, υ Ν *« o ÉS

c Φ

JQ

E 3

z

(a)

Molecular size- Molecular size-FIG. 7.2. Size distribution of polymer molecules.

(a) Uniform distribution. (b) Random (exponential) distribution.

Condensation Polymerization

The formation of linear polymers by the polycondensation of saturated difunctional compounds, such as dibasic acids and glycols, follows an entirely different kinetic scheme. Instead of the fast chain reaction involved


in addition polymerization (which leads to the presence of long chain polymers right at the start of the reaction) chain growth proceeds stepwise. In the formation of polyesters, each step is a simple chemical reaction between —OH and —COOH groups. The reactivity of such groups is considered to be independent of the size of the molecule to which they are attached. Consequently, the probability of reaction of all functional groups is the same and is independent of the degree of reaction. The first step obviously produces dimer, and continued monomer addition would lead successively to trimer, tetramer, etc. At the same time, however, inter-action occurs between growing polymer molecules and the product is, in fact, the result of all the step reactions,

/-mer + y-mer -> (/ -f j) mer.

From what has been said above, all these random reactions can be described by a single bimolecular rate constant equal to that for the first dimer-forming step. The overall rate of condensation decreases as the polymer chains are built due to exhaustion of reactive groups but, in theory, never falls to zero. In practice, it is customary to add a slight stoicheiometric excess of one or other of the functional groups to ensure that the reaction completely ceases at a desired degree of polymerization. As a result of the purely random reaction between all molecules present, there is a random distribution of molecular sizes present at all steps of the reaction and the distribution curves may be quite similar to those of some vinyl polymerizations.

Special consideration may be given to condensation polymers of the urea-formaldehyde and phenol-formaldehyde types. These differ pri-marily from most linear or branched polymers in that they are usually prepared in two stages. In the first stage the molecules present are of a low degree of polymerization and often occur as syrupy liquids. The full-size molecule is then built up in the second or moulding stage to a cross-linked network which is non-fusible and insoluble. Such polymers are usually referred to as thermoset.

Non-random Distribution

If reaction can be restricted to one between polymer molecule and monomer and if this reaction is slow compared to the initiation step, then a much sharper distribution would be expected. This situation is approached in the polymerization of ethylene oxide initiated by hydroxy groups. In theory, the number of chains is determined by the concen-tration of hydroxy groups initially present and the degree of polymeri-zation should continually increase as the reaction proceeds and the ethy-lene oxide is used up. Some /raAzsetherification appears to occur in practice to modify this simple picture, but the distribution of molecular sizes is nevertheless much sharper than for normal condensation or addition polymerization.

It is only among naturally occurring polymers that we find examples of I


molecules of uniform size. It is likely that proteins in vivo are characterized not only by a unique stereospecific structure but also by a single molecular size. Isolation may be expected to involve a slight degree of degradation and the sedimentation analysis generally reveals the presence of a small number of homodisperse fractions. Nature's skill in producing unique molecular sizes has not been successfully duplicated in the laboratory, however.

Long Chain Branching

Considerable broadening of the molecular size distribution is frequently observed in synthetic polymers and the most potent cause for this is the possibility of branching. This possibility arises in vinyl polymerization through chain transfer with the polymer molecule. This process involves abstraction of a hydrogen atom from a previously formed polymer mole-cule by a growing free radical which is thereby terminated. The hydrogen-abstracted polymer is at the same time converted into a free radical capable of adding to monomer and initiating a branch chain, whose length may be comparable with that of the main chain (Fig. 7.3).

H H H H H — C — C — C — C — C — dead polymer

H H H H H

H H H + · C — C — C — growing chain

H H H

H H H H H -> — C — C — C — C — C — hydrogen abstraction

H H · H H

H H H + HC — C — C —

H H H

H H H H H -> —C —C —C —C —C

H H | H H H C H

I H C H

I H C H

I FIG. 7.3. Formation of a branched polymer by chain transfer.

Branching can only occur in condensation polymers if suitable tri-functional compounds are deliberately added. Such compounds are in-cluded when a crosslinked network structure is the desired end product, so, in this instance, branching may be regarded as an intermediate stage of

H H H + HC — C — C -

H H H

growth of a branch

L O N G C H A I N POLYMERS 117

crosslinking. Branched chain molecules also occur frequently in nature, for example, in amylopectin and glycogen. All these branched molecules are characterized by extremely broad distributions and, as a consequence, by extremely large ratios of weight average to number average molecular weight (see page 132).

STRUCTURAL VARIATIONS IN HOMOPOLYMERS

The term homopolymer is conventionally applied when a single mono-mer is polymerized inasmuch as it is reasonable to expect this to lead to chain molecules with an identical repeating unit. In practice, full identity is never attained Even if only 0-1 per cent of the repeating units differ from the norm, great differences in physical-chemical behaviour may result. It is instructive to consider what new groups might arise in vinyl polymers as a result of relatively infrequent reactions during polymeri-zation. These are:

(a) End groups. These may be initiator fragments, inhibitor fragments or unreacted double bonds. The concentration of such end groups is, of course, intimately related to the chain length, and this concentration is normally so low as to be negligible. Important contributions of end groups are encountered, however, in thermal and photochemical depolymerization phenomena.

(b) Branch points. A distinction is normally made between branches a few atoms long which occur in the monomer such as the —COOC4H9 group in polybutyl acrylate, and those which are created in the polymeri-zation process through chain transfer and usually contain at least several monomer units. The latter branches may be very long and comparable in size with linear polymer molecules or they may be short as in the occasional 1:2 addition of butadiene molecules when the branch is a pendent vinyl group. In both cases, profound chemical and physical changes may be caused. Long chain branches, in particular, not only modify the size distribution as mentioned above, but also give rise to more spherical molecules having much lower melt and solution viscosities than purely linear molecules of the same molecular weight. Short chain branches may confer chemical reactivity and may also prevent a high degree of crystal-Unity.

(c) Head-to-head and tail-to-tail linkages. These arise from the non-symmetrical structure of most vinyl monomers. Although the normal mode is head-to-tail addition, as expected from consideration of the resonance stabilization of polymeric radicals, occasional variations may occur. Their occurrence is probably of insufficient frequency to have a very profound effect on physical properties but they may constitute weak links so far as resistance to thermal degradation is concerned. The nature of these links is illustrated below for a monomer CH2 = CXY (Fig. 7.4). It will be obvious that this structural irregularity is confined to unsym-metrical monomers. It cannot occur with polyethylene (C2H4)n or poly-tetrafluorethylene (CaF^n.


X X X X X I I I I I

— CH2 — O + CH2 = C -> — CH2 — C — CH2 — C — CH2 — C — I I I I I

Y Y Y Y Y (a) Normal head-to-tail arrangement favoured by steric consideration and

increased resonance stabilization of the radical — CH2 — CXY compared with the radical — CXY — CH2.

X X X X X I I I I I

— CH2 — O + C = CH2 -> — CH2 — C — C — CH2 — C — CH2 — I I I I I

Y Y Y Y Y {b) Head-to-head arrangement, unlikely due to steric factors.

X X X X X I . I l I I

— C — CH2 + CH2 = C -* — C — CH2 — CH2 — C — CH2 — C — I I I I I

Y Y Y Y Y (c) Tail-to-tail arrangement, unlikely due to insufficient stability of radical

— CXY — CH2.

X X X X X X I . I I I I I

- C - C H 2 + C H 2 = C-> - C - C H 2 - C H 2 - C - C - C H 2 - C H 2 - C -I I I I I I

Y Y Y Y Y Y (t/) Regular head-to-head, tail-to-tail arrangement still more improbable than

above occasional links. X X X X X I l I I I

— CH2 —C- + -C —CH2 > — CH2 — C — C — CH2 — C— CH2 — I l I I I Y Y Y Y Y

{e) A single head-to-head unit, which must be present if growing radicals terminate by coupling or combination.

X X X X I I I I

— CH2 — O + C — CH2 -> — CH2 — CH + C = CH — I I I I

Y Y Y Y (/) An unsaturated end, formed when growing radicals terminate by dispro-

portionation.

FIG. 7.4. Configuration of monomer units in a long chain polymer.


Regularity in Configuration

Vinyl monomers of structure CH2 = CHR or CH2 = CXY give rise to chains in which every other chain carbon atom is asymmetric. Optical isomerism is inherent in such molecules but will be manifested only if the same configuration is retained by all the units. Although such molecules are common in nature, proteins being the best example, synthetic polymers are generally composed of randomly oriented units, and methods have only recently been discovered of preparing polymers containing regularly oriented sequences. Such polymers have been named (a) isotactic poly-mers or (b) syndiotactic polymers, depending on whether (a) all the sub-stituents lie on one side of a theoretical plane containing the main C—C chain or whether (b) they lie alternately above and below this plane. These alternative arrangements are shown in Fig. 7.5. In three dimensions, where the C—C—C valency angle is 109-5° these forms are not interchangeable since rotation of a particular C—C bond will inevitably throw the adjacent C atom out of the plane.

In reality, a planar disposition of the main chain in isotactic polymers is by no means likely due to steric interference between neighbouring R groups, and these molecules tend to assume, in fact, a helicoid configura-tion. Nevertheless, the relationship of the R groups to the main chain remains regular, either all on the right hand of the C—C zig-zag or all on the left hand.

The R groups are much further apart in the syndiotactic structures and do not interfere with a possible planar configuration. Short sequences of syndiotactic structures are accordingly believed to be more common than isotactic sequences among free radical initiated polymers, which, by and large, consists of irregular short sequences of the two.

It must be emphasized that configurational isomerism only arises when assy metric carbon atoms are present. Different molecular forms in this sense are not possible in polyethylene, polytetrafluorethylene, polyvinyli-dene chloride, polywobutylene, polyamides or in polyesters.

c c c — C — C — C — C — C — C — C — C — C — C — C — C —

c c c random (atactic) polymer

— C — C — C — C — C — C — C — C — C — C — C — C —

c c c c c c isotactic polymer

c c c — C — C — C — C — C — C — C — C — C — C — C — C — C — C —

c c c c syndiotactic polymer

FIG. 7.5. Random, isotactic and syndiotactic forms of a polypropylene molecule (H atoms omitted for clarity).


CRYSTALLINITY

Examples of polymers of very high molecular weight exhibiting perfect crystallinity abound in nature, e.g. among the proteins, but none exist among synthetic polymers. Four factors may condition the extent of crystallization of polymers: (i) the intermolecular forces, (ii) the regularity of chain contours, (iii) the segmental size and flexibility, (iv) rate of cooling during crystallization. Even when the intermolecular forces are very low, crystallization may occur—the best example is that of polyethy-lene. If crystallinity is absent or incomplete, this must be attributed there-fore to the remaining factors, namely contour irregularity, insufficient internal flexibility, or very rapid cooling which freezes in an irregular arrangement. Polytetrafluorethylene is an example of a highly crystalline synthetic polymer. The intermolecular forces are moderate and the high degree of crystallinity is a consequence of the shape uniformity.

Structural irregularities prevent propagation of crystalline order through-out the entire mass and limit the maximum size of the crystallites. The melting point of these depends on their size and for a highly irregular structure no stable crystals may occur at room temperature. The effect of structural irregularities is perhaps paralleled by the phenomena of dis-locations among metals due to impurities but such irregularities in poly-mers may be much more abundant and widespread, since they are unable to move and aggregate.

The existence of crystals shorter than the polymer molecules is detected in cellulose and in all crystalline synthetic polymers. A picture is created of a single polymer molecule threading its way through regions of order and regions of disorder. The regions of order are normally termed crystallites, to emphasize their small size compared to ordinary crystals. The proportion of material contained in crystallites or ordered regions is termed the degree of crystallinity. Scattering of light occurs between the crystalline and amosphous regions, in each of which phases the refractive index differs, and as a consequence crystalline polymers are normally opaque or translucent, in contrast with the fully amorphous or glasslike transparent polymers such as polystyrene or polymethyl methacrylate. In these glass-like polymers, crystallization is inhibited by shape irregularity due to the mixed configurations of successive chain units. New methods of synthesis, e.g. for polystyrene, have given isotactic structures which do crystallize and consequently have very different physical properties.

Reasons other than these must be sought to explain lack of crystallinity in polymers not containing asymmetric C atoms. Indeed, lack of crystal-linity in such polymers is rare, the most important example being poly-isobutylene (which, however, crystallizes readily on stretching). Partial lack of crystallinity may be attributed to inadvertent structural irregularities to steric hindrance of sidegroups and to low temperature stability of any crystals formed. Examples of irregularities are the occasional long branch units, the methyl groups and the double bonds occurring in low density polyethylene. The first are formed by chain transfer, the next probably by electron or proton transfer and the last by scission or disproportionation.


Whatever their mode of formation, they are undoubtedly present and explain the high proportion of amorphous material in comparison with high density polyethylene (page 219) or polymethylene made from diazo-methane in which these structural irregularities are virtually absent. The size of the individual crystallites may be limited by the presence of branches; since smaller crystallites usually melt at lower temperatures than larger crystallites, the presence of branches will reduce the melting tem-perature of polymers. Branched polyethylene, for example, melts some 20° below linear polyethylene. Moreover, the range of melting tem-peratures is much greater because of the wider dispersion of crystal size.

FIG. 7.6. Volume-temperature curves, showing melting of long chain polymers. (a) Volume-temperature curve for crystallites. (b) Volume-temperature curve for amorphous structure. (c) Volume-temperature curve for molten (liquid) polymer. A Melting curve for polymer with large crystals. B Melting curve for polymer with smaller crystallites of varying sizes.

On the other hand, cases are known of non-isotactic asymmetric polymers which nevertheless crystallize readily. In polyvinyl alcohol, for example, the H and OH groups are located at random but the similarity in atomic radii of these groups permits them to be accommodated in the regular lattice without undue distortion. Polyvinyl acetate with an identical distribution of side-groups is amorphous since the longer acetate groups can no longer be accommodated in a crystal.

Oriented Polymers

Both crystalline and amorphous polymers can be oriented either during cooling from the melt or by applying mechanical stresses at lower tem-peratures. The individual molecules then tend to lie predominantly in one direction, as can be shown from an x-ray diffraction photograph. In crystalline polymers the crystals themselves can be oriented.


Polymers which have been highly oriented in one direction acquire a fibrous structure, and have a high tensile strength along the fibre axis. At right angles they are relatively weak since only Van der Waals' forces are available to hold adjacent molecules together, whereas along the fibre axis it is the primary chemical bonds which resist the stress. Oriented crystalline (or partially crystalline) polymers of high strength occur naturally (e.g. cellulose) or are formed by stretching synthetic polymers (e.g. nylon, Terylene). At room temperature rubber is amorphous in structure. For high degrees of stretching, the molecular chains line up parallel to the applied stress and in this position are able to crystallize even at room temperature. The oriented crystals then reinforce the rubber, giving it good tensile properties at high elongation. On releasing the external stress, the crystals usually melt, and the rubber returns to its original length.

Partially crystalline polymer

Partially crystalline oriented polymer

FIG. 7.7. Random and oriented crystallinity; dotted lines show outline of some crystalline regions.

Amorphous Solid Polymers

Polymers which appear to have solid properties as judged by their mechanical behaviour may yet be non-crystalline as shown by x-ray diffraction. Such polymers can have an irregular arrangement of groups about the main chain and be held together by weak Van de Waals' forces only. They owe their rigidity to the entanglements between the chains; these cannot disentangle under stress, being too stiff at the temperature of


measurement. As the temperature is raised the molecules become very flexible and, given a sufficient time, can flow very slowly under applied stresses. The difference between solid and liquid is therefore not sharp as in the case of crystalline materials but depends on the time during which the stress is applied. In such polymers, the transition between solid and liquid takes place over a range of temperatures and the corresponding transition is known as a second order transition. This can be defined as the lowest temperature for which significant disentanglement and flow can occur in the time interval of the experiment. When alternating stresses are applied at high frequency, the transition temperature will therefore be higher than when the stress is applied at very low frequency. The difference is shown, for example, by the difference in elastic modulus of many poly-mers when measured statically or dynamically; the latter measurements usually give a higher modulus.

A further restriction on the flow of molecules past one another may be imposed by the presence of hydrogen bonds which link molecules together as in nylon. The hydrogen bonding of polymer molecules can serve as a model for hydrogen bonding in biological systems, and is believed to be very sensitive to radiation.

STRUCTURAL VARIATION IN COPOLYMERS Further changes in physical and chemical properties can be introduced

by introducing several monomer units in a single molecule. Such long chain polymers are then referred to as copolymers as distinct from homo-polymers.

Random Copolymers

When a vinyl polymer is formed from a mixture of two monomers, the composition depends on the composition of the feed and the monomeric reactivity ratios. These factors also govern the probability of a particular monomer adding to a given radical at any time and this probability is in fact directly proportional to the instantaneously formed polymer com-position. When each monomer enters into the polymer in significant proportions, the probability of finding a long sequence of one or other monomer is very small.

These copolymers are known as random copolymers in which the sequence lengths of monomers obeys simple statistics distribution laws, and therefore do not exceed a few units at most. In effect, the repeating unit of this polymer has lost its identity and the properties of the individual homopolymers are generally completely submerged.

When both the reactivity ratios are much smaller than unity, very pro-nounced alternation in the arrangement of the monomer must result. In other words, the sequence length rarely differs from unity. Such a case occurs in styrene-maleic anhydride copolymers.

Block Copolymers

Special means must be employed to produce copolymers in which the sequences of each type are long. One typical method involves the use of a


long-lived radical which will continue to propagate if transferred to another monomer. Alternatively, different polymer molecules may be end-linked together by chemical means. Such copolymers are known as block copoly-mers, the term block signifying a long block or sequence of monomer units of a given type. Generally, the term is further confined to essentially linear polymers containing alternating long sequences each of the monomers. Where the molecules are branched, there is no tendency for the monomers in the branches to differ profoundly from those in the main chain.

Graft Copolymers

It is easy to visualize another form of long sequence copolymer in which one of the monomers is disposed as long branches emanating from a continuous backbone of the other monomer. These branch-like sequences are bordered at one end only by monomer units of the other type. Such copolymers can be made by developing initiating centres among the internal units of the parent polymer and allowing these to interact with monomer. Addition of the initiating centres to the monomer takes place and branch chains are propagated.

Despite the obvious geometric distinction, the properties of block and graft copolymers have much in common. From a consideration of behaviour it may be best to group these types together as non-random or long sequence copolymers to distinguish them from random copolymers in which the sequence of each monomer is short. The properties of long sequence copolymers differ radically from those of random copolymers, insofar as the individual homopolymer properties obtrude to a far greater extent. Moreover, it has recently been shown that the physical properties of long sequence copolymers depend not only on the relative proportion of each component but also on the number and length of the individual sequences. This observation has no parallel among random copolymers.

Isotactic Polymers and Long Sequence Copolymers

An interesting parallel can be drawn between the molecular arrange-ment in copolymers, and in homopolymers where each unit is assymmetric. In the latter case we can write the units as D and L units, depending on their orientation within the chain. Then if the units in the copolymer are A and B, the following arrangements are possible in a linear molecule:

ABBBAABABAAB DLLDLDDLLLD Random copolymer Random (atactic) homopolymer

A B A B A B A B D L D L D L D L D L Regularly alternating copolymer Syndiotactic homopolymer

A A A A A B B B B B D D D D D L L L L L Long sequence (block) copolymer Isotactic homopolymer

(with occasional irregularities)

L O N G C H A I N P O L Y M E R S 125

The introduction of any structural irregularity into a molecular chain increases the difficulty of crystallization. The presence of asymmetric units in a random homopolymer reduces the size of any crystallites formed, and (as in conventional polystyrene) may prevent these from forming even at very low temperatures. If, however, the units are distributed in some regular manner, some degree of crystallization may still be possible. Although examples are rare among vinyl copolymers, condensation poly-mers such as polyesters and polyamides, consisting of regularly alternating copolymers, form stable crystals which impart temperature resistance and strength to the material. The same may be expected of syndiotactic poly-mers. In block copolymers the length of each crystallite will be restricted by the length of the sequence so that a range of properties is possible depending on this length. Graft copolymers, on the other hand (parti-cularly when produced by radiation which gives branches of irregular length) might be expected to be largely amorphous unless the branches are far apart.

Non-carbon-chain Polymers

Discussion has hitherto been confined to polymers of carbon which exhibit a rather unique ability to link with neighbouring carbon atoms to form stable long chains. All organic polymers suffer from lack of resistance to high temperature and the conclusion is inescapable that the provision of high temperature insulators will require the development of inorganic polymers.

The most important of these at the present time are the silicones. These are based on a silicon-oxygen backbone and the structure may be regarded as intermediate between organic polymers and the inorganic glasses.

GH3 C/H.3 v^ri3 C^H.3 I I I I

— Si — O — Si — O — Si — O — Si — I I I I

C113 CH3 CH3 CH3

Dimethyl siloxane polymer

These molecules are formed by hydrolysing alkyl chloro-silanes, when the hydroxy-silanes initially formed are converted instantly to siloxane chains. The properties of these silicones vary with the molecular weight and degree of branching and with the nature of the alkyl substituent.

Other types of inorganic polymers are polymerized sulphur and selenium (which have weak chain-links), polymerized phosphonitrilic chloride, fluoride or organic derivatives, polymers of boric oxide and boron nitride and the various natural silicates. In the case of polyphosphonitrilic chloride, the monomer is unknown and the chain is only found as a cyclic trimer and tetramer. On exposure to temperature of about 300°C, these


compounds decompose and form long chain rubberlike black insoluble products of probable structure shown below.

ci ci

P+ - / \ -

N N C l \ 1 I + / C 1

CK \ _ / XC1 N

Cyclic trimer

Cl 1 +

— P —N I 1

Cl

Cl - 1 + — P —

! 1 Cl

■ N "

1+ N — P — N ! 1 1 1 Cl | 1+ 1 +

Cl — P — Cl Cl — P — Cl 1 1 1 Cl | I - 1+ 1-

N — P — N 1

Cl Cyclic tetramer

Cl Cl - 1+ - 1+ — P —N —P —

1 I 1 1 Cl Cl

Long chain polymer

With the exception of the siloxane polymers, little radiation work appears to have been carried out on these inorganic materials, but they would be expected to show some of the properties of ionic crystals when exposed to radiation.

CHAPTER 8

MOLECULAR WEIGHT DISTRIBUTION

THE most striking effect observed when long chain polymers are subjected to high energy radiation is undoubtedly the change in properties caused either by linking molecules together (which increases the average mole-cular weight and degree of branching, and which may lead to the formation of a three-dimensional network) or by the fracture of the main chain (causing a reduction in average molecular weight). Whether one or other of these alternative effects occur depends on the chemical structure of the polymer, but as far as is known, not on the type of radiation used.*

A number of properties of polymeric systems depend less on their chemical structure (which may intervene as a parameter) than on the molecular weight distribution and, in the case of a three-dimensional net-work, on the location and distribution of crosslinks. The viscosity of a linear polymer, for example, depends on its average molecular weight and the variation about this average as well as on its configuration. The soluble fraction, swelling in solvents and elastic modulus of a network, can be calculated from statistical and thermodynamic considerations. Confirma-tion of these theoretical predictions has been sought by measurements on chemically crosslinked systems, but radiation offers a more convenient and quantitatively reproducible method of producing such systems. Conversely, by comparing the calculated and observed properties of irradiated poly-mers, quantitative data on the changes produced by radiation can be obtained.

Of fundamental importance are the concepts of average molecular weight and molecular weight distribution. The notation defined in this chapter can be used to describe both the initial conditions and those resulting from radiation induced changes when a transition from a long chain to a network polymer takes place. The assumption is made that all monomer units are identical and equally likely to become crosslinked or fractured by radiation. This assumption will not be true for the end groups (which are relatively infrequent except in highly branched polymers) and for low molecular weight material of a similar chemical nature when end corrections may be necessary. The analysis can nevertheless be extended to copolymers if, in comparison with the distribution of the different monomer units, the average distance between crosslinks or fractures is sufficiently large to justify a random statistical approach.

* Irradiation in the presence of oxygen often leads to additional main chain fracture. or crosslinking via peroxide bridges.

127


DEFINITION OF MOLECULAR WEIGHT DISTRIBUTION

The size of a polymer molecule may be defined either by its molecular weight M, or by the number u of repeating chain or monomer units (often called mers), each of molecular weight w, which it contains. For a polymer molecule the number of repeating units u is large, so that the different molecular weight of the end groups, which need not be identical with the repeating units, may usually be ignored. Then

M = uw. (8.1)

From the point of view of statistical analysis it is often the number of repeating units u in a molecule rather than its molecular weight M which is important. Much of the following analysis will therefore be expressed in terms of w, which is referred to as the degree of polymerization of the molecule. Equation (1) can be used to express the results in terms of molecular weight M.

In practice, the polymer molecules of a specimen cover a range of sizes, and it becomes necessary to consider both the average molecular weight, and the distribution of molecular weights about this average. There are several ways of determining this average, each depending on a different physical or chemical property, and leading to a different numerical value. For most polymeric materials several molecular weight averages must therefore be envisaged: the number, weight, viscosity and z averages. These are defined below in terms used for the subsequent radiation work.

In a specimen let n(u) or n(M) represent the number of polymer mole-cules each of which comprises u monomer units or has a molecular weight M. When no ambiguity arises the expression in brackets may be omitted. Then if n(u) or n{M) is known as a function of u or M, the molecular weight distribution is fully specified. The total number A0 of molecules in the specimen is by definition

oo A0 = Σ n(u) (8.2)

u= 1

while the total number of chain units in the specimen is Ax where 00

Ax = Σ n(u)u (8.3) u= 1

and the total molecular weight of the specimen is wA±. For analytical purposes it is useful to extend the series to generalized

parameters A09 Au A2, . . . Ai although only the first two have a direct physical meaning. These parameters are defined in terms of the number n(u) of molecules present in a specimen which contain u units;

A0 = Ση(ιή; Ax = Σ«(ί/)#; Α2 = Σ«(«)«2; Az = ΣΑΖ(«)«3

and in general At = Σ/ι(κ) U*.

MOLECULAR WEIGHT DISTRIBUTION 129

In terms of the number n(M) of molecules of molecular weight M the définition becomes

Ai = Ση(Μ) Μ'/νν'". (8.4)

The molecular weight distribution is completely defined by the series of parameters A0, Au A2, . . . A\ and they may be considered as an alternative to n(u) as a method of expressing this distribution.

Number Average

The number average molecular weight (N.A.M.W.) is obtained by dividing the total molecular weight of the specimen by the number of molecules in it. It is generally represented by the notation Mn and defined by the equation

Mn = Ση(Μ) Μ/Ση(Μ) = wAJAo. (8.5)

A molecule of this weight is termed a "number average" molecule and will contain MJw chain units. This is referred to as the number average degree of polymerization and is written u±:

Ul = MJw = AJAo. (8.6)

The number average is the average obtained when the molecular weight is determined by methods which do not depend on the length of the molecule—e.g. end group determination, osmotic pressure, ebulliosopic, isopiestic and cryoscopic measurements.

Weight Average

Many properties of a polymer, such as its viscosity in solution, or its mechanical strength, are largely determined by the longer molecules in a specimen, and the appropriate averages must be biased in favour of these longer molecules. The weight average molecular weight (W.A.M.W.) M^ as derived from light scattering measurements is defined as follows :

Mw = Σ/ι(Μ) Μ2/Ση(Μ) M = wAJA^ (8.7)

One can also speak of a "weight average" molecule, as one of molecular weight Mw. This will contain MJw units which is the weight average degree of polymerization* u2.

u2 = MJw = AJAX = Zn(u)u2^n(u)u. (8.8)

* The suffixes to the number average, weight average, etc. (ul9 u2 , . . .), degree of polymerization should match those for the number average, weight average, etc. (Mw, Mw, . . .), molecular weight. However, the analysis requires the sequence to be continued indefinitely, so that it is preferable to use a numbered sequence. The notation used here relates the u suffixes directly to the expressions involving the A sequences. It may be hoped that eventually the numbering notation will be extended to the average molecular weight; thus Mn = Mx\ Mw = M2; Mz = M3, etc.


z Average

A third average, termed the z average molecular weight, arises in ultracentrifuge or sedimentation studies. By definition

Mz = Ση(Μ)Μ3ΙΣη(Μ)Μ2 = wA3/A2. (8.9)

A z average molecular weight molecule has a molecular weight equal to this average Mz and therefore contains Mz/w units; this is accordingly termed the z average degree of polymerization uz,

«3 = MJw = A3/A2 = ^n{ü)uzßn{u)u\ (8.10)

This notation can be readily generalized for any required order of the average; such a generalization is required for the theory of solubility given in Chapter 9.

In general, we may refer to an average molecular weight of degree "i""(Mt·), to an " / " average molecular weight molecule, and to an " / " average degree of polymerization (wj,

Mi = wAi/Ai-i = Ση(Μ)Μΐ/Ση(Μ)Μΐ-1 (8.11)

W = Ai\Ai_x = Σ/ιΟΟι/'/Σ/Κκ)«*-1. (8.12)

The series ul9 w2, w3,. . . ut;. . . consisting of the various average degrees of polymerization, forms an alternative method of defining the molecular weight distribution. The first three values are found directly by experi-mental methods (e.g. osmotic pressure for uu light scattering for u2 and sedimentation rate for w3). Higher members of the series are not deter-mined directly but can in theory be deduced from solubility/radiation dose curves (see page 145.)

Viscosity Average

A very convenient method of measuring average molecular weights of polymers is based on the viscosity of dilute solutions. The limiting vis-

7)s—η0

cosity number or intrinsic viscosity [η] is defined as the limit of as the concentration c tends to zero; η5 being the viscosity of the polymer solution, η0 that of the solvent. For a polymer of uniform molecular weight M, the experimental relation is often found to hold:

[η] = kMa (8.13)

where k and a are parameters depending on the chemical structures of polymer and solvent and on the temperature. Values of k and a have been determined for a number of polymers. For polymers for which k and a are known a determination of [η] leads immediately to the viscosity average molecular weight.

M O L E C U L A R W E I G H T D I S T R I B U T I O N 131

When the molecules are not of uniform size, the same equation (8.13) can be used to give an average molecular weight Mv provided that the value of Mv {the viscosity average molecular weight) is defined as follows:

ΜΌ = { Ση{Μ)Μ1+α/Ση(Μ)Μ}1Ια. (8.14)

The corresponding viscosity average degree of polymerization uv is given as

uv = MJw = ^n(u)u1+a^n{u)u } l l a . (8.15)

If the series of parameters A0, Al9 A2 is extended to fractional values

A1+a=?*n{u)u1+a (8.16)

and uv = {Al+JAdlta. (8.17)

Special Types of Molecular Weight Distribution

In a uniform or monodisperse molecular weight distribution, when all the polymer molecules are of equal size, n{u) = 0 for all values of u except the average ux. Then A0 = ΣΛ(«) and A{ = Hn(u)u* = ΗΙ*ΣΛ(«) = UÂQ, so that U{ = Ai/Ai-1 = ul. All degrees of polymerization are therefore equal, as are the number average, weight average, z average and viscosity average molecular weights

Ui = u2 = u3 = uv = ut (8.18) Mn = Mw = Mz = Mv = Mit

Most polymers as prepared do not have such a uniform molecular weight distribution, and to obtain even a reasonably close approximation, a series of fractionations is necessary. A more usual distribution is termed a random or exponential distribution, since it may be obtained by random fracturing of an infinite chain. It is also obtained as a result of many polymerization reactions. The molecular weight distribution follows the statistical equation with the appropriate number average ux

"i\ Uj Ul*\ Uj

and since for long chain polymers ux > 1, u > 1

n(u) = (AJuS) exp ( - i//Wl). (8.19)

For such a distribution At = iux A%-x and u{ = iux.

r^t Ul K2 U3 U: Thus r = _ 2 = = T = ' " = 7 (8 ,20)

Mn Mw Mz Mi and _ = _ = _ = . . . = _ .

K

132 A T O M I C R A D I A T I O N AND P O L Y M E R S

For the viscosity average, integration of equation (8.15) with the exponen-tial function gives for Mv

Μυ = Μη{(α+1)Γ(α+1)} (8.21)

H,

1-9

1-8

1-6

< - * - "

0-3 0-4 0-5 0-6 a

0-7 0-8 0-9 1-0

FIG. 8.1. Ratio of MvJMn for a random distribution in terms of the parameter a in the viscosity relation [η] = kMa.

where T(\+a) is the gamma function a\. The ratio MJMn is shown in Fig. 8.1 for a range of values of a. In practice a usually lies between 0-6 and 1, depending on the configuration of the molecule in solution. For a = l,Mv = 2Mn = Mw; while for a = 0-8 Mv = 1-908 Mn = 0954 Mw. In most cases the viscosity average Mv lies close to Mw, the weight average which, for a random distribution, is always twice the number average.

A more general molecular weight distribution is one in which the ratios of the average degrees of polymerization are as follows :

λ w2

"λ+ϊ '\+2 λ+3 λ + / - 1 (8.22)

The average molecular weights will of course follow the same ratios. If λ = 1, this distribution becomes the usual random or exponential as described above. If λ tends to infinity, it becomes a uniform one, while at the other extreme, if λ tends to zero

«2 ; 1 1 - uz = - iii 2 3

1 i-\

Ui (8.23)

and uju2 tends to zero. This distribution may be called the pseudo-random distribution or the weight-random distribution.

The same molecular weight distribution can be defined by the number n(u) of molecules of degree of polymerization u:

n(u) = C(«/w1)x"1 exp ( -XM/ W I ) (8.24)

where λ is the parameter defining the type of distribution and C is merely

MOLECULAR WEIGHT DISTRIBUTION 133

a normalizing factor. In either notation λ defines the sharpness of the distribution. As λ increases, so does the sharpness of the distribution and the various average molecular weights become more nearly equal.

Table 8.1 summarizes the notation used in this book. It should be pointed out that in certain papers the molecular weight distribution of a polymer is expressed not in terms of the number of molecules of a certain size, i.e. n{u) or w(M ), but rather in terms of the weight of molecules of a certain size, i.e. in terms of the quantity Mn(M) or un(u). This change of notation does not of course invalidate the arguments presented here, but the expressions for molecular weights distribution, etc., are modified.

Table 8.1. Notation used for Molecular Weights

Molecular weight of monomer unit Molecular weight of polymer molecule Number of monomer units in polymer

molecule

w M

u(=M/w) Number of molecules

with u monomer units or of weight M

Parameters of molecular weight distribution

Average molecular weight

In general:

number (NA.M.W.) weight (WA.M.W.) z fcA.M.W.) I

viscosity Number of units in average molecule

number average weight average z average / average viscosity average

Number of polymer molecules in specimen Number of monomer units in specimen Special molecular weight distributions :

uniform

n{u) n{M) Ao, Ai9 A2i . . . Ai Ai = Σ«(ί/)κι = Ση(Μ)ΜίΙ\νί

Μη (= MJ = Ση{Μ)Μ/Ση(Μ) Mw (= Μ2) = Ση(Μ)Μ2/Ση(Μ)Μ Μζ (= Ms) = Ση(Μ)Μ3ΙΣη(Μ)Μ2

Mi = Ση(Μ)ΜίΙΣη(Μ)Μί-1

Μυ = [Ση{Μ)Μ1+αΙΣη{Μ)ΜΥ'α

Degree of polymerization ux (= MJw) = AJA0

(= MJw) = A2jAx (= MJw) = AJA2 (= M » = AilAi-i (= MJw) = (Al+JAJV*

u2

Hi

A Ax

«1 = W2 = W3 =

random

pseudo random

"i = 2W2 = 3W3 = . · · = -Mi

"i/«2 = 0; u2 = -w3 = ^w4 = . . . - rui

CHAPTER 9

THE PROPERTIES OF A CROSSLINKED NETWORK

MANY polymers, after exposure to small doses of high energy radiation, show an increase in viscosity, average molecular weight and degree of branching. These changes are due to dimerization—the linking of mole-cules together—and the reaction has also been observed in low molecular weight compounds such as «-paraffins. As the reaction proceeds, closed loops are formed and produce a three-dimensional network polymer, the properties of which are very different from those of the original linear or branched material. This transition in properties from a one- to a three-dimensional structure is also observed in the vulcanization of rubber, in the chemical crosslinking of polymer molecules such as copolymers of styrene and divinyl benzene, and in the gelation of polyesters.

Polymers can be considered as consisting of an assembly of units (not necessarily identical) which are mono-functional, di-functional or poly-functional. The mono-functional units (—A) are needed to end the chain and therefore act as terminal units. Di-functional units (—A—) extend the chain but do not provide any means of linking to other molecules to form a network. This function is assumed by the tri-functional (—A—)

I or higher functional units which also occur in branched polymers. The possibility of network formation depends on the degree of functionality, i.e. on the ratio of poly-functional units to mono-functional units. A small discrepancy arises from the possibility of intramolecular crosslinks, i.e. links of one poly-functional unit in a molecule with another such poly-functional unit in the same molecule.

In the theory as given here, the term crosslink is taken to refer to a tetrafunctional link or junction point, binding two long molecules together side by side:

—A—A—A—A—A— . . . I

—A—A—A—A—A— . . .

The two monomer units indicated by primes are referred to as the cross-linked units while the crosslink itself, which is similar to other C—C bonds, is shown in heavy type. In a crosslink, each crosslinked unit is tri-functional. This chapter is concerned with the formation and properties of networks formed of such tetrafunctional junction points.

134

PROPERTIES OF A CROSSLINKED NETWORK 135

Other forms of linking may also occur as, for example, in most branched molecules

—A—A—A'—A—A I

A I

A I

A

This type of link or junction point is tri-functional in character and only one tri-functional unit A'—is involved. To distinguish this process from crosslinking, the term endlinking has been coined (although in fact it involves the end of one molecule linking to the side of another). Networks formed with this type of link are discussed in Chapter 11.

If crosslinks are assumed to occur at random, the resultant change in molecular weight, solubility, elastic modulus and swelling in solvents can be deduced theoretically from the number of crosslinks. The first two properties depend on the probability that an individual molecule is linked to its neighbour and the relevant variable is the overage number of cross-links per molecule. Elasticity and swelling, on the other hand, depend on the thermodynamic behaviour of flexible chains restrained at both ends and the equilibrium position is determined by the chain length between crosslinks, the average kinetic energy (i.e. the temperature) and the applied stresses. Here the relevant factor is the density of crosslinks along the molecular chain or alternatively the average molecular weight between successive crosslinks. The theories, as usually worked out for these latter properties, are appropriate for molecules initially of infinite molecular weight, and the initial finite molecular weight then only arises as a correction factor to allow for end effects.

The formulae enabling these changes in properties to be related to the radiation dose are given in this chapter while the more detailed derivation of molecular weight changes and solubility is presented in the Appendix. Full details of the derivation of the formulae for elastic modulus and swelling has been published in a number of books on polymer chemistry.

Crosslinking Density

The basic assumption made in radiation theory is that crosslinks are located at random along the molecular chains. Within each unit they may favour individual bonds, but the distance between successive cross-links must obey the statistical laws of a random distribution to a sufficient degree of accuracy. This assumption is to some extent implicit in the nature of the interaction between radiation and matter, which produces ionization and excitation at random, and has been justified by many experimental results. In special systems where energy transfer over long distances occurs, or where the arrangement of chain units is non-random, this assumption may have to be qualified.

The proportion of main chain units crosslinked by a radiation dose r is


referred to as the crosslinking density and is denoted by q. For a polymer specimen containing Ax chain units the number of crosslinked units will therefore be qAu but the number of crosslinks is only qAx\2 since each crosslink involves the linking together of two units.

For high degrees of crosslinking, where the number of initial molecular ends is very small compared with the number of crosslinked units, the perturbing effects of these free ends may be ignored as the first approxi-mation. The specimen can then be considered as a network, or as a series of closed loops each consisting of several molecular chains. Each molecular chain is terminated at each end by a crosslinking unit; con-versely each crosslinked unit ends two such chains. The total number of chains must therefore be equal to the number of crosslinked units.* For a specimen containing AL units (each of formula or molecular weight w), and therefore comprising qAl crosslinked units, there will be qAx mole-cular chains comprising on the average AJqAi or \/q units. The corre-sponding molecular weight of each chain is w/q. In the literature, this (number) average molecular weight between crosslinks is denoted by the symbol Mc. Mc is a fundamental quantity on which depend the elastic properties and swelling behaviour of a fully crosslinked network.

Mc = w/q. (9.1)

In many experiments, it has been found that the crosslinking density q is proportional to the radiation dose r and is independent of the radiation intensity. In this case, it is convenient to write

q = W, (9.2)

where q0 is a constant representing the susceptibility of a polymer system to crosslinking. q0 is in fact the proportion of monomer units crosslinked per unit radiation dose.

q0 can be expressed in terms of the G value for crosslinking. If the radiation dose r is expressed in megarads, unit dose corresponds to the absorption of 0-624 x 1020eV per gram or 0-624 x 10 2 Vx 1 66 x 10~24 = l-04x 10~4w eV per chain unit of molecular weight w. By definition, this energy absorption produces q0 crosslinked units. An energy absorption of 100 eV will therefore give 100^0/104x 10~4Η> crosslinked units. By defi-nition this is the G value for crosslinking (expressed as the number of units crosslinked per 100 eV absorbed).

G (crosslinked units) = 0-96 x 106 q0/w. (9.3)

Since each crosslink between two molecular chains involves two cross-

* This only applies to tetrafunctional links or junction points. For trifunctional junction points the number of chains is 1-5 times the number of junction points, if end effects are ignored. For tetrafunctional junction points the number of chains is twice the number of junction points.


linked units, one on each of these chains, the G value for the formation of a crosslink will be half this value.

G (crosslink) = 0-48 x 106 qQ/w (9.4)

and Mc = w/q = w/q0r = 0-48 x 106/Gr (9.5)

where r is expressed in megarads. q0 and G are quantities which are determined primarily by the chemical

structure of a polymer; as will be seen later, they may also depend to a small extent on the physical state of the material during irradiation, e.g. its temperature. They depend little if at all on the length of the polymer molecule.

Fracture Density

In polymers which degrade under radiation due to main chain fracture, a quantity analogous to the crosslinking density may be defined. This is the fracture density represented by p and is the probability of a main chain fracture occurring per chain unit. The exact location of this fracture is unimportant from the present point of view; it may occur within a single unit or on the bond linking it to a neighbouring unit. The essential feature is that each such fracture results in a single polymer molecule becoming split into two. Here again, the experimental evidence indicates that main chain fracture occurs at random along a homopolymer chain although not necessarily at random within each chain unit. By analogy with short chain molecules, differences in susceptibility to radiation damage may be expected towards the end of long chain polymer molecules, but in view of the large number of units per molecule, such differences may usually be neglected. The fracture density p is usually found to be proportional to the radiation dose r so that it is convenient to put

P=Por (9.6)

whereto is the proportion of main chain units fractured per unit radiation dose. p0 can be related to the G value for main chain fracture by a formula analogous to that for crosslinking.

G (fracture) = 0-96 x 106p0/w. (9.7)

The value for G is not doubled as for crosslinking since only one molecule is involved in each main chain fracture.

Crosslinking Index; Crosslinking Coefficient

For high degrees of crosslinking when the disturbing effect of the end groups can be ignored as a first approximation, radiation-induced changes in certain physical and chemical properties are dependent on the cross-linking density q or the related quantity Mc. However, for gel formation and solubility changes in crosslinked polymers, these end effects are of prime importance. It is the average number of crosslinked units per molecule which must then be considered rather than the average distance


along the molecular chain between successive crosslinked units. Since the lengths of the polymer molecules in a specimen will not usually be equal, this average can be related either to the number average, the weight average or higher averages as defined previously. Up to the present it has been sufficient to consider only the first two averages :

(a) The number of crosslinked units per number average molecule is referred to as the crosslinking index and is denoted by γ.

(b) The number of crosslinked units per weight average molecule is referred to as the crosslinking coefficient and is denoted by δ.

The notation used is capable of extension to other averages should they become necessary, e.g. in the study of branching.

For a polymeric system with a number average degree of polymerization #1 (N.A.M.W. Mn = wuj and a weight average degree of polymeri-zation u2 (W.A.M.W. Mw — wu2)

γ = qu1 and δ = qu2. (9.8)

If q is proportional to the radiation dose r

T = q*uxr and δ = q0u2r (9.9)

while - = — = -r-r. (9.10) T wi Mn

For a uniform distribution, (λ = co, equation 8.22) γ = δ; for a random distribution (λ = 1) γ = δ/2; while for a pseudo random distribution (λ = 0) γ/δ = 0.

For degrading polymers one can define in a similar way a fracture (or scission) index pux and a fracture (or scission) coefficient pu2. So far no special notation for these quantities has proved necessary.

Table 9.1. Notation for Crosslinking and Degradation

Proportion of units crosslinked q Number of units crosslinked in a specimen

with Ax units qAx

Number of units crosslinked per average molecule number average (crosslinking index) qux = γ weight average (crosslinking coefficient) qu2 — δ

Proportion of units crosslinked per unit radiation dose q0 (= q/r)

Average molecular weight between crosslink Mc = w\q = w/q0r If r is expressed in megarads G (crosslinks) = 0*48 X 10e qQ/w

Mc = 0-48 xl06/Gr. Density of main chain fractures (ratio of

main chain fractures to chain units) p Density of main chain fractures per unit

dose Po = p/r If r is expressed in megarads G (fracture) = 096 x Wpjw


INCREASE IN AVERAGE MOLECULAR WEIGHT DUE TO CROSSLINKING

When a crosslinking polymer is irradiated, the links formed between molecules decrease the number of separate molecules, and increase their average size. For molecules initially all of uniform size Flory (1942) has calculated the number of dimer, trimer, etc., molecules formed as a function of the crosslinking index γ. His results, given in the Appendix, give the weight fraction Wu W2. . . Wi of each of these consti-tuents. Using the notation given in the previous chapter, these may be defined in terms of the initial degree of polymerization ux ;

Wi = «(wi) uJHin(ui)ui W2 = «(2«i) lu^niiiîix

and in general Wi = n(iUl) /«χ/Σ/ΐίι/Οί/!. (9.11)

The graph of Wi against / is shown in Fig. 9.1 for different values of the crosslinking index γ (which for a uniform distribution equals the crosslinking coefficient δ). The number of initial molecules unaffected by radiation decreases exponentially, while the number of molecules of any

O 0-5 1-0 1-5 2-0 2-5

Crosslinking index,y

FIG. 9.1. Crosslinking of uniform molecules. Formation of molecules with 2, 3, 4, . . . times the initial molecular weight. When γ exceeds unity a divergent series is

formed.

given larger size increases to a maximum, then falls off again as these molecules are themselves linked to further molecules. If γ < 1 the weight fractions Wi form a convergent series; if γ > 1 the series diverges, and molecules, theoretically of more than infinite degree of polymerization,


appear. These latter molecules are in fact closed networks of a three-dimensional character, in which some primary molecules are linked back on themselves via other molecules. This closed assembly of molecules constitutes a network or gel, whose properties are fundamentally different from the single, or multiple but finite molecules containing uu 2uu 3ul9 etc., units. The figure shows that for an initially uniform molecular weight distribution, network formation commences when there is, on an average, one crosslinked unit per molecule.

If the initial molecular weight is non-uniform, the same process of net-work formation can take place. The point at which an insoluble network first begins to form is termed the gel point, and the corresponding radiation dose is the gelling dose (rgei). As the density of crosslinking increases beyond this point the amount of network material increases in extent while the remaining material (consisting of all the polymer molecules unaffected by radiation, and of those linked together in pairs, triplets, etc., but still of finite size) decreases.

The network structure is referred to as the gel while the remaining soluble fraction is termed the sol. As is shown in the Appendix, the gel point is determined by the simple condition δ = 1, i.e. the specimen first starts to become insoluble when there is an average of one crosslinked unit per weight average molecule. This is true whatever the initial mole-cular weight distribution.

CHANGES BELOW THE GEL POINT (δ<1) Up to the gel point, a polymer is still completely soluble and the only

effect of radiation is to increase the average molecular weight and degree of branching. Each link decreases the number of separate molecules by one (linkages between different units of the same initial polymer molecule

5?

Ό

υ o £ Φ en σ £_ Φ > <

wu3

wu2

0 0-5 1-0 1-5 Crosslinking coefficient, ό

FIG. 9.2. Increase in number average, weight average and z average up to the gel point. Beyond this point the number average of the sol fraction falls. (Arbitrary

initial distribution.)

Z average

wu3 1

(T^ry

—

Jl

Awu2

1) /oo(rf=l)

7 Weight average

/ T^3 |

———

WU]

v,<i\Naverage

Gel ^ ^ point *H

t i I


being ignored in this theory). The molecular weight distribution of a polymer will therefore be modified even by very small amounts of radia-tion-induced crosslinking. Since certain properties of polymers (such as its viscosity) depend on this distribution, it is necessary to consider the manner in which this distribution varies with the radiation dose, i.e. with the crosslinking density q.

The required molecular weight distribution of the irradiated material can be expressed either in terms of the number of molecules of a given size (i.e. the description in terms of n(u) given on page 128) or alternatively in terms of the number average, weight average, etc., degree of polymeri-zation ult w2, «3. . . which can be measured directly. For an initially uniform distribution, the change in molecular weight distribution can be described in terms of n(u) but for a more complex initial distribution no solution in this form is available. The alternative method of expressing distribution in terms of uu u2, « 3 . . . or A0i Au A2, Az.. . can, however, be used, and the number average, weight average, z average of an irradiated material can be derived whatever the initial molecular weight distribution and whatever the degree of crosslinking (Charlesby, 1954).

Consider a polymer specimen containing initially Ax units distributed among A0 polymer molecules. If this specimen is subjected to radiation giving rise to a crosslinking density q, the number of crosslinked units will be Aiq. Each crosslink (two crosslinked units) reduces the number of separate molecules by one if internal linking (i.e. linking of a molecule to itself) is ignored. A crosslinking density q will therefore reduce the number of molecules from A0 toA0-Âiq (some of these molecules will necessarily become branched). The total number of units Αλ is unaltered so that the number average of units per molecule after radiation is now

« / = AS/A*' = AiKAo-iAtf) (9.12)

where the parameters for a crosslinked material are denoted by a prime while the parameters for the unirradiated material are left unaltered. Thus, since γ = qux by definition, and ux = AJAQ,

UI' = κι / (1-έγ) = « i / ( l - t e« i ) = uÂ\ - g - ή

(9.13)

The number average molecular weight of an irradiated material therefore increases with crosslinking up to the gel point but still remains finite. For an initially uniform distribution γ = δ and at the gel point (when δ = 1) γ = 1 so that Mn' (gel point) = 2Mn. For a random distribution γ =δ/2 and at the gel point Mn' (gel point) = 1-33 Mn. For a pseudo-random distribution γ = 0 (since γ = δ^/«2; δ = 1 and ujuz = 0), and ux' does not increase. These values are the maximum values for « / .

or from (9.2):

Similarly

1 1 q 1 q0r Ux ux 2 Ui 2

Mn'= MJ{\-h).


The corresponding calculations for other parameters of the molecular weight distribution are less readily deduced. They may be derived from the condition for gel formation (δ = 1) and lead to similar results whatever the initial distribution (see Appendix) :

*.' - W O - S ) ; MJ = MJ(l-$) (9.14)

1 1 1 —, = q = — — qor from equation (9.2) "2 Wa u*

and u/ = «3 /α -δ ) 2 ; M/ = Μ,/(1-δ)2. (9.15)

At the gel point, when δ = 1 both u2' and uz' rise to infinity. At lower densities of crosslinking, the values of MJ and M / are unambiguously fixed by δ. Thus at a crosslinking density half of that required to reach the gel point, δ = 0-5 and MJ = 2MW while Mx' = 4M* for all distributions.

DOSE FOR INCIPIENT GELATION

The theory given in the Appendix shows that, whatever the initial molecular weight distribution, three-dimensional network formation first begins to occur, and the specimen to become partly insoluble, at a cross-linking density corresponding to one crosslinked unit per weight average molecule. Thus the gel point is given by the equation

S=qu2 = \. (9.16)

Replacing the crosslinking density q in terms of the radiation dose rge

for gelation (equation 9.2) q = q0 Agei,

equation (9.16) becomes

#o*Vgei = l ; rgei = \/q0u2. (9.17)

or rgelMw = wfq0.

Since q0 can be expressed in terms of the G value for crosslinks (equation 9.4)

rgel = 0-48 x 106/G wu2 = 0-48 x 106/G Mw (9.18)

if rgei is expressed in megarads, and G refers to the number of crosslinks formed per 100 eV.

If the G value for crosslinking does not vary appreciably with molecular weight, the product rge\ Mw is constant;

rgei Mw = 0-48 x 10eIG. (9.19)

If the value of Mw is known, a determination of the gelling dose rgei immediately gives the G value for crosslinking. Conversely, if G is known for a polymer, a determination of rgei leads to a determination of the


weight average molecular weight. If neither G nor Mw are known, the determination of the gelation dose for a number of polymeric samples (A, B, C) of similar chemical structure can be used to compare their weight average molecular weights Mw. Thus

Mw(A)rgel(A) = Mw(B)rgQl(B) = Mw(C)rgel(C). (9.20)

SOLUBLE FRACTION

δ = 1 represents the minimum amount of crosslinking required to form incipient gel. As δ increases above 1, the initial specimen can be separated by conventional solution techniques into two constituents. The first con-stituent or sol is still soluble in the usual solvents and consists both of the original molecules which on statistical grounds were unaffected by radiation and of those molecules which were linked together by radiation to form branched molecules of finite size. These linked molecules will necessarily be larger in molecular size and fewer in numbers than the original molecules of

1-00 0-801 0-60 0-50 0-40

0-30

0-20

0-10 0·08| 0-06 o-oa 0-04 0-03

0-02

0-011

° \K\ KM

0-4 0-6 1-0 2 3 4 5 6 8 10 14 20

Crosslinking coefficient, 6 = number of crosslinked units per weight average molecule

FIG. 9.3. Decrease in soluble fraction when δ exceeds 1. (a) initially uniform distribution (b) initially random distribution (c) initially pseudo-random distribution.

When δ is less than unity the polymer is completely soluble in all cases.

which they are composed, but they will still be soluble. The second con-stituent or gel comprises those molecules which have been linked together to form a three-dimensional network polymer of unlimited size. This


network or gel, in which every atom is bound to every other by a series of primary bonds, cannot be dissolved or melted without chemical decomposition, although the meshes of the network can to some extent be separated (i.e. swollen) by solvents for the uncrosslinked polymer. At the gel point, the amount of gel material is infinitely small. As the density of crosslinking is increased, the gel fraction rises and forms an

100%5 8 0 % f

6 0 %

40%

« 20%

o υ σ t_

— if)

10%

8 %

6 %

4 %

3 %

2%

N \Λ

\ \

I

v \

\l

Γ\Ι5

Nk¥

"W M \ \ \ \ \ M 1

K I N I I N N 1

H\N Hi

JK>7. J307.

J50% H60%

- 70%

H80%

j 9 0 %

-J92% -^94%

i 9 0 %

H 98%

J99% 1 2 3 4 5 6 7 8 1012 15 20

Crosslinking coefficient S

FIG. 9.4. Some calculated sol/δ curves. I n molecules of uniform size. II n molecules each containing u monomer units + nJ2 molecules each

containing 2« monomer units, III n molecules with u monomer units each -f nJA molecules with 4«

monomer units each, IV n molecules with u monomer units each + nj 10 molecules with 10M

monomer units each, V random distribution.

increasing proportion of the total. The relative proportions of sol (s) and gel (g) can be predicted from the initial molecular weight distribution and the crosslinking density. Fig. 9.3 shows the manner in which the solubility changes as the crosslinking coefficient δ increases. These curves were calculated for the three different initial molecular weight distributions defined on page 132 (i.e. uniform, random, pseudo-random). All these curves show an initial decrease in solubility starting at the gel point (δ = 1)


but the subsequent slopes differ. The more nearly uniform the initial distribution, the more rapidly does the solubility decrease.

A general expression is derived in the Appendix, relating the gel fraction g ( = 1 —s) to the crosslinking coefficient δ, in terms of the initial molecular weight distribution (defined in terms of the degrees of polymerization u2, «3, «4, etc.). This expression can be written

* = fc-^ - 8 ' g ' + 1 ^ 8 » * ' - . . . (9.21) 2! «a 3! w2

If values of u29 u3, w4, etc., can be obtained for a polymer, the value of g for any value of Sg can be calculated, and δ obtained by simple division. Fig. 9.4 shows the curves for some polymers calculated in this way. The influence of polydispersity on the sol dose curve can be readily seen.

In several cases, an explicit expression can be derived from this general formula for the relationship between sol fraction and the crosslinking coefficient δ. If the initial distribution is uniform, the formula becomes

j = e x p ( - s n ^ 7 ) (9.22)

while, if the initial distribution is a random one

* = 1/(1+γ-γ*)» (9.23)

which can be written in a simpler form

s + Vs~= 1/Y = 2/δ. (9.24)

For a pseudo-random distribution the formula becomes particularly simple

s = 1/δ. (9.25)

In all cases the formulae only apply if δ > 1. Below this value s = 1.

Differences between Crosslinking in Sol and Gel

Although crosslinks are produced at random throughout the system, their distribution as between the sol and gel fractions are different. A polymer molecule with no crosslinked units is necessarily left in the sol, while the more crosslinks are carried by a molecule, the greater is its chance of being found in the gel. Whatever the initial distribution, the crosslinking density in the sol qs (see Appendix)

qs = sq (9.26)

where q is the overall average and s is the sol fraction. The crosslinking density qg in the gel is higher

qg = (\+s)q. (9.27)

This can be checked from the overall average

sqs + (\-s)qg =q.

As crosslinking proceeds, qs tends to zero and qg to q.


The initial average molecular weight of the polymer molecules which form the gel is also higher than those remaining in the sol. This is to be expected since the longer molecules will tend to have a higher number of crosslinked units, thereby increasing their chance of becoming attached to

Crosslinking coefficient δ

Oosslinking index, y

FIG. 9.5. Various parameters for an initially random distribution. s = sol ; ys = crosslinking index for sol molecules Yg = crosslinking index for molecules in gel Mng/Mn ratio of initial number average of molecules forming the gel to the

overall initial average Mn'\Mn ratio of number average of sol molecules to overall initial average.

the gel. In the extreme case of a uniform distribution there is of course no difference, but for a random distribution, the initial number average molecular weight of the molecules which, after crosslinking, form the gel is given by the expression

Mn (gel molecules) = Mn (1 + Vs)

while for the molecules eventually remaining in the sol, the initial average is MnVs. The final number average molecular weight in the sol

Mn' (sol molecules) = Mn Vs/ (1 - hys3/2). (9.28)


The number of crosslinked units per number average molecule entering the gel, or remaining in the sol, are likewise different. The value for Mcg, the average molecular weight between crosslinks in the gel, is smaller than the overall average Mc

Mcg = Mqg = W<7 (1 +s) = Meld +s). (9.29)

For this random distribution, the results are shown in Fig. 9.5. The soluble fraction s is constant at 100 per cent up to γ = 0-5 or δ = 1 and then decreases rapidly. At the gel point, the number average molecular weight Mn increases to a maximum of 1 -33 of its initial value Mn. The number average molecular weight of the sol molecules then decreases rapidly. The initial number average molecular weight Mng of the molecules forming the gel initially is twice the overall average (longer molecules being favoured for crosslinking) but then sinks to the initial figure Mn as most of the molecules are linked into the gel. The crosslinking index yg

of the molecules forming the gel, and the crosslinking index γ$ of the sol molecules are also shown. For a dose corresponding to four times the gelling dose (γ = 2; δ = 4) there is an average of only 0 1 crosslinked unit per sol molecule: for highly irradiated polymers branched molecules in the sol are therefore rare, unless present in the initial material.

Solubility/Dose Plots

Much of the experimental work on radiation effects in polymers has in-volved the accurate determination of solubility curves. Several methods of plotting these have been suggested, the choice being primarily determined by the parameters it is intended to investigate. A useful plot for lightly irradiated specimens is one of gel fraction (g) vs. dose. This should in theory give a sharp cut at the gel point, enabling the gelling dose to be measured accurately. An alternative is to plot g against g x dose, which parallels the theoretical formula (9.21), apart from a constant factor: B/r. The initial deviation from a linear plot then gives a value for uju2.

A frequent method is to plot log sol against log dose. This provides a nearly linear plot for a uniform distribution, and a linear plot for the pseudo-random distribution (9.25). Examples of these plots are shown in Fig. 13.10. In this case the unknown ratio of crosslinking coefficient to dose does not affect the shape of the curve, and only produces a lateral displace-ment. It is therefore very simple to compare this type of plot with a series of calculated curves, traced on transparent paper to the same scale, to determine the type of molecular weight distribution present in the initial material. This method of plotting also places adequate emphasis on data for high doses, when the sol fraction is small.

A plot of s -f- Vs against reciprocal of dose should give an accurately linear plot if the initial distribution is a random one (equation 9.24). When the dose is infinite, s + Vs = 0. This gives one point on the curve, while the gelation dose can be readily determined from the intercept with s + Vs = 2. The slope 2/q0u2 determines the ratio of dose to crosslinking coefficient.

L


One of the advantages of this technique of plotting solubility curves is that while the theoretical curve is only linear for a random distribution, it is approximately so for many other forms of molecular weight distri-bution. Fig. 9.6 shows the calculated curves for uniform, random and pseudo-random distributions. The slope of the approximately linear curve can be taken as an approximate measure of the degree of uniformity of the distribution.

FIG. 9.6. Theoretical curves relating soluble fraction s and crosslinking coefficient δ for an initially uniform, random or pseudo-random distribution.

If chain fracture as well as crosslinking results from radiation, the soluble fraction s never disappears, but as the dose increases, s tends to a value depending on the ratio of crosslinking to main chain fracture. Plots of s -f Vs against \\r do not then tend to zero, but to a value equal to this ratio. Simultaneous crosslinking and main chain fracture is discussed in Chapter 11

EFFECT OF CROSSLINKING ON ELASTIC PROPERTIES

Elasticity and Entropy

In the absence of constraints, the arrangement of units in a flexible chain is dictated primarily by probability considerations. The elastic properties of a crosslinked network arise from the entropy decrease which results when external constraints impose a less statistical probable arrange-ment on the units.

The free energy F of a system depends on its internal energy U and on the entropy S

F=U-TS (9.30)

where T is the absolute temperature. temperature

For an extension e at constant

(dF\ = idu\ _T(dS\ [de JT \de)r \deJ7

(9.31)


In many systems it may be assumed that the internal energy of a polymer chain does not significantly change with extension. This will be a valid approximation for long flexible chains, with only weak forces between them. In that case the external force /

H a 4 = - r ( £ ) r <9·32» where the entropy S is related to the probability P of a given configuration of the polymer chain

AS = kMnP (9.33)

and k is the Boltzmann constant. The problem is therefore one of calculating the probability P of various

polymer chain configurations.

Single Chain

For a single chain consisting of n chain units (or monomers) each of length λ the distance r between ends can vary from 0 to nk. The probability of any intermediate distance r is given by a Gaussian distribution

rm / \fmj

P{r)ar = ^ r - exp ( - ^ ) d | ' - ) (9.34)

where rm is the most probable distance between the ends of the chain. This probability distribution fails for high extensions, when the length r approaches the maximum physically possible, i.e. nk. This is a limitation on the following theory, which cannot therefore be applied when a polymer chain approaches a fully extended position. The value of rm is equal to λ V3n/2, and the ratio of rm to the maximum possible extension

rjl „x Λ- <9J5)

Thus for a polymer chain comprising 100 units, the maximum possible extension is (200/3)* or 8-17 times. This simple model assumes that each unit in the polymer chain is completely rigid, but that the link between successive units is fully flexible. In practice, this second condition does not hold. Valency angles determine the orientation of successive units relative to each other so that a linear array of chain units is impossible and the maximum length is less than nk. Moreover an energy barrier must be overcome if successive units rotate about the bond between them. The first limitation (i.e. valency angle) can be overcome by reducing the


equivalent length of each unit. If Θ is the valency angle the value of rm

becomes

r - , = ? " x , } ? 5 n ! · (9·36> 3 l + c o s 6

In polymers with a carbon backbone the C—C—C valency angle is 109-5° so that rm

2 = 4«λ2/3. The second limitation is more serious, and is dealt with by considering the chain as formed of segments each of which is rigid; successive segments are, however, free to take up any orientation relative to one another. Each segment will comprise several monomer units, the number of monomer units per segment being chosen to give the appropriate degree of flexibility to the whole chain. Measurements of viscosity and of birefringence of paraffin chains lead to values of about 15 carbon atoms per segment, but the figure will obviously alter with temperature; at higher temperature, when the effective barrier to rotation is reduced, a smaller number of units per segment will be required to confer on it the required flexibility. The softening of many polymers may be ascribed to this increased flexibility.

A further restriction arises from the finite volume occupied by the atoms which leads to an increased equilibrium size. This has not been taken into account in the theory. For low values of r the values for P(r) should be reduced, and the true value of rm will be greater than that calculated above.

Three-Dimensional Network

The calculation outlined above for a single polymer chain has been extended to a three-dimensional network by Kuhn (1934,1936,1939,1942), Wall (1942, 1943), Treloar (1943-46, 1949), Flory (1944, 1950, 1953) and others. It was assumed that all polymer chains are extended in the same proportion, a model independently justified by James and Guth(1943). The value for the entropy change when a network is extended by a factor a in one direction is found to be

Δ5 = - | M ( a 2 + 2 / a - 3 ) (9.37)

for N chains. The volume of the specimen is assumed to remain constant (as found experimentally) so that, the extension ratio in one direction being a, the compression in the two directions at right angles is 1/VôT

One cubic centimetre of a crosslinked polymer, of density p will contain 6*02 x 1023 p/Mc chains (between links) each of number average molecular weight Mc.

Then

Δ 5 = - - ^ ( α * - 2 / α - 3 ) (9.38) 2MC


when changes in internal energy U are ignored.* a represents the length, in the stressed direction, of unit length in the

original specimen. The extension e per unit initial length equals a—1. Then for small extensions

f=3pRTe/Mc (9.40)

and the corresponding elastic modulus

E = 3 pRT/Mc

Using equation (9.1), this can be expressed in the form

E = 3 pRTq/w (9.41)

while if the crosslinking density q is proportional to the radiation dose r (equations 9.2, 9.4) expressed in megarads:

E = 3 pRTq0 r/w = 6-24 x 10"* pÄ7VG(crosslink) (9.42)

The elastic modulus E is then proportional to the radiation dose; from the ratio the value of q0 or of G can be immediately deduced.

For higher degrees of extension, where terms in e2 cannot be ignored, the theoretical expression can be used in the alternative forms:

f=PRT(oc-\l^)q/w = pRT(K-l/oL*)q0r/w = 2-08 x 10-6 pRT(oi- 1/a2) rG(crosslink) (9.43)

when r is expressed in megarads. A plot of/against a— 1/a2 should then give a straight line, from whose slope q0 or G(crosslink) can be deduced.

Owing to the considerable assumptions and simplifications made in deriving this theory, a note of caution must be sounded in applying it to quantitative measurements on irradiated polymers. The theory cannot be applied to polymers in which crystallinity or stiff chains and entanglements prevent the free motion of units passing each other. Again, for very high degrees of crosslinking, the average length of the chains may fall well below that required to justify the assumption of a Gaussian distribution in chain length, while for low degrees of crosslinking, end effects become

* In the derivation by Guth and James (1957) the right-hand side of this equation must be reduced by a factor of two. This doubles the G values derived from the above expression.

where R is the gas constant. The relationship between applied stress per unit area and elongation e at constant temperature

(9.39) f==äF^_TäS = pRT( n de de Mc\ a2/


important due to the initial finite molecular weight and appropriate corrections must be applied. These and related problems have been studied in considerable detail in the classical theory of polymer chemistry and some of the conclusions relevant to the properties of irradiated polymers are summarized here.

End Effects

Tn a network formed by linking molecules together, some chains will be formed, terminated at both ends by crosslinked units. Other chains will be limited at one end by a crosslinked unit, at the other end by a free end, i.e. one of the initial ends of the molecule. Only the former chains can be acted on by the applied stress, the latter acting merely as a diluent or plasticizer. Flory (1944, 1953) has derived an expression for the active chains, i.e. those limited at both ends by crosslinks.

Consider a specimen containing initially A1 monomer units and A0

molecules. It is assumed that these molecules are then linked together into a network. The minimum of crosslinks needed to constitute all these molecules into a single molecule is A0— 1, since each link can reduce the number of separate molecules by one. Any additional crosslinks are then available to form internal loops in the network, i.e. chains limited at each end by a crosslink. Each additional crosslink provides two crosslinked units and therefore increases the number of chains by two. Thus, for a total crosslinking density q in a specimen with Ax units, there will be Axq/2 crosslinks, of which only Axq\2—(A0— 1) will be available for forming a three-dimensional network. The average number of units uc per chain is therefore

Mc Ax

w A1q—2Aii-\-2

(there being two crosslinked units, and two chains per crosslink). The corrected value of Mc in terms of the initial N.A.M.W. Mn, and the value MCoo for infinitely long molecules (when this correction is not needed) is therefore

1 _ * _ Arf—lAo+l _q 2 ,Α.1Λ 77 — — A \AQ^>L)

Μ^ wuj wAl w wuL

_ J _ _ ^ _ = 1_ L 2MC„ \ 1 Λ _ 2 Mc» Mn Mc„ \ Mn ) Mc„ \ γ

Inserting this value for Mc in the formulae (9.41) for E gives

E = 3 pRT\ - 1 - — —) = 3 pRTw-1 (q-2/u,)

\MC„ MnJ = 3 pRTqoW1 (r—2/qouJ = 6-24x 10-6

9RTG(r-0'96xlO*/GMn) (9.45)

(9.44)

where G is the number of crosslinks per 100 eV. This end correction does


not alter the slope in a plot of E vs. r\ the effect of the initial finite molecular weight merely shifts the curve and increases the abscissa at the origin to a dose equivalent to 2/q0Ui. A similar conclusion applies to the relationship between f and r for larger extensions :

/ = pRTw-Kx-ll*2) q0 (r-2/qoU,)

= 208X 10-6 ?RT(oL-l/a2)G ( r -0 -96x 10*/GMn). (9.46)

This correction for end effects is not entirely justifiable. It ignores the existence of a sol fraction, the difference in molecular weight of the molecules going into the sol and gel (longer molecules have a greater probability of being crosslinked), the difference in crosslinking density as between sol and gel, and the existence of numerous free ends in branched molecules. Attempts to allow for these additional effects have been made, and lead to somewhat similar correction factors. It cannot yet be said that a fully satisfactory correction factor has been established; in view of other factors which intervene when the crosslinking density is low, this gap in our knowledge is less serious than would appear.

Entanglements

Chains sufficiently tangled together may, without being crosslinked, provide a considerable hindrance to the motion of molecules past each other. There will result an increase in the elastic modulus which has not been calculated and is generally represented by a parameter g > 1,

E = 3 pRTg/Mc (9.47)

Earlier work on highly-elastic polymeric systems linked together by chemical means have led to values for the parameter g of between 1 and 3. The introduction of such a parameter has not yet been found essential in the case of radiation induced crosslinking.

In this connexion it may be useful to mention the effect of cross-linking in producing cyclic structures. These structures arise from internal linking, i.e. a monomer unit being linked to another unit further along the same chain. This process is generally ignored in the theory of gel forma-tion; it constitutes a loss of linkages which would otherwise be used in linking separate molecules together to form a gel. Such internal links form loops, which because of the limited flexibility of the polymer chain normally comprise a number of monomer units. Such loops would therefore surround other molecules and restrict their movement in the somewhat similar manner to a crosslink. In an extreme case a number of such loops could be linked together in the same manner as the individual loops in a steel chain. Each molecule would then remain finite in size, although the behaviour of the whole system of such linked loops might be similar to that of three-dimensional network of considerable flexibility. In this respect such internal linkages form entanglements equivalent to crosslinks and may perhaps be considered as such. Once the gel point


has been passed, the formation of such internal links becomes progres-sively less important, as the distinction between internal and external links fades.

High Elongation

At low and medium elongations, of up to about 250 per cent, agreement of theory with observed stress-strain curves for vulcanized rubber is good; above this limit the stress increases more rapidly than given by theory. In the case of rubber this has been ascribed to crystallization as the isoprene units become oriented. In other polymers where no crystallization occurs, this explanation cannot be offered. Failure of the theoretical relationship is to be expected on purely statistical grounds, when the simple Gaussian probability distribution (equation 9-34) no longer represents a valid approximation. Kuhn and Grun (1942) have developed a more accurate derivation, which involves the Langevin function

Urlnk) = coth(r/«X)-«X/r, (9.48)

where rjnk is the ratio of the distance between chain ends to its maximum value. At low values of rink this expression approximates to the value obtained from the Gaussian distribution; at higher values of rjnk the stress for a given extension is considerably greater. This theory assumes n > 1, and will therefore fail if the number of segments per chain is small, i.e. for very highly crosslinked polymers. Treloar (1949) has shown that for a chain with six segments it fails at 50 per cent of the maximum extension, for a chain of 25 segments at 60 per cent and for a chain of 100 segments at 80 per cent.

For a typical polymer of molecular weight 10\ and of degree of poly-merization 1000, end correction terms would become small at a cross-linking density of about 10-20 crosslinked units per molecule (i.e. 1-2 per cent crosslinking). If the flexibility of the polymer requires 10 monomer units per segment, the number of segments between crosslinks is only about 5-10. The maximum extensibility is then only about threefold, and the Langevin function will fail at half this value. It cannot therefore be said that, even in the absence of crystallinity, the elastic properties at high deformation can be predicted quantitatively.

TENSILE STRENGTH

No mathematical theory has yet been generally accepted to account for the observed tensile strength of crosslinked polymers. The tensile strength depends not only on the density of crosslinking, but on the possibility of crystallization, on filler action and on initial molecular weight. In many polymers which crosslink under irradiation, a maximum tensile strength occurs at the same crosslinking density as when crosslinking is produced by the more conventional chemical techniques. In crystallizable polymers such as rubber, the subsequent decrease in strength as further crosslinks are introduced may be due to interference of these crosslinks with


crystallization. In polymers which do not crystallize when oriented this decrease may arise from the distribution of chain lengths and consequent variation in their maximum extensibility. Under uniform extension, the shorter chains reach their maximum extension first, and carry a dispro-portionate part of the load. The greater the density of crosslinking, the shorter is the chain between such links, and the smaller is its possible extension. Even for flexible long chains this ratio is of the order of Vn (equation 9.35). For very short chains it is much smaller.

Fine filler particles can increase the stiffness and the tensile strength of many polymers but the mechanism by which this reinforcement takes place is not yet clear. The introduction of small rigid spherical particles would in itself raise the elastic modulus in the same manner as it increases the viscosity of a fluid. The expression derived by Guth and Gold (1938) leads to a reinforcement factor for the elastic modulus E

E = E0 (1 + 2·5ν/+14·1ν/2) (9.49)

where E0 is the modulus in the absence of filler, and v/ is the volume fraction of filler. In certain cases the reinforcement achieved is however considerably greater, and would appear to arise from bonds of unspecified type between polymer molecule and filler.

The improvement in tensile strength produced by the introduction of fillers may be largely due to physical causes, the individual filler particles diverting an incipient crack or tear into a direction more nearly parallel to the applied stress, and hence less harmful. Alternatively some un-specified surface reaction between filler (e.g. carbon black) and polymer molecule may serve to repair any damage caused by a high local concentra-tion of stress.

SWELLING OF CROSSLINKED POLYMERS

Most amorphous polymers, if not crosslinked, dissolve readily in a variety of organic solvents; in the same solvents, crosslinked polymers, or at least the gel fraction, can only swell to an extent determined by the density of crosslinks and the type of solvent used. Crystalline polymers, on the other hand, do not dissolve and are barely swollen unless the tem-perature is raised to a point at which some crystalline regions start to melt. Polyethylene, for example, can be readily dissolved in benzene or toluene only at temperatures above about 60-80°C, although irradiated polyethy-lene will then only swell. In good solvents, molecular weight has little influence on the solution characteristics; in poor solvents it may be critical, lower molecular weight polymers being soluble whereas higher molecular weight polymers only swell.

The study of the solubility of polymers constitutes a well-developed branch of polymer science which has been extended to the study of poly-mers crosslinked chemically, or by irradiation. Here we are primarily concerned with the relation between the extent of swelling of a crosslinked polymer in a good solvent and the degree of crosslinking.


When a polymer molecule is dissolved, there will be a heat of mixing due to the interaction energy of polymer and solvent and, in addition, a change in the entropy of the system. Expressions for these have been derived (see Flory, 1953; Frith and Tuckett, 1951). Even in a crosslinked polymer there is still a tendency for solvent molecules to enter the system, and the polymer network swells. This swelling takes the form of three-dimensional stretching of the individual chains; their configurational entropy is thereby decreased in a manner similar to that for an externally applied stress. This entropy decrease will become greater as the degree of swelling increases. An equilibrium condition is therefore reached between these two competing effects. The degree of equilibrium swelling depends on the heat of mixing, the entropy of dilution and the entropy of the elastic network. The smaller the average molecular weight Mc between crosslinks the smaller will be the degree of equilibrium swelling.

The equation giving the equilibrium swelling (Flory, 1953) is

In (1-χ)+χ+μχ2+^-Ιχ1ΐ*--) = 0 (9.50) Mc\ 2 /

where p is the polymer density, Mc the average molecular weight between crosslinks, v is the molar volume of the solvent and x is the polymer volume fraction of the swollen gel. Its reciprocal V(= l/x) is the swelling ratio, i.e. the volume of swollen polymer divided by its original volume (assumed completely crosslinked). The parameter μ is the heat of mixing term, arising from the heat of interaction between solvent and monomer units in the polymer chain. Unless there is a very high density of cross-linking resulting in an appreciable chemical change, μ should be a constant for a given polymer-solvent combination, μ was originally defined theoretically and it can be derived from osmotic pressure measurements. However, it is best considered as an empirical quantity, which varies with temperature;

μ = ß + oc/i?r.

In equation (9.50) the last term arises from the elastic deformation of the network, the others from the heat and entropy of mixing. In some derivations, the last member (x/2) is omitted.

If the degree of swelling is large (x small) the expression can be simplified

(0·5-μ)Λ:2 = pvxl'3/Mc (9.51)

or replacing x by 1/K, the reciprocal of the swelling ratio

j/5/3 = (ο·5-μ)Μ,/Ρν. (9.52)

In terms of the crosslinking density q ( = w/Mc)

F5 / 3 = (0·5-μ)Η>/ρν<7 (9.53)

or if q is proportional to the radiation dose

y*i* = (0·5-μ)>ν/ρν^0Γ = 0-48 x 106 (0-5^)/pvG(crosslink>\ (9.54)


This expression only holds within the limits of the approximations used (e.g. x/2 < x1/3). For a swelling ratio of 8, x/2 is 1/16 and x113

is i , so that deviations may be expected even at high swelling ratios. It will also fail at low swelling ratios of about 2 or 3 when the expansion used for ln(l— x) is no longer a sufficiently good approximation. Within these limits, a log-log plot of V against r should give a straight line of slope—5/3. This relationship is often observed to hold for irradiated polymers at doses well above the gel point. Fig. 9.7 shows the curve usually obtained when V is expressed as the swollen weight divided by the initial dry weight (both sol and gel). Close to the gel point, this relation-ship fails because of the existence of a soluble fraction which cannot swell, the difference in average molecular weight between sol and gel molecules and the difference in crosslinking density. These corrections are discussed

>

O t-O) c

en o

» log dose

FIG. 9.7. A typical swelling/dose curve. Weight swelling ratio = swollen weight/initial dry weight.

in detail on page 287. To some extent, the usual Flory correction (see equation 9.44) can be used as a correction for the free ends (molecular chains terminated at one end only by a crosslink), but the situation is complicated by the dependence of the gel fraction on the weight average molecular weight whereas the Flory correction involves the number average. In theory a study of the swelling ratio of irradiated polymers near the gel point may lead to information on the initial molecular distri-bution, but no such experimental studies have been reported. It is usually preferable not to attach much quantitative significance to the swelling data when the crosslinking coefficient δ is less than about 2, especially as in this region the gel fraction varies very rapidly with radiation dose.

REFERENCES CHARLESBY, A., / . Polymer Sei. 11, 513, 521, 1953. CHARLESBY, A., Proc. Roy. Soc. A222, 542, 1954. FLORY, P. J., Chem., Rev. 35, 51, 1944. FLORY, P. J., / . Amer. Chem. Soc. 69, 30, 1947. FLORY, P. J., / . Chem. Phys. 18, 108, 1950. FLORY, P. J., / . Phys. Chem. 46, 132, 1942. FLORY, P. J., Principles of Polymer Chemistry, Cornell, 1953. FLORY, P. J. and REHNER, J., / . Chem. Phys. 11, 521, 1943.

^ x /Part ial ly >/Lsoluble

/ ^ ^ s p e c i m e n

Gelling dose

^ S l o p e = - 0 - 6

Deviations a t V low swelling ^

ratio * \


FRITH, E. M. and TUCKETT, R. F., Linear Polymers, Longmans Green, 1951. GUTH, E. and GOLD, O., Phys. Rev. 53, 322, 1938. GUTH, E., JAMES, H. M. and MARK, H., Advances in Colloid Science 2, 1946. GUTH, E. and JAMES, H. M., / . Polymer Sei. 24, 479, 1957. HUGGINS, M. L., Ind. Eng. Chem. 35, 216, 1943. HUGGINS, M. L., / . Phys. Chem. 46, 151, 1942. JAMES, H. M. and GUTH, E., / . Chem. Phys. 11, 455, 470m 1943; 15, 669, 1947;

21, 1039, 1953. JAMES, H. M. and GUTH, E., / . Polymer Sei. 4, 153, 1949. KUHN, W., KolloidZ. 68, 2, 1934; 76, 258, 1936; 87, 3, 1939; 101, 248, 1942. KUHN, W. and GRÜN, F., KolloidZ. 101, 248, 1942. KUHN, W. and KUHN, H., Helv. Chim. Ada 26, 1934, 1943. STOCKMAYER, W. H., / . Chem. Phys. 12, 125, 1944. TRELOAR, L. R. G., Trans. Faraday Soc. 39, 36, 241, 1943; 40, 59, 109, 1944;

42, 83, 1946. TRELOAR, L. R. G., Physics of Rubber Elasticity, Oxford Univ. Press, 1949. WALL, F . T., / . Chem. Phys. 10, 132, 485, 1942; 11, 527, 1943.

CHAPTER 10

THEORY OF POLYMER DEGRADATION

Degradation and Depolymerization

Whereas certain polymers, after exposure to high energy radiation, show an increase in molecular weight, leading to the formation of a three-dimen-sional network, other polymers show a reduction in average molecular weight, with corresponding changes in viscosity and mechanical properties. These changes can be ascribed to radiation-induced fracture of the main chain, with a rearrangement of the atoms near the point of fracture to stabilize the end-groups. This process, termed degradation, is essentially different from the process of depolymerization (often produced by thermal means) whereby a change in one of the bonds allows the molecule to revert wholly or in part to the original monomer. In radiation induced degra-dation little or no monomer is produced even after extensive main chain fracture. Depolymerization is a chain reaction, involving many of the bonds present in a polymer molecule, whereas degradation only affects atoms in the neighbourhood of the fracture site, and is best shown by a progressive reduction in the average molecular weight. At the same time as main chain fracture, rearrangement of chemical bonds near the point of fracture and the liberation of small chemical groupings take place, which can be revealed by chemical means. These rearrangements are determined by the chemical structure of the monomer unit ; the changes in molecular weight depend on the chemical structure only in so far as this will determine whether crosslinking or degradation takes place, and the energy required (expressed as a G value for the reaction). It is therefore possible to consider separately the statistics of polymer breakdown by radiation and the chemical changes produced. In this chapter the changes in average molecular weight and the molecular weight distribution are analysed. These may then be compared with the changed properties of the irradiated polymer, such as its viscosity and mechanical strength.

Distribution of Main Chain Fractures

The simplest case to consider from the analytical point of view occurs when a single radiation induced event—ionization or possibly excitation —results in a single fracture of the main chain of a polymer, and, after possible rearrangements near the point of fracture to stabilize the new end-groups, the formation of two shorter polymer molecules. Incident energy is absorbed equally readily at any one of the monomers of the chain, and in the absence of energy transfer over long distances, each monomer unit is equally liable to fracture. This will not necessarily be true for the units near the end of a polymer chain, where reactivity and bond strengths may

159


be appreciably different from internal units. Evidence from chemical degradation shows that this end effect does not extend far along the chain. Early work by Kuhn et al. (1930, 1932) on the hydrolysis of cellulose and its lower molecular weight homologues cellobiose, cellotriose and cellotetrose showed that the reaction could be explained by assuming that only the two terminal groups were more lightly bound, by a factor of three, than internal linkages, and that all their internal linkages were equally likely to be hydrolysed in molecules comprising up to 10 monomer units. Wolfram, Sowden and Lasettre (1939) extended this work to the hydrolysis of methylated cellulose and found no change in reactivity when the degree of polymerization was reduced threefold from 150 to 50. Matthes (1943) showed by hydrolysis of the polyamide poly-e-caproamide that splitting of equivalent bonds occurred uniformly when the degree of polymerization was reduced from 220 to 6. Similarly, Flory (1940) found that the alcoholysis of the polyester decamethylene adipate proceeded with constant reactivity of the inter-unit bonds over the range of 40-15 monomers per molecule. In radiation induced degradation of polymers, where energy capture occurs at random, and where the degree of poly-merization is usually very high (greater than 1000), the effect of increased reactivity of end-groups may usually be neglected. If necessary, end corrections could be made formally by adding on an equivalent number of monomer units, but in practice they have never yet been found necessary.

A second possible restriction to the following theory arises if the poly-mer molecules to be irradiated exist in different environments. Thus a different degradation rate may occur at the surface of an irradiated poly-mer, due to the presence of oxygen. Again, different degradation rates have been observed in high molecular weight cellulose (degree of poly-merization of about 1500) degraded by chemical methods (Husemann and Schulz, 1942), but here the effect may be due to alternating regions of crystalline and amorphous structure with different susceptibilities to chemical attack. In parallel work by Saeman, Millett and Lawton (1952), and by Charlesby (1955), in which high energy electrons were used to degrade cellulose, no such effect has been observed.

The problem to be studied relates to the changes in average molecular weight (number, weight or viscosity) and to the modified molecular weight distribution resulting from radiation-induced main chain fracture occurring at random. The same general problem arises in connexion with polymer degradation resulting from thermal, chemical or ultrasonic treat-ment, and a number of papers have been published analysing these changes. In most mathematical analyses (Kuhn, 1930, 1932; Klages, 1932; Durfee and Kertesz, 1940; Mark and Simha, 1940; Montroll and Simha, 1940; Sakurada and Okamura, 1940) it is assumed that initially all molecules are equal in size, and that all main chain bonds are equally liable to fracture. Jellinek and White (1951) have extended this to the case where the probability of fracture depends on chain length, and Simha (1941) has taken into account the increased probability of chain fracture occur-ring near the ends of molecules. Later analyses by Watson (1953) and

THEORY OF POLYMER DEGRADATION 161

Dump (1954) mainly refer to an initially uniform or random distribution. The general distribution was studied by Charlesby (1954).

In this chapter a mathematical analysis is given of the changes in average molecular weight, and molecular weight distribution, as a result of main chain fracture occurring at random along the polymer molecule. The notation used is defined in Chapter 8. p represents the radiation induced fracture density (i.e. the number of main chain bonds broken per monomer unit) and is therefore equal to half the number of molecular free ends produced by radiation. p0 is the corresponding fracture density per unit radiation dose.

The parameters of the initial molecular weight distribution are repre-sented by uu u2f u3... or A0, Au A2... as before, while the corresponding parameters for the final distribution are represented by primes # / , u2\ U% . . . AQ , A\ , y42 . . .

The analysis falls under three headings : (a) Changes in the number average degree of polymerization ux (num-

ber average molecular weight Mn). These changes do not depend on the initial molecular weight distribution, and the results are therefore generally valid.

(b) Changes in all the molecular weight parameters for an initially random molecular weight distribution. In this case the random nature of the distribution will be maintained.

(c) Changes in molecular weight parameters for any other initial distribution—in particular a uniform one. At sufficiently high degrees of radiation these parameters approximate to those in (b) above.

REDUCTION IN NUMBER AVERAGE MOLECULAR WEIGHT

Consider all the monomer units initially present (i.e. AJ in the specimen linked together to form a single closed chain. By breaking this chain at A0 points chosen at the appropriate places, the initial distribution of A0

molecules of the appropriate lengths can be obtained. This still leaves AÎ—AQ bonds between monomer units open to degradation.*

On irradiation, a fraction p of these AX—AQ bonds will be fractured, each such fracture giving rise to an additional molecule. After radiation, the total number of molecules therefore increases to

A0' = Ao+piA.-Ao) (10.1)

while the number of units involved is unaltered

Λ ' = Ax.

* Main chain bonds can either be broken between adjacent monomer units, within these units, or in both places. Mathematically there is a slight difference in that bonds between adjacent monomers can only be broken once, and once this occurs, the probability of a second break at the same point vanishes. In practice, the distinction is of no importance for polymers, since the chance of several independent breaks at the same bond is negligible for the fracture densities considered.


The number average molecule in the degraded polymer has « / monomer units where

« / = AS/AS = A1/[A0+p(Al-A0)]

1+/*«!-1) \+pux since ux>\. (10.2)

This result is independent of the distribution of main chain fractures both in the initial and in the irradiated specimen, and therefore does not depend on the initial molecular weight distribution, nor on the distribution of the fractures within each irradiated molecule. Osmotic measurements of the decrease in the number average molecular weight Mn' with radiation can therefore only lead to a determination of the fracture density p in relation to the radiation dose. To obtain any information on the distribution of these radiation induced fractures along the molecular chain, it is necessary to study the molecular weight distribution by other means, e.g. by viscosity measurements or light scattering techniques, which depend on other averages.

Virtual Degradation

It is often a simplification to consider the final molecules as being derived from a closed chain of Ax monomer units, fractured at first to give the initial molecular weight distribution (with parameters A0, Au A2. . .), and then fractured again by irradiation to give the final distribution (A0'9 Ai, A2' . . .). The concept of a virtual initial degradation is a very convenient one mathematically, since it enables many distributions to be related directly to an initially infinite molecule, which is much simpler as a starting point for analysis.

To obtain A0 molecules from a cyclic molecule containing A± monomer units requires AQ-\-\ main chain fractures; the corresponding virtual frac-ture density is therefore Pi = (A0+\)/Ai or since A0>\, pi = A0/Ai = 1/tfi. Further fracture due to radiation reduces the number average degree of polymerization to « / where

« / = «i/(l+P«x) = H(P+PÙ (10.4) and Mn = w/(p-\-pi).

One of the advantages of introducing this concept of virtual degradation is that for high degrees of degradation, when few of the molecules initially present remain undegraded, the distribution of molecular weights approxi-mates to that obtained by random fracture of an infinite cyclic chain with a total fracture density p+p» This is true whatever the initial distribution

The number average molecular weight after irradiation (Mn') is related to its initial value Mn by equations (8.6 and 10-2)

(10.3) , , , Mn Mn M» = T - ; = ΓΊ—vr~r

1 +pui 1 +pMn/w


provided that p is large enough. Then the number of irradiated molecules n' containing between u and u+du monomer units is

n'du = Aiip+pi)2 exp ( -u Jf+pùdu. (10.5)

For low values of p, this is not usually true; the distribution of mole-cular weights does depend on the initial distribution. In general, the con-cept of a virtual radiation dose is most useful in two cases :

(i) at high degrees of degradation (p>pi), when equation (10.5) applies to all initial distributions;

(ii) at low degrees of degradation, if the initial molecular weight already follows a random distribution.

INITIALLY RANDOM DISTRIBUTION Any molecular weight distribution which is initially of a random charac-

ter will retain this character, although with different parameters, if it is subject to further random fracture. It has been shown above that for any distribution after fracture the number average degree of polymerization Ui is related to its initial value (equation 10.4)

Hi = « i / 0 +/>Wi).

Since for a random distribution the weight average u2 or u2 is twice the number average, it follows that

u* =u2l(\+pu2l2) (10.6) so that Mw' = MJ(l +pMw/2w).

Similarly, for the z average which is three times the number average

K»' = KS/(1+/WS/3) (10.7)

Μ / = Μ,/(1+/>Λ/,/3κΟ.

Introducing the concept of virtual fracture density pi ( = l/wx)

w»' = 2w/ = 2/(p+pi) (10.8) Ha' = 3I<J>+Pi)

and, as is often more conveniently used

1/M/ = (j>+pi)/2w. (10.9) Similarly 1/M/ = (p+pdßw.

A corresponding expression applies to the viscosity average molecular weight, but the coefficient depends on the ratio Mv\Mn given by equation (8.21).

If the fracture density p is directly proportional to the radiation dose r (equation 9.6) there is a linear relation between \\MW (or 1/M/, 1/M/) and r\ thus

1/M/ = (p0r+Pi)/w = p0(r+r0)2lw = 0-52x 10"6(r + r0)G (10.10) and 1/M/ = (p0r+Pi)/w - p0(r+r0)/w = l-04x \0~\r+r0)G

M


where A*0(= Pi/p0) is the "virtual" radiation dose, necessary to fracture an infinitely long chain to give the initial molecular weight distribution; G refers to the number of main chain fractures per 100 eV of energy absorbed. A plot of \\MW' (or 1/M/, \\MV') against radiation dose r should therefore give a straight line if the fracture density is proportional to radiation dose. This is usually found to be the case. The slope of the curve gives the energy absorbed per main chain fracture, while the intercept at the origin enables the initial number average molecular weight to be deduced.

GENERAL RELATION

The change in the number of molecules AQ' with fracture density (equation 10.1) does not depend on the location of fracture within the molecule but only on their number. In calculating the general distribution term A/, it is however necessary to know the distribution of all fractures, of which only those due to radiation will be located at random; the analysis given below will also apply to copolymers where irregularities in structure either occur at random or with a periodicity very small compared with the total length of each molecule.

Consider a molecule with u monomer units in a specimen with a radiation induced fracture density/?. If there is an increase dp in the fracture density, the probability of the molecule being unaffected is 1 — uàp. If it is fractured, the two molecules formed will have lengths xu and (1— x)u where x can have all values between 1/w and 1 — 1/w (or approximately 0 to 1) with equal probability. In the expression for Ai or ΣΛ'(Μ)«' the term «* must now be replaced by

l

w'(l - udp) + udp I [(*«)'+ (1 - xu)'] dx

x=0

ui—ui+1\ 1 — -—: Id/?, a decrease of ui+1 —-r dp. \ l + i/ i + l or ul—

The increase in A{ due to the additional fracture density dp is obtained by summing this expression for all molecules

so that the recurrence relation is obtained

(10.11)

This fundamental equation enables the parameters A2', Az\ A/, etc., to be calculated from their initial value by means of a Maclaurin expansion.

AAi* = -Ση'(μ)ι*+ι ττ4 Φ = -A'i+i 1τ-\ dp

<L4/_ i - l ., dp i+l


If F(x) is a function of x, its value can be deduced from its differential coefficients when x = 0 :

Ai can be considered as a function of the fracture density p, with the value Ai dit p = 0. Then

A' A A. ίάΑΛ -L P2 (U%Ai'\ + .. P = 0

The differential coefficients can be deduced from the recurrence equation (10.11). For A2', for example,

\ dp J p = 0 3

IVAA = _ 1 ίάΑΛ = 1 \ dp* ) p = 0 ~3\ dp Jp = 0 6 4

and in general (dJAA 2 ( - i y \ d ^ / ^ 0 (y+l)(y+2) ; + 2 '

Then

Λ ' = Λ , -^pA z + f^2^- ^ Q / * 3 ^ + · · · + c+2)lpJAi+2'

Dividing by Αλ' (= AJ gives the weight average degree of polymerization w2', in terms of the parameters of the initial distribution :

1 _1_ l 2

«2 = u2--pu2u3 + —p2UiU3u2- . . .

2( - iy . • - · + c+2)\pJUi+2 Uj+1 ' ' ' "2 + ' ' * (10.14)

Similar expressions with different coefficients can be obtained for the higher parameters. Thus

A*'= Α,-^ρΑ, + ^ Μ , + . . . + y ^ y ( - / 0 ' " 1 4 + 2 + . . . (10.15)

Several special cases may now be considered. For an initially uniform

(10.12)

(10.13)

166

distribution ux — u2 = uz = bution at random


. Hence after fracturing such a distri-

w2' = « i - £/Wi2 + Ϊ 2 ρ 2 ί / ι 3 _ 2(-py 0'+2)! UiJ+1 + - . .

2«!

' β 5 (g-p«, _ 14-^nj (10.16)

and MJ 2Mn

ip/PiV (e-P'Pi-l+p/pi)

in terms of the initial (virtual) fracture density pi or w/Mn; p/pi represents the average number of radiation induced fractures per initial molecule. This expression has been derived independently by Sakurada and Okamura (1940).

For an initially random distribution ul=-^-= -? = . . . - and 2 3 j

«>'= 2«!---3!/>!/!«+ j 2 4 ! / > 2 " i 3 - · · ·

= 2ull(l+pud=2l(p+pi)

as previously derived (equation 10.8).

The decrease in weight average molecular weight with increasing fracture

2-0*

1-5

1-0

0-8

0··6

0-5

0-4

0-3

0-2

^

Λ Square

CM U

^

niforr

andom

~U+4U

m N ^ \

0 0-5 1-0 2-0 3-0 4-0 6-0 8-0 Breaks /initial molecule {puQ, ,

Mn/2 Mn/3 Mn/A Mn/Q H /10 NAMW of degraded molecules

FIG. 10.1 Decrease in weight average for various types of initial distribution.


density is shown in Fig. 10.1 for four different initial distributions. For an initially random distribution (heavy full curve) the decrease is uniform and linear. For an initially uniform distribution it approximates to a random distribution only after an average of about five fractures per initial molecule. Mixtures of two uniform distributions occupy an inter-mediate position. From the shape of the curve relating weight average molecular weight to radiation dose, it is theoretically possible to derive information as to the initial molecular weight distribution.

An alternative form of plot, namely the reciprocal molecular weight

φ"

/

Ψ '4 f>

1 /?J

/ /on

/ No. aver ^υ,/υ'

■âge

d is t r ibu t ions^^

/Uni form

ψ

4-OII

U + 4U

jAWeight \ l y ^ a v e r a g e

//\ \ /SkRandom J dist nbu t ion

ill II

%LJp 1 2 3 4 5 6 7 8 = virtual breaks/initial molecule i~pu,) fracture

FIG. 10.2. Variation in reciprocal number and weight average with chain fracture density (for various types of initial distribution).

fracture density, is shown in Fig. 10.2. The curve obtained is always linear whatever the initial distribution if the reciprocal number average is plotted. If the reciprocal weight average is plotted, it is only linear when the initial distribution is random.

CHANGES IN INTRINSIC VISCOSITY

The reduction in intrinsic viscosity as a result of radiation induced fracture has been studied in the case of initially random and uniform distributions.

Random Distribution

An initially random distribution remains so after further random


fractures. From the relation between Mn and Mv (equation 8.21 and equation 10.4)

[η]_' J M ^ [η] I Mv

*Αα= {^\a= (pi X (10.17)

or if the fracture density p is proportional to the radiation dose r

(r+r0) e [ηΓ = ro* fo] (10.18)

where r0 is the virtual radiation dose corresponding to the initial distribu-tion. A log-log plot of [η]' against (r+r0) should therefore give a linear plot of slope -a. Experimental evidence to substantiate this equation is given in the chapters on degrading polymers.

Uniform Distribution The changes in intrinsic viscosity of an initially uniform polymer, as

radiation induced fractures located at random are added, have been analysed by Dump (1954) following the analytical method described above. Two equivalent forms are derived for Ai+a in terms of the fracture density p, and the initial virtual fracture density /?*(= l/«i), and from these the ratios of the intrinsic viscosities [η]7[η] are calculated for three

2-00

1-60

1-20 1-00

0-80 I ~

0-601

0-40

t Ö 0*30

£ l £ : J0-20L

0*15 I-

0-10

0-08

0-06 h

0-04

K -

' ^ X

.._ I - " '"Î

. l.__.l_J_._LLU4 Extrapolated from | ! high fracture density

^

- -

s

I

M ! Γ

! i

"" "

I

I I

I

' " " " " [ ■ "

- - —-

r\fr

I— — — ■

..__._

- - - - - -

- ■—

——

—

—

—

^ σ = 0 ·

<?~1

SÛf

-

- r ' Γ ■I - ' -

s

5 !

= 0 •6 7

I !

-[-

M

30 50 70 100 1 2 3 4 5 6 8 10 15 20

P/ff =pu}

Average number of radiation induced fracture per initial molecule

FIG. 10.3. Decrease in intrinsic viscosity with fracture density for various values of a; [η] = kMa (initial distribution uniform).


values of the coefficient a. These results shown plotted on a log-log scale in Fig. 10.3 are equivalent to a log-log plot of [η] against r + r 0 . Fo r high values of the fracture density (p >ρϊ) these curves approximate to straight lines, as would have been obtained if the initial molecular weight distri-but ion had been random. I t is seen that once there are on the average some four or five radiation-induced fractures per initial molecule, there is little to distinguish between the curves and those for a r andom distribution, except that the linear port ion does no t extrapolate back to unity. This difference arises from the fact that for the same initial Mn or fracture density pi, a uniform distribution has a lower intrinsic viscosity than has a r andom distribution. By a linear extrapolation of such curves it is therefore possible to at tain an estimate of the degree of non-randomness in the initial polymer.

R E F E R E N C E S

CHARLESBY, A., Proc. Roy. Soc. A224, 120, 1954. CHARLESBY, A., / . Polymer Sei. 15, 263, 1955. DURFEE, W. H. and KERTESZ, Z. I., / . Amer. Chem. Soc. 62, 1196, 1940. DURUP, J., / . Chim. Phys. 51 (2), 64, 1954. FLORY, P. J., / . Amer. Chem. Soc. 62, 2255, 1940. FREUDENBERG, K. and BLOMQVIST, G., Ber. 68, 2070, 1935. HUSEMANN, E. and SCHULZ, G. V., Z.physik. Chem. B52, 1, 1942. JELLINEK, H. H. G. and WHITE, G., / . Poly. Sei. 6, 745, 1951. KLAGES, F., Z. Phys. Chem. A159, 357, 1932. KLEINEST, T. and MOSSMER, V., Monatsheft 79, 442, 1948. KUHN, W., Ber. 63, 1503, 1930; Z. Phys. Chem. A159, 368, 1932. KUHN, W., FREUDENBERG, K., DURR, W., BOLZ, F. and STEINBRUNN, G.,

Ber. 63, 1510, 1930. MARK, H. and SIMHA, R., Trans. Faraday Soc. 36, 611, 1940. MATTHES, A., / . Prak. Chem. 162, 245, 1943. MONTROLL, E. W. and SIMHA, R., J. Chem. Phys. 8, 721, 1940. SAEMAN, J. F., MILLETT, M. and LAWTON, E. J., Ind. Eng. Chem. 44, 2848, 1952. SAKURADA, I. and OKAMURA, S., Z. Phys. Chem. A187, 289, 1940. SCHULZ, G. V. and HUSEMANN, E., Z. physik. Chem. B52, 23, 1942. SIMHA, R., / . Appl. Phys. 12, 569, 1941. TOBOLSKY, A., / . Polymer Sei. 26, 247, 1957. WATSON, W. F., Trans. Faraday Soc. 49, 1369, 1953. WOLFRAM, M. L., SOWDEN, J. C. and LASETTRE, E. N., / . Amer. Chem. Soc. 61,

1072,1939.

CHAPTER 1 1

ALTERNATIVE METHODS OF NETWORK FORMATION

THE formation of an insoluble network is not necessarily related to the presence of crosslinks of the type analysed in Chapter 9. Such networks can also arise, e.g. in condensation polymerization, in the curing of unsaturated polyesters with or without monomers, and in grafting, without there necessarily being a random distribution of tetra-functional junction points, or crosslinks. Some of these alternatives are described below.

These reactions can be usefully followed by the increase in molecular weight up to the gel point, and the subsequent decrease in soluble fraction. It is not yet possible to give a full analytical summary of these parameters for the various possible methods of network formation.

CONDENSATION REACTIONS A theory of network formation arose early in the study of condensation

polymers, obtained by reacting difunctional or polyfunctional acids and alcohols. From difunctional monomer units, only linear or cyclic polymers can be formed. Let / represent the functionality of each monomer, and assume that a proportion a has been utilized. Each bond or link formed in this way reduces the number of molecules by one, but involves two monomers. The number of bonds produced is therefore oLfAJl, where Ax

is the number of monomers. The number of molecules is therefore decreased from Ax to A1—\oifA1 and the degree of polymerization becomes

Ki = l / ( l - i « A When polyfunctional units are present ( />2) , the possibility arises of gel formation. If such a unit is linked to a neighbouring polyfunctional unit there remain /— 1 groups on the latter available to react. Of these a proportion a will do so. If a( /—1)>1, the sequence of links can grow indefinitely to form a network whereas if a( /—1)<1, the sequence peters out. For gel formation therefore

α > 1 / ( / - 1 )

and the gel point is defined by the condition

ac = l / ( / - l ) (11.1)

when the degree of polymerization

«i = 1/(1-KO = 2(f- l)I{f-2). (11.2) 170

ALTERNATIVE METHODS OF NETWORK FORMATION 171

Stockmayer (1943, 1945) has shown that the weight average degree of polymerization

w. = (l + a ) / { l - * ( / - l ) } 01.3) which becomes infinite at the gel point, when a = l/(/— 1).

This theory gives the gelation point, but no details on solubility charac-teristics, or density of crosslinking. In the radiation field, little work has been carried out on condensation reactions, for which this theory is most appropriate. For long chain polymers, where the functionality for cross-linking per molecule differs, depending on the molecular weight distribution, it is preferable to consider the process of network formation in closer detail, as in Chapter 9.

SIMULTANEOUS CROSSLINKING AND DEGRADATION

In most long chain polymers, a major effect of exposure to high energy radiation is either the fracture of side chains leading to the formation of crosslinks, with an increase in average molecular weight and the formation of a network structure, or alternatively the fracture or scission of main chain bonds, resulting in a reduction in average molecular weight. The possibility arises of both these effects occurring simultaneously; this state of affairs has been observed to occur in the irradiation of polyethylene and of other polymers in the presence of oxygen. In this section, the changes in average molecular weight and solubility are considered where both crosslinking and degradation occur at random, both being propor-tional to the radiation dose. Work along these lines has been published by Charlesby (1953), Watson (1953), Schultz (1955) and Horikx (1956).

Formally, the two processes can be considered as occurring consecutively rather than simultaneously. At first, only main chain fracture (i.e. degra-dation) takes place to the extent corresponding to the radiation dose r, and results in a modified molecular weight distribution. The second step then consists in crosslinking this new distribution, likewise to an extent depending on the radiation dose r. This separation of the two processes (which normally occur simultaneously) is only justified by the random nature of the two processes.

In the particular case where the initial molecular weight distribution is of the random type, the first process (random chain fracture) will not alter the type of distribution, but merely reduce the weight averages from a value Mw to Mw'. Subsequent crosslinking can then be treated as pre-viously, but in this case the gel point occurs when there is one crosslinked unit per molecule of weight average Mw\ a quantity which itself decreases with radiation dose. If crosslinking is a less likely process than degradation, the gel point will never be reached, however high the radiation dose.

If dashed letters are taken to represent the molecular weight distribution after degradation but before crosslinking, then the values for w / , ^ ' , ^ ' , etc., are given from the theory of degradation (10.4, 10.8):

Ui = l/(pi+p0r) = ujil+pouj) u%' = 2/(j>i+por) = «2/(l +PoUir).


The crosslinking coefficient δ' is then given by the equations

δ' = qu2 = q0ru2' = <7o«2>V(l-f/?0«i>*)

= δ/(1 +p0ulr) = W+2P- δ) (11.4)

where δ is the crosslinking coefficient in the absence of any degradation (Po = 0).

Using double dashed letters to indicate the final values after both degradation and crosslinking, equation (9.13) gives for the number average degree of polymerization u-C

1 1 0 1 . 1 1 , / <lo\ ίΛΛ ,, ux ux 2 ux 2 Ux \ 2 }

while equation (9.14) gives for the weight average degree of polymeri-zation u2

± = l - * = ~1- + - * = - + ^ - 0 - 4 · . (11.6) «2 ih w2 2 u2 \ 2 ]

Both 1/w/' and \\u%" are therefore linear with radiation dose r but have different coefficients. The gelling dose rgei is obtained from the condition that u2 becomes infinite, i.e.

— = rge\(qQ-pol2). u2

The relation between sol fraction s, and crosslinking coefficient δ' (or crosslinking index γ'), is from (9.24 and 11 -4)

s + Vs = 2/δ' = Ι/γ'

— 2 ^ +Pouir) __ 1 +PoUir q0u2r q0Uxr

= Polqo + l/qoihr.

(11.7)

The gelation dose rgei equals ll(2q0—p0)iix as against lj2q0Ux in the absence of simultaneous fracture.

The effect of fracture on the soluble fraction is equivalent to cross-linking only by a reduced radiation dose r\ where r' = r/(l-\-p0Uir). Unlike r, r cannot increase indefinitely; its limiting value is 1/poUi. Then

s + Vs = 2/δ = 2\qQu2rf = 2(1 +p0Uif)lqoU2r

as before. Fig. 11.1 shows the change produced in the shape of the sol/δ (or reciprocal dose) curve, for various degrees of main chain fracture.


For high doses, r tends to infinity and s+Vs has a limiting value of Polio, when the soluble fraction is

J«, = Polio + 0-5-0-5 (l+4/>oteo)*. (Π.8)

If p0>2q0, s cannot decrease below 1, and the polymer remains completely soluble, whatever the radiation dose. Since there are two crosslinked units per crosslink, this condition states that unless the number of radiation

FIG. 11.1. Simultaneous crosslinking and degradation. Figures denote ratio of degradation to crosslinking (pQ/q0) for an initially random distribution.

induced crosslinks exceeds the number of radiation induced main chain fractures, no network will be formed. However even under this condition, when no gel is obtained, the specimen will be profoundly modified by radiation as linear molecules are replaced by increasingly branched ones.

In the general case, where the initial molecular weight distribution follows an arbitrary law, the problem is still theoretically soluble by the same methods. The initial distribution defined by A0, Al9 A2. . . etc., becomes modified by fracture to one defined by A'0, A\9 A\ . . . in the manner described in the chapter on degradation. The soluble fraction s is then calculated for this new distribution using the general treatment of Chapter 9. For large radiation doses, however, this procedure is un-necessary. When the number of fractures per average initial molecule is large (greater than about three) the initial distribution no longer matters greatly and may be replaced by a random distribution of the same initial number average molecular weight. The relation between sol fraction and radiation dose in terms of the parameters p0 and q0i as given above, can then be used. For an initially random distribution a plot of s + Vs against 1/r gives a straight line of slope l/#0«i and an intercept p0/q0 (r -> oo); for a non-random distribution the plot of s+ Vs against 1/r will not be linear but will be asymptotic to this plot.

Fig. 11.2 shows the effect of simultaneous crosslinking and main chain fracture on the crosslinking coefficient δ (referred to the initial weight


average) in the case of initially uniform, random, and pseudo-random distributions.

O CD

<- σ

CO

Pseudo-random Random

1 7 /

>^ Uniform

0-5 1-0 1-5 Po/Qo - r Q t ' ° of fracture to crosslinking

FIG. 11.2. Increase in crosslinking coefficient 8 at the gel point due to simultaneous fracture.

ENDLINKING

In network formation by crosslinking, molecules are linked by lateral bonds to form a tetra-functional junction point.

A network can also be formed when the end of one molecule is linked to the side of another. This process, termed endlinking, gives rise to a tri-functional junction point | . Each such link therefore involves the reaction between the end of one molecule, and a side group on the other. The precise chemical process by which endlinking can occur has not been studied in any detail. It may be envisaged as arising from a reaction between two radicals, one terminal on one molecule, the other lateral on the other; alternatively it may be formed as a result of the attack of an active end group of one molecule on a neighbouring neutral mole-cule. In either case problems of chemical mechanism arise which are similar in character to those involved in any explanation of crosslinking. That some process of endlinking does occur in irradiated polymers is, however, shown by the formation of intermediate products CM+rH2w+2r+2 from irradiated long chain paraffins CMH2M+2 ( l<r</ i—1) . Crosslinking can only give rise to the dimer C2wH4n+2, the trimer C3»H6n+2, etc.

The mathematical analysis of the process of endlinking has been carried by Charlesby (1955) in the case of a random molecular weight distribution


(equation 8.20). This assumption has the advantage that random main chain fracture will eventually give rise to such a distribution whatever the initial distribution of molecular weight.

If the number of molecules (after radiation) is A, the number of free ends is 2A. Of these, a proportion a is assumed to be capable of forming endlinks. Thus there will remain, after endlinking, 2Acc links and 2,4(1 —a) free ends. The sol fraction is shown to be

«-(£)'■ <"·»» By definition gel formation first occurs when s becomes less than unity.

The gel point is therefore determined by the condition

or a - 0 - 2 5 . (11.10)

Assume now that in the specimen all radiation induced main chain fractures give free ends which are sufficiently active to lead to end links. The initial ends of the molecules are taken to remain inactive. Then for a specimen containing initially A0 molecules (2A 0 stable free ends) and A1 monomer units, there will be formed

2A,w x 104 x 10-« Gr

active end groups (w being the monomer weight, G the number of main chain fractures per 100 eV absorbed, r the radiation dose in megarads). The proportion of all end groups which are active

α =2^1>νχ1·04χ10-6σΓ/(2^1>νχ1Ό4χ10-6σ/ '+2Λ). (Π .Π)

Incipient gel formation, which occurs at a = 0-25, then corresponds to a radiation dose

rge l 2>wAx 104xlO-6G

_ 0-32 x l 0 6 _ 0-64 x lO 6

~ MnG MWG

so that rgQlMw = 0-64 x 106/C (11.12)

This formula is to be compared with that for crosslinking (9.19)

rge\Mw =0'96xlO*/G

where G now refers to the number of crosslinked units per 100 eV of energy absorption.


The relation between sol fraction s, and radiation dose r for endlinking by radiation

Λ \ 3 Λ Η Ί · 0 4 Χ 1 0 - ^

(3-12X10-6MMGA·)-2 (11.13) if r>rge\.

Fig. 11.3 compares the sol-fracture curve for endlinking with the sol-crosslinking curve for an initially random distribution. It is seen that the shape of the two curves do not differ greatly, and experimentally the end-linking curve could be readily confused with that for crosslinking with a slightly modified initial molecular weight distribution.

Radiat ion- induced f ractures/ in i t ia l (number average) molecule 0-25 0-5 1 2 3 4 5

UUj

60

4-U

20

10 8 6

4

2

1

\ \ \ λ \ s

\ V

\ \ \ \ \ \ \ \

\

\ \ \ \

FIG.

0-2 0-33 0-5 1 2 3 4 5 0-40-67 1 2 4 6 8 10

Links/init ial (number average) molecule upper scale, crosslinking lower scale, endlinking

11.3. Comparison of sol-dose curves for crosslinking — , (initial distribution random). (From Charlesby, 1955.)

and endlinking

A fuller analysis (Charlesby, 1955) gives values for the decrease in the number of sol molecules, of endlinks in the sol and gel, and of the density of links in the sol or gel fraction as the radiation dose is increased. Thus, the number average molecular weight increases from its initial value Mn to a maximum value 1·5ΜΜ at the gel point and subsequently decreases (in the sol). For endlinked networks, the average molecular weight between links


Mc = 3Mn v 7 ( l - s ) / ( l - V 7 ) 5 / 2 (11.14)

which when the density of endlinks is high (s -> 0) gives

Mc = 3Mn/312 X 10-6 MnGr = 0-96 χ W/Gr. (11.15)

This expression is identical with that derived for crosslinking, if G is taken to refer to the number of crosslinked units per 100 eV absorbed.

If only a fraction a of the ends produced by main chain fracture are active, and form endlinks

_ a2Axw x l 04x10-6Gr α ~ 2Axw X 1-04 x lO-«Gr+2A0

/ ΐ - α \ 2 (2A0+(l-ä)2A1wxV04xl06Gr\2

so that s = ^—J ^ 3 * 2 ^ χ 1-04 x l O - G r )

and s = 1 if rgQ{ = 0-96 x 10«IMnG(4a—1). (11.16)

No gel formation is therefore possible if a < 0-25. Again the minimum sol fraction for an infinitely large dose

Joo =(l-a)2/9a2. (11.17)

This result is to be compared with that derived above (equation 11.8) for simultaneous crosslinking and degradation.

It may be concluded that, at least mathematically, there is little to choose between crosslinking and endlinking as an explanation of network forma-tion, solubility, swelling or elastic modulus. Such quantitative differences as occur may be readily obscured by minor differences in molecular weight distribution.

The results caji be expressed in the form that for incipient gel formation one endlink is required per three inactive molecular ends, i.e. one active end in four ends. This derivation applies only to an initially random distribution. No analysis has been published for an arbitrary molecular weight distribution.

NETWORK FORMATION AS A CHAIN REACTION

Insoluble networks may be formed by a chain reaction, e.g. by irradiation of unsaturated polyesters, or by excessive grafting. The characteristics of such networks differ fundamentally from those produced by the cross-linking or endlinking process, in which each link requires separate activa-tion by one (or possibly two) acts of ionization or excitation. In the net-work formation considered here considerable changes in solubility are produced by very low doses showing that a chain reaction is involved; a single ionization or excitation gives rise to a number of links. Further-more, the relation between sol fraction s and crosslinking index or


coefficient (γ or δ) differs widely from that observed by conventional crossHnking. In crosslinking there is an approximately linear relation between log s and log γ (depending on initial molecular weight distribution) whereas in the chain reacting system an approximately linear relation is obtained between log s and γ. A mathematical analysis of this type of reaction (Charlesby, 1957) is based on the following assumptions:

(i) radiation initiates a chain reaction, analogous to a polymeri-zation, each chain formed linking together a number of the polymer molecules initially present;

(ii) the growth of the polymerization chain is inhibited by some inherent process such as resonance stabilization, and therefore differs from conventional polymerization, in which chains termi-nate by combination, disproportionation or chain transfer, and where a dependence on radiation intensity is observed.*

Conventional crosslinking

(5 crosslinks or 10 cross I inked units)

L/~ Conventional crosslinking. Each link requires one (or two) separate radiation events. For the five polymer molecules shown 5 to 10 radiation events are required.

Linking by a chain reaction

B

Chain reaction in network formation. A polymerization chain initiated at A grows until inhibited at B. A single radiation event at A links together the five polyester molecules (/^-Ό·2).

FIG. 11.4. Network formation by conventional crosslinking, and by a chain reaction.

If the inhibitor effect is represented by / the average number of links per chain is (1 — /)//. Analysis then shows that the crosslinking coefficient at the gel point

i Vi-1 2(l·^/)

sgei=_-^7 τττ—* ULI»)

both for an initially uniform or random distribution. vx is the number of possible sites per molecule for linking, so that the factor νγΙ(νχ—1) is a

* The analysis for such chain reactions, leading to the growth of linear or branched polymers, is given in Chapter 22.

ALTERNATIVE MËÎHÔDS ÖF NETWORK FORMATION 179

correction factor for molecules with a limited number of such sites. For a long chain polymer v2 > 1, and if / is small

8gei~//2 (11.19)

as compared with $gei = 1 for conventional crosslinking. Each polymerization chain is assumed to be initiated by a single radiation

event. If the value for chain initiation is G

8 = 1-04 xlQ-*Gr MJi (11.20)

where i is an "amplification" factor, arising from the number of links resulting from each initiation. This relation is to be compared with

δ = V04xlO-GGr Mw

for conventional crosslinking, where G now refers to the number of crosslinked units per 100 eV absorbed.

The gel point is then related to the gelation dose

r* = W 1 0 4 X 1 0 - G M , = - ^ ^ ^ ^ ^

~0'5xl0*i*/GMw. (11.21)

The gelation dose is therefore smaller, by a factor of the order of i2, than the gelation dose for crosslinking. If ί ' ^ Ο Ί , this leads to a very small gelation dose of a fraction of a megarad, even for relatively low molecular weight polymers.

The shape of the sol/dose curve is of the form

.y = exp(-Y) (11.22)

for an initial uniform molecular weight (Fig. 11.5). Only slight deviations occur for other molecular weight distributions, or molecules of finite size. A solubility of 37 per cent is obtained if γ —' 1, or if the dose

r ~ 10« i/GMw (11.23) compared with

r ~ WIGMW

for conventional crosslinking. The ratio is only /, as against i2 for incipient gel formation. The exponential decrease in solubility also results in a very rapid change in network structure for small doses.

N


0*8 X \ N

1 2 Crosslinking index y

FIG. 11.5. Solubility of a network formed by a chain reaction.

Top curve: initial distribution uniform. Bottom curve: initial distribution random.

Figures in brackets indicate the average number of sites υ per molecule capable of forming a link, and the inhibitor effect /. The number of molecules involved in each chain is \ji. Of particular interest are the low crosslinking index for gel formation, and

the similar shape of the curves down to about 30 per cent solubility.

(From Charlesby, 1957)


R E F E R E N C E S

BASKETT, A. C , Simposio Internaz. di Chimica Macrom., Milan-Turin, 1954. CHARLESBY, A., / . Polymer Sei. 11, 513, 1953; Proc. Roy. Soc. kill, 60, 1954;

A231, 521, 1955; A241, 495, 1957. FLORY, P. J., Principles of Polymer Chemistry, Cornell, 1953. FRITH, E. M., and TUCKETT, R. F., Linear Polymers, Longmans Green, 1951. HORIKX, M. M., J. Polymer Sei. 19, 445, 1956. OKAMOTO, H., and ISIHARA, A., Polymer Sei. 20, 115, 1956. SHULTZ, A. R., Nucl. Eng. and Sei. Congress, Cleveland, Dec. 1955. STOCKMAYER, Advancing Fronts in Chemistry, Reinhold, 1945. STOCKMAYER, / . Chem. Phys. 11, 45, 1943. WATSON, W. F., Trans. Faraday Soc. 49 (11), 1369, 1953.

CHAPTER 12

RADIATION-INDUCED CHANGES IN ORGANIC MOLECULES

A CONSIDERABLE number of chemical reactions initiated in organic mole-cules by exposure to high energy radiation have been studied in the solid, liquid and gaseous state, as well as in solution. Certain of these may serve as models for reactions occurring when long chain polymers are irradiated.

The precise sequence of events leading to the observed changes is still under very active discussion. The initial acts are primarily ionization, excitation and the production of free electrons ; the ejection of an atomic nucleus is far less frequent even with very high energy radiation, and such nuclear displacements may be neglected. Many of the subsequent chemical reactions are, however, typical of radicals, i.e. uncharged mole-cules with an unpaired electron. It is therefore often assumed as a working hypothesis that the electron is captured by an ion to produce a highly excited molecule. If this process takes place sufficiently rapidly, the individual atoms will not have had time to change their interatomic distances (Franck-Condon principle). The excited molecule may have a potential energy greater than when the atoms are further apart, and may consequently decompose. An alternative hypothesis is that the molecule breaks down while still in the ionic form, and then recaptures an electron.

In most organic systems the method of decomposition is not unique and a range of products is obtained. Attempts have been made to deduce the breakdown process from the observed final products. Other methods of investigating the intermediate reactions include the study of irradiated solutions, and the incorporation of additives such as iodine or diphenyl picryl hydrazyl (DPPH) which act as scavengers for the radicals formed in the intermediate stage, and give modified products (Chapter 27). This work will not be discussed here ; only the final products obtained from the radiation of simple organic structures will be described in so far as these may be compared with the chemical changes observed in irradiated may be compared with the chemical modifications observed in irradiated polymers.

Among the chemical reactions observed to occur under radiation are oxidation and reduction, decarboxylation and deamination, halogenation, nitration and acetylation. Changes in isomerism, addition and conden-sation polymerization, and dimerization have also been reported. The presence of oxygen nearly always has a considerable influence on the products obtained. A summary of such reactions is shown in Table 12.1. Several excellent reviews on the subject have been published (Collinson and Swallow, 1955, 1956; Tolbert and Lemmon, 1955).

182

RADIATION INDUCED CHANGES IN ORGANIC MOLECULES 183

Table 12.1. Some Chemical Effects of Radiation

Substance irradiated

Methane

Ethylene

Acetylene

Vinyl compounds

Chloroform

Methyl iodide

Primary alcohols

Hydroxy acids

Carboxylic acids

Benzene

Conditions

Pure Oxygen present Iodine present In aqueous solution, oxygen present

Pure In aqueous solution, oxygen present

Pure Oxygen present

Pure or in solution, normal dose rate Pure, very high dose rate

Oxygen present

Pure

Pure, a-particles Pure, ß- or γ-radiation In aqueous solution, oxygen present

In aqueous solution, oxygen present

Pure

In aqueous solution, oxygen present In aqueous solution

Pure Chlorine present

In aqueous solution, oxygen present In aqueous solution, hydro-gen present

Major products detected

Ethane, hydrogen C0 2 , water Methyl iodide, hydrogen Formaldehyde, CH3OOH

Polyethylene CH3CHO

Polymer, some benzene co2, co

Polymer

Low molecular weight pro-ducts

CCI3 OOH

Ethane, iodine

Hydrogen, glycol, aldehyde Glycol, no aldehyde Aldehyde

Keto acid

Hydrogen, unsaturated acid, C0 2 , hydrocarbon Per-acids

Dicarboxylic acid, hydrogen

Some polymer 1 : 2 : 3 : 4 : 5 : 6 - hexachloro-cvc/tfhexane

Phenol

Diphenyl


Table 12.1—continued

Substance irradiated

Oxidation-reduction indicators

Steroids

Amino acids

Thiols

Polymer [CH2—CHR]M

Polymer [CH2— CR1R2]„ Ri, R2 Φ H

Polysaccharides

Nucleic acids

Conditions

In aqueous solution, oxy-gen present In aqueous solution, air-free, added organic sub-stance present

In organic solvent, oxy-gen present In aqueous solution, no oxygen present

In aqueous solution

In aqueous solution, espe-cially with oxygen present

Pure

Pure

Pure or in solution

Pure or in solution

Major products detected

Irreversible oxidation

Reduction

Oxidation at certain posi-tions only Hydrogénation, dehy-droxylation

Ammonia

Disulphide

Crosslinking

Degradation

Degradation

Degradation

(Based on Collinson and Swallow, 1956)

PARAFFINS The simplest system to consider is that of the long chain paraffin mole-

cule, either linear or branched. Here the major changes are: evolution of hydrogen and of lower molecular weight paraffins; the production of unsaturated molecules such as ethylene; and the linking together of mole-cules (dimerization). Recently Dewhurst (1956) and Davison (1957) have drawn attention to the formation of products intermediate between the original molecule and its dimer.

These changes can be ascribed to the fracture of C—H and C—C bonds, and the subsequent reaction of the radicals formed. Table 12.2 shows the G values for some «-paraffins, i.e. the number of the original molecules altered, or the number of molecules produced per 100 eV of energy cap-tured. The major product of irradiation is hydrogen, and the evolution of each hydrogen molecule is accompanied by the formation of a double bond C = C in the main chain, or by the linking together of two molecules either side by side (crosslinking) or end to side or end (endlinking).


Cyclization may be considered as a special form of the latter. The irradia-tion of methane, for example, leads to hydrogen evolution and the forma-tion of ethane, possibly by a reaction between methyl radicals

CH4* > CH3- + H CH3· -f- CH3·—> C2H6

H + H > H2

An alternative mechanism may be abstraction by a highly energetic H atom (Ht) ; two methyl radicals are then produced by a single ionization or excitation:

CH4* > CH3· + Hf Hf + CH4 > H2 + CH3-CH3- + CH3--> C2H6

An ionic reaction may also be envisaged : CH3+ + CH4->C2H5- + H2

The formation of unsaturation in a linear paraffin may be expected to occur with a highly energetic hydrogen atom

C2H5 CH2 CH2 C2H5 > C2H5 C H CH2 C2H5 -f- Ht C2Hs C H CH2 C2Hs -}- Ht -*■ C2Hs CH CH C2Hs ~\- H2

-> C2H5 C H = CH C2H5 -f- H2

This process may be visualized as an alternative to the linking process : if the hydrogen atom attacks a neutral molecule it produces a radical mole-cule which will eventually link to a neighbour; if it attacks a radical molecule unsaturation may occur. Recent experiments have indicated that some hydrogen molecules are evolved directly by a "molecular" process, and would therefore form an unsaturated molecule without requiring the existence of a separate H atom.

Of considerable interest is the large proportion of hydrogen evolved from irradiated paraffins, and indeed from most irradiated polymers. If the products evolved were to depend only on the number of bonds avail-able for fracture, and on their strength, a much higher proportion of low molecular weight hydrocarbons would be expected to form, since the C—C bond is weaker than the C—H bond. That this does not in fact occur has been ascribed in part to the "cage" effect. When C—C bonds are broken, the large fragments obtained are held in position by the neigh-bouring molecules, and are very likely to react with one another, or recombine. When C—H bonds are broken, the H atom has a much better chance of diffusing away before recombination takes place. This explana-tion does not, however, account for the observed effects in the irradiation of paraffins in the gaseous state, where hydrogen production is the pre-dominant reaction, although no cage effect can limit the products formed in the gas.

The published data on the irradiation of paraffins include the use of various sources, of different ionizing power, but the results expressed in

Tab

le 1

2.2.

G

Val

ues

for

Dec

ompo

siti

on

of P

araf

fins

Com

poun

d

Met

hane

E

than

e Pr

opan

e B

utan

e /z

-Hex

ane

«-H

epta

ne

77-D

ecan

e /z

-Tet

rade

cane

C

yc/o

hexa

ne

Stat

e

Gas

G

as

Gas

G

as

Liqu

id

Liqu

id

Liqu

id

Liqu

id

Liqu

id

Rad

iati

on

6 M

eVa(

rado

n)

6 M

eVa(

rado

n)

6 M

eVa(

rado

n)

6 M

eVa(

rado

n)

017

MeV

(ele

ctro

ns)

1-3

MeV

(y)

Elec

tron

s 0-

17 M

eV e

lect

rons

0-

17 M

eV e

lect

rons

1-

5, 0

17

MeV

ele

ctro

ns

20 M

eV d

eutr

ons

2 M

eV e

lect

rons

G (

deco

mp.

)

8-6

6-6

6-6

6-9 ? 7-2-

7-6

6-8

G(H

2)

6-5

50

4-3

4-8

41

4-9

(+ 0

-2)

4-2

3-5

3-4

4-4

5-1,

5-4

5-

25

G(C

H4)

10

1-17

0-

88

0-33

0-

41 (

+ 0

1)

0-22

00

9 00

6 00

9

G(C

2H6)

1-3

0-68

0-

96

0-69

(±

01

)

0-21

G (

othe

rs)

(C3H

8)0-

34

(C=

C)4

±2

Ref

eren

ces:

Tol

bert

and

Lem

mon

(19

55);

Kre

nz (

1955

). Se

e al

so D

ewhu

rst

(195

7): G

(H2)

= 5

±0-2

fro

m C

e to

C1&

.

186 ATOMIC R A D I A T I O N AND POLYMERS


terms of a G value for hydrogen production do not differ very greatly as between proton, deuteron and a-particles, which give a high density of ions in a liquid or solid, and fast electrons or γ-particles, where the density is considerably smaller. This may be seen in some of the earlier results shown in Table 12.2 and particularly in the data for cyclohexanc examined by Schüler and Allen (1955). For 2 MeV electrons they found a G value for hydrogen production of 5-25; for a- and deutron irradiation using a cyclotron, similar values of between 51 and 5-5 were obtained, although the density of ionization was a hundredfold greater. This inde-pendence of yield on the ionization density contrasts with the observed variation by a factor of over three, in the G value for the oxidation of ferrous sulphate in aqueous solution.

This independence of yield also applies to the rate at which energy is absorbed within the system. The same products are obtained, whether the same amount of energy is absorbed in a matter of seconds, as with a fast electron beam, or in days, as with weak γ-radiation.* This greatly simplifies the use of G values, since the conditions of radiation need not be fully specified, and results obtained under one set of conditions can be readily extended to very different conditions of radiation.

The G value for hydrogen production decreases slightly with increasing molecular weight of «-paraffins, while the methane yield decreases very rapidly as the length of the molecular chain grows. These values can be extrapolated to very high molecular weight paraffins, but their application to irradiated polyethylene must be treated with caution, since some ethylene polymer contains side chains which are very liable to decom-position or evolution under radiation.

Recent measurements by Davison (1957) has thrown some doubt on the G values for the hydrocarbon products from the irradiation of paraffins. Using a gas Chromatographie technique, he obtained evidence of the formation of unsaturated compounds such as ethylene, and of inter-mediate products. His results are summarized in Table 12.3. Davison found that the yields are independent of dose, at least at low dose rates,

Table 12.3. G Values for Some Paraffins

Paraffin

jz-Pentane /z-Hexane «-Heptane 2:2-Dimethylbutane

G(H2)

4-5 4 0 3-9 2-2

C(CH4)

021 012 009 009

C(C2H4)

0 4 022 018 0 04

G(C2H6)

0-75 036 025 014

G(C3H6)

0-75 0 2 0C9 001

G(C3H8)

0-75 0-36 0-27 002

* Very different conditions are present in the electrical discharge. Although ioniza-tion still occurs, the products obtained are very different from those found by exposure to high energy radiation. Wiener and Burton (1953) found that in a high intensity electrical discharge the gaseous products from methane are hydrogen and acetylene. At lower pressures (2 mm Hg) a flow discharge gives much larger amounts of ethane (Fischer and Peters, 1929).


and also of intensity, thereby showing that these products are not obtained by secondary effects of radiation on the primary product.

The absolute accuracy of these results is estimated at about 10 per cent, but the relative values are more accurate. An interesting feature is that, in contradiction to the earlier data, products such as ethane and propane have much greater yields than methane. This contradicts the statistical theory according to which all C—C bonds are equally likely to be broken.

Branching

In irradiated paraffins the number of radicals produced is not greatly affected by branching, but the distribution of final products is very different. By measuring iodine uptake in irradiated paraffins Weber, Forsyth and Schüler (1955) deduced the radical yield relative to that in «-heptane. Their results are summarized in Table 12.4, and show very little difference as between linear branched and cyclic hydrocarbons. The number of radicals formed is twice the loss of iodine molecules given in the table.

The effect of branching or cyclization on the hydrogen yield is shown in Table 12.5. These results were obtained by Schoepfle and Fellows at an unspecified dose rate using a 170 kV electron beam, at a current of 300μΑ for 30 min. The yield of methane in cyclohexanc is lower than in the normal hexane, as would be expected since in a cyclic structure two C—C bonds must be broken to allow methyl or méthylène radicals to be produced. The presence of branching greatly reduces the hydrogen yield in dimethyl or trimethyl hexane, while the methane yield is greatly increased. Branches are particularly susceptible to radiation fracture and this tendency is also observed in many long chain polymers. The increased percentage of product shown as not volatile at liquid air temperature is not explained.

Table 12A. Effect of Molecular Structure (Linear Branched or Cyclic) on Total Radical Yield (Measured by Disappearance of Iodine)

«-Pentane «-Hexane

«-Heptane

«-Octane

«-Nonane

G(--h) Linear

3-7, 4-2 3-8

3.4*

3 3 , 4-3,

3-7 Cyclic

Cyc/öpentane Oc/ohexane

3-8, 3-4 2-6-3-3

3-5

Source: Weber,

G(-h) 1

Branched /sopentane 2-methylpentane 3-methylpentane 2:2-dimethylbutane 2:3-dimethylpentane 2:4-dimethylpentane 2:3-dimethylhexane 2:4-dimethylhexane 2:2:4-trimethylpentane 2:2:5-trimethylhexane

3-9, 4 0 , 4-3 3-7, 3-2 3-5, 3-9 4-4, 4 1 4-2, 4-4 4 1 , 3-9 4-4, 4-5 3-7, 4 0 4-2 4-4, 4 0

Cyclic branched Methylcyc/öpentane Methylcyc/ö hexane

Forsyth and Schuler (1955). * Standard.

3-7, 3-8 3-1,3-2


Table 12.5 Gaseous Products from Irradiated Paraffins (0*17 MeV Electrons)

Paraffin

w-Hexane //-Heptane «-Octane Λ-Decane Λ-Tetradecane cjc/öHexane 2:5-dimethylhexane 2:2:4-Trimethylhexane Methylcyc/öhexane

cm3 gas at S.T.P.

57-6 51-4 48-3 41-6 34-9 45-8 49-8 50-3 39-2

% Hydrogen

66-3 76-9 78-8 78-9 911 88-9 421 351 82-8

% Methane

5-3 3-9 2-8 21 1-6 1-3

116 152 31

% Non-volatile in liquid air

27-9 192 18-3 18-2 69 90

46-2 48-2 130

Source: Schoepfle and Fellows (1931).

Dimerization Of particular importance to the polymer field is the formation of dimers,

by linking together of pairs of paraffin molecules. The process may be considered as due to the combination of two radical chains (hydrogen atoms omitted for clarity).

C—Ç—C—C C — C — C — C I

c—C—c—c c—c—c—c Because of the different mobilities one would expect to observe marked differences in the product, depending on whether irradiation takes place

80

8 60 -4-> o

c/) υ

- 40 _£> "iö o υ ω· *> M 20 ω ç k

0 1000 2000 4000 Dose, megarads

FIG. 12.1 Increase in viscosity of an alkyl benzene due to crosslinking. Initial M.W. 250.


in solid or in liquid paraffins. No striking differences have been reported, and the results of Charlesby (1954) on the pile irradiation of liquid and solid paraffins described on p. 209 indicate no discontinuity in G values as the molecular chain length is increased, from paraffins molten under radiation conditions to those which were still solid.

The effect of dimerization is to increase the average molecular weight, and to lead eventually to the formation of a closed network. These changes are most conveniently followed by physico-chemical techniques, similar to those used for the study of irradiated polymers ; they are there-fore conveniently discussed at the same time as those in polyethylene (Chapter 13). The G values from dimer formation derived from these results of the order of 3-5, are in fair agreement with estimates based on chemical analyses.

The phenomenon of dimerization (or crosslinking) applies to a large number of organic molecules, and is of considerable importance when organic molecules are used in reactors, owing to the sharp increase in viscosity resulting from the increase in molecular weight (Fig. 12.1).

Unsaturated Aliphatic Molecules

With few exceptions the changes produced by radiation in a range of olefin and acetylene molecules have not been studied in the same detail as have the paraffins. Many of the reactions observed are complicated by chain reactions which lead to polymer formation. This is notably the case for ethylene, whose polymerization to polyethylene has been studied in considerable detail, and for acetylene which is transformed to a solid like cuprene. For such reactions the overall yield may be very considerable, and G values for the polymerization of ethylene of up to several thousand have been reported in the literature. However, most of these reactions are chain reactions, in which radiation only intervenes in the initiation process. The subsequent reaction of propagation is independent of the radiation and any other means of initiation would lead to the same high yields. Only the initial stages of the polymerization process should there-fore be considered as specific to radiation.

In those aliphatic molecules which do not show a chain reaction under irradiation, the yield of hydrogen is only about one quarter of that of the corresponding paraffin molecule. Part of this difference may arise from the tendency of the hydrogen produced to hydrogenate unsaturated bonds. There is tentative evidence that whereas olefins with a low degree of unsaturation show an increase in the unsaturation as the radiation dose is increased, this is not the case for molecules which are originally highly unsaturated. For olefins irradiated in the nuclear reactor at Harwell, the critical figure for unsaturation was about one C—C per 20 carbon atoms. Below this degree of unsaturation, exposure to high energy radiation increased the degree of unsaturation; above it, the effect of radiation was to decrease the degree of unsaturation.

In unsaturated aliphatic compounds, dimerization may occur in a similar manner to that observed in irradiated paraffins. The energy required for


the formation of a link between two molecules has been measured as a function of chain length and of the position of the double bond along the chain (Charlesby, 1955). G values for linking decene 1 and octadecene 1 are 10-9-10-3 respectively. In the case of the octadecene series, the G values decrease as the double bond moves towards the centre of the mole-cule and approximate to the values observed for fully saturated octa-decane. The same tendency is found in the acetylene series, unsaturation towards the end of a molecule rendering the molecule much more sensitive to radiation. Table 12.6 summarizes these results. In addition to changes in unsaturation and dimerization, changes in isomeric structure have also been observed in the octadecene series. Whether the eis- or trans-isomer is irradiated, the product tends towards an equilibrium mixture of eis and trans containing about 35 per cent of trans material. The ds-isomer is slightly more reactive than the trans-isomcr in that dimerization occurs more readily.

Table 12.6. G Value per cross linked unit

Compound

n-paraffins Cio c18 ^ 2 8

C32 olefins C10 (-18

eis Ci g trans C18

acetylenes C10 C17

C18 C22 C28 C32 diene

2:5-dimethyl hexadiene 1-5

6 6 * 7 0 7 8

139

1

109 10 3

— —

14 3 18 3 170

Position of unsaturation

2 3 4 5 6 7

9 2 7-3 6 9 — 6 3 5 9 9-8 7-7 7 0 — 6-4 6 1

8

6 1 —

9

6 4 6 1

7-1(11) 7.4(9) 84(i6)

Figures in brackets indicate position of unsaturation. Source: Charlesby (1955).

* Taken as standard.

Aromatic Hydrocarbons Aromatic hydrocarbons are remarkably stable to radiation and the

yields, expressed in terms of molecules changed or gases evolved, are often


lower by an order of magnitude than the yields for corresponding aliphatic hydrocarbons. This radiation protection has been ascribed to the resonant structure of the benzene ring which enables considerable excitation energy to be absorbed without bond rupture. It may perhaps be considered that the ejection of an electron still leaves a sufficient number of bonding electrons to give the system a stable structure. This protection is not confined to the molecule itself and may extend to neighbouring non-aromatic hydrocarbons if the lowest excited state of the molecule is above that of benzene (see Chapter 26). Due to the reduced protection offered, the extent of the reaction increases with the length of the alkyl side chain. For example, the G value for hydrogen evolution in benzene is 0Ό36, in toluene 0-13 and in ethyl benzene 0-18, while the G values for production of methane are 0-0012, 0-008 and 0-03 respectively.

Polymer formation may take place on irradiation but the yields for benzene and toluene are low (0-76 and 1-28 respectively). Styrene poly-merizes but less readily than methyl methacrylate.

Table 12.7 shows the G values for radical production, measured with iodine or diphenylpicryl hydrazyl as a scavenger; for normal paraffins the corresponding values are about 7. The total amounts of gas evolved by aromatics such as benzene, toluene, are only about 5 per cent of those from «-paraffins (Schoepfle and Fellows, 1931), while naphtha-lene, anthracene and diphenyl give very much lower yields.

Table 12.7. Comparison of Radical Yield in Aromatics and Aliphatics

Benzene Toluene Xylene Ethyl benzene Heptane cyc/oHexane

Iodine1

[ G ]

066 2 4 2-5 2-8 6-8 6 6

DPPH 2

[G]

115, 0-83

2 0

6 9

1 Weber, Forsyth, Schuler (1955). 2 Bouby et ai, (1955), etc., corrected for G(Fe) = 15-5. 8 Wild (1954).

Car boxy lie Acids

The effect of radiation on carboxylic acids has been studied in some detail and the effect of chain length and of temperature on the final products have also been investigated.

The major gaseous products of irradiation are hydrogen, carbon monoxide and dioxide and water, due to decarboxylation, and hydrogen evolution from C—H fracture along the main chain. Methane and ethane are only found in appreciable concentrations (above 1 per cent) in irradiated acetic acid. In palmitic acid, for example, the yield of CO and C 0 2 is about equal to the yield of hydrogen, although the energy absorbed


directly in the acid end group only constitutes about one-sixth of the total energy absorption. Main chain fracture of C—C bonds appears to be almost negligible. These effects offer a good example of the reaction loosely termed energy transfer; although energy is absorbed at random along the chain, the end group suffers a disproportionate amount of change.

Table 12.8 shows the G values for C02, CO and hydrogen formation and in addition the proportion of the total weight constituted by the terminal —COOH group. The decrease in the total G value for carbon monoxide and dioxide is roughly parallel with the decrease in the fractional weight of the COOH group in the specimen (except for acetic acid).

An interesting observation is that decarboxylation tends to increase with temperature. C 0 2 production increases from a G value of 1-4 at —70°C to 2-8 at 45°C.

As might be expected from the low reactivity of aromatic compounds under radiation, benzoic acid is relatively stable to radiation. G values for H2, CO and C 0 2 production are low: 00026, 00025 and 0-286 respectively. Polymer formation is the most frequent occurrence, with a G value of 0-3.

It has been suggested that hydrocarbons found in nature may arise from the decarboxylation of aliphatic acids by naturally occurring radio-activity. Such radiation would also be expected to produce dehydrogena-tion. The absence of hydrogen from the products found in nature may be due to diffusion, as in most irradiated polymers hydrogen is readily lost. Dehydrogenation would also be expected to result in crosslinking and unsaturation in the remaining products.

Table 12.8. Radiation Changes in Carboxylic Acids 6 Me Va (radon) radiation

CwH2w_t-iCOOH

Acetic Octanoic (Caprylic) Dodecanoic (Laurie) Hexadecanoic (Palmitic) Eicosanoic (Arachidic) Docosanoic (Behenic) Triacontanoic (Melissic)

n

1 7

11

15

19

21

29

M.W.

60 144

200

256

282

341

453

G value

H2

0 9 14

1-5

1-7

14

12

2-3

CO

0 5 0 5

0 4

0 3

0 2

0 1

0 2

co2

2-8 2-2

1-5

1-3

0-7

0 5

0 8

Weight fraction

COOH CnH2n+iCOOH

0-75 031

0-22

0176

016

013

010

From Breger and Burton (1946), Breger (1948), Burton (1949).


Aliphatic Alcohols

McDonell and Newton (1954) have carried out a detailed study of the reaction products obtained when alcohols are irradiated with 28 MeV helium particles. The principal site of bond rupture is at the carbinol-carbon atom, the overall G value for alcohol dissociation being approxi-mately 5. Hydrogen gas, hydrocarbons and carbonyl are the major products. The primary oxidized products are aldehydes and glycols as well as ketones for secondary alcohols. Table 12.9 shows some of the yields obtained; very full details are given in the original paper. In a later paper, Newton and McDonell (1956) discuss the effect of varying the energy input. Hydrogen, carbonyl products and vie glycols showed a reduced yield at the higher doses, presumably due to the presence of modified products.

Table 12.9. Major Gaseous Products from Irradiated Alcohols G values (28 MeV a particles)

Alcohol

Methyl Ethyl H-Propyl /ôPropyl «-Butyl wöButyl 5-Butyl /-Butyl

/7-Octyl «-Decyl

( - M ) *

5-8 4-73 606 4-73 418 4-83 606 411

—

Total gas

4-48 4-47 3-86 4-35 4-26 3-99 4 51 3-48

3-68 3 63

H2

3-46 3-46 2-80 2-71 3-59 2-77 261 1 24

3-48 3-47

CO

0-23 O i l 010 0083 0066 0100 0060 0046

0052 0 044

CH4

036 0-43 067 114 0055 0142 0-37 1 60

0022 0018

C2H6

0014 017 0-54 017 0035 0017 086 0-52

0021 0036

* Minimum value for disappearance of alcohol.

Effect of Oxygen and of Hal ides

Due to the high electron affinity of oxygen and to its reaction with free radicals it can readily modify many radiation-induced reactions. Hydro-carbons irradiated with a particles are dehydrogenated, whereas in the presence of oxygen, carbon dioxide and water is produced, and the usual dehydrogenation is reduced or suppressed (Lind, Bardwell and Perry, 1926). Bach (1955) has summarized work on the irradiation of hydro-carbons in the presence of oxygen. Table 12.10 shows the yield of various peroxides in the irradiation of hydrocarbons; RxOOR2 is usually the most frequent product and in this respect it differs from photochemical or autoxidation reactions which give hydroperoxides. The reaction may be considered as arising from a combination of a peroxide radical Ri02· with a hydrocarbon radical R2% giving a peroxide crosslink RiOOR-j. Acids and carbonyl compounds are also formed at a rate which is initially independent of temperature and intensity of radiation, showing that these

RADIATION I N D U C E D C H A N G E S IN O R G A N I C M O L E C U L E S 195

reactions occur from the initial material and not from a product formed by radiation. The usual radiation protection mechanism in benzene operates to a far less marked extent in oxidation reactions.

Table 12.10. Oxidation of Hydrocarbons (G values)

RiOORa ROOH H 2 0 2 Total peroxides Carbonyl compounds Acids

«-Heptane

2-2 1-2 0 3 3-7 2 0 0 4

iso Octane

1-3 0-7 0-3 2-3 1-2 0 6

cyclo-Hexane

0 2 1 0 1-2 0 6 0 2

Toluene

1-2 0-4 0 2 1-8

Benzene

0 2 - 0 3 0 02-0-3 0 5 045 0

Source: Bach (1955).

Bach also compared the behaviour of acetic acid under radiation both in the absence and in the presence of air. In the initial stages of the reaction, oxygen prevents the appearance of methane, hydrogen or carbon monoxide, presumably by reacting with the radicals which produce them and diverting them into the formation of peroxides. In the presence of oxygen, formaldehyde methyl and acetyl hydroperoxide, acetyl and hydrogen peroxide are formed, the G value for the products totalling 3-34. The yield of carbon dioxide is unaffected (G = 2-4) by the presence of oxygen while the G value of other gases liberated is reduced from 3 to negligible amounts. In the irradiation of alcohols, however, oxygen greatly increases the G values for gaseous products, and new products—acids, aldehydes and water and peroxides are formed with good yields. The G values appear to vary with radiation intensity, indicating some form of chain reaction. Proskurnin and Barelko (1955) observed a large yield of oxidation products in benzyl alcohol, in spite of radiation protection by the benzene ring; this would again indicate a chain reaction. This may take the form of an attack of a peroxide radical on a neutral molecule.

H H

CeH5 O + 02 -> C6H5 C — O — O I I

OH OH H H H

C6H5 COO· + C6H5CH2 OH -> C6H5 C — OOH + C6H5 O I

OH OH OH Halogen compounds are particularly sensitive to radiation. Measure-

ments of the G values for radical production have been made, using o


DPPH or iodine as a scavenger for the radicals (see Chapter 27). Chain reactions are also observed, as in the chlorination of benzene or toluene (Harmer, Anderson and Martin, 1954; Cox and Swallow, 1956). Chlori-nation can occur both as addition to the nucleus, or as substitution in the side chain. Chlorine substitution in paraffins and in polyethylene has also been found to occur, with a high yield (Charlesby and Cox).

Other Compounds

A variety of more complex organic compounds have been irradiated including amino acids, various gaseous compounds, esters, ammonium salts, amines, oils and tars. No systematic deductions can be made at this stage of the products formed which are of interest to the radiation of polymers. However, the irradiation of simple model compounds is likely to become of increasing importance in the investigation of radiation mechanisms in polymers. The effect of physical state is also of considerable interest and more comparisons of the products obtained from the same material, irradiated as a gas, a liquid and a solid, are needed.

REFERENCES Reviews ALLEN, A. O., Ann. Rev. Phys. Chem. 3, 57, 1952. COLLINSON, E. and SWALLOW, A. J., Quart. Rev. Chem. Soc. 9, 311, 1955. COLLINSON, E. and SWALLOW, A. J., Chem. Reviews 56, 471, 1956. LIND, S. C, Chemical Effects of Alpha Particles and Electrons, 2nd Ed., Chemical

Catalogue Co., N.Y., 1928. TOLBERT, B. M. and LEMMON, R. M., UCRL 2704; Rad. Res. 3(1), 52, 1955. SACHS, F., Effects of α- β, γ and x-rays on organic compounds, Bibliography

Y904, 1952 WEISS, J., Ann. Rev. Phys. Chem. 4, 143, 1953. WILLARD, J. E., Ann. Rev. Nucl. Sei. 3, 193, 1953. WILLARD, J. E., Ann. Rev. Phys. Chem. 6, 141, 1955.

References ANDERSON, L. C , BRAY, B. G. and MARTIN, J. J., International Conference on

Peaceful Uses of Atomic Energy, Vol. 15, p. 235, Geneva, 1955. BACH, N., International Conference on the Peaceful Uses of Atomic Energy,

Geneva, 1955, 7, 538. BOUBY, L., CHAPIRO, A., MAGAT, M., MIGIRDICYAN, E., PREVOT-BERNAS, A.,

RHENISH, L. and SEBBAN, J., International Conference on Peaceful Uses of Atomic Energy, Geneva, 1955, 7, 526.

BREGER, I. A. and BURTON, V. L., J. Amer. Chem. Soc. 68, 1639, 1946. BREGER, I. A., J. Phys. Chem. 52(3), 551, 1948. BURTON, V. L., J. Amer. Chem. Soc. 71, 4117, 1949. CHARLESBY, A., Proc. Roy. Soc. A222, 60, 1954. CHARLESBY, A., Rad. Res. 2(1), 96, 1955. CHARLESBY, A. and Cox, R. A., unpublished. Cox, R. A. and SWALLOW, A. J., Chem. & Ind. (Rev.) 1277, 1956. DAVISON, W. H. T., Chem. & Ind. (Rev.) 662, May 1957; Chem. Soc. Sp. Publ.

(9), 151, 1958.


DEWHURST, H. A., / . Chem. Phys. 24, 1254, 1956; J.Phys. Chem. 61,1466,1957. DONALDSON, D. M. and MILLER, N., J. Chim. Phys. 52, 578, 1955. FISCHER, F. and PETERS, K., Z. Phys. Chem. A141, 180, 1929. FORSYTH, P. F., WEBER, E. N. and SCHÜLER, R. H., / . Chem. Phys. 22, 66, 1954. HARMER, D. E., ANDERSON, L. C. and MARTIN, J. J., Chem. Eng. Prog. Symp.

50(11), 253, 1954. HONIG, R. E. and SHEPPARD, C. W., / . Phys. Chem. 50, 119, 1946. KRENZ, F . H., Nature, Lond. 176, 1113, 1955. LIND, S. C , BARDWELL, D. C. and PERRY, J. H., / . Amer. Chem. Soc. 48, 1556,

1926. LIND, S. C. and BARWELL, D. C , / . Amer. Chem. Soc. 48, 2335, 1926. MCDONELL, W. R. and NEWTON, A. S., / . Amer. Chem. Soc. 76, 4651, 1954. MCDONELL, W. R., A.E.C., UCRL 1378, July 1951; / . Chem. Phys. 23, 208,

1955. MCLENNAN, J. C. and PATRICK, W. L., Canad. J. Res. 5, 470, 1931. MCLENNAN, J. C , PERRIN, M. W. and IRETON, H. J. C , Proc. Roy. Soc. A125,

246, 1929. MANION, J. P. and BURTON, M., / . Phys. Chem. 56, 560, 1952. NEWTON, A. S. and MCDONELL, W. R., UCRL 3317, 1956. PATRICK, W. N. and BURTON, M., J. Amer. Chem. Soc. 76, 2626, 1954. PREVOST-BERNAS, A., CHAPIRO, A., COUSIN, C , LANDLER, Y. and MAGAT, M.,

Disc. Faraday Soc. 12, 98, 1952. PROSKURNIN, M. A. and BARELKO, E. V., Collection of Papers on Radiation

Chemistry, Academy of Sciences, U.S.S.R., Moscow, 1955. SCHOEPFLE, C. S. and FELLOWS, C. H., Industr. Engng. Chem. {Industr.) 23, 1396,

1931. SCHÜLER, R. H., J. Phys. Chem. 60, 381, 1956. SCHÜLER, R. H. and ALLEN, A. O., / . Amer. Chem. Soc. 11, 507, 1955. SCHÜLER, R. H. and HAMILL, W. H., J. Amer. Chem. Soc. 74, 6171, 1952. SHEPPARD, C. W. and BURTON, V. L., / . Amer. Chem. Soc. 68, 1636, 1946. TUNITSKY, N. N., KUPRIYANOV, S. E. and TIKHOMIROV, M. V., Collection of

Papers on Radiation Chemistry, Academy of Sciences, U.S.S.R., Moscow, 1955. WEBER, E. N., FORSYTH, P. F. and SCHÜLER, R. H., Rad. Res. 3, 68, 1955. WHITEHEAD, W. L., GOODMAN, C , and BREGER, I. A., J. Chim. Phys. 48, 184,

1951. WIENER, H. and BURTON, M., / . Amer. Chem. Soc, 75, 5815, 1953. WILD, Disc. Faraday Soc. 12, 127, 1954. WILLIAMS, R. R. and HAMILL, W. H., / . Amer. Chem. Soc. 72, 1857, 1950. WILLIAMS, R. R. and HAMILL, W. H., Rad. Res. 1, 158, 1954.

CHAPTER 13

POLYETHYLENE

POLYETHYLENE has only been known for two decades but its combination of chemical, mechanical and electrical properties has given it a unique position among commercial long chain polymers. At room temperature it is insoluble in all organic solvents and is resistant to acids and alkalis. By varying the average molecular size, weight distribution and degree of branching, it is possible to obtain soft and waxlike, tough and flexible, or harder but more brittle materials. Electrically, polyethylene has high resistivity and dielectric strength and an extremely low power factor. It can be readily extruded, moulded and calendared.

Polyethylene was first produced commercially by the polymerization of ethylene under conditions of high pressure and temperature; this resulted in a highly branched polymer. More recently, catalytic processes have been developed which enable ethylene to be polymerized at much lower temperature and pressure. This leads to a more regular structure of the molecule and less branching, this being reflected in changed density and melting properties. To distinguish between these various types of poly-ethylene, it has become usual to refer to them either as high pressure and low pressure polyethylene (depending on their method of manufacture) or alternatively as low and high density polyethylene (the more linear poly-ethylene having a higher density). The original high pressure process, if suitably adapted, can also lead to higher density material in which the degree of branching and the density lie intermediate between the earlier high pressure materials such as Alkathene (Grade 2 or 7) and the later low pressure Ziegler or Phillips materials.

At room temperature, polyethylene is partly crystalline and it is the crystalline regions which, by binding neighbouring molecules together by secondary forces, are largely responsible for giving mechanical strength to the polymer. As the temperature is raised, these crystalline regions pro-gressively melt and the polymer becomes softer and more flexible until at the temperature at which all crystals have melted it is transformed into a viscous fluid.

The average size of the crystal regions is largely determined by the degree of branching, which interferes with crystal growth. In highly branched polyethylene, individual crystals tend to be small and possibly distorted and consequently have a low melting point. The proportion of non-crystalline material is also high and this reduces the density of the polymer. In the more linear polyethylenes, the large size of the individual crystal and the small amount of amorphous material results in polymers which are considerably stifTer and more brittle and have a higher density.

198

POLYETHYLENE 199

The melting properties of polyethylene may be compared with those of normal or branched paraffins which can be considered as low molecular weight forms of polyethylene. As the molecular weight of normal paraffins is increased, the melting point rises to a limiting value of about 137°C. Some forms of polyethylene only become fully molten at a temperatuie close to this limiting value, indicating the large size and degree of regularity of the crystals. However, some decrease in crystallinity does take place at

CH3

CH2

H H H H C H 2 H H H H -C—C—C—C—C C— Ç— C— C-

H H H CH2H H ÇHgH H

CH2 ÇH2

CH3 CH2

CH3

(a) Highly branched polyethylene

H H H H ÇH2 H H -C—C—C—C—C C—C-

H H H H H H H

(b) Lightly branched polyethylene

Crystalline region in a lightly branched polythene

Smaller crystalline regions in a highly branched polythene

FIG. 13.1. Effect of branching on crystallite size.

temperatures down to about 100°C due to the melting of the smaller crystalline regions. Low density (highly branched) polyethylene having much smaller crystals begins to show a significant decrease in crystallinity at a temperature of about 60-70°C, and the largest crystals melt at about 115°, which is about 20° below the theoretical limit for a linear paraffin.

A completely linear polymethylene, free of all branches, can be prepared from diazomethane. This polymer (unfortunately not available commer-cially) may be regarded as the ultimate form of polyethylene, i.e. exhibiting

Tabl

e 13

.1.

Com

para

tive

Pro

pert

ies

of S

ome

Typi

cal

Pol

yeth

ylen

es

at a

bout

20°

C

Den

sity

El

astic

mod

ulus

(ps

i)

Cry

stal

line

mel

ting

poin

t (m

ax.)

Coe

ffic

ient

of

vol

ume

expa

nsio

n xl

O4

Type

Am

orph

ous

0-87

*

10*

Low

den

sity

0-92

18

000

115°

C

9

Alk

athe

ne

Gra

de 2

-7

Inte

rmed

iate

0-94

67

000

5-7

Alk

athe

ne

H.D

.

Inte

rmed

iate

-h

igh

0-94

5 75

000-

1250

00

134°

C

Her

cule

s

Hig

h de

nsity

0-95

90

000

4-8

Zieg

ler

Hig

h de

nsity

0-96

20

0000

3-9

Mar

lex

Cry

stal

line

101

136-

5°C

3-

5

' Val

ues f

or a

mor

phou

s ar

e ca

lcul

ated

. O

ther

figu

res

will

var

y w

ith g

rade

, bat

ch a

nd t

empe

ratu

re.


POLYETHYLENE 201

the maximum density and crystallinity. In assessing the effects of radiation, it is convenient to consider the monomer unit of polyethylene as being CH2 rather than C2H4 so that the results for polyethylene (C2H4)n may be directly compared with those for polymethylene (CH2)W and with paraffins containing an even or odd number of carbon atoms. The polyethylenes available for study thus range from a highly branched material to one which is essentially linear in character and from very low molecular weight w-paraffins (such as decane) to polymers of very high molecular weight containing more than 104 carbon atoms per weight average molecule. A direct comparison should be made between /7-paraffins and unbranched (high density) polyethylene and between branched paraffins and branched (low density) polyethylene, but published data on radiation effects in the newer high density polyethylenes with very low degrees of branching are still rather scant.

Many of the radiation effects observed in low molecular weight paraffins have been summarized in the previous chapter and serve as a useful guide to the chemical changes which may be observed in irradiated polyethylene. The physical changes are complicated by the complex nature of the polymer which is not yet fully understood. The presence of both crystalline and amorphous regions, the occurrence of side groups often of unspecified length along the main chain as well as the uncertainty in the molecular weight distribution renders polyethylene a difficult system for quantitative study.

The following major effects observed when polyethylene is irradiated are analogous to those produced in low molecular weight paraffins :

(i) The evolution of hydrogen and low molecular weight hydro-carbons.

(ii) The formation of C—C bonds between molecules (dimerization or crosslinking). These bonds convert the polymer into one which is partly insoluble and infusible.

(iii) An increase in unsaturation. At low doses, the degree of un-saturation is proportional to the dose but eventually tends to a maximum value.

(iv) Destruction of crystallinity. The percentage of material present in the crystalline form at room temperature decreases and even-tually disappears with increasing dose.

(v) Colour changes. In common with many other polymers, poly-ethylene acquires a yellow tinge on irradiation.

(vi) Oxidative reactions, particularly near the surface, when the irradiation is carried out in the presence of oxygen.

Of these changes, the formation of crosslinks is by far the most impor-tant from the point of view of modification of physical properties. The extent of the physical changes depends primarily on either the average number of crosslinked units per molecule (γ or δ) or on the density (q) of crosslinking, i.e. the proportion of carbon atoms linked to other mole-cules. It is convenient to consider the changed properties of irradiated polyethylene as falling into four main ranges depending on the values of


γ or δ and of q. This division follows the changes in molecular weight, solubility, swelling and elastic properties discussed theoretically in Chapter 9.

1. At very low radiation doses, corresponding to less than one cross-linked unit per weight average molecule (δ< 1), the major effect of crosslinking is an increase in the degree of branching and in the average molecular weight, often leading to an increased tensile strength. The polymer is still completely soluble and melts and flows at about the same temperature but the melt (bulk) viscosity is higher. This pattern of behaviour applies to low density poly-ethylene in the approximate range 0-2 megarads and to high density polyethylene in the approximate range 0-10 megarads, depending on initial weight average molecular weight.

2. In the second range, the polymer consists of two phases, a sol and a gel, the latter forming a three-dimensional network which is insoluble and infusible. The crosslinking index δ is greater than unity but q is still very small. The mechanical properties in this range are modified by the reduced degree of crystallinity and by the presence of a network. At temperatures below the melting point of the crystallites, the mechanical properties of the polymer are still largely determined by the residual amount of crystallinity, but at higher temperatures only the radiation induced crosslinks in the gel hold the molecules together and the properties of the irradiated polymer are those of a rubber-like material with high elongation at break. The properties of the latter system can be largely predicted in terms of the theory of rubber-like deformation (page 151). For low density polyethylene, this range extends from a few megarads up to several hundred. The corresponding values of crosslinking density q lie between 0-1 per cent and 5 per cent approximately.

3. A transition region occurs when crystallinity has been almost entirely destroyed and the mechanical properties are determined primarily by the crosslinking density q. The average molecular weight between successive crosslinks is too small to allow high elastic deformation and the elastic modulus varies but slowly with temperature. The properties corresponding to this density of crosslinking are intermediate between those of the rubber-like state above and the glass-like state outlined below, and may perhaps be referred to as the cheese-like state as indicative of the manner in which the material tends to crumble under stress.

4. At very high radiation doses corresponding to very dense cross-linking, polyethylene acquires a glass-like structure characterized by high elastic modulus and low deformation at break; breaks show conchoidal fracture. This behaviour is generally observed when the density of crosslinking exceeds about 1 carbon atom in 10 (#>10 per cent).

POLYETHYLENE 203

By means of radiation it is therefore possible to pass through a series of solid state structures each with typical mechanical properties, from a crystalline polymer, through one which is amorphous and highly elastic into a glass-like material. The mechanical properties shown by irradiated polyethylene depend very markedly on the radiation dose and where crystallinity is present, on the temperature of testing.

It is convenient to review the available information on irradiated polyethylene under the following main headings :

(i) the effect of radiation-induced crosslinking, e.g. changes in mole-cular weight, solubility, swelling and infusibility;

(ii) reduction in crystallinity; (iii) radiation induced chemical changes; (iv) mechanical and electrical properties.

EFFECT OF CROSSLINKING Molecular Weight Changes below the Gel Point

For very low radiation doses insufficient to produce a network structure, the effect of radiation is merely to increase the average molecular weight and degree of branching. Changes in degree of crystallinity are generally negligible. The effect of crosslinking is to increase the viscosity and the tensile strength but the polymer remains completely fusible and soluble. Conventional chemical methods can therefore readily be used to measure changes in molecular weight, etc.

Although theoretical calculations on the change in molecular weight are available (pages 141-142), no detailed quantitative study of an experimental character has been published for irradiated polyethylene, the major diffi-

.1 1 i i 2-6

2-4 1000

Mn'

2-2

2-0 0 100 200

Dose, megaroentgen

FIG. 13.2. Change in Mn of irradiated octacosane (electron irradiation). (From Miller et al, 1956.)

culty being that of measuring accurately molecular weight distribution. To obtain an estimate of the crosslinking density q, it has usually been

l\ V

\

\

v

\


necessary to use as a model a long chain paraffin of uniform and known molecular weight and to extrapolate the results to high molecular weight material. This method has been adopted by Charlesby (1954) in studying the changes in fusibility of «-paraffins and by Miller, Lawton and Balwit (1956) in their investigation into molecular weight changes. In the latter experiments pure octacosane (C28H58) was subjected to electron irradiation and the increase in number average molecular weight Mn

f was measured cryoscopically in cyc/ohexane. The reduction in number average mole-cular weight Mn observed by Miller et al. (Fig. 13.2) can be expressed in the form

- i - , = - Î - ( l - 1 0 2 x l O - 3 r ) Mn Mn

where the radiation dose r is given in megaroentgen defined as an energy absorption in octacosane of 83-8 x 106 ergs/g. This expression is of the same form as that given by the theory of crosslinking (equation 9.13)

Mn Mn Mn

Then γ = 2·04χ 10-3r, qQux = 2·04χ 10~3 and since ux = 28 for octaco-sane when CH2 is taken as the unit, q0=l'3x 10_5/megaroentgen. When the unit radiation dose is chosen as 1 megarad or 108 ergs/g, the value of q0

must be increased by a factor —— to 8-7x 10~5. From equation (9.4) the 83*8

corresponding value of G per crosslink is 3. In addition to crosslinking, unsaturation is produced by radiation and

it is of interest to compare these results with comparable measurements on analogous unsaturated molecules. Some measurements (Charlesby, 1955) on the increase in M»' of several octadecenes (C18H36) using ebullioscopic methods are shown in Fig. 13.3. The samples were irradiated in vacuum in the BEPO pile at Harwell and the units referred to are pile units equivalent to about 45 megarads. For these specimens γ = unity at doses of between 7-5 and 10 pile units or 340-450 megarads, depending on the position of unsaturation. Then

<7oWi = 2-2 to2-9xl0- 3

if the radiation unit is 1 megarad. For octadecenes, ux = 18, and hence <7o = 1-2 to l-6x 10-4 leading to G values of 4-3 to 4-7 for crosslinking. These G values are somewhat higher than those for octacosane, the difference arising primarily from the presence of double bonds at various positions along the chain. Although the radiation intensity differs by a factor of about 104 as between electron radiation and pile radiation, the q0 values for «-paraffins and olefins are equal within a factor of 2 or less.

POLYETHYLENE 205

JO_ M'

8 10 pile units

FIG. 13.3. Increase in molecular weight of irradiated olefins (C18H36).

2 4 6

Radiation dose,

Viscosity and Flow Properties The increase in molecular weight with crosslinking results in an increased

bulk viscosity above the melting point of the crystals. The theory gives the increased molecular weight of the product but this cannot be related

103-

107 s

0-5

K= 2 ^ 2

V ^ 1

-7-0

0

K)c

K):

XT

10c

1er3 10"2 K)-1 1-0 10 Shear gradient, sec-1

FIG. 13.4. Effect of shear rate on viscosity of irradiated polyethylene at 250°C. Full line—Alkathene 2 (branched, low density).

Broken line—Marlex (linear, high density). Figures denote radiation dose in megarads of 2 MeV electron radiation.


to the change in bulk viscosity as the irradiated material is more highly branched and the flow of molten polyethylene is non-Newtonian.

Fig. 13.4 shows the effect of shear rate on the viscosity of irradiated polyethylene in the molten condition. Both low density and high density polyethylene are considered. The effect of low radiation doses is to increase the average molecular weight and the degree of long chain branching. This causes an increase in viscosity at a given shear gradient and an increased shear rate dépendance. Similar effects are observed in both Alkathene and Marlex, and at doses close to the gel point when long chain branching is present in both materials the viscosities tend to equalize.

The Davenport Grader is widely used in standardizing the grade number of polyethylene. This instrument allows measurement of the extrusion rate at a temperature of 190°C under standard conditions of pressure; if the reciprocal flow rate is plotted against the dose, a linear relationship is obtained over a limited range of doses. The effect of radiation can then be seen as a reduction in the equivalent grade number (Fig. 13.5).

vo

0-8

? 0-6

|

*o 0-4 .o a

0-2

O 0-2 0-4 0-6 0-8

Dose, megarads

FIG. 13.5. Increased viscosity of irradiated polyethylene. Measured at 190°C in the Davenport Grader. Ordinate gives ratio of flow of irradiated material to that of

unirradiated material.

This method cannot be utilized near the gel point, however, and the flow rate remains finite even beyond this point, probably because some cross-links are destroyed during the extrusion process. There is, however, a marked difference in the appearance of the extruded crosslinked poly-ethylene which shows uneven flow and formation of small globular structures. Gel Formation

The gel point in polymers may be defined as the minimum degree of crosslinking at which an insoluble fraction first appears. According to the theory of crosslinking given in Chapter 9, this point corresponds to one

POLYETHYLENE 207

crosslinked unit per weight average molecule (δ = 1) if there is no simul-taneous main chain fracture. The direct determination of the threshold for gelation in polyethylene is, however, attended by a number of experi-mental difficulties and most of the published estimates of the gel point rely on an extrapolation procedure using such properties as viscosity, solubility, fusibility or swelling.

If the radiation dose at the gel point rgei is expressed in megarads, the condition δ = 1 can be written (equations 9.17, 9.19)

rge\Mw = w/q0 = 0-48 x 106/C7

where q0 is the proportion of carbon atoms crosslinked per megarad, G is the number of crosslinks formed per 100 eV absorbed and the unit is CH2(H> = 14). With the value deduced above for octacosane q0 = 8-7x 10-5, and G = 3:

rge\Mw = 1-6 xlO5.

Measurements of rge\Mw for a range of w-paraffins and polyethylenes have been made by Lawton et al. (1954), Charlesby (1954) and by Busse and Bowers (1958).

Lawton et al. subjected a variety of polyethylenes of molecular weights ranging from 7,000 to 35,000 to 0-8 MeV electrons and measured the minimum doses at which the samples retained their shape as a swollen strip in toluene at 90°C. The experimental relationship derived by Lawton et al. can be expressed in the general formula (r in megaroentgens)

r = 1-8 x l O W - 1 · 3 2

or rM = 1 ·8χ10βΜ-°·3 2

The product r M therefore varies from about 0-6 x 105 to 1 x 105 for a five-fold range in molecular weight. For paraffin wax of molecular weight about 480, the experimental value of rM was 4 x 105, while for a paraffin compound Alcowax 6 with a reported molecular weight of 2500, rM was equal to 1*3 x 105. These results indicate a threefold variation of cross-linking density per unit dose (q0) over nearly a hundredfold range in molecular weight, and G values of up to 6. However, the molecular weight used by Lawton is the number average whereas the location of the gel point depends on the weight average molecular weight; for polyethylene, the latter may be several times greater. Furthermore, the test applied gives not the gel point (when only an infinitely small amount of polymer is transformed into a three-dimensional network) but the somewhat higher degree of crosslinking, at which there is sufficient amount of a stable and continuous network to be capable of being handled. The extent to which δ must exceed unity depends on the molecular weight and on the precise conditions of test; a network formed of small molecules requires a higher proportion of crosslinks per molecule to achieve the required rigidity.


The above results may therefore exaggerate the variation of q0 and G with molecular weight.

Charlesby (1954) studied the sudden change in fusibility which occurs when a network first starts to form. Specimens of polyethylene, Winno-thene (a lower molecular weight paraffin) and linear paraffins of uniform molecular weight were irradiated in vacuum in the BEPO nuclear reactor at temperatures of 70°C. The melting point of the irradiated specimens was determined from their flow properties while still in the sealed glass con-tainer. The shape of the melting point-radiation curve for several «-paraffins is shown in Fig. 13.6. The cause of the initial drop in melting

70i

6 0

> e ^

50

4 0

30

·§ 20| o.

101

■ >200 eC

V ^ _ J > 1 6 0 °C-2$t

-ισ

- 2 0

-301

C12 H26

x.x<;

>200°C

— /H '^-Χχ.Χ-Χ · **

0 10 20 30

Radiation dose, pile units

FIG. 13.6. Melting point of irradiated «-paraffins.

point is not clear—it may arise from radiation induced main chain fracture producing some lower molecular weight components. After a radiation dose which depends on the initial molecular weight, the melting point curve reaches a minimum and then rises slowly. At a critical dose, a sharp change in melting behaviour is observed. Below it, the material melts readily and flows when heated; above it, the specimen no longer flows even at considerably higher temperatures of about 200°C. This radiation dose may be taken to correspond to the formation of an infinite network

POLYETHYLENE 209

which comprises only part of the specimen but which is swollen by and retains the remaining molecules, preventing them from flowing. The corresponding radiation dose r/ needed to promote infusibility is therefore somewhat greater than the theoretical dose required to form an infinitely small amount of network. For the observed change in fusibility, the amount of network formed must be sufficiently large to incorporate the remaining sol fraction.

Values of the minimum irradiation dose for infusibility r/ and of the product rjM are given in Table 13.2. The radiation doses are expressed

Table 13.2. Radiation Dose for Infusibility of Paraffins

Paraffin Cn

c7 ^ 1 0

C12 C24

^-28

( -32

Q{4 ^-3 6

M

100 142 170 338 394 450 478 506

r (pile units) fluid solid

49-7 50-7 28 30 27 28 10-5 108 9-5 10-2 7 7-4 6 7 6-8 7

r/(mean) pile units

50-2 29 27-5 106 9-8 7-2 6 5 6 9

rf megarads

2260 1300 1240 477 440 324 293 310

rfM xlO-5

2-26 1 85 211 1 61 1-73 1 46 1-40 1-57

both in pile units and in megarads assuming one pile unit is equivalent to 45 megarads. The lower figure for r represents the highest observed dose at which the specimen flows readily, the higher figure being that at which a non-flowing material is observed. The value of r/ is taken as the mean of these values, and r/M lies close to rge\M for octacosane (l-6x 105).

These results may be slightly on the high side since a finite amount of gel is needed to form a network capable of retaining the sol fraction. An upper estimate for the error involved can be obtained by assuming that the true gel point corresponds to the minimum melting point. This decreases the values for rf by about 10-20 per cent and the corresponding average value for rge\Mw then becomes 1-3 x 105. With this value

qQ = 1-1 x 10-4 and G (crosslink) = 3-8.

Busse and Bowers (1958) determined the gelation points often branched polyethylenes by observation of solutions in a-chloronaphthalene at 150°C ; these showed a slight optical heterogeneity as a network was formed. The values of Mw were determined by light scattering, and the product rgt\Mw

equalled 8Ό6χ105 to better than 25 per cent. The constant product indicates that in these polyethylenes G (crosslink) is approximately inde-pendent of molecular weight. However the product is considerably higher than the values derived above and corresponds to a very low value of 0-5 for G (crosslink). The cause of this marked discrepancy is not known; it may be due to the method used for ascertaining the gel point of polymers having a very wide molecular weight distribution. It is perhaps significant that this formula was not found to apply to linear polyethylenes, which have a sharper molecular weight distribution.


Solubility

In theory, an accurate method of determining the gel point consists in extrapolating the sol/radiation dose curve to 100 per cent solubility. Unfortunately for low density polyethylene, the very wide molecular weight distribution gives a curve with a very small slope near the gel point. The results for several linear (M, Z) and branched (/) commercial polyethylene samples are shown in Fig. 13.7. These are samples of

- ^ 1 M—

| r—J

VJT ^ML

**!<.

FP tt l \ |Mv \

χΛ^

Ι Π ^ V ^

\

S N, r

1 ni MM p IN J X NJ

( f*^

τί] l \7

1 10 100 4 0 0 Radiation dose, rnegarads

FIG. 13.7. Solubility-radiation curves for linear and branched polyethylene. The smaller slope for specimen / indicates a wider molecular weight distribution; the

lower value of rgel indicates a high value of Mw. (From Charlesby, von Arnim and Callaghan, 1958.)

approximately similar bulk viscosity in the molten state and the more highly branched material (/) requires a lower gelation dose than do the linear materials. If q0 and G (crosslink) are independent of molecular weight and degree of branching, this implies a higher weight average molecular weight for the branched material. Thus, in spite of their higher mechanical strength, linear materials may have a smaller weight average molecule. The solubility data can provide no information on the number average molecular weight.

Once the gel point has been reached, an increasing fraction of the polymer becomes insoluble. The shape of the sol/dose curve is directly related to the initial molecular weight distribution (page 145), and assuming that crosslinking density is proportional to radiation dose, an accurate determination of the sol fraction as a function of dose provides information which is difficult to obtain by other means. Some experi-mental curves for various types of polyethylene are shown in Fig. 13.7. The steeper slopes of the curve for specimens M and Z as compared with

POLYETHYLENE 211

specimen / are a direct indication of the sharper character of their initial distribution. For specimen /, the sol fraction approaches the gel point at a small angle indicating the presence of a small amount of polymer of very high molecular weight. Simultaneous Scission and Crosslinking

In polyethylene, the interpretation of the sol/dose curves may be com-plicated by two factors: the simultaneous occurrence of main chain scission as well as crosslinking, and the oxidative degradation during irradiation or subsequently on extraction of the sol fraction.

It has been shown (page 172) that where the effect of radiation is to promote crosslinking only, the limiting value of the sol fraction for large radiation doses is zero, whatever the initial molecular weight distribution. However, where both crosslinking and main chain fracture (degradation) occur simultaneously at random and in proportion to the radiation dose, crosslinking predominating, the sol fraction tends to a limiting non-zero value characteristic of the ratio of fracture to crosslinking.

Some early values (Charlesby, 1954) suggesting simultaneous fracture and crosslinking in polyethylene are given in Fig. 13.8, together with the

Crosslinking index

_ ~ Ρ · 5 Λ 1*0 2-0 4-0 10 20 40 _ ,

o '.♦-> υ ö

o

0-05 0-1 0-2 0-5 1 2 3 4 5 Radiation dose, /?,pile units

FIG. 13.8. Sol and gel fraction of irradiated polyethylene. 0 Observed values.

■ Theoretical curve assuming a random probability weight distribution initially. (Numbers represent ratio of chain fracture to crosslinking).

Theoretical curve assuming uniform initial distribution. (No chain fracture.)

theoretical curves for various ratios of fracture to crosslinking. From these data, a ratio of about 0-35 may be deduced for the polyethylene studied. This value agrees with the products formed when low molecular

p


weight paraffins (butane, propane, ethane) are subjected to deuteron or a-particle radiation (Honig and Sheppard, 1946; Lind and Bardwell, 1924, 1926), but is apparently inconsistent with the low yields of methane, ethane, propane and butane observed in the oc-bombardment of hexa-decane in the solid state (Breger, 1948) This discrepancy may perhaps arise in part from the different susceptibilities to fracture of C—C bonds along the chain near the ends or at points of branching.

Baskett and Miller (1954) showed that radiation causes main chain fracture in the absence of air by extracting all the soluble fraction from a lightly crosslinked specimen, which was then re-irradiated and re-extracted. The growth of a small soluble fraction was taken as evidence of chain scission in the gel fraction but part of this may be due to degradation occurring in the extraction process. The limiting gel fraction obtained in this way (Fig. 13.9) for a low density polyethylene was 2-4-2-8 per cent

v A X & ιμ-

106 J_

107

Dose,

108 109

rad

FIG. 13.9. Soluble material in irradiated polyethylene gel fraction. (From Baskett and Miller. 1954.)

corresponding to a fracture/crosslinking ratio of between 0-18 and 0-20. By assuming a ratio of 0-2 and making plausible assumptions as to the

number average molecular weight Mw, Baskett (1955) was able to derive from the observed sol/fraction curves molecular weight distributions which agree well with a proposed mechanism for polymer growth involving chain transfer to dead polymer molecules. The ratio Mw to Mn varied from 5 to 10 (as against 2 for a random distribution); this is indicative of a very wide molecular weight distribution. In their studies on the effect of oxygen on the crosslinking of low density polyethylene, Alexander and Toms (1956) found that when specimens were irradiated in vacuo the soluble fraction tended to zero as the radiation dose increases. From this they concluded that in the absence of air no chain fracture takes place. Plots of the solubility of linear and branched polyethylene (Fig. 13.7) seem to follow the same pattern. However, a more satisfactory method of using solubility data consists in plotting s-\-Vs against 1/r as on page 173. For an initially random distribution, this should give an accurately linear plot, while for non-random distribution in which both crosslinking and degradation occur, the curve should eventually approxi-

POLYETHYLENE 213

mate to a linear plot whose intercept at \\r — zero gives the ratio /?0/<7o of degradation to crosslinking (equation 11.7) :

S+Vs = (l+/?oi/i>*)/<7o"i>*

which equals p./q0 when \\r equals zero. The curves shown in Fig. 13.10 were deduced for four fractionated

samples of polyethylene subjected to electron radiation in vacuo. As 1-5 ^—

1-0

+ to

0-5

0 0-01 0-02 0-03 0-04 0 Ό 5 O 0 6 \/r 100 50 25 20 15 r

r , megarads FIG. 13.10. Limiting solubility of irradiated polyethylene (plot of s -f VTagainst 1/r).

(From Charlesby and Pinner, 1958.)

predicted, they become asymptotic to straight lines for doses exceeding 15—25 megarads and tend to a limiting value of 0-32 ± 0Ό2. This would therefore be the value of p0/g0 for these branched polyethylenes.

It is not established that the same ratio holds for linear polyethylenes which do not have radiation sensitive regions in the neighbourhood of branching points. Linear paraffins show both C—C and C—H bond fracture, the ratio for hexane being approximately the same as that obtained for polyethylene above, but these C—C breaks occur preferentially at the second and third C—C bonds along the paraffin chain. Other C—C bonds are less liable to fracture. It has not yet been decided whether the overall ratio of fracture to crosslinking in a linear paraffin decreases indefinitely as the length of the chain is increased (and the proportion of end groups therefore diminishes) or whether it tends to a limiting value as for branched polyethylenes. Swelling

Unirradiated low density polyethylene only dissolves in organic solvents, such as benzene, xylene or toluene, at temperatures above about 60°C when an appreciable fraction of the crystallites begin to melt. For high density polyethylene, higher temperatures may be necessary. On sub-

s& V

<

V/ k ^ j

) /

>

-—"""'

r

'"""""Â


sequent cooling, the polymer does not stay in solution but forms a flocculent precipitate.

After being subjected to radiation doses beyond the gel point poly-ethylene is only partly soluble, but above about 60°C the insoluble fraction swells to an extent depending on the radiation dose. Fig. 13.11

20

10 I x

X IS.

X

107 2x107 5x107 108

Radiation dose, roentgen FIG. 13.11. Swelling of irradiated polyethylene at constant exposure time (200 hr).

Variable γ-intensity. Xylene solvent. . κ 5 / 3 = 1 ·6Χ10 9 /Λ

( 20i

15

10

7

5 4 3

2

> 5

Pp (P

]

£ s s 2-C

P\

Crosslînkîng index

5-0 10 2 0 I > I

Theoretical curve for ^randorr

Ns I P \ P

r

i distribution 1 | 1 -ft,

V W\ W'

v/P 1 1 1 1 1

1 1 1

±

K \ I N A/

s

7>v£\ «

50

V v

100

p "W _Li 0-05 0·1 0·2 0 ·4 1·0 1-5 3-0 5 ·0 7-0 10

Radiation dose , pile units

FIG. 13.12. Swelling of high (P) and low (W) molecular weight polyethylene. Solvent, xylene at 120°C.

■ Calculated swelling curve for polyethylene (P) assuming initially random distribution.

(From Charlesby, 1954.)

POLYETHYLENE 215

shows the swelling ratio* V plotted against the radiation dose on a log/log scale. The full curve represents the simplified theoretical relationship (equation 9-54) which predicts a slope of —0-6 when end corrections can be ignored. In this experiment, polyethylene was subjected to γ-radiation of varying intensity but constant exposure time. Corresponding swelling curves for polyethylene and Winnothene subjected to pile radiation of constant intensity for varying periods are shown in Fig. 13.12. At suffi-ciently high doses, when the influence of the initial molecular weight becomes small, the two curves approximate to one another, showing that the values of q0 and G for crosslinking are independent of molecular weight. These curves show that while an insoluble network is formed at very low doses of radiation, considerably higher degrees of crosslinking are required to produce a polymer with low swelling characteristics.

CRYSTALLINITY CHANGES

X-ray and electron diffraction patterns of polyethylene show a sharp ring pattern superimposed on diffuse haloes. The ring pattern is typical of that produced by ^-paraffins in a crystalline lattice while the haloes are similar to those obtained from paraffin molecules in the liquid state. Except for the regions near the edge of crystallites where there is a tran-sition in structure, polyethylene may therefore be considered as composed of crystalline and amorphous regions similar in molecular arrangement to those in solid or in liquid paraffins. The individual crystallites are smaller than the length of a single polyethylene molecule and each molecule must thread its way through several crystallites and the intervening amorphous regions, thereby binding the crystals together. The mechanical properties of polyethylene are directly related to the size and number of the crystallites and the length of the amorphous chains between them. Attempts have been made to estimate the relative proportions of the crystalline and amorphous regions from the intensity of the x-ray pattern. Hermans and Weidinger (1949) measured the peak intensities while Krimm and Tobolsky (1951) considered the total area under the intensity curve. Matthews, Peiser and Richards (1949) compared the degree of crystallinity of polyethylenes of various molecular weights with their densities at 20°C. Their results indicate that increases in density are directly related to increased degrees of crystallinity.

Calorimetric measurements have been carried out by Raine et al. (1945) and by Dole et al. (1952) with the same object in view. A further method used by Hunter and Oakes (1945) consists in studying the variation in specific volume (reciprocal of density) with temperature.

The degree of crystallinity deduced depends on the molecular weight

* In theory, one should measure the ratio of swollen volume to gel fraction (which is alone capable of swelling), but in practice it is more convenient to measure the swelling ratio V in terms of the original dry volume. At crosslinking densities suffi-ciently high for end effects to be ignored, the sol fraction is very small and the difference may usually be ignored; at lower crosslinking densities, the major cause of error is due to end effects and the formula is inapplicable.


and degree of branching of the polymer as well as on the experimental method used; it varies only slightly with the previous history of the specimen since, unlike many polymers, polyethylene cannot be readily quenched from the melt. For low density polyethylene at room tem-perature, about 50-60 per cent of the polymer is crystalline. As the temperature is raised, this percentage decreases and reaches zero at about 110-120°C, i.e. at the melting point of the largest crystalline region. High density polyethylene shows a higher crystalline fraction (about 80 per cent) at room temperature and this decreases to zero at about 130-135°C, close to the theoretical melting point of an infinite «-paraffin. There would therefore appear to be little hope of a marked increase in the crystalline melting point of any linear or branched polyethylene.

The effect of radiation on the crystallinity of polyethylene is best followed by a comparison of the x-ray diffraction patterns of specimens irradiated to varying extents and by a study of their density as a function of temperature.

X-ray Diffraction Patterns

X-ray diffraction patterns obtained from a series of irradiated low density polyethylene samples at room temperature are shown in Fig. 13.13. As the radiation dose increases, the sharp ring pattern characteristic of crystalline regions becomes weaker, but there is no marked change in their spacings* and little tendency for the rings to broaden. At the same time, the halo pattern produced by the amorphous regions becomes very intense. These changes show that radiation destroys crystallinity but not, as might be expected, by structural disturbances of the lattice such that the pattern becomes more diffuse and broadens into haloes. On the contrary, the crystals become fewer but retain their initial regular structure almost unaltered until their number and size is too small to give an observable pattern. At all stages, there is a sharp distinction between crystalline and amorphous structure. The same distinction may perhaps be expected to occur in the process of melting other polycrystalline solids.

The pattern of a highly irradiated polyethylene which has lost all its crystalline character differs insignificantly from that of //-paraffins in the liquid state. The halo spacings stem from the average distances between carbon atoms either in the same or adjacent molecules and, apart from local distortions, would therefore not be expected to differ in liquid and in crosslinked paraffins.

A high degree of crosslinking is necessary to destroy all traces of crystallinity. For specimens irradiated at 70-80°C in the BEPO nuclear reactor, an energy input of at least 10-14 pile units (or about 450-630 megarads) was needed to produce a polyethylene showing no crystallinity at room temperature. This radiation dose may be expected to crosslink about 5-7 per cent of the carbon atoms {q ~ 0Ό5-0Ό7). Even at this high dose, partial recrystallization may take place when polyethylene is

* Slichter and Mandell (1958) have reported very slight changes in unit cell dimen-sions at very high doses.

FIG. 13.13. X-ray diffraction patterns of irradiated polyethylene showing destruction of crystalhnity. (a) 0 radiation, (b) 4 pile units, (c) 9 pile units, (d) 13 pile units,

(e) 16 pile units. (From Charlesby, 1953.)

Facing p. 216

POLYETHYLENE 217

cooled below room temperature. If, however, the polyethylene is irradiated at higher temperatures (Ross, 1954) close to its melting point, a lower dose will achieve the same object. This may be partly due to the higher efficiency of crosslinking (see the temperature effect discisêd on m*^ 234), but is

V30i

1-25

1·20

M5

WO

1-05

40 60 80 100

Temperature ,

FIG. 13.14. Effect of temperature on specific volume of low (a), intermediate (b) and high density (c, d) polyethylenes.

(tti, n) expansion curve for crystalline regions. va calculated volume for liquid paraffins.

mainly caused by the destruction of the larger crystals which alone exist at temperatures near the maximum melting point. The smaller crystals having melted at the higher radiation temperatures do not re-form on cooling because of the restraints imposed by the crosslinks.*

* Specimens irradiated at room temperature, also show a reduction in crystallinity when subsequently annealed (Dole and Howard, 1957, Williams et al., 1958, Wood-ward et al., 1957).


When all crystallinity has disappeared, polyethylene becomes trans-parent apart from a yellow coloration which is due to chemical changes such as polyene formation.

Specific Volume

A more quantitative approach to the changes in crystallinity resulting from radiation can be made by measurements of the specific volume as a function of temperature (Charlesby and Ross, 1953; Charlesby and Callaghan, 1958). Fig. 13.14 shows the change in spécifie volume of some low density and high density polyethylenes as the temperature is raised. Increase in specific volume is due in part to the thermal expansion of both crystalline and amorphous regions but is mainly caused by the dis-appearance of the crystalline regions, which have a higher density than the amorphous regions which they produce on melting. For low density polyethylene (a) with a wide range in crystal sizes this melting process extends over a considerable temperature range, but ends fairly abruptly at a temperature of about 115° when all the crystals have disappeared. This temperature may be taken as the melting point of the largest crystallites

9V2

18

24

33-36

41-5

72-7

149

0 20 40 60 80 100 120 140 160 Temperature, °C

FIG. 13.15. Specific volume of irradiated polyethylene. Alkathene 2. Pile irradiation. Figures show radiation dose in pile units.

present in the original material. Above this temperature, the expansion is approximately linear and corresponds to the thermal expansion of the liquid paraffin. For high density polyethylene (c, d) where most of the

POLYETHYLENE 219

crystals are larger, the melting range is shorter and extends to temperatures close to 136°C.

In irradiated polyethylene, the specific volume-temperature curve follows the same general pattern, but (at least when pile radiation is used) the temperature of transition is progressively lowered indicating that the maximum melting point of the crystallites is reduced by radiation (Fig. 13.15). Such a reduction would be expected if radiation reduces the crystallite size or regularity of crystal structure and thereby lowers the melting temperature. The major difference from unirradiated material is that above the maximum melting point, irradiated polyethylene, instead of forming a viscous liquid, turns into a rubber-like solid. Radiation cannot therefore be said to raise the melting point of the crystals but it does prevent the amorphous polymer from flowing at low stress.

For very low radiation doses, there is little difference between the specific volume-temperature curves of unirradiated and irradiated material, showing that very little change has occurred in either size or number of crystallites.* Moreover, the coefficients of thermal expansion above the maximum melting temperature are also very similar. The density of a lightly crosslinked amorphous polyethylene network is therefore almost identical with that of polyethylene in the liquid phase. At higher radiation doses, greater differences appear. In addition to a decrease in the maximum melting temperature, the magnitude of the volume change is reduced. The former effect is due to the reduction in average crystal size while the latter is caused by the smaller amount of crystalline material present at any given temperature.

Above the melting point, irradiated polyethylene has a lower specific volume than unirradiated material. This reduction in volume is caused not by any changes in crystallinity but arises from the contraction in dimensions due to the formation of crosslinks. The distance between two carbon atoms is reduced by crosslinking from about 4Â to only 1-54Â and carbon atoms near such links are also drawn together, resulting in a denser structure.

For very highly irradiated material where no crystallites remain even at room temperature, there are only slight traces of any temperature tran-sition in the specific volume-temperature curve which now represents the expansion of a completely amorphous material. With increasing radiation dose and density of crosslinking, the thermal expansion is progressively reduced. In polyethylene in which all carbon atoms are crosslinked, thermal expansion can only occur by extension of C—C bond lengths or

C

κ\ by distortion of C C valency angles, the force constants being far greater than those arising from Van de Waals' binding forces. For such a highly irradiated polymer, one would expect thermal expansion to be

of a much lower order. * Dole and Howard (1957) found little change in the spécifie heat in samples of γ-irradiated polyethylene, but noted a slight lowering of the melting point (1-5°C) after 50 megareps.

of a much lower order.


The relation between specific volume v of polyethylene and the external pressure Pc can be represented by the formula

(ν-ν0)(Λ+Λ·) = RTIM

which is somewhat reminiscent of the equation of state for a gas. Here v0 is a minimum volume (in theory, the specific volume at 0°K), i \ is equi-valent to an internal pressure and M is the molecular weight (28) per unit C2H4. The reduction in specific volume due to links between mole-cules may be considered as arising from an additional internal pressure or alternatively as a reduction in the number of units able to expand as the temperature rises. Charlesby (1953) has shown that a formula of this character represents the specific volume-temperature relation for a cross-linked polyethylene in the amorphous state, when Pi is replaced by Pi + β<7, q being the proportion of carbon atoms crosslinked and β is a suitable parameter. This work has been amplified in a more recent paper by Charlesby and Callaghan (1958).

The reduction of crystallinity with temperature and with radiation dose can be expressed numerically by considering the specific volume v of the polymer as comprising a weight fraction / of amorphous polyethylene of specific volume va and a weight fraction (1 —/) of crystalline polymer of specific volume vc.

V =fva + (1—/)VC = Vc +f(va-Vc).

vc can be deduced from the x-ray spacings of the crystal lattice. At room temperature vc = 1-00 for low density polyethylene and 1-005 for high density polyethylene and increases by about 4 x 10-4 per degree rise in temperature (curves m, n in Fig. 13.14). The value of va cannot be deduced directly from the x-ray spacings of the halo pattern because their precise significance in terms of the molecular arrangement is not clear. However, there appears to be no marked difference between the amorphous pattern of polyethylene at room temperature, the corresponding pattern above the melting point (when all crystallinity has disappeared) and the amorphous pattern of short chain «-paraffins which are liquid at room temperature. This similarity in molecular arrangement suggests a deri-vation of va at various temperatures by two alternative methods:

(i) by extrapolating the specific volume of liquid polyethylene to temperatures below the melting point;

(ii) by extrapolating the specific volume of low molecular weight «-paraffins which are liquid at a given temperature to a high (n = 1000) or infinite molecular weight at the same temperature as in Fig. 13.14.

The similarity in the values obtained by these two methods serves to justify the procedure. Then from measurements of v at any temperature, the corresponding degree of crystallinity (1 — / ) may be deduced. This is shown in Fig. 13.16. The same procedure may be adopted for estimating the decrease in crystallinity with radiation dose as a result of radiation. In this case, however, the value of va at any temperature can only be

POLYETHYLENE 221

20 40 60 80 100 120 140 Temperature °C

FIG. 13.16. Crystallinity of low density (a), intermediate (b) and high density (c, d) polyethylenes.

ou

40

•I 30 σ

-4->

o 20

10

0 ^ / 2

SN?

X4V2

\ 0

^ 2 N

\_ \

\

\ 20 40 60 80 100 120

Temperature, °C

FIG. 13.17. Crystallinity of irradiated low density polyethylene. Figures show dose in BEPO pile units.

obtained by extrapolation of the specific volume above the melting point to lower temperatures. Fig. 13.17 shows the reduction of the crystalline fraction derived in this way for low density polyethylene subjected to various amounts of pile radiation. In specimens subjected to 9-5 units of BEPO reactor radiation, only a few per cent of the crystals remains at room temperature and these completely melt at about 35°C. At this dose, the average chain between successive crosslinks comprises about 20 carbon


atoms ; it is perhaps of interest that the corresponding «-paraffin molecule (eicosane) having a chain length of 20 carbons melts at 38°C.

The melting characteristics of an unirradiated low density polyethylene may be represented by that of a series of crystallites of «-paraffins CwH2M+2 with the appropriate distribution of values of n (Charlesby and Callaghan, 1958). This distribution is shown in Fig. 13.18. When the melting characteristics of pile irradiated polyethylene are transformed into the appropriate equivalent distribution of «-paraffins, the effect of radiation is seen to be a preferential destruction of the larger crystals which may then form smaller crystals. For electron irradiation, giving an equal energy absorption in the polymer, the destruction of crystallinity is possibly far less severe because in the pile densely ionizing protons produced by fast neutron-hydrogen collisions give small regions of considerable energy dissipation. It is, however, difficult to reconcile this with the apparently random distribution of crosslinks in pile irradiated polyethylene.

FIG. 13.18. Equivalent paraffin crystal distribution in irradiated low density polyethy-lene. (Figures indicate radiation dose in pile units.) Broken curve indicates changed distribution for electron radiation at 80°C 150 megarads equivalent to 3 pile units.

Effect of Crystallinity on Density of Crosslinking

In the crystalline regions, molecular chains are more tightly packed and less mobile than in the amorphous regions, which can be considered as semi-liquids. Crosslinks (which require a considerable local rearrange-ment of molecular chains) may therefore be expected to occur preferen-tially in the amorphous regions. To investigate this point, measurements of crosslinking density on low density (highly crystalline) and high density (branched, lower crystallinity) polyethylene have been carried out, in each case under similar conditions of irradiation and testing.

The methods used include determination of elastic modulus above the melting point (Fig. 13.31) swelling measurements (Fig. 13.19) or hydrogen evolution and unsaturation (Charlesby and Davison, 1957, Epstein, 1957,

POLYETHYLENE 223

Charlesby, v. Arnim and Callaghan, 1958, Schumacher, 1958, Waddington' 1958). In most of these experiments the doses used were insufficient to cause any significant change in the degree of crystallinity. None of these experiments show any significant differences for the G value for cross-linking for the wide variety of polyethylenes studied, in which the amorphous fraction varied by a factor of four.

It could be argued that radiation may produce radicals which are trapped in the crystalline regions, and which only promote crosslinking when the polymer is heated or swollen, during the measurements of G. The presence of such trapped radicals has been reported by Lawton et al. (1957) for low density material. However the paramagnetic resonance measurements of Abraham and Whiffen (1958) are of direct relevance, in that they reveal the presence of trapped radicals in irradiated low density material, but none in irradiated high density, highly crystalline polyethy-lene. Only when the latter is exposed to air during or after irradiation is there evidence of peroxide radicals. It can be that the observed radicals are trapped at branch points present in the low density polymer (which are particularly sensitive to radiation) and that such trapped radicals are not directly relevant to the crosslinking reaction.

/

/ /

N

A / /

\

i

/ / / / A

(\\

7

l\ l M ^ r

7

\ 1 VL yi \ ^ s . V.

Λ s <4 V

T/^·/^ / 1 z T ^

1 10 100 4 0 0 Radiation dose, megarads

FIG. 13.19. Effect of crystallinity on crosslinking and swelling properties. At high doses, when end effects are negligible, similar swelling ratios are observed.

Z, M—highly crystalline. /—low crystallinity.

C H E M I C A L C H A N G E S The doses required in the industrial crosslinking of polyethylene and of

other long chain polymers usually only amount to a few megarads ; in the absence of a chain reaction the chemical changes produced are relatively small, since for a chemical reaction in which G changes are produced per


100 eV absorbed only G micromoles are obtained per gram of material subjected to 1 megarad. In polymers small changes in molecular weight due to crosslinking or degradation can be readily traced by physical methods, but other chemical changes are more difficult to follow. The methods used include analysis of the gases evolved, changes in weight of the irradiated specimen, infra-red and ultraviolet spectroscopy and chemical titration. Direct chemical analysis is rendered difficult by the infusibility and insolubility of irradiated polyethylene. By using model compounds such as paraffins, comparative results may be obtained on soluble materials subjected to much higher doses; in applying these results to polyethylene, attention must be paid to the more complex structure of the polymer as well as to secondary reactions due to the effect of radiation on the primary products.

Weight Changes and Gas Evolution

It was early observed that polyethylene rods subjected to nuclear reactor radiation in the presence of air show weight changes, and evolve gas. By irradiating specimens of varying surface to volume ratio, the expression for the weight changes (AM) was shown to contain terms depending on the surface area exposed (A), or on the mass of polymer irradiated (M).

0 10 20 30 4 0 Radiation dose

FIG. 13.20. Change in weight of pile-irradiated polyethylene. Effect of surface to mass ratio {A/M).

+ AjM=2$'3 X A\M= 13-4 O Δ D Λ/Μ=2·7, 2-9 cm'/g.

AM = - 0 -7 x 10~3 M r + 5 x 10~6 Mr2+0-17 x 10~8 Ar~4x 1 0 6 Ar2

where r is expressed in pile units (Charlesby, 1952). The terms Mr and Ar, which predominate except at very high doses, may be ascribed mainly to

POLYETHYLENE 225

οί

OC*

O (

o / o

Ξ°ο

/

I τ

> c o o /

o

c

)

> o I

/ /

OI}DJ uoqjDo o} uaßojp^H

s^jun

a|id

'a

so

p

uoi^DipD

y

FIG

.13

.21.

Dec

reas

ein

hydr

ogen

/car

bon

ratio

inir

radi

ated

poly

ethy

lene

.


hydrogen evolution and to surface oxidative reactions. The formula fails for films less than a few millimetres in thickness and this may therefore be considered as the limiting depth of the surface effects. The G value for hydrogen evolution, derived from the first term, is about 6-7, but this is certainly an overestimate, due to other causes of weight loss. Miller et al. (1956) derived G(H2) = 5-7 for low density polyethylene, 5-4 for poly-methylene and 4-3 for octocosane.

A number of G(H2) values have recently been reported, and show less scatter than do the G values for crosslinking. According to Tolbert and Lemmon (1954) the G(H2) value for a series of //-paraffins tends to a limiting value of 3-2, but Dewhurst (1957) has obtained values of 4-7 to 5-2 (with no significant trends) in the range C6 to C16. Other G(H2) values for simple hydrocarbons are given in Table 12.2.

In polyethylene Dole et al. (1957) obtain a value of 3-75, Charlesby and Davison (1957) give values of about 3-1 for temperatures from — 196°C to + 100°C. Schumacher (1958) obtains 2-5 for high density, and 2-9 for low density materials (difference probably not significant), this G value being independent of dose up to about 350 megarads of x-rays, when secondary effects appear.

Changes in the hydrogen-carbon ratio in polyethylene specimens result from the preferential loss of hydrogen, due either to crosslinking or changes in unsaturation (see page 229). At the radiation levels usually employed this change is small, but values of H/C, obtained by micro-chemical analyses, over an extensive range of reactor radiation doses, are shown in Fig. 13.21. The initial slope appears to be greater than the average, and may be due to the limiting degree of unsaturation which is attained at high doses.

Gas analyses carried out on a number of different specimens, and under different radiation conditions, agree in showing that hydrogen evolution far exceeds that of all the other hydrocarbons combined. Serious quanti-tative differences have, however, arisen as to the minor products (Table 13.3). By analogy with the irradiation of linear and branched paraffins, one would expect branches attached to the main polymer chain at a tertiary carbon to be particularly susceptible to radiation and this indeed is found to be the case. Harlen et al. (1955) irradiated a series of poly-methylenes with branches of known length prepared from mixtures of the appropriate diazoalkanes. The gaseous hydrocarbons (in addition to hydrogen) evolved following electron irradiation at 150°C, were largely determined by the structure of the side chains introduced into the material; some unsaturated hydrocarbons were also produced as well as a very appreciable amount of methane. Although no quantitative data are given, it is stated that the short side chains are removed selectively. This offers a unique method of estimating the presence of side chains in polyethylene, and the method has been applied by Harlen to two com-mercial high pressure polyethylenes. The presence of unsaturated short hydrocarbon chains in the gases evolved may be accounted for by some hydrogen transfer reaction to the main chain. The precise mechanism is

Tab

le 1

3.3.

Gas

Evo

luti

on f

rom

Ir

radi

ated

Pol

yeth

ylen

e an

d P

araf

fin

(Per

cent

age

of t

otal

—un

satu

rate

s in

bra

cket

s)

Hyd

roca

rbon

ev

olve

d

H2

C c 2

c 3

Q c 5

c 6

c 7

c 8

Alk

athe

ne

98 0-5

0-5

0-5

0-5

Cha

rles

by e

t al.

19

52

Dup

ont

98-6

0-

2 0-

7 01

(05

) 0-

35

Dol

e et

al.

19

54

Poly

ethy

lene

s

Dup

ont

97 0-4

0-9

0-2

(0-2

) 1-

0(0-

2)

Dol

e et

al.

19

54

Br.

H.P

. Po

lyth

ene

97 0-2

0-8

(0-6

5)

0-2

(004

) 0-

6 (0

08)

015(

003)

00

7 (0

07)

005

(008

)

Har

len

et a

l.

1955

Am

. H.P

. Po

lyth

ene

97 01

6 0-

84 (

0-4)

00

4(01

2)

0-74

(0-

2)

015(

008)

0-

2 (0

04)

005

Har

len

et a

l.

1955

Com

mer

cial

85

Law

ton

et a

l.

1954

Poly

met

hyle

ne

99-4

0

1 00

5 (0

-3)

001

(008

) 00

3 (0

01)

Har

len

et a

l.

1955

99-9

005?

005?

Mill

er e

t al.

19

56

Oct

acos

ane

91 0-5

21

1-3

0-9

M

0-7

0-6

Mill

er e

t al

. 19

56

See

also

Sch

umac

her

(195

8), P

arki

nson

et a

l. (1

958)

.

Q

227 P O L Y E T H Y L E N E


however still unsettled, as is the formation of unsaturation in the irradiation of polyethylene and paraffin molecules generally.

1·Οι 1 1—]—! 1—:—ΓΊ 1-Or

0-8

a o o c 3 O

I 0-41 > ■5 0

^

A /

l

\ I

—

!..

**?

0-8j

0-6

0-4

0-2

0

A

/ 1 1 \ 11 f I>i1l

2 S ^ ^ ^ 0 ? C 1 C 2 C 3 C 4 C 5 C 6 C 7

American British FIG. 13.22. Gas evolution of hydrocarbons from irradiated branched polyethylenes

saturated. unsaturated. d , C2, C3 etc., represent number of carbon atoms in hydrocarbon.

(From Harlen et al., 1955.)

The possibility that the observed scission of polyethylene chains does not occur at random, but arises at points of weakness along the chain such as branching points, has long been suggested. It is known from mass spectroscopy and from the irradiation of linear paraffins that the last few carbon bonds in the chain are particularly liable to fracture, giving rise to methane, ethane and propane, etc. In branched paraffins there is a greatly increased fracture of side chains. The existence of branches on low density polyethylene, but not to any appreciable extent on poly-methylene, would then account for the considerable difference in the ratio of hydrocarbons to hydrogen evolved as shown in Table 13.3. Both octacosane and low density polyethylene, with a relatively high ratio of side chains or ends to C—H bonds, have a correspondingly high ratio of hydrocarbon to hydrogen evolution. Unsaturation

R R

Apart from the formation of crosslinks C — C the most notice-

R R able chemical change which occurs when long chain paraffins are irradiated in the absence of air is an increase in main chain unsaturation. The increase in /ra/w-unsaturation in electron-irradiated polyethylene is shown in Fig. 13.23. Similar changes have also been observed in the irradiation of olefins, the tendency being for the amount of unsaturation to change towards a limiting value. Octadecene, for example, shows little change in unsaturation (Charlesby, 1955) and it may therefore be provisionally assumed that the limiting value corresponds to about 5-5 per cent un-saturation (i.e. one C = C bond per 18 C—C bonds). In agreement with this value, oleic acid (C9 = C8COOH) bombarded with deuterons

POLYETHYLENE 229

shows little tendency to be reduced to stearic acid (Ci?—COOH) (Burton, 1949). In rubber with a much higher degree of unsaturation initially present there is a decrease in unsaturation on irradiation.

The presence of unsaturation has a significant effect on the properties of irradiated polyethylene—particularly its susceptibility to subsequent oxidation. Several attempts have been made to estimate the amount of

0-25

8 0-20 ό

° 0-15 υ c 5 ο·ιο o «Λ < 0 · 0 5

0 100 200 3 0 0 400 500 megaroentgen

FIG. 13.23. Increase in /ra/jj-unsaturation. (From Lawton, Zemany and Balwit, 1954.)

unsaturation produced, but unfortunately chemical methods are difficult to apply in a crosslinked polymer. Most of the work has been confined to infra-red studies of irradiated film, and to the measurement of bromine or iodine uptake.

In the infra-red spectrum, three main bands appear, related to double bond structure:

— CH = CH — r/O/w-vinylene; 10·35μ RCH = CH2 terminal unsaturation (vinyl); 11·0μ R \

C = CH2 pendent méthylène (vinylidene) ; 11·25μ.

R The latter two correspond to pendent groups at the end of, or along, the chain, while inms-vinylene unsaturation occurs within the main chain, when two adjacent hydrogen atoms are removed. Cw-vinylene unsaturation cannot be readily detected by infra-red methods, and lack of information on its concentration is a serious weakness of the infra-red technique. A further uncertainty arises in the conversion of the observed optical absorption into concentration of double bonds, the extinction coefficient varying with the state of specimen, its temperature, etc. For this reason most attempts at absolute measurement have been checked by the bromine or iodine values.

> /

/

/

A

A X

/

Tabl

e 13

.4

Uns

atur

atio

n an

d B

ranc

hing

in

Thre

e Ty

pica

l P

olye

thyl

enes

{u

nirr

adia

ted)

Den

sity

T

ype

Uns

atur

atio

n: C

= C

per

100

0 C

Tot

al u

n-sa

tura

tion

M

ain

chai

n tr

ans

(a)

Ter

min

al

(b)

Side

cha

in

pend

ent

(c)

Met

hyl

grou

ps C

H3

per

1000

C

Ter

min

al

Tot

al

met

hyl

ends

B

ranc

hes

met

hyl +

et

hyl

I.C

.I,

high

pr

essu

re

Zie

gler

low

pre

ssur

e

Phil

lips

low

pr

essu

re

Low

Hig

h

Hig

h

Supe

r D

ylan

M

arle

x 50

0-6

0-7

1-5

0-1

(17%

)

0-3(

43%

)

1-41

(9

4%)

C =

C

—

00

9(1

5%

) 0

41

(68

%)

0-22

(32%

) 0-

18(2

5%)

00

15

(1%

) 0-

075(

5%)

c=c

21-5

3

<l-

5

-C- II c

Infr

a-re

d ab

sorp

tion

(μ)

10

-35

101

110

4-6

-1-5

—C

H3

7-25

16-9

<1

-C

—

I

CH

3

C2H

5

(eth

yl)

130

Dat

a ba

sed

on D

. C

. Sm

ith,

Ind

. E

ng.

Che

m.

48,

1161

, Jul

y 19

56.

230 ATOMIC RADIATION AND P O L Y M E R S

Proc

ess

11-2

5

DY

NH

POLYETHYLENE 231

In the study of octacosane and polymethylene by Miller et al. (1956), no observable unsaturation prior to radiation was found. In low density polyethylene all three forms of double bonds are found to exist. Rugg et al. (1953) report 60 per cent vinylidene groups and about 20 per cent each of vinyl and /ra/w-vinylene groups in a specimen of unirradiated DYNH low density polyethylene: a total of 0-38 x 10~4 mole/g. Bromi-nation of similar unirradiated material by Miller et al. (1956) led to a total initial unsaturation of 0-54 x l O - 4 mole/g, i.e. about one double bond per number average molecule of molecular weight 20,000. Dole et al. (1954), working on a Dupont polyethylene film of Mn = 32,000 deduced from infra-red data a total of 1Ό3 double bonds per number average molecule.

The subsequent changes on irradiation have been studied by Dole et al. (1954, 1958); Lawton, Zemany and Balwit (1954); Miller et al. (1956); Ballantine et al. (1954); Black and Charlesby (1955, 1958); Luy and Schumacher (1956); Busse and Bowers (1958); Schumacher (1958).

Main Chain Unsaturation The results obtained by Miller et al. for polymethylene and octa-

cosane indicate no radiation induced formation of vinyl and vinylidene double bonds, but some formation of /raws-unsaturation. From infra-red and bromination experiments values of 0-81 x 10~4 mole/g (trans) and 0-85 x lO - 4 mole/g (total) unsaturation were obtained for octacosane sub-jected to 200 megaroentgens, the agreement obtained being taken to indicate that c/s-unsaturation was not formed under the conditions of the radiation. The average corresponds to a G value for unsaturation of 1-9.

In polyethylene a G (unsaturation) value of 2-2 was obtained by bromi-nation measurements ; because of the unknown state factor (depending on crystallinity) less reliable values of 1-2 or 2-4 were derived from infra-red measurements. A parallel set of infra-red observations made on irradiated polymethylene gave identical results, showing that unsaturation is not modified by branching.

Dole et al. (1954) carried out parallel experiments in the Argonne nuclear reactor, but their radiation doses were calibrated in terms of hours of exposure in a "goat hole" of the pile. An approximate equivalent dosage can, however, be deduced from the hydrogen evolution. Assuming a G value for hydrogen production of 5-7, and a rate of gas evolution of 8-7x 10-7 mole/g per "goat hole hour" 96-98-6 per cent being hydrogen, 1 goat hole hour is equivalent to about 0-15 megarads of electron or gamma radiation. On this basis the maximum exposure of 1,170 hr, used by Dole, is equivalent to 175 megarads, and would deposit 583 eV per polymer molecule (A/w = 32,000) to produce the (infra-red) observed value of 10-2 trans-aoxxbte bonds. The G value for /ra^s-unsaturation would then be 1-75, in satisfactory agreement with the results of Miller et al. (1956). The value obtained by bromination experiments, however, is 2-5 times this value (leading to a G value of 4-4), twice the value obtained by Miller. Dole et al. interpret the difference between trans- and total


unsaturation as arising from the presence of c/s-unsaturation, not observed in infra-red work. This does not explain the difference in bromination values obtained by Dole and Miller; Miller suggests that the difference arises from bromine substitution which unless corrected for may lead to considerable error.

Black and Charlesby (1955, 1958) measured unsaturation by an iodine method. To reduce the difficulty of penetration of the Wijs solution into a crosslinked network, thin polyethylene film (0-5 and 1-5 mil film) was used. In addition parallel experiments were carried out with hydrocarbons in which the same radiation doses still left the specimens completely soluble. The results showed that the same amount of unsaturation was produced in various paraffins (C10, C34, C36) as in polyethylene and poly-methylene; in methyl stéarate the production of unsaturation was some-what less. The unsaturation produced did not vary significantly with film thickness, nor with oxygen concentration, the figure being 0173 C = C bonds per 100 C atoms, per unit pile radiation, and 00034 C = C bonds per 100 C atoms, per megarep of 4 MeV electron radiation. The ratio indicates an equivalence of 51 (±10 per cent) megareps of electron irradiation per pile unit in fair agreement with the values obtained by other means. These values correspond to a G value of unsaturation of 2-3.

</) E o σ c

α υ O O ^-

h CL

<J II

u

£ KJ

1-5

1-0

0-5

I I • Polyethylene a Tetratriacontane Δ Hexatriacontane * Methyl stéarate © n deçàne

9*°

X*

sK S* Δ

i

100 500 200 300 400 Dose, megareps

FIG. 13.24. Increase in /ra/«-unsaturation in several hydrocarbons.

Later measurements by Dole, Milner, and Williams (1958) give G value for transvinylene of 2-2, 1-8 and 1-5 for various linear and branched polyethylenes. If an allowance is made for the simultaneous destruction of vinylene, these values are increased by 0-2 for a branched polyethylene; Schumacher (1958) obtained about 1-25, independent of the type of poly-ethylene, while Charlesby and Davison (1957) found 1-2-1-5 with only a slight temperature dependence. There is a general agreement that the G value decreases with dose, the increase in trans-unsaturation following an exponential law approximately

A(C=C) = Amax(C=C) [1-exp (-r / r 0 )] .

POLYETHYLENE 233

The limiting concentration Amax(C=C) is greater in the more crystalline polyethylene (Dole et al., 1958). The existence of a limiting value for the trans-unsaturation may be due to some form of radiation-induced hydro-génation by the hydrogen evolved. Directly supporting this suggestion is the observation of Schumacher (1958) that trans-\'my\zn.Q unsaturation produced by 250 megarads is reduced by 30 per cent when hydrogen at a pressure of one atmosphere is initially present. One would then expect the limiting concentration to depend on a number of factors, e.g. dose and specimen size in relation to that of the container.

If H 2 is produced by crosslinking, and by changes in unsaturation

G(H2) = G(crosslink) + G(C=C).

According to these results the proportion of H2 evolution arising from crosslinking would constitute about 60 per cent of the total. However, Dole's results indicate that unsaturation is the major effect and cross-linking accounts for only 20-40 per cent of the total hydrogen produced.

The overall loss of hydrogen can be measured by changes in the H/C ratio of the irradiated polymer. The decrease in this ratio is rapid up to about 10 pile units (500 megareps) and then is approximately halved. At first hydrogen evolution results both from increased unsaturation and from crosslinking. Once the degree of unsaturation reaches a maximum value (at about this radiation dose) only hydrogen evolution from cross-linking continues. The magnitude of the changes in H/C (Fig. 13.21) agrees approximately with the results deduced from fratfs-unsaturation measurements.

Pendent Double Bonds The other forms of unsaturation initially present in branched poly-

ethylene (but not in polymethylene or octacosane) usually show a rapid decrease with radiation dose. Vinylidene double bonds disappear after 15 megaroentgens (Miller et al., 1956) or about 20 megaroentgens (Dole et al., 1954), while the small amount of vinyl double bonds disappears after 50 megaroentgens (Miller et al., 1956) or increases very slightly (Dole, et al., 1954; Black and Charlesby, 1955). A detailed study of changes in vinyl and vinylidene unsaturation is given by Dole, Milner and Williams (1958), who report an approximately exponential drop in concentration, with initial G values of 3-7 (vinyl) and 1-8-9-6 for vinylidene. These high G values may result from a very effective energy transfer, from some form of hydrogénation, or form a participation in crosslinking, as suggested by Dole and Keeling (1953) and Pearson (1956).

Effect of Temperature during Irradiation

The degree of crosslinking produced in polyethylene by a given radiation dose increases with the temperature of radiation. Black (1956) deduced a G value for crosslinking in low density polyethylene by measuring the elastic modulus at 140°C when the disturbing effect of crystallinity is absent. Specimens subjected to γ-radiation at temperatures from —196° to +73°C showed a change in G value by a factor of about 4. The


effect was confirmed in a detailed investigation in which the cross-linking density in electron irradiated samples of both low and high density polyethylene was estimated using three different techniques—elastic modulus, swelling and solubility (Charlesby and Davison, 1957). To avoid any interference from possible oxidative degradation, the samples were thoroughly outgassed prior to radiation. The results obtained by the three different techniques showed the same variation with temperature from —196° to the melting point and the shape of the curve was similar to that obtained by Black although the radiation intensities differ by a factor of about 105. The G value for crosslinking at room temperature (2-5-3Ό) agrees well with the values deduced earlier for crosslinking in w-parafrins. The difference in the absolute G values for crosslinking between the two sets of experiments may be due to small systematic errors in dosimetry but the temperature factors shown in Fig. 13.25 are more

°c

•HOP 0 ~60 -120 -196 Γ 1

a

* >.

r 1

1 1

* ■ * - ·-

^

X -

NT

.._..

V

> \

»«

1

» _ . -

^ Π V

' - » ,

1

%

— — —

> h

— ,

-u<-

- · —1

\=c ■

»H2-J

H-

— ^ x i

I ZL 20 4 0 6 0 8 0 100 120 140

104/7" FIG. 13.25. Effect of temperature on crosslinking, unsaturation and hydrogen evolution.

G values are given for hydrogen evolution ( · ) , formation of /r#«.s-unsaturation (x), and loss of initial unsaturation ( + ) ; they were all measured on the same samples (20 megarads).

G values for cross-links X were deduced from elasticity ( 4 after correcting for end-effects; 25 and 40 megarads), from swelling (0) and solubility (□) by comparison with the dose required to give the same effect at room temperature and normalized to G(X) = 2-5 at 0°C (20 megarads).

G(X) values deduced from elasticity measurements for γ-irradiation in air are shown (v ) for comparison, after correcting for G(FQ) = 15-5.

reliable. Above the melting point, differences in crosslinking density appear as between measurements based on solubility and on swelling data. The latter show a decrease in G value for crosslinking which may

o o

IUI

5

1·0

0-5

POLYETHYLENE 235

represent a change in the crosslinking mechanism as between the solid and liquid phases.

In parallel with these experiments, G values for unsaturation and for hydrogen evolution were derived. These show little change with tem-perature. One must therefore conclude that the simple equation assuming that hydrogen arises from crosslinking and from unsaturation

<7(H2) = G (crosslink) + G (unsaturation)

cannot hold. One tentative explanation is that both main and side chain fracture occur and that some degree of endlinking superimposed on cross-linking produces a change in molecular weight which accounts for the observed solubility, elasticity and swelling characteristics. The observation of Lawton that radicals are trapped in the crystalline regions is perhaps relevant although it is difficult to explain the lack of any difference in the observed reactions as between low density and high density polyethylene which differ by a factor of about 4 in amorphous content.

The formation of /raws-unsaturation which is readily revealed by infra-red spectroscopy was found to occur during the irradiation process even at low temperatures and not subsequently on warming up. It appears unlikely that hydrogen abstraction reactions can occur as readily at — 196°C as at + 100°C and the formation of unsaturation in polyethylene may therefore be attributed at least in part to a molecular process in which a hydrogen molecule is formed from an individual polyethylene molecule. This conclusion is in agreement with the results of Dewhurst and of Davison on low molecular weight paraffins.

Oxidation The physical and chemical properties of polyethylene can be seriously

modified when irradiations are carried out in air. Oxygen reacts readily with radicals, reduces the extent of crosslinking and forms carbonyl com-pounds whose stability under heat is less than that of polyethylene itself. A number of papers dealing with the irradiation of polyethylene have discussed the effects produced when radiation is carried out in the presence of oxygen.

The formation of an oxidized film on the surface of polyethylene rods, subjected to BEPO reactor radiation, was reported in 1952 (Charlesby). This surface layer was waxy in appearance, and after 26 units of radiation corresponded to an empirical formula C3H50. It can be readily removed, but its presence accounts for changes in ultraviolet fluorescence, and can give irradiated polyethylene a more hydrophilic surface, suitable for printing.

Dole et al. (1954) irradiated thin films of polyethylene in the Argonne nuclear reactor. The specimens were subjected to three doses of radiation, either in air or in vacuum; differences were observed between these two sets of specimens. In the presence of air, there was an increase in weight although this increase was less than the oxygen uptake measured chemically, the difference being due to the loss of hydrogen and other


gases. Changes in the infra-red spectrum occurred, notably at about 5·9μ, due to the carbonyl stretching frequency, and at about 2·9μ probably due to the hydroxyl group. The carbonyl absorption was observed to increase with radiation dose but to tend towards a maximum value.

Ballantine et al. (1954) subjected thin polyethylene film (4 mil) to electron and γ-radiation at 25°C and at — 18°C. In addition to unsatura-tion (discussed above) an increased absorption in the infra-red spectrum at 2·9μ and 5·85μ was observed. These two wavelengths correspond to OH and C = 0 absorption as previously reported by Dole. The intensity of these bands increased with radiation dose and with temperature of irradiation. By comparing these intensities with thicker film (10 mils) irradiated under similar conditions, Ballantine showed that most of the hydroxyl and carbonyl absorption was concentrated in the surface layers.

In his work on the mechanical properties of irradiated polyethylene at temperatures above the melting point, Chapiro (1955) found that the mechanical strength at high temperature of 2 mm films, subjected to γ-radiation from a cobalt source, depended not only on the dose but also on the dose rate when oxygen was present during the irradiation treatment. Oxygen was found to reduce the degree of crosslinking and to form peroxide bridges between molecules. These bonds decomposed at temperatures of about 150°C. Under normal conditions, polyethylene contains a certain amount of oxygen and if the radiation intensity is low so that the time needed to reach a certain dose is protracted, further oxygen can diffuse into the system. The extent of the oxygen effect will therefore depend on the exposure time as well as on the dose. For example, at a radiation intensity of 432 roentgens/min, the efficiency in air of cross-linking was reduced to about one-third of that observed in vacuum. At very high radiation intensities, as with electron irradiation, the oxygen effect is largely confined to the dissolved oxygen.

To compare the effect of radiation conditions Black and Charlesby (1955) carried out measurements on polyethylene films of various thick-nesses subjected to nuclear reactor radiation at Harwell, to Co60y-radiation and to electron irradiation from a 4 MeV accelerator. Changes due to the presence of oxygen were traced by infra-red, ultraviolet, oxygen absorption, electrical and chemical measurements and by the weight changes.

In the infra-red spectrum, absorption maxima ascribable to oxygen-containing bonds were observed at wavelengths of about 2-9, 5-9, 11-5 μ together with a group in the neighbourhood of 8·5μ. None of these maxima were observed when the radiation was carried out in a vacuum.

The hydroxyl absorption at about 3μ could be ascribed to the presence of alcohols or acids. The intensity of this band varied in approximately linear fashion with the radiation dose up to about 200 megarads but above this value showed signs of tending to maximum. At the same time, there was a slight shift in the wavelength of this maximum, which could be due to the absorption of hydroperoxide in the absorption band. On subsequent heating after irradiation, the absorption in this region tended to decrease.

The infra-red spectrum at 11·5μ gave evidence of the presence of

POLYETHYLENE 237

peroxides. A direct chemical measurement gave the following values for the peroxide concentration on 10 mil film irradiated in air in the BEPO reactor (Table 13.5).

Table 13.5. Peroxide and Hydroperoxide Content of Irradiated Polyethylene

Radiation (pile units)

0 0-6 1-2 3-2

Peroxide found O—O per

1

100 C atoms

0015 008 010 0-21

The formation of carbonyl compounds such as aldehydes and ketones was traced from the optical density of the carbonyl band at 5·8μ. The optical density for films of varying thickness subjected to various radiation doses is shown in the following Table 13.6. The figures in brackets are calculated assuming O.D = 5 x 10_3r + 2-5 x 10~6r2d where d is the thick-ness in mils.

Table 13.6. Carbonyl Absorption for Various Film Thickness

4 MeV electron dose in megarads Optical density at 5 8μ Calculated

surface effect

5 20

100 200

film thickness in mils 2 1 4 1 7

0031 0116 0 485 (0-55) 1-200(1-20)

0055 0159 0-616 (0 60) 1-42 (1-40)

0077 0199 0-671 (0-67) 1 68 (1 70)

0025 010 0-50 10

The optical densities quoted differ from those given earlier by Ballantine et al. (1954) for films of similar thickness and subjected to electron beam irradiation at about the same temperature. These investigators also found considerable differences between γ- and electron radiation at the same temperature. These differences indicate that the rate of oxygen attack varied according to the conditions of radiation.

Ultraviolet measurements also reveal considerable changes due to the formation of carbonyl groups, these being most readily revealed by changes in the ultraviolet absorption spectrum in the neighbourhood of 265 πιμ. When heated, polyethylene which had previously been irradiated in the presence of oxygen shows an increased rate of oxygen uptake; in particular the carbonyl increases at the expense of the hydroxyl group. The general infra-red background is increased due to the formation of


ethers and carboxylic compounds. A general increase in the ultraviolet absorption is also observed, particularly in the region of 225 and 260 ηψ.

Even when radiation takes place in vacuum, carbonyl formation may subsequently take place (Schumacher, 1958). This effect is greatest for irradiations carried out at lower temperature (10°C) and is affected by subsequent annealing. It may be associated with trapped radicals, or with the decrease in unsaturation.

Black and Charlesby observed an increase in peroxides to a maximum value on heating and then a decrease. This would agree with the observa-tions previously made by Chapiro on the breakdown of such bridges at temperatures of about 150°C, leading to radical formation.

The changes produced by heating irradiated polyethylene can also be followed by observing the weight changes. In many cases, these show an initial loss of weight due to the evolution of volatile material. There follows an induction period depending on the presence of antioxidants, followed by a steady increase to a maximum value as the material oxidizes.

The effect of oxygen on the solubility of polyethylene irradiated in air has been studied by Alexander and Toms (1956). For thick specimens such as rods, the difference in solubility is relatively small even when the dose is acquired over a period of many hours (Table 13.7). In thin specimens, the effect is much more serious and could result either from an increase in the number of chain fractures, or a reduction in the number of crosslinks. St. Pierre and Dewhurst (1958) have concluded in favour of the latter, unstable peroxide bridges being formed in lieu of crosslinks.

100, 8 0

6 0

4 0

10

ol 1 2 4 6 810 20 40 60 100 128

megarads

FIG. 13.26. Decrease in sol fraction with electron dose for 18μ specimen irradiated in air or in vacuum.

(From: Alexander and Toms, 1956.)

The effect of film thickness on the degree of crosslinking is shown in Table 13.7; the thinnest films irradiated in air show the greatest oxygen effect while when the same thin films are tightly packed, diffusion of oxygen is hindered and interference with crosslinking is greatly reduced.

\

Air

Vacuum

POLYETHYLENE 239

Table 13.7. Effect of Oxygen on Crosslinking and Solubility

Specimen

Rod 1 cm dia. Film 50μ thick Film 18μ

50μ ΙΟΟμ 175μ

Radiation dose

Reactor ( ^ 2 5 megarads) Reactor ( ^ 2 5 megarads) Electrons (18 megarads) Electrons (18 megarads) Electrons (18 megarads) Electrons (18 megarads)

Sol fraction %

Irradiated in air

28 60 74 69 49 38

Vacuum or

nitrogen

23 28 41 36 34 33

Tightly packed wads

38 35 33 31

From Alexander and Toms (1956). Electron radiation at 3 megarads/min, 1 MeV.

MECHANICAL AND ELECTRICAL PROPERTIES In considering the mechanical properties of irradiated polyethylene, it

is necessary to take into account not only the radiation dose r which

2 0 0

O x

0-4

0-2

2-93

6 0 100 Temperature, °C

FIG. 13.27. Variation of E with temperature and radiation dose (dynamic values) (From Charlesby and Hancock, 1953.)


determines the proportion q of crosslinked monomer units but also the temperature of measurement, since this largely determines the degree of crystallinity present in a given type of polyethylene. An example of the variation in the elastic modulus with temperature and with radiation dose is shown in Fig. 13.27 for polyethylene irradiated in the BEPO reactor (Charlesby and Hancock, 1953). The figure shows the sudden change which occurs in the modulus near the temperature at which crystallinity completely disappears in the more lightly irradiated polyethylenes. For highly irradiated materials, however, no crystallinity is present even at room temperature, and the transition is far less marked or does not occur at all. It is therefore convenient to consider separately the elastic pro-perties of polyethylene at temperatures above the melting point of the crystalline regions (when the elastic properties are dominated by the presence of radiation induced crosslinks) and the same properties below the crystalline melting points, when the elastic properties arise from the combination of radiation induced crosslinks and crystalline regions.

Elastic Properties Above the Melting Point The theory of elastic deformation of an elastic network has been sum-

marized above (Chapter 9). Theory gives a relationship for the stress / required to elongate a specimen by a factor a (a being the ratio of the elongated length / to its original length /0). This expression can be written in the form

where the important variables are the absolute temperature T (which also affects p to a minor extent), the elongation represented by the factor a, the degree of crosslinking expressed in terms of the molecular weight between crosslinks Mc and the initial finite molecular weight Mn occuring in the Flory correction factor (1 — 2McjMn). From the radiation point of view, this expression can be written in a more convenient form (equation 9.46)

/ = ?RTw^ (α-1/α2) qoÇr-2/qouO

which can be tested in a very extensive manner by studying the elastic stress/'as a function of the variables a, T, r and ux. To avoid interference from crystallinity in polyethylene, it is necessary to carry out these measurements at temperatures above the melting point.

Shape of Stress-Strain Curve The general shape of the stress-strain curve at constant temperature

and radiation dose is represented by the relationship

f<x a—1/a2.

Fig. 13.28 shows the observed relationship, the experimental values being represented by crosses while the theoretical curve is indicated by a full line. This curve was obtained for polyethylene subjected to 40 megarads of

POLYETHYLENE 241

radiation. Lower radiation doses permit a much higher elongation of the stressed material and the data obtained by Lawton, Balwit and Bueche on polyethylene after about only 15 megarads of radiation is shown in Fig. 13.29 (150°C curve). For elongations beyond about 100 per cent the additional factor 1/a2 becomes very small and there is often an approxi-mately linear relationship between stress and strain up to the breaking point.

en

in 3|

έ 2 1

~ Ί — I I I I I Γ STRESS/STRAIN CURVE

High density polyethylene 40 megarad Test temp. 140° U ^

/+

I xT + +Observed values-

Theoretical curve

10 20 3 0 4 0 50 60 70 80 9 0 100 110 % elongation

FIG. 13.28. Stress-strain curve for polyethylene subjected to electron radiation at 20°C.

(From Charlesby, von Arnim and Callaghan, 1958.)

3 0 0 0

2 0 0 0

0)

d 1000

fî-81°C ßp.s.i. χ2 ·5

p.s.i.xl

..·-·

VL ^

+ 27VC

I

100

+ 150°C-• Exp. o Calc.

500 2 0 0 3 0 0 4 0 0 % elongation

FIG. 13.29. Stress-strain curve for low density polyethylene subjected to 15 mega-roentgens electron radiation at room temperature, tested at — 81°C, +27°C and

+ 150°C.

Radiation Dose

According to equation (9.46), the elastic modulus should vary directly with the radiation dose, apart from a correction factor depending on the initial molecular weight. For very high doses of radiation, this factor may be ignored. The results obtained by Charlesby and Hancock (1953) and by Baccaredda et al. (1956) using low density polyethylene subjected to a wide range of radiation doses in the BEPO nuclear reactor are shown in


Fig. 13.30. In this case, both static and dynamic measurements were carried out, the higher values obtained by dynamic measurements arising from the internal viscosity of the polymer. (This difference also occurs in unirradiated polymers and is connected with the flexibility of the mole-cular chain.) Theoretical curves have been calculated on the basis of the theory of rubber elasticity. Since the exact value of q0, the number of monomers crosslinked per unit radiation dose is not known accurately, two values are considered: qQ = 0-5x10~2 and 0-35 xlO-2 when unit radiation dose is the unit BEPO pile dose of approximately 50 Mrad. (In terms of the megarad, the corresponding values are 1 x 10~4 and 0-7 X 10-4

and the G values for crosslinking are 3-4 and 2.4.) It will be seen that at low and medium radiation doses the observed

points follow the theoretical curve approximately, but at much higher doses of the order of 20 pile units or above the elastic modulus is very much greater than that predicted by the theory of rubberlike elasticity. This departure from theory is not unexpected. The theory of rubberlike elasticity relies on the existence between successive links of long flexible chains distributed approximately according to a Gaussian distribution. At the high radiation doses shown in the figure, the average number of carbon atoms between successive crosslinks is only of the order of 10 or less. Such chains are too short to apply the statistical theory and are in

Percentage of carbons crosslinked, ςΐ0=0·ΟΌ5

0*5% 1*0% 5-0% 10% 25%

1010

«V 109

£ υ <υ c ■σ

^ 8 108

1

— x Dynamic measureme — + Static measurements

> /

/ /

x

//

—£L / /

/ / : / / <Λ/+ < / /

i

rv ΊΛ ; /

-p >ntsx£

r~ / / , /

( a ) /

UÈ -7 /

/, ; /

/ / / /

/ / / / / / /

1 5 10 20 50 100 Radiation dose, pile units

FIG. 13.30. Elastic modulus at 150°C. - theoretical curve for rubberlike elasticity assuming q0 = 0-5 X 10~2 (a) and

0-35 XlO-2 (6)·

P O L Y E T H Y L E N E 243

any case insufficiently flexible. The departure from the theoretical curve may therefore be considered as due to the non-applicability of the theory to networks with very high densities of crosslinking. The properties of such highly irradiated polyethylene approximate to those for a glass-like structure; the elastic modulus is high, the maximum elastic deformation low, and fractures have the same conchoidal structure as for rods of glass.

Temperature

The variation of elastic modulus with temperature is shown in Fig. 13.27. Above the usual crystal melting point, there is a slight temperature increase in the modulus (predicted in equation 9.46) for polyethylene irradiated to give it rubber-like elastic properties. Highly irradiated polyethylene, on the other hand, does not show any such temperature dependence, the elastic modulus continuing to fall slowly with increasing temperature. The theory of elastic properties of highly crosslinked materials has not yet reached the same stage of development as that for a rubberlike network, and the experimental data obtained for highly irradiated polyethylene may prove useful for comparison with such theories.

End Corrections

When the radiation dose is low, many of the molecules are tied into the network by only one crosslink. Such molecules cannot contribute to the elastic resistance to deformation and must be considered as long chain plasticizers. To evaluate the effective chain length between crosslinks, it is therefore necessary to subtract from the total number of crosslinks those needed to link together all the molecules into a single network. This amounts to a reduction in the total radiation dose by an amount r0

which depends on the number of molecules to be linked per gram of material, i.e. on the initial molecular weight.

Equation (9.46) can be written in the form

f=9RTw-H*-ll*2)qo(r-r0)

where r0 = 2/qo^ = 0-96 x \0&/GMn. Here, G represents the number of crosslinks produced per 100 eV absorbed.

According to this equation, a plot of .//(a—1/a2) against the radiation dose r should give a linear curve with a positive intercept r0y this intercept depending on the polymer chain length. Plots of this character are shown in Fig. 13.31. For a low density polyethylene, r0 was found to be equal to 11 megarads, as against a calculated value 2/<7<,Wi of 12*5 megarads. For an early form of high density polyethylene, r0 = 8 megarads, while for a later version r0 is close to zero. The values of Ui for these polymers are unknown. It should be pointed out that these values of r0 differ from the gel point obtained by solubility measurements; r0 as given here depends theoreti-cally only on the number average molecular weight, whereas the solubility measurements depend on the weight average. Moreover, there is no justification for extending this method of correction to low densities of crosslinking since a finite elastic modulus may be obtained when only a

R


small proportion of the molecules have been linked together to form a network. The correction as given here assumes that all molecules have been linked together.

k L _ iZ I I I I I

0 25 50 75 100 125

Dose, megarads

FIG. 13.31. Effect of initial molecular weight on crosslinking (test temperature 140°C). (From Charlesby, von Arnim and Callaghan, 1958.)

Determination of G Values for Crosslinking

The observed relationship between stress and strain can be used to derive values for the density of crosslinking per unit radiation dose q0

and hence for the G value for crosslinking. Since there is some doubt as to the validity of the end correction it is preferable to deduce q0 and G values from the more highly irradiated specimens, when this correction is small.

A further cause of error arises from the possibility of chain entangle-ments. For very long chains, those entanglements may serve as effective crosslinks, preventing molecules from flowing past one another unless the stress lasts sufficiently long for thermal vibrations to free the chains and allow them to slip past one another. The effect of entanglements has been represented by Flory by an additional factor g in the expression for the applied stress (equation 9.47).

Assuming various values of g, Lawton, Balwit and Bueche (1954) com-pared the number of crosslinks produced by a given radiation dose in polyethylene specimens of various molecular weights. With g = 1, the number of crosslinks formed by a given dose was found to be independent of molecular weight, whereas for g = 3 there is less crosslinking at higher molecular weights. The values obtained are shown in the following table for specimens irradiated at 15 and 30 megarads, using elastic measurements at both low and high elongations.

The agreement obtained between the measurements of elastic modulus at low elongation and the measurements of tensile strength at high elongation serves to verify the shape of the stress-strain curve predicted

POLYETHYLENE 245

Table 13.8. Number of Crosslinks per Unit Volume (xlO"20)

Elongation

Low g = 1 g=l

High g = 1 g = 3

Low g = 1 ^ = 3

High g = 1 g=l

Radiation dose (Mrad)

15 15 15 15 30 30 30 30

Specimen and molecular weight

DYLT 12000

0473 0-457

— — — — — —

DYNH 21000

0-361 0-292 0-367 0-294

—. — — —

DXH35 35000

0-435 0-248 0-318 0-208 0-684 0-334 0-709 0-339

Alathon 5 19000

0-391 0-32 0-381 0-316 0556 0-375 0-547 0-372

From Lawton, Balwit and Bueche (1954).

on theoretical grounds. From these measurements, G values of between 3-6 and 4-8 are obtained assuming g = 1, and G values of between 2-5 and 3 if g = 3. However, in carrying out these calculations a very large correction is needed for the end effect discussed above and this reduces the reliability of these values. The results obtained can be represented most satisfactorily by taking the entanglement factor g as equal to 1. This is in line with elastic measurements carried out on other polymers for which an alternative measurement of G is possible.

Further measurements of elastic modulus of high and low density polyethylene subjected to electron radiation were carried out by Charlesby, von Arnim and Callaghan, (1958). In this case, data were obtained over a much wider range of radiation doses so that the correction for the initial molecular weight is much smaller. Fig. 13.31 shows that the modulus obtained in this way is proportional to the radiation dose r if a correction r0 is made for the initial dose needed to form a closed network. Although the amount of amorphous material under the radiation conditions used varied by a factor of between 3 and 4, the slopes of these curves are similar, indicating that the q0 and G values for crosslinking are independent of the degree of crystallinity.

Elastic Properties at Room Temperature At room temperature, the elastic properties are largely dominated by

the presence of crystallinity. The shape of the curves obtained by either static or dynamic measurements are shown in Fig. 13.32. The dynamic measurements may, of course, be dependent on the frequency of measure-ment since this affects the viscosity of the system. In the figure, it is seen that at low radiation doses the elastic modulus is very little affected by radiation, the changes produced being due to a combination of a small degree of crosslinking and a small reduction in crystallinity. As the radiation dose is increased, crystallinity is reduced and this reduction has a greater influence than the simultaneous increase in crosslinking density. As a result, the elasticity decreases until a radiation dose of about 10 pile


units when crystallinity has almost disappeared. At about these radiation doses, the elasticity of polyethylene even at room temperature agrees with that to be expected from the theory of rubberlike elasticity. Further increases in the radiation dose cause a very rapid rise in the modulus. This rise, which is greater than that predicted by theory, is due to the

o

+ X

Stat ic compression measurements Dynamic at about 1kc/sec Dyne

->

3mic

- N ,

at v

"\+^

^ ^ s

3 k c -

*+

\ \ \ \

\ \ \

M

7kc/

>

/ / /

Jr {; Y/

// //\

sec

f

A Ά

/ - * "

/

X+

0 1 2 4 10 20 4 0 100 200 Radiation dose, pile units

FIG. 13.32. Elastic modulus at 20°C. Theoretical expression for rubberlike elasticity with q0 = 0-5 X 10~2 (a) and

0-35xlO-2(Z>).

formation of a very dense crosslinked network to which the theory of rubberlike elasticity does not apply. The departure from theoretical predictions is similar to that observed at the higher temperatures.

Elastic Modulus at Intermediate Temperatures

The radiation region in which rubberlike elasticity is observed is limited on the one hand by the need to keep the degree of crosslinking low, to avoid entering the cheeselike or glasslike state; and on the other hand by having absorbed sufficient radiation to destroy all crystalline regions at the temperature of measurement. Radiation has been seen to destroy the larger crystals preferentially (page 223), and therefore increasing the dose will extend the elastic region. In Fig. 13.33, due to Baccaredda et al. (1956), the effects of temperature and radiation dose are shown. At 60°C there is only a limited region in which the irradiated polyethylene behaves as a rubber. At 80°C and at 100° the region becomes progressively more extensive towards lower radiation doses. Below the elastic region the modulus is higher than that predicted due to the residual amount of crystallinity, not destroyed by radiation. Above this region it again exceeds the theoretical value owing to the very high density of linking, exceeding that to which the theory can apply. Since this density of cross-

POLYETHYLENE 247

20

10

5

2

1-0

0 -5 0-3 0-2

0

/ / ü3

f 1 1

-f— //

60 °C

Ξ=Ξ s

-u^.

A 80 X,

+1-_n^

/ I pfcr / f

J œ τ^

_

^ • ^ " ^ Ί Ί

100 °C /

I= j

/ I ι

s*

20 4 0 40 20 4 0 0 20 Radiation dose , pile units

FIG. 13.33. Effect of temperature on elastic modulus. o observed. theoretical expression for rubberlike elasticity.

linking does not depend on temperature, changes in the latter cannot affect the extent of agreement at the higher doses. This is confirmed by the data shown in the figure.

Shape of Stress-Strain Curve at Room Temperature

Fig. 13.34 obtained by Lawton, Balwit and Bueche shows the stress-

3000

2000

1000

o 6 N

§p«:i:=:--:

100 200 300 4 0 0 500 % elongation

6 0 0 700

FIG. 13.34. Tensile strength of low density polyethylene at room temperature. R shows dose of electron radiation in roentgens.

(From: Lawton, Balwit and Bueche, 1954.)

strain curve for a low density polyethylene subjected to various radiation doses from a 800 kV electron beam and then tested at room temperature. The initial sharp rise in the curve corresponds to the straining of the specimen against the forces between crystallites. Above a certain load which depends on time, these can become oriented and cold flow takes place. In low density polyethylene, the force required to continue drawing is close to or sometimes above that required to initiate cold flow. In high density polyethylenes, on the other hand, a greater force is required to


initiate cold flow (by necking the material) than is needed to continue drawing once it has commenced. This difference is reflected in the shape of the curve. In this region of high elongation, crystals are being oriented along the stress direction and the deformations are permanent in character; the polyethylene is then transformed into an oriented fibre-like structure with considerable strength along its length.

The effect of radiation on this pattern of behaviour is shown in Fig. 13.35. For low doses of the order of 10 megarads, there may be an increase in the elongation at break and in the tensile strength. Increased radiation doses, however, render the material more difficult to cold draw. The elongation at break is considerably decreased even though the tensile strength suffers little or no change. An initial increase in elongation and tensile strength has been observed in other lightly irradiated materials, e.g. rubber. It may possibly be explained as due to the increase in average molecular weight.

4800

4000

3200

2400

lOUU

800

ÎL |U:25

1 ,

(b) High d

10

^ 5

ensity poly«

0

^7

2 U " C I sthylene |

0 250 500 750 1000 Percentage elongation

FIG. 13.35. (a, b) Effect of radiation on cold flow and elongation at break at room temperature.

Doses in megarads of electron radiation at 20°C. Creep

The presence of crosslinks in the amorphous regions in polyethylene has a marked effect on the resistance to creep. Irradiated polyethylene has more elastic recovery than unirradiated material and even a small amount of radiation may be very effective in improving the properties when creep is to be avoided. Results obtained by Ballantine et ai (1954) may be quoted in this connexion. Their results on tensile creep (at 30°C),

POLYETHYLENE 249

Table 13.9. Creep and Recovery

Low density polyethylene subjected to γ-radiation at 25°C.

Dose

megareps

0 1 5

10 20 40 64

Tensile creep 30°C 10 sec 104 sec

compliance cm2/dynexl09

1-56 2-33 1-42 1-80 1-15 1-57 0-93 1-15 109 1-20 0-81 106 0-80 0-80*

Deformation length after 1 min 15 min

compression at 132°C % of initial thickness

0-22 012 0-30 016 0-47 0-37 0-50 0-41 0-65 0-61 0-78 0-69 0-84 0-84

% Recovery after 104 sec creep

in 10 sec in 104 sec (30°C)

504 69-5 78-6 100 73-5 79-5? 71-5 100 76-5 100 85-5 100

Failed.

recovery after 1Ü4 sec creep and deformation under compression (60 kg for a 1 in. dia. disk) are given in Table 13.9.

The table illustrates the reduced creep under stress, and the improved recovery resulting even from low radiation doses.

Elongation at Break and Tensile Strength The effect of radiation is to increase the tensile strength at room tem-

perature for radiation doses up to about 10 megarads; above this it decreases slowly (Fig. 13.36). The elongation at break also rises slightly,

700%

600%|

5 0 0 %

400%

3 0 0 %

200%

100%

20 4 0 6 0 80 100 120 140 Radiation dose, megarads

FIG. 13.36. Tensile strength and elongation at break of irradiated polyethylene. Low density polyethylene, molecular weight 21,000. Irradiated with 800 kV electrons,

tested at room temperature. (From Lawton et al., 1954.)


then decreases rapidly. This reduction in elongation at break with increased density of crosslinking is also shown in many other polymers.

Due to the loss of crystallinity, the tensile strength of unirradiated poly-ethylene falls off very rapidly with temperature (Fig. 13.37). The effect of radiation is to improve the tensile properties at high temperatures although

6000

5000

4000

3000

2000

1000

© Non- i r radiated x Irradiated -15x106 rep

J Λ \ χ

\ ~ " Έ —

—V\ V

-Λ^ -VJ - 5 0 0 +50 100 150 Test temperature , °C

FIG. 13.37. Tensile strength of low density polyethylene at various temperatures.

201 0 0-2 0-5 1-0 2-0

Radiation dose, 5-0 10

pile units

FIG. 13.38. Tensile strength of low density polyethylene at various radiation doses, and test temperatures. All measurements at a constant rate of strain of 1-8 in/min.

POLYETHYLENE 251

considerable doses are required to obtain tensile strengths at high tem-peratures in any way comparable with those for the unirradiated material at room temperature (Fig. 13.38). Above the usual melting point, the tensile strength does not appear to vary greatly with temperature. Irradiated polyethylene should therefore not be used in conditions where high temperature and high tensile strengths are required simultaneously. From the practical standpoint, the major effect of radiation on mechanical behaviour is to prevent polyethylene from flowing when heated above its melting point.

Sisman and Bopp (1951) have studied the elastic properties at room temperature of low density polyethylene irradiated in the Oak Ridge pile. The elastic modulus and shear strength increase slowly from about 1018 nvt (equivalent to about 700 megarads or 5 per cent crosslinking) and very rapidly from 6 x 1018 nvt. The impact strength decreases from lower doses of about 0-1 x 1018 nvt (about 0-5 per cent crosslinking) and the hardness from about 0-4 x 1018 nvt. Comparable data have been given by Bresee et al. (1956) using Co60 γ-radiation at an intensity of 1-2 to 3-6 x 104 r/min, and for doses of 0, 100 and 1000 megarads. Harrington (1956) has compared the change in elongation, tensile strength and weight in a number of types of polyethylene, subjected to 0, 5, 10, 50, 100 and 150 megarads and subsequently measured at 25°C. (See Chapter 31.)

Effect of Radiation Conditions

The influence of oxygen on the chemical changes produced by irradiation has already been referred to (pages 236 onwards). By reducing the number of crosslinks, and by forming unstable peroxide bridges, oxygen also influences the mechanical properties. Figure 13.39 shows such changes in thin polyethylene films, in which oxygen diffusion occurs readily over the long exposure period used. At high radiation intensities, as when electron

υ o Φ

1_ 3

■8. 3 1_

O

i-3

E Q. E Φ 1-

300.

250

200

150

υ 0-5 1-0 1-5 2-0 2-5 Dose, megaroentgen

FIG. 13.39. Comparison of temperature of rupture of thin polyethylene specimens. V = irradiated in vacuum 42-3 to 423 roentgens/min. A = irradiated in air 432 roentgens/min.

(From Chapiro, 1955.)

VI A


beams are used, the effect is far smaller because of the short exposure times available for oxygen diffusion.

The effect of irradiation temperature on the mechanical properties is due to the different crosslinking efficiency (page 234), as well as to different degree of crystallinity present. Ross (1954) obtained amorphous rubber-like polyethylene at lower doses when irradiation was carried out near the crystalline melting point. Furthermore when irradiation is carried out at room temperature, and the specimen is then annealed above the melting point, there is a subsequent reduction in crystallinity at room temperature (Woodward et ai, 1957, Dole and Howard, 1957, Williams et al., 1958). Presumably the strains imposed by the formation of crosslinks can be relieved at the higher temperature, and unstable crystalline regions, once allowed to melt, no longer reform. Schlichter and Mandell (1958) have also found that oriented films are more readily affected by radiation than unoriented material.

Little attention has been paid to other forms of radiation, in which ion pairs are formed in very close proximity. Shinohara et al. (1958) have used deuteron irradiation but have only studied changes in carbonyl con-centration and vinylene unsaturation. Sella and Trillat (1958) used low voltage ion beams such as H+, H2+, H3

+ and oxygen ions, and observed changes in the a dimension of the orthorhombic cell, together with a reduction in crystallinity. With hydrogen ions, unsaturation is decreased, as would be expected if some form of hydrogénation takes place. An interesting feature was the preferential attack of oxygen on the centre of spherulites, or in highly stressed regions. However no information is available on the effect of these changes on the mechanical properties.

Lawton, Balwit and Bueche investigated the effect of molecular weight of the starting material. Their results for the tensile strength at room temperature are shown in Fig. 13.40. It will be seen that there is an initial rise, and that the maximum corresponds very approximately to the "virtual radiation dose" r0 defined on page 244, which gives the correction for the finite initial molecular weight.

0 50 100 150 200 megaroentgen

FIG. 13.40. Ultimate tensile strength at different initial molecular weights (tests at room temperature 800 kV electrons).

POLYETHYLENE 253

Memory Effect An interesting and often amusing property of lightly irradiated poly-

ethylene has been termed the memory effect. It relies on the fact that on irradiation a crossiinked network is formed with a definite equilibrium state. When other constraints such as crystallinity or external stresses are removed, the polymer will return to the same molecular arrangement as it had during irradiation. Thus, at room temperature, a lightly irradiated tube or sheet does not show any obvious differences from unirradiated specimens, but on heating above the melting point (to destroy crystallinity) a highly elastic specimen is obtained, which can be readily deformed. On cooling the deformed specimen, it will recrystallize into this new shape, which it will retain permanently until reheated. If, subsequently, the tem-perature exceeds the melting point of the crystals, the specimen imme-diately returns to the shape it possessed during radiation. This retention of a memory of its initial shape provides a most striking demonstration of the existence of a network structure superimposed on any crystallinity. The memory effect in polyethylene is quite distinct from that observed in highly viscous polymers, where the deformation under load is partly plastic, partly elastic; in that case, the degree of recovery depends on the time of application of the stress and is never complete.

The memory effect can be utilized for a variety of applications, such as encapsulation. A sheet or film of irradiated and orientated or stretched polyethylene is bound over an object and then heated. The removal of crystalline constraints allows the polyethylene to contract on to the specimen, taking up its external shape.

Electrical and Mechanical Strength An interesting relationship between mechanical and electrical strength

of polyethylene has been established by Stark and Garton (1955). For

?

% 4

•4->

ω o b 2 υ ω

o_

< FIG. 13.41. Electric strength of polyethylene irradiated with 4 MeV electrons; values

calculated from the initial sample thickness. This figure may be compared with the mechanical properties shown in Fig. 13.27.

(From: Stark and Garton, 1955.)

o o

-

Unir - poly

1

c\

"ddiated :hene

1

A o

1

3 0 0

o

^B

150 ΓΤ

megara

xP-

negarads

ds 6

4 0 80 120 160 200

Temperature, C


unirradiated polyethylene, the electrical strength decreases with rise in temperature until the melting point is reached. Theories of intrinsic electric strength predict a more rapid fall with temperature in irradiated material. Stark and Garton observed that for low density polyethylene after irradiation, the electric strength/temperature curve approximates in shape to the change in the elastic modulus, falling with temperature until the crystallites have all melted and then remaining approximately constant (Fig. 13.41). This points to a direct relationship between electrical and mechanical cause of breakdown. This behaviour can be readily explained if the cause of breakdown is assumed to be mechanical, the electric field serving to compress the polyethylene to an extent which varies with its modulus.

In a specimen of initial thickness /0 to which is applied a constant voltage V, the stress across the specimen is

8 Ï \7 / where ε is a dielectric constant of the material and t is its compressed thickness. Stark and Garton assumed an exponential relationship between the stress and strain and therefore write

= £log(/0//)

where E is the Young's modulus under compression. The quantity t2 log (t0/t) has a maximum value when t0/t is about 0-6

and this defines the maximum strain which can be sustained without mechanical breakdown. The maximum electric field which can therefore be sustained is (4πΕ/ε)ϊ e.s.u., or in terms of the applied voltage

Vmax = î02-lVË^

assuming that the intrinsic electric breakdown does not occur at a lower field strength. Good agreement is obtained between the observed electric strengths and those calculated from the above assumptions, indicating that in the specimen failure is due to mechanical causes. A full quantitative treatment would, however, necessitate a more accurate relationship between mechanical stress and strain.

Electrical Properties

Changes in the electrical properties of polyethylene following irradiation are very minor in character and little published information is available on this subject. For low density polyethylene at room temperature, the permittivity rises very slightly, the increase being less than 1 per cent at frequencies between 103 and 108 c/s for a radiation dose of 100 megarads. The power factor is more affected by radiation and increases about threefold over the same frequency range; however, at frequencies of about 1010 c/s, no increase has been observed. At higher temperatures,

8π \

POLYETHYLENE 255

both power factor and permittivity decrease but show no discontinuity as the maximum melting temperature is passed. These slight changes in electrical properties of low density polyethylene may be due in part to the formation of unsaturation by radiation, but probably arise mainly from small amounts of oxidation. The incorporation of antioxidants reduces the small deterioration in performance.

Some forms of high density polyethylene polymerized in the presence of a catalyst show poor electrical properties due to the presence of residua) catalyst. In these materials, no significant changes have been found as a result of radiation.

The effects of very high doses of radiation, likely to be experienced when polyethylene is exposed to pile radiation for periods of weeks, are more marked and are considered on Chapter 31.

Permeability In view of the small changes at room temperature in chemical structure,

elastic modulus and crystallinity caused by doses corresponding to a few crosslinks per molecule, large changes in permeability are not to be expected. Sobolev et al. (1955) found no difference in gas permeability for a dose of 10 megaroentgens, and a reduction of about half, for a dose of 100 megaroentgens. This reduction can be ascribed to a decrease in the diffusion constant, when polymer chains are tied together. Greater relative differences are to be expected at higher temperatures. On the other hand Bent (1957) found that the permeability to a wide range of organic solvents of low density polyethylene was increased at lower temperatures by radiation and reduced at higher temperatures. Increased solubilities were also observed, but it is difficult to account for these differences by the small reduction in crystallinity due to radiation.

REFERENCES Crosslinking ABRAHAM, R. J., and WHIFFEN, D. H., Trans. Faraday Soc. 54, 1291, 1958 BASKETT, A. C , Symp. Intern. Chim. Macromoleculare, Milan, Turin, 1954. (La

Ricerca Scientifica, 1955.) BASKETT, A. C , and MILLER, C. W., Nature, Lond. 174, 364, 1954. BREGER, I. A., J. Phys. Coll. Chem. 52, 551, 1948. BUSSE, W. F., and BOWERS, G. H., / . Polymer Sei. 31, 252, 1958 CHAPIRO, A., J. Chem. Phys. 52, 246, 1955. CHARLESBY, A., Proc. Roy. Soc. A222, 60, 1954. CHARLESBY, A. and CALLAGHAN, A., J. Phys. Chem. Solids 4, 306, 1958. CHARLESBY, A., and PINNER, S. H., Proc. Roy. Soc. A249, 367, 1959. CHARLESBY, A., VON ARNIM, E. and CALLAGHAN, L., J. Appl. Rad. Is. 3, 226, 1958. EPSTEIN, L. M., J. Polymer Sei. 26, 399, 1957. HONIG, R. E. and SHEPPARD, C. W., / . Phys. Chem. 50, 119, 1946. LAWTON, E. J., BALWIT, J. S. and BUECHE, A. M., Industr. Engng. Chem. (Anal.)

46, 1703, 1954. LAWTON, E. J., BALWIT, J. S. and POWELL, R. S., Amer. Chem. Soc. Meeting,

Miami, 1957; / . Polymer Sei. 32, 257, 277, 1958. LIND, S. C. and BARDWELL, D. C , S. Amer. Chem. Soc. 48, 2335, 1926. LIND, S. C. and BARDWELL, D. C , Science 60, 364, 1924.


MILLER, A. A., LAWTON, E. J. and BALWIT, J. S., / . Phys. Chem. 60, 599, 1956. MUSSA, C , La Ricerca Scientifica 26(4), 1177, 1956. WADDINGTON, F . B., J. Polymer Sei. 31, 221, 1958.

Crystallinity Changes

CHARLESBY, A. and CALLAGHAN, L., / . Phys. Chem. Solids 4, 227, 306, 1958. CHARLESBY, A., Phil. Mag. 44, 578, 1953. CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A217, 122, 1953. DOLE, M., HETTINGER, W. R., LARSON, N. R. and WETHERINGTON, J. A.

J. Chem. Phys. 20, 781, 1952. DOLE, M. and HOWARD, W. H., / . Phys. Chem. 61, 137, 1957. HERMANS, P. H., Kolloid Z. 120, 3, 1951. HERMANS, P. H. and WEIDINGER, A., J. Polymer Sei. 4, 709, 1949. HUNTER, E. and OAKES, W. G., Trans. Faraday Soc. 41, 49, 1945. KARPOV, V. L., Session of Academy of Science of U.S.S.R. on Pacific Applica-

tions of Atomic Energy. Section of Chemical Science 3, Moscow 1953. KARPOV, V. L. and ZVEREV, B. L, Collection of Papers on Radiation Chemistry,

Academy of Sciences, U.S.S.R., 215, Moscow, 1955. KRIMM, S., J. Phys. Chem. 57, 14, 1953. KRIMM, S. and TOBOLSKY, A. V., J. Polymer Sei. 7, 57, 1951; Text Res. J. 21,

805, 1951. MATTHEWS, J. L., PEISER, H. S. and RICHARDS, R. B., Acta Cry stall. 2, 85, 1949. RAINE, H. C , RICHARDS, R. B. and RYDER, H., Trans. Faraday Soc. 41, 56,1945. REDING, F. P. and LOVELL, C. M., / . Polymer Sei. 21, 157, 1956. Ross, M., A.E.R.E. M/R, 1401, 1954. SLICHTER, W. P., and MANDELL, E. R., / . Phys. Chem. 62, 334, 1958 WILLIAMS, T. F., MATSUO, H., and DOLE, M., / . Amer. Chem. Soc. 80,2595, 1958.

Chemical Changes

ALEXANDER, P., and TOMS, D., / . Polymer Sei. 22, 343, 1956. BALLANTINE, D. S., DIENES, G. J., MANOWITZ, B., ANDER, P. and MESROBIAN,

R. B., J. Polymer Sei. 13, 410, 1954; B.N.L. 294, March 1954. BLACK, R. M., Nature, Lond. 178, 305, 1956; / . Appl. Chem. 8, 159, 1958. BLACK, R. M. and CHARLESBY, A., A.E.R.E. M/R 1818, 1955 (and unpublished). BURTON, V. L., J. Amer. Chem. Soc. 71, 4117, 1949. CHAPIRO, A., J. Chim. Phys. 52, 246, 1955. CHAPIRO, A., J. Polymer Sei. 23, 377, 1957. CHARLESBY, A., Proc. Roy. Soc. A215, 187, 1952. CHARLESBY, A., Rad. Res. 2, 96, 1955. CHARLESBY, A. and DAVISON, W. H. T., Chem. & Ind. (Rev.) 232, 1957. DEWHURST, H. A. ,7 . Phys. Chem. 61, 1466, 1957; 62, 15, 1958. DOLE, M., Radiation Dosimetry Symposium IV. p. 120, Army Chemical Centre,

Maryland, 1950. DOLE, M. and KEELING, C. D., / . Amer. Chem. Soc. 75, 6082, 1953. DOLE, M., KEELING, C. D. and ROSE, D. G., / . Amer. Chem. Soc. 76,4304,1954. DOLE, M., KEELING, G. D. and ROSE, D. G., Chem. Eng. News 32, 1342, 1954. DOLE, M., MILNER, D. C. and WILLIAMS, T. F., J. Amer. Chem. Soc. 79, 4809,

1957; 80, 1580, 1958. HARLEN, F., SIMPSON, W., WADDINGTON, F. B., WALDRON, J. D. and BASKETT,

A. C , / . Polymer Sei. 18, 589, 1955. HORNBECK, R. F. and PARKINSON, W. W., ORNL 2413, August 1957. LAWTON, E. J., ZEMANY, P. D. and BALWIT, J. S., J. Amer. Chem. Soc. 76, 3437,

1954.

POLYETHYLENE 257

MILLER. A, A., LAWTON, E. J. and BALWTT, J. S., / . Phys. Chem. 60, 599, 1956. OAKES, W. G. and RICHARDS, R. B., J. Chem. Soc. 2929, 1949. PEARSON, R. W., Chem. & Ind. (Rev.) 903, 1956. RUGG, F. M., SMITH, F. F. and WARTMAN, L. H., / . Polymer Sei. 11, 1, 1953. SCHUMACHER, K., Koll. Zeits. 157(1), 16, 1958. SLOVOKHOTOVA, N. A. and KARPOV, V. L., Collection of Papers on Radiation

Chemistry, Academy of Sciences, U.S.S.R., 197, 206, Moscow, 1955. SNOW, A. I. and MOYER, H. C , J. Chem. Phys. 28, 1222, 1958. ST. PIERRE, L. E., and DEWHURST, H. A., J. Chem. Phys. 29, 241, 1958. WINOGRADOFF, N. N., Nature, Lond. 165, 123, 1950.

Mechanical Properties

BACCAREDDA, M., BORDONT, P. G., BUTTA, E. and CHARLESBY, A., La Chimica e

Vlndustria 28, 561, 1956. BACCAREDDA, M. and BUTTA, E., / . Polymer Sei. 22, 217, 1956. BALLANTINE, D. S., DIENES, G. J., MANOWITZ, B., ANDER, P. and MESROBIAN,

R. B., B.N.L. 294, March 1954. BALLANTINE, D. S., DIENES, G. J., MANOWITZ, B., ANDER, P. and MESROBIAN,

R. B., J. Polymer Sei. 13, 410, 1954. BRESEE, J. C , FLANARY, J. R., GOODE, J. H., WATSON, C. D., and WATSON, J. S.,

Nucleonics 14(9), 75, 1956. CHAPIRO, A., J. Chim. Phys. 52, 246, 1955. CHARLESBY, A., and HANCOCK, N. H., Proc. Roy. Soc. A218, 245, 1953. DEELEY, C. W., KLINE, D. E., SAUER, J. A. and WOODWARD, A. E., J. Polymer

Sei. 28, 109, 1958. DOLE, M., KEELING, C. D., and ROSE, D. G., / . Amer. Chem. Soc. 76,4304, 1954. HARRINGTON, R., Nucleonics 14(9), 70, 1956. LAWTON, E. J., BALWIT, J. S. and BUECHE, A. M., Industr. Engng. Chem. 46(8),

1703, 1954. LUY, H. and SCHUMACHER, K., Zeits f. Ang. Physik. 8(5), 222, 1956. PARKINSON, W. W. and KIRKLAND, W. K., ORNL 2413, August 1957. RYAN, J. W., Nucleonics 11(8), 13, 1953. SISMAN, O. and BOPP, C. D., ORNL 928, 1951. STARK, K. H. and GARTON, C. G., Nature, Lond. 176, 1225, 1955. WOODWARD, A. E., DEELEY, C. W., KLINE, D . E. and SAUER, J. A., / . Polymer

Sei. 26, 383, 1957.

Miscellaneous

BENT, H. A., J. Polymer Sei. 24, 387, 1957. BLACK, R. M., Electrical Manufacturer October 1957. CHARLESBY, A. and BAIN, T., Brit. Plast. April 1957. CHARLESBY, A. and PINNER, S. H., Industr. Plast. Mod. 9-10 November-Decem-

ber 1957 FUSCHILLO, N. and SAUER, J. A., / . Chem. Phys. 26,1348,1957; / . Appl. Phys. 28,

1073, 1957. MUSSA, C , Ric. Sei. 26(4), 1177, 1956. SCHNEIDER, E. E., Disc. Faraday Soc. 19, 158, 1955. SELLA, C , and TRILLAT, J. J., C.R. Acad. Sei., Paris 246, 3246, 1958. SHINOHARA, K., AMEMIYA, A. and DANNO, A., J. Phys. Soc, Japan 13(6), 604,

1958. SOBOLEV, I., MEYER, J. A., STANNETT, V. and SZWARC, M., / . Polymer Sei. 17,

417, 1955.

CHAPTER 14

RUBBER Rubber as an Elastomer

It is customary to divide long chain polymers into "elastomers" and "plastomers", depending on their elastic properties at high deformation. Although rubber is perhaps the outstanding example of an elastomer, it only exhibits high elastic deformations under suitable conditions. In the raw state, or at low temperatures when it crystallizes, or when compounded as in ebonite, it does not show this behaviour. The property of high elasticity is determined primarily by the flexibility of the molecular chains, and their restriction by an adequate, but not excessive, number of cross-links. Chemical structure only intervenes in so far as it permits adequate flexibility of chains, and the formation of bridges or crosslinks between them. The high elasticity properties of rubber can thus both be created by the formation of crosslinks by radiation (when insufficient are originally present) or it can be destroyed by excessive crosslinking.

Conventional Curing of Rubber

Raw rubber is widely distributed in nature in small concentrations. The major source is para rubber, obtained from the rubber tree Hevea Brasiliensis. The rubber latex obtained from this tree consists of a suspen-sion of rubber hydrocarbon in an aqueous fluid together with some stabilizing colloids which protect the rubber molecule against oxidation. When this suspension is dried or coagulated by chemical means, crêpe rubbei is obtained. This crêpe rubber can be swollen and eventually dissolved in many organic solvents, such as benzene or xylene; a small residue may be found which was at first thought to indicate that rubber consists of two distinct constituents, but it appears that the insoluble residue consists largely of rubber molecules which have been lightly linked together. On breaking down crêpe rubber on a mill, the molecular weight is decreased and, with the exception of the protein material, the rubber is rendered completely soluble. In this form, rubber is not useful as an elastomer. To acquire satisfactory mechanical properties, bridges or crosslinks must be formed between molecules, i.e. it must be "vulcanized". This process, discovered by Goodyear, consists in compounding the rubber with sulphur and heating to 130°-150°C for several hours. The process can be speeded up by the use of accelerators such as tetramethyl thiuram disulphide which usually operate by a free radical mechanism. Although other chemical methods of producing the same change in physical pro-perties have been devised, modifications of the Goodyear technique are still the most important industrially. The sulphur becomes chemically

258

RUBBER 259

combined with the rubber molecule, forming intermolecular bridges. The mechanical properties of the vulcanized rubber depend largely on the percentage of combined sulphur, maximum tensile strength being usually obtained with a mix containing about 2 per cent of sulphur. Higher quantities give a weaker rubber of higher elastic modulus, while an entirely different material—ebonite—is obtained with about 40 per cent of sulphur corresponding roughly to one sulphur atom per monomer (isoprene) unit. It is interesting to compare ebonite with highly irradiated rubber to dis-cover whether its properties are essentially due to the high degree of cross-linking, or whether they arise from a new chemical compound of sulphur and rubber.

The change in properties of rubber on vulcanization—reduced solubility, increased tensile strength and good elastic recovery after high elongation— are typical of the results to be expected from crosslinking, but the mechanism of formation of these sulphur links has not yet been univer-sally agreed. Indeed, several processes may be taking place simultaneously involving bridges consisting of one or more sulphur atoms and possibly direct carbon-carbon linkages between adjacent molecules.

homers of Rubber

Rubber molecules have been shown to consist of isoprene units —CH2—CH = C(CH3)—CH2— in the ds-configuration, the monomer units being in the head-to-tail position. Gutta-percha and balata are naturally occurring isomers of rubber, and are predominantly in the trans-configuration. The difference in mechanical properties would arise from the different conditions for crystal formation and chain flexibility and possibly from differences in molecular weight. It is far more difficult to crosslink gutta-percha by chemical methods but radiation treatment can give a well vulcanized form of gutta-percha. This difference may arise from the possible changes in isomerism caused by irradiation.

EARLY WORK As early as 1933 Newton claimed a patent for the vulcanization of

rubber by short exposures to 250 kV cathode rays. Subsequent radiation work in the 1940s arose largely from the need to obtain materials capable of use in nuclear reactors without serious deterioration in their physical properties.

Davidson and Geib (1948) studied a number of natural and synthetic rubbers which had been subjected to nuclear radiation. Their results on the synthetic butyl rubber will be discussed later in the chapter on poly/söbutylene. Changes produced in natural rubber (smoked sheet) were enhanced by incorporating ammonium borate, when the boron under slow neutron bombardment gives rise to highly energetic a-(He) particles :

B10 + «(slow) -> Li7 + He4.

The major effects arise from the dense ionization produced by these alpha particles, as well as from the recoil protons when a hydrogen atom

s


suffers a collision with a fast neutron, and from electrons released by Compton scattering of a γ-photon. The changes observed were, however, found to correspond to only a slight degree of vulcanization.

Subsequent work on the radiation treatment of rubber and of rubber mixes can be considered to fall under three main headings, depending on the main emphasis of the work. Under the first heading can be considered work of a basic character on the radiation of pure unvulcanized rubber, designed to measure the chemical changes produced and the relation between crosslinking and radiation dose. This work is therefore directly comparable with that carried out on other polymers. A second set of experiments is concerned with the irradiation of rubber previously vul-canized by chemical methods, and is largely intended as part of a search for radiation resistant materials likely to be of value in or around nuclear reactors. Under the third heading fall experiments in which unvulcanized rubber incorporating other materials such as fillers are subjected to radiation curing. These are intended as a method of studying quantita-tively the nature of the reaction between rubber molecules and the filler. The results obtained may be compared with radiation cured rubber with-out filler, and with similar compositions in which the cure is carried out chemically.

CROSSLINKING OF UNVULCANIZED RUBBER

At low doses of radiation, insufficient to cause gel formation, the theory given in Chapter 9 predicts a rise in the molecular weight with a corre-sponding increase in viscosity. Beyond the gel point, the molecular weight of the soluble fraction falls as the larger molecules are linked into the network. This behaviour is shown in smoked rubber samples subjected to pile radiation. Fig. 14.1 shows the initial rise in the intrinsic viscosity

π — i — i — i — i — i IIGel point

6

•t 4 o υ ω

■> o 'ω ρ

0 10"2 2 χ Κ Γ 2

Radiat ion dose , pile un i ts

FIG. 14.1. Intrinsic viscosity of soluble fraction of irradiated rubber. Gelling dose: Specimen I; r ei = 10-3 pile units

Specimen II: rge i= 2x 10~3 pile units

—V \

Gel

\

\ Doint

V

i \ I

RUBBER 261

of the soluble fraction followed by a rapid fall once the gel point has been passed. Quantitative deductions from these data are, however, difficult to make since the intrinsic viscosity is not related in a simple manner to the molecular weight of the branched molecules formed. The relatively small initial rise is due to a slight amount of crosslinking already present before irradiation; its effect can be represented approximately by an appropriate virtual radiation dose r0 added to the effective radiation dose r.

In some early experiments by Charlesby (1954), the relationship between crosslinking index γ and radiation dose r (in pile units) for a given rubber was expressed by the equation

γ - 0-55 + 45r = 45(r + 0012)

where 0-55 represents the degree of crosslinking present in the specimen prior to irradiation and 0012 pile units is the virtual radiation dose r0 required to correct for this initial crosslinking. The subsequent decrease in solubility of this sample is shown in Fig. 14.2. The steepness of the

100i 80

60

«,ο

40

30

20l

M v

ii 3-0 O 0 ·5 1-0 2-0

Crosslinking index FIG. 14.2. Decrease in solubility of irradiated rubber.

calculated for an initially uniform distribution. calculated for an initially random distribution.

X observed values.

curve is related to the initial molecular weight distribution, and for an initial uniform distribution a much more rapid decrease in solubility (shown by the broken line) would be expected. Accurate measurements of the gel fraction as a function of dose may be expected to provide useful information on the shape of the initial distribution curve in various rubbers.

The swelling properties of the crosslinked network depend on the density of crosslinking q or the average molecular weight between crosslinks Mc. Fig. 14.3 shows the volume-swelling ratio for irradiated rubber in benzene at 20°C. At low doses, the swelling increases with dose since an increased fraction of the total specimen is found in the network. Once the soluble fraction becomes negligible in amount, the observed relation between swelling ratio V and radiation dose r (measured in pile units) can be represented by the equation

K5/8 « 22-6/r


which agrees with the predicted relationship (equation 9.54). A direct com-parison of the swelling in benzene of irradiated rubber with the results obtained by Gee (1946) on rubber crosslinked by chemical means, indicates that unit pile radiation crosslinks some 0-7 per cent of the isoprene units,

40 30

20

f 31

1 1 1 1 !

10-2 2X10"2 5xKT2 10"1 2x10_1 SxlO"1 1

radiation dose, pile units FIG. 14.3. Swelling of irradiated rubber in benzene. Full line; I75'3 = 22-6/r.

(From Charlesby and Groves, 1954.)

i.e. q0 = 0Ό07 and Mc = 9700/r. Earlier experiments on rubber swollen in decane led to a lower value for Mc (= 6000/r) when the solvent-polymer interaction parameter μ was taken as 0-443. Then qQ =0-011. Fig. 14.4

2x105

105

5x10'

K 2x10

5X103!

2x103k

X N.

l·

^

-

S^x

N. x

\ x 1

0-01 0-1 1·0 r, pile units

10

FIG. 14.4. Variation of molecular weight between crosslinks (Mc) and radiation dose r (deduced from swelling in decane). The slope of — 1 indicates that the number of

crosslinks is proportional to the dose. (From Charlesby, 1954.)

RUBBER 263

shows that the crosslinking density is proportional to the radiation dose over a range of at least 100.

The elastic properties of a rubberlike network can be predicted theoreti-cally (Chapter 9). At low elongations, the theory gives a value for the elastic modulus E (equation 9.45) which can be written in the form

E = 3 9RTq0w-\r-r0)

where r0 is the correction needed for the initial finite molecular weight of the material; the entanglement factor g is assumed to be equal to 1. Experimental values of the modulus for smoked rubber specimens sub-jected to pile radiation (Charlesby and Groves, 1954) are shown in Fig. 14.5. Except at very low doses where end effects and entanglements are

2-4

2-0

! i i

-

-

-

25 °C

! ; [

! s jS i

: ! i

A \ ! ! ! / \ Ε = 1 ·5χ10 6 *7 ·5 x l O V

. 1 ,

S 1 1

i 1 I 1

E 1-6

Φ

£ 1-2

* 0 -8 Uj

0-4 0-8 1-2 1-6 2-0 2-4 2-8 Radiation dose, pile units

FIG. 14.5. Elastic modulus of pile irradiated rubber. Measurements at 25°C and 75°C (From Charlesby and Groves, 1954.)

significant, there exists a linear relationship between £ a n d ras predicted by theory. When the temperature of measurement is raised, the initial part of the curve also becomes linear, presumably due to a reduction in chain entanglements. The equation to the curve is

£ = 1 · 5 χ 106 + 7 ·5χ 106r

where the radiation dose r is expressed in pile units. The initial term 1-6x10 may arise from a small degree of crosslinking existing prior to radiation Comparison of this experimental relationship with the theoretical value leads to a value for q0 of 8 x 10-3 when the unit radiation dose is taken as the pile unit. If radiation doses are expressed in megarads the equivalent value of q0 is about 1 -8 x 10~4.


At high elongations, the relationship between stress and strain is no longer linear so that no constant value can be given for the elastic modulus E. The theoretical expression (equation 9.43) can be written

f=pRT(x-lloL*)q0r/w

when end effects and chain entanglements are ignored. The experimental results of Charlesby and von Arnim (1957) are shown in Fig. 14.6 for two

OJ

F υ D l

-* H-.

18

16

14

12

10

ö

6

4

.

/

/

/ , ,/

/ /

A

1 +

/ /

S

£

+ 1 1

+ 1 1

1 + [

/ *

ΛΓ\ rr

i 20 megarad

1 2 3 4 5 6 7 8 9 o c - — 0

a 2

FIG. 14.6. Stress-strain curve for irradiated rubber (no filler). Full line shows the theoretical curve;/proportional to a— 1/a2.

levels of electron radiation. In these curves, the stress / and the strain factor (a—1/a2) are proportionate to one another except at high elonga-tions where incipient crystallinity and limited chain length between cross-links reduce the elongation at a given stress. From the slope of the curve, values of q0r may be deduced. The value obtained for q0, the proportion of isoprene units crosslinked per megarad, is 1-4 x 10-4.

When rubber is irradiated while kept in a stress oriented condition, different moduli parallel and at right angles to the direction of the initial stress are obtained. G Value for Crosslinking

From the various values of q0 given above, the G value for crosslinking can be deduced. Table 14.1 shows that the values obtained, although scattered, are lower than those for polyethylene, and do not vary signi-ficantly with radiation intensity or type of radiation. The most probable value is

G(crosslink) = 1-2

and appears to be constant over a wide range of radiation doses.

RUBBER 265

Table 14.1. Crossiinking Density q0 and G Value per Crosslink in Irradiated Rubber

Method

Comparative swell-ing

Swelling in decane Modulus at low

elongation Stress-strain high

elongation

Radiation

BEPO reactor

BEPO reactor BEPO reactor

2MeV electrons

Intensity rads/min

5x l0 4

3x l0 4

5x l0 4

106-107

<7o (per pile

unit)

7xl0~3

l l x l0~ 3

8x l0 - 3

—

q0* (per equiv't

megarad)

1-55 xlO"4

2-4 xlO-4

1-8X10-4

1·4χ10-4

G = 0-48 x 106#o/w

11

1-7 1-3

1

♦Assuming 1 pile unit is equivalent to 45 megarads.

Chemical Changes Surprisingly little attention has been paid to the chemical changes pro-

duced when rubber is irradiated. As in most polymers, the major con-stituent of the gases produced is hydrogen (Charlesby, 1954) with much smaller amounts of methane, and traces of ethylene and propane. The amount of hydrogen evolved is lower than would be expected on the basis of two hydrogen atoms being liberated per crosslink formed, and part of the difference may be ascribed to a reaction of hydrogen with the rubber, causing a reduction of unsaturation (Fig. 14.7). This rapid reduction in

3 1 1 90

o 80 II

o 70

° 60 ϊ ^ σ 40 ί_ B 30 σ £ 20 Ώ

10 1 2 Radiation dose, pile units

FIG. 14.7. Reduction in unsaturation of irradiated natural crêpe rubber.

unsaturation is in accord with studies on paraffins and olefins, according to which there exists a limiting degree of unsaturation (about 1 double bond per 20 carbons) towards which irradiated polymers tend as the radiation dose is increased.

Changes in isomerization may be expected, as have been observed in simple olefins, and in polybutadiene (Golub, 1958). However there is no evidence of radiation-induced degradation, since the solubility does not


tend to a finite value as the dose is increased (equation 11.8). Rather surprisingly, there is as yet no evidence that oxygen plays as important a part in radiation-induced reactions in rubber as it does in polyethylene, when it combines with radicals to give peroxide bridges. That radicals are formed in rubber has been clearly shown by Turner (1958) who studied the reduced crosslinking efficiency of rubber when additives, capable of acting as radical acceptors, were incorporated in the mixture prior to irradiation. However, even with high concentrations of these additives, the crosslinking density never sank below one-third of its maximum value. This residual crosslinking could be due to local high primary radical con-centrations in each δ electron track, to a residual reactivity of the combined additive, to some form of energy transfer, or finally to a contribution to crosslinking from a non-radical source (e.g. an ionic reaction). Similar experiments in other polymers seem highly desirable.

The location of the crosslinks on the isoprene units has not yet been determined, but the bonds formed, which must be direct C—C links, would be expected to be more stable than the sulphur bridges in chemical vulcanization, and the ageing properties of an irradiated rubber are stated to show some improvement.

IRRADIATION OF CURED RUBBERS As part of their extensive tests on a series of plastic materials, Bopp and

Sisman (1953) irradiated various natural rubber compositions in the Oak Ridge nuclear reactor. The rubbers were prepared from the mixes shown in Table 14.2. The compounded rubbers were then cured at 292°F for 25 min (32 A43) or at 310°F for 20 min (PIS IC9) and subsequently irradiated for varying periods. The results obtained therefore refer to a

200

Φ

O

> 1 100 •if [c *♦-o

0 0-001 0-01 0-1 1-0 10

Radiation dose, units of 1018nvt FIG. 14.8. Tensile strength and elongation of cured rubber (Oak Ridge pile radiation

\Qu;nvt = 600 megarads). (From: Bopp and Sisman, 1955.)

cured rubber, in which intermolecular links already present are due to sulphur bridges. One would therefore expect radiation to promote further crosslinking beyond the optimum value. At a dose equivalent to about 50 megarads, the tensile strength of composition 32A43 had fallen '

Tab

le

14.2

. C

ured

R

ubbe

r C

ompo

sitio

ns

Nat

ural

rub

ber

Zinc

oxi

de

SRF

carb

on b

lack

St

earic

aci

d Ph

enyl

nap

hthy

lam

ine

Sulp

hur

Cap

tax

Pla

stic

izer

s L.

P. o

il D

ioct

yl p

htha

late

D

ioct

yl s

ebac

ate

Ant

ioxi

dant

s D

ibut

yltin

dila

urat

e D

odec

yl m

erca

ptan

Le

ad p

erox

ide

Chl

oran

il F

iller

A

sbes

tos f

ibre

Nat

ural

rub

ber

32A

43

P1S1

C9

100

100

5 5

70

20

1 3

1 3 3

0-6

1

Plas

ticiz

ed r

ubbe

r P1

S2C1

P1

S2C2

P1

S2C3

100

100

100

5 5

5 30

30

30

1

1 1

3 3

3 1-5

1-5

1-5

30

30

30

Rub

ber

with

ant

ioxi

dant

17

FBB1

97

16RC

B1

100

100

5 5

70

70

1 1

1 1

3 0-6

10

10

10 5

Fille

d ru

bber

17

FBB1

94

100 5 1 1 3 0-

6

250

From

Bop

p an

d Si

sman

, 19

53.

(Thi

s pa

per

incl

udes

dat

a fo

r a

num

ber

of o

ther

nat

ural

and

syn

thet

ic r

ubbe

r co

mpo

sitio

ns.)

RUBBER 267


about 10 per cent, while the elongation was reduced by about 25 per cent from its initial figure of 420 per cent. Exposures equivalent to about 1000 megarads reduced both tensile strength and elongation to very small values. Further radiation above 6000 megarads increased the tensile strength to several times its initial value, this corresponding to a very high degree of crosslinking. This rapid increase indicates that a second type of network is being formed, of high strength, which may perhaps be compared to ebonite, but unlike the latter, it is formed in the presence of relatively small amounts of sulphur. It would therefore appear that the physical properties of ebonite do in fact arise from the high density of linking and are not necessarily bound up with the formation of a sulphur-rubber compound of approximate formula (C5H8S)W. The properties of ebonite may therefore be comparable with those of highly irradiated rubber, polyethylene and other highly crosslinked systems.

Bopp and Sisman found that if the rubber is irradiated in a compressed condition, only partial recovery takes place when the stress is subsequently released. This is tentatively ascribed by them to radiation induced fracture and crosslinking occurring simultaneously. However, a network, what-ever the means of crosslinking, will tend to revert to its shape during the process of crosslinking when the external stress is removed. If the material is chemically crosslinked in one shape, and further crosslinked by radiation in another, then subsequently in the absence of external stress it will take up an intermediate shape, depending on the ratio of the two densities of crosslinking. This technique may even offer a possible method of com-paring crosslinking densities induced chemically and by radiation.

The effect of irradiation cured rubber compositions including plasti-cizers was also studied by Bopp and Sisman. Into a standard rubber mix comprising smoked sheet, standard vulcanizing agents and carbon black there was incorporated a plasticizer (Table 14.2). The mixture was then cured at 310°F for 20 min. In each case subsequent exposure to radiation decreased the tensile strength and elongation at break. The smallest radiation dose studied (0-25 x 1018 nvt or about 150 megarads) reduced the tensile strength by a factor of between 2 and 4 while the elongation at break was reduced by factors varying between 5 and 10. Fig. 14.9 shows these results for the composition including 30 parts of dioctyl phthalate as a plasticizer (PIS2C2), as well as for other mixes given in Table 14.2.

The use of antioxidants as a method of stabilizing rubber against the effect of radiation was also envisaged by Bopp and Sisman. Specimens prepared from the mixes shown in Table 14.2 showed appreciable decreases in both tensile strength and elongation at break, even for doses of only 0-25 x 1018 nvt, equivalent to about 150 megarads. There was no evidence that the presence of these materials protected the rubber molecule against the effect of radiation.

The reinforcement of rubber with asbestos fibre (250 parts per 100 parts rubber) but no carbon black filier, gave a weaker rubber, with lower elongation at break (Mix 17FBB194). Irradiation of the mixture after vulcanization did not further decrease the strength, although the elonga-tion at break was greatly reduced. These results indicate that the use of a

RUBBER

CM

c — onnn

en c Q)

V) 1000

•s , c |2

> \

\ \~Na

•

7 \ > "

λ \ \

ural rubber (32A43)

*JRubber+asbestos ^ J V - i i y F B B I Q ^

Rubber + x. \antioxidant > \ ( 1 6 R C B 1 )

Λ-JJ Rubber -h plasticiser (P 1S2C2)

%

I 0-5 1-0

Radiation dose, 10 n v t

4 0 0 %

3 0 0 %

8

2 0 0 %

en c £ Lu

100%

0-5 1-0 Radiation dose, 1018 nvt

FIG. 14.9. Reactor radiation of cured rubbers.

269


rubber with a mineral filier will not give an elastic material resistant to radiation.

Ryan (1953) carried out somewhat analogous experiments on a range of plastics and rubbers, including natural rubber. The radiation source used consisted of a series of radioactive uranium slugs which had been removed from the Hanford reactor after use, and were stored in a pit to lose most of their radioactivity before chemical treatment. The radiations extended over a period of some six months, and the energy absorbed would correspond to an approximate radiation dose of some thousand megarads. Such doses are far too great for the radiation curing of con-ventional polymers to give high elastic properties, and the tensile strength and elongation at break of the irradiated precured rubber decreased from some 850 psi and 800 per cent elongation to 150 psi and about 10 per cent elongation. The elastic modulus was increased about fortyfold.

Gehman and Hobbs (1954) subjected a series of rubber and synthetic elastomeric substances to γ-irradiation from a cobalt 60 source at the University of Michigan. The total doses given amounted to about 100 megarads, accumulated at the rate of some 105 rads/hr, and at a temperature of about 20°C. Among the specimens studied there were specimens of natural rubber milled smoked sheet, and two mixes of smoked sheet plus the usual chemical vulcanizing agents (sulphur, 3 part; zinc oxide, 5 parts; stearic acid, 1-5 or 3 parts per 100 of smoked sheet) with or without carbon black (50 parts HAF) and accelerators.

The milled sheet which had not been vulcanized prior to irradiation showed improved properties such as tensile strength and elongation at break, as compared with unvulcanized rubber. The values were still very low, however. The other specimens which had previously been cured by a conventional vulcanization of 10-30 min at 275-311°F showed a decreased elongation at break, and in most cases a reduced tensile strength. In all cases the modulus increased. The results are all consistent with an increased density of crosslinking, the dose used being too high for optimum mechanical properties.

PHYSICAL PROPERTIES OF RADIATION-CURED RUBBER

Early work by Charlesby et al (1954) on unvulcanized rubber is concerned largely with the evaluation of density of crosslinking and its effect on solubility swelling and elasticity. This work is summarized on pages 261-266.

Jackson and Hale (1955) subjected a series of natural and synthetic rubbers to gamma radiation both from spent fuel rods and from the cobalt source installed at the Materials Laboratory of Wright Air Development Centre. The average dose rates were about 3 x 106 roentgens/hr from the fuel rods and 5-2 x 105 roentgens/hr from the cobalt source. The effect of three doses was studied (10, 30 and 40 megaroentgens) on the mechanical properties, the ageing and the swelling of rubbers containing varying amounts and type of carbon black including Philblack A, MT Thermax and HPC Channel Black.

Tabl

e 14

.3.

Impr

ovem

ent

in T

ensi

le S

tren

gth

(psi

) du

e to

Irr

adia

tion

of F

illed

Nat

ural

R

ubbe

rs

Car

bon

blac

k or

filie

r

Philb

lack

A (f

ast

extru

sion

furn

ace

carb

on b

lack

) M

T Th

erm

ax (

med

ium

ther

mal

c.b

.) H

PC c

hann

el b

lack

(ch

anne

l c.b

.) Si

lène

EF

(cal

cium

sili

cate

) K

alva

n (c

alci

um c

arbo

nate

)

Car

bon

blac

k et

c; p

arts

per

100

par

ts r

ubbe

r 20

40

60

(a)

(b)

(c)

256

630

307?

16

7 49

2 50

2 13

9 61

2 73

8 23

9 72

2 10

42

137

300

287

(a)

(b)

(c)

500

1845

18

09

217

750

1150

38

9 75

0 10

50

503

947

1517

24

9 78

0 72

5

(a)

(b)

(c)

702

2030

16

90

410

1630

19

95

893

1630

19

47

835

1465

17

25

542

670

1607

Rad

iatio

n do

se

(a)

10 m

egar

ads,

(b) 3

0 m

egar

ads,

(c) 4

0 m

egar

ads.

(Fro

m J

acks

on, W

. W. a

nd H

ale,

D.,

1955

.)

RUBBER 271


The main differences observed as compared with conventional chemical vulcanization were the improved ageing characteristics of the irradiated materials. This was especially striking in the case of an experimental acrylate material, which is particularly difficult to vulcanize by chemical methods. Improvements in the heat ageing and oil ageing of a synthetic rubber (Buna N) were also found.

Jackson and Hale give a considerable amount of data on the effect of incorporating carbon black or fillers into natural rubber and the three synthetic rubbers, GR-S, Neoprene WRT and in Hycar 1001 (Buna M). The improvements in the tensile strength of natural rubber incorporating varying amounts of carbon black or other fillers and subjected to three radiation doses, 10, 30 and 40 megaroentgens, are shown in Table 14.3.

Fillers

Gehman and Auerbach (1956) investigated the mechanical properties of unvulcanized rubber compositions, crosslinked by radiation. For uncompounded rubber the maximum tensile strength obtained fell short of the best that could be obtained by chemical vulcanization. When carbon black was incorporated in the mix, radiation produced rubbers with tensile strengths of over 2000 psi even for doses as low as 10 megarads. Most of their work was, however, devoted to a study of the effect of incorporating CaC03, BaS04 and ZnO as fillers. The change in tensile strength with radiation dose is shown in Fig. 14.10, while Fig. 14.11 shows

01 I I I I 1 10 100

Dose, megarep FIG. 14.10. Effect of radiation dose on tensile strength.

that the effect may be related to the specific volume of the filler. Unfor-tunately, no details are available of the average size of the filler particles, so that any differences due to changes in the surface/volume ratio cannot be investigated. The change in modulus is related to the atomic number of the metal in the filler, though not to its specific volume (Fig. 14.12).

RUBBER 273

12

**-P 8 σ> ö

Q.

2 4 In Φ

«Λ

ι φ

'"" 0

1

Βα SO„ /

/ /

ZnO ·

/ / f

Ca C 0 3 ·

0·1 0-2 0-3 0-4 Specific volume of filler, cc/gm

FIG. 14.11. Effect of specific volume of fillers on tensile strength per unit of radiation.

Carbon Black Reinforcement Charlesby, Burrows and Bain (1958) carried out a detailed investigation

into the improvements obtained by incorporating carbon black into a

20 3 0 4 0 50 Atomic No. of metal in filler

FIG. 14.12. Relation of atomic number of filler to modulus per unit of radiation.

rubber mix, which was subsequently crosslinked by radiation. Several types of carbon black particles were used, most of the work being confined to Philblack 0, a heavy abrasion furnace black (HAF). The parameters studied included type and concentration of carbon black; other additives including sulphur, stearic acid and accelerators used in conventional chemical vulcanization; the type and intensity of radiation and the dose. Several types of rubber were also compared.


Radiations were carried out both with 2 MeV electrons and with pile radiation. These differ in the intensity and period of radiation (of the order of a few minutes in the former and of a day in the latter for the same accumulated dose), in radiation temperature (20-30° as against 80°C) and in the type of radiation (electrons as against ys and fast neutrons). In spite of these considerable differences, the changes produced by a given dose were not significantly different when account is taken of the wide variation in experimental conditions.

Except at very low radiation dose (10 megarads or less) when the number of crosslinks per molecule is low, end effects are small and the stress for a given strain was found to be approximately proportional to the radiation dose, in agreement with theoretical predictions (equation 9.42). However, at elongations beyond about 100 per cent, the extension was no longer proportional to the load, but increased far less rapidly. This behaviour, which is only shown in pure gum stock at much higher elongation (Fig. 14.6), has also been observed in rubber compositions vulcanized by conventional chemical methods. The cause of this behaviour is not clear; it may be due to some interaction between carbon black and rubber molecules, or to the test conditions (which were carried out at a finite rate of extension as used in industrial testing, and are therefore not typical of static conditions), or again to the limited extensibility of the rubber chains between successive carbon particles.

Fig. 14.13 shows a typical load-extension curve, obtained from a

< L

:

V

< /

y /

D

f

50 100 200 400

Extension in °/o FIG. 14.13. Shape of load-extension curve up to the breaking point.

X25 megarads; +40; O 60; D 100; Λ 150 Mix: 100 parts smoked sheet, 50 parts Philblack 0

Load/dose = (2-3 extension)2.

RUBBER 275

number of similar specimens irradiated to various extents. In the figure the ratio load/dose for a given extension is approximately constant, but on the log-log scale used, the points above 100 per cent extension fall on a straight line of slope 2. Most of the mixes studied give the same type of plot, so that the experimental results can be expressed

L = (Fx)2r

where L is the applied load in psi, r the radiation dose in megarads, and x the extension per unit length. F is a parameter which characterizes the mix. Values of F for a number of mixes are shown in Table 14.4 for both electron and reactor radiation. The comparable values of F for chemical vulcanization have been derived as described below.

The elongation at break decreases very rapidly with increase in the radiation dose (Fig. 14.14). In a few specimens there is an initial increase

1000

800

g 600 -Ω

Ö c 400

ΦΟ

~o c

U 200

20 40 60 80 100 120 140 160 Radiation dose, megarads

FIG. 14.14. Elongation at break of rubber-carbon mix (electron radiation). Figures indicate parts of Philblack 0 per 100 parts of smoked sheet rubber.

in elongation, probably due to the rise in the average molecular weight prior to the formation of a network. If the initial molecular weight is low, this would result in an increased tensile; a similar effect has been observed in polyethylene (Lawton, Balwit and Bueche, 1954). If this initial increase is ignored, the relation between elongation at break (e) and the dose (r) can usually be expressed in the form

log e = Ar + BF' + C

where A, B, C are suitable parameters independent of mix, and F' is a new parameter, characterizing the mix in terms of its elongation at break.

T


Some of the F' values are shown in Table 14.4, and approximate to the F values obtained from the shape of the load-extension curve.

At small doses the tensile strength is low. It increases to a maximum value, and then falls again, as shown in Fig. 14.15. In the large range of

20 40 60 80 100 120 140160 Dose, megarads

FIG. 14.15. Tensile strength of irradiated carbon rubber mixes (electron radiation). Figures indicate parts of Philblack 0 per 100 parts of smoked sheet rubber.

specimens studied, this maximum always occurs at a dose of about 50 megarads. The independence of dose for maximum tensile strength can be predicted from the two previous equations. From them one deduces (assuming F = F')

log T = log r + 2Ar + 2 log F + 2BF + 2C.

Hence the maximum tensile occurs when dT/dr = 0 or r (maximum tensile) = —0-4343/2Λ = 47 megarads

since for all mixes A = —4-6 x 10~3. A similar derivation can be used to determine the concentration of

carbon black to give maximum tensile, the relevant condition being d77dF = 0

or 2 xO-4343/F + IB = 0; F = —0-4343/5 ~ 3 since B = —0-142.

The relation between Fand carbon black concentration can be obtained from Table 14.4. The concentration of carbon black for maximum tensile derived in this way (50-60 per cent in a typical case) is very similar to that used in chemical vulcanization.

RUBBER 277

Although the reinforcement of rubber by carbon increases with the density of these crosslinks, it appears to be independent of the method by which these crosslinks are introduced into the system.

Table 14.4. F and F' for Radiation and Chemical Vulcanization

Carbon Black

Philblack 0

Philblack 0

Philblack 0

P33

Parts per 100 rubber

25

50

100

50

100

Additives

2s — a

— — 2s b a

— — 2s — — a

— a

Cure

Electron Reactor Chemical Electron Reactor Electron Electron Chemical Electron Reactor Electron Electron Reactor Chemical Reactor Chemical

F Stress/ Strain

1-4 1-7 1-4 2-3 2-6 2-4 20 2-05 5-2 61 4-8 11 1-2 105 1-6 1-4

F' Elongation

at break

1-5 1-75 0-6 2-35 2-75 2-45 2-3 1-55 5-2 61 4-8 11 10 0-42 1-5 0-92

Additives : 2s 2 parts stearic acid b 2 parts stearic acid 3 parts sulphur a various vulcanizing agents (see original paper).

(From Charlesby, Burrows and Bain, 1958.)

Radiation and Chemical Curing Sulphur is an essential ingredient in the conventional chemical curing

of rubber with carbon black. It might therefore be suspected that its presence would have some effect on radiation vulcanization. In fact this is not the case, and sulphur does not appear to play an important part in reactions resulting from exposure to radiation. For example, in a mix of 100 parts of smoked sheet rubber, and 50 parts of Philblack 0, the incor-poration of three parts of sulphur prior to radiation has no significant effect either on the stress-strain curve, the modulus or the elongation at break. This absence of any marked effect on radiation vulcanization is also found when certain other chemicals used in conventional chemical curing are incorporated in the mix prior to irradiation.

To compare the mechanical properties of irradiated rubber-carbon mixes, with similar mixes cured by conventional chemical methods, it is essential to compare specimens having the same degree of cure. In irradiated rubbers this can be determined quantitatively from the radiation dose. Published data on the mechanical properties of rubbers cured by conventional chemical techniques can be used if the degree of cure is


translated into an equivalent radiation dose; for example, it may be assumed that the degree of cure is that required to achieve maximum tensile strength. From the work on irradiated rubber this dose is known to be about 47 megarads, so that one may apply to chemically vulcanized material the same formula as for radiation cured rubber assuming the polymer has received the equivalent of 47 megarads. The values of F, F' for chemically vulcanized rubber deduced either from load/extension curves or from the elongation at break are shown in Table 14.4. The differences in Fas between chemical cure, pile radiation and electron radia-tion do not exceed the variations to be expected as a result of differences in mixing technique, minor ingredients such as sulphur, and testing condi-tions. There would therefore appear to be no great difference in the mechanism of carbon black reinforcement whatever the method of cure. Significant differences do, however, appear in F' values which depend on the elongation at break; radiation vulcanized specimens have a lower elongation at break and lower tensile strength. This difference, which has also been remarked on by Gehman and Auerbach (1956), may arise from the different nature of the bridges between rubber molecules. Radiation induced crosslinking gives direct C—C bonds whereas chemical vulcaniza-tion gives longer bridges involving sulphur.

Type of Reinforcement The reinforcement observed when carbon or fillers such as zinc oxide

are incorporated in an elastic network may result either from the effect of inert particles on the distribution of internal stresses, or from the formation of chemical bonds between the rubber and the active surface of the particles. Guth and Gold have shown theoretically that small spherical particles of an inert character will increase the modulus by a factor 1 + 2-5 v/ + 14-1 Vf2 where v/ is the volume fraction of the inert filler (see page 155). At a given elongation, this effect should be revealed by an equivalent increase i n F 2 .

F2(reinforced) = F2(l + 2-5 vf + 14-1 v/).

Fig. 14.16 shows the increase in the value of F with carbon black concen-tration in smoked sheet. For two of the blacks tested (P33, Sterling FT) the reinforcement obtained agrees with that calculated from the Guth and Gold formula (full line). In other blacks, the reinforcement is far greater and would indicate some form of surface interaction. It is of interest to note that chemically vulcanized mixes incorporating these blacks show the same distinction, although perhaps to a lesser extent. The extent of reinforcement available can be very considerable; the incorporation of an equal weight of Philblack 0 into rubber increases its modulus for a given radiation dose by a factor of 100. Nevertheless, the dose for maxi-mum tensile is almost unaffected.

Radiation Treatment of Rubber The crosslinking of rubber by radiation would at first sight appear to

offer several advantages over conventional vulcanization. The product can be purer with no undesirable tastes or smells. Thick specimens can

R U B B E R 279

be treated to give a material of uniform elastic properties and the treat-ment can be very rapid and continuous if high-powered sources are used.

By a suitable variation of the radiation dose across the specimen, rubberlike materials can be prepared whose elastic properties vary in any predetermined manner. Radiation treatment of orientated materials can render this orientation permanent (Charlesby and von Arnim, 1957) and

5 - 0

"3·Π

o*c\

1-0

0-5

0-3

• Sterling FT v P 33 a Philblack 0 + Spheron 9 x Sterling V Δ Vulcan 3 o Vulcan 6

1

[/.

D

_ ^ l "

f '-

s

X < t l

^

,Λ trz

^^

,4-U π

^

•-"""

»

*Λ η.

^

^ Μ Ϊ

Observed curve for reinforcing carbon blacks

D Chemically cured Philblack 0

» Chemically cur ?d P 3 3

A ~ · ^ c

-

1

_

ir

5

3

?

1

5

0 10 20 30 4 0 50 60 70 80 90 100

Parts carbon black/100 parts rubber

F I G . 14.16. Increase in elastic modulus by carbon black.

the elastic modulus then depends on the direction of stress. Crosslinking can be carried out over a wide range of temperatures and the temperature of treatment can be chosen as that most suitable for other parts of an industrial process. Materials may be incorporated which would normally interfere with the conventional chemical curing cycle. The use of radiation for the treatment of re-claimed rubber has also been suggested. The main disadvantages of the process arise from the present high cost of the capital equipment and the high doses of radiation needed to achieve maximum tensile strength. Furthermore, there are strong indications that this maximum tensile strength is lowered by the different character of the bridges between the rubber chains. These disadvantages should not be considered as insuperable. The use of radiation as an industrial tool is still in its infancy and the cost of high energy radiation is decreasing


rapidly. To reduce the cost of radiation treatment, further suitable additives may be incorporated in the mix to increase its reactivity. The flexibility of this form of treatment is such that rubberlike block and graft copolymers with useful new properties may be both produced and crosslinked by radiation.

Apart from its direct application in industrial processing, radiation is a valuable method for promoting reactions quantitatively and its use in the research laboratory may provide a deeper insight into the mechanisms of fracture and of reinforcement of rubber, and hence lead indirectly to improved rubber compositions.

SYNTHETIC RUBBERS A number of papers deal with the effect of radiation on synthetic

rubbers, but with the exception of butyl rubber no systematic investigation has been carried out. This material is a copolymer of butadiene and /sebutylène, the butadiene units serving primarily to allow the chains to be linked together. The proportion of butadiene units in the copolymer is very low, and the effects of radiation are dominated by main chain fracture of the /söbutylene units, as in poly/söbutylene (page 330). Bopp and Sisman (1955) found a reduction in both modulus and tensile strength, the latter falling to a fifth of its original value after a pile dose equivalent to only 50 megarads.

Butadiene polymers can be readily crosslinked by radiation (Charlesby and Groves, 1954), but the process is accompanied by a change in iso-merism (Golub, 1958). The copolymer of butadiene and styrene known commercially as GR-S rubber, Buna S or Hycar is more resistant to radiation, due to the radiation protection offered by the styrene group (Bauman and Glantz, 1957). Bopp and Sisman have given some mechanical data for the irradiated copolymer. Acrylonitrile copolymerized with butadiene appears to show some radiation resistance, but the modulus increases rapidly with radiation dose (Ryan, 1953, Bauman and Glantz, 1957).

Neoprene or polychloroprene is very sensitive to radiation, particularly in the nuclear reactor, where the absorption of energy is greatly increased by slow neutron capture by chlorine atoms. Neoprene is readily cross-linked (Charlesby and Groves, 1954), becomes insoluble, and acquires a higher modulus (Ryan, 1953). Some mechanical properties of irradiated neoprene obtained by Bopp and Sisman (1953) are shown in Fig. 14.17. They show a higher modulus for a dose as low as 004x 1018 nvt (about 100 megarads) and improved compression set.

Other synthetic rubbers whose mechanical properties at room tempera-ture have been investigated by Bopp and Sisman (1953) included Hycar P.A. (polyacrylate) Hycar OR (butadiene-acrylonitrile) Silastic (silicone rubber) Thiokol (in which sulphur occurs as part of the main chain) and the copolymer Vulcollan. Several of these are discussed under the appro-priate chapter headings. In all cases there was an increase in Shore hardness on irradiation, often allied to a decreased tensile strength and elongation at break. In a few cases, notably poly butadiene and Hycar

RUBBER 281

OR-15 and OS-10 an increase in tensile occurred, at least over a limited range. These results can all be understood as being due to an increased density of crosslinking, which may exceed that needed for maximum tensile strength (as found for rubber). At high doses several polymers including Hycar OR-15, OS-10 and PA-21, Neoprene and Hypalon showed an increase in tensile strength, together with a very low elongation at break. These changes can perhaps be associated with a transition in

3000

"i 2000 Q.

8 5 1000

I V S ' I I I I I

0 VO 2-0 3-0 4-0 5-0 Strain in tension, in/in

FIG. 14.17. Increase in modulus and compression set of irradiated Neoprene. (From Bopp and Sisman, 1953.)

structure towards the glass-like state (or ebonite) as the density of crosslinking is increased.

The effect of fillers on the mechanical properties of various types of elastomers—natural, butadiene-styrene, butadiene and nitrile has been studied by Kuzminskii, Nikitina and Karpov (1957). The source used was an x-ray tube of 8 kV, providing a radiation intensity of about 0-5 mega-reps/min. The total doses accumulated ranged from 30 to 600 megareps, most of these doses being far greater than is needed to produce high elasticity. In contrast to much of the previous work on rubber, the curves obtained showed a steady increase in tensile strength with dose.

The effect of additives on the properties of the butadiene-styrene copolymer was studied in detail. Sulphur and tetramethyl thiuram disulphide, which act as inhibitors for polymerization, reduced the effect of radiation by about 20-40 per cent when present in low concentration of about 1 per cent. Presumably they combine with the radicals produced by radiation, and prevent crosslinking. Other additives such as mercapto-benzothiazole and diphenyl guanidine had little effect. Filler such as zinc oxide, present at a concentration of 10 per cent may increase the modulus by about 50 per cent. Carbon black was also found to act as a reinforcing agent in butadiene. This reinforcement is not primarily due to particle size, since lamp-black, with the largest particle size (lowest surface area) was most effective. Some form of surface chemical reaction must therefore be envisaged.

JO

* L O P Γ"/ *> 1 **** / /

Λ / //

f o Breaking point


30 60 90 120 150 180 210 240 270 Dose in megareps

FIG 14.8. Effect of carbon black on the elastic modulus of butadiene polymer (1) -f- 20 parts lamp-black. (2) -f 40 parts lamp-black. (3) + 75 parts lamp-black. (4) + 100 parts lamp-black. (5) + 120 parts lamp-black. Δ -f- 120 parts channel. X + 120 parts stove-black. O + 120 parts thermal-black.

(From Kuzminskii, Nikitina and Karpov, 1957.)

DufTey (1958) has used filters to improve the tear resistance and tensile strength of polyvinyl methyl ether elastomers. For carbon-black filled polymers, maximum strength was reached at a dose of 50 megarads (corresponding to that in natural rubber). With silica-filled polymer the dose for maximum tensile was lower. Magnesia proved to be a better filler than might be expected from its behaviour in other elastomers. Further details on irradiated commercial elastomers are given in Chapter 31.

RUBBER 283

R E F E R E N C E S

BAUMAN, R. and GLANTZ, J., / . Polymer Sei. 26, 397, 1957. BOPP, C. D. and SISMAN, O., ORNL. 1373, 1953; Nucleonics 13(7), 28, 1955;

ibid. 13(10), 51, 1955. CHARLESBY, A., A.E.R.E. M/R 1185, 1953; Nature, Lond. Ill, 167, 1953; Plastics

18, 70, 1953; Rubb. Age, Lond. 34, 453, 1953; Atomics 5(1), 12, 1954; Rev. Gn. Caoutch. 32(1), 39, 1955; Rubb. Chem., Lond. Technol. 28, 1, 1955; Trans. Pias. Inst. 23, 133, 1955.

CHARLESBY, A., BURROWS, J. and BAIN, T., Rheology of Elastomers, p. 122, Pergamon Press, 1958.

CHARLESBY, A. and VON ARNIM, E., / . Polymer Sei. 25, 151, 1957. CHARLESBY, A. and GROVES, D., Proceedings of the Third International Rubber

Technology Conference, 317, London, 1954. CLARK, G. L., Brit.J. Radiol. 23, 112, 1927. DAVIDSON, W. L. and GEIB, I. G., / . Appl. Phys. 19, 427, 1948. DAVIDSON, W. L. and GEIB, I. G., Rubb. Chem. Techniol. 22, 138, 1949. DUFFEY, D., Ind. Eng. Chem. 50(9), 1267, 1958. EISLER, S. L., R.I.A. 53-4519 AD 24914 (ORDTX 10), Nov. 1953. GEE, G., Trans. Faraday Soc. 42, 33, 585, 1946. GEHMAN, S. D. and HOBBS, L. M., Rubber World 130, 643, 1954. GEHMAN, S. D. and AUERBACH, L, / . Appl. Rad. Isotopes 1, 102, 1956. GOLUB, M. A., / . Amer. Chem. Soc. 80, 1794, 1958. HOBBS, L. M., FLETCHER, D. W. and BROWN, D. E., COO. 196, Sept. 1953. JACKSON, W. W. and HALE, D., Rubber Age 77, 865, 1955; WADC-TR-55-57,

1955. JONES, S. S., Gen. Elect. Rev. 57(4), 6, 1954. KARPOV, V. L., Conference on the Peaceful Uses of Atomic Energy, July 1955

(in Russian), Academy of Sciences, U.S.S.R. KRASNIKOV, A. L., J. Exp. Theor. Phys. U.S.S.R. 9,1343,1939, p. 13, English Ed.,

Consultants' Bureau, New York, 1955. KUZMINSKII, A. S., NIKITINA, T. S. and KARPOV, V. L., / . Nucl. Energy 4, 268,

1957; Atomnaya Energiya 1(3), 137, 1956; Soviet J., Atomic Energy 3, 431, 1956.

LAWTON, E. J., BAL WIT, J. S. and BUECHE, A. M., Industr. Engrg. Chem. (Anal.) 46, 1703, 1954.

NEWTON, U.S.P. 1906402, May 2, 1933. RYAN, J. W., GEL 54, 1952; Nucleonics 11(8), 13, 1953. STOCKMAN, C. H., HARMON, D. J. and NEFF, H. F., Nucleonics 15(11), 94, 1957. TURNER, D. T., / . Polymer Sei. 27, 503, 1958.

CHAPTER 15

POLYSTYRENE

POLYSTYRENE is a clear transparent plastic with good resistance to most forms of chemical attack although it is dissolved in aromatic and chlori-nated solvents. The molecular weight of linear polystyrene can be readily deduced from the intrinsic viscosity of such solutions.

In the absence of an inhibitor, polymerization of the monomer styrene (or vinyl benzene) occurs slowly even at room temperature and to prevent this spontaneous reaction an inhibitor is usually added. The reaction is speeded up by higher temperature and by the presence of catalysts. Poly-merization can also be initiated by exposure to ultraviolet light or to high energy radiation. Much of the work on radiation-induced reactions of monomers has been devoted to styrene which has been shown to polymerize by a radical mechanism (Chapter 22).

In conventional polystyrene, the phenyl groups are arranged at random on either side of the polymer chain; this random distribution prevents crystallization even at very low temperatures and the structure of the polymer as revealed by x-ray diffraction is amorphous in character. At room temperature, polystyrene is a material of high modulus, low elon-gation at break and negligible cold flow. These properties are due to the rigid nature and entanglement of the molecular chains at low temperatures. As the temperature is raised, molecular flexibility increases and at about 80°C polystyrene distorts very readily under load, while at higher tem-peratures it flows as a viscous liquid unless crosslinked. The temperature limitation may be reduced to some extent by the copolymerization of styrene with other monomers. Other copolymers, such as butadiene-styrene, are much more flexible and are used as synthetic rubbers.

It has recently become possible to produce a form of polystyrene in which the phenyl groups are arranged regularly on one side of the polymer chain. This isotactic polystyrene can be crystallized and the mechanical properties, being dominated by these crystals, differ very considerably from those of the random (conventional) polystyrene.

Polystyrene can be crosslinked by exposure to high energy radiation or chemically by copolymerizing it with small amounts of divinyl benzene. It is of interest to compare radiation-induced crosslinking in polystyrene with that occurring in polyethylene and in rubber. In polystyrene the rigid structure prevailing at room temperature inhibits the motion of even small molecules through the specimen; in rubber the chains are highly mobile and diffusion of reactive entities is almost as readily achieved as in a viscous fluid. In polyethylene, considerable differences may be expected as between the crystalline and the amorphous regions. In spite

284

POLYSTYRENE 285

of these considerable differences, the radiation-induced changes appear to follow the same general pattern in all three types of polymeric structure.

Crosslinking and Degradation

Early experiments on the solution properties of irradiated polystyrene show that the polymer becomes crosslinked and partially insoluble when subjected to nuclear pile radiation (Charlesby, 1953). The solubility in toluene or benzene of polystyrene rods was found to be independent of the solvent but »o decrease rapidly with radiation dose (Fig. 15.1). This

lOOi

FIG. 15.1. Solubility of irradiated polystyrene.

Theoretical curve ■

X Sol fraction in benzene. + Sol fraction in toluene.

random distribution random distribution uniform distribution

gel point = 0-55 pile units. gel point = 0-5 pile units. gel point = 1 pile unit.

decrease in solubility followed theoretical predictions for an initially random molecular weight distribution (equation 9.23) assuming that the crosslinks occur at random and in proportion to the radiation dose. It did not follow the calculated relationship for an initially uniform mole-cular weight (equation 9.22). At high doses, solubility tended to zero, indicating the absence of degradation additional to the observed cross-linking. An interesting feature found in these early experiments was the large dose needed to reach the gel point, this in spite of the high initial molecular weight of the polymer. This high resistance to radiation-induced changes is due to the protection effect of the benzene rings in the molecule, and parallels the observations made in solutions of low molecular weight organic compounds with benzene.


These observations were extended to a study of the swelling charac-teristics of the crosslinked polymer. The simplified theoretical formula (9.54) was found to represent adequately the observed swelling charac-teristics at medium levels of crosslinking. At low degrees of crosslinking, however, a number of corrections must be applied to this simple relation-ship:

40

o 3 0

a I_ σ»20 c a Ϊ 15

if 10 ■>

> 8

6

0 c

~°+r

• \

s 3

& N <sj

*&Px

I > s

0-5 0-8 1-0 1-5 2-0

Radiation dose 3-0 4-0 6-0 8-0

y pile units

FIG. 15.2. Weight swelling ratio and radiation dose: B, first set, swollen in benzene: T, first set, swollen in toluene. Second set swollen in toluene: (X ) 4 days. (O) 2 weeks.

(+ ) 14 weeks.

(i) In calculating the swelling ratio, only the gel fraction should be considered since this alone can swell without dissolving. It is, however, more usual to take the swelling ratio V as the swollen weight divided by the initial dry weight. A correction for the soluble (non-swelling) fraction must be included.

(ii) Approximations are made in deriving the simplified formula (9.51) from the more accurate version (9.50). A full calculation of the latter gives a much more complex variation of the degree of swelling with crosslinking density.

(iii) As shown in equations (9.26) and (9.27), there exists a difference between the crosslinking index of the gel and that of the material as a whole. Similarly, the average molecular weight of the molecules in the gel is greater than the average for the whole specimen. At low densities of crosslinking, these two factors must be considered.

(iv) Only crosslinks resulting in closed loops are effective in swelling; corrections must therefore be applied to allow for links needed to form a gel before a closed network is obtained.

When allowances for all these corrections were made, a more satisfactory fit was obtained with the observed data at low degrees of crosslinking (Fig. 15.3).

POLYSTYRENE 287

Feng and Kennedy (1955) studied the changes in electrical conductivity and in average molecular weight of polystyrene and other polymers which

2x10'

Mc

5x10' 0 1 2 3 4 5 6 8

Radiation dose , r pile units

FIG. 15.3. Variation of Mc with radiation dose r\ (O) experimental values with corrections (i) and (ii) ; (—, - ) calculated with corrections (iii) and (iv).

had been subjected to ß-radiation from a strontium 90 source. Irradiation was carried out both in air and in vacua and the molecular weight changes were followed by measurements of intrinsic viscosity. From these measure-ments, Feng and Kennedy deduced that in polystyrene simultaneous crosslinking and degradation takes place, a conclusion at variance with the results of earlier solubility measurements. Furthermore, for specimens irradiated in air, they deduced a radiation intensity dependence CO for the rate of degradation intermediate between / and I0'5 (Table 15.1), while for crosslinking in vacuo a relationship varying as I°'& was obtained. (In the paper, the process is termed polymerization but is in fact dimerization or crosslinking since no chain reaction is involved and exposure to radiation leads primarily to branched molecules.)

These conclusions are difficult to reconcile with the results of other and more direct measurements of crosslinking both in polystyrene and other polymers. Errors may be due both to the uncertainty in deriving mole-cular weights from intrinsic viscosity measurements of branched mole-cules and to the wide range in energy absorption of the ß radiation from a strontium source. The intensity effect of crosslinking in vacuum, reported by Feng and Kennedy, is at variance with the good agreement between G values for crosslinking polystyrene obtained with pile radiation (Charlesby), with γ-radiation (Wall and Brown) and electron radiation (Shultz et al.). All these methods lead to a G value per crosslink of 0Ό5 (±001) although the radiation intensities involved vary by a factor of


about 105. For degradation in air, however, an intensity effect may well be present; the rate of main chain fracture can then depend on the diffusion rate for oxygen, the exposure time and the geometry of the system.

Table 15.1. Degradation of Polystyrene Irradiated in Air (Strontium 90 ^-radiation)

Fracture rate (10~4 bonds/hr) Intensity (megarads/hr) Rate/intensity (10-4 bonds/megarad) Rate/(intensity)i*

4 019

21 9-2

217 1-2 0094 0049

23 24 7-1 5-4

0-8 0025

32 5

0-45 0013

34 5

♦Arbitrary units. (From: Feng and Kennedy, 1955.)

Shultz et al. (1956) studied the changes in molecular weight of thin films of polystyrene irradiated in air with 1 MeV (peak) electrons. Measure-ments of gel formation, of weight average molecular weight Mw' and of intrinsic viscosity [η]' were obtained and compared with theoretical calculations.

The gel point for a polymer of initial weight average molecular weight Mw = 1-15 x 106 was found to be 10-3 megareps. From this value, an energy deposition of 1710 eV/crosslink was deduced, in satisfactory agree-ment with the approximate values deduced by Charlesby for pile radiation and by Wall and Brown for cobalt γ-radiation. Degradation was also observed, the ration of chain scission to crosslinking being 0-35; this figure is higher than is consistent with the earlier pile data of Charlesby. The difference may, however, be due to the presence of oxygen in the thin films which would cause degradation during the irradiation in air, as previously reported by Wall and Magat (1953). Shultz et al. related the increase in Mw' to crosslinking density, assuming both crosslinking and degradation to occur at random and in proportion to the radiation dose. Equation (11.6) can be written in the form

Λ7-? = Ί Ϊ 7 - + ( Ρ Ο / 2 - * Ο ) - (15.1) Mw' Mw w

The data obtained (Fig. 15.4) indicates that the relationship between 1JMJ and r is linear, as predicted by this formula. This tends to confirm the assumption that both crosslinking and degradation occur simul-taneously in proportion to the radiation dose. The extrapolated value of 9-4 megareps for the point at which Mw' becomes infinite is in satisfactory agreements with the gelling dose (10-3 megareps) deduced from solubility measurements.

Intrinsic viscosity measurements also show a rise with radiation dose but here comparison with theory is less easy to obtain. An approximate equation for the viscosity of branched polymer molecules has been derived by Stockmayer et al. (1949, 1953). Fig. 15.5 shows a comparison of these theoretical predictions with the observed relationship. Agreement is fair

POLYSTYRENE 289

up to within 80 per cent of the gelation point; above this point, theory predicts a rapid drop in viscosity which is not, however, observed. This indicates a failure of the theory at these high degrees of branching.

2 4 6 8 10 Γ , megareps

FIG. 15.4. Reciprocal weight-average molecular weight, \\MW\ of an electron-irradiated polystyrene film plotted against radiation dose, r.

Radiation may provide a useful method of obtaining polymers with accurately known degrees of branching, suitable for studying its effect on such properties as viscosity.

wo

1-30

1·9Π

1·1Π

(M'w

* r \

.0-74 /

yS C

I

f

Theory

/ * ft

I

Y 4 observe I

I

o /

kd I

! I 0-2 0-4 0-6 0-8

r/ra gel

FIG. 15.5. Electron-irradiated polystyrene. Observed and theoretical ratios of intrinsic viscosities of irradiated and non-irradiated polystyrene in benzene. A plot of (Mw'JMtv)0"" is included to indicate approximately the depression of intrinsic

viscosity due to radiation-induced branching.


Effect of Oxygen

Wall and Magat (1953) report an observation of Landler on the irradiation of polystyrene in the French nuclear reactor at Chatillon. A slight decrease in intrinsic viscosity was found (from 2-60 to 2-42); this however was probably due to the radiation-induced main chain fractures produced in the presence of atmospheric oxygen exceeding the number of crosslinks produced.

Alexander and Toms (1956) report an oxygen effect in the irradiation of polystyrene in the form of films or of rods. In the latter case oxygen penetration is limited and gel formation occurs more readily due to reduced competition between crosslinking and degradation. With thin films irradiated in air degradation predominates and no gel is formed (Table 15.2).

Table 15.2. Effect of Oxygen on Crosslinking in Polystyrene {Pile radiation)

Dose (megarads)

60

120

200

Form

£ in. rod Thin film i in. rod Thin film \ in. rod Thin film

Sol fraction % Air Vacuum

31 100 14

100 14

100

2 52 1

23 0-5

23

(From: Alexander and Toms, 1956.)

The effect of oxygen on pile irradiated polystyrene was also studied by Sears and Parkinson (1956) using infra-red measurements to determine changes in structure. Polystyrene was irradiated in vacuum in the Oak Ridge reactor, the dose accumulated being equivalent to 105 megarads. The bands associated with the presence of benzene rings lost much of their intensity but this could be due to the very high dose used. When air was admitted OH and C—O bonds were produced (Fig. 15.6). The measure-ments indicate that the reactive entities such as radicals or unsaturated compounds formed during radiation persist for long periods at room tem-perature. These may be related to the change in colour of polystyrene on irradiation. The yellow colour formed in this way is, however, unlikely to be due to trapped free radicals since it persists in irradiated polystyrene when this is swollen in benzene, under conditions which allow free access of air.

The effect of oxygen on radiation-induced changes in polystyrene can be related to an early observation of Winogradoff (1950), who reported a change in fluorescence characteristics on the surface of x-ray irradiated polystyrene.

Jech (1954) also observed a change in the surface properties of poly-styrene subjected to a particle bombardment, which resulted in a decreased

POLYSTYRENE 291

J

s

JP(

f

- \ 1)

iwr -»lys f o r e

hyr< ΙΓ

»ne rac Jiat

ÎA'Â'.i

'on

1 n J

II

Λ

11

r rlilT» iln

fi

ΛΑ n

ï 1)1

h r

l 1

contact angle for water droplets. Since the change was only observed in the presence of air, it can be ascribed to a surface oxidation effect.

Using infra-red measurements Steigman et al. (1953) also found that oxygenated products are formed when polystyrene is irradiated in air. Polystyrene powder irradiated in air or in vacuum reacted to a greater extent with DPPH than did unirradiated samples, indicating the presence of radicals.

100,

80

60

40

20

o' 100,

80

60

' 40

20

o' 100.

80

60

40

20

0' 100,

\

\ N,

J ' 100 x10 9 r o d s \ ^ m e a s u r e d in He Vj

I I I I I

Λ

A I/ I 1 ! 1 1

t

4 \J

\ k^ J

J" Ar \l\ v \ s\S \l\ \l\

Λ

1 h

\ \ Λ /

I

X

Sa

ex

L

m e s a m p ] f t e r a i r Dosure for 17 h our s

e \ Λ / ^ /

V ^ • - - ~r λ A ■> Λ

ΐ 1 vAf

4000 3000 1000 800 600 2000 1800 1600 1400 1200 Wave number , cm-1

FIG. 15.6. The infra-red spectrum of polystyrene after irradiation and exposure to air. (From: Sears and Parkinson, 1956.)

Deuter ated Polystyrene

Wall and Brown (1957) attempted to study the mechanism of cross-linking in polystyrene by investigating the G value for crosslinking and for gas evolution in deuterated specimens. Irradiations were made with the National Bureau of Standards cobalt 60 source, and the gel point deduced from the increased viscosity. At room temperature, the energy absorbed per crosslink in polystyrene was about 2000 eV (1860-2200), with somewhat higher values for the deuterated material. When irradiated

u


at — 196°C, the energy required per crosslink was increased by approxi-mately 50 per cent, confirming the temperature effect reported in other polymers (Fig. 15.7).

2-4 ■— ~

2-2

2-0

1·

0 2 4 6 8 10 Dose, megarads

FIG. 15.7. Temperature effect in crosslinking of polystyrene by γ-radiation. • room temperature radiation.

X—196°C radiation. Both irradiations in vacuum; arrows indicate gel point.

(From: Wall and Brown, 1957.)

Table 15.3 shows the effect of deuteration on the energy absorbed per crosslink, and on the gas evolution. If crosslinking involves the abstraction of a hydrogen in a specific position, there would be an increased energy requirement of at least 40 per cent for polystyrene deuterated in that portion, with little change for other forms of deuterated polymer. The observed results show no such simple relation.

Table 15.3. Energy Dissipation in y-irradiated Polystyrenes

Sites of deuteration

α,β,β

ß.ß α

β Ρ Nil Nil Nil

Μυ

973000 793000 1118000 1148000 1032000 1176000 1252000 326000

EQQ

2370 + 150 2970 + 500 2550+150 2150 + 100 3300 + 550 1860+ 20 2050 + 200 2200 + 250

E{G)

7650 5450 5240 3830 4730 4490 3910 —

Μυ initial viscosity average molecule weight. E(X) energy absorbed per crosslink (in eV). E(G) energy absorbed per molecule of H2, HD or D2 formed (in eV).

(From: Wall and Brown, 1957.) (Site of deuteration—see Fig. 15.8.)

-

\-

h

J*

<

1

"* 1

__] L

it

i *·'**

1

1

1

POLYSTYRENE 293

Table 15.4. G Values for H andO Production in Irradiated Polystyrene

Site of deuteration

α,β,β ß,ß

α ß Ρ

Nil

G(H)

xlO-3

17-2 27-1 34-6 470 40-6 44-6

G(D)

xio-3

9 6-5 3-5 5-2 1-6 —

G(H+D)

XlO"3

26-2 33-6 381 52-2 42-2 44-6

- C D f e - C D - - C D 2 - C H - - C H 2 - C D -

0 0 0 - C H D - C H - - C ^ - C H -

0 0 D

/3 P FIG. 15.8. Polystyrene deuteration sites.

(From: Wall and Brown, 1957.)

Table 15.4 shows the G values for hydrogen and deuterium production in various deutero-polystyrenes. From these values Wall and Brown deduce the G values for fracture of C—H and C—D bonds (Table 15.5), and the isotope effect. For ordinary polystyrene hydrogen evolution is computed to be

22-3 per cent from the a-position 39-2 per cent from the ß-position 38*5 per cent from the benzene ring.

No single position can therefore be selected as the site of crosslinking.* The lower G value for hydrogen production than for crosslinking would indicate some degree of hydrogénation, a deduction in agreement with that advanced to account for the limiting degree of unsaturation in irradiated polyethylene. However, Wall and Brown emphasize the considerable difference in isotope effects in deuterated polyethylene and polystyrene, which they conclude must arise from a fundamentally different mechanism.

* From paramagnetic resonance measurements Abraham and WhirTen (1958) con-cluded that radical formation in the a position is favoured, but their G value of 0-2 is greater than the G value for crosslinking.


Table 15.5. G Values for Specific C—H and C—D Bond Rupture

Bond position

a ß P r f

G values x 103

C—H C—D 9-9 3-5 8-7 3-25 3-9 1 65 3-3 (1-5) 2-3 1-6

Isotope effect

2-8 2-7 2-4

(2-2) —

r represents ring hydrogens other than para. (p). f represents increased effect due to presence of a D atom

on same carbon.

Mechanical Properties of Irradiated Polystyrene

Bopp and Sisman (1951, 1956) measured the stress-strain curves of various commercial samples of polystyrene, as a function of radiation dose. Measurements were made according to a standard procedure (A.S.T.M. D638-49T). The results obtained for Amphenol (unmodified polystyrene), Styron 475 (black-pigment filled polystyrene) and Royalite (a styrene-acrylonitrile copoJymer) are shown in Fig. 15.9. Amphenol

0 0-02 0-12 0-14

Strain FIG. 15.9. Mechanical properties of commercial types of polystyrene after pile

radiation. Doses are in units of 1000 megarads. X denotes breaking point.

shows no appreciable change in modulus at 25°C for doses as high as 9100 megarads, corresponding to a very high density of crosslinking (about one crosslinked unit per 10 monomer units); the elongation at break is, however, decreased slightly. Styron 475 shows a greater reduction in the elongation at break, with an increased modulus and ultimate tensile strength. In this material radiation has reduced the flow characteristics of the material, possibly by bonding with the pigment filler. In the copolymer Royalite there is an increase in modulus, and for a dose of

POLYSTYRENE 295

200 megarads an increased elongation at break—indicating the effect of crosslinking on a more flexible system. While the mechanical properties of Royalite are improved by radiation, the necessary doses are probably too high to be of immediate commercial value; these high doses are necessary due to the radiation protection offered by the benzene ring.

While there is relatively little change in the mechanical properties of irradiated polystyrene at room temperature, there is some evidence of an increased softening point. This increase is observed for doses sufficient to produce gel formation, but not a tightly knit network. Fig. 15.9 based

4-2r

£ 4-Ol·

g 3·8|-

■σ 3·6|

3-4

3-2

3-0

2·θ|

2-6

> , \ s

■ - —-

^Ν42

1 \ \ \ \ \

— ]

, \

\ \ 1

υ 20 40 60 80 100 120 °C

FIG. 15.10. Elastic modulus of unirradiated and irradiated polystyrene (42 pile units). (From: Baccaredda et ai, 1956.)

on data published by Baccaredda et al. (1956) shows an increase of some 10°C. This increase is observed with a relatively small dose of about 60 megarads, and increases further by only a few degrees when the dose is raised by a factor of 10.

Colichman and Fish (1957) have also drawn attention to an improve-ment in mechanical properties in polystyrene, copolymerized with and crosslinked by divinyl benzene. When polymerization of a styrene-divinyl benzene mixture is initiated by radiation rather than by conventional chemical techniques, a larger concentration of divinyl benzene can be incorporated without causing a loss in mechanical properties. Other factors of importance are temperature and oxygen during the radiation treatment. While the increase in crosslinking density may be expected to give a harder material, it is not yet clear why radiation should be preferable to conventional methods for initiating the reaction.

Protection

Polystyrene is one of the most radiation resistant of long chain polymers; although it crosslinks, the energy required (about 2000 eV/crosslink) is nearly 100 times greater than in most linear polymers such as polyethylene.


This resistance is due to the benzene ring which protects by a "sponge" effect in much the same way as in solutions of benzene and other aromatics. This protection not only covers the styrene monomers unit, but may also extend to neighbouring groups as, for example, in copolymers of styrene and /stfbutylene (Alexander and Charlesby, 1955). In these copolymers the degradation which to butylène polymer suffers under radiation is greatly reduced by the presence of neighbouring styrene units. Synthetic rubber compositions involving styrene (such as styrene-butadiene copolymer) are likewise fairly resistant to radiation (Bauman and Glantz, 1957). a-Mcthyl styrene on the other hand degrades readily under radiation.

Krenz (1955) studied protection effects and energy transfer in specimens of polystyrene including small amounts of anthracene. The energy absorbed by x- or γ-rays in the polystyrene was transferred to the anthracene which fluoresced.

The subject of protection against radiation effects is discussed in fuller detail in Chapter 29.

REFERENCES ABRAHAM, R. J., WHIFFEN, D. H., Trans. Farad. Soc. 54, 1291, 1958. ALEXANDER, P. and CHARLESBY, A., Radiobio logy Symposium, Liege, August

1954, p. 49, Butterworths, 1955. ALEXANDER, P. and CHARLESBY, A., Proc. Roy. Soc. A230, 136, 1955. ALEXANDER, P. and TOMS, D., / . Polymer Sei. 22, 343, 1956. BACCAREDDA, M., BORDONI, P. G., BUTTA, E. and CHARLESBY, A., Chim. e

lndustr. 28, 561, 1956. BAUMAN, R. and GLANTZ, J., / . Polymer Sei. 26, 397, 1957. BOPP, C. D. and SISMAN, O., ORNL 1373, 1953. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955; 14(3), 52, 1956. CHARLESBY, A., / . Polymer Sei. 11, 513, 521, 1953. COLICHMAN, E. L. and FISH, R. F., Nucleonics 15(10), 134, 1957. DURUP, J., / . Chim. Phys. 51, 64, 1954. FENG, P. Y. and KENNEDY, J. W., J. Amer. Chem. Soc. 11, 847, 1955. HORNBECK, R. F., KIRKLAND, W. K., PARKINSON, W. W. and SISMAN, O.,

ORNL 2413, August 1957. JECH, C , Z. Phys. Chem. 203, 309, 1954. KRENZ, F. H., Trans. Faraday Soc. 51, 172, 1955. SCHNEIDER, E. E., Trans. Faraday Soc. 19, 158, 1955. SISMAN, O. and BOPP, C. D., ORNL 928, 1951. SISMAN, O., Plastics Technol. 1, 345, 1955. SEARS, W. C. and PARKINSON, W. W., / . Polymer Sei. 21, 325, 1956. SHULTZ, A. R., ROTH, P. I. and RATHMANN, G. B., / . Polymer Sei. 22,495, 1956. STEIGMAN, J., BRILL, R., AROND, L., BENDER, A., CORTH, R., GOODMAN, J., and

COPPERMAN, A., AD. 16978, AD.28234, July-October 1953. STOCKMAYER, W. H. and FIXMAN, M., Ann. N.Y. Acad. Sei. 57, 334, 1953. WALL, L. A. and MAGAT, M., J. Chim. Phys. 50, 308, 1953a. WALL, L. A. and MAGAT, M., Mod. Plastics 30(11), 111, 1953b. WALL, L. A. and BROWN, D. W., J. Phys. Chem. 61, 129, 1957. WINOGRADOFF, N. N., Nature Lond. 165, 123, 1950. ZIMM, B. H. and STOCKMAYER, W. H., J. Chem. Phys. 17, 1301, 1949.

CHAPTER 16

SILICONES General Properties

The group of polymers known as silicones form a chemical intermediate between organic plastics and inorganic glasses. The main chain of these polymers consists of alternating silicone and oxygen atoms and therefore contains no carbon atoms, whereas the side chains consist of groups similar to those found in organic polymers such as polywöbutylene or methyl styrene. They also differ from many organic elastomers in that no unsaturation is possible in the main chain.

Extensive work on these silicones was carried out at an early date by Kipping, who noted the possibility of polymerization by intermolecular condensation. Silicones may be produced via the action of Grignard reagents on SiCl4 to give chlorosilanes, which are then hydrolysed. Possible intermediates may be hydroxysilanes, e.g. SiMe2(OH)2 which eliminate water to yield siloxane chains. The length and structure of siloxane chains depend on the relative proportion of the alkyl or aryl groups in the chlorosilane. In the methyl chlorosilanes, for example, chain propagation arises from the presence of SiMe2(Cl)2; SiMe3Cl acts as a chain terminal and its concentration largely determines the molecular weight, while SiMe(Cl)3 or Si(Cl)4 can act as crosslinking agents.

Silicones may either take the form of linear or of cyclic polymers (SiMe20)rt where n may be as low as 3. The repeating unit is usually

Me Me I I

—SiO— or —SiO— I I

Me Ph

the phenyl silicones having better temperature properties. Silicones are prized principally for their stability at high temperatures and for the small variation of their viscosity with temperature. Linear or cyclic silicones are soluble in many common organic solvents but are insoluble in water and the lower alcohols. Suitably formulated they are used as water repellents.

Silicone liquids or greases are available covering a vast range of viscosities, these being directly related to the molecular weight. Silicones with suitable vulcanizing agents and fillers may serve as high temperature rubbers, permitting use for long periods at temperatures above 200°C. Furthermore these rubbers have a low brittle point of —65 to —80° C. Silicones suffer from a very low tensile strength which can, however, be

297


improved by incorporating fillers, when tensile strengths of 600 psi or more are currently obtained.

In suitable solvents, silicones may be used as adhesives or as thermo-setting varnishes for impregnating. By incorporating aluminium powder, high temperature resisting enamels can also be produced.

Molecular Weight Determination

As a starting material for studying radiation effects on polymers, silicone fluids show a number of advantages, not the least of which is the very wide range of molecular weights readily available. Dimethyl silicones are usually specified by their bulk viscosity, which can range from 0-65 centostokes (the lowest trimethyl end-group silicone), to several million centistokes (for a degree of polymerization of several thousands). The average molecular weight can be determined either from the bulk viscosity, or from the limiting viscosity number (intrinsic viscosity) using selected solvents.

Barry (1946) studied the relationship between these properties and the molecular weight, as determined by light scattering or osmotic pressure in the higher molecular weight region, and by end group titration or direct chemical analysis for the lower molecular weight material. By analogy with earlier work on polyesters and polyamides, Barry deduced a relation-ship between the bulk viscosity at 25°C measured in centistokes, for mole-cular weights above 2500 :

log1(fl (cs at 25°C) - 1 + 0-0123 Mw* (16.1)

where the bulk viscosity η is assumed to depend on the number average molecular weight Mn. Subsequent work has indicated that the bulk viscosity depends more closely on the weight average molecular weight Mw which gives greater emphasis to the longer molecules. More extensive data can be shown to be represented by a formula of the type

logics = A logl0 Mw + C. (16.2)

A formula of this same type has been shown by Fox and Flory (1951, 1954) to represent the bulk viscosity of poly/s6>butylene and polystyrene at temperatures above the second order transition point. In a plot of bulk viscosity against molecular weight (on a log-log scale) two regions may be observed. Above a critical value which depends to some extent on the polymer, the parameter A is constant while the parameter C varies with temperature and possibly with the type of polymer. Below this critical molecular weight, different values of A and C are found. The transition from one regime of viscous flow to another occurs at a molecular weight of 17,000 for poly/sobutylene and at 38,000 for polystyrene. Above these molecular weights the value found for A was 3-40 in either case, while below them A equals 1-75 for poly/söbutylene and 1-65 for polystyrene.

SILICONES 299

Analogous regions are found for the dimethylsiloxane fluids, the transition occurring at a molecular weight of 30,000. Above this value, A is found to be equal to 3-7 (Charlesby, 1955) or 3-64 (Warwick et al, 1955) while below it equals 1-39 (Charlesby) and 1-43 (Warwick). The relatively close agreement in the value of A above the critical figure of the order of 30,000 for three very different polymers would appear to indicate some common process of flow of long chain materials independent of polymer structure. Bueche (1952) has produced a theory of viscous flow showing a critical value of the molecular weight which leads to a somewhat similar formula. The parameter A, according to Bueche's theory, is equal to unity below this critical value and 2-5 above it. These theoretical values are therefore lower than those observed by Fox, Flory, Charlesby and Warwick.

Barry also derived an equation relating to the intrinsic viscosity [η] to the molecular weight of a fractionated sample. His formula can be written

[η] = 2 x l 0 - 4 M ° · 6 8 (16.3)

for solutions of dimethylsiloxanes in toluene at 25°C. This formula is similar in character to that found for the intrinsic viscosity of a large number of long chain polymers (equation 8.13).

Table 16.1. Bulk Viscosity of Long Chain Polymers logn^ = A log10 Mw+C

Polymer

Poly/sobutylene

Polystyrene

Dimethyl silicones

Poly/wbutylene

Polystyrene

Dimethyl silicones

M.W.

>17,000

>38,000

>30,000 >40,000

4000-17,000

4000-38,000

200-30,000

A

3-40

3-40

3-7 3-64

1-75

1-65

1-39 1-43

C at temp. T

-13-52 at 217°C

- 1 3 - 4 a t 2 1 7 ° C

-15-56(S) at25°C -15-44(P)a t40°C

- 5 - 3 8 a t 2 1 7 ° C

-5-28(S) at25°C -5 -54(P)a t40°C

Reference

Fox and Flory (1948, 1951)

Fox and Flory (1948, 1950, 1954)

Charlesby (1955) Warwick, et al.

(1955)

Fox and Flory

Fox and Flory

Charlesby Warwick, et al.

(S) Viscosity measured in stokes. (P) Viscosity measured in poises.

Crosslinking in dimethylsiloxanes

Most polymers with two groups attached to a single atom in the main chain become degraded when exposed to high energy radiation. This is


notably the case for polywobutylene, which can be directly compared with the silicones

/ H Me \ / Me \ — C — C — — O — Si —

\ H Me Jn \ Me J» poly/söbutylene polydimethylsiloxane

The changes in solubility, flow properties and elasticity indicate however that the silicones are crosslinked by radiation. The dimethylsiloxanes crosslink very readily, whereas the phenyl siloxanes which are more radiation-resistant require higher doses, an effect directly comparable to the radiation protection found in polystyrene. The dimethyl siloxanes offer a convenient system in which to study radiation reactions, since the initial molecular weight can be readily obtained from viscosity data, the polymer is readily soluble in many organic solvents including benzene, xylene and methyl ethyl ketone, and the properties do not vary greatly with temperature. As the main chain atoms (Si and O) differ from those of the side chain (C, H) it becomes possible to distinguish simply between main chain and side chain fracture.

Small radiation doses increase the viscosity of the polymer, but it still remains soluble. The increase in molecular weight can be calculated from the number of crosslinks formed (equations 9.13, 9.14) but does not agree with the values derived from the changes in bulk or intrinsic viscosity, using the expressions given in equations (16.2, 16.3). This discrepancy arises from the formation of branched molecules, whose viscosity is different from that of linear molecules of the same average molecular weight. The relationship between viscosity and degree of branching has been studied quantitatively (Charlesby, 1955) using radiation as a means of producing controlled degrees of branching.

The increase in number average molecular weight up to the gel point has been studied quantitatively by Bueche (1956). Low molecular weight silicone polymers, containing only five or ten silicon atoms were subjected to electron radiation for doses up to 230 megareps, the corresponding increases in Mn being measured cryoscopically in benzene and cyc/<?hexane. As predicted by theory (equation 9.13), a linear relation was obtained between 1/Mn and r, and from the slope Bueche deduced a G value per crosslink of about 4-5. In a later study, using cryoscopic methods St. Pierre Dewhurst and Bueche (1958) obtained a G value of 2-5 (±0*4) for cross-linking in pentadimethyl siloxanes.

As the radiation dose is increased, the gel point is reached, when there is an average of one crosslinked unit per weight average molecule (δ = 1). At this point there are sudden changes in the solubility and the flow properties of the crosslinked polymer. From the known initial molecular weight Mw and the radiation dose for gel formation, the energy absorbed per crosslink can be derived. Table 16.2 shows the relationship between the gelling dose rgei (determined by the formation of a small amount of insoluble material) and the initial molecular weight, for dimethylsiloxanes

SILICONES 301

Table 16.2. Radiation Dose rgQ\for Incipient Gel Formation

Bulk viscosity, η (cS at 25°C)

100,000 30,000 12,500

1000 50 10

Weight average molecular wt.

(Mw)

106,000 80,000 62,000 26,400

3900 1200

Number of monomers per weight average molecule (w2)

1430 1080 840 360 53 16

Minimum radiation dose for gel forma-

tion (/-gel)

3-7 xlO-2

5-lxlO-2

7x l0 - 2

12X10-2

72xl0-2

3-2

>gel"2

53 55 59 43 38 51

/■gel in pile units.


subjected to mixed radiation in the BEPO nuclear reactor at Harwell. The unit of radiation dose is the pile unit, approximately equivalent to 45 megarads. The quantity /-geiw2 is approximately independent of initial molecular weight, so that from equations (9.17), (9.18) it follows that q0 and G for crosslinking are also independent of chain length.

As soon as a small amount of insoluble material is formed by cross-linking, the polymer no longer flows. This behaviour arises from the presence of a small amount of a three-dimensional network ; near the gel point only a small fraction of the total has been converted, and to account for the lack of observable flow it must be assumed that this small gel fraction is capable of absorbing the remainder of the polymer to form a non-flowing material.

Fig. 16.1 shows the minimum radiation dose (in pile units) required to prevent flow in dimethylsiloxanes of different initial bulk viscosities. Also plotted is the relationship between viscosity and initial molecular weight (full line). Except at very low molecular weights, the viscosity-molecular weight curve passes through the points relating viscosity to gel point. The radiation dose rj (in pile units) needed to prevent flow is related to the weight average degree of polymerization

ffu2 = 42

whereas for the formation of an insoluble fraction (Table 16.2)

rgeiw2 = 50.

It may be concluded that unit pile dose will crosslink between 2 and 2-4 per cent of the monomer units and that this figure is independent of the initial molecular weight.


Weight average Molecular weight

m 5

10

i r>4

10

1000

V

100

;o

1

103

I 104

I

o Incipient gel formation i-H Onset of infusibility

Viscosity / molecular weight curve J

— H 1

H /

/ /

/

/

/ '

7 7 7 /

105

i

20 10 4 2 1 0-4 0-2 0-1 0-04 Radiation dose, pile units

FIG. 16.1. Irradiated silicones. Effect of bulk viscosity and molecular weight on gelation dose. Experimental values O ; ι-ι show dependence of gelation dose on bulk viscosity. The curve shows relation between bulk viscosity and molecular weight.

Solubility

As crosslinking proceeds beyond the gel point, the soluble fraction decreases at a rate which depends on the initial molecular weight distri-bution (page 145). Fig. 16.2 shows the soluble fraction plotted as a function of radiation dose for a number of commercial silicones. The point at which an insoluble fraction first appears (0-12 pile units for a 1000 cS fluid) depends only on the weight average Mw, while the initial slope of the curve depends on the ratio of z average to weight average (Mg/Mw). On the same curve are shown the shape of the curves to be expected from an initially uniform, and an initially random distribution. The experimental results represent a distribution which is somewhat wider than random.

SILICONES 303

100%

0-04 0-1 0·4 Radiation dose,

1-0 2-0 pile units

FIG. 16.2. Effect of crosslinking on solubility of various silicone fluids. Left: Calculated solubility curves. Right: Observed values.


Swelling

Although the gel is no longer soluble, it does swell in organic solvents to an extent which depends on the degree of crosslinking and on the inter-action constant μ between polymer and solvent.

Fig. 16.3 shows a set of values of the swelling ratio V plotted on a log-log scale against the radiation dose r (expressed in pile units) for a number of silicones. Except at low doses when end effects cannot be ignored, all the points fall approximately on a straight line of slope —0-6. The observed relationship

V5'3 = 8-5/r

is similar in form to the theoretical relationship (9-54) indicating that q0

and G are independent of Mw and R over the range considered. Comparing the experimental and theoretical formulae leads to a relationship between q and μ

q0 = w(0-5— μ)/8·5ρν

where q0 is the density of crosslinking per unit pile dose.

Warwick has used the swelling ratio of a dimethylsiloxane polymer, subject to a single radiation dose of 10 megareps from 2 MeV electrons to derive a value for q0 (see page 306).


o 1° o

Φ

i

1 2

/ / X

1 1 1 1

X

\ \\ 1 1 1

1 11

+ ' x N i 1 I 1

+ 1

1 1 1 1 1 1 1

s \

X

+ o

•

OOOOO cS 1000 cS 30 0 0 0 cS 50 cS

V 5 /3 = 8 . 5 / r

7 +>v

!

}

0-04

20

10:

R Φ 5 £

E 2. 5 H2

0-1 Radiation

0-2 dose,

1-0 0-4 pile units

FIG. 16.3. Swelling of irradiated dimethylsiloxanes in xylene at 20°C, (From Charlesby, 1955.)

2-0

Elastic Modulus

Bueche (1956) studied the elastic modulus of a series of polydimethyl-siloxane fractions as a function of the radiation dose. The ratio of applied stress t o a - 1 / α 2 (which Bueche denotes by A) should be proportional to the radiation dose r (equation 9.43). Fig. 16.4 shows that this is the case except at low densities of crosslinking, where the free ends, due to the initial finite molecular weight, can no longer be ignored. The theoretical relation then leads to a G value of approximately 2-3. Apart from this work no systematic study of the elastic properties of unfilled silicones

**·"'

• o

Δ

I

a ^ ^

^l

^ ° r

•520000 o 890000 fraction □ 439000 fraction Δ 220000 fraction

10 20 30 rx10"e

4 0 50 60

FIG. 16.4. Dependence of Elastic Modulus on dose. Electron irradiation of unfilled dimethylsiloxane polymers.

(From Bueche, 1956.)

SILICONES 305

appears to have been made, and only a few miscellaneous values forq0 and G have been obtained (Charlesby, 1955; Warwick, 1955). In the absence of fillers, silicone rubber produced by the irradiation of high molecular weight fluid has a very low tensile strength, and tears very readily. Energy per Crosslink; G Value

For unfilled silicones, the energy absorbed per crosslink formed can be deduced either from the gelling dose (if the initial weight average Mw is known), from the elastic modulus (assuming no entanglements) or from the swelling ratio (if the interaction parameter μ is available).

The data on the gelation dose (Charlesby, 1955) using pile radiation can be summarized as follows (one pile unit being taken as equivalent to 45 megarads)

r/Mw = 140 x 103 (from flow properties) rgeiMw = 166 x 103 (from sol fraction)

where the doses for infusibility or insolubility /y, rgei are measured in megarads.

Since rge\Mw = wlqQ (equation 9.17) and w = 74, the corresponding values of q0 are 5-3x 10~4 and 4-5x 10~4. Elastic measurements (Charlesby, 1955) lead to an approximate value of rMc of l-4x 105 and a q0 value of 5·3χ 10~4 (±0-5) in good agreement with the above. This indicates that chain entanglements cannot play an important part in the observed elasticity at low deformation. The corresponding G values per crosslink are 3-4 (q0 = 5-3 x 10~4) and 2-9 (q0 = 4-5 x 10""4).

The results of Bueche (1956) give G = 4-5 (q0 = 7x 10~4) from cryo-scopic measurements, and G = 2-3 (q0 = 3-5 x 10~4) from elastic data. Later data by St. Pierre et al (1958) lead to G = 2-5 and q0 = 3-9 y 10"4.

Warwick (1955) deduced G values of 2-8 and 2-3 for pure silicone irradiated with 10 megareps of 2 MeV electrons, the measurements being based on swelling and elastic moduli respectively. These various values can be considered as being in fair agreement, bearing in mind the different materials and radiation conditions used, and the approximations made in applying the theory. It may be concluded that for dimethylsiloxanes, the degree of crosslinking is proportional to the radiation dose and is independent of molecular weight (except possibly for very low molecular weight polymers) and of the type and intensity of the radiation, over a range of intensities from 1 megarad in a few seconds (electron radiation) to several hundred megarads in a matter of days (pile radiation). Warwick has also found crosslinking to occur with deuterons from a cyclotron beam, with γ radiation from a radioactive cobalt 60 source, and x-rays from a conventional x-ray machine, although accurate dose measure-ments were not available.

A conventionally cured silicone requires a crosslinking density corre-sponding to an Mc value of about 15,000. Since Mc = wlq0r<^>14/5x 10_4r, the corresponding radiation dose would be about 10 megarads.


Chemical Changes

Only a limited amount of work has been published on the chemical changes resulting from exposure to high energy radiation. The stabili-zation of the polymer against radiation, when phenyl side groups are present, is directly analogous to the radiation stability of polystyrene, but stands in contrast to the rapid decomposition of α-methyl styrene polymer. Warwick studied the changes in the tetramer (octamethylcyc/ötetra-siloxane) and observed increases in molecular weight, viscosity, density and refractive index which he ascribed to the formation of ethylene bridges between two tetramer molecules

-Si — CH2 — CH2 — Sic although the presence of méthylène bridges

-Si —CH2 —Sic could not be ruled out.

An alternative approach involves analysis of the gases evolved during radiation. These may either be trapped inside the polymer, forming small bubbles, or they may diffuse to the surface. Mass spectrometric analysis of the gases liberated in this way were made on a linear silicone (Charlesby, 1955), on the cyclic tetramer (Warwick, 1955), on the pentamer (St. Pierre et al, 1958), and on a commercial silicone varnish (Ryan, 1954). The results indicate primarily the evolution of hydrogen and methane. The ethane found could arise from the combination of two methyl radicals, and the final constituents would undoubtedly vary with the radiation dose, high doses affecting the primary gaseous products. A significant feature is the ratio of hydrogen to carbon which differs only slightly in all these experiments. The observed gas evolution can only arise from radiation-induced fracture of both Si—C and C—H bonds, and the lack of fracture of Si—O bonds, in spite of the similar bond energies :

Si—C Si—O C—H 57-6 89-3 87-3 kcal/mole

According to the experimental data, C—H bonds must be fractured three times as frequently as Si—C bonds.

Crosslinks between chains can occur by the elimination of two methyl groups, giving Si—Si bonds, by the elimination of one methyl and one hydrogen atom to give méthylène bridges Si—CH2—Si; or by the elimi-nation of two hydrogen atoms only, leaving an ethylene bridge

CH3—Si—CH2—CH2—Si—CH3 I I

The corresponding ratio of H to C in the evolved gases would be 3,4 and infinity. The observed values of about 6 would indicate the presence of

SILICONES 307

some ethylene linkages, as well as méthylène and possibly direct Si—Si bridges. However in the infra-red work of Kantor (reported by Bueche, 1956) no evidence for ethylene linkages in octamethyltetrasiloxane was found, méthylène links being twice as frequent as direct Si—Si links.

Table 16.3. Gases Evolved {per cent of Total)

Material

H2 CH4 C2H6 H/C Radiation Dose in megarads

Charlesby

Long chain pure silicone

41 47 12 6-3

Pile 200

Warwick

Cyclic tetramer

34 60 4-6 6-2

Electrons 52-5

Ryan

Silicone varnish

51-3 38-3

? &6

Pile Several thousand

St. Pierre

Pentamer

31 47 22

4-2 Electron ?

Temperature Effect

As in the case of polyethylene, crosslinking of silicone fluids is found to depend on the temperature at which radiation is carried out. Measure-ments of the dose required to cause gel formation in a 1000 cS dimethyl-siloxane fluid using 2 MeV electrons at a dose rate of 1 megarad/sec give the following G values :

150°C G = 4-1 20°C G = 3 1 0°C G = 2-8

- 7 8 ° C G = 2-6

These G values refer to the number of crosslinked units per 100 eV absorbed. The G values per crosslink are only half these quantities and are therefore lower than those quoted above, but relative values, which are of importance in the present connexion, show a variation of the same order of magnitude as for polyethylene. The temperature dependence for crosslinking of silicones is, however, of more fundamental interest as it cannot be due to secondary effects such as changes in crystallinity; furthermore, the low temperature coefficient of viscosity of silicone fluids renders it less likely that the temperature effect is related to the mobility of molecular chains.

Fillers

Silicone polymer without fillers, crosslinked by radiation forms rubber-like materials with good extensibility, but with very poor tensile strength and tear characteristics. Greatly improved properties can be obtained by irradiating a mixture of silicone and fillers such as silica powder, or even carbon black whose presence would interfere with conventional chemical vulcanization.

x


Results for a silicone (100 parts) mixed with 35 parts of silica of 21 ιημ particle size are given by Warwick. His values of M deduced for γ-and electron irradiated samples from their swelling and elastic modulus are shown in Fig. 16.5. A relationship of the form

0 + \

< \ o

Γ . x J

Measurements Swelling Modulus'

x Co γ o + 2 MeV ·

_ electrons

• v

\ X \ \ ^ \ ;

T\

1 2 5 10 20 50

Dose, mega reps

FIG. 16.5. Radiation dose and Mc values for filled silicones. (100 parts silicone, 35 parts silica.)

- M c = 8x 104/r Errors at low doses may be partly due to end-effects.

holds when r is expressed in megarads. This leads to q0 = 9-2x 10~4 and G = 5-8. In view of the concordant results obtained both from swelling and from elastic modulus, it may be assumed that the higher value of G arises from polymer-filler links, formed by radiation.

According to the theory developed by Einstein and extended by Guth and Gold (page 155) the elastic modulus of a mixture of rigid spherical filler particles and polymer should be increased by a factor

1 + 2-5v/+ 14-lv/2

where v/ is the volume fraction of filler. With the filled silicone studied by Warwick this would increase the modulus by a factor about 50 per cent, and help to explain the apparent increase in the G value. The theory would not, however, account for the decreased swelling ratio.

The effect of radiation on silica or carbon filled silicones is shown in Table 16.4. In spite of the considerable difference in radiation intensity and exposure time there is no systematic difference as between electron and gamma radiation. The Shore hardness rises continuously with dose, the tensile strength reaches a maximum at about 10 megarads, and the elongation at break falls from about 2 megarads.

Experiments were also carried out with x-ray radiation using a copper

SlLICONES 309

target at 35,000 volts and 20 milliamps or a tungsten target at 50,000 volts and 20 milliamps. The low efficiency of utilization of energy input using x-ray radiation is shown by the time taken; 2-5 hr in the case of the copper target and 20-25 min for the tungsten target. This is about 5000 times longer than the time needed to produce the same effect with electrons using a lower total energy input and a wider beam scan.

In assessing the effect of radiation on mechanical properties of radiation, the presence of a filler is of considerable importance. The possibility of increasing the tensile strength by the use of such fillers as carbon which cannot conveniently be incorporated in the usual silicone rubbers is a matter of some interest. The theory of filler action is not yet properly understood, one theory being that radicals present on the surface of the particle act as temporary crosslinks with the rubber molecule. In this case radiation may increase the number of crosslinks in a filled material.

Table 16 A. Mechanical Properties of Irradiated Filled Silicones 100 parts silicone -f 35 parts silica filler

Radiation in megaroentgen

or megarep

125 (γ) 2 (electrons) 5 ( γ ) 6 (electrons) 10 (electrons) 20 (electrons) 25 (γ) 40 (electrons)

Shore hardness

18 15 27 26 29 43 53 52

Tensile psi

135 153

1180 742 876 679 916 561

% Elongation at break

550 750 750 605 580 250 158 117

Carbon-filled silicone rubber 6 (electrons) 10 (electrons) 20 (electrons)

27 35 47

709 787 581

805 435 200

(From Warwick, 1955.)

A further advantage of using radiation as a method of curing silicone fluids is that once radiation has ceased no further reaction takes place and the extent of cure is completely determined by the radiation dose. The material itself is much purer as no catalysts or vulcanizing agents need be present. The use of filler does, however, appear to be essential for useful mechanical strength.

Ageing

A comparative study of the ageing properties of dimethylsiloxanes cured both by chemical methods and by radiation has been carried out by Osthoff et al. (1954). When a specimen is held under tension at an elevated temperature stress relaxation takes place. This is


associated with stressed portions of the chain fracturing and then re-form-ing in an alternative and relaxed condition. The presence of small amounts of potassium hydroxide from the polymerization process, or benzoyl peroxide from the vulcanizing process, catalyses this relaxation in the presence of water vapour oxygen or carbon dioxide in the air. When crosslinking takes place by radiation in the absence of such catalysts stress relaxation is greatly diminished; none was observed at 130°C in the presence of carbon dioxide or water vapour.

At 250°C some chain scission does occur in radiation-cured silicones, possibly due to the presence of water vapour and carbon dioxide. The same effect will also occur in peroxide cured material, but will be com-pensated to some extent by further curing at the same temperature.

Technical Materials

Bopp and Sisman (1955) irradiated previously cured silastic rubbers and observed mechanical changes consistent with the production of further crosslinkages. Silastic 7-170 irradiated in air had its breaking elongation reduced to half by a pile radiation equivalent to 60 megarads, while for Silastic 250, the initial breaking elongation of 320 per cent was reduced to half by a radiation dose of about 100 megarads. When speci-mens were irradiated in a compression jig, the subsequent elastic recovery was likewise reduced and amounted to only 27 per cent for a 40 megarad radiation dose; no recovery was observed at 230 megarads as against 98 per cent for unirradiated material.

Ryan ( 1954) studied the effect of pile radiation on various components incorporating silicone varnish. Irradiations were carried out in the Oak Ridge pile for 2-3 x 1018 nvt at a temperature of 25°C and at Brookhaven for 6-3 x 1018 at a higher temperature of 160-180°C. Irradiation at the higher temperature produced heavy crazing in silicone-împregnated cable coils but no crazing in a silica glass varnish, which was, however, em-brittled. In this latter case, the tensile strength and elongation at break were reduced by a factor of between 25 and 50 per cent. Deformation and creep under load of silicone varnishes were greatly decreased.

Klein and Mannal (1955) have reported an increased dielectric strength in silica mica insulation whereas Ryan noted a decreased breakdown voltage in silicone-impregnated mica tape.

All these changes agree qualitatively with an increased formation of crosslinks due to radiation, but for a detailed assessment of results in any specific application a study under the required radiation conditions and shape of specimen is essential.

REFERENCES ANON., NP 5218, University of Pittsburgh, 1954. BARRY, A. J., J. AppL Phys. 17, 1020, 1946. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955. BOPP, C. D. and SISMAN, O., ORNL 1373, 1953. BUECHE, F., / . Chem. Phys. 20, 1959, 1952; 21, 1850, 1953; 22, 603, 1954. BUECHE, F., / . Polymer Sei. 19, 297, 1956.

SILICONES 311

CHARLESBY, A., A.E.R.E. Rep C/R 1231; M/R 1456; M/R 1556; Nature, Lond. 173, 679, 1954; Proc. Roy. Soc. A222, 542, 1954; A230, 120, 1955, J. Polymer Sei. 17, 379, 1955.

Fox, T. G. and FLORY, P. T., / . Amer. Chem. Soc. 70, 2384, 1948; J. Appl Phys. 21, 581, 1950; / . Phys. Chem. 55, 221, 1951 ; / . Polymer Sei. 14, 315, 1954.

GUTH, E. and GOLD, O., Phys. Rev. 53, 322, 1938. HUNTER, M. J., WARRICK, E. L., HYDE, J. R. and CURRIE, C. C , / . Amer. Chem.

Soc. 68, 2284, 1946. HYDE, J. F., U.S. Patent 2490357. KLEIN, P. H. and MANNAL, C , A.I.E.E., Oct. 3, 1955. LAWTON, E. J., BUECHE, A. M. and BALWIT, J. S., Nature, Lond. 172, 76, 1953. MANNAL, C , Nucleonics 12, 49, 1954. OSTHOFF, R. C , BUECHE, A. M. and GRUBB, T. C , / . Amer. Chem. Soc. 76,

4659, 1954. RYAN, J. W., Mod. Plastics 31(8), 148, 1954. ST. PIERRE, L. E., DEWHURST, H. A., BUECHE, A. M., GE Report 58-RL-1913,

1958. WARWICK, E. L., Ind. Eng. Chem. 47, 2388, 1955. WARWICK, E. L., PICCOLI, W. A. and STARK, F. O., / . Amer. Chem. Soc. 77,

5017, 1955.

CHAPTER 17

OTHER CROSSLINKING POLYMERS POLYVINYL CHLORIDE

THE MONOMER is usually prepared from acetylene and hydrochloric acid or from dichlorethane ; at room temperature it is a gas which can readily be liquefied. Polymerization is usually carried out in solution or emulsion using a peroxide catalyst. Polymerization may also be initiated by exposure to ultraviolet light or high energy radiation (Mund et ai, 1949, 1951, 1953; Chapiro, 1950, 1957).

Polyvinyl chloride has a more regular structure than most vinyl polymers and this is shown, for example, by the narrow haloes obtained from x-ray diffraction. While these haloes are not as sharp as the ring pattern obtained in the x-ray diffraction study of partially crystalline polymers such as polyethylene, they do reveal the presence of very small regions of pseudo-crystalline structure. These ordered regions may account for the good dimensional stability, the brittleness and the good solvent resistance of the polymer at room temperature. At higher temperatures when these pseudo-crystalline regions have largely melted, polyvinyl chloride is soluble in many solvents, although solutions usually gel or precipitate out on cooling.

In its pure form, polyvinyl chloride is brittle and tends to decompose near the softening point. Commercial products therefore include stabili-zers, plasticizers, fillers and often pigments and these may profoundly alter the properties of the polymer. In studying the effect of radiation on the commercial materials, it is therefore necessary to distinguish between changes produced in the relatively pure (rigid) polymer, in the more flexible, highly plasticized product and in copolymers such as those formed with vinylidene chloride.

From the radiation point of view, the presence of chlorine would be expected to increase the radiation sensitivity as compared with, say, polyethylene. High G values are often obtained in the irradiation of chlorine-containing organic compounds (Table 27.1). Moreover, the presence of only one chloride atom on alternate carbon atoms in the main chain would be expected to place the polymer in the class of crosslinking materials. On the other hand, polyvinylidene chloride, which is often associated with polyvinyl chloride in commercial materials may be expected to degrade.

Crosslinking v. Degradation

Lawton et al (1953) listed polyvinyl chloride as one of the degrading polymers, whereas Charlesby (1953) found crosslinking to occur. Karpov (1955) subsequently confirmed the occurrence of crosslinking. This

312

OTHER CROSSLINKING POLYMERS 313

difference is undoubtedly due to the effect of oxygen during irradiation. In the experiments of Lawton et ai, radiations were carried out with thin specimens, subjected to 800 kV electron bombardment, in which oxidation could play a considerable role. In the experiments of Charlesby, carried out in the BEPO reactor at Harwell, 0-5 in. dia. rods were used containing the minimum amount of additives in the form of lead salts and plasticizer needed to make the mixture workable. In these thicker rods, oxidation would be largely confined to the surface. Crosslinking was demonstrated both by the decreased solubility after irradiation and by the elastic properties of the irradiated rods at higher temperatures. Whereas unirradiated rods heated under stress bend and flow, irradiated rods under the same conditions suffer a finite amount of deformation, which is largely recoverable when the applied stress is removed. Being sub-stantially independent of time, this recoverable deformation must arise from the formation of a three-dimensional network with elastic properties.

Changes in Mechanical Properties

Early work by Burr and Garrison (1948) on the irradiation of polyvinyl chloride was concerned primarily with the change in mechanical properties of commercially available material, when subjected to nuclear pile radiation.

Sisman and Bopp (1953) describe in detail the changes produced by pile radiation in the stress-strain curve, the shear strength, the impact strength and hardness at room temperature of polyvinyl chloride and of copolymers such as vinyl-vinylidene chloride (Saran) and vinyl chloride-acetate. In the case of a commercial vinyl chloride polymer (Geon 2046)

10000

_. 8000

6000

ί! 4000 if)

2000

0 5 10 15 20 % strain

FIG. 17.1. Stress/strain curve for vinyl chloride-acetate copolymer. (Figures show pile doses converted to equivalent megarads.)

(From Bopp and Sisman, 1955.)

there was a reduction in the elongation at break after a dose of about 1017 mt, and a decrease in tensile strength after a dose of 0·7χ 1018 nvt. The vinyl-vinylidene chloride copolymer showed considerable reduction in tensile strength after much lower doses as well as a decreased elastic modulus. In the case of the vinyl chloride-acetate copolymer, there was

/ $

A\ / ^

V^ V

L ^

χ 800 4200

X

60

„12 , 200%


a considerable increase in the elongation at break, the curve indicating cold flow properties not present in the original polymer. These observa-tions can be explained in terms of crosslinking of the vinyl chloride and acetate groups and degradation of the vinylidene group.

Byrne et al. (1953) measured the change in mechanical properties of \ in. thick samples of non-plasticized polyvinyl chloride, subjected to cobalt γ-radiation at an intensity of 25000 r/min. At room tempera-ture some improvement in shear and impact strength was observed at doses of above 200 megaroentgens, the impact strength rising by about 100 per cent and the shear strength by about 30-40 per cent for a radiation dose of 300 megaroentgens. The tensile strength decreased by some 10-20 per cent at 100 megaroentgens and thereafter remained constant. These differences were ascribed by the authors as possibly arising from plastici-zation due to the smaller molecules formed by degradation; crosslinking was not considered.

The most detailed study of irradiated polyvinyl chloride is that given by Chapiro (1956). The mechanical properties were studied on thin fibres, both unorientated and orientated (partially crystallized) by stretch-ing, and subjected to γ-radiation from a cobalt source. In specimens

11-2 mega roentgen

o 2 +10 t_

c

c 0)

X

ω - 1 0

- 2 0 O 100 150 200 250

eC FIG. 17.2. Extension of irradiated filaments of polyvinyl chloride in air or in vacuum

showing effect of crosslinking in vacuo and degradation in air. (From Chapiro, 1956.)

irradiated in air, and subsequently heated under constant tension, there was a retraction at about 120°-150°C, possibly due to the destruction of orientation and crystallinity, followed by an elongation and break of the specimen at about 160°-170°C. The higher radiation doses corresponded to a lower breaking temperature. With specimens irradiated in vacuo on the other hand, the more highly irradiated specimens (more than

. 8-3 ~\ megaroentgen

vacuum

41 megaroentgen

(vacuum)


20 megarads) did not break, even at temperatures above 250°C. More-over, the elongation under constant tension was approximately independent of temperature above 150°-200°C. This indicates clearly that the polymer irradiated under vacuum becomes crosslinked. The doses required to achieve a crosslinked network are, however, much higher than for most crosslinking polymers; this may arise from a low average molecular weight, a considerable degree of radiation stability, a different and relatively inefficient method of crosslinking, or from competing reactions not present to the same extent in other polymers.

Chemical Changes

A detailed study of the chemical effects of high energy radiation on the pure polymer is complicated by the subsequent reactions of the chlorine or hydrogen atoms produced. These react with other polymer molecules in a chain reaction, liberating hydrochloric acid and causing changes in colour long after radiation has ceased. The nature of these reactions is not specific to radiation-induced changes; similar reactions occur when the pure polymer is heated to near its melting point, and are combated by the incorporation of a stabilizer such as lead stéarate. In basic research on the effect of radiation, the incorporation of such stabilizers is a consider-able disadvantage since they complicate the reaction in an ill-defined manner.

When a transparent or lightly coloured polyvinyl chloride sheet without stabilizer is subjected to a few megarads of radiation (electrons or γ-rays) it becomes somewhat coloured, usually with a yellow tint, similar to that obtained after lengthy exposure to light. After standing, the colour of the irradiated specimen darkens progressively over a period of days. This change may be explained by the formation of polyenes formed by the successive removal of chlorine and hydrogen atoms.

The changes in colour on or after irradiation arise from two causes: (i) the change due to the polymer itself, which is revealed by an increase in absorption in the ultraviolet and violet end of the spectrum, and (ii) the appearance of new bands in the visible region between 600 and 800 ιημ which have only been observed for plasticized polymer.

The former type of change is observed in many polymers including poly-ethylene and polystyrene and may be readily ascribed to conjugated un-saturation, due to the successive removal of H and Cl. The presence of trapped free radicals is unlikely to account for these colour changes since the colour persists if the polymer is swollen in a solvent. In the case of polyvinyl chloride irradiated in a vacuum, a further absorption band at about 490 ηιμ also appears after some time.

Byrne et al. (1953) studied the evolution of halogen from polyvinyl chloride and other halogenated polymers, as well as the change in their mechanical properties. Although radiation may break C—Cl bonds, the chlorine liberated in this manner must still diffuse through the specimen


before being evolved at the surface, and during this evolution secondary reactions may occur to modify the nature of the gases reaching the surface. Diffusion through a thick specimen will also be slower. The reaction may therefore appear to be dependent on dose rate or on specimen thickness.

To take this diffusion into account, Byrne et al irradiated the polymer both in the form of | in. cubes and as shavings about 5-10 mils in thick-ness. The specimens were placed in a sodium hydroxide solution and irradiated with γ-radiations from a cobalt 60 source, at a nominal intensity of 25000 r/min. The caustic solution was subsequently withdrawn and analysed for chloride concentration.

The results show an approximately linear relation for the polymer irradiated in the form of shavings, but a non-linear relation, corresponding to a lower yield of chlorine, was found when the polymer was irradiated in the form of cubes. The G value of 7 for chlorine evolution from the shavings is a lower estimate. For carbon tetrachloride the G value lies close to 10.

The degree of crosslinking produced by radiation of polyvinyl chloride is relatively low; Chapiro (1957) quotes a G value for crosslinking of 0-6, which is far smaller than the G value for chlorine evolution. To account for this suprisingly low value, Chapiro considers that the forma-tion of crosslinks is due not to a direct reaction between two radicals as in other polymers but rather to a reaction between a radical and an un-saturated bond formed by radiation. These radiation-induced radicals can be retained in specimens irradiated in vacuum but may subsequently be destroyed by heating.

Colour Changes

On subjecting polyvinyl chloride compositions containing pigments or dyes to radiation, considerable changes in colour can be produced by doses of a few megarads or less. Henley (1954) has suggested the use of this property as the means of dosimetry. The change in pH obtained by irradiating polyvinyl chloride film produces a change in light absorption of a dyestuff incorporated in the film. Pinner and Swallow (1957) have subjected a number of dyed or pigmented polyvinyl chloride compositions to electron radiation and observed the wide range of colour changes. By using a suitable absorbing mask it proved possible to print a pattern throughout the specimen several millimetres in thickness. Suggested applications for this include printing of patterns throughout a plastic sheet subjected to heavy wear, and printing on wrappings with no physical contact.

Wippler (1958) found that the crosslinking of the polymer is greatly simplified by irradiating it in a suitable solvent (see Chapter 25), in which case little or no colour formation occurs on irradiation. However, a deep brown-violet colour is observed on drying, which disappears again on reswilling. This observation is difficult to reconcile with any explana-tion involving trapped radicals or conjugated double bonds.


POLYACRYLATES Polyacrylates constitute a group of rubberlike polymers, of structure

— CH2 — CH —

whose behaviour under radiation contrasts with that of the methacrylate polymers _ C H 2 - C ( C H 3 ) -

I COOR

which degrade readily under radiation. Shultz and Bovey (1956) studied the processes of crosslinking and

degradation in a series of acrylate polymers. Initial weight average mole-cular weights Mw were obtained by light scattering, and the gel fractions determined by extraction of the sol. Irradiations by means of a 1 MeV

1 2 4 6 8 10 20 40 60 80 â= r/rqe[

FIG. 17.3. Soluble fraction vs. crosslinking coefficient δ for electron-irradiated acrylate polymers Δ p o l y m e t h y l a c r y l a t e

Π poly/2£o-pentylacrylate. O · poly/ΕΤΛ-butyl acrylates.

Solid lines show calculated curves for various ratios p0/q0 of fracture to crosslinking.

electron beam from a resonant transformer were carried out in air, and the presence of oxygen may account for the simultaneous crosslinking and degradation observed.

Fig. 17.3 shows the sol fraction plotted against the crosslinking coeffi-cient δ. These observed data follow the curves of solubility calculated on the assumption of simultaneous crosslinking and degradation (full lines), p0/q0 being the ratio of main chain fracture to crosslinked units. The derived values for energy absorbed per crosslink (Ec) and per main chain fracture (Ed) are given in Table 17.1. With the exception of the tert.-bu\y\


acrylate polymer, all the values of Ec agree closely, leading to a G value for crosslinking of about 0-5. The product rgQ\Mw (equation 9.19) is also substantially constant at 1-lxlO6. The simultaneous occurrence of degradation is shown convincingly in Fig. 17.3, and may be explained either as due to the presence of oxygen, or as being a basic reaction of the acrylate system; a repeat of some of the measurements under vacuum or nitrogen can readily decide the matter.

The much higher energy required to crosslink tert.-butyl acrylate polymer may be ascribed to the absence of a hydrogen atom in the a position to the alcoholic oxygen; irradiation of simple alcohols shows that the a hydrogen is especially sensitive to radiation. It is not due to any shielding effect by the three methyl groups, since «ôpentyl acrylate, in which shielding by methyl groups may also occur, crosslinks as readily as other acrylates with little or no such shielding. On this view, most of the crosslinks have the following pattern.

• · · — CHg — CH — · · · I COOCRx

I COOCR2

I CH2 — C H

Table 17.1. Energy for Crosslinking and Degradation in Poly acrylates

Acrylate polymer

Methyl

n-Butyl isoButyl sec-Butyl ter/.-Butyl

neoVentyl

MwX\0~5

6-9 4-7

10-6 80 9-5

11-4 5-7 5-7

101 7-4

''gel megareps

1-45 2-5 0-87 115 1-30 30 6-4 5-7 3-8 1-35

Ec eV

87 103 80 80

107 300 317 282 335 87

Ed

eV

580 573 530 615 535 430 530 470

(840) 348

Pol (Jo

015 018 015 013 0-20 0-7 0-6 0-6

(04) 0-25

rge\Mw

megareps

1-OxlO6

1-2x10« 0-9 xlO6

1-2x10« 1-2x10« 3-4x10« 3-6x10« 3-3X106

3-8x10« 10x10«

fgel = calculated gelation dose assuming no degradation. ECy Ed = energy per crosslinked unit, or per main chain fracture.

Source: Shultz and Bovey, 1956.

Schultz and Bovey also studied 1-1 di-hydroperfluorobutyl acrylate polymer, over a wide range of molecular weights (Mw from 3·2χ 10« to 2-8x 104). Little oxygen degradation was observed but at higher doses a reversal takes place in the sol/dose curve probably due to degradation resulting from fluorine-containing fragments. This phenomenon would be similar in some respects to the degradation of PTFE under radiation (Chapter 20). The low value of rge\Mw (about 0-6 x 106, as compared with 1-1 x 106 for the other acrylates) is due to the higher radiation sensitivity of the halogen compound.


POLYACRYLONITRILE Changes produced in polyacrylonitrile following electron radiation have

been investigated by Burlant and Taylor (1958). In nitrogen, crosslinking ensues, and H2 and HCN are evolved, with low G values (0-2 and 0-04). Poly-a-methacrylonitrile on the other hand degrades, and gases evolved comprise H2 (G~0-9), HCN (G~ 1-1) and CH4 (G—0-3). This difference is illustrative of the general rule that polymers of structure —CH2—CHR— crosslink, and those of structure —CH2—CRjRa— degrade. The only exception observed so far is polyvinyl alcohol, with R equal to OH; this degrades in the solid form (Danno, 1958), but crosslinks in aqueous solution.

NYLON Nylon is a generic term for polyamides having a repeating unit

— NH — (CH2)y — NHCO — (CH2)* — CO — produced as a condensation product from a diamine and a dibasic acid. The polymers are largely crystalline and can be readily oriented to give strong fibres of high melting point. A typical example, nylon 66, is soluble in hot formic acid or phenol, and melts at about 260°C. When the values of x and y in the above formula become large, the properties of the corresponding nylon tend towards those of polyethylene.

Relatively little work has been published on the effects of radiation on nylon. Sisman and Bopp (1951 and later) have published details of the change in mechanical properties at room temperature, due to exposure in the Oak Ridge reactor. Fig. 17.4 shows the reduction in elongation at

10 0 0 0 | - / « n v t = 1 2 . 8 x 1 0 1 ö

8 000

^ 6000

ω 4 000

2000

.18 •14x10' 1-19 x10u

Non-irradiated

• Breaking point

0-2 J _

0-6 0-8 0-4 Strain

FIG. 17.4. Stress-strain curves for pile irradiated nylon FM-3003. 1018 nvt = 103

megarads. (From Sisman and Bopp, 1951)

break, and the increase in modulus due to crosslinking. For these nylons doses of the order of 100 megarads or more are needed to produce very significant changes.


Charlesby (1953) and Lawton et al. (1953) have reported that nylon crosslinks under radiation. This tendency is shown by such changes as decreased solubility, increased heat resistance, etc. The doses required for insolubility are higher than those in polyethylene, due in part to the lower molecular weight of nylon.

The tendency to crosslink has been challenged by Little (1952, 1954) who carried out some detailed work on highly irradiated nylon 66. The dose used was 50 BEPO pile units or approximately 2500 megarads (the dose for insolubility is some 50 times less). After this dose the specimen could be swollen in formic acid without dissolving, but the addition of dilute hydrochloric acid did result in solution, from which she concluded that the structure formed was not crosslinked. However, hydrochloric acid may be expected to cause hydrolysis, breaking the main chain, so that this evidence is questionable.

The effects of cobalt 60 γ-rays and of 2 MeV electrons on polycapro-lactum (nylon 6) have been examined recently (Majury and Pinner, 1958). The polymer of initial molecular weight Mn =20,000 became partially in-soluble in m-cresol at a threshold dose of about 35 megarads, from which it was calculated that the net G value for crosslinking was 0-35. At the same time, irradiation caused an increase in the number of amino end-groups (G = 0-6) but a decrease in the number of carboxyl end-groups (G = 0-3). The release of amino end-groups revealed that chain cleavage was occurring at the same time as crosslinking. The reduction in the carboxyl group count showed that independent decarboxylation was occurring and/or that the amide groups were cleaved by a radical mech-anism, leaving a ketonic radical which became stabilized without forming an acid group.

The effect of pile radiation on the dynamic mechanical properties of 66 nylon have been studied by Deeley et al. (1957). In the unirradiated state, nylon shows four absorption peaks in its elastic spectrum, whose positions depend on the temperature and frequency of resonance. These peaks are related to the onset of various degree of mobility of the mole-cules. Of these the highest (a) peak is due to the melting of crystallites. After the specimen was irradiated in the Brookhaven nuclear reactor for a dose of 0-3 x 1018 nvt this peak was found to move to lower temperatures, while at higher temperatures the elastic modulus increased. These results are consistent with the conclusions reached from the behaviour of irradiated polyethylene, i.e. radiation destroys crystallinity but causes crosslinking, which binds the molecules together even at temperatures above the crystalline melting point. The destruction of crystallinity was also shown by infra-red measurements and by a decrease in density. As the radiation dose is further increased, crystallinity is completely destroyed, the necessary radiation dose being about 4 x 1018 nvt. Greater mobility is also allowed in the amorphous regions.

From the elastic measurements Deeley et al. concluded that crosslinking occurs less readily in nylon 66 than in polyethylene (about 4 per cent as against 10 per cent for the same dose), and that furthermore it is not


proportional to the radiation dose. It should however be mentioned that even for polyethylene, there is no linear relation between crosslinking and dose at high degrees of crosslinking, when the theory of rubber-like elasticity no longer holds.

POLYETHYLENE TEREPHTHALATE

Polyethylene terephthalate (Terylene or Dacron) is a polyester formed as a condensation product of ethylene glycol and terephthalic acid.

(— O — CH2 — CH2 — O — CO — C6H4 — CO —)„.

When drawn it can exhibit considerable strength along the orientation axis of the molecule. From the presence of lengths of paraffinic chain and of phenyl groups in the molecule structure, one would expect to observe crosslinking but a considerable degree of radiation protection. Some early results indicated the development of a small degree of crosslinking (Charlesby, 1953), but further and more detailed work (Todd, 1954) has shown that the reactions may be more complex.

Bopp and Sisman (1953) studied the change in mechanical properties at room temperature of the polymer film (Mylar) after irradiation at the

150

1 100 j5

"ç 'S 5 0

0 0-01 0-1 1-0 10 100

Radiation dose, 1018 nvt FIG. 17.5. Physical properties of polyethylene terephthalate. (Terylene or Mylar film.

Oak Ridge pile. Both tensile strength and elongation at break show a decrease to about half the original value for a dose of about 1018«v/ ( ^ 500 megarads) consistent with the occurrence of main chain fracture.

Todd (1954) studied the solubility of two samples of the polymer in ö-chlorophenol, as well as the limiting (intrinsic) viscosity of the solution. With pile radiation doses as high as 100-400 megarads there was com-paratively little change in the intrinsic viscosity, indicating the relatively high stability of the polymer to radiation, the highest dose corresponding to a decrease of less than 50 per cent. (For polymethyl methacrylate, for example, the intrinsic viscosity decreases 100-fold for only one-tenth of this dose, and 400 megarads would give a viscosity average molecular

| Curve Property *lnitial value 1 Tensile strength 25 000 p.s.i. 2 Elongation 50 %


weight of only 3000.) Measurements of the Huggins constant (deduced from the initial slope of the viscosity-concentration curve of a polymer solution) indicated a small rise with radiation; this is often accepted as a measure of branching, although Todd suggests it may be due to changes resulting in decreased solubility. The decrease in the Vicat softening point observed by Todd may also be explained in terms of increased branching. In either case the results indicate considerable radiation-resistance of the polymer to radiation.

Teszler and Rutherford (1956) investigated the effect of pile and gamma radiation on fibres of polyethylene terephthalate, drawn to varying extents. By contrast with previous workers, appreciable effects are reported for very small doses, significant changes being claimed for slow neutron doses as Iowas 1014AZV/ (equivalent to 0-1 megarad or less). Changes in the irradiated fibres were followed by melting point measurements, solubility in ra-cresol and mechanical strength or modulus. The authors claim that the fast neutrons in the reactor cause considerable degradation (but no data are published on this point), and therefore chose reactor conditions in which the ratio of slow to fast neutrons was high. The slow neutrons react with hydrogen atoms, giving rise to a γ-ray of high penetration which produce the observed changes. In view of the low rate of energy loss of such γ-rays, the changes claimed for low neutron doses are even more remarkable, the maximum doses used being 1017 nvt for reactor work and 10 megarads from cobalt irradiation.

No appreciable change in solubility was observed but there was a possible slight decrease in softening point, after a dose of 1017 nvt (about 50 megarads). Increases in elastic modulus were taken as an indication of crosslinking. It is difficult to see how the introduction of the very few crosslinks which the lower doses may produce can cause a significant improvement in mechanical properties. Changes in the degree of crystal-linity or of orientation resulting from radiation conditions account more readily for the observed effects. The authors do conclude that initially some crosslinking occurs, but that at high draw ratios degradation (chain scission) predominates, reducing the tenacity of the fibres by 20 per cent and forming carboxyl groups.

Little (1954) subjected polyethylene terephthalate to pile radiation at Harwell, the dose used (15 x 1017 neutrons/cm2) being equivalent to about 750 megarads. This very high dose would be sufficient to destroy all crystallinity in polyethylene, and to promote sufficient crosslinking to bring it into the glasslike state. In the case of polyethylene terephthalate the polymer irradiated in the fibres had indeed lost all their strength and begun to powder, while chips of the polymer were brittle. Little or no change in the crystalline pattern was observed however. When irradiated in the non-crystalline state and subsequently annealed at 140°C, crystalline patterns were produced, indicating that the effects of radiation are probably smaller than in polyethylene.

An oxygen effect was also observed. In the absence of oxygen during irradiation a number of —COOH end-groups are formed whereas in the presence of air the groups —OH and C = 0 group are obtained.


POLYPROPYLENE Long chain polymers of propylene have only recently become available.

Its structure is intermediate between polyethylene and poly/sobutylene, with one methyl group on every alternate carbon. Under radiation its behaviour also lies between that of polyethylene, which crosslinks readily with little main chain fracture, and poly/stfbutylene, in which main chain fracture is the main reaction. Black and Lyons (1957) found that cross-linking only slightly exceeds degradation, and as a result both the dose to gel, and the limiting sol fraction are high. The data obtained show a linear relation between s + V s and 1/r, similar to that in Fig. 9.6 (Charlesby and Pinner, 1958). The gases evolved consist almost entirely of hydrogen (99 per cent), and the G(H2) value is slightly greater than for high density polyethylene (Hornbeck and Parkinson, 1957). Waddington (1958) has confirmed the high gelation dose, and deduced a net G (crosslinking) value of between 0-6 and 1-3.

REFERENCES

ANON., Chem. Engng. News 33(16), 1655, 1955. BLACK, R. M. and LYONS, B. J., Nature, Lond. 180, 1346, 1957. BOPP, C. D. and SISMAN, O., Nucleonics 13(10), 51, 1955. BURLANT, W. J. and TAYLOR, C. R., J. Phys. Chem. 62, 247, 1958. BURR, J. G. and GARRISON, W. M., AECD 2078, 1948; AECD 3634, 1948. BYRNE, J., COSTIKYAN, T. W., HANFORD, C. B., JOHNSON, D. L. and MANN,

W. L., Industr. Engng. Chem. (Anal.) 45(11), 2549, 1953. BYRNE, J. and MANN, W. L., K981, November 1952. CHAPIRO, A., / . Chim. Phys. 47, 747, 764, 1950; 53, 895, 1956. CHAPIRO, A., Industr. Plast. Mod. 41, January 1957. CHARLESBY, A. and PINNER, S. Η., Proc. Roy. Soc. 1958 (in the press). CHARLESBY, A., Nature, Lond. 171, 107, 1953; Plastics 18, 70, 142, 1953;

Trans Plast. Inst., Lond. 23, 133, 1955; Nature, Lond, 111, 167, 1953. COMBRISSON, J. and UEBERSFELD, J., C.R. Acad. Sei. (Paris) 238, 1397, 1954. DANNO, A., J. Phys. Soc. Japan 11, 609, 614, 1958. DEELEY, C. W., WOODWARD, A. E. and SAUER, J. A., / . Appl. Phys. 28(10),

1124, 1957. HENLEY, E. J., Nucleonics 12, 62, 1954. HORNBECK, R. F. and PARKINSON, W. W., ORNL 2413, August 1957. HUGGINS, M. L., / . Amer. Chem. Soc. 64, 2716, 1942. KARPOV, V. L., Conference on the Peaceful Uses of Atomic Energy, Chemistry

Section, p. 3, Moscow, 1955. LAWTON, E. J., BUECHE, A. M. and BALWTT, J. S., Nature, Lond. 172, 76, 1953. LITTLE, K., Nature. Lond. 170, 1075, 1952; 173, 680, 1954. MAJURY, T. G. and PINNER, S. Η., / . Appl. Chem. 8, 168, 1958. MUND, W. et al., Bull. Class. Sei., Acad. Roy. Beige 35, 656, 1949; 37, 333,

696, 1951. MUND, W. et al Bull. Soc. Chim. Beige 62, 109, 645, 1953. PARKINSON, W. W., SEARS, W. C. and SISMAN, O., ORNL 2413 August 1957. PINNER, Η. and SWALLOW, A. J., Plastics, June-July 1957. RYAN, J. W., GEL 54 (1952). RYAN, J. W., Nucleonics 11(8), 13, 1953.

Y


SHULTZ, A. R., and BOVEY, F . A., / . Polymer Sei. 22, 485, 1956. SISMAN, O. and BOPP, D. C , ORNL 928, 1951; 1373, 1953. SISMAN, O., Plastics Technol. 1, 345, 1955. TESZLER, O. and RUTHERFORD, H. A., Textile Res. J. 796, October, 1956. TODD, A., Nature, Lond. 174, 613, 1954. WADDINGTON, F. B., / . Polymer Sei. 31, 221, 1958. WALL, L. A., and MAGAT, M., / . Chim. Phys. 50, 308, 1953. WIPPLER, C , J. Appl. Rad. Isotopes 2, 187, 1957; J. Polymer Sei. 29, 585, 1958;

Radioisotopes in Scientific Research, Vol. 1, p. 175, Pergamon Press, 1958.

CHAPTER 18

POLY/SOBUTYLENE Poly/sobutylene is the simplest long chain polymer which degrades under radiation. It is prepared by the polymerization of butène, a by-product of the petroleum industry, and is sold under the trade names of Vistanex, Oppanol or Isolene. Although low molecular weight material can be prepared by radical reactions, high molecular weight polymer is obtained by low temperature ionic polymerization using Friedel-Crafts catalysts, such as boron trifluoride or aluminium chloride. Recent work (see Chapter 22) has shown that it can also be polymerized ionically by radiation.

Poly/sobutylene is soluble in hydrocarbons but not in acetone or alcohol. At room temperature, it is not attacked by acids, alcohols or ozone, but tends to break down on exposure to strong sunlight. It has excellent water resistance and is outstanding in impermeability to gases. Fillers such as carbon black may be incorporated and poly/sobutylene may be compounded with rubber or polyethylene. Poly/sobutylene retains its rubberlike properties over a wide temperature range from about - 7 0 ° C t o H-150°C.

Being fully saturated, poly/sobutylene cannot be crosslinked chemically and therefore suffers badly from cold flow. To reduce this flow, a copoly-mer of isobutem with a few per cent of isopvene or butadiene is used, the unsaturation of the woprene or butadiene allowing vulcanization in a manner similar to that for rubber. This latter copolymer is known as GR-I or butyl rubber.

Poly/sobutylene has been found to have a head to tail structure, two methyl groups being located on alternating carbons of the main chain. x-ray studies of the polymer show that it has an amorphous structure at room temperature but that it crystallizes on stretching. These crystals do not consist of molecules in the usual zig-zag planar formation with a repeated dimension of 2-54Â as is usual for polymer molecules with a saturated C—C main chain. The repeat distance is found to be 18-6Â and a helical arrangement of the molecule is therefore likely, due to steric hindrance of the methyl groups. Poly/^butylene may be readily depoly-merized thermally and a large amount of monomer distilled off. Steric hindrance between methyl groups may account in part for the low thermal stability of the polymer.

The molecular weight of the polymer is related to both the bulk viscosity η and the intrinsic viscosity [η] at a given temperature. Fox and Flory (1948, 1951) found a relationship of the type η = AMn where A depends on temperature and n on the average molecular weight range (Chapter 16). A more convenient method of measuring the molecular weight is in terms

325


of the intrinsic viscosity [η] which is related to the viscosity average mole-cular weight by equation (8.13)

[η] = kMa.

The values of k and a depend on the temperature and on the solvent; for solutions in di-wobutylene at 25°C, k = 3-6 x 10~4, a = 0-64 over the range 6,000-1-3 x 106 (Flory, 1943). In carbon tetrachloride at 30°C, k = 2-9 x lO"4, a = 0-68 (Fox and Flory, 1949).

EFFECTS OF RADIATION At an early date, Davidson and Geib (1948) irradiated natural and

synthetic rubbers in the nuclear reactor, and found that in the case of butyl rubber a slight degradation occurred. Subsequently, Bopp and Sisman (1953, 1955) studied the change in mechanical properties of various irradiated butyl rubbers. Their results are best understood by considering

1500i 1 1 1 1 ,

1 2 3 4 5 Strain

FIG. 18.1. Stress-strain curves for irradiated butyl rubber. (Figures give dose in units of 1018 n\t, and correspond to 0, 12, 24, 48 megarads.)

(From Bopp and Sisman, Nucleonics, 1955.)

the effect of high energy radiation on wobutylene in the absence of the small percentage of crosslinking monomeric units.

Charlesby (1953) using pile radiation and Lawton, Bueche and Balwit (1953) using 0-8 MeV electrons found that poly/jobutnee was one of the polymers which degrade under radiation. A detailed study of the processes involved has been given by Alexander, Black and Charlesby (1955).

In these experiments, specimens of polywobutylene from different sources were subjected to various forms of high energy radiation—γ-rays from two cobalt 60 sources of 100 and 400 curies, electrons from a 4 MeV linear accelerator, and mixed radiation from the BEPO nuclear reactor at Harwell. Changes in molecular weight were observed by measurements of intrinsic viscosity [η] which give directly the viscosity average molecular weight Mv.

Main chain fracture is directly related to the reduction in the number average molecular weight Mn but only indirectly to Mv. Measurements of the decrease in Mn can be made by osmometry but are usually less accurate and far more time consuming than measurements of Mv. The decrease in Mn gives information on the number of radiation-induced

POLY/SOBUTYLENE 327

fractures but not on the location of these fractures along the chain. On the other hand measurements of the viscosity average Mv or the weight average molecular weight Mw do depend on the location of such radiation-induced fractures, fractures lying near the end of a long chain molecule having little effect on the viscosity. It was therefore preferable to study the variation in Mv with radiation and compare the results obtained with those calculated on the assumption of random location of main chain fracture, in which case it is possible to obtain a direct relationship between Mv and Mn. This procedure gives information on both the number and location of the radiation-induced fracture.

Fracture Density deduced from Intrinsic Viscosity The decrease in number average degree of polymerization («/) as a

result of main chain fracture is given by equation (10.4). If the number of fractures is proportional to the radiation dose r9 this expression may be written in the very convenient form

— = Pi+Por 1 =Po(r+r0) (18.1)

where pi is the reciprocal of the initial number average degree of poly-merization i/i, and r0 is defined as equal to Pi/p0. r0 is a virtual radiation dose, namely the dose which would be required to produce a polymer with the observed initial distribution from one which is infinitely long. r0 can be considered as a parameter depending on the initial number average molecular weight.

This relationship holds whatever the initial molecular weight distri-bution. If the initial distribution is random, the viscosity average mole-cular weight Mv is proportional to the number average molecular weight Mn\ the same is true of the viscosity average and number average degrees of polymerization

Mv uv ' ' __ (β + 1)Γ(β + 1) Mn «i

from equation (8.21). The ratio Mv/Mn can be read directly from Fig. 8.1. Thus if the intrinsic viscosity measurements are carried out in carbon tetrachloride solution at 30°C, a = 0-68 and Mv/Mn = 1-85. Then

1 1 1 1 M?~û7~ VS5û7 " F85^56 Po ( ' + r o ) (18*2)

where w9 the molecular weight of the chain unit C4H8 is 56. A plot of 1/M/ against (r+r0) should therefore give a linear plot of slope 0-96x 10_2/70. The intercept gives r0, which is directly related to the initial number average molecular weight. The G value for main chain fracture can then be deduced from equation (9.7)

G(fracture) = 0-96 x I O W H ' = 1-7 X 104/>0. If the initial molecular weight distribution is not a random one, this


formula cannot be applied except at high radiation doses when radiation-induced fractures have reduced the initial distribution to a random one. Departure from a linear curve at low doses would therefore indicate that the initial distribution is not random. If the plot of -j-r-, against (r+r0) is not linear at high doses, it must be concluded that radiation-induced

1

/

^L·

/ /

/

for a non random distri

bution

VQ5Mn

^Mv for an initially random distribution

Radiation dose r ·

FIG. 18.2. Increase in-r^with dose (a) initially random distribution, (b) initially non-random. r0 = virtual dose.

fractures do not occur at random or alternatively they are not pro-portional to the radiation dose.

Experimental results for the degradation of poly/stfbutylene are shown in Figs. 18.3 and 18.4. Plots are made of -— against (r+r0) using

My a log-log scale which gives equal importance to each measurement. Fig. 18.3. shows the results obtained for γ-radiation carried out at 20°C while Fig. 18.4 represents the effects of electron radiation at the same temperature but at much higher radiation intensity. In each case, the appropriate correction r0 has been added to the measured radiation dose to allow for the initial molecular weight. This correction factor is usually very small compared with the true radiation dose r.

The results for three different specimens are plotted in each curve, each specimen having a different value of r0 according to its initial average molecular weight. The linearity of the curves and their slope of +1 show that the density of fracture is proportional to radiation dose, that it is independent of molecular weight over a 100-fold range and that the initial

POLY/5OBUTYLENE 329

10'

100

50

£ ü

ιθ 1U "~7 V

5

2

1

+

+ /

/

I

l/i Δ

. I 0-5 1-0 2-0 3-0 5-0 10

Dose 9 megaroentgen 20 30 50

FIG. 18.3. Degradation of poly/sobutylene by γ-radiation at 20°C. X + Δ O three different samples measured at four radiation intensities.

(From Alexander, Black and Charlesby, 1955.)

10°

OC\C\

1ΠΟ

50

20

10

5

2 »

• /

s À

/

10 20 50 100

Dose, Megarep

FIG. 18.4. Degradation of poly/söbutylene by electron radiation at 20°C. (From Alexander, Black and Charlesby, 1955.)


distribution is approximately random. represented by the equations :

The experimental results can be

- = 2*5 0+A*O) (γ radiation)

106

—- = 2-7 (r+r0) (electron radiation) Mv

(18.3)

(18.4)

where r and r0 are expressed in units of 1 xlOV or 1 xlO6 rep. Within experimental error, there is no significant difference in the effect of electron beams and of γ-radiation in so far as their ability to fracture molecular chains is concerned.

By comparing these equations with that calculated theoretically (18.2), an average value can be derived for p0i the fracture density per unit radiation dose (in megarads) :

Po = 2-9xl0-4

while from equation (9.7) the corresponding G value for main chain fracture is 5 ( ± 5 per cent) over the entire range extending down to molecules comprising only 40 monomer units.

A corresponding plot was obtained by subjecting various samples of

1000

500|

10°

200

100

50

20

10i

5

/

/

/

/

A

/

S

i

y ä

γ

0-01 0-05 0-2 0-5 1-0 2-0 Pile units

5-0 10

FIG. 18.5. Degradation of polywobutylene by pile radiation. X + Δ three different specimens

Circled marks denote vacuum irradiation. (From Alexander, Black and Charlesby, 1955.)

POLY/S0BUTYLENE 331

poly/jobutylene to BEPO pile radiation. can be represented by the relationship

The results shown in Fig. 18.5

106

— = 110 (r+r0) My

where r-\-r0 are expressed in BEPO pile units. By comparing this relation-ship with those obtained for electron and γ-radiation, it is found that unit pile radiation has the same effect as a γ- or electron radiation dose of 44 million roentgens. A figure very close to this was obtained by com-paring the effects of γ- and pile radiation on polymethyl methacrylate and polyethylene. However, the comparison is only a practical one since the conditions of radiation, in particular the temperature, are appreciably different, pile irradiation taking place at about 70-80°C and γ-radiation at 20°C, and this affects the fracture rate.

Unsaturation In addition to main chain fracture, radiation produces increased un-

saturation in poly/wbutylene. This unsaturation may be revealed by

1000

500

200

100

106

Mi

0-05 0-1 0-2 0-5 1-0 2-0 5-0

Double bonds/100 monomer units FIG. 18.6. Unsaturation and main chain fracture. Unsaturation at high and low

temperature. — average at 20°C. γ-Radiation Pile radiation

Δ —190°C T · 70°C + O — 80°C X + 70°C D + 90°C (From Alexander, Black and Charlesby, 1955.)


infra-red measurements and by direct chemical titration. Using Wij's reagent, the degree of unsaturation is found to be directly proportional to the radiation dose (Fig. 18.6), at least up to values of about one double bond per six monomer units. Results indicate that each radiation-induced main chain fracture is associated with 1-87 C—C bond. It has not yet been determined whether the formation of unsaturation is directly related to main chain fracture or whether two processes take independently by different processes. When the main chain is fractured, some rearrangement of the chemical groups near the point of fracture would be expected to occur, but although probable there is no direct evidence that this re-arrangement involves the formation of a double bond. Infra-red measure-ments show that the main change arises from an increase in the number of RiR2C = CH2 groups, but there is no evidence of the formation of main chain or terminal unsaturation such as RiHC^CHRa, RHC=CH2. At the same time, there is no change in the colour of the specimen, whereas in most polymers a dense yellow or brown colour would be produced by the radiation doses considered here. This discoloration is also absent when silicone fluids are irradiated, and would therefore appear to be associated with main chain unsaturation.

1125/* 10-85/* 10-58/*

wavelength FIG. 18.7. Infra-red absorption in the 11μ regions.

(From Alexander, Black and Charlesby, 1955.)

Gas Evolution Gases evolved during irradiation comprise mainly hydrogen, methane

and zstfbutylene. The proportion of wobutylene is small initially but increases rapidly with radiation dose. It would appear, therefore, that the direct effect of radiation is to split off the hydrogen and methyl groups which then react to form methane, while the /söbutylene arises from two independent main chain fractures on successive units in the chain.

This process of main chain fracture is quite different to that observed when polyisobutylène is degraded thermally. In the latter case, consider-able amounts of the monomer are produced, whereas in the case of radiation fracture the amount of monomer is very small compared with the number of main chain fractuies. With radiation only a small amount

POLYISOBUTYLENE 333

of atomic rearrangement is necessary to stabilize the molecule, and unlike thermally induced depolymerization, no chain reaction is involved.

Temperature Effect

The amount of degradation produced by radiation is found to vary with temperature. This effect appears to be a fundamental one and has also been observed in biological systems. The number of main chain fractures is still found to be proportional to radiation dose but the energy absorbed per break varies with temperature. Whereas the average energy absorbed per break amounts to 20 eV at 20°C, it rises to 45 eV at - 196°C and drops to 10 eV at 90°C. This temperature effect does not arise from

r ——

1

•

•

°c FIG. 18.8. Effect of temperature on behaviour of polywobutylene and certain dry

bacteria under radiation. + poly/sobutylene energy per main chain fracture (γ-radiation) O inactivation of E. Coli B (vacuum dried) 50 kV x-rays • ditto lyophilized (Bachofer) D Red cell catalase inactivation by 3-7 MeV deutrons ■ ditto 1 MeV deutrons (Setlow and Doyle)

(All measurements relative to — 196°C.) (From Alexander, Black and Charlesby, 1955.)

thermal depolymerization since specimens heated for three days at 70°C reveal negligible changes in molecular weight. The temperature effect is of considerable interest in any study of the mechanism of radiation changes and will be discussed separately.

Fig. 18.8 shows the relative energy required to degrade poly/^butylene at various temperatures (taking a value of 1 at the temperature of liquid nitrogen). The curve shows the same temperature dependence as is observed in the inactivation by radiation of certain biological specimens.

Protection

Copolymers of /^butylène and styrene were irradiated by Alexander and Charlesby (1955) to determine the effect of protection by the benzene ring. Radiation protection was found to extend for a distance of some 4-5 carbon atoms along the chain (see Chapter 29).


Table 18.1. Effect of Temperature on Degradation of Polyisobutylene

Temperature °C

- 1 9 6 - 8 0

20 70 90

G (main chain fracture)

2-2 3-7 5 8-3

10

To* observed

3-4X106

l l x l O 6

0-55 xlO6

r0 calculated

3-55 xlO6

105 x 10e

0-5 XlO6

* These are "virtual radiation doses" to allow for the finite initial molecular weight.

R E F E R E N C E S ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. A232, 31,

1955. ALEXANDER, P. and CHARLESBY, A., Proc. Radiobiology Symposium, Liege, p. 49,

1954, Butterworths, 1955. ALEXANDER, P. and CHARLESBY, A., Proc. Roy. Soc. A230, 136, 1955. ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955; 13(10), 51, 1955. BOPP, C. D. and SISMAN, O., ORNL 1373, 1953. CHAPIRO, A., J. Chim. Phys. 53, 306, 1956. CHARLESBY, A., Plastics, May, 1953. DAVIDSON, W. L. and GEIB, I. G., / . Appl. Phys. 19, 427, 1948. FLORY, P. J., / . Amer. Chem. Soc. 65, 372, 1943. Fox, M. and FLORY, P. J., J. Amer. Chem. Soc. 70, 2384, 1948. Fox, M. and FLORY, P. J., / . Phys. Chem. 53, 197, 1949; 55, 221, 1951. KARPOV, V. L., Conference on the Peaceful Uses of Atomic Energy, p. 33 ; Moscow,

July 1955. KARPOV, V. L., Nucl. Sei. Abstr. 9, 7725, 1955. LAWTON, E. J., BUECHE, A. M. and BALWTT, J. S., Nature, Lond. 172, 76, 1953.

CHAPTER 19

POLYMETHYL METHACRYLATE

METHYL METHACRYLATE monomer, a colourless liquid boiling at 100°C, can be polymerized by chemical catalysts such as benzoyl peroxide or by radiation (Chapter 22). Polymerization in solution or emulsion is also possible, and leads to the formation of a granular or fine powder. Various forms of the polymer are sold under such trade names as Perspex, Lucite, Plexiglas or Diakon moulding powder. These may differ in molecular weight and in the incorporation of plasticizer. The polymer is transparent to visible light and to the near ultraviolet, although after radiation increased absorption is noted in the high-energy side of the spectrum.

The polymer shows an amorphous structure when studied by x-rays and this may be ascribed to an irregular arrangement of the side groups on either side of the main chain. At room temperature, the material is rigid, but scratches readily. It softens at about 70°-120°C, depending on the conditions of test as well as on the molecular weight and the presence of plasticizer.

Polymethyl methacrylate is one of the polymers whose changes under radiation have been most closely studied. The molecular changes produced are in many respects typical of most methacrylate polymers—fracture of the main chain, leading to a reduction in average molecular weight, together with decomposition of the side chain. The repeating unit in the methacrylate polymers is [CH2—C(CH3)(COOCnH2M+i)] and is to be compared with the acrylate series (page 318)

[ C H 2 - C H (COOC„H2M+1)]

in which the predominant effect of radiation is crosslinking. Sisman and Bopp (1951, 1955) have reported on the change in mechani-

cal properties after specimens were irradiated in the nuclear reactor at Oak Ridge. For radiation doses of up to 0 1 x 1018 nvt, there was no change in the elastic modulus at room temperature, but the tensile strength decreased rapidly by 20 per cent at 001 x 1018 nvt (about 8 megarads) down to 10 per cent of its initial value at 0 1 l x 1018 nvt (about 80 mega-rads). The shear strength and impact strength followed a less rapid decrease and there were no appreciable changes in the specific gravity, water absorption, hardness or electric properties for these radiation doses.

Charlesby (1953) and Lawton, Bueche and Balwit (1953) found that polymethyl methacrylate degrades under radiation, becoming more readily soluble. The changes in molecular weight due to main chain fracture were studied quantitatively by Alexander, Charlesby and Ross (1953,

335


1954), while effects of side chain fracture were described qualitatively by Charlesby and Ross (1953). Further work on the changes in mechanical properties and absorption spectra was reported by Chapiro (1956) while Wall and Brown (1956, 1957) studied the influence of benzene, air and other additives on the trapped radicals, revealed by paramagnetic resonance (Schneider et al, 1951, 1955; Abraham and Whiffen, 1958). Several authors have reported a temperature effect in main chain fracture, the G value depending on the temperature of irradiation.

Decrease in Molecular Weight

In the experiments of Alexander, Charlesby and Ross (1953, 1954), specimens of polymethyl methacrylate, either in the form of rod or powder, were subjected to mixed radiation from the BEPO nuclear reactor or to pure γ-radiation from a radioactive cobalt source. The specimens were then dissolved in chloroform and the viscosity average molecular weights Mv' determined from the intrinsic viscosity [η] of the solution. (Bischoff and Desreux, 1952, 1953, Meyerhoff and Shulz, 1951 and Meyerhoff 1953).

[η] =4 ·8χ10- 5 Μ,° · 8 .

The results for pile irradiated specimens are shown in Fig. 19.1 where r is the radiation dose in pile units and r0 is a small correction factor,

5 X 1 0 6I

3x106

2x106

10Ö

5x l0 5

3x105

2x105

105

5x104

N V

ex

Y Y e\ \

S A 0-01 0-03 0-1 0-2 0-3

pile units 0-5

FIG. 19.1. Variation of viscosity-average molecular weight with radiation dose. X rods (r0 = 0Ό07); · rods (r0 = 0-OC6); O outside shavings from rods; + rods

heated to 150°C after irradiation; Δ powder (r0 = 0-0175); — Mv = 2-6 x 10-4 (r _[- r o ) . (Dose in pile units.)

(From Alexander, Charlesby and Ross, 1954.)

POLYMETHYL METHACRYLATE 337

depending on the initial molecular weight of the unirradiated polymer. The relation observed can be expressed in the form :

^ - , = 38-5xlO-6(^+ô). (19.1) iK/y

For γ-radiation a similar type of relation is observed:

-^- = 0-83xlO-6(H->\)) (19.2)

where the radiation dose r is expressed in megaroentgen and r0 is again a small correction factor to allow for the finite molecular weight Mv of the initial unirradiated material

r0 = 1-2X107M,.

A comparison of these equations representing the effect of pile radiation and of γ-radiation shows that one BEPO pile unit has the same effect as 46 megaroentgens or 43 megarads, in so far as the degradation of poly methyl methacrylate is concerned.

The linear relationship between 1 /Mv' and r over an extensive range of doses indicates that the number of fractures produced is proportional to the radiation dose and that these fractures occur at random in the mole-cular chain. Calculation of the values of q0 and G for main chain fracture runs parallel to that outlined in the chapter on poly/sobutylene, except that the exponent a in the expression for the intrinsic viscosity is 0-8, so that

^ - ' = - ' = 1 - 9 0 8 Mn lit

and w the molecular weight of each chain unit C5H802 is 100. Then, for a random distribution of fractures (equation 18.2)

W = H M ) X *('+'·>· ( 1 9 · 3 )

Comparison with the experimental results for γ-radiation gives

Po = 1·908χ100χ0·83χ10-6 = 1-6x10-*

when the unit dose is 1 megaroentgen. When unit dose is expressed in megarads

Po = 1-7x10-*. Then from equation (9.7)

G(fracture) = 0-96x 10* p0/w = 1-6

over a range of molecular sizes from an average of 20,000 down to 260 monomer units.

These results, obtained by Alexander, Charlesby and Ross for J in. rods for thin outside shavings, for polymer powder (more subject to any surface oxidation effects) and for specimens heated to 150°C after radiation, showed no significant dependence on specimen thickness.


Wall and Brown (1956) carried out similar measurements with speci-mens prepared from distilled and degassed monomers polymerized at 60°C and then cast from benzene solution. After further heat treatment to remove residual radicals the samples were subjected to gamma radiation either in air or in vacuum. The G values for a variety of specimens, shown in Table 19.1, are compared with those for Alexander, Charlesby and Ross and some recent values for electron radiation where the intensity is some 105 times greater. No significant intensity effect is observed. The increase in G value observed when heating specimens FB-1 and FB-2 to 120°C might arise from the removal of residual benzene (otherwise retained in the polymer) which can act as a radiation protective agent. Inherent stresses in biaxially stretched material also appear to have no effect. The magnitude of the oxygen effect (a reduction in the number of fractures for a given dose) overlaps the variation between different speci-mens, but appears to reduce the fracture density by about one-third.

Table 19.1. G Values for Main Chain Fracture ofPoly me thy I Methacrylate at about 25°C

Sample

Film FB-1 FB-1 FB-2 FB-2 Commercial Ί

sample > (oriented) J

Commercial Ί sample >

(unoriented) J \ in. rod shavings Thin sheet Film

Vacuum treatment before radiation

hr 100°C 20 100°C 20 120°C 20 100°C 20 120°C 20

\ 100°C 20 I 120°C 20

\ 100°C 20 L 120°C 20

G values irradiation in

Vacuum Air

1-23 0-645 1-70 0-894 200 1-25 1-99 1-37 2-48 1-52 2-28 2 31 2-23 2-23 216 2-16 1-6 1-6

1-5 1-7

Reference

(a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (b) (c) (d)

(a) Wall and Brown (1956), γ-radiation. (b) Alexander, Charlesby and Ross (1953, 1954), pile γ-radiation. (c) Charlesby, electron radiation (unpublished). (d) Shultz et al. (1956), electron radiation.

Shultz, Roth and Rathmann (1956) studied the degradation of poly-methylmethacrylate and tert.-butyl methacrylate both by measurements of the weight average and viscosity average molecular weight. Samples of the former were prepared by three techniques: thermal polymerization at 93°C in the absence of oxygen, at room temperature in the presence of some oxygen, and by exposure to ultraviolet light. Films were subjected to


high intensity radiation from a 1 MeV (peak) electron beam. The recipro-cal weight average molecular weight, deduced from light scattering (which gives Mw) and from intrinsic viscosity measurements in benzene (giving Mv) is linear with radiation dose, as shown in Fig. 19.2.

If in addition to main chain fracture, some crosslinking occurred, polymer molecules would become increasingly branched, and their intrinsic viscosity would fall increasingly below that of linear molecules of the same weight. The weight average determinations however would still show a

2 4 6 8 10 r , megareps

FIG. 19.2. Molecular weight of irradiated polymethyl methacrylate x from light scattering, O from intrinsic viscosity.

(From Schultz et al., 1956.)

linear dependence on dose (equation 11.6). The experimental data show that, although individual points are somewhat scattered, there is no systematic tendency for the viscosity and weight average measurements to drift apart. It must therefore be concluded that crosslinking does not play a significant part in irradiated polymethyl methacrylate.

The G value (1-7) obtained for main chain fracture by the high intensity electron radiation used by Shultz et al. agrees well with that found for γ-irradiation at much lower intensities. For tert-buiy\ methacrylate the corresponding G value is somewhat higher (2-3).

Gaseous Products

If blocks of polymethyl methacrylate are irradiated for upwards of two pile units (or about 100 megarads) at a temperature exceeding 70°C, bubble formation takes place within the specimen, which swells. This bubbling or foaming can also take place after radiation, if the irradiated block is heated to a suitable temperature, depending on the radiation dose. It has not been observed to occur at temperatures below about 70°C, however large the radiation exposure (Ross and Charlesby, 1953).

This effect arises from the decomposition of the ester side chain. Analysis of the gas shows the presence of hydrogen, carbon monoxide and dioxide, and methane, and very small amounts of other gaseous products. As in polyethylene, a surface effect is observed in that for thick specimens

z


FIG. 19.3. Transparent block on left is irradiated polymethyl methacrylate. Foamed block on right shows the effect of subsequent heating on an irradiated block.

(From Ross and Charlesby, 1953.)

Table 19.2. Gases Evolved by Radiation Degradation and Thermal Depolymerization

All samples preheated for 20 hr at 100°C radiation dose 6-25 megarads, heated after radiation (where indicated below), 3 hr at 125°C

Product

Hydrogen Carbon dioxide Methyl formate Carbon monoxide Methane Methyl methacrylate

(monomer) Methanol Benzene

Irradiated in vacuum

1-37 2-07 0-57 2-83 3-56 00

006

Heated only

0-78

—·

— 11-7

246

Irradiated in vacuum, then heated

4-67 7-9 —

— 45-6

6-4 227

Irradiated in air,

then heated

5-5 16-2 —

— 152

222 All values in table are in μπιοΐβ^; for irradiated samples they are close to six times

the corresponding G value. (From Wall and Brown, 1956.)

FIG. 19.4. Bubbling effect in irradiated Perspex (polymethyl methacrylate), showing edge effect.

FIG. 19.5. (a) Specimens irradiated and then heated, showing free zone. (b) Two specimens irradiated 0-3 units, cut in half and heated for 3, 6, 9 and 12 min.

Bubble formation continues to edge where this is cut after radiation (From Ross and Charlesby, 1953.)

Facing P. 340

FIG. 19.7. (a) and (b). Internal fissures and cracks produced in polymethyl metha-crylate left standing for a few weeks after irradiation.


the weight loss of lightly irradiated samples depends on the surface area, and not the mass of polymer irradiated. (Alexander, Charlesby and Ross, 1954.) No monomeric methyl methacrylate is obtained unless the irra-diated polymer is heated to about 125°C, showing that radiation does not result in depolymerization comparable with that obtained by pyrolysis (Table 19.3). However, the vacuum irradiated polymer is more sensitive to subsequent thermal depolymerization than unirradiated material, possibly because of the presence of long-lived free radicals, which may also account for the small post-effect reported by Wall and Magat (1953).

The gaseous products formed by side chain decomposition could arise during the radiation process itself, or subsequently on heating. Evidence in favour of the former hypothesis is as follows :

No foaming takes place near the edge of an irradiated block, possibly because the gases produced by radiation have diffused out. This edge effect is not observed if the edges are cut after radiation although the gaseous products would be equally capable of diffusion to the new surfaces formed (Figs. 19.4 and 19.5).

An irradiated block of polymethyl methacrylate, on subsequent solution in a suitable solvent, releases trapped gases although no heating has taken place.

On this view, the effect of radiation is to break both main chain and side chain bonds. The disintegration products are held as separate atoms or molecules within the solid. When the temperature is raised above the softening point, the high internal pressure is able to overcome the cohesion of the polymer molecules, and these fragments come together to form bubbles. The higher the dose, the greater the internal pressure and the reduction in length of the long chain polymers ; the temperature necessary for bubbling is therefore reduced.

Trapped Radicals Paramagnetic resonance determinations (Schneider et ai, 1951, 1955;

Combrisson and Uebersfeld, 1954; Abraham et al, 1958) have confirmed the presence of trapped radicals. These may cause degradation on subsequent solution of the polymer or alternatively cause the fragments to recombine. To investigate this behaviour Wall and Brown dissolved irradiated specimens in a solvent containing a monofunctional terminator capable of reacting with these radicals and thereby suppressing their reactions. For vacuum irradiated samples they failed to find any effect on the amount of degradation; this may have been due to a very low concentration of trapped radicals. When, however, the experiment was repeated with specimens irradiated in air, a slow decrease in intrinsic viscosity was observed extending over a day or more. This they ascribed to a. breakdown of peroxide structures formed in the polymer, this break-down being promoted by such additives as tert.-buiyl catechol, hydro-quinone and dimethyl aniline.

An alternative chemical test for the presence of trapped radicals consists in using irradiated polymer as a means of initiating polymerization in monomer. At a temperature of 29°C induced polymerization was found


to be initiated by polymethyl methacrylate irradiated and kept in vacuo. The effect was much smaller for specimens irradiated in air, or in vacuo containing a small amount of inhibitor such as benzoquinone (Fig. 19.6).

Minutes Minutes F I G . 19.6.

(a) Polymerization of methyl methacrylate by irradiated polymer at 29°C. Curve a 10 megarads 1-65 g polymer/1.; curve b 10 megarads 1-25 g/1. -f- hydroquinone; point

c · no irradiated polymer present. (b) Polymerization of methyl methacrylate by irradiated polymer at 70°C. Curve a 10 megarads in vacuo stored in vacuo for 7 days; curve b, 10 megarads in vacuo stored in air for 7 days; curve c, 10 megarads in air stored in air for 7 days; curve d, un-

irradiated. (From Wall and Brown, 1956.)

On the other hand, the activity at 70°C of air-irradiated polymer was much greater than that of vacuum-irradiated material, presumably due to the breakdown of the peroxide structure at this temperature, which thereby liberated radicals capable of initiating polymerization. The concentration of peroxide structures was estimated by Wall and Brown as at least 10~3

mole per 10 megarads, while that of the trapped radicals in vacuum irradiated specimens amounts to at least 10~5 mole/1. This concentration was calculated to drop 100-fold in solution in about 1 sec, but was esti-mated to need some 4 months to decrease by the same amount in solid polymer. Combrisson and Uebersfeld (1954) have in fact observed a decrease in the paramagnetic resonance response after a period of about 15 days. Abraham et al. (1958) only observed this decrease when their specimen was heated to 80°C. Similar paramagnetic resonance spectra were observed by these workers for a number of other methacrylate polymers, with G values of between 1 and 3, i.e. comparable with the number of main chain fractures. After examining the previous theories, they concluded that the radical responsible was most probably

COOR R — CH2 — C ·

CH3


Temperature Effect The early experiments of Alexander, Charlesby and Ross (1953)

indicated that the G value for main chain fracture varied with temperature. This effect has since been confirmed both for the degradation of polymethyl methacrylate and polyisobutylene and for the crosslinking of polyethylene and silicones. Wall and Brown (1957) report a reduction in the G value from 1-23 at 25°C to 0-50 at — 196°C when the specimen is irradiated in vacuum, but in air, when the yield is lower, the effect of temperature is far less marked. A recent set of G values at various temperatures for sheets of polymer, subject to electron beam radiation, is shown in Table 19.3. The G value at 20°C for electron radiation is, within experimental error, equal to that for γ-radiation described above, although the dose rates differ by a factor of about 105, the electron radiations being complete in a few seconds, the γ-radiations requiring several days for the same energy absorption. If any intensity effect exists, possibly related to the after-effect caused by trapped radicals, it must be smaller than the dosimetry errors.

Table 19.3. Temperature Effect in Degradation of Polymethyl Methacrylate

Temperature

-196°C -78°C

0°C 20°C 25°C

100°C Source:

G

0-75 1-4 1-5 —

2-3 (a)

0-5 — — —

1-23 —

(b)

(a) Charlesby and King, electron radiation. (b) Wall and Brown (1956), γ-radiation.

Mechanical Properties

After radiation at temperatures too low to permit foaming or bubbling, polymethyl methacrylate appears to be unaffected apart from certain slight colour changes to be discussed below. If a block of such irradiated material is left standing for a period of weeks or months, it will, however, show internal fissures or cracks and may even fall apart into fragments. This effect may arise from the progressive opening of fissures formed during the initial radiation, or from successive depolymerization in the region of excessive stress incurred during radiation. The depolymerization of polymethyl methacrylate under conditions of stress is known to occur even in the absence of radiation and is often referred to as stress-cracking or crazing. A further possibility is that radicals, trapped within the specimen, are able to react slowly.

Sisman and Bopp (1951, 1955) have studied the changes in mechanical


properties of the polymer subjected to a range of radiation doses in an nuclear reactor (Fig. 19.8). Their doses are expressed in terms of neutron

Strain, % FIG. 19.8. Stress-strain curve for irradiated polymethyl methacrylate. Figures give

slow neutron flux in Oak Ridge pile. 1018 nvt = 700 megarads. (From Bopp and Sisman, 1955.)

flux; converting these to equivalent gamma radiation, and thence to mole-cular weight (using equation 19.2), gives directly the relationship between these properties and molecular weight, for a random distribution (Fig. 19.9). The mechanical properties begin to fall off rapidly for a degree of polymerization of below 500.

Neutron f l ux , n v t

0 2xl01 5 5x1015 10162x1016 5x1016 1017

M 0 6 ) 3 x 1 0 5 2x105 105 5x104 2x104 104

Mn (approximate)

FIG. 19.9. Decrease in tensile strength and elongation at break (in terms of NAMW of irradiated polymethyl methacrylate).

POLYMETHYL METHACRYLATE 345 Chapiro (1956) studied the elongation and rupture of specimens of

polymethyl methacrylate, irradiated by γ-rays from a cobalt source and from a high intensity, low voltage (37 kV) x-ray machine. The results on the elongation at various temperatures is shown in Fig. 19.10, and indicates a sudden elongation at a temperature which decreases with increasing radiation dose. These results may be converted to a direct relationship between temperature and molecular weight. However, the temperatures observed are far in excess of the softening temperature of the polymer and indeed above the temperature needed for thermal degradation, and have no fundamental meaning. They can best be considered as a measure of the bulk viscosity of the polymer, which would be reduced by the smaller molecular weight of the irradiated specimens and by the greater ease of thermal depolymerization.

Chapiro also reports a small intensity effect on the temperature for rupture. This effect was, however, only observed at low doses, of 3 mega-rads or less, and amounts to a variation of ± 25 per cent over a range of intensities from 57 roentgens/min to 72,500 roentgens/min.

Mechanical tests on these specimens, which were 1 mm in thickness,

500i

400h

300l·

200r

100

3-25

2-3 r 0-821

1°

150 2 0 0 250 3 0 0 °C

FIG. 19.10. Extension of a specimen of polymethyl methacrylate under a constant load of 1-4 g/mm2. Figures indicate dose in megaroentgens.


showed no difference as between radiation in air and in a vacuum. Any oxygen effect, such as reported by Wall and Brown, must therefore be confined to the surface layers only. Spectrum

Changes in the ultraviolet and visible spectrum are observed following radiation, and the absorption increases, particularly in the ultraviolet. A slight yellow coloration is also observed. When plasticizers or inhibition are present, as in commercial samples, a short-lived fluorescence may be


observed after electron radiation, together with transient colours giving absorption in the 700-800 ιημ region. The changes in the spectra observed by Alexander, Charlesby and Ross and by Chapiro have not been explained in any detail. These spectra may arise from electrons trapped at F centres, free radicals or conjugated double bond systems. For comparable doses, the coloration in polymethyl methacrylate appears to arise from different causes than that produced in irradiated polyethylene or polystyrene; its rapid disappearance in irradiated polymethyl methacrylate argues against the presence of conjugated double bands. On exposure in air the colour gradually fades from the outer surface inwards, a sharp line of demarcation being observed. This fading is associated with the diffusion into the polymer of oxygen, which can react with the chromophore groups. The colour also disappears readily on heating, although some evidence of visual absorption may remain, possibly due to a secondary cause of coloration. There appear to be serious differences in the ultra-violet and visible spectra reported by different authors, so that the spectrum appears to depend on the purity of the sample. The change in optical absorption of a pure or dyed polymer has been suggested as a method of dosimetry (Fowler and Day, 1955; Day and Stein, 1951).

Protection

Alexander, Charlesby and Ross (1954) have reported a protection effect when various additives such as aniline, thiourea or benzoquinone are present in the polymer during radiation. The addition of 10 per cent of these reduces the amount of degradation by a factor of about two or three. This effect is different to that reported by Wall and Brown, where the presence of tert.-bx\Xy\ catechol in a solvent reduced the molecular weight of samples of the polymer irradiated in air, due to interaction with the peroxide bridges. This protection effect operates in the solid; protection effects in irradiated solution of polymethacrylic acid have also been reported (see Chapter 25) when the effect of radiation is primarily on the solvent. The solvent radicals formed then attack the polymer, but competitive reactions with the additive reduces their effect on the polymer.

REFERENCES

ABRAHAM, R. J., MELVILLE, H. W., OVENALL, D. W. and WHIFFEN, D. H., Trans. Faraday Soc. 54, 1133, 1958.

ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954, ALEXANDER, P., CHARLESBY, A. and Ross, R., A.E.R.E. M/R 1269, 1953. BEVINGTON, J. C. and CHARLESBY, A., Ric. Sei. 25, 1955. BISCHOFF, J. and DESREUX, V., Bull. Soc. Chim. Belge 61, 10, 1952. BISCHOFF, J. and DESREUX, V., J. Polymer Sei. 10, 437, 1953. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955; 13(10), 51, 1955. BURR, J. G., et al, A.E.C.D., 3634, 1948. CHARLESBY, A. and Ross, M., Nature, Lond. 171, 1153, 1953. CHARLESBY, A., Nature, Lond. Ill, 167, 1953. CHARLESBY, A. and BEVINGTON, J. C , A.E.R.E. M/M 85, 1954. CHAPIRO, A., / . Chim. Phys. 53, 295, 1956.


COMBRISSON, J. and UEBERSFELD, J., C.R. Acad. Sei. (Paris) 238, 1397, 1954. DAY, M. J. and STEIN, G., Nature, Lond. 168, 644, 1951. FOWLER, J. F. and DAY, M. J., Nucleonics 13(12), 52, 1955. HORNBECK, R. F., KIRKLAND, W. K., PARKINSON, W. W. and SISMAN, O.,

ORNL, 2413, August 1957. KARPOV, V. L., Conference on the Peaceful Uses of Atomic Energy, Chemistry

Division, p. 3, U.S.S.R. Academy of Sciences, Moscow, 1955. KARPOV, V. L., Nucl. Sei. Abstr. 9, 7725, 1955. LAWTON, E. J., BUECHE, A. M. and BALWIT, J. S., Nature, Lond. 172, 76, 1953. MEYERHOFF, G. and SCHULZ, G. V., Makromol. Chem. 7, 294, 1951. MEYERHOFF, G., Naturwiss. 40, 106, 1953. Ross, M. and CHARLESBY, A., Atomics 4(8), 189, 1953. Ross, M. and CHARLESBY, A., A.E.R.E. M/R 1067, 1953. SCHNEIDER, E. E., Disc. Faraday Soc. 19, 158, 1955. SCHNEIDER, E. E., DAY, M. J. and STEIN, G., Nature, Lond. 168, 645, 1951. SHULTZ, A. R., Nuclear Engineering and Science Congress, December 12-16,

1955, Cleveland, Ohio. SHULTZ, A. R., ROTH, P. I. and RATHMANN, G. B., / . Polymer Sei. 22, 495, 1956. SISMAN, O. and BOPP, C. D., ORNL 928 (1951). UEBERSFELD, J., C.R. Acad. Sei. (Paris) 239, 240, 1954; Ann. Physique (Paris) 1

(XIII), 395, 1956. WALL, L. A. and BROWN, D. W., / . Phys. Chem. 61, 129, 1957. WALL, L. A. and MAGAT, M., J. Chim. Phys. 50, 308, 1953. WALL, L. A. and BROWN, D. W., J. Res. Nat. Bur. Stand. 57(3), 131, 1956. WRIGHT, J., Chem. &Ind. (Rev.) 1026, 1955.

CHAPTER 20

POLYTETRAFLUORETHYLENE (PTFE)

POLYTETRAFLUORETHYLENE (PTFE) is a long chain polymer (CF2—CF2)n of high symmetry which can fit into a highly crystalline structure. The main applications arise in fields where its high melting temperature, wide working range of temperature (—100° to -f-300°C), inert chemical nature or low coefficient of friction are sufficiently important to overcome the initial high cost and difficulty of manufacture.

The melting point of PTFE, i.e. the temperature at which the crystals melt and the polymer acquires an amorphous structure, is about 330°C (Renfrew and Lewis, 1946; Rigby and Bunn, 1949). x-ray diffraction studies have revealed several transitions below this temperature at 20° and 30° (Bunn and Howells, 1954). The occurrence of sudden changes in structure so far below the melting point is probably unique in long chain polymers and may be explained in terms of the relatively smooth and cylindrical shape of the polymer molecule. The changes in x-ray diffraction pattern at these temperatures are consistent with rotation and longitu-dinal displacement along the chain axes, the shape of each molecular configuration being largely unaltered.

Radiation-induced Changes in PTFE

Although a number of papers have been published dealing with the changes produced in PTFE by exposure to high energy radiation, no clear picture of the processes which occur has yet appeared. The experi-mental difficulties in studying the reaction arise largely from the non-reactive nature of PTFE which makes it very difficult to trace any chemica changes. For some time, there was indeed doubt as to the structure of the polymer itself; the suggestion was made that during extrusion cross-links are formed and this would explain the loss in mechanical properties in reprocessed polymer. The deterioration is now ascribed to a small amount of depolymerization or degradation. In view of these difficulties, most experimental work has been directed towards measuring the change in mechanical properties, loss in weight and the nature and amount of gases evolved. Paramagnetic resonance measurements (Schneider, 1955; Ard et al., 1955; Abraham and Whiffer, 1958) and measurements of electrical conductivity (Liversage, 1952; Fowler and Farmer, 1954; Feng and Kennedy, 1955) have also been carried out.

Many of the changes observed are not completed when irradiation is stopped but continue progressively over periods of weeks. This behaviour may arise from the reaction of residual radicals in the system, from oxygen attack on these radicals, from progressive fracture of bonds due to internal

348

POLYTETRAFLUORETHYLENE (PTFE) 349

stresses set up by occluded gases or from the slow rate of diffusion to the surface of these gases. Somewhat similar structural deterioration may be observed in polymethyl methacrylate if this is left standing over a period of months after irradiation. Change in Mechanical Properties

Bopp and Sisman (1951, 1955) measured the change in the elastic modulus, the tensile strength, elongation at break and other mechanical properties of PTFE at room temperature following exposure to pile radiation at Oak Ridge. Some of their results are summarized in Fig. 20.1.

FIG. 20.1. Stress/strain curves for pile irradiated Teflon (polytetrafluorethylene). (From: Sisman and Bopp, 1951.)

At radiation doses as low as 0Ό05 x Î018 nvt, equivalent to about 5 mega-rads of γ-radiation, PTFE loses most of its strength. A marked deteriora-tion in tensile strength of thin foil for doses as low as 0-1 megarad of electron radiation has been observed (S. H. Pinner, private communi-cation), but this deterioration did not increase with time after exposure.

The decrease in zero tensile strength of PTFE (involving a creep fracture test carried out above the crystalline melting point) has been used as a measure of radiation damage (Nishicka et al, 1958). Significant decreases are observed for Co 60 γ doses as low as 104 rads, and at 1 Mrad the zero tensile falls by a factor of about 50. Somewhat similar changes have been observed in the melt viscosity, which decreases by a factor of 200 for a dose of only 0-3 Mrads (Matsumae et al., 1958). These doses are considerably smaller than those needed to produce marked changes in the mechanical properties of other polymers which degrade under radiation (e.g. poly-methyl methacrylate, Fig. 19.9). If, for example, the G value for degra-dation is about 3 (as in many polymers), a radiation dose of 01 Mrads would produce only one fracture per 6-4 x 104 C atoms, whereas according to Berry and Paterson (1951) there are only about 104 C atoms in an average molecule. One would not expect this very low fracture density in a highly crystalline material to have a profound effect on its mechanical properties.

Unusual changes in crystallinity have also been reported by Nishicka


et al, for doses as low as a few thousand rads, above which an increase in crystallinity is reported. These changes cannot be explained as simply due to the very infrequent main chain fractures. One may perhaps ascribe the radiation sensitivity of PTFE to some form of chain reaction (some-what analogous to the dehydrochlorination of PVC), to a continuous dis-integration of the lattice under load stress set up by occluded gases, or to the preferential disruption of molecular chains between crystallites. A further possibility is that for useful mechanical properties, unusually high mole-cular weights are essential to compensate for the low intermolecular forces ; in this case very few breaks per molecule would have a considerable influence.

Chemical Changes The inert chemical nature of PTFE renders difficult an assessment of

the chemical changes produced by radiation using conventional chemical methods. Ryan (1953, 1954) has observed changes in the infra-red spectrum which may be ascribed to the formation of unsaturation

— C = C — + F 2 ; — CF = CF2 F F

although the relationship between unsaturation and dose or dose rate has not been derived. The presence of unsaturation was also checked directly by oxidation measurements.

The gases evolved when PTFE, sealed in evacuated silica vessels, is subjected to pile radiation have been analysed by mass spectroscopy and shown to consist mainly of SiF4 and CF4 in roughly equal amounts (Charlesby, 1952). The presence of the SiF4 could arise from the attack of evolved fluorine on the glass container. Alternatively, Woodley has shown that the reaction

CF4 + SiOa -> SiF4 + C 0 2

proceeds very rapidly in the presence of radiation. The presence of small amounts of C0 2 in the gases analysed would confirm this reaction. The considerable amount of residual CF4 observed may have diffused out of the specimen after the cessation of radiation.

The loss of weight during irradiation in the BEPO pile has been measured by Charlesby both as a function of the surface area or mass and of the radiation dose. For PTFE sheet of greater thickness than 0-2 mm (area/ mass greater than 44 cm2/g) the weight loss was found to be independent of the mass of irradiated polymer, and to vary as the surface area and the square of the radiation dose r, expressed in pile units;

Δ Μ = —1·25χ10-Μτ»

where A is the surface area exposed. For thinner specimens, however, the weight loss depended not on the area of the specimen but on the total mass irradiated. The variation with the square of the radiation dose no longer held at high doses of the order of 20 pile units, when the specimens began to break up during radiation, exposing larger surfaces.


The quadratic dependence on dose can be related to the probability of independent C—C breaks at neighbouring points along the chain, the assumption being that radiation causes main chain fracture and that the CF4 gas evolved arises from two such fractures occurring independently but falling close together.* The subsequent loss of weight would then depend on diffusion of CF4 of the surface. At higher temperatures this diffusion should increase and show a greater weight loss. This was actually observed when the specimen was heated after radiation. On the other hand, for sufficiently thin films (less than 0-2 mm thickness) in

0-04r

0-06

FIG. 20.2. Weight loss of pile irradiated PTFE. (From Charlesby, 1952.)

*At high fracture densities the theory would indicate a departure from a quadratic dose dependence, as is in fact observed.


which all the CF4 liberated by radiation can diffuse to the surface, no surface effect and no diffusion control were observed.

Both Ryan (1953, 1954) and Worrall (1957) measured the fluorine evolved directly by radiation. The PTFE specimens were irradiated in the presence of dilute NaOH or distilled water and the amount of fluoride in the water determined by chemical analysis. With cobalt 60 γ-irradiation, Ryan deduced a relationship of the form

Δ Μ = 305X10-V1151

where ΔΜ is the weight of fluorine evolved per gram of PTFE and r is the radiation dose expressed in megaroentgen. The effect of surface area of the specimen on the rate of evolution of fluorine was not investigated. Although most of the fluoride obtained was evolved in the course of the irradiation, further evolution took place during a post-radiation period of some 30 days. The amount of fluorine evolved after radiation was proportional to the dose up to 200 megarads and became approximately

2·0|

Radiation dose, megaroentgen

FIG. 20.3. Fluorine evolved during a post-radiation period of sixteen weeks. Due to the limited diffusion rate, thicker specimens evolve more fluorine after irradiation.

(From Worrall, 1957.)

constant at higher doses. Presumably for the longer exposure times the fluorine liberated during the first period of irradiation was able to diffuse out during the latter part, so that for long exposure times steady state conditions prevail.

Worrall (1957) measured the fluorine evolved from specimens of varying thickness (surface area 46, 12, 8 and 2 cm2/g) both when subjected to


cobalt 60 γ-radiation and BEPO pile radiation at Harwell. For thick specimens (of area 12cm2/g or less) subjected to γ-radiation the total amount of fluorine evolved depended on the surface area A and the radiation dose (r) expressed in megarads

A(F~) = 2-5x10-° Ar1'2

whereas for the thinner specimens of surface area 46 cm2/g, the corre-sponding loss per weight M was

A(F-) = 22xl0-6 Mr12.

The critical ratio of surface area/mass would therefore be Ac = 22/2-5 = 8-8 cm2/g. Thus for films thicker than about 1-5 mm, fluorine evolution would be restricted by the rate of diffusion at the dose intensities used. For similar specimen weight Worrall found that thinner specimens evolved about six times as much fluorine as the more massive ones. If the remaining fluorine is present as a gas within the specimen, considerable internal pressures can be built up.

Parallel experiments were also carried out by Worrall on PTFE im-mersed in water and irradiated in the BEPO nuclear reactor. For these specimens the fluorine evolution was related to the radiation dose r expressed in pile units

0-1

100

Ig

PTF

E

Ό

1

Î 1-0

0-1

A(F-) = 014 x 10-3.

, A ,o 2 ,„ Cur

A B

7

ve \+

.1

/ ■

f

i

/ 1

γ

i 8 cm2 /g ) 2 cm2 /g

1

/ ■

1 j

/ ■ /

1

Ar1'*

A

/

A/

/ /

/

1-0 10 Radiation dose, pile units

FIG. 20.4. Pile irradiation of PTFE. Total fluorine evolved (From Worrall, 1957.)

100


while the total evolution after allowing the specimens to stand for some weeks reached the value

A(F~) =0-17xlO-Mr 1 , a .

This formula is of the same character as that derived for γ-radiation. A direct comparison of the coefficients gives an equivalent for the pile unit of 34 megaroentgen of γ-radiation. This is lower than the pile equivalents deduced from the degradation of poly/söbutylene or polymethyl metha-crylate or from the unsaturation of polyethylene. In these latter polymers, a contribution arises from the collision of fast neutrons with hydrogen atoms, fast protons being produced which promote dense ionization. In PTFE, however, the amount of energy which can be absorbed from these fast neutrons is much smaller, owing to the absence of hydrogen atoms ; the amount of energy transferred per collision to the carbon or fluorine atoms is far smaller (p. 44), and the number of such collisions is also reduced.

The contribution from pure γ radiation is also smaller in PTFE than in hydrocarbons, since energy deposition is primarily due to Compton scattering, and is proportional to the electron density, and this density is lower in PTFE. Thus in a pure γ-radiation field PTFE only absorbs 87 per cent of the energy absorbed by the same mass of water. In the mixed radiation from reactors the difference will be greater, and can serve as a measure of the relative contributions of γ and fast neutrons to the absorbed dose.

Direct measurements of energy deposition from the BEPO pile have been made by Anderson (see Worrall, 1957), using calorimetric methods. The deposition in C and F atoms are shown in Table 20.1. The total energy deposition in PTFE per pile unit amounts to 1·68χ 1021 eV/g as com-pared with a value of 1 -71 x 1021 eV/g deduced by Worrall from comparative measurements of fluorine evolution in the pile and in a known γ-field.

Table 20.1. Calorimetric Determination of Energy Deposition in PTFE (BEPO pile; 1 pile unit = 1017 thermal neutrons/cm2)

Substance C F

(C2F4)n

Energy deposition in eV/g per pile unit

Fast neutron Gamma Total 0-30X1021 1-47X1021 l - 77x l0 2 1

0-26X1021 1-40X1021 l - 6 6 x l 0 2 1

0-27X1021 1-41 x lO 2 1 1-68X1021

Comparison of Weight Loss and Fluorine Evolution There appears to be a marked discrepancy between the loss in weight

of irradiated PTFE and the amount of fluorine evolved. The measure-ments of Charlesby in which PTFE was irradiated in air or vacuum lead to a weight loss varying as the square of the radiation dose r2 whereas both the measurements of Ryan and of Worrall indicate an evolution of fluorine varying as r1'15 or r1*2. The discrepancy cannot arise from the effect of radiation intensity since Worrall found the same dose dependence for pile radiation and for gamma radiation where there is nearly a 100-fold


difference in intensity. The discrepancy must therefore be ascribed to the different products measured and to the different conditions of irradiation. The products measured were respectively CF4 and fluorine and it is not certain which of these is the primary product. The course of the reaction may therefore proceed in very different ways in the presence of either oxygen or of water. The paramagnetic resonance experiments of Ard, et al. (1955) show that oxygen can react with the radicals trapped in PTFE. Reactions may also occur with water and indeed increases in weight have been observed in PTFE when irradiated under these conditions. These increases may be due to water uptake in the small fissures formed by radiation; these fissures will permit water to penetrate within the material and interact with any reactive entities. Furthermore, the effect of radiation on the water itself may be relevant, radicals produced in the water being capable of further reaction. It would therefore appear advisable to carry out measurements on fluorine evolution when the PTFE is not immersed in water.

All the results obtained agree in stressing the importance of surface/ volume ratio and indicate that the reaction is diffusion controlled over a period of weeks.

Mechanism of Degradation

In spite of its similarity in structure to polyethylene, PTFE degrades rapidly under radiation; Wall (1956) has explained this apparent incon-sistency as due to difference in bond energies. In polyethylene hydrogen abstraction reactions are energetically possible, since to break a C—H bond 87 kcals/mole of energy is required, but 100 kcals/mole are recovered by the formation of a H—H bond.

H + —CH2— ~> H2 + —CH— + 13 kcals/mole.

In PTFE the C—F bond is stronger (107 kcals/mole) while the F—F bond is weaker (39 kcals/mole). The reaction would then be endothermic

F H CF2— -> F2 + —CF 68 kcals/mole.

The formation of unsaturation, either main chain (—CF=CF—) or terminal ( CF=CF 2 ) is difficult to explain as an abstraction reaction, owing to the unfavourable energy relation. It may either be assumed that fluorine evolution is in part a molecular process, as in polyethylene and simple paraffins (Dorfman, 1956; Charlesby and Davison, 1957) or that the formation of terminal unsaturation is an integral part of the process of main chain fracture, as suggested by Alexander, Charlesby and Ross (1954) in the case of polymethyl methacrylate.

An alternative explanation of chain fracture is that strains in the main chain, caused by steric interference of the large F atoms, weaken the C—C bond, and therefore favour main chain fracture.

In many hydrocarbons and long chain polymers, the chain configuration is that of a plane zig-zag, carbon atoms being in /ra/w-configuration with a distance between alternative carbons of 2-54 Â. This is notably the case for polyethylene. In PTFE the bulky dimensions of the fluorine atoms

AA


attached to each carbon prevent this arrangement and a less favourable configuration is observed, with carbon atoms forming a non-planar zig-zag arrangement along a helix. The distance between alternating carbon atoms is also increased to a value of 2-595A to accommodate the fluorine atom.

C C The distortion of the \ / bond angle from the tetrahedral angle

C 109-5 to 116° would reduce the stability of such a system which is under constant tension. This distortion is one possible explanation for the fact that whereas polyethylene crosslinks under radiation, PTFE degrades very readily. According to this view, polychlorotrifluorethylene (CFC1— CF2)M with similar steric hindrance would also degrade whereas Polyvinyl-chloride (CH2 CH Cl)n would tend to crosslink or degrade depending on whether successive units take up a head-to-head or head-to-tail structure and on the chain configuration. Values of the bond reactivity in polymers, if deduced from non-distorted model structures, may be very misleading in predicting the pattern of behaviour of these polymers under radiation.

There are considerable discrepancies in the results of paramagnetic resonance in irradiated PTFE, reported by Schneider (1955), Ard, Shields and Gordy (1955), Gordy (1955) and Abraham and Whiffen (1958). Schneider, for example, observed a broad 3-line spectrum, whereas Abraham and Whiffen found no evidence of a spectrum when irradiation was carried out in vacuum. If, however, air was present during or after irradiation, absorption spectra appeared, the time required being dependent on the state of subdivision of the polymer. Ard, Shields and Gordy observed a clear 8-line spectrum, which would correspond to the odd electron moving equally in the field of seven F atoms, e.g.

CF3 — CF2 — Ç — C F 2 — .

However it appears unlikely that the number of branched molecules present in PTFE is adequate to account for the intensity of the spectrum.

The apparent absence of paramagnetic absorption in highly crystalline polymers such as PTFE, Kel-F and linear polyethylene may possibly be due to a very broad spectrum which cannot be resolved. The presence of some reactive entity is necessary to explain the appearance of an absorption spectrum when oxygen is admitted after irradiation. The slow diffusion of oxygen into an irradiated specimen and its subsequent reaction with these active entities may account for the long-term mechanical breakdown often reported.

POLYMONOCHLOROTRIFLUOROETHYLENE Work on polymonochlorotrifluoroethylene (CFC1—CF2)n, (Kel-F or

Fluorothene) parallels closely that on PTFE. Sisman and Bopp (1951) found no great change in the elastic modulus of the polymer after pile radiation for doses of up to 0·2χ 1018 nvt (about 100 megarads for this polymer), but the elongation at break was already halved by a dose of only 006x \0 l s nv t .


5 OOO

4 0 0 0

5 3 0 0 0 m

8 2000 Φ t.

S 1000

L I I 1 1 1 1

0 0-2 0-4 0-6 Strain, in /in

FIG. 20.5. Stress-strain curves for pile irradiated Fluorothene (CFC1—CF2)n.

Byrne, Costikyan, Hanford and Mann (1953) studied the reaction in some detail, and compared the results with changes produced in both plasticized and non-plasticized Fluorothene with those in polyvinyl chloride and carbon tetrachloride. In the last case the evolution of halides would be less hindered by diffusion problems. Samples of the polymer, in the form of thin shavings or cubes in a sodium hydroxide solution, were sub-jected to cobalt γ-radiation at about 25000 r4min. The results obtained are qualitatively similar to those described above for PTFE. The existence of a diffusion-controlled reaction is shown by the difference between halide evolution from cubes and shavings. Chlorine and fluorine evolution was about equal in spite of the different amounts present in the polymer.

The effect of plasticizer (low molecular weight Fluorothene) on gas evolution was negligible for thin shavings of the polymer, but increased it in thicker samples. Thus diffusion rate is partly determined by the mole-cular weight of the specimen. The effect is shown in Table 20.2, where

Table 20.2. Maximum Rate of Halogen Evolution millimoles halide/g. 109r ( = 1-04 G)

Form

Cubes F Cl

Shavings F Cl

Maximum total

Fluorothene (C2F3Cl)n Plasticized Unplasticized

1 0-4 1 0-4 2-5 3 2-5 3 5 6

Polyvinyl chloride

(C2H3C1) n

1

1 7

CC1 4

9 9

the maximum rates of production of halides are compared. 1 millimole per g per 109 r corresponds to a G value close to unity.

The change in mechanical properties observed by Byrne et al. follow those given by Sisman and Bopp (1951); the tensile and impact strength

n v t = 0 - 0 6 x l 0 1

n v t=0-11x10, c

Non irradiated

< Breaking load

n u t - Π · 9 v 1 0 K


decrease to a small fraction of the initial value for a dose of some 50 megarads. A corresponding decrease in shear strength requires a higher dose of about 300 megarads. The authors also discuss the implications of halogen evolution on the possible life of Fluorothene under pile radiation conditions.

Goodman and Coleman (1957) have studied the dielectric breakdown of Kel-F films, irradiated in air. Electrical breakdown occurs when the film degrades to a yellow powder. The dose needed for this degree of radiation damage depends on the radiation intensity, varying as the root of the latter. Thus at 2 megareps/hr, a dose of 250 megareps is needed to produce zero tensile strength ; at an intensity of 180 reps/hr, the correspond-ing dose falls to about 1-5 megareps.

The cause of this intensity-dependence is not known ; it can be due to a physical breakdown of the irradiated polymer, which may take a long time to develop (as for example in polymethyl methacrylate). It would appear incorrect to relate the long term physical damage only to the immediate chemical changes produced by radiation since these show no such marked intensity dependence. Internal strains set up by occluded gases, and reactions of trapped radicals may cause further damage, and it would be advisable to study the change in physical properties at various periods after the initial exposure, to trace any such effects.

REFERENCES ABRAHAM, R. J. and WHIFFEN, D. H., Trans. Faraday Soc. 54, 1291, 1958. ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. ARD, W. B., SHIELDS, H. and GORDY, W., / . Chem. Phys. 23, 1727, 1955. BERRY, K. L. and PETERSON, J. H., J. Amer. Chem. Soc. 73, 5195, 1951. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955. BOPP, C. D. and SISMAN, O., ORNL 1373, 1953. BUNN, C. W. and HOWELLS, E. R., Nature, Lond., 174, 549, 1954. BYRNE, J., COSTIKYAN, T. W., HANFORD, C. B., JOHNSON, D. L. and MANN,

W. L., Ind. Eng. Chem. 45, 2549, 1953. CHARLESBY, A., A.E.R.E. M/R 978, 1952. CHARLESBY, A. and DAVISON, W. H. T., Chem. & Ind. 232, 1957. DORFMAN, L. M., J. Phys. Chem. 60, 826, 1956. FENG, P. Y. and KENNEDY, J. W., / . Amer. Chem. Soc. 17, 847, 1955. FOWLER, J. F. and FARMER, F. T., Nature, Lond., 174, 136, 1954. GOODMAN, J. and COLEMAN, J. H., J. Polymer Sei. 25, 253, 502, 1957. LIVERSAGE, W. E., Brit. J. Radiology 25, 434, 1952. MATSUMAE, K., WATANABE, M., NISHIOKA, A. and ICHIMYA, T., / . Polymer Sei.

28, 653, 1958. NISHIOKA, A., TAJIMA, M., OWAKI, M., / . Polymer Sei. 28, 617, 1958. RENFREW, M. M. and LEWIS, E. E., Ind. Eng. Chem. 38, 870, 1946. RIGBY, H. A. and BUNN, C. W., Nature, Lond., 164, 583, 1949. RYAN, J. W., Modern Plastics 31(2), 152, October 1953; Ind. Plastiques Modernes

6(6), 40, 1954. SCHNEIDER, E. E., Disc. Faraday Soc. 19,158,1955 ; / . Chem. Phys. 23,978, 1955. SISMAN, O. and BOPP, C. D., ORNL 928, 1951. WALL, L. A., Society of Plastics Engineers J., p. 17, March 1956. WOODLEY, AEC Report HW 40142. WORRALL, R., A.E.R.E. M/R 2159, 1957.

CHAPTER 2 1

CELLULOSE

CELLULOSE ( C 6 H I 0 O 5 ) « is the name given to a considerable variety of closely similar and very complex carbohydrates which form the main constituents of the cell walls of plants and thus constitute the framework of the plant organism. Structurally, cellulose can be considered as an unbranched polymer of high molecular weight, repeating units being anhydroglucose rings. The regular structure of the polymer molecule allows crystals to be formed and according to x-ray measurements cellulose is about 70 per cent crystalline. This high degree of crystallinity is probably associated with hydrogen bond formation between adjacent molecules, and these strong intermolecular forces are sufficient to permit considerable expansion of the unit cell without disruption during swelling by strong acids and alkalis.

Cellulose has no melting point and is fairly resistant to thermal degra-dation. The ß-glucoside link is labile towards hydrolytic agents and continued exposure to such reagents leads to progressive reduction in the molecular weight, leading ultimately to the disaccharide, cellobiose.

Cellulose is insoluble in all organic solvents but forms addition com-plexes with acids and alkalis and with cuprammonium and similar complex ions which are water-soluble.

Cellulose derivatives, however, are non-crystalline and dissolve readily in organic solvents. Substituents in general decrease molecular chain stiffness and increase solubility and fusibility, the effect being greater the larger the substituents group.

The cuprammonium process is used in the manufacture of regenerated cellulose fibers. Other treatments of cellulose include esterifification, etherification, xanthation, acetal formation, oxidation and degradation. Rayon and Cellophane are principally made by the xanthate process. This involves treatment of cellulose with sodium hydroxide solution and carbon dissulphide, spinning or extruding the ripened solution into the desired form, and then coagulating with acid.

Nitrocellulose is made by nitration of cellulose and represents the oldest and possibly best known plastic. Its chief disadvantage is inflammability.

Cellulose acetate is made by the treatment of cellulose with acetic anhydride and sulphuric acid. It resembles nitrocellulose in toughness but is non-inflammable. It has rather high water sorption which gives rise to dimensional instability. This is much less pronounced in other cellulose esters such as cellulose acetate butyrate. Cellulose ethers are made by reaction of soda cellulose with alkyl halides. One example is ethyl

359


cellulose which resembles cellulose nitrate and cellulose acetate butyrate in toughness. The most important cellulose ether is carbomethoxy cellulose which is used very widely as a sizing and thickening agent. It is obtained by reacting soda cellulose with chloracetic acid.

Degradation of Cellulose

As a result of exposure to high energy radiation, cellulose and related materials suffer main chain degradation and other chemical changes of a less well-defined character. Degradation is readily shown by the increased solubility and reduced viscosity. Other changes include decreased mechanical strength.

The rate of degradation in cotton linters (containing 97-7 per cent a cellulose) and wood pulp (containing 960 per cent a cellulose) has been traced by Saeman et al. (1952). Samples were irradiated with 800 kV (peak) electrons and their viscosity was measured in cupriethylenediamine to determine the reduced molecular weight. The formula used was the simple Straudinger relationship between the intrinsic viscosity and the degree of polymerization ux

«i = k fo]

when k = 190. The curves obtained at low radiation doses differed for the two types of material but approached a common curve at 10 megarads

Charlesby (1955) used the same data with a relationship of a more general form (8.13)

er = kMv'*

to relate the measured intrinsic viscosity and the viscosity average of the degraded cellulose. For a random distribution

M / / M n, = {(fl+l)r(fl+l)}1/«

but in the case of cellulose, a is not known accurately. The intrinsic viscosity can be expressed in terms of the radiation dose (equation 18.1)

[ηΓ = kMn'a(a+l)T(a+l)

= kwau1'a(<a+l)T(a+l)

= kwa (fl+1) T(a+\)lpoa(r+r0)« (21.1)

Thus, a log-log plot of fo]' against r+r0 should give a straight line of slope — a; r0 represents the virtual dose needed to produce the initial average molecular weight.

A plot of [η]' against r + r 0 using the data of Saeman et al. (1952) is shown in Fig. 21.1, with and without an appropriate value for the small initial correction term r0. With this correction term, a linear relationship is observed over the entire range, justifying the assumptionthat the fracture density is directly proportional to the radiation dose.

CELLULOSE 361

m 1-0

0-6

0-4 0-3

0-2

0-1

-. . """—-> —. ""---—

N ·

X S ^ ^ - ■ ^ 4 v .

"***·> ^

!_ iV^JVersus i υμ versus / τ ι0

+ Wood pulp · Wood pulp

x Cot ton l in ters ■ Co t ton l i n t e r s

^ν \

/ > ν

Ν.

\ ^

-

0-1 0-2 0-5 10 20 50 100

Radiation dose r or r + rQ> megaroentgen FIG. 21.1. Degradation of cellulose by radiation.


- [η] = 4-9 (r + r0)~° 71 (r in megaroentgens) (r0 =0-75 megaroentgens for wood pulp, 1 megaroentgen

for cotton linters.)

This result is of further significance since it may be taken to indicate that no difference arises in the rate of fracture as between crystalline and amorphous regions. There is no tendency for the cellulose chain to be first broken in the amorphous regions and only subsequently in the crystalline ones, as is the case when depolymerization or degradation is induced by chemical means. The similarity in the curves for cotton linters and for wood pulp shows that one is measuring a common property of the cellulose backbone, namely its sensitivity to radiation-induced fracture.

The experimental results can be written in the form

M ' = 4-9(r+A-0)-°·71 (21.2)

from which the value for a of 0-71 in the equation (21.1) can be derived immediately. This value is somewhat lower than other derivations (0-77, 0-81) but is relatively unambiguous. The value of k was deduced from other data as being about 5·9χ 10~4 but is far less certain. Using these values of k and a, a value of/?0 can be obtained from equations (21.1) and (21.2)

p0 = 1-6x10-

when the unit dose is the megaroentgen, or about 1·8χ 10~3 when the unit dose is the megarad. This leads to a value for G (main chain fracture)


of about 10. This unusually high value, as compared with poly/sobutylene and polymethyl methacrylate, renders suspect the value of k which was derived indirectly. This result indicates the value of radiation analysis as a means of deriving a value for a in a degrading polymer and its inherent inability to give k without either some independent calibration of molecular weight or of G value for degradation.

An independent check on the value of p0 derived above can be made by comparing the calculated decomposition with that observed in various cellulosic materials by Saeman et al. An exponential exp (—p0r) is used at high doses since a monomer unit can only be decomposed once. Table 21.1. shows that the observed decomposition rate of carbohydrates follows this exponential relation.

Table 21.1. Calculated and Observed Decomposition (Assuming fracture density p0 = 016 per cent per megaroentgen)

Calculated decom-position (%):

100 p0r 100 exp(— p0r)

Observed decom-position (%):

Cotton linters Wood pulp Wood Glucose

Radiation dose r (megaroentgens)

1

016 016

— — — —

10

1-6 1-6

2 5 3 2

50

8 7-7

12 10 — —

100

16 14-8

14 17 9

14

500

80 55

44 — — —

Accepting this value of p0, the initial molecular weight can be derived from the value of r0, required to produce the linear plot in Fig. 21.1. By definition r0 = pi/p0 — l//?0«i (p. 164). Values quoted in the literature are compared with the number average degree of polymerization ul9

obtained from r0.

Degree of Polymerization (wi) Cotton Linters (raw)

(cleaned) (bleached) (calculated)

Wood pulp (calculated)

mical Changes

1400 1200-1300

700 625

600-1000 830

Hermans (1949) Hermans (1949) Hermans (1949) (i/i = l//?o r0)

Hermans (1949) («! = llp0r0)

In addition to degradation of the main chain, Saeman et al. (1952) found evidence of chemical decomposition by radiation, leading to

CELLULOSE 363

an improved sugar yield when the irradiated sample was treated by the conventional batchwise dilute acid hydrolysis. It was shown that glucose and the carbohydrates in cellulose were both decomposed at the same rate, whereas in the presence of lignin in wood pulp cellulose decomposition was less rapid. Hydrolysis in 0 1 N sulphuric acid at 180° was more rapid in heavily irradiated materials; this was ascribed to the destruction of crystallinity by radiation. The yield of sugar was also greatly increased by radiation, but to affect a significant improvement, depolymerization down to a chain length of about 200 glucose units or an equivalent radiation dose of about 10 megarads was needed. To obtain a water soluble material, requiring a degree of polymerization estimated at only 6, a dose of 500 megarads would be required.

In related work, Lawton et al. (1951) studied the effect of electron radiation on basswood. The cellulose after irradiation become vulnerable to the attack of bovine rumen bacteria, small changes in the production of volatile acids being observed at 10 megarads and reaching a maximum at 100 megarads. The lignin component of the wood was less affected by radiation and may have served to protect the cellulosic component to some extent. These doses are, of courses, far too high to be of commercial value.

Chemical changes in related carbohydrates have been studied by Khenokh (1955) by means of infra-red radiation.

DEXTRAN In studying the effect of electron radiation on dextran, Price, Bellamy

and Lawton (1954) investigated the changes in the weight average as determined by light scattering, the reduction in the intrinsic viscosity and the increase in the number of end groups as determined by chemical analysis. From their results, they concluded that the energy per break was constant over the range 5-100 megarads. A comparison of number and weight averages showed a considerable degree of branching. The change in weight average molecular weight due to irradiation doses of up to 100 megarads corresponded to a decrease in molecular weight by a factor of over 104. A material of molecular weight (50,000) useful for clinical purposes as a blood extender can be achieved with a radiation dose of 80 megarads, the yield of useful material being possibly higher than that obtained by acid hydrolysis.

MECHANICAL PROPERTIES At an early date Winogradoif (1950) a reduction in tensile strength T

of cellulose acetate of the form

T = To exp (—rlr0)

where T0 is the initial strength and r is the x-ray dose given in air. No value of r0 is given. Winogradoif also observed a decrease in crystallinity, the evolution of gases and the formation of fissures giving a brittle material.


In their compilation of data on the mechanical properties of irradiated plastics, Sisman and Bopp (1951, 1955) give data on a number of cellulose derivatives—ethyl, proprionate, acetate, nitrate and acetate-butyrate. The samples were irradiated in the Oak Ridge pile and the doses expressed in nvt units, a dose of 1018 nvt corresponding very approximately to 103

megarads. For most of these polymers, there was little significant change in the density, weight or water absorption for doses of at least 0-1 x 1018 nvt. The elastic modulus likewise remained almost unaltered at this dose level. There was only a slight increase in the Rockwell hardness for the nitrate and the acetate, the other polymers remaining unaffected. The observed reduction in elongation can be ascribed to the shorter polymer molecules resulting from radiation. Table 21.2 summarizes the data on the decreased tensile strength, and the impact and the shear modulus, as well as the dose at which the tensile strength effectively sinks to zero. For cellulose nitrate, for example, a dose of 100 megarads reduced several of these properties by about 50 per cent. On the assumption of a G value for main chain fracture of 5 (this figure being only a rough estimate), this dose would result in an average chain length of about 16 monomer units. This may be taken as an indication of the segment length of these materials. Figs. 21.2 and

5 0 0 0

4 0 0 0

U)

°" 3 0 0 0

U) ω <v £ 2000 ω

1000

υ 5 10 15 20 25 St ra in , %>

FIG. 21.2. Stress-strain curves for cellulose acetate irradiated in Oak Ridge pile. Figures indicate dose in units of 1018 nvt. X indicates breaking point.

(From Sisman and Bopp, 1951.)

21.3 show some typical stress-strain curves while Fig. 21.4 shows the reduction in mechanical properties with dose. With the exception of the modulus, these depend on the average molecular size, which is reduced by radiation.

xO-11

0*05

U- o

CELLULOSE 365 40001

3000

(0 £L

: 2000

Φ L

if) 1000

1% Strain per cent

FIG. 21.3. Stress-strain curves for cellulose propionate. Figures indicate dose in 1018 nvt of Oak Ridge pile irradiation, Asterisk shows breaking point. Tensile strength

is reduced by radiation; modulus is unaffected. (From Sisman and Bopp, 1951.)

Dose in 10 n v t FIG. 21.4. Relative change in properties of ethyl cellulose.

(From Sisman and Bopp, 1951.) Curve No.

1 2 3 4 5

Property Tensile strength Elongation Elastic modulus Shear strength Impact strength

Initial value 6000 lb/in2. 40 per cent 2-1 XlO5 lb/in2. 6700 lb8in2. 2-0 ft-lb/in. of notch.

Gilfillan and Linden (1955) measured the tenacity of cotton and viscose after subjecting filaments to neutron and γ-radiation. They con-firmed the absence of crosslinking, and also found that as far as their mechanical properties are concerned, these materials are more sensitive to radiation than nylon or polyacrylonitrile.

Chapiro (1956) carried out some measurements on cellulose acetate subjected to γ-radiation doses of the order of 20 megarads or less. The temperature at which specimens broke under a given stress was found


Table 21.2 Cellulose Derivatives

Radiation for 50

Ethyl Proprionate Acetate Nitrate Acetate-butyrate

Shear

70 40

200 100 70

per cent decrease*

Impact

40 40 70

100 50

Tensile

50 40

200 100 70

Elongation

20 40f 70 40 50

Radiation* for zero tensile

140 200 400 140 100

*Approximate values in megarad equivalent. f No appreciable cold flow.

Initial Values

Ethyl Proprionate Acetate Nitrate Acetate-butyrate

Shear psi

6700 2900 6400 8800 4000

Impact ft-lb/in. of

notch

20 11 1-37 2-75 3-3

Tensile psi

6000 2600 5300 7600 4200

Elongation at break

%

40 1-6

20 30 60

(Based on Sisman and Bopp, 1951.)

to decrease uniformly with dose. No intensity effect (57-580 roentgens/min) nor any large oxygen effect was observed. Stable colour changes were obtained for doses above 3 megarads while the polymer becomes brittle at about 20 megarads.

Glegg and Kertesz (1956) claim to have discovered an after-effect in irradiated cellulose and pectin, the viscosity of the irradiated samples (after about 0-1 and 2 megarads respectively of cobalt 60 γ-radiation) decreasing over a period of 10-30 days by 106 and 25 per cent respectively of the immediate change. Combrisson and Uebersfeld (1954) and Abraham and Whiffen (1958) have also reported on the presence of trapped radicals which can be detected by paramagnetic resonance, and which may account for any after effects.

REFERENCES ABRAHAM, R. J. and WHIFFEN, D. H., Trans. Faraday Soc. 54, 1291, 1958. BOPP, C. D. and SISMAN, O., Nucleonics 13(7), 28, 1955. CHAPIRO, A., / . Chim. Phys. 53, 295, 1956. CHARLESBY, A., / . Polymer Sei. 15, 263, 1955. COMBRISSON, J. and UEBERSFELD, J., C.R. Acad. Sei. (Paris) 238, 1397, 1954. GILFILLAN, E. S. and LINDEN, L., Textile Res. J. 25(9), 773, 1955. GLEGG, R. E. and KERTESZ, Z. I., Science 124, 893, 1956; / . Polymer Sei. 26,

289, 1957.

CELLULOSE 367

HERMANS, P. H., Physics and Chemistry of Cellulose Fibres, p. 101, Elsevier, Rotterdam, 1949.

KHENOKH, M. A., Dokl. Akad. Nauk SSSR, 104, 746, 1955. LAWTON, E. J., BELLAMY, W. D., HUNG ATE, R. E., BRYANT, M. P. and HALL, E.,

TAPPI 34(12), 113A, 1951. PRICE, F. P., BELLAMY, W. D. and LAWTON, E. J., / . Phys. Chem. 58, 821, 1954. SAEMAN, J. F., MILLETT, M. A. and LAWTON, E. J., Industr. Eng. Chem. (Anal. )

44, 2848, 1952. SISMAN, O. and BOPP, C. D., ORNL 928, 1951. SISMAN, O., Plastics Technol. 1, 345, 1955. WINOGRADOFF, N. N., Nature, Lond. 165, 72, 123, 1950.

CHAPTER 22

POLYMERIZATION THE PREVIOUS chapters have been mainly concerned with the use of high energy radiation to modify polymers already formed. In this chapter we shall discuss the use of radiation to produce these polymers from their monomeric constituents. The two processes are fundamentally different; in the modification of polymers the amount of chemical change produced is generally small and is proportional to the radiation dose over a wide range; it is also independent of radiation intensity. Each crosslink or chain fracture requires a separate act of ionization or excitation, although in special cases several chemical changes may result from each of these acts. Radiation induced polymerization, on the other hand, is a chain reaction in which a large number of chemical changes may follow each single act of ionization or excitation.

RADIATION AS AN INITIATOR OF POLYMERIZATION

The polymerization of monomers involves at least three separate stages, i.e. chain initiation, chain propagation and chain termination, and these may be modified by further reactions such as chain transfer. Radiation intervenes primarily only in the initiation stage, acting as a means of starting the reaction which then continues independently of it. This is no longer true at very high intensities where primary radicals produced by radiation can intervene directly in the termination mechanism. The number of growing chains which can react with each other also depends on the radiation intensity, which is therefore of considerable importance in polymerization.

Polymerization of monomers can occur under a variety of conditions ; as a liquid, in the gas phase, as a solid, in solution, in emulsions or dis-persions. Such reactions have been studied using chemical or photo-chemical techniques of initiation and a wealth of information on reaction rates under a variety of conditions is available for workers studying radiation-initiated polymerization, since the kinetics of propagation and termination are usually similar.

Polymerization can be initiated either by a radical or by an ion. Although radiation produces ions and excited molecules as a primary act, most of the polymerization reactions already studied have been found to proceed by a radical mechanism, the radicals being produced indirectly from the ions or excited molecules. Only very recently has evidence been produced which suggests an ionic polymerization reaction induced by radiation; most of the published information refers to radical polymeri-zations of vinyl monomer. Formation of polymers by condensation does

368

POLYMERIZATION 369

not appear to be a chain reaction which can be initiated by radiation and comparatively little work has been carried out on such polymers.

A number of books are available dealing with the kinetics of polymeri-zation initiated by chemical or photochemical means. The present chapter is therefore largely confined to the specifically radiation aspects of the subject, some of which are also relevant in other fields of radiation research, e.g. in the measurement of radical yield.

The use of radiation polymerization has a number of distinct advantages as compared with the usual chemical techniques. The latter require catalysts which may be incorporated in the polymer and then remain as an impurity which may continue to react. With radiation, on the other hand, no impurities are introduced although trapped radicals may still be present in the solid polymer. The temperature conditions needed for initiation by catalysts are not necessarily those most suitable for chain propagation, whereas with radiation the initiation step is almost tem-perature independent and that reaction temperature may be chosen which is most suitable for the propagation step. Radicals can be produced uniformly throughout the system whatever its physical state and poly-merization in the solid state is possible. During chemical polymerization, the exotherm produces a rise in temperature which results in an increased rate of dissociation of the chemical catalyst. In radiation polymerization, this temperature rise has little effect on the initiation step, the number of primary radicals depending only on the instantaneous radiation intensity. Much closer control of the reaction is therefore possible. Much higher concentrations of radicals can be produced instantaneously, sufficient to swamp the effect of any inhibitors incorporated in the mixture. Indeed, it has been possible to obtain polymerizations with radiation which, due to the presence of certain additives, cannot be obtained by chemical methods. Less stringent reaction conditions may be adequate for radiation initiated polymerization; thus in the gaseous polymerization of ethylene, lower pressures and temperatures can be used, giving a more linear material. (Anderson et al, Bretton and Hay ward, and Medvedev.)

There remains as a further objective the production of polymers by radiation which cannot be obtained by conventional chemical methods. Work along these lines is being pursued very actively although little has been published on the subject.

Early Work

Early observation of polymerization arose in connexion with the study of a particle bombardment of gases. Both saturated and unsaturated gases were studied, and the results expressed in terms of the ratio MjN of the number of molecules modified (M) to the ions produced (N). High values of this ratio led Lind to a concept involving the formation of clusters of molecules around an ion. In spite of its early date, much of the work of Lind et al. in America (1924, 1926) and of Mund (1925, 1926) in Belgium is of direct relevance to date. Coolidge (1925) carried out analogous work making use of a high energy electron beam, while Heising 1932, 1935, 1939) studied the polymerization of several simple olefins


such as propylene and butylène. Subsequently, more detailed investi-gations were carried out by Hopwood and Phillips (1938, 1939, 1940) in Britain, Rexer (1944) in Germany, Joliot (1940) in France, in which such typical monomers as methyl methacrylate and styrene were subjected to neutron, γ- or x-ray irradiation.

With the increasing availability of high intensity sources of radiation— nuclear reactors, radioactive isotopes and high-voltage accelerators— there has been a considerable growth of interest in the subject. Work is currently proceeding along several complementary routes: the investi-gation of reaction kinetics of radiation-induced polymerization, the study of reactions in the solid state or in emulsions and the search for new types of reaction.

FACTORS AFFECTING^ POLYMERIZATION RATE As far as the kinetics of the reaction are concerned the dependence on

radiation dose of the polymer formed is far more complicated than is, for example, that of crosslinking or degradation of polymers ; it involves such factors as the radiation intensity, the physical state of the monomer (which may be present as a solid liquid or gas, in solution or in an emul-sion), the solubility of both monomer and polymer, the degree of con-version, the temperature of irradiation. Various kinetic schemes have been proposed to account for the influence of these factors. Although these schemes often give a valuable insight into the most important factors influencing the reaction under given experimental conditions, they do not account quantitatively for the apparently anomalous behaviour observed in certain systems. Further information on reaction kinetics in such systems could be of considerable value, can not only for polymerization studies, but in other fields of radiation chemistry where the reactions of the radicals produced by radiation are influenced by their surroundings.

Much of the work on radiation induced polymerization is concerned with the irradiation of monomers either in the bulk form, or in solution. The behaviour of such systems is very markedly affected by the intensity of radiation. Most of the work has made use of radioactive cobalt sources, giving intensities of the order of 10—103 rads/min, when the rate of polymerization (or of disappearance of monomer) is often proportional to the root of the intensity (70*5). This intensity dependence is typical of many thermally-induced polymerizations, in which the dependence on catalyst concentration C varies as C 0 5 . Where departure from the 70'5

intensity dependence occurs, it is often possible to determine whether this also occurs with thermal catalysts, or whether it is specific to radiation-produced radicals.

Among the advantages of studying polymerization kinetics with radia-tion is the enormous range of intensities which can be covered, from a fraction of a rad per minute to one million or more rads per second.*

*A radiation intensity of 1 megarad/sec will release 1·04χ 10~3 G moles of radicals per sec, per 1000 g. Since the G value for radical production may often exceed 10, instantaneous radical concentrations can be achieved which are quite unobtainable by chemical methods.

POLYMERIZATION 371

This extensive range greatly facilitates comparison of alternative kinetic schemes, although it also discloses discrepancies not otherwise revealed. In the following sections we shall be mainly concerned with the more theoretical aspects which are directly relevant to the reactions in irradiated polymers.

KINETICS OF RADIATION POLYMERIZATION The study of radiation-induced polymerization has been largely confined

to vinyl monomers which polymerize by a free radical mechanism. The monomer has been irradiated either in the pure state, or in solvents ; other methods of polymerization have been studied in far less detail. The basic reactions are summarized in Table 22.1 ; certain of the relevant rate con-stants k can be compared with, or derived from, chemically catalysed systems.

The rate of formation of free radicals (Stage 1) is directly proportional to the radiation intensity /, and depends on the G value for radical forma-tion in the monomer and (where irradiation is carried out in solution) on the G value for the solvent. Both these factors are included in the para-meter kx in the table (Stage 1) thus

*i = fcr [Gm(M) + GS(S)] (22.1)

where kr is a constant depending only on the units in which the radiation intensity is expressed, and Gm, Gs are the G values for radical formation in monomer and solvent. In most cases the transfer reactions (7) and (8) are ignored as they do not modify the polymerization rate, though they do affect the molecular weight distribution and degree of branching.

The radicals formed by radiation can undergo several reactions ; they may initiate polymerization (Stage 2), they may terminate growing polymer chains (Stage 5) or they may combine with each other (Stage 6). It has not proved possible to produce a general solution of these simul-taneous differential equations, and in practice one assumes that one or other of these reactions predominates. Chapiro et al. (1955) have analysed the various alternatives on the assumption that steady state conditions are rapidly attained.

The concentration of primary radicals eventually reaches an equili-brium value, depending on their production by reaction (1) and possibly (7) and their elimination by reaction (2), (5) and (6). If the time to reach this equilibrium value is very protracted, the analysis may be further complicated.

In the crosslinking of polymers, the number of crosslinks produced depends only on the total dose absorbed, so that the rate of formation of crosslinks is directly proportional to the radiation intensity /. In poly-merization the rate of polymerization (or the rate of disappearance of monomers) can often be expressed in the form

BB

Tabl

e 22

.1.

Rea

ctio

ns

in R

adic

al

Pol

ymer

izat

ion

(R)

Prim

ary

or in

itiat

ing

radi

cal c

once

ntra

tion

(RM

) C

once

ntra

tion

of g

row

ing

poly

mer

rad

ical

(P-

) ch

ains

(M

) M

onom

er c

once

ntra

tion

/ R

adia

tion

inte

nsity

k 1

...

Rat

e co

nsta

nts

P

Stab

le p

olym

er

(S)

Solv

ent c

once

ntra

tion

(P)

Poly

mer

con

cent

ratio

n (e

xpre

ssed

in m

onom

er u

nits)

Stag

e

0)

Rad

ical

for

mat

ion

(2)

Chai

n in

itiat

ion

(3)

Chai

n gr

owth

(4)

Mut

ual

term

inat

ion

(5)

Term

inat

ion

by p

ri-m

ary

radi

cals

(6)

Com

bina

tion

of p

ri-m

ary

radi

cals

(7)

Tran

sfer

rea

ctio

n

<8)

Tran

sfer

rea

ctio

n

Solv

ent o

r m

onom

er

+ ra

diat

ion

-> ra

dica

ls

R+

M-+

RM

-

RM

n+M

->R

Mn+

i

RM

n+R

Mm->

RM

m+

nR

at

d(R

-)

d(R

M)

at

at

d(M

) =

kj

=

k 2(R

)(M

)

= k

P (

RM

) (M

) dt

d(

RM

)

d(R

') dt

RM

n+R

Mm

^RM

nH+

RM

m^

CH

=CH

2 -

RM

n'+

R'^

RM

nR

R+

R->

RR

RM

n +

M->

RM

nH+

M·

dt d(

RM

·) kt

(R

M)2

dt d(R

M-)

dt

= k

t' (R

M)2

Form

atio

n of

radi

cals

from

so

lven

ts or

mon

omer

A

ttack

of r

adic

al o

n m

ono-

mer

A

ttack

of g

row

ing

chai

n on

m

onom

er

Term

inat

ion

by

com

bina

-tio

n Te

rmin

atio

n by

dis

prop

or-

tiona

tion

d(M

-)

d(R

M)

dt

dt

k 5(R

M-)

(R

) A

dditi

on o

f rad

ical

to g

row

-in

g ch

ain

)2 Lo

ss o

f prim

ary

radi

cals

= k 7

(RM

')(M

) Tr

ansf

er t

o m

onom

er

d(R

M)

d(P

-)

RM

n+P

->R

MnH

+P

' -

dt

=

-^-J

= k

8 (R

M)

(P)

Tran

sfer

to

poly

mer


POLYMERIZATION 373

where the power n depends on the conditions under which the polymeri-zation takes place. Usually n is less than unity, so that in terms of yield for a given total dose, low intensities will be favoured. Moreover, the range over which this relationship holds will be far more limited than the extensive range over which crosslinking has been shown to be proportional to the intensity.

In several polymerization experiments, the rate of polymerization has been observed to vary approximately as / (n= 1). This has been interpreted in terms of the discrete nature of the ionization tracks produced by each primary high energy particle. If the intensity of the incident beam is sufficiently low, free radicals will not be distributed at random throughout the system, but will be grouped round these tracks, leaving gaps with no radicals. Each polymer chain, which must have a radiation-induced radical at each end, will draw both these radicals from the track of the same incident particle. Under these conditions the rate of polymerization will be proportional to the number of incident particles, and there will be no overlap or interference between them. It is doubtful whether suffi-ciently low intensities have been used in these experiments to show this track effect, especially when the rapid diffusion rate of the radicals and free electrons formed around each track is considered. It will therefore be assumed that the radicals produced by radiation are in fact distributed at random.

Polymerization under Steady State Conditions

At low intensities, the reaction of the primary radicals with monomer is far more probable than with growing chains, or with other radicals. Once steady state conditions apply, the polymerization will then proceed by stages (1), (2), (3) and (4) (Table 22.1) and termination of the growing polymer chains is uniquely by mutual termination. The appro-priate equations for steady state conditions are

dS^l= kJ-k2(R-)(M) = 0 (22.2) at

^ ^ = k,(R-)(M)-kt(RMY = 0, (22.3)

and the rate of polymerization

^ = - —■ = UR-){M) + kp(RM-)(M). (22.4)

The solution is

i£>_ *,, + */ /! ' ,„, (22.5) m


For high degrees of polymerization the first term can be ignored. Then replacing kj by its value as given above

-£- = j * (M)kr*[Gm{M) + GS(S)]* /*

-*W» (*'«)·(■+ «»>)· (22.6)

where the last factor is only included when the irradiation takes place in a solvent which under the influence of radiation can give rise to free radicals capable of initiating polymer growth.

100,

FIG. 22.1. Dose rate dependence for the polymerization of vinyl acetate, methyl methacrylate and styrene monomers.

dose rate dependence as /°·5.

+ Chapiro, A., / . Chim. Phys. 47, 747, 764, 1950. • Chapiro, A., and Wahl, P., C.R. Acad. Sei., Paris 238, 1803, 1954. O Landler, Y., Thesis, Paris, 1954. X Ballantine D. S., et al, BNL 141, 1951; BNL 229, 1953; Chem. Eng

Progr. 50(11), 267, 1954. ' *'

■ Nikitina, T. S., and Bagdasarian, Ch. S., Radiation Chemistry, p. 183, Acad. Sei. U.S.S.R., Moscow, 1955; Medvedev, S. S. , / . Chim. Phys. 52, 677, 1955.

♦ Chapiro, A., and Migirdicyan, E., / . Chim. Phys. 52, 439, 1955. Data for higher intensities has recently been given by Chapiro and Sebban-Danon, / . Chim. Phys. 54, 776, 1957.

In Fig. 22.1 the observed conversion rate of methyl methacrylate, of vinyl acetate and of styrene (at low conversion rates) are plotted as a function of the radiation intensity. On this log-log plot the observed slope agrees with the above theoretical relation up to intensities of 10M03 r/min beyond which it decreases. This decrease at higher inten-sities can be interpreted as arising from the high concentration of growing

POLYMERIZATION 375

chains such that a significant proportion of primary radicals react with them, rather than with the monomer to initiate further polymerization. In this range of intensities reaction (5) competes with reaction (4) and (2). At even higher intensities most of the growing polymer chains are terminated by primary radicals. The reactions then proceed by stages (1), (2), (3) and (5), leading to the following equations for the steady state:

d(R-) at

dQRM) at

= kJ-k2(R')(M)-k5(R')(RM·) = 0

= k2(R-)(M)-k5(RM)(R') = 0

with the approximate solution

d(-M) d(P) — ^ — = - ^ τ = kpkt{M)*lkt

(22.7)

(22.8)

(22.9)

which is independent of radiation intensity. Increases in the radiation intensity cause more polymer chains to grow, but these are of shorter length. In the extreme case most of the radicals will be eliminated by direct combination with each other (Stage 6) while the remainder will form short polymer chains RMnR by stages (1), (2), (3) and (5). This has been observed at the very high dose rates achievable with electron accelerators. The results of Majury (1955) obtained with a series of very intense electron pulses from a Capacitron are shown in Table 22.2; most of the polymerization may well be due to residual radicals remaining after irradiation.

Table 22.2. Polymerization of Methyl Methacrylate at High Intensities

Total dose (reps)

150 xlO8

1500 xlO3

150 xlO3

150 xlO3

15 xlO4

No. of pulses in dose

12 2 1

12 12 1 1

% polymerization after 4hr 40 hr

0-85 0-92 0-98 0-95 0-85 0-98 0-68

408 4-79 4-92 5-43 408 4-92 3 51

Non-steady Conditions

Conditions may arise which do not allow the assumption of steady state conditions of polymerization, as, for example, when the polymer formed is insoluble in its monomer or the solvent used, so that the growing chain is precipitated from solution and mutual chain termination (Stage 4) becomes difficult or indeed impossible. Such polymerizations may be characterized by a long induction period, and a considerable post-radiation


polymerization effect (since the radicals needed to terminate chain growth are no longer available). In these cases the observed dependence of polymerization rate on intensity is of the form

4-^»/Λ 0-5 <#ι<1. at

Dump and Magat (1955) have studied the relationships for non-steady conditions. Where mutual termination of growing chains is possible

100

i_ 10

1 10 100 1000 10 000 Radiation intensity, r/min

FIG. 22.2. Anomalous intensity effect. • Acrylonitrile. O Vinyl chloride. + Acrylonitrile polymerized in the solid state.

(Stages 1, 2, 3, 4) Magat (1955, 1956) derived an expression stated to account for the anomalous polymerization behaviour of vinyl chloride. In the case of acrylonitrile, termination is by the primary radicals only (Stages 1, 2, 3, 5) ; Dump and Magat (1955) deduced an expression showing that over a 16-fold range of intensities the conversion rate was proportional to the 0-8 power of the intensity.

Anomalous polymerization rates of vinyl chloride were observed by Mund, Van Meersche and Momigny (1953) who irradiated the monomer in the gas phase with a-particles. The yield was directly proportional to the intensity; this might be expected to result from the regions of high ionization intensity in alpha radiation or from chain termination on the walls of the vessel. Chapiro (1956) irradiated the liquid monomer with Cobalt 60 γ-rays, observing an acceleration of the reaction with time, and a post-radiation polymerization effect, characteristic of autocatalytic reactions. Similar results have also been observed when the monomer is polymerized by chemical methods. Chapiro suggested that the anomalous behaviour of vinyl chloride polymerization is in part due to the greater radiation sensitivity of the polymer, the radicals formed on it themselves contributing to further radical initiation. Schindler and Breitenbach (1954)

Λ

1

• 1 ! / //

A*£-SA J^/V

//*

1 i

J

*

*% —

POLYMERIZATION 377

found an intensity dependence of 7058, and Chapiro one of I059. In a homogeneous solution Tkatchenko et al. (1951) found a (initiation)05

dependence relationship, while Burnett and Wright (1954) observed a variable power in a tetrahydrofuran solution, depending on temperature. The abnormal behaviour of vinyl chloride polymerization is therefore not specific to radiation, and arises whenever polymerization takes place in a medium in which the polymer is not soluble.

Polyacrylonitrile is not soluble in its own monomer, although it is soluble in dimethyl formamide. In the latter case under the appropriate condition of concentration the dependence of polymerization rate on intensity (7055) is not very different (Prevot-Bernas, 1953) from that assuming stationary state conditions, and mutual chain termination (Stage 4). In most cases, however, the coefficient is considerably higher (I0'8 about) and marked post radiation effects are observed. Collinson and Dainton (1952) polymerized acrylonitrile with x-rays and γ-rays in D 2 0 solution, the results indicating that the D atoms liberated in the solution by radiation participate in the reaction. The opposite conclusion was reached by Fiquet and Bernas (1954), who irradiated solutions of acrylonitrile, methacrylonitrile and methyl methacrylate and were unable to detect the presence of D atoms in the polymer by means of infra-red analyses. The variation with x-ray or γ-ray intensity observed by Collinson and Dainton lay between 70·25 at high intensities and 70"95 at low intensities. Bernstein et al. (1953) obtained an intensity dependence of approximately I0'1 with gamma irradiation of the pure monomer. Using chemical catalysts in pure monomer Thomas and Pellon (1954) found a dependence on catalyst concentration (C) of between C 0 7 5 to C0 8 2 , while Prevot-Bernas and Sebban (1956) using gamma radiation obtained an intensity dependence as P 76 to I0'79. In a concentrated aqueous solution the latter found a dependence as 70·73, while in dilute solutions the variation was as 70"85. With the exception of irradiations carried out in dimethyl formamide solution, all these results indicate an intensity variation of about I0'8, over a thousand-fold range in intensity. The theory suggested by Durup and Magat (1955), assuming a non-stationary state and termination by primary radicals, gives the required intensity dependence but only over an intensity range of 1-16. Prevot-Bernas and Sebban-Danon (1956) were also unable to reconcile their experimental results with a theory of Thomas and Pellon (1954) involving a monomolecular occlusion mechanism of termination. It therefore appears that in polyacrylonitrile the termination step is retarded by poor solvent conditions giving a long lifetime to the growing radicals, but no theory has yet been able to explain all the experimental results quantitatively (Bamford, Barb and Jenkins, 1952).

Data on the polymerization rate after irradiation (post-effect) are given by Bensasson and Bernas (1957, 1958).

Gel Effect

At high degrees of conversion the polymerization reaction speeds up, and polymer molecules of rapidly increasing molecular weight are formed. This is due to the Trommsdorf effect which arises from the increasing


viscosity of the mixture, reducing the mobility of the growing polymer chains, and hence rendering less likely mutual termination of two such

^ '

^s*·' ^ ^

f 1

1

D

0 ~ 10 20 30 40 Exposure t ime, hr

FIG. 22.3. Gel effect in styrene irradiated at 72°C, 4000 r/min. (From Ballantine et al BNL 229, 1953.)

chains. The increase in polymerization rate will itself cause a rise in tem-perature due to the exotherm, and this in its turn may cause a further increase in reaction rate. In thermally initiated polymerization the number of radicals formed by the catalyst will also increase and the reaction may therefore "run away". With a radiation initiated reaction, no such danger exists, as the rate of radical production is controlled by the radiation intensity only. Fig. 22.4 shows the sudden increase in polymerization rate at high degree of conversion due to the increased immobilization of the growing chains and the corresponding temperature rise.

Other effects can occur when high degrees of conversion are obtained. Radiation gives rise to radicals on the polymer chains, which may either lead to the formation of a branched material, or in certain polymers, to crosslinking and the formation of an insoluble network. Thus Okamura et al. (1956) have reported an increasing polymer yield per unit dose, when vinyl acetate is irradiated. The average molecular weight, the viscosity and the Huggins constant (slope of the viscosity/concentration curve) all show increases, which the authors ascribe to increased branching even at low degrees of conversion.

Effect of Temperature

In thermally-initiated polymerizations, changes in temperature will affect the initiation, propagation and termination stages. Increases in temperature give rise to an increased number of primary radicals and may lead to a reduced average molecular weight. In radiation initiated radical polymerization, the number of radicals formed depends only on the radiation intensity and not appreciably on. temperature. (In ionic poly-merization the temperature dependence may be very different.) A rise in temperature will then increase the propagation rate and give rise to higher molecular weight polymers and larger conversion rates.* Table 22.3 shows this increase in molecular weight and conversion rate for methyl methacrylate and styrene. The activation energies of polymerization

* An increase in temperature may also increase the termination rate.

POLYMERIZATION 379

100

80

60

40

έΌ *ΛΤ '^

(α)

" χ

>

/ χ

3 6 9 12 15 18 21 24 Megarep

§100,

k 80

δ 60

Β 40 C

£ 20

^Γ '

(b) f

Megarep

36

30

24,

(c)

ϋ I

12 24 Megarep

36

8G

70

* - χ -

(d)

1 II

/ \ ^_

2 3 4 5 6 7 8 Megarep

FIG. 22.4. Gel effect in the polymerization of styrene. (a) Conversion of styrene at room temperature (0-19 megareps/hr). (b) Conversion of styrene at 72°C (0-245 megareps/hr). (c) Exotherm during polymerization at 27°C. (d) Exotherm during polymerization at 72°C.

(From Ballantine et al., 294, 1954.)

deduced by Ballantine et al. (1953, 1954) for gamma initiation (6-7 kcals/ mole for styrene, 4-25 kcals/mole for methyl methacrylate) are similar to the values (5-1, 5-3, 7-4 kcals/mole for styrene, and 4-7 kcals/mole for methyl methacrylate) previously obtained for photochemical initiation. In the latter case only excited molecules are formed and polymerization occurs by a free radical mechanism. The similarity in activation energies indicates that radiation induced polymerization is likewise free-radical in character in these monomers.

Table 22.3

Monomer

Styrene Styrene Styrene Methyl methacrylate Methyl methacrylate Methyl methacrylate

. Temperature Dependence of Polymerization

Temperature °C

- 1 8 ° C 25 70

- 1 8 ° C 25 70

Dose rate reps/min

4400 3100 3100 4400 4100 4100

Conversion rate % per Mrep

0-36 2-2

10-4 10 32-4 84

Molecular weight*

1 2 x l 0 3

42X103

200 x l O 3

70-150 x lO 3

160 x 1 0 s

250-360 x lO 3

(From Ballantine et al, 1954.)

* Excluding high conversion values due to Trommsdorf effect.


Tn a later paper (Ballantine et al., 1956) the variation with temperature of the G value for radical initiation is studied in the polymerization of styrene and of methyl methacrylate. The data used were those previously given by Ballantine et al. (1954), and showed an 70'5 intensity dependence up to a dose rate of at least 5000r/min, whereas the earlier data by Chapiro, Magat et al. (1955) (Fig. 22.1) indicated a failure of the 70 5

intensity dependence beyond about 1000 r/min. The rate of initiation d(R')/dt (measured in units of moles/1, sec) was calculated from the overall rate of polymerization by the relation deduced by Tobolsky et al. (1952, 1953, 1955).

d(R-)ldt = 2A' RP2I[M]2

where RP is the measured overall rate of polymerization and A'=5-68 x 10~6

exp [12-46 kcal/RT] for styrene or 502 x 10~5 exp [9-35 kcal/RT] for methyl methacrylate. Within experimental error the G values for radical production and for subsequent initiation of polymerization obtained in this way did not vary significantly with temperature over the range -18°C to+72°C (Table 22.4).

Comparable G values deduced for ß-radiation by Seitzer and Tobolsky (1955) and for gamma radiation by Prevot-Bernas et al. (1952) are also shown in the table. Seitzer and Tobolsky suggest that the different G values for each monomer arises from the difference between radical initiation as between beta and gamma radiation, but it may in fact be due to the different assumptions made in deriving G values from the initial data, or in the measurement of the radiation dose.

POLYMERIZATION IN A SOLVENT When irradiation of a monomer is carried out in the presence of a

solvent, the radicals produced in the latter may initiate further polymeri-zation. The appropriate factor is given in the expression for the poly-merization rate (Equation 22.1). The G value for radical formation in the solvent is involved in this factor, and from observed polymerization rates in solution, G values for radical formation have been deduced (see Chapter 27). However, owing to protection and energy transfer reactions (Chapter 29) the simple addition of the appropriate G values for each of the constituents is not always justified. Chain transfer reactions must also be considered. When the monomer is diluted in a relatively inert solvent (Gs < Gm) such as toluene, the monomer concentration is lowered, Stage 2 in Table 22.1 is rendered less likely and the maximum intensity at which the 70'5 law still applies is decreased. When Gs is much greater than Gw irradiation in solution may greatly increase the yield.

The course of the reaction will be affected by the solubility of the polymer which may be very different from that of the monomer. This is the case for styrene and methyl methacrylate in some solvents ; the growing polymer chains retract within themselves, thereby greatly reducing the chance of mutual termination. The chains will therefore grow by the diffusion of monomers until a free radical is able to terminate the chain. A similar state of affairs may arise in polymerization within a solid.

Tabl

e 22

.4.

G V

alue

s for

Pol

ymer

izat

ion

of S

tyre

ne

and

Met

hyl

Met

hacr

ylat

e

Mon

omer

Styr

ene

Met

hyl

met

hacr

ylat

e

Rad

iatio

n

T Y

Ύ

P Y Y Y Y ß Y

Tem

p. °

C

72

25

-18 30

-5

15

70

25

-18 30

-5

15

Inte

nsity

r/

min

4100

32

00

4400

4100

42

00

4400

Poly

mer

izat

ion

rate

m

/1 s

ecxl

O5

5-84

10

1 0-

24

49-9

21

1 7-

25

Rat

e of

in

itiat

ion

m/1

sec

x 10

8

4-40

20

8 2-

74

28-4

36

0 53

-8

G*

prim

ary

radi

cals

0-78

0-

46

0-56

4-88

5-

74

7-68

et

0-22

314

Gl

0-97

16-7

* G

val

ues

from

Bal

lant

ine

et a

l. (1

956)

, bas

ed o

n po

lym

eriz

atio

n da

ta g

iven

in

this

tab

le.

t G

val

ues

from

W.

H. S

eitz

er a

nd A

. V. T

obol

sky

(195

5).

t G

val

ues

from

Pre

vot-B

erna

s et

al.

(195

2), c

orre

cted

to

a G

val

ue (

ferr

ous

sulp

hate

) of

15-

5.

P O L Y M E R I Z A T I O N 381


Ballantine etal. (1954) studied the polymerization of 7V-vinylpyrollidone by gamma radiation under a variety of conditions. The polymer is used as a blood plasma substitute and can be polymerized chemically or by light. However, for its use as a blood plasma substitute, it was thought that a more rigorous control on molecular weight distribution, and on purity, could be achieved by the use of radiation. Among the variables studied were temperature, radiation intensity, type and concentration of solvent. The results shown in the following table indicate that radicals produced in certain solvents, such as water, can initiate polymerization, whereas those formed in alcohol are less likely to do so. This reduction may, however, also be due to interaction of the growing chain with the solvent, reducing the molecular weight.

Table 22.5. Polymerization Rate ofN-vinylpyrollidone

Solvent

25%MeOH 1 75%H20 / 50%MeOH 1 50%H2O / 75%MeOH 1 25%H20 /

100% MeOH 100%EtOH 100%/^PropylOH 100%/é?r/.-ButylOH

Intensity r/min

9000

9000

9000 7500 7500 6000 6000

Monomer con-centration %

5

5

5 5-30 5-30 5-30 5-30

Rate of polymerization %/min

5-5-60

3-3-3-6

1-67-1-73 0-5-0-8 0-7-0-95 0-6-0-85

0-95-1-1

The low yield obtained in the polymerization of ethylene has been ascribed to a low initiation rate, rather than a low efficiency of chain propagation (Medvedev, 1957). Higher yields of the polymerization reaction can therefore be expected when the reaction takes place in solution. Overall G values obtained with ethylene under pressure were increased by a factor of about 20 in the presence of methyl alcohol, cyc/ohexane or heptane. Further experiments were also carried out on the emulsion polymerization of ethylene.

The rate of initiation can be further increased by the use of benzoyl peroxide (Krongauz and Bagdasarian, 1958), which, in the presence of benzene decomposes very readily due to an energy transfer process from the excited benzene molecule. With cyc/öhexane and ethyl acetate, however, no such process occurs, decomposition of the benzoyl peroxide being due to radicals formed in the solvent.

POLYMERIZATION IN THE SOLID STATE

When monomers are irradiated in the solid state, polymerization may either take place in the solid, or trapped radicals may be formed which react subsequently, when the specimen is dissolved or heated near its

POLYMERIZATION 383

melting point. In the latter case the effect of the trapped radicals on the subsequent polymerization reaction is equivalent to giving the liquid monomer an equivalent dose at a much higher intensity sufficient to produce the same high radical concentration instanteously. In this case

"£*v° 3

Solidification

\ O 1 >v 1

\

\ \

0 -20 - 4 0 - 6 0 - 8 0 -100 Temperature, C

FIG. 22.5. Effect of physical state on polymerization of methyl acrylate. 2-5 megareps of electron radiation.

(From: Schmitz and Lawton, 1951.)

most of the radicals will combine with one another, and low degrees of polymerization are to be expected.

Schmitz and Lawton (1951) subjected a vinyl monomer (tetraethylene-glycol dimethacrylate) to electron radiation at various temperatures, and followed the reaction by the exotherm during and after cessation of radiation. The yield increased with dose, and decreased with increasing intensity at a given dose, as is to be expected for a vinyl polymerization reaction (Fig. 22.5). As the temperature was reduced the extent of the reaction (as measured by the exotherm) diminished, and ceased at about — 50°C to — 55°C, the temperature at which the material sets to a glass. Below this temperature the radicals formed were trapped, but subsequently reacted to cause polymerization as soon as the specimen was warmed to a semi-fluid state. Majury (1955) was also able to retain trapped radicals in irradiated frozen methyl methacrylate, which subsequently reacted on heating. Burlart and Adicoff (1958) have reported a discontinuity in the polymerization rate of vinyl stéarate near the monomer melting point.

The polymerization of acrylamide in the solid state has been subjected to a number of investigations (Mesrobian et al, 1954; Ballantine,1954; Henglein and Schulz, 1954, 1955). The monomer has a melting point of 84°C, but gamma irradiation at about 3500 r/min, and at temperatures of 5°C, 35°C and 55°C gave high degrees of conversion for doses of the order of 10 5 -5xl0 6 rads (Mesrobian-ef aL, 1954). At the same time crystallinity was partially destroyed, to yield amorphous polymer. The continued existence of crystalline regions in a partly polymerized specimen is somewhat surprising. Further work by Restaino et al. (1956) showed


that except possibly at the highest intensity used, neither the molecular weight of the product, nor the rate of conversion per megarad, were affected by radiation intensity at low conversions (Table 22.6). The reaction rate was unaffected by the presence of oxygen, but was increased slightly by the presence of glass (possibly due to the extra electrons emitted) and to a greater extent by the presence of a small amount of water, or by an increase in temperature, the overall activation energy being 4-7 kcal/mole.

Table 22.6. Effect of Temperature and Intensity on Solid State Polymerization of Acrylamide

(Cobalt 60 gamma radiation)

Intensity rads/min

5000 5000 5000 5000 5000

50 500

3500 4800 4900

14000

Temperature °C

0 20 35 50 65

35 35 35 35 35 35

Initial conversion

%/hr

11 1-9 2-4 3-6 5-3

0-3 3-5

151 220 25-5 42-1

M of polymer

. 1-73 1-73 1-71 1-75

1-56 1-73 1-68 1-74 1-77 1-68

Fraction converted

per megarad

— — — —

0-92 107 0-69 071 081 0-47

It is difficult to reconcile the observed polymerization characteristics (such as the lack of a dose rate effect on molecular weight and conversion rate) with existing theories of polymerization. Restaino et al. (1956) suggests that polymerization proceeds along the tracks of the primary ionizing particles, although at the doses used the distributions of ions might well be expected to be random. Ballantine et al. (1954) and Schulz and Henglein (1955) suggested that polymerization occurs preferentially along certain crystallographic directions. This is in agreement with the proposal of Stannett and Szwarc (1953), who emphasized the importance of spatial orientation.

Other polymers which have been polymerized in the solid state are shown in Table 22.7. The high polymerization rates of acrylic acid and of its metallic salts are noteworthy. In the case of vinyl stéarate both the conversion rate and the average molecular weight are higher when irradiations are carried out in the liquid state.

Lawton, Grubb and Balwit (1956) compared the polymerization rate of an organosilicon monomer (hexamethyk^c/ötrisiloxane) below and above the melting point. A high intensity electron beam giving from about 106

to 107 r/min was used, and an intensity effect was observed. Although

POLYMERIZATION 385

Table 22.7. Polymerization of Monomers in the Solid State

Monomer

Aery 1 amide A W methylene-

&/sacrylamide Methacrylamide Acrylic acid Methacrylic acid Acrylate calcium Acrylate potassium Acrylate barium Vinyl stéarate Vinyl carbazole Maleic Anhydride Allyl amine

hydrochloride Allylamine picrate Stilbene

Temperature °C

30

30 30

- 1 8 - 1 8

35 35 35 26 30 30

30 30 30

Intensity rep/hr

190 x lO 3

190 x10 s

190 x lO 3

3 1 0 x l 0 3

310 x lO 3

190 x lO 3

190 x 1 0 s

190 x 1 0 s

190 x 1 0 s

220 x10 3

220 x10 s

220 x10 s

220 x10 3

220 x10 s

Initial conversion

% per hr

12-8

1-6 0-7

> 4 0 0 >400 181 56-8 75-1 200 1 1 * Of

0Î 0Î 0Î

Conversion per megarep

/o

67-5

8-4 3-7

>1300 >1300

95 300 395 105 5

* Induction period of 20 hr. t No polymerization up to 100 hr (>200 megarads). Î No polymerization up to 25 hr ( > 50 megarads).

(From Restaino et al. 1954, 1956.)

this may be taken as an indication of a free radical mechanism, it proved impossible to initiate the reaction with a chemical free radical initiator. Furthermore, the polymerization reaction proceeded rapidly below the melting point, but fell to negligible values above it.

The exotherm measured during irradiation in the solid state is strong evidence that the reaction takes place in the solid, and not on subsequent heating or during dissolution. Polymerization in the solid state appears to obey different kinetics to that in liquids and its study is likely to lead to useful information on the behaviour of molecules in solids.

EMULSION POLYMERIZATION Ballantine et al. (1954) compared the polymerization of styrene in an

emulsion, with bulk polymerization of the monomer. In spite of a reduced radiation intensity, the emulsion polymerized about 100 times as rapidly as the pure monomer, and the molecular weight was also increased. The increased efficiency may be partly accounted for by the additional radicals formed in the water, which can then initiate styrene polymerization (this is an illustration of the indirect effect discussed in Chapter 25) and partly by the higher G value for the production of radicals in water. Moreover, the reduction in termination rate increased the molecular weight, a larger number of monomers being polymerized per initiating radical. The activation energy of the reaction in the emulsion was found to be 3-7 kcal/mole.


Table 22.8. Bulk and Emulsion Polymerization of Styrene

Method

Bulk Bulk Bulk Emulsion Emulsion Emulsion

Intensity r/min

4400 3200 4100 1000 1000 1000

Temperature °C

- 1 8 30 72 25 35 45

Conversion rate % per hour

01 0-42 2-53

36 45 54

Mol. weight of polymer (x 103)

17-26 38-110

165-348 800-1500 885-1726

1200-2060 (From Ballantine et al, B.N.L. 294, 1954.)

POLYMER YIELD Unlike the situation for crosslinking or degradation, the yield of polymer

formed by radiation cannot be expressed even approximately in terms of moles per megarad of radiation. The yield depends on a variety of factors, some of which have been discussed above. As an indication of what has been achieved in practice, the following data are given for rough com-parative purposes only.

The relative susceptibility of various monomers to polymerization by γ-rays is shown in Table 22.9, based on a paper by Chapiro (1957). These values could be altered considerably by changing the experimental con-dition such as temperature or dose rate.

Callinan (1954, 1956) has subjected a series of monomers to gamma radiation of about 8000 r/min, and studied the properties of the polymers obtained. Equivalent G values for the overall reaction have been calculated (Table 22.10). These may be readily altered by conditions of polymeriza-tion, and may in any case be underestimates as the total dose was often in excess of that needed for full conversion. Callinan also produced a number of copolymers, the dose needed being intermediate between that required for the polymerization of each constituent.

Results on the polymerization of styrene and methyl methacrylate (Ballantine, Colombo, Glines and Manowitz, 1954) are given in Fig. 22.1. In their work on the polymerization of vinyl pyrollidone, the same authors obtained conversion rates of some 10 per cent for a dose of 0-1 megarads, and higher yields when the monomer was irradiated in solution (in which case more radiation energy is absorbed by the solution, per gram of monomer treated).

A considerable amount of effort has been devoted to the radiation-induced polymerization of ethylene on an industrial scale. The products obtained often compare favourably with those obtained by the earlier high-pressure technique; the lower pressure and temperature conditions favour the formation of a more linear polymer, allowing a higher degree of crystallinity and as a result a higher melting point and improved mechanical properties. Recent catalytic techniques (Ziegler, Phillips and Natta) allow somewhat similar material to be produced without the use of radiation but difficulties may arise in removing the catalyst.

POLYMERIZATION 387

The yields obtained vary very considerably with experimental conditions. G values of 1000 or more have been obtained, and with certain monomers conversions of several hundred per cent per megarad are already obtainable. Costs of polymerization are therefore comparable with those based on chemical methods.

Table 22.9. Polymerization of Monomers 1000 r/min gamma radiation 20°C

Butadiene* Styrene Methyl methacrylate Acrylamide (solid) Acrylonitrile Vinyl chloride Methyl acrylate* Vinyl acetate

% conversion per hour

001 0-2 4 6 9-5

15 18 27

%per megarad

0-2 3

67 100 160 250 300 450

* Extrapolated from low intensities. The per cent conversion figures relate to low degrees of conversion only.

Table 22.11. Polymerization of Perfluorinated Monomers

Monomer

Perfluoropropylene

Perfluorobutadiene

Perfluoroacrylonitrile

Perfluoro /sobutene Perfluoroamylpropylene

1: 1-Dihydroperfluoro-butyl acrylate

Dosage (Mrep)

208-7

225-6

196-4

220 231

220

Per cent poly-

merized

30

55

20

25-30 15-20

100

Yield in lb per kWh

1-5

2-5

1

1-3 0-8

4-5

Properties

Trimer or tetramer boilingatl75°-180°C terminal and internal unsaturation.

Low m o l e c u l a r weight, unsaturated, largely dimer, melts at 115°-125°C.

Low m o l e c u l a r weight softening at 90°-100°C.

Viscous liquid. Liquid boiling at 170°-

200°C at 2-3 mm, with no unsatura-tion, remainder un-distillable.

Highly crosslinked rubber softens and degrades at 150°C.

(From Ballantine et al.t 1954.) CC

Tabl

e 22

.10.

Pro

pert

ies

of P

olym

ers

Pro

duce

d by

R

adia

tion

Mon

omer

Ethy

lene

(at

1200

lb/ in

2 )

Met

hyl

met

hacr

ylat

e M

ethy

l acr

ylat

e

Styr

ene

Met

hyl s

tyre

ne

Vin

yl p

yrol

'idon

e A

cryl

onitr

ile

Dos

e in

m

egar

ads

36

15

10

24

11

38 2-2 1-2

Yie

ld (

%)

12 5

42

10

0 10

0

100

100

100

100

Equi

vale

nt G

—.

1000

20

00

600

1000

250

6000

40

000

Prop

ertie

s

Hig

her

mel

ting

poin

t th

an l

ow d

ensit

y po

lyet

hyle

ne,

due

to

less

bran

chin

g.

Hig

her

heat

dis

torti

on te

mpe

ratu

re th

an c

onve

ntio

nal

poly

mer

. H

ighe

r de

nsity

tha

n co

nven

tiona

l po

lym

er;

cros

slink

ed a

nd

rubb

erlik

e.

Hig

her

dist

ortio

n te

mpe

ratu

re

than

co

nven

tiona

l po

lym

er;

tinte

d by

radi

atio

n, f

aded

afte

r he

atin

g.

Den

ser,

high

er

dist

ortio

n te

mpe

ratu

re

than

co

nven

tiona

l po

lym

er.

Den

ser

than

con

vent

iona

l po

lym

er.

(Fro

m C

allin

an, 1

954,

195

5, 1

956.)


POLYMERIZATION 389

Polymerization of Fluorocarbons

Attempts to polymerize perfluorinated monomers other than perfluor-ethylene using thermal means of catalysis have so far proved unsuccessful. Ballantine et al. (1954) subjected a number of monomers to high doses of gamma radiation, but were unable to obtain useful yields (Table 22.11). The products were of low molecular weight and did not have the desired properties. In view of the great sensitivity of PTFE to radiation, it must be assumed that for these monomers the propagation rate is very low. The extremely high doses used indicate the possibility that ionization has occurred in each monomer unit, and that no chain reaction is involved. The low yield (expressed in pounds of product per kWh of energy absorbed) is indicative of the low efficiency of the process, at least under the experi-mental conditions adopted.

IONIC POLYMERIZATION

Until recently it was widely felt that, although high energy radiation gives rise to ions in large number, all radiation-induced polymerizations proceed via a radical mechanism. Strong evidence for radiation-induced ionic polymerization has now been obtained (Davison et ai, 1957). Although the reaction has been found to occur in a number of monomers, it has only been studied in detail in the case of /sobutene.* As for ionic reactions the yield is greatly increased at low temperatures, and moreover is approximately independent of radiation intensities over a range of from 105 to 1, thus showing that the termination of the growing chains is not due to radiation-induced radicals (Table 22.12). However, the conversion rate (per cent per megarad) is not proportional to the radiation dose and compounds produced by radiation may be partially responsible for limiting the length of the polymerization chain. Other additives can also affect the overall G value for the reaction (Table 22.13). The G value for initiation cannot be readily deduced from the overall G value and the degree of polymerization, since the average molecular weight of the poly-mer formed is itself subsequently reduced by radiation-induced scission, but at low doses, when this factor is less important, the G values for initiation appear to be small.

Further work involving catalysts has shown that the overall yield can be greatly enhanced by incorporating some inorganic compounds in the form of powder (Charlesby and Worrall, 1958). Table 22.14 shows the effect of various additives both on the overall G value for conversion, and on the viscosity average molecular weight. From the latter the number average degree of polymerization ux was derived (equation 8.21), and from G (overall) and uu the G value for initiation deduced. This value is a maximum, since it does not allow for radiation-induced chain fracture, or chain transfer reactions during polymerization. Table 22.15 shows the effect of additive concentration, of type and intensity of radiation, while

* Burlant and Green (1958) failed to discover an ionic contribution in the poly-merization of styrene -methyl methacrylate and other monomers, but obtained some evidence for its existence in isoprene.


Table 22.16 shows that at low doses, when radiation-induced fracture and inhibition are small, the yield is proportional to dose, and the G value for initiation is constant. The most interesting feature of this work is the increase in yield obtained in the presence of simple inorganic compounds, which would not be expected to have such a marked effect on radiation-induced reactions of organic compounds. The results shown in Tables 22.15 and 22.16 would indicate that the increased response is due to a higher G value for initiation, rather than a reduced termination rate (although the possibility of chain transfer reactions cannot be entirely discarded). The lack of any significant dependence on radiation intensity or type of radiation is worthy of note. Polyethylene powder which might be expected to increase the yield for radical reactions, and give rise to graft polymers, is found to act as an inhibitor for the reaction.

It may be surmised that the reaction proceeds on the surface of the inorganic compound, which is present in the form of a fine powder. The ability to increase the yield of radiation-induced organic reactions by the use of inorganic catalysts is a feature of considerable interest; there is as yet no reason to believe that such reactions are confined to ionic poly-merization reactions.

Table 22.12. Effect of Radiation Intensity on the Polymerization ofisoButene at-7$-5°C

Dose (megarads)

1 5

10 20

Percentage conversion

gamma electron

4-4 4-0 111 13-5 17-8 20-2 241 270

Conversion rate (percentage/megarad) gamma electron

3-1 3-5 1-4 1-6 0-8 10 0-5 0-5

Mvxl0~3

gamma electron

610 660 145 178 77 99 41 56

Y-Radiation from Co60 at 103 rads/min. Electron radiation (2 MeV) at 108 rads/min.

Table 22.13. Effect of Additives on the Polymerization of isoButene

(5 megarads, 2 MeV electron radiation)

Additive

Compound

Nil Oxygen Benzoquinone Di/sobutene Carbon tetrachloride Propane

/o

010 0-54 1-23

34-4 870

Conversion %

13-5 0-7 0-8 0-3

21-7 11-9

Mvxl0"3

178 — — —

181 76

(From Davison, Pinner and Worrall, 1957.)

POLYMERIZATION 391

Table 22.14. y-Itradiation ofisoButene in the Presence of Solid Additives

Radiation intensity 930 rads/min Dose 1 megarad Temperature -78-5°C

Additive (percentage weight

fraction)

Nil 0 Calcium oxide 61-8 Magnesium oxide 47-9 Aerosil 1-9

5-7 Sodium bicarbonate 57-3 Carbon black 32-4 Polyethylene powder 30-8 Zinc oxide*

Percentage of

isobutene converted

5-3 72-7 401 12-9 21-1 21-6

4-8 0-7

MrXlO- 3

513 303 469 687 923 662 456 small

G value (overall) for conversion (a) (b)

928 928 4750 8040 3570 6850 2160 2200 4290 4540 1560 3650 551 815

85 70

G value for initiation

(maximum) (a) (b)

019 0 1 9 1-6 6 1 0-8 1.6 0-32 0-32 0-48 0-51 0-24 0-56 013 0 1 9

* See Table 22.15, 22.16. (a) Based on total energy absorbed in mixture. (b) Based on energy absorbed directly in zsobutene.

(From Worral and Charlesby, 1958.)

Table 22.15. Effect of Additive Concentration on Polymerization of isoButene

Additive ZnO Dose 1 megarad Temperature — 78-5°C.

2 MeV pulsed electron radiation; maximum intensity about 108 rads/min at 400 pulses/sec.

1*3 MeV γ-radiation; uniform at 930 rads/min.

Additive concentration (percentage

weight fraction)

0 14-4 231 28-1 36-2 45-7

0 13-5 24-8 410 530

Radiation

Electron Electron Electron Electron Electron Electron

Gamma Gamma Gamma Gamma Gamma

/soButene converted

(percentage of total

/sobutene)

4-9 20-6 43-2 45-9 53-8 59-9

5-3 28-9 55-4 95-9 97-9

MvXlO-5

541 538 402 399 342 342

513 297 404 388 297

G value (overall) for conversion

(a) (b)

830 830 3010 3510 5680 7400 5640 7830 5860 9170 5670 11400

928 928 4270 4940 7120 12900 9600 16300 7930 16900

G value for initiation (maximum)

(a) (b)

016 0 1 6 0-56 0-65 1-5 1-9 1-5 2 1 1-8 2-8 1-7 3 1

019 019 1-5 1-7 1-8 2-4 2-6 4-4 2-8 6 0

(a) Based on total energy absorbed in mixture. (b) Based on energy absorbed directly in wobutene.


Table 22.16. Effect of Dose on Polymerization in the Presence of Zinc Oxide (41 per cent). -^-Radiation at 930 rads/min.

Temperature—78-5°C

Dose (megarads)

0 1 0-37 0-54 0-80 100

Percentage of /sobutene converted

12-7 34-6 59-2 86-6 95-9

G value (overall) for conversion

12700 9430

11100 10800 9600

M v x l 0 - 3

541 390 404 407 388

G value for initiation (maximum)

2-4 2-5 2-9 2-8 2-6

R E F E R E N C E S ANDERSON, L. C , BROWNELL, L. E., LEWIS, J. G., MARTIN, J. J., et al. (Univer-

sity of Michigan, Ann Arbor), C0086, 90, 91, 124, 196, 198; AECU 2889, 2981; Chem. Engng. Progr. 50(5), 249, 1954; Nucl. Engng. 62, 1954; Inter-national Conference on the Peaceful Uses of Atomic Energy, Geneva 15, 235, 1955; Mod. Plast. 32(7), 94, 1955.

BAGDASARIAN, Ch. S. and NIKITINA, T. S., Collected Papers on Radiation Chemistry, p. 183, Academy of Sciences, U.S.S.R., Moscow, 1955.

BALLANTINE, D. S., Mod. Plast. 32(3), 131, 1954. BALLANTINE, D. S. and MANOWITZ, B., B.N.L. 229, 1953 (Styrene, methyl metha-

crylate); B.N.L. 294, 1954 (Emulsion polymers, N vinylpyrollidone, fluoro-carbons, solid state polymerization); B.N.L. 317, 1954 (A/-vinyl pyrollidone).

BALLANTINE, D. S., COLOMBO, P., GLINES, A. and MANOWITZ, B. (Polymerization of styrene and methyl methacrylate), Chem. Engng. Progr. Symp. 50(11), 267, 1954.

BALLANTINE, D. S., GLINES, A., METZ, D. J., BEHR, J., MESROBIAN, R. B. and RESTAINO, A. J., / . Polymer Sei. 19, 219, 1956.

BAMFORD, C. H., BARB, W. G. and JENKINS, A. D., Nature, Lond. 169, 1044, 1952.

BAMFORD, C. H. and JENKINS, A. D., Proc. Roy. Soc. A216, 515, 1953; AA 228, 220, 1955.

BENGOUGH, W. I. and NORRISH, R. J. W., Proc. Roy. Soc. A200, 301, 1950. BENSASSON, R. and PREVOT-BERNAS, A., / . Chim. Phys. 53, 93, 1956; 54, 479,

1957; J. Polymer Sei. 30, 163, 1958 BERNSTEIN, I. A., FARMER, E. C , ROTHSCHILD, W. G. and SPALDING, F. F., / .

Chem. Phys. 21, 1303, 1953. BOUBY, L., CHAPIRO, A., MAG AT, M., et al., International Conference on the

Peaceful Uses of Atomic Energy, Geneva, 1955, 7, 526. BRASCH, A., HUBER, W. and WABY, A., Science, 1955; Nucleonics, 1955. BRETTON, R. H. and HAYWARD, J. C , et al. (Yale University), Chem. Engng.

Progr. 50(13), 73, 1954; NYO 3309, 1952; NYO 3310, 1952; N Y 0 3311, 1952; NYO 3312, 1953; NYO 3313, 1955.

BURLANT, W. and ADICOFF, A., J. Polymer Sei. 27, 269, 1958. BURLANT, W. and GREEN, D. M., J. Polymer Sei. 31, 227, 1958. BURNETT, G. M. and WRIGHT, W. W., Proc. Roy. Soc. A221, 28, 1954. CALLINAN, T. D., ONR Symposium Report, ACR 2 Washington 1954; Elect.

Engng., N.Y., 74, 510, 1955; J. Electrochem. Soc. 103(5), 292, 1956.

POLYMERIZATION 393

CHAPIRO, A., C.R. Acad. Sei., Paris 228, 1490, 1949; 229, 827, 1949; / . Chim. Phys. 47, 747, 764, 1950; 53, 512, 1956; Industrie des Modernes Plastiques, 8(9), 67, November 1956 (includes a list of references).

CHAPIRO, A. and MAGAT, M., Actions Chemiques et Biologiques des Radiations 3, 65, 1958.

CHAPIRO, A., MAGAT, M., PREVOT-BERNAS, A. and SEBBAN, J., J. Chim. Phys. 52, 689, 1955.

CHAPIRO, A., MAGAT, M., SEBBAN, J. and WAHL, P., Simposio Inter, di Chim.

Nfacrom., Milan 1954; La Ricerca Scientifica, Interscience, p. 73, 1955. CHAPIRO, A. and MIGIRDICYAN, E., / . Chim. Phys. 52, 439, 1955. CHAPIRO, A., SEBBAN-DANON, J., J. Chem. Phys. 54, 276, 1957. CHAPIRO, A. and WAHL, P., C.R. Acad. Sei., Paris 238, 1803, 1954. CHARLESBY, A., PINNER, S. H. and WORRALL, R. (in press). COLICHMAN, E. L. and FISH, R. F., Nucleonics 15(10), 134, 1957. COLLINSON, E. and DAINTON, F. S., Disc. Faraday Soc. 12, 212, 1952. COLLINSON, E. and SWALLOW, A. J., Chem. Rev. 56(3), 471, 1956. COOLIDGE, W. D., Science 62, 441, 1925. DAINTON, F. S., / . Chim. Phys. 48, 182, 1951. DAINTON, F. S., and COLLINSON, E., Ann. Rev. Phys. Chem. 2, 99, 1951. D A VISON, W. H. T., PINNER, S. H. and WORRALL, R., Chem. & Ind. (Rev.), 1274,

1957. DRIGO, A. and DE MARCO, L., La Ricerca Scientifica, 27(3), 721, 1957. DURUP, J. and MAGAT, M., J. Polymer Sei. 18, 586, 1955. EIDUS, Y. T. and PUZITSKII, K. V., Usp. Khim. 22, 838, 1953. FIQUET, F. and BERNAS, A., J. Chim. Phys. 51, 47, 1954. GARRISON, W. M. J., Chem. Phys. 15, 78, 1947. HEISING, G. B., J. Amer. Chem. Soc. 54, 2328, 1932; / . Phys. Chem. 39, 1067,

1935; 43, 1207, 1939. HENGLEIN, A. and SCHULZ, R., Z. Naturf. 9b, 617, 1954. H O P WOOD, F. L., Brit. J. Radiol. 13, 221, 1940. HOPWOOD, F. L. and PHILLIPS, J. T., Proc. Phys. Soc. 50, 438, 1938; Nature,

Lond. 143, 640, 1939. JOHNSON, D. H. and TOBOLSKY, A. V., / . Amer. Chem. Soc. 74, 938, 1952. JOLIOT, F., B.F., Patent No. 451, 131, 1939, 966760, 1940. KRONGAUZ, V. A. and BAGDASSARIAN, T. S. (in press). LANDLER, Y., Bull. Soc. Chim. 4, 543, 1956. LAWTON, E. J. GRUBB, W. T. and BALWIT, J. S., / . Polymer Sei. 19, 455, 1956. LIND, S. C. and BARDWELL, D. C, Science 60, 364, 1924; 62, 422, 593, 1925. LIND, S. C , BARDWELL, D. C. and PERRY, J. H., J. Amer. Chem. Soc. 48, 1556,

1926. LINDSEY, H., BROWN, D. E. and PLETCHER, D. W., Bull. Amer. Phys. Soc. 29,

14, 1954. LOEWE, S., Science 114, 555, 1951. MAGAT, M., Bull. Soc. Chim. 4, 535, 1956; / . Polymer Sei. 16, 491, 1955, 19,

583, 1956 MAJURY, T. G., J. Polymer Sei. 15, 297, 1955. MEDVEDEV, S. S., / . Chim. Phys. 52, 677, 1955 ; Radioisotopes in Scientific Research

1, 161, Pergamon, 1958. MESROBIAN, R. B., ANDER, P., BALLANTINE, D. S. and DIENES, G. J., / . Chem.

Phys. 22, 565, 1954. M U N D , W. and KOCH, W., Bull. Soc. Chim. Belg. 34, 241, 1925; / . Phys. Chem.

30, 289, 1926.


M U N D , W., VAN MEERSSCHE, M. and MOMIGNY, J., Bull. Soc. Chim. Belg. 62,109, 1953.

NIKITINA, T. S. and BAGDASARIAN, Ch. S., Zh. Fiz Chim., U.S.S.R. (in press). OKAMURA, S., YAMASHITA, T. and HIGASHIMURA, T., Bull. Chem. Soc. Japan

29(6), 647, 1956. PREVOT, A., C.R. Acad. ScL, Paris 230, 288, 1950; 233, 366, 1951. PREVOT-BERNAS, A., C.R. Acad. Sci.9 Paris 237, 1686, 1953. PREVOT-BERNAS, A. and SEBBAN-DANON, J., / . Chim. Phys. 53, 418, 1956. PREVOT-BERNAS, A., et al. Disc. Faraday Soc. 12, 98, 130, 1952. RESTAINO, A. J., MESROBIAN, R. B. BALLANTINE, D. S. and DIENES, G. J.,

Simposio Inter, di Chim. Macrom., Milan, 1954; La Ricerca Scient ifica, Interscience, 1955.

RESTAINO, A. J., MESROBIAN, R. B., MORAWETZ, H., BALLANTINE, D . S., DIENES, G. J. and METZ, D . J., / . Amer. Chem. Soc. 78, 2939, 1956.

REXER, E., Reichsber. Phys. Beihefte Phys. 1, 111, 1944. SCHINDLER, A. and BREITENBACH, J. W., Simposio Inter, di Chim. Macrom.,

Milan, 1954; La Ricercia Scientifica, Interscience, 1955. SCHMITZ, J. Y. and LAWTON, E. J., Twelfth Congress LU.P.A.C., New York,

1951; Science 113, 718, 1951. SCHULZ, R. and HENGLEIN, A., Angew. Chem. 67, 232, 1955. SCHULZ, R., HENGLEIN, A., STEINWEHR, H. E. and BAMBAUER, H. U., Angew.

Chem. 8, 232, 1955. SCHULZ, R., RENNER, G., HENGLEIN, A. and KERN, W., Makromolek. Chemie

12, 20, 1954. SEITZER, W. H., GOECKERMANN, R. H. and TOBOLSKY, A. V., / . Amer. Chem.

Soc. 75, 755, 1953. SEITZER, W. H. and TOBOLSKY, A. V., NP 5306, 5348, 5349, 1954; / . Amer.

Chem. Soc. 77, 2687, 1955. STANNETT, V. and SZWARC, M., / . Polymer Sei. 10, 587, 1953. SUN, K. H., Mod. Plast. 32(1), 141, 1954. THENARD, P., C.R. Acad. Sci.9 Paris 78, 219, 1874. THOMAS, W. M. and PELLON, J. J., / . Chem. Phys. 21, 1303, 1953; / . Polymer

Sei. 13, 329, 1954. TKACHENKO, G. V., KHOMINKOVSKI, P. M. and MEDVEDEV, S. S., Zh. Fiz. Khim.

U.S.S.R. 25, 823, 1951. TOBOLSKY, A. V. and BAYSAL, B. J., / . Polymer Sei. 11, 471, 1953. TOBOLSKY, A. V. and OFFENBACH, J., J. Polymer Sei. 16, 311, 1955 WORRALL, R., CHARLESBY, A., / . Appl. Rad. Isotopes 4, 84, 1958.

CHAPTER 23

GRAFT AND BLOCK COPOLYMERS MOST of the published research on the irradiation of polymers has been concerned either with the formation of the polymer itself, or its subsequent modification by crosslinking or degradation. In either case basic work has been largely directed towards the treatment of a homogeneous material, only a single monomer being considered. More recently attention has been focused on the possibility of producing special forms of copoly-mers by means of radiation.

In the simplest form of copolymer, the monomer units A and B are distributed at random along the chain. Examples are the copolymerization of a mixture of styrene and methyl methacrylate. (Schmitz and Lawton, 1951; Ballantine et al, 1953; Seitzer, et ai, 1953; Lindsey, et al, 1954.) In such random copolymers the properties often tend to be intermediate between those of the two constituents.

When the arrangement of the two (or more) monomer units is not random, special properties may emerge. We shall not consider conden-sation polymers, produced, for example, by the reaction between difunc-tional acids and alcohols to give a polyester of structure

. . . — Ac — Al — Ac — Al — Ac — . . . since in this case the monomer unit is best considered as the combination (—Ac — Al—) which repeats regularly. Other forms of alternating arrangement — ABABABAB — will not be discussed here for the same reason.

Several types of non-random arrangement are of particular importance. In the block copolymer, successive monomers tend to be of the same type, so that a sequence of a number of monomer units A is followed by a sequence B.

— A AAAABBBBAAAABB AAAAA — This definition applies primarily to the statistical arrangement of monomer units in the main chain; side chains where present play no special role. In graft copolymers, on the other hand, the main chain consists essentially of a homopolymer, i.e. of a sequence of units A, whereas the side chains are predominantly composed of units B.

B B B

— AAAAAA AA AAAAAA — B B B B B

395


In certain cases these two definitions may overlap, and indeed other definitions of graft and block copolymers have been used based on the method of preparation. The author feels, however, that the terminology should relate to the structure of a system, rather than its mode of pre-paration, since it is the former which confers on a copolymer its special properties. In this chapter we shall be largely concerned with the formation of graft copolymers in which radiation promises to be an exceptionally versatile tool.

A very different type of arrangement is obtained if bridges are formed between molecules, equivalent to lengthy crosslinks

— AAAAAAAAAAAA — B B B B B — AAAAAAAA — B B

— AAAAAAAAAA — B

— AAAAAA — B B

in a homopolymer system. Such a system would have an infinite molecular weight, be insoluble, and have a finite elastic modulus. Examples of such systems are the cured forms of polyester-styrene mixtures, and vulcanized rubber if the sulphur chains are sufficiently long. The special properties of such systems are sufficiently marked to consider them as falling in a separate category, and a term which might be used to describe them is network copolymers.

When it is not possible to distinguish between homopolymer, block or graft polymer and network polymer, one may refer to deposited polymer as being the total amount of monomer reacted.

Block copolymers can be formed by radiation, and by the effect of ultrasonics. The polymers are initially present either as an intimate mixture or a solution of two homopolymers. The effect of radiation or of ultra-sonics is to disrupt these chains, leaving free radicals at the ends. By linking these together a certain proportion will form block copolymers.

— AAAAAAAAAA — -> — AAAA· + -AAAAAA —1 — BBBBBBBBBB > — BBBBBB· + BBBB — J "*

— AAAABBBB — — BBBBBB AAAAAA —

The properties of such copolymers may be of particular interest for such uses as fibres in the textile industry, but little information on the products formed in this way by the use of radiation has been published. The radiation energy requirements can in theory be calculated, since each two changes in a sequence requires at least two main chain fractures. In many cases

GRAFT AND BLOCK COPOLYMERS 397

the corresponding G values for degradation are known, but an allowance must be made for the possibility of recombination to give homopolymer and for reaction with the solvent.

In graft copolymers a homopolymer chain AAAAAA is irradiated in the presence of a monomer unit B. Radicals formed on the polymer chain A can then initiate side chain polymerization of monomer units B:

— AAAAAAAAA > — AAAAAAAAA —

-> — AAAAAAAAA — B B B

To achieve the maximum degree of grafting it is advisable for the polymer A to be present in excess, or for initiation of radicals on B to be less frequent than that on A; in other cases a considerable amount of homopolymer BBBBBBB may be formed. The doses required for grafting may be very low since each radical can initiate a long polymerization chain. At the same time it is advisable to avoid the formation of bridges between homopolymers A, either by direct crosslinking, or via a chain of B units, since the properties of such a network copolymer are very different from those of a graft, and may render it difficult to process. This may be achieved by the use of low intensities which favour polymerization, as compared with crosslinking.

There is not as yet any satisfactory theoretical means of relating the properties of graft copolymers to those of the constituents. Not only is the number of possible combinations of the two constituents A and B extremely large, but in addition the length of the side chains B, and their distance apart, will be of importance. At present a considerable amount of research effort is being devoted to the study of certain selected grafts. It appears likely that several practical applications of radiation in this field will precede any comprehensive theory.

The presence of trapped radicals in irradiated solids and the resultant post-radiation effects have been known for some years. In 1955, Restaino et al. mentioned the possibility of polymerization in a two-phase system such as methyl methacrylate in styrène as a possible method of deriving graft copolymers, but did not give any experimental details. Chapiro (1956) in his study of irradiated polyethylene gave two methods of pre-paring graft copolymers on polyethylene, subsequently extended to other polymer systems. In the first method polyethylene is irradiated in the presence of the monomer, with grafting on to the radicals produced on polyethylene. The second (post-irradiation) method relies on the fact that when polyethylene is irradiated in air peroxide bridges are formed, which become unstable at a temperature of about 150°C, and give rise to radicals. The polymer is irradiated in the presence of oxygen and is subsequently heated in the presence of monomer, the radicals formed by


decomposition of peroxide bridges being used for grafting purposes. This has the advantages that the grafting itself can be carried out at any time, away from the radiation source, and the risk of homopolymer formation is greatly reduced. The form of the original polymer may be conserved in the process, which is an advantage when the copolymer is difficult to mould or process.

(radiation to + 0 2 | produce peroxides) O

O H

Stage (2) peroxide + monomer B -> ^ — — ~ ^ — ™ (heat treatment to | decompose peroxides) O

BBBBB

The irradiation of polymer in vacuo to produce trapped radicals, which can subsequently react with monomer, may also be envisaged. Little has been published on this technique, possibly because of the low concentra-tions of radicals.

Some of the advantages of graft polymers claimed by Chapiro (1956) are as follows:

(i) acrylonitrile grafted on to polyvinyl chloride increases its softening point;

(ii) methyl methacrylate grafted on to polyethylene results in a harder product, transparent and amorphous;

(iii) acrylonitrile grafted on to polymethyl methacrylate gives a trans-parent copolymer which is insoluble in most solvents ;

(iv) the superficial grafting of acrylamide on to polyethylene gives a hydrophilic surface which is printable.

These constitute only a few of the potential applications of the grafting technique.

The use of radiation as a means of producing graft material has aroused considerable interest in industrial circles, but comparatively little of their work has been published as yet. Several papers have appeared by scientists working at Brookhaven and Brooklyn Polytechnic, describing some grafted materials using γ-radiation (see below). The use of high-intensity radiation sources has been investigated by Charlesby and Pinner (1956-57). Chapiro (1957) has described the swelling properties of poly-ethylene grafted with acrylonitrile.

The problems which require investigation can be considered to fall under four headings :

(a) preparation of the material for grafting; this involves such questions as compatability and diffusion rate;

(b) a consideration of radiation techniques, e.g. the effect of intensity and dose, and the G values for radical formation of the constituents;

I o o

Stage (1)


(c) the structure of a grafted system, i.e. the number and length of branched chains, the formation of bridges, the effect of residual crystallinity;

(d) the assessment of the physical and chemical properties of grafted materials in terms of their chemical constituents, and of their structure.

Little basic information on these aspects has been published as yet, although a considerable increase may be expected in the near future as work at present in progress becomes available. In recent papers by Burlant and Green (1958) and Turner (1958) very high values are esti-mated for the length of polymethyl methacrylate branches in grafted materials.

FIG. 23.1. Expansion of polyethylene grafted with acrylonitrile. Figures show the grafting ratio (total weight/initial weight). During grafting the initial shape is

conserved. (From Chapiro, 1957.)

GRAFTING FROM MONOMER AND FROM SOLUTION Several papers have appeared which give preliminary data on some

grafted systems. Table 23.1 summarizes some of the data obtained by Ballantine et al. (1956) for a number of polymer-monomer systems subjected to γ-radiation at intensity levels of about 3000-5000 r/min. In all cases, grafting took place in an atmosphere of nitrogen and was measured by subsequent solution and precipitation. The increased polymerization rate of styrene in the presence of polymethyl methacrylate can arise both from the Trommsdorf effect in a highly viscous system (which reduces the probability of mutual chain termination) and from the greater number of radicals produced on the polymethyl methacrylate as

Tabl

e 23

.1.

Gra

fting

of

Som

e P

olym

er-M

onom

er

Syst

ems

Roo

m te

mpe

ratu

re: C

obal

t 60

irrad

iatio

n at

abo

ut 3

500-

5000

rep/

min

Poly

mer

Poly

met

hyl m

etha

cryl

ate

Poly

met

hyl m

etha

cryl

ate

Poly

-2: 5

dic

hlor

osty

rene

H

evea

rub

ber

Poly

dim

ethy

l silo

xane

Teflo

n Po

lyet

hyle

ne

Poly

ethy

lene

Po

lypr

opyl

ene

Initi

al

mon

omer

Styr

ene

Styr

ene

Styr

ene

Styr

ene

Acr

ylon

itrile

M

ethy

l m

etha

-cr

ylat

e St

yren

e St

yren

e A

cryl

onitr

ile

Styr

ene

Initi

al

wei

ght

% of

m

onom

er

100 21

75-2

35-4

50

-9

301 ? ? ? 7

Dos

e (m

egar

ep)

1-75

0-52

0-

25

3-6

% c

onve

rsio

n of

mon

omer

2-2

730 —

59-7

91

-5

98 7 7 7 7

% c

onve

rted

mon

omer

in

copo

lym

er

—

16-3

—

20-4

49

-2

30-6

4-

9 10

-2

23-8

30

-2

Rem

arks

—

Tota

l gr

aftin

g D

egra

datio

n of

po

lym

ethy

l m

etha

cryl

ate

Tota

l gr

aftin

g To

tal

graf

ting

Parti

al g

rafti

ng

Surfa

ce g

rafti

ng

Tota

l gr

aftin

g To

tal

graf

ting

Tota

l gr

aftin

g So

urce

: B

alla

ntin

e et

ai,

1956

.


G R A F T AND BLOCK C O P O L Y M E R S 401

compared with styrene for a given energy input. At high concentrations of polymethyl methacrylate, however, no useful graft was obtained due to the degradation of the polymer under radiation. Styrene was found to graft readily on to polyethylene at doses of the order of 1 megarad, giving a suffer material with a lower elongation at break and a reduced tensile strength.

In the grafting of acrylonitrile on to polyethylene (Chen et al., 1957) the high reactivity of the monomer results in the formation of a consider-able amount of homopolymer which is highly undesirable for grafting. An improvement is obtained by irradiating the polyethylene film in water saturated with acrylonitrile. At constant radiation intensity (3000-6000 r/min) the amount of grafted material is not proportional to the dose; however, a graft containing 10 per cent (molar) of acrylonitrile was obtained from a saturated solution subjected to a dose of 0*3 megarads at 3000 r/min. With 2-vinyl pyridine an extremely rapid rate of grafting on to polyethylene was observed at a dose of 0-1 megarad, but the product obtained was brittle. By irradiating a 20 per cent solution of the monomer, vinyl carbazole was grafted on to polyethylene giving a graft with improved solvent resistance and a higher softening point than polyethylene and a reduced brittleness as compared with polyvinyl carbazol. Table 23.2 compares the values obtained for various grafts with those for unirradiated and irradiated (non-grafted) polyethylene.

Table 23.2. Some Properties of Polyethylene-Vinyl Carbazol Graft Copolymers

Composition* of graft

(% vinyl carbazole)

0 0 9-3

16 7 25-9 341 35-6 53-7

Dosage (mega-reps)

0 30

3-5 1-36 2 0 4

11-2 11-2 321

Dissipation factor 1 kc at

25°C(%)

006 006 —

011 011 012 012 —

Tensile strength (lb/in2)

2480 3150 2660 2360 —

2650 2770 3600

Elongation at break

(%)

700 500 660 505 — 310 337 45

Drape temperature

(°C)

80-104 90-104 98-112

— — —

103-138 165-220

* Calculated as weight of vinyl carbazol divided by total weight of graft copolymer X 100.

Source: Chen et ai, 1957.

Surface grafting of styrene on to PTFE (Teflon) has been achieved, the amount of grafted styrene being proportional to the dose up to at least 0-2 megarad (when 3 per cent styrene was grafted on to a 1 mil film of PTFE). Whereas PTFE shows a reduced tensile strength on radiation, styrene-grafted PTFE shows no such deterioration, possibly because the radicals formed in the polymer react with the styrene monomer. The


presence of a grafted styrene surface increases the adhesion of the material (to Scotch Boy transparent self-adhesive tape) if at least a few per cent of styrene graft is present. The ink retention of the graft is also improved. Vinyl pyrollidone from an aqueous solution has also been grafted on to PTFE but in this case the rate of grafting appears to be proportional to the amount of grafted monomer already present. A 10 per cent molar grafted vinyl pyrollidone on a 1 mil PTFE film was obtained by subjecting

0-0011 i 1 1 M I N I 1 i 1 i i l ! ! l 104 105 106

Total dose , r

FIG. 23.2. Grafting of styrene to PTFE (Teflon). Film thickness of PTFE, 1 mil. γ-Intensity, 3000 r/min.

L j j 1 ! I i ! i I i I 0 103 104 105 106 107

Dose, r

FIG. 23.3. Surface grafting on 1 mil PTFE. Effect of styrene on physical properties. (From Chen et al.9 1957.)

the monomer (in 20 per cent aqueous solution) to 0-6 megarads at 2900 r/min. The surface grafting on to materials, especially fibres, may be of considerable interest from the technical point of view.


Acrylonitrile has been grafted to dimethyl silicone rubber and to Hevea gum rubber. In the former case, doses of the order of 0-5 megarad sufficed to produce a 30 per cent graft to acrylonitrile and this resulted in a reduction in the swelling in heptane or toluene by a factor of 2.

Grafted materials appear to be very suitable for use as exchange mem-branes. Sulphonated styrene-polyethylene graft copolymers have been prepared for use as cation exchange membranes, polyethylene serving to limit the swelling of the membrane and to give it adequate mechanical strength. Anion exchange membranes have also been prepared by intro-ducing quarternary amine groups into a polyethylene-styrene graft copoly-mer or by quarternization of a vinyl pyridine-polyethylene graft.

Henglein and Schnabel (1957) have also reported on the formation of surface grafts which can show osmotic pressure effects.

EFFECT OF RADIATION INTENSITY The rate of grafting may be determined by the radiation intensity, or

by the rate of diffusion of further monomer into the system as that originally present is removed by grafting. Chen et al. (1957) describe some measurements for styrene grafts on polyethylene, at dose rates of between about 330 and 5000 rads/min, and for two different film thicknesses of polyethylene—4 and 10 mil. The results shown plotted in Fig. 23.4

a 0-8

Φ 0-6

0-4

0-2

0 5 1Q Time, hr

FIG. 23.4. Effect of film thickness and dose rate on grafting of styrene to polyethylene, (From Chen et al, 1957.)

O · D ■ 4 mil films at 20000, 65000, 172000 and 300000 r/hr Δ A V T 10 mil film at 20000, 65000, 172000 and 275000 r/hr

indicate that the amount of grafted styrene is proportional to the radiation dose in all cases. The variation with intensity and with film thickness (which affects the diffusion rate) is more difficult to interpret. On the thicker film, the rate of grafting is independent of intensity, indicating that the reaction is diffusion-controlled. On the thinner film, grafting (when measured in grams per megarad) is perhaps more effective at lower intensities as is to be expected if the limiting factor is now polymerization

DD


rate. However, the most difficult feature to understand in these experi-ments is the higher rate of grafting in the thicker film. Conditions for polymerization within a solid matrix have not as yet been adequately studied; the above anomalies emphasize the need for further investigation of the effect of film thickness and of diffusion rate.

In a more recent paper, Charlesby and Pinner (1957) investigated the grafting of a number of monomers on to polyethylene, and studied the rate of grafting over an extensive range of radiation intensities, from about 103 rads/min, using cobalt γ-radiation, up to about 106 rads/sec, using 2 Mev electrons from a Van de Graaff accelerator. No difference between the two types of radiation is anticipated, but as in polymerization, the radiation intensities would be expected to produce a considerable difference in the amount and distribution of grafted material. A further difference arises from the fact that whereas the γ-radiation was constant in intensity, the electron radiation was delivered in a series of pulses depending on the sweeping of the electron beam across the specimen and on the passage of the specimen under the beam. Any point of the specimen received a number of pulses of electron radiation each lasting for about 250 μ$εο at a dose rate of 107 rads/sec. Such pulses recurred at intervals of 2-5 msec giving an overall average of 106 rads/sec. By passing the specimen through the beam several times, with intervals of about 1 min between passes, the overall average was further reduced to only 106 rads/min. The correct average radiation intensity to consider therefore depends on the lifetime of the radicals. If this is shorter than 25(^sec, the relative radiation intensity is 107 rads/sec. If it is much longer than several minutes, non-steady conditions prevail, but the average intensity can be taken as 1 megarad/min during the exposure period.

In their experiments Charlesby and Pinner (1957) irradiated various

30

i-

E 20

o a ■σ a> 't 10 o a û

1-5 2-5 4 6 8 10 Dose, megarad

FIG. 23.5. Effect of radiation intensity on grafting rate. (Vinylidene chloride on polyethylene.)

(From Charlesby and Pinner, 1957.) © gamma radiation 500 rads/min C X-rays 60 000 rads/min O electrons 0-2 megarads/pass (1 min) f electrons 1 megarad/pass (1 min)

L· /

-·—

y s*

<

«

'-""""'

>—

^^jt i"


types of polyethylene (differing in the degree of branching and hence in amount of amorphous material present) which had absorbed an equili-brium amount of monomer. This equilibrium absorption was largely determined by the proportion of amorphous material present in poly-ethylene, as well as by the temperature. Furthermore, styrene and vinyli-dene chloride were absorbed far more readily than methyl methacrylate, vinyl acetate or acrylonitrile monomer.

On irradiating the polyethylene samples, those swollen with styrene or vinylidene chloride showed a degree of grafting directly proportional to the radiation dose, even at the very high radiation intensities obtained with the electron accelerator. Furthermore, the amount of grafted material was approximately proportional to the root of the radiation intensity, from which it might be inferred that termination occurred by a bimolecular process (Fig. 23.5). With the other monomers the amount of grafted material rapidly tended to a limiting value (Fig. 23.6). This was ascribed to the small amount of monomer initially present in the swollen polymer, or to its rapid polymerization due to a high G value. Further absorption of monomer was slow, so that the grafting is essentially limited by the low diffusion rate. Grafts produced while the polymer was immersed in monomer were unsatisfactory as grafting tended to occur near the surface

Dose , megarad

FIG. 23.6. Grafting rate of styrene, vinyl acetate and methyl methacrylate showing limitation imposed by equilibrium sorption.

(From Charlesby and Pinner, 1957.)

as further monomer entered the system, such surface grafting leading to non-homogeneous products and surface crazing. This situation could be improved by submitting the specimen to a series of small radiation doses between each of which it was allowed to re-establish equilibrium with the monomer. In this case, the amount of deposited polymer was roughly proportional to the total radiation dose for most monomers (Fig. 23.7).

To avoid this diffusion control, experiments were carried out with a polyethylene base in the form of fine powder. Under these conditions,


the rate of conversion of monomer (methyl methacrylate) was far greater than in the absence of a substrate.

An increase in conversion rate has also been observed by Sebban-Danon (1958) in styrene-polywöbutylene grafts. As the degree of conver-sion is increased, the dépendance on intensity / changes from 70,5 to 70,8

about, due to the change from bimolecular to monomolecular termination.

FIG. 23.7. Effect of intermittent dose and inversion on total grafting rate. (From Charlesby and Pinner, 1957.)

O MM · VA 3 St C VC.

As compared with low-intensity sources, high-intensity sources have the disadvantage of lower efficiency in terms of grafted material per unit radiation dose. This reduction in yield is, however, not excessive and compares favourably with the use of these high intensity sources for poly-merization. The properties of grafts formed with low-intensity and high-intensity sources may be expected to differ, even for products containing the same overall percentage of graft material; low-intensity sources will favour the production of fewer branches, each of greater length. With high intensity radiation the procedure may be simplified since inhibitors present in the monomer to stabilize it need not be removed.

GRAFTING AND CRYSTALLINITY

The properties of polyethylene grafted with acrylonitrile has received special attention. Because of the crystallinity in the former such studies can be expected to provide information on the sites of grafting, and on the structure of the grafted material. Chapiro (1957) has studied the swelling properties of such grafts in dimethyl formamide, which only dissolves polyacrylonitrile with difficulty. The swelling ratio is found to depend not only on the ratio of grafting (copolymer/polyethylene) but also on the temperature at which grafting takes place (using the pre-irradiation technique) and on the time for swelling.


Swelling only occurs at temperatures above 110°C, which is needed to destroy crystallinity in polyethylene. Some grafts rapidly swell to a limiting value, others show swelling in two stages, while others again dissolve completely. Fig. 23.8 shows the swelling ratio for grafts formed at temperatures up to 135°C, and Fig. 23.9 those at higher temperatures. In all cases the maximum swelling occurs for a grafting ratio of about 8, and such samples grafted at a temperature of about 95°-l 35°C can dissolve. Lower and higher degrees of grafting don ot give a soluble product. The explanation of this behaviour is associated with the stability of polyethy-lene crystals, and with the formation of a network polymer. At a low grafting temperature, many polyethylene crystals remain unaffected. On subsequent heating in solvent, the insolubility of these parts of the poly-ethylene chains greatly reduces the swelling ratio. At temperatures of 95°-135°C a more uniform grafting can take place, and the graft polymer

Soluble

30 40 50 60 70 80 Grafting ratio

FIG. 23.8. Effect of grafting ratio and temperature of grafting on swelling ratio. initial swelling. swelling limiting.

Temperature of grafting: 46-87° and 95-135°C. (From Chapiro, 1957.)

152-170°C

1 10 20 30 40 50 Grafting ratio(acrylonitrile + polyethylene/polyethylene)

FIG. 23.9. Effect of grafting ratio and temperature of grafting on swelling ratio. initial swelling. swelling limiting.

Temperature of grafting: 152°, 170°, 190°. (From Chapiro, 1957.)


can dissolve. At even higher temperatures (and degrees of grafting) the mobility of the monomer is increased, and there is an increased probability of mutual chain termination of acrylonitrile branches. Bridges formed between the polyethylene molecule convert the graft into a network struc-ture, with a finite degree of swelling which depends on the number and size of the network loops (Chapiro, 1958).

In parallel experiments by Charlesby and Callaghan, measurements of residual crystallinity in acrylonitrile grafts on polyethylene were made by studying the specific volume-temperature curves. For low density poly-ethylene this curve shows a transition at about 115°C, the highest melting temperature of the crystallites (Fig. 13.15). Changes in the shape of this curve would indicate the effect of grafting on the size and number of these crystallites.

In the experiments both methods of grafting suggested by Chapiro were used. 0Ό5 mm polyethylene film was subjected to 6 megarads of cobalt radiation (at 5000 rads/min) in the presence of air, and then grafted with acrylonitrile at 160°C. Alternatively, the film was irradiated in the presence of acrylonitrile at room temperature. The specific volume-

1-30

1-25

1-20

1-15

MO

ω E 2 1-05

u

% 1-00

a to

0-95

0 - 9 0

0-85

2 0 4 0 6 0 8 0 100 120 140 160 °C

FIG. 23.10. Specific volume of polyethylene, of polyacrylonitrile and of grafts, showing elimination of crystallinity. (Figures show total weight/weight of polyethylene.)

(Charlesby and Callaghan, unpublished.)

-

*Λ

/ y

_ ^ + - ^

>

/

vA

/

r f%-3

>olyeth-

2-3

" / J

Polyac

ylene

/ /

V

COI

.... ryloni

1

1-7

t r i le

G R A F T AND B L O C K C O P O L Y M E R S 409

temperature curves for these grafts are shown in Fig. 23.10. In the more lightly grafted polymer there is little change in the transition at 115°C showing that crystals are not affected by grafting, the branches occurring primarily in the amorphous regions. In the grafts prepared at 160°C acrylonitrile can penetrate throughout the specimen; but can only form grafts at the sites of oxygen attack. It must therefore be assumed that the branches are initiated only in the amorphous (highly branched) regions susceptible to oxygen attack and do not interfere with subsequent recrystal-lization of the more linear portions. In more highly grafted material (grafting ratio about 2-3) the transition is greatly diminished, while for a grafting ratio of 5 it is completely lacking. This must be due to mingling

Tabl

e23

.3.

Poly

mer

izat

ion

and

Gra

fting

onPo

lyet

hyle

ne

Dos

era

teC

onve

rsio

nra

te(p

erce

ntpe

rm

egar

ad)

Poly

mer

izat

ion

Gra

ftin

gR

efer

ence

Rad

s/m

inR

ads/

pass

.H

r/m

egar

adSt

yren

eM

ethy

lSt

yren

eM

ethy

l(a

t1

pass

/min

)m

etha

cryl

ate

met

hacr

ylat

e

102

-16

712

·535

0-

-(a

)10

3-

173

80-

-(a

)10

4-

1·7

0-7

20-

-(a

)-

0·2

x10

60·

08-

1·9

-19

0(30

)*(b

)-

106

0·02

0-16

-6-

1(1'

3)*

-(b

)4

x10

3-

0-2

--

?(20

)*-

(c)

*Fi

gure

sgi

veco

nver

sion

per

cent

base

don

initi

alm

onom

erab

sorb

ed;

inbr

acke

tsth

egr

afte

dm

onom

eras

afr

acti

onof

tota

lgr

aft

(mon

omer

plus

subs

trat

e).

Ref

eren

ces:

(a)

Bou

byet

al.,

1955

.(b

)C

harl

esby

and

Pinn

er,

1957

_(c

)B

alla

ntin

eet

al.,

1956

_


of the long branches with the non-grafted linear stretches of polyethylene, preventing the latter from recrystallizing. As a result an amorphous graft copolymer is formed.

COMPARISON OF POLYMERIZATION AND GRAFTING The effect of the substrate in radiation grafting can best be shown by

comparison of grafting rate with the straight polymerization of the mono-mer under identical conditions of radiation intensity. Most information is available for the polymerization of styrene and methyl methacrylate (Chapter 22). At low intensities (below about 1000 rads/min) these poly-merize at a rate which varies as the half power of the radiation intensity / so that the amount of monomer polymerized per megarad varies as 7-0*5. At higher intensities, the concentration of radicals is such that many are lost either by combining with growing chains or with each other and as a result the rate of polymerization varies far less rapidly with intensity. The yield of polymerized material per megarad then also becomes very low. Table 23.3 shows some published data on the yield of polymerized material at various radiation intensities from cobalt irradiation as well as the very low values for polymerization at the very high intensities obtained from an electron accelerator. In grafting, however, the conversion rate, even at these very high intensities, remains high and is still comparable with that obtained at much lower intensities when pure monomer is irradiated. This emphasizes the importance of the substrate, both in providing radicals which initiate chain growth and in impeding the termination mechanism which limits the length of the chain. The relatively high yields obtained

Table 23.4. Comparison of Methyl Methacrylate and Styrene

G values for radical initiation

Polymerization rate % monomer loss/ hr

Grafting rate % deposited/megarad

r(°C)

15 25 30-5

20 70 25

- 1 8

25

Dose rate

(r/min)

—

100 4000 4000 4000

106

Methyl metha-crylate

16*7 36 3-14

2-2 18 7-6 2-6

23

Styrene

0-97 208 0-22

009 21 0-36 0086

1

Ratio

18 18 14

25 8

20 30

23

References

(a) (b) (c)

(d) (b) (b) (b)

(e)

References: (a) Prevot et al, 1952 (corrected to ferrous G value of 15-5). (b) Ballantine et al, 1956. (c) Seitzer and Tobolsky, 1955. (d) Chapiro et al, 1955. (e) Charlesby and Pinner, 1957.


in such grafts renders more economic the use of high intensity radiat ion obtained from electron accelerators.

Table 23.4 shows the G values previously obtained for polymerization of methyl methacrylate and of styrene. These values indicate that methyl methacrylate produces radicals about 15 times as readily as does styrene. The rate of polymerization, i.e. ra te of disappearance of monomer , depends bo th on the G value for radical product ion and on the propagat ion and terminat ion rate. Here again, ratios of between 8 and 30 have been observed. When the two same monomers are irradiated in the presence of polyethylene to form grafted material , the reaction rates are likewise observed to be in the approximate rat io of about 23. This may be taken to indicate that many of the grafts are initiated not on the polyethylene bu t on the monomer consti tuent of the graft.

R E F E R E N C E S ANGIER, D. J. and TURNER, D. T., / . Polymer Sei. 28, 265, 1958. BALLANTINE, D. S., COLOMBO, P., GLINES, A. and MANOWITZ, B., B.N.L. 229,

1953; Chem. Engng. Progr. 50(11), 267, 1954. BALLANTINE, D. S., COLOMBO, P., GLINES, A., MANOWITZ, B. and METZ, D. J.,

B.N.L. 414ÇT 81), 1956. BALLANTINE, D. S., GLINES, A., METZ, D. J., BEHR, J., MESROBIAN, R. B. and

RESTAINO, A. J., / . Polymer Sei. 19, 219, 1956. BOUBY, L., CHAPIRO, A., MAGAT, M., MIGIRDICYAN, E., PREVOT-BERNAS, A.,

RHEINISCH, L. and SEBBAN, J., Int. Conf. Peaceful Uses of Atomic Energy, Geneva, 1955, 7, 526.

BURLANT, W. J. and GREEN, D. H., / . Polymer Sei. 28, 252, 1958. CHAPIRO, A., Chim. Industr. 76(4), 754, 1956; / . Polymer Sei. 23, 377, 1957;

Indus. Mat. Pias. 8(12), 34, 1957; 29, 321, 1958. CHAPIRO, A., et al, J. Chim. Phys. 52, 689, 1955. CHARLESBY, A. and PINNER, S. H., Conference on Graft Polymers, Paris, 1956;

Industr. Mat. Pias. 9(9), 30, 1957; 9(10), 43, 1957. CHEN, W. K. W., MESROBIAN, R. B., BALLANTINE, D. S., METZ, D . J. and GLINES,

A., Ann. Symp. Colloid Soc, Madison, June 1956; Int. Symp. Macromol. Chem., Rehovoth, 1956; / . Polymer Sei. 23, 907, 1957.

COCKBAIN, E. G., PENDLE, T. D. and TURNER, D. T., Chem. & Ind., 759, 1958. HENGLEIN, A., SCHNABEL, W. and HEINE, K., Angew. Chem. 70, 461, 1958. HENGLEIN, A. and SCHNABEL, W., Naturwissenschaften, 13, 376, 1957; Makromol.

Chem. 25(1/2), 119, 1957. LINDSEY, H., BROWN, D. E. and PLETCHER, D. W., Bull. Amer. Phys. Soc. 29,

14, 1954. MAGAT, M., / . Chim. Phys. 52, 709, 1955. MESROBIAN, R. B., BALLANTINE, D. S. and DIENES, G. J., Symp. Pure Appl.

Chem., Milan, 1954, p. 14. RESTAINO, A. J., MESROBIAN, R. B., BALLANTINE, D. A. and DIENES, G. J., Simposio

Inter, di Chim. Macrom., Milan, Turin, 1954; La Ricerca Scientifica, 1955. SCHMITZ, J. V. and LAWTON, E. J., Twelfth Congress I.U.P.A.C., New York, 1951. SEBBAN-DANON, J., / . Polymer Sei. 29, 367, 1958. SEITZER, W. H., GOECKERMANN, R. H. and TOBOLSKY, A. V., / . Amer. Chem.

Soc. 75, 755, 1953. SEITZER, W. H. and TOBOLSKY, A. V., / . Amer. Chem. Soc. 77, 2687, 1955. TURNER, D. T., Chem. & Ind., 995, 1958.

CHAPTER 24

CURING OF UNSATURATED POLYESTER RESINS

THE term polyester resin is loosely applied to a solution of an unsaturated polyester in a vinyl monomer. The polyester itself is formed by conden-sation of one or more dibasic acids with one or more dihydric alcohols, the unsaturation usually being incorporated by selecting maleic acid as one of the acid components. The vinyl monomer may be vinyl acetate, methyl methacrylate, or more usually styrene. The mixture is cured by the incorporation of a small amount (0-005-0-1 per cent) of catalyst such as benzoyl peroxide, which on heating gives rise to free radicals. The curing process consists in a copolymerization reaction involving the vinyl mono-mer and the unsaturation sites in the polyester. It differs from conven-tional polymerization in that the monomer forms bridges between the polyester molecules, and the fully-cured material is a highly crosslinked three-dimensional network structure. The cure does not result in any volatile products, little or no external pressure is required and to set off the reaction heat is not always essential.

Polyester resins are most frequently used in conjunction with glass fibre reinforcement, the fibres giving mechanical strength to the laminate while the polyester acts as a binding agent. The resultant structures have high impact strength, chemical resistance and low density. Polyester resins are also used for embedding electronic components.

Polyester resins can also be cured by radiation in which case no catalyst is required. Since the process is a chain reaction, proceeding via unsatura-tion in the polyester, the doses needed are small, of the order of a few megarads or less. In the absence of a chain reaction, doses exceeding 100 megarads would be needed to achieve the same high density of network formation.

There are several essential differences between the network polymers formed in this manner, and those produced by conventional crosslinking described in earlier chapters. One major distinction is that links between polymer molecules can no longer be assumed to occur at random; if a polyester molecule is linked to a neighbouring polyester molecule via a vinyl chain it is highly likely to form part of an extensive polymerization chain, linking it to a number of such molecules. The conditions for gel formation and for a given degree of insolubility will therefore be different.

Polyester molecules are usually of relatively low molecular weight, and there may be only a few unsaturation sites per molecules. This corresponds to a very low degree of polymerization, and it is no longer possible to assume that the number of links per molecule is small compared with the

412

CURING OF UNSATURATED POLYESTER RESINS 413

number of sites available for crosslinking. Many of the approximations used in crosslinking studies are then no longer justifiable. The average polymerization chain length may be expected to depend not only on radiation intensity (as in conventional polymerization) but also on other conditions such as temperature, viscosity of the mixture, monomer con-centration and residual polyester unsaturation, all of which vary in the course of the reaction. No simple proportionality can therefore be expected between radiation dose and number of links produced.

The physical properties of a cured polyester-vinyl mixture depend both on the chemical structure of the constituents, and on the length of the chains between successive junction points, whether these chains form part of the polyester molecule itself, or of the vinyl polymerization chain. A fundamental problem is to relate the number and size distribution of these chains to the ensuing physical properties, a problem somewhat akin to that involved in the characterization of graft copolymers.

Investigations of radiation-cured polyester resins may be considered to follow three main lines :

(i) The search for an improved product; as compared with conven-tional methods of curing, radiation can be expected to provide more accurate control of the initiation step, a greater flexibility in choosing the temperature and rate of the reaction, and a higher density of initiating radicals.

(ii) The study of polymerization kinetics in a complex system of industrial importance.

(iii) A method of producing a range of network structures in which the mechanical properties may be related to composition. This should eventually provide a firmer theoretical understanding of the glass-like state. At present the properties of such highly-crosslinked networks are far less well understood than are those of lightly linked flexible chains, showing rubber-like elasticity.

As yet only a few papers have appeared on the subject of radiation-cured polyesters, either alone, or in admixture with a vinyl monomer. These have been primarily devoted to a comparison of radiation and thermal cure, or to the influence of radiation conditions (dose, intensity, inhibitor concentration) on the curing process.

NETWORK FORMATION BY A CHAIN REACTION Early work on a somewhat related system was carried out by Schmitz

and Lawton (1951) who subjected tetraethylene glycol dimethacrylate (TEGMA) to high-voltage electron radiation. In network formation in polyesters, styrene or another vinyl monomer is needed owing to the difficulty of forming a homopolymer with maleic or fumaric unsaturation. In the system studied by Schmitz and Lawton the reactive groups are vinyl, which can propagate through polyaddition chains, so that no vinyl monomer need be added. The presence of several unsaturations per mole-cule allows the formation of a three-dimensional network, as compared with the (possibly highly branched) long chain molecules formed in the


usual polymerization reactions. In some respects this system can serve as a model for the radiation-initiated cure of network polymers.

Schmitz and Lawton followed the course of the reaction in TEG MA by means of the exotherm of the polymerization reaction, which gave rise to temperature increases of up to 100°C. After only 2-5 megarads, 45 per cent of the double bonds had been utilized, indicating the high efficiency of this chain reaction. The reaction was found to proceed at temperatures as low as — 55°C. Below this the initial material is a glasslike solid; the radicals produced by radiation at these lower temperatures were frozen in, but caused some polymerization on subsequently warming up the speci-men. The reaction was inhibited by benzoquinone or oxygen, indicating that it proceeds by a free radical mechanism.

COMPARISON OF RADIATION AND THERMAL CURE

Callinan (1954, 1955, 1956) has compared some of the properties of commercial polyester mixtures in which the cure (or polymerization) is initiated either by radiation or by thermal decomposition of conventional chemical catalysts. The radiation doses (of unspecified intensity) were obtained from a 2000 curie Ca 60 source, and the initial polyester mixtures, present as clear viscous liquids, were solidified by doses of only 0-2-0-4 megarad. The advantages claimed by Callinan for the radiation cure were higher density, reduced water absorption, lower dielectric loss at high temperature and increased softening point. Increasing the radiation dose leads to an increased hardness, colour and a higher heat distortion tem-perature. These improvements can be expected from a more highly cross-linked network.

The temperature at which radiation is carried out affects the properties of the product. At low temperatures (—40°C) only a soft gel is obtained, whereas the same dose (0-33 megarad) at 25°C results in a hard solid. At higher temperatures the dose needed for cure is reduced, but internal strains may be produced on subsequent cooling. The effect of temperature is to be ascribed to the changed reactivity and mobility of the monomer, rather than to a different initiation rate. Large specimens, giving an increased exotherm, may therefore behave differently to small specimens subjected to the same dose.

As in conventional curing, oxygen acts as an inhibitor, and specimens irradiated in air have a tacky surface, which eventually hardens or can be dissolved.

A more detailed comparison of the physical properties of a typical polyester mixture, cured by radiation or by a thermally initiated catalyst, was made by Charlesby and Wycherley (1957). The polyester was prepared from maleic and phthalic anhydride and ethylene glycol, while the vinyl monomer was styrene. The mixture was subjected to a series of exposures from a high-intensity pulsed electron beam, the maximum instantaneous intensity per pulse being about 10 megarads/sec, with 400 pulses/sec, and intervals of 30 sec between successive doses of 1-3 megarads. Measurements were made immediately after irradiation, and repeated after 2 days to


permit most of the trapped radicals to react. It was confirmed that this period was sufficient to allow most of the reaction to go to completion.

The extent of cure was followed by measurements of (i) Young's modulus (dynamic), (ii) static modulus, (iii) hardness, (iv) tensile strength, (v) elongation at break, (vi) density and (vii) solubility. Fig. 24.1

en c it) &

c

10-000

6000

2000 /

/ / /

.

\ Γ

I

40 i

| 3 0 |

I 20|

10l·

\ \ \ \ \ \

H

v 10Oh

y*ul· O'Uh

7Ό{-

6-0l· 5· Oh

\ \\

1 \> \

f> H L< )

—

\'dCidr

1-280l·

1-278H

1-274L

1 L

I r o i o ~° U" ft o * ° E E 6

üJJ[ 4 c £ 2

y

7 ^ /

f

~ ^ r ^ " " r

12 Catalyst

I ÖUr

160

140

120

-V

— ( ! T r r !

5-ol· 4-6

4-2F

3-4

' < > ^ _J

Dose, megarad

3 4 5 6 7 8 9 10 Catalyst Dose, megarad

FIG. 24.1. Properties of an electron-irradiated polyester-styrene mixture. immediately after radiation

after standing Polyester : maleic and phthalic anhydride + ethylene glycol ; Acid number 46 7; Mn = 890; 2 6 ethylenic double bonds/molecule Styrene: 30 per cent by weight of total Δ = chemical catalytic cure.

(From Charlesby and Wycherley, 1957.)

shows the change in most of these properties with radiation dose. In the same figure are shown (Δ) the corresponding values for the same polyester mixture fully cured by the conventional catalytic technique. A dose of 3 megarads was sufficient to form a solid block or gel, but the properties examined showed further change either on standing, or on being subjected to increased doses up to about 7 megarads. The curves show that all these methods lead to the same approximate values for the minimum dose needed to achieve a full degree of cure, and that there are no substantial


differences in resultant properties as between radiation and thermal cure. A range of commercial polyesters resins could also be cured at the same radiation intensity by doses of between 4 and 8 megarads. Colichman and Scarborough (1958) also found that styrene-modified polyester syrups required doses of 5-10 megarads, and that the products were no better than those cured thermally.

Although these doses are substantially higher than those quoted for polyester curing with γ-radiation, the difference is to be expected from the much higher intensities used with electron radiation. At these high intensities, the rate of radical production is high, and when termi-nation occurs by bimolecular combination, the polymerization chains are short. After the cessation of irradiation sufficient trapped radicals or polymerizing chains are left to continue reacting and give the ageing effects shown in Fig. 24.1. At doses above 9 megarads no ageing was observed ; lack of further reaction may be due to the lack of mobility in the cured system, or to exhaustion of reactive groups. Evidence for the latter is provided by the limiting solubility, and by lack of further reaction on heating. However, after 10-15 min at 100°C, the dark straw colour induced by radiation faded or tended to disappear.

TRANSITION TEMPERATURE The elastic properties of polyesters containing varying amounts of

styrene, and subjected to various radiation doses or to thermal treatment for curing, were studied by Charlesby and Fukada (1958). At room tem-perature, and up to about 60°C, fully-cured specimens showed a high modulus, independent of dose, and corresponding to a highly crosslinked network typical of the glasslike state. At high temperature, the modulus fell to a much lower value, representative of rubber-like elasticity. In the intervening temperature range, covering about 30°C, the cured polyester showed transition properties (most conveniently summarized in the term "cheeselike") : modulus intermediate between the rubberlike and glasslike state, high mechanical damping, low elongation at break and a tendency to crumble. These transitions can be readily explained as due to the increasing flexibility of the chains (polyester or polystyrene) between junction points; only at higher temperatures is this adequate to allow high elastic deformations. The temperature at which these transitions occur is related to the polymerization chain length, and therefore can serve as a measure of cure.

The temperature of this transition depends on the applied frequency. In dynamic testing the time available for a chain to deform depends on the frequency of the applied stress. High elastic damping occurs when this frequency lies close to the deformation frequency of the molecular chain, and this determines the transition temperature range. In fully cured specimens studied by Charlesby and Fukada, no changes in tem-perature or frequency of maximum damping were observed on heating or further irradiation, showing that in these specimens the average chain length was not substantially altered by this treatment. The same was found to be true of thermally cured polyesters. For polyesters only


partially cured by radiation, subsequent heating caused a further, perma-nent rise in the transition temperature, due to further polymerization, even in the absence of catalyst. It would therefore appear that the limited degree of cure obtained at low doses was, at least in part, due to radicals becoming trapped in the increasingly viscous material, and that further reaction occurred when the higher temperature restored mobility.

20 40 60 80 100 120 140 160 FIG. 24.2. Elastic modulus and damping measured with both rising and falling tem-

perature: O — rising temperature, Δ — falling temperature. Ein dynes/cm2. (From Charlesby and Fukada, 1958.)

K I N E T I C S O F N E T W O R K F O R M A T I O N I N P O L Y E S T E R S

By studying the effect of resin composition, inhibitor concentration and radiation conditions (e.g. intensity) on the dose needed for full curing, an insight may be gained into the polymerization conditions in the system.


The data obtained may be compared with corresponding data on poly-merization of the vinyl monomer alone, or in a grafted polymer. Work along these lines has been carried out by Ballantine and Manowitz (1956) who studied the effect of radiation intensity, and by Charlesby et al. (to be published) who also investigated the influence of inhibitors. In related work, Charlesby et al. (1958) studied the polymerization characteristics of the polyester itself, in the absence of vinyl monomer.

Ballantine and Manowitz (1956) subjected a series of commercial samples of unsaturated polyester plus styrene to γ-radiation at intensities of a few thousand rads/min, the cure being followed by the temperature rise due to the exotherm.

In normal curing, where the chemical catalyst such as benzoyl peroxide is initiated by heat, there is an induction period followed by a rapid tem-perature rise. This may amount to as much as 200° in a few minutes and results in a non-uniform or distorted material. If the initial material is at a higher temperature, the decomposition of the catalyst may be more rapid and the temperature rise even greater. With radiation at the levels used by Ballantine and Manowitz (1200-13,500 rads/min) the time for cure was longer, the temperature rise more gradual and the maximum temperature reached by the polyester lower. Ballantine and Manowitz used the reciprocal of the time needed to attain the peak temperature as a measure of rate of cure and found that (apart from a constant) this varied as the root of the radiation intensity. Unlike the situation for a chemical catalyst system where the rate of decomposition of catalyst and hence of initiating radical formation is temperature dependent, the maximum exotherm produced in a radiation cure occurred at the same time whatever the temperature of the initial material (Fig. 24.3).

Experiments with other monomers—vinyl acetate and methyl metha-crylate—were carried out using polyester mixtures containing various amounts of polypropylene phthalate or maleate. In spite of the greater reactivity of these vinyl monomers in conventional polymerization, no improvement was observed as compared with polyesters incorporating styrene. Whereas a full cure of the latter could be achieved with doses of about 0-3-0-5 megarad at the radiation intensities used, with vinyl acetate or methyl methacrylate as the monomer much higher doses were often inadequate to provide a full cure, as measured either by insolubility or hardness. This difference is due to the lower reactivity of these monomers with maleic acid derivatives; instead of network formation a considerable degree of homopolymer formation may take place.

Measurements of the dose needed for gel formation in polyester-styrene mixtures were carried out by Charlesby and Wycherley (unpublished). In these experiments, the effect on the dose of factors such as radiation inten-sity, inhibitor concentration and degree of unsaturation of the polyester were studied. Although the transition to the gel state could be followed by the increase in viscosity, it was found to be more convenient, and suffi-ciently accurate, to determine the gel point by inspection after some pre-liminary runs. The formation of a non-flowing specimen is very sharply defined, although it is less well-marked in the presence of oxygen.


200

5C i*iO

1ΠΠ

i I y

\

40 60 Time, mîn

FIG. 24.3. Effect of initial temperature on exotherm. Upper curve : Chemical cure with peroxide.

Increased initial temperature gives more rapid reaction. Lower curve: Irradiated at 3500 r/min. Reaction time is unaltered.

(From Ballantine and Manowitz, 1956.)

Fig. 24.4 shows that the reciprocal of the gelation time for gel formation (i.e. the curing rate) in an outgassed mixture gives a linear relation when plotted against A//, where / is the radiation intensity. The same relation-ship was found in the presence of air, although longer exposure times were

Specimen outgassed before radiation

Specimen irradiated in air

20 40 60 80 sfïy (rads/min]

FIG. 24.4.

EE

0 20 40 60 _ „ / Γ , (rads/min)/2

Gelation time vs. intensity for a commercial polyester (subjected to γ-radiation in vacuum or air).


needed. It is of interest that (as in the data obtained by Ballantine and Manowitz) the curves obtained did not pass through the origin. In the presence of varying concentrations of benzoquinone, the gelation dose is increased. Fig. 24.5 shows that the gelation dose rg can be expressed in the approximate form

** = α(β) + β \ /7+γ ,

where (Q) is the concentration of benzoquinone, and α, β, γ are suitable constants.

0-4

0-3 r> o i. o cr> <u E

. 0 - 2 ω 10 o •Ό C

,o '■p J3

10 20 30 40 50 60 70 80 V T , (rads/min)/2

FIG. 24.5. Effect of benzoquinone concentration and radiation intensity on gelation dose.

benzoquinone concentration H- 1-15 x 10-5 O 4-95 x 10~5

Δ 2-12 x 10-5 + 7-64 x 10 5

Π 3-67 x 10-5 Δ 10-24 X 1 0 5

Benzoquinone can be expected to act either as an inhibitor, combining with the primary radicals produced by radiation, or as a retarder, reacting with the growing polymerization chains and reducing their average length. In the former case the gelation dose is increased by an amount directly proportional to inhibitor concentration, assuming the inhibitor is all destroyed in the process. If the reaction of the benzo-quinone is primarily to react with, and terminate growing vinyl poly-merization chains in competition with a bimolecular termination step, then its effect will depend on its concentration and on the constants of the kinetic reaction. At high concentrations it is effectively the only quantity determining polymerization chain length, and the reaction


becomes substantially independent of radiation intensity. At lower concentrations the situation is complicated by competitive reactions.*

If the assumption is made that all the benzoquinone molecules react with primary radicals, being destroyed in the process, an increased gelation dose of 0-25 megarad corresponds to the removal of about 8x l0~ 5 of benzoquinone per gram of polyester—a G value for disappearance of benzoquinone of 2-8 which appears reasonable. It must be stressed that this calculation is a considerable oversimplification, and ignores several competing reactions which may be of importance.

For a full cure of the polyester mixtures studied by Charlesby and Wycherley (1957) a dose of 7 megarads was found necessary. This corre-sponds to an average energy deposition of 0-65 eV per polyester molecule. Taking the severe criterion that in a fully cured mixture each polyester molecule is involved in only one polymerization chain, the G value for such changes is 100/0-65 or 155. This very high value is typical of a chain reaction, and is likely to be an underestimate, since in a fully cured speci-men it is likely that more than one double bond will have reacted per polyester molecule. Assuming that the G value for radical initiation is about 3, the degree of polymerization is seen to be of the order of 50. The overall G value of the reaction will be increased by carrying out the irradiation at lower intensities, this increase being due to a rise in the degree of vinyl polymerization ux rather than to a change in the G value for initiation.

The conditions for gel formation by a chain reaction require further consideration, since they differ in a number of respects from those for network formation by random crosslinking. It has not yet been clearly established whether, for a given polyester-vinyl monomer mixture, the degree of cure depends only on the percentage of monomer consumed, or whether it is also affected by the average length of the vinyl polymerization chain ux.

In a theoretical analysis summarized in Chapter 11, Charlesby (1957) found that the shape of the solubility-crosslinking index curve varies but little with effective polyester length (number of double bonds per mole-cule) or molecular weight distribution, for values of the solubility s from about 80 to 20 per cent, and in this range approximates to

s = e x p ( - y ) where γ is the number average of unsaturated bonds utilized per polyester molecule (see Fig. 11.5). This relationship is not seriously modified by the total degree of unsaturation of the polyester molecules, nor by the average degree of polymerization ux of the polymerization chain (assumed to be large). On the other hand the conditions for incipient gel formation (defined theoretically as the formation of a molecular chain of infinite molecular weight) do depend directly on the average degree of vinyl poly-merization uu the product YWX being of the order of unity at the gel point.

* Tn a recent paper Shultz (1958) has estimated the length of the intermolecular links by subjecting a thermally cured methacrylate polyester to the degradatre effect of radiation, and measuring the increased solubility.


In the case of a polyester-vinyl monomer, the average vinyl chain length Wi will depend on the radiation intensity, on the presence of transfer agents, on the nature of the termination step and on the degree of cure. The conditions for cure are therefore seen to be complex, and different results will be obtained depending on whether cure is defined in terms of an infinite viscosity or of a definite degree of insolubility.

REACTIONS OF UNSATURATED POLYESTERS The radiation reactions of unsaturated polyesters in the absence of

vinyl monomer have been studied by Charlesby et al (1958), and compared with those induced by a chemical catalyst. The effects on the reaction rate of oxygen, temperature of irradiation, radiation intensity and of additives were investigated.

Two phases of the reaction can be distinguished. At first there is a rise in molecular weight due to branching; this can most conveniently be followed by bulk viscosity measurement. Once the gel point is passed the solubility decreases rapidly. Unlike the behaviour of polyester-vinyl mixtures, these reactions (in the absence of the vinyl monomer) are sub-stantially independent of radiation intensity (Figs. 24.6 and 24.7), only

5 0

40

*c

C D

0-1 0-2 0-3 0-4 0-5 0-6 Dose, megarad

FIG. 24.6. Rise in viscosity prior to gel formation. Room temperature irradiation of polydiethyleneglycol maleate

O 5600-6000 rads/min V 1100-1600 rads/min Δ 3200-3600 χ 644 □ 1900-2400

varying by a factor of two or less when the radiation intensity is increased from about 102 rads/sec of γ-radiation, to about 10e-108 rads/sec for electron radiation from a Van de Graaff accelerator. Other differences

~

-

-

1 1 1 1 1

~AJ

1


\\J\J

8 0

70

6 0

5 0

;-* 4 0

c o

"ö ω 2 0

15

10

' — s ?

S ^

1 I

K tfN>

>

7

3 ;

V

\ \

4

> V

D (

o \

ί >\

7 É

; i i

B $

..._.t

5 10 1

r o -

>

1 12 13 1 Dose , megarad

FIG. 24.7. Decrease in solubility after gelation of unsaturated polyester (For symbols see Fig. 24.6.)

10 15 20 Dose , megarad

25

FIG. 24.8. Effect of polyester unsaturation on insolubility (Van der Graaff electron radiation)

Average number of— C = C — bonds per molecule Δ 1-6, O 2-9, □ 4-3.


are that the reaction rate is almost unaffected by the presence of oxygen, and tends to decrease at higher temperatures.

These properties (particularly the absence of an effective intensity dependence) render the reaction superficially analogous to a conventional crosslinking reaction. There are fundamental differences however. Experi-ments with similar polyesters with varying degrees of unsaturation con-firmed that the reaction proceeds primarily through the unsaturated bonds (Fig. 24.8). Even if all the absorbed energy is funnelled to these bonds the dose required for incipient gel formation (about 0-5 megarad) can only provide about 0 1 to 0-2 eV per molecule, which is far too low to provide one crosslinked unit per weight average molecule (the condition for gelation by random crosslinking). A second distinction is in the shape of sol-dose curve which approximates to the relation : log s proportional to dose.

This behaviour can be explained by assuming that a chain reaction takes place via the double bonds, but that the termination step is not a bimole-cular reaction. One possibility is some form of resonance stabilization, or a degradative chain transfer reaction, whereby a radical of greater stability is produced, unable to continue the chain reaction.

Mathematical analysis of this type of network formation (Charlesby, 1957), leads to the gelation and solubility curves having the observed characteristics. Comparison with the experimental data gives a G value for radical production of 11-4, and a short poly-addition chain length averaging 4-9.

The effect of additives on the reaction is to increase the dose for gelation, but not to affect the subsequent decrease in solubility. They must therefore be largely effective as inhibitors in the first stages of the reaction only.

Charlesby et al.y (1958), also studied the properties of the system when gelation was initiated by a chemical catalyst. The solubility decreased to a limiting value depending on the initial catalyst concentration, again indicating a limited chain reaction. The effect of additives on the reaction, and a comparison of radiation and thermal cure, are discussed in Chapter 29. These experiments emphasize the importance of vinyl monomer in the reactions of unsaturated polyesters. In their absence only a short-chain reaction occurs; in its presence the degree of polymerization is considerably greater, and the termination step is bimolecular.

ADVANTAGES OF RADIATION CURE The experiments described above enable a comparison to be made

between radiation and thermally initiated catalysts as a means of curing polyester mixtures. Radiation treatment has a number of advantages; the initiating radicals can be introduced into the system at any required rate, and can be stopped at any stage. In thermal cure, the maximum number of initiating radicals is limited by catalyst concentration ; the rise in temperature due to the exotherm of the polymerization causes an increased decomposition rate of the catalyst, so that once a reaction starts it is virtually uncontrollable. The use of radiation avoids the difficulty in


thermal cure of ensuring good dispersion of inhibitor, catalyst and pro-moter throughout the mixture, which are necessary to produce a uniform cast. The speed of the reaction can be altered over a very wide range, and the resultant product need not contain catalyst and promoter residues. By carrying out the reaction slowly, the exotherm can be reduced, and excessive strains on cooling avoided.

The ease of control rendered possible by the use of radiation has suggested the use of a two-stage process. The fluid polyester mixture containing a catalyst is set by a small radiation dose, to give flexible semi-cured sheets, films, etc. These may be stored, shaped and the cure completed by further thermal treatment.

The curing reaction may cease due to the exhaustion of catalyst, of monomer or of unsaturated sites in the polyester, or to the lack of mobility in the system. The first possibility does not arise in radiation cure, when the number of initiating radicals can be increased indefinitely by increasing the dose. Most of the published results reveal the existence of a fairly well-defined limit to the degree of cure, above which neither an increase in temperature nor in initiating radical concentration produce any further marked changes. This would point to a system in which monomer or unsaturation has been exhausted. However, other resin mixtures may exist in which the cure is restricted by the lack of mobility of the monomer. In such systems an increase in temperature, accompanied by an increased dose, should give an improved product.

An interesting feature of the experiments is the sharp distinction in reaction kinetics as between unsaturated polyesters with and without the addition of monomer. The latter systems show many of the features of some conventional vinyl monomer polymerizations (such as the dependence on 70'5), but the reaction is complicated by the changes in viscosity and the presence of other reactive groups, and by the variation in their con-centration as the reaction proceeds. As yet insufficient information is available for a quantitative determination of network structure in these systems. The use of radiation offers a novel approach to such investi-gations.

REFERENCES BALLANTINE, D. S. and MANOWITZ, B., B.N.L. 389 (T 73), May 1956. CALLINAN, T. D., ONR Symposium ACR-2, p. 24, 1954; Elect. Eng., 74, 510,

1955, Elect. Equip., July 1956; Insulation, p. 12, August 1956. CHARLESBY, A. and FUKADA, E., Rheology of Elastomers, p. 150, Pergamon

Press, 1958. CHARLESBY, A., Proc. Roy. Soc. A241, 495, 1957. CHARLESBY, A. and WYCHERLEY, V., / . Appl. Rad. Isotopes 2, 26, 1957. CHARLESBY, A., WYCHERLEY, V. and GREENWOOD, T. T., Proc. Roy. Soc,

A244, 54, 1958. CHARLESBY, A., WYCHERLEY, V. and GREENWOOD, T. T., to be published. COLICHMAN, E. L. and SCARBOROUGH, J. M., / . Appl. Chem. 8, 219, 1958. GORDON, M., GRIEVESON, B. M. and MCMILLAN, I. D., J. Polymer Sei. 17, 107,

1955; 18, 497, 1955; Trans. Faraday Soc. 52, 1012, 1956. SCHMITZ, J. V. and LAWTON, E. J., Science 113, 718 1951. SCHULTZ, A. R., / . Amer. Chem. Soc. 80, 1854, 1958.

CHAPTER 25

IRRADIATION OF POLYMERS IN SOLUTION IN previous chapters, the effect of radiation on polymers in the solid state has been shown to lead to crosslinking or degradation as well as to other chemical changes. The corresponding reactions of polymers in the liquid state do not appear to be substantially different, as may be inferred from the crosslinking of elastomers, in which the individual chain segments have considerable mobility and can in some respects be considered as liquid over very limited regions. Likewise the corresponding reactions of liquid paraffins do not differ substantially from those of polyethylene in the solid state.

The irradiation of polymers in solution, however, presents a new aspect in that the chemical changes produced in the polymer may result either from the direct effect of radiation on the polymer molecule, or they may follow reactions between the polymer and the reactive entities formed in the solvent by radiation. Thus, high energy radiation can produce a direct effect on the polymer molecule and an indirect effect via the solvent. The irradiation of polymers in the dry state can only give rise to the direct effect.

The relative importance of direct and indirect action has been widely discussed in radiobiology, which is largely concerned with the effect of radiation on complex macromolecules immersed in an aqueous environ-ment. To distinguish between direct and indirect effect, three main criteria have been adopted, based on the concept that the direct effect is substantially immune to external influences whereas the indirect effect can be more readily modified.

(i) The possibility of chemical protection by additives. It is assumed that no interference with the direct effect is possible but that pro-tection against the indirect effect can be offered by additives which compete with the polymer for the reactive entities (such as radicals) formed in the solvent.

(ii) The effect of concentration. The direct effect on the polymer mole-cule is independent of its concentration in solution, whereas the indirect effect becomes increasingly important as the concentration diminishes. In a solvent containing a weight fraction c of macro-molecules, the energy absorbed by the solvent is proportional to 1 — c, and the energy which can therefore be transferred per macro-molecule is proportional to (1 — c)/c, assuming no loss in other ways. If c is small, this fraction becomes 1/c and can be very large when c is small. This concentration dependence can account for the large effects produced in dilute solutions by relatively small radiation doses.

426

IRRADIATION OF POLYMERS IN SOLUTION 427

(iii) The physical state. The temperature, for example, can be expected to play a prominent part in the indirect effect but would have little or no influence on the direct effect of radiation on the macro-molecule.

The value of these criteria has been considerably weakened by the recent work on long chain polymers which often serve as convenient models for macromolecules of biological interest. When irradiated in the solid state, long chain polymers can only change as a result of the direct effect, but the evidence presented by Alexander, Charlesby et al. shows that radiation protection can still be offered both against crosslinking and degradation by small amounts of additive molecules. Moreover, the influence of oxygen on the biological effect of radiation runs parallel to the oxygen effect observed in many polymers irradiated in the solid state. The influence of polymer concentration in solution is far more complex than was previously considered and will be described below. The effect of temperature has also been demonstrated in solid polymers both for cross-linking and for degradation (Black, 1956; Charlesby and Davison, 1957; Alexander, et al, 1954; Alexander et al., 1955). In the case of poly/tt?-butylene, for example, the temperature dependence of degradation parallels closely that previously observed for the inactivation of bacterio-phage. As a result of these phenomena which are now well established for polymers, the sharp distinction previously drawn between direct and indirect effect has become far less convincing.

Among the many investigations which have been published on radiation effects on aqueous solutions of macromolecules of a biological character, those relating to the effects produced in nucleic acid solutions have attracted considerable attention (see references). These show marked changes in the viscosity of the solutions. Light scattering measurements (Peacocke and Preston, 1958) and intrinsic viscosity measurements (Pea-cocke and Cox, 1957) have shown that the molecular weight decreases and is inversely proportional to the square of the radiation dose. This is con-sistent with the random rupture of sugar phosphate linkages within the twin helices of the nucleate, the molecular weight decreasing when two breaks in each strand occur opposite to each other. The cross-linking hydrogen bonds of the nucleate were found to be very sensitive to γ-radiation (Cox and Peacocke, 1957). Chemical changes such as the liberation of ammonia have also been observed at high levels of irradiation.

Comparatively little work has been published on the irradiation of solutions of high polymers. Most of the quantitative data can be con-sidered in three groups :

(i) Alexander and Fox (1952-54) studied the behaviour of very dilute aqueous solutions of polymers, this work being largely intended to help in the interpretation of biological reactions.

(ii) Charlesby and Alexander (1955-57) studied the behaviour of water-soluble polymers over a wide range of concentrations and analysed quantitatively the various phenomena of crosslinking, swelling, degradation and radiation protection. Detailed studies at critical


concentrations were made by Berkowitch et al. (1957) and Danno (1958).

(iii) Wall and Magat (1953), Chapiro, et al (1955) investigated the behaviour of polystyrene in organic solvents. This work was extended by Henglein et al. (1956-58) and by Durup (1958).

DEGRADATION IN DILUTE AQUEOUS SOLUTIONS A number of polymers have been irradiated in very dilute aqueous

solutions; both the low concentrations used and the low doses needed to produce appreciable changes in viscosity confirm that the observed effects arise from the indirect effect (Alexander and Fox, 1952, 1953, 1954; Charlesby and Alexander, 1957).

Polymethacrylic acid has been studied in the concentration range 0Ό1-0T per cent. The polymer was irradiated both in the acid form and when partially neutralized by the addition of NaOH. The effect of low doses of radiation is to cause a decrease in visccsity, but only if oxygen is initially present in the solution (Table 25.1—(a)). At high doses, some degradation

Table 25.1. Degradation of Polymethacrylic Acid by x-Rays (Initial Mw = 2-1 xlO6)

Polymer con-centration %

0025 0025 0025 005 01 0025 0025 0025

Ionization degree (a)

00 00 0-6 0-6 0-6 0-6 0-6 0-6

Radiation conditions

Air Nitrogen Air Air Air Nitrogen Nitrogen Nitrogen

Dose (r)

1000 1000 1000 2000 4000

103

104

105

Decrease in viscosity %

45 4

67 62 64 3

16 85

Remarks

(a) Effect of oxygen.

(b) Effect of concentration.

(c) Effect of high doses.

a denotes the fraction of—COOH groups neutralized by NaOH; at a = 0-6, the pH of the solution is 7-1. All viscosities measured under standard conditions (see Alexander and Charlesby, 1957).

was observed in its absence, but this could be ascribed to the formation of hydrogen peroxide in the solution, thereby giving rise to an equivalent effect (Table 25.1—(c)).

The effect of concentration on the observed changes in viscosity is highly significant. In the presence of air, the dose required to produce a given change in viscosity (measured under standard conditions) is directly proportional to the concentration, indicating clearly that the changes arise from an indirect effect (Table 25.1—(b)). Unfortunately, the flow properties of dilute polymethacrylic acid are non-Newtonian and solution viscosity data cannot be used directly as a quantitative measure of changes in the density of main chain fracture. This was derived on an absolute scale by light scattering measurements and confirmed that the decrease in viscosity observed arises from degradation, and not from an internal linking of


polymer molecules, which would also result in a decreased viscosity. The results plotted in Fig. 25.1 show that the density of main chain fractures is proportional to the dose, and lead to a G value of 1 Ί for the number of main chains fractured per 100 eV absorbed in the solution. This value is in very good agreement with that obtained by other means.

x10 20

E 15 £

en 10 I

"o σ £_

5 .E σ sz o c Ό Σ

0 500 1000 1500 2000 2500 3000 Radiation dose, rads

FIG. 25.1. Weight changes in irradiated solutions of polymethacrylic acid; 0-355 per cent solution, ionized. Mw determined by light scattering. Number of main chain

fractures includes those in initial polymer, i.e. the total number of molecules. (From Alexander and Charlesby, 1957.)

In certain experiments, discrepancies arose which could be traced to the presence of peroxide groups in the initial polymer, these peroxides acting as points of subsequent weakness. As in other polymers, these peroxide groups could be destroyed by heating.

Other water-soluble vinyl polymers show degradation when irradiated in dilute solution in the presence of oxygen, but some degradation may arise even when irradiation is carried out in an oxygen-free atmosphere. In the acid form, polyacrylic acid degrades at nearly the same rate in both cases, although when ionized by NaOH, it only degrades in the presence of oxygen. Polyacrylamide behaves in much the same way as polyacrylic acid in the non-ionized form, degrading in the presence of oxygen and rather less rapidly in its absence. Polystyrene sulphonate is degraded equally readily in the absence as in the presence of oxygen. Poly vinyl alcohol and poly vinyl pyrollidone in 0 1 per cent solution show a drop in viscosity when irradiated in the absence of oxygen and a greater drop in its presence, but the situation is complicated by the aggregation of these polymers in solution.

All the experiments show that when irradiated in very dilute solution, there is a decrease in viscosity, resulting from an indirect effect of the radicals produced in water. In the case of polymethacrylic acid, irradiated in the presence of oxygen, this decrease has been shown to arise from the

1-5!

10°

0-5

| 1 1 1 - τ ^


increased number of main chain fractures. In other polymers, this con-clusion is not unavoidable; the decreased viscosity may also be due to some form of internal linking, giving rise to a molecule of smaller swept volume.

As required for an indirect effect, the observed changes are inversely proportional to polymer concentration, down to values of 0Ό25 per cent (or less) in the case of polymethacrylic acid. This strict proportionality, even at very low concentrations, indicates the large number of molecular collisions which a radical produced in water can make while still retaining its ability to react with a polymer molecule. There is some divergence of view on the radicals involved, and on the role played by oxygen. Alexander and Fox (1954) consider that oxygen is necessary for degradation, whereas Baxendale and Thomas (1958) concluded that the G value for fracture was decreased from 1Ό5 in the absence of air, to 0-64 in its presence. Addition of H 2 0 2 approximately doubled the degradation rate, due to modified reactions in the water.

The effect of oxygen and hydrogen peroxide first becomes detectable at concentrations of 10~6 to 10~5M, and there is no increased effect above 10-3M.

EFFECT OF POLYMER CONCENTRATION IN AQUEOUS SOLVENTS

Most of the water soluble polymers studied by Charlesby and Alexander (1955, 1957) degrade in very dilute aqueous solution but, unlike poly-methacrylic acid, crosslink when irradiated in the solid. At some inter-mediate concentrations one would therefore expect a change-over from one pattern of behaviour to another; one possible suggestion is that direct

—r-\ \ \ \

—

\

I \ \ \

.1— lr''

r > '

Λ''

0-5 1 2 5 10 20 50 Concentration, %

FIG. 25.2. Dose for gelation of polyvinyl pyrollidone. Jfjj range of values for gelation. (From Charlesby and Alexander, 1955.)

action causes crosslinking and indirect action results in degradation. How-ever, for solutions of polyvinyl alcohol, polyvinyl pyrollidone, polyacry-lamide or polyacrylic acid, it was found that all show crosslinking and the


formation of an insoluble network at concentrations as low as 1 per cent; this low concentration and the small doses required for gelation demon-strates clearly that the crosslinking must arise from an indirect effect. The phenomenon of crosslinking in solution is quite striking in appearance. When the dilute solution is irradiated, its viscosity rises relatively slowly, then with remarkable suddenness a gel structure is formed which fills the specimen tube. This gel may be removed from the container and swollen further in water; the limiting degree of swelling attained being determined by the initial concentration of polymer and the dose. That a true network is formed can be shown by drying the gel and re-swelling, when the same limiting degree of swelling is reached.

If the gelled specimen, still immersed in its container, is irradiated further, the equilibrium degree of swelling is reduced until it falls below that able to take up all the water in the initial solution. At this stage the gel exudes the excess water, and breaks away from the container wall, but retains the shape of the latter. Increased radiation causes further shrink-ing. This behaviour would indeed be expected from an increased density of crosslinking.

The relation between minimum dose for gelation and polymer concen-tration in Fig. 25.2 shows a marked transition at about 0-5 per cent. Above a polymer concentration of 1 per cent, the gelation dose increases with concentration as is to be expected from the indirect effect; the larger the number of polymer molecules, the smaller the share of each in the energy absorbed by the solvent. The outstanding feature of this curve, which is typical of other polymers, is the sharpness of the transition at about 0*5 per cent concentration. At lower concentrations, no tendency to gel formation is observed, and instead of increasing at first, the viscosity decreases steadily with dose.

(a) (b) (c)

FIG. 25.3. Formation of a crosslinked gel by irradiation of a solution. (5 per cent polyvinyl alcohol in water).

(a) solution in open vessel (b) crosslinked gel, swollen in water, fills vessel (c) after further irradiation, the density of crosslinking rises, and the gel which

can no longer absorb all the water, shrinks away from the walls of the vessel.


As with crosslinking in solid polymer, the minimum dose for gel formation is independent of radiation intensity and is little affected by oxygen.

Table 25.2. Effect of Radiation Intensity on Gelation Dose Poly vinyl Pyrollidone

Radiation intensity (r/min)

3100 1720 1030 575

Gelation dose (megarads)

2 per cent solution

0-89-10 0-82-1-1 0-81-10 0-92-1-0

15 per cent solution

2-4-2-6 2-5-2-8 2-5-2-7 2-5-2-7

(Source: Charlesby and Alexander, 1955.)

D E G R A D A T I O N I N O R G A N I C S O L V E N T S The degradation of polystyrene and polymethyl methacrylate in organic

solvents was investigated by Wall and Magat (1953) who followed the reaction by the decrease in intrinsic viscosity. This reaction occurred in the presence of oxygen, which can react with the polymer radicals formed by fracture and produce an after-effect. According to Chapiro et al. (1955, 1957) an effect may also be obtained at high doses in the absence of

Έ I 5x I0-5

jt-Q.

L °\s

«5 0-5xl0~5

"E 105 2 X 1 0 5 3X105 4x105

2 Initial molecular weight

FIG. 25.4. Influence of molecular weight of initial polymer on number of breaks per molecule. Polystyrene in chloroform.


oxygen, due to the direct action of radiation on the polymer molecule. The number of chain breaks is directly proportional to the dose and to the initial molecular weight (Fig. 25.4), i.e. the energy per break is indepen-dent of molecular weight.

The post-effect was studied by Fox (1953, 1955) and by Durup (1958); the latter distinguishes two processes, one giving a short-lived


intermediate, the other (hydroperoxide) being relatively stable at ordinary temperature.

Henglein et al. (1957, 1958) investigated a wide range of solvents and found that in poor solvents polystyrene crosslinking is favoured above a 20 per cent concentration, whereas in good solvents only degradation occurs. Additives such as iodine, DPPH, benzoquinone, reduce the degree of crosslinking. In solutions of polymethacrylic esters, oxygen may increase or reduce degradation, depending on the solvent.

The work of Wippler (1958) on the irradiation of poly vinyl chloride in organic solutions shows some interesting features. Whereas the dry polymer crosslinks with difficulty, in certain organic solvents it crosslinks readily at low doses, forming a swollen gel. During the reaction gases are evolved, but the effect of oxygen is relatively small. The degree of cross-linking varies with the nature of the solvent, being negligible in cyclo-hexanone, and very high in dimethylformamide. Most of the work was carried out in tetrahydrofurane.

Below the gel point the reciprocal of the weight average molecular weight (1/Ma/) obtained by light scattering measurements decreases with the dose, in accordance with equation (9.14).

Mw (1 Mw

-qûtf)

The corresponding G value per crosslink is 2-15, a value of the same 2·0 Γ

20 30 40 100/Λ; r in mega reps

FIG. 25.5. Gel formation in PVC dissolved in tetrahydrofuran (20 per cent PVC, cobalt 60 γ radiation).

(Data from Wippler, 1958.)


order as for many other polymers. However, the viscosity average molecular weight shows no corresponding rise as crosslinking proceeds; on the contrary it may even diminish. This can be understood in terms of internal crosslinking, which could diminish the viscosity of a solution as the molecules became increasingly branched.

The occurrence of simultaneous crosslinking and degradation in these solutions is best illustrated by a plot of s + Vs vs. \\r (Fig. 25.5) (equations 9.24, 11.7). Except possibly close to the gel point, the plot obtained is effectively linear, but does not tend to infinity at large doses. The extra-polated value of s + Vs is 0-15, which is also the degradation-crosslinking ratio p0/q0 (equation 11.7). The slope of the curve is \/q0uu and if the initial distribution is assumed random, u1 can be deduced from the light scattering value of Mw. This leads to a G value for crosslinking (equation 9-4) of 2-5. The G value for degradation with p0lq0 equal to 015 is there-fore 2x0-15x2-5 or 0-75, and the effective G value (corresponding to q0—po/2 in equation 11.6) is 2-5—0-375 or 2-125, in excellent agreement with the value deduced from the light scattering measurements below the gel point.

This accord with theoretical predictions over the entire range of reactions shows that once adequate mobility has been acquired by the polyvinyl chloride molecule, crosslinking and degradation proceed as in other polymers irradiated in the solid form.

Little effect of polymer concentration was observed over the range 15-80 per cent, and the increased ease of crosslinking, as compared with the dry polymer, must therefore be ascribed to the greater mobility of the polymer molecules in solution, rather to an indirect effect (as in water-soluble polymers).

With initially linear polymers which degrade under radiation, no difficulties arise in the interpretation of viscosity average molecular weight, and Henglein et al. (1957) has made use of this technique to study the degradation of polymethylacrylic esters in a variety of solvents. Protection effects were observed; for example the G value for main chain fracture, which is 1-2 in ethyl acetate, dropped to 0-48 in benzene (in the absence of air) presumably due to energy transfer leading to radiation protection (see Chapter 29). In chlorine-containing solvents such as chloroform or carbon tetrachloride, the G value is enhanced to 4 and 6.4. This increase is ascribed to the reaction of the chlorine evolved from the solvent 4 with the polymer molecule.

MECHANISM OF POLYMER REACTIONS IN AQUEOUS SOLUTION

The behaviour of long chain polymers irradiated in solution shows a number of distinct features which must be accounted for by any theory of radiation mechanism. The main features involved are: (i) a decrease in viscosity at low concentrations such as corresponds to main chain fracture; (ii) crosslinking at higher concentrations; (iii) a sharp transition from


one pattern of behaviour to the other, occurring in a very limited concen-tration range; (iv) an increased radiation dose needed for gelation at higher concentrations at least in aqueous solutions.

Among the possible reactions which may be considered are: (a) a distinction between direct and indirect action, the former causing

crosslinking, the latter degradation; (b) the formation of a network by endlinking as distinct from cross-

linking; (c) the formation of unstable centres on the polymer molecules which

can either crosslink or degrade depending on the environment ; (d) competition between internal and external links in the molecular

structure. The assumption that direct action results in crosslinking whereas

indirect action leads to degradation, can be ruled out by the great difference in G values required to give a transition at a concentration as low as 1 per cent; this will require a difference in G values of the order of 100 to 1 for direct and indirect action. Moreover, the dose for gel formation by direct action should be constant or even decrease at high concentrations and this is certainly not the case.

The theory of endlinking outlined in Chapter 11 has been proposed as an alternative to crosslinking. Whereas crosslinking assumes the fracture of side chains, and the formation of a bond between molecules lying side by side, endlinking assumes the fracture of main chains, and the formation of activated endgroups which can combine with neighbouring molecules. Crosslinking gives rise to tetrafunctional, endlinking to trifunctional junction points. In either case an insoluble network can be formed, so that the existence of such a network is not conclusive evidence of cross-linking. If the process of endlinking is assumed to occur, it may well depend on concentration; at high concentrations, the active endgroup has a high probability of meeting another molecule and forming an endlink; at very low concentrations the probability of such encounters is reduced and the fractured molecules may stabilize themselves by an internal rearrangement, leading to a degraded product.

For an initially random molecular weight distribution, incipient gel formation by endlinking can be shown to occur when there is an average of one main chain fracture per three molecules, the two fractured ends linking themselves on to adjacent molecules to form trifunctional junction points. There are therefore two active polymer ends per six inactive ends needed to form an incipient gel.

If only a proportion a of the radiation fractured chains form endlinks, the remainder resulting in inactive groups, the condition for incipient gel formation becomes

2fa = i[2f(l-a) + 2A0] or

Sfa = 2f+2A0

f=AlAa-X) FF


where/is the number of radiation induced fractures, and A 0 is the number of polymer molecules initially present. The corresponding gelation dose (equation 11.16)

rgei = 0-96 x 106/M„G (4a-1)

This equation applies when polymer molecules alone are irradiated. In solution the number of molecules per gram of solution is reduced by a factor c equal to the polymer concentration

rs = 0-96 x 106 c/MnGs (4a-1)

where rs is the gelation dose for the solution, and Gs is the number of main chain fractures per 100 eV absorbed in the solution; both Gs and a may vary with concentration. If in the absence of a solvent it is assumed that all polymer fractures result in endlinking, a = 1 when c = 1. The gelation dose in solution as compared with the dry polymer is reduced in the ratio

rs _ 3G /'gel (Aa-\)GS'

The observed variation of rs with concentration c is shown in Fig. 25.6 20r

10

ί« 5

o <y

I O

\

I I \ I JL *■

^Y* ^ ^F Λ" H

0-5 1 2 5 10 Concentration-

50 100

FIG. 25.6. Top curve: gelation curve for poly vinyl pyrollidone.

Bottom curve: calculated value of a assuming G=GS (note that this curve shows no sudden transition).

(From Charlesby and Alexander, 1955.)

(top curve). If the energy absorbed per fracture is independent of concen-tration, the variation of a with concentration may be deduced from these experimental results (bottom curve of Fig. 23.6). It is seen that the curve obtained is smooth, and that the sudden transition at about 0-7 per cent concentration no longer appears. It may therefore be concluded that the

IRRADIATÎÔK ÔF POLYMERS IN SOLUTION 437

observed changes in the gelation curve can be explained in terms of a gradual reduction in the proportion a of main chain fractures which cause endlinking; in particular, the sudden transition observed in many poly-mers at about 1 per cent can be explained mathematically without assum-ing any abrupt change in the physical process. This suggestion therefore has the advantage of mathematical and physical simplicity, but it must be added that so far the evidence for endlinking is indirect and is largely confined to irradiated low molecular weight paraffins where intermediate products have been observed. One objection made to this theory states that if endlinks are produced by the reaction between a fractured end on one molecule and a radical on the second molecule, the product may depend on radiation intensity. This objection does not arise if the reaction is one between the fractured end group and a neutral molecule.

The third hypothesis suggested above assumes competition between net-work formation and degradation, but in this case the link is of the conven-tional crosslinking type. The theory assumes that the active sites pro-duced on polymer molecules have a limited life, and can only form a crosslink if a second neutral molecule becomes available for crosslinking within this time. In other cases, the activated polymer molecule is assumed to degrade. The dose for gelation with this model can be deduced (Alexander and Charlesby, 1957):

10e ac rgei = —

GMna 4—5 exp(—ac)

where a is related to the time available before an activated molecule rearranges to give a stable degraded product. On the assumption that for polyacrylic acid G^l as it is for the dry polymer, the value deduced for a is 50 and the calculations not only show that the minimum dose for gelation occurs at 1-2 per cent but also give a correct value for the minimum dose required for gelation. Fig. 25.7 shows the agreement obtained with this calculation on the assumption that a = 55 for polyvinyl pyrollidone.

2X106

σ t_

* 4X105

Φ

Q 2xiCr

105

0-5 1 2 5 10 Concentration, %

FIG. 25.7. Gelation curve for polyvinyl pyrollidone. Experimental, I. Theoretical, a= 55.

(From Alexander and Charlesby, 1957.)

I 1

\ I

V I vi— ^

l-^

A Λ

'


The final theory considered here assumes that both direct and indirect action lead to crosslinks being formed. An activated unit in one polymer molecule can link itself on to another monomer whether this be on the same molecule or on the neighbour. In the former case, one can refer to an internal link (which produces a cyclic structure in the polymer mole-cule) ; in the latter case the usual form of external link is produced which eventually gives rise to a network. About each monomer unit one can therefore consider two concentrations, the external concentration c being due to monomer units on other chains, while the internal concentration Ci arises from monomer units of the same molecule. Ci depends primarily on the chain configuration and on the molecular weight of the polymer but is little, if at all, affected by the external concentration c. The ratio of links formed which occur between monomer units on different mole-cules is therefore cl(c+Ci). The number of activated monomer units per gram capable of crosslinking is 0-625 xl01 8Gr with the appropriate value for G. Of these a proportion a can give rise to crosslinks. Both G and a may depend on concentration. The total number of effective links, i.e. those able to link to other molecules, is therefore

0-625 x\018Gmcl(c+a)

and the number of crosslinked units* per weight average molecule is equal to

2 x 1 04 x 10~6GrocMw/(c+ a).

For incipient gel formation, this quantity must equal unity (equation 9.16). Then the gelation dose

rge l = 0-48 X 10&(c+Ci)/G<xMw.

By making various assumptions as to the variation of G and <x with concentration c, Charlesby and Alexander (1955) obtained curves of rgei against c which showed a minimum, but these curves gave no evidence of the sudden transition which is the most striking feature of the gelation dose-concentration curve.

A more detailed consideration of the process of internal linking may, however, lead to a different conclusion. In a solution, the molecular chains are in a constant state of motion and even though the average molecular arrangement is constant, there is a continuous change in the distance between individual units. This change imposes a bias on the system in that internal crosslinks can only be formed when two monomer units come into close proximity even though their average distance apart may be considerably greater. Once the internal crosslink is formed, the two chains are linked together permanently and this imposes a constraint

* Assuming two crosslinked units per link, only one of which is "activated" directly by radiation.


on adjacent units. As more internal links are formed, the molecule shrinks and the internal concentration C( round each unit increases. One can therefore visualize the position arising where above a critical concen-tration most links formed are external and the macrogel is formed, whereas below it sufficient internal links are formed to give rise to molecules of decreasing volume which rapidly decreases the chance of further external links being formed. The mathematical formulation of these conditions has not yet been derived so that no quantitative comparison is possible. One would expect the transition to occur at a concentration c which is of the same order as C{.

At present, these three mechanisms (endlinking, crosslinking vs. degra-dation, internal linking) explain qualitatively most of the experimental data on crosslinking and degradation in solution. The observed decrease in viscosity in solution is, however, not a decisive test of polymer degrada-tion, since this decrease may also be due to the reduction in swept volume of each molecule due to internal linking. Some very recent data obtained by Berkowitch et al. (1957) in irradiated poly vinyl alcohol may be mentioned in this connexion. In this work the changes in viscosity and in weight average molecular weight (from light scattering measurements) were investigated in detail for concentrations in the neighbourhood of the transition at 0-5 per cent. For concentrations sufficient to give a gel, a steady rise in the viscosity with dose was observed. For slightly more dilute solutions, the viscosity showed an initial rise followed by a more rapid fall, while for very dilute solutions the viscosity fell continuously with radiation. Moreover, the weight average molecular weight for a non-gelling polymer rose with dose to a maximum and then remained constant. These latter observations indicate a limited amount of crosslinking both external and internal, followed by internal linking only. The maximum weight average molecular weight reached would therefore give an indica-tion of the overlapping volume of different polymer molecules in solution, and the difference between gelling and non-gelling concentration would be due to the distinction between macro- and microgel. It would still be necessary to explain the sharpness of the transition for systems which contain a very wide distribution of molecular sizes.

Similar results were obtained by Danno (1958) who also favoured the formation of internal links. However, to explain the complex nature of the viscosity/dose curve, he assumed a combination of crosslinking and degradation, but it is difficult to reach any firm conclusion, purely from intrinsic viscosity data, owing to the presence of branched molecules.

Perhaps the most striking feature already revealed in this work is the reversal from crosslinking to apparent degradation which can occur in irradiated polymer solutions as a result of relatively minor changes in concentration or pH. It may be inferred by analogy that in the field of radiobiology equally small changes in experimental conditions may pro-duce completely disproportionate changes in the behaviour of biological materials under radiation; in investigating such changes, even semi-quantitatively, the environment must be closely specified.


REFERENCES Nucleic Acid in Solution CONWAY, B. E., Brit. J. Radiol. 27, 36, 1954. CONWAY, B. E. and BUTLER, J. A. V., / . Chem. Soc. 834, 1952. Cox, R. A., OVEREND, W. G., PEACOCKE, A. R. and WILSON, S., Nature, Lond.

176, 919, 1955. Cox, R. A. and PEACOCKE, A. R., / . Chem. Soc. 2499, 2646, 1956; 4724, 1957. DANIELS, M., SCHOLES, G. and WEISS, J., Experientia 11, 219, 1955. PEACOCKE, A. R. and Cox, R. A., Trans. Faraday Soc. 53, 250, 1957. PEACOCKE, A. R. and PRESTON, B. N., / . Polymer Sei. 31, 1, 1958. SCHOLES, G., STEIN, G. and WEISS, J., Nature, Lond. 64, 709, 1949. SCHOLES, G. and WEISS, J., Biochem. J. 53, 367, 1953; 56, 65, 1954.

Aqueous Solutions ALEXANDER, P. and CHARLESBY, A., J. Polymer Sei. 23, 355, 1957. ALEXANDER, A. and Fox, M., / . Chim. Phys. 50, 415, 1953. BAXENDALE, J. H. and THOMAS, J. K., Trans. Faraday Soc. 54, 1515, 1958. BERKOWITCH, J., CHARLESBY, A. and DESREUX, V., J. Polymer Sei. 25, 490,

1957. CHARLESBY, A. and ALEXANDER, P., / . Chim. Phys. 52, 699, 1955; Int. Symp.

Macrom. Chem., Rehovoth, 1956. DANNO, A., / . Phys. Soc. Japan 13(7), 722, 1958. Fox, M. and ALEXANDER, P., J. Chim. Phys. 52, 710, 1955. KHENOKH, M. A., Zh. Obsch. Khim. 11, 776, 1941; 17, 1024, 1946.

Organic Solvents CHAPIRO, A., Industrie des Plastiques Modernes, p. 34, February 1957. CHAPIRO, A., DURUP, J., FOX, M. and MAGAT, M., Symposia Inter, di Chim.

Macrom., Milan, Turin, 1954; La Ricerca Scientifica 24, 207, 1955. DURUP, J., / . Chim. Phys. 51, 64, 1954; 54, 739, 746, 1957; / . Polymer Sei. 30,

533, 1958. Fox, M., C.R. Acad. ScL, Paris 237, 1682, 1953; Radiobiol. Symp., Liege, 1954,

p. 61, Butterworths, 1955. HENGLEIN, A. and BOYSEN, M., Makrom. Chem. 20, 83, 1956. HENGLEIN, A., BOYSEN, M. and SCHNABEL, W., Zeits. Physik. Chem. 10, 137,

1957. HENGLEIN, A. and SCHNEDIEER, C , Rad. Res. 2, 128, 1958; Zeits. f. Phys. 18, 56,

1958. HENGLEIN, A., SCHNEIDER, C. and SCHNABEL, W., Z. Phys. Chem. 12, 339, 1957.

1957. SCHMITZ, J. V. and LAWTON, E. J., Twelfth Congress LU.P.A.C., New York,

1951. WALL, L. A. and MAGAT, M., J. Chim. Phys. 50, 308, 1953; Mod. Plast. 30,

(July), 111, 1953. WIPPLER, C , Radioisotopes in Scientific Research 1, 174, 1958, Pergamon Press.

Temperature Effects ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. A232, 31,

1955. ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. BLACK, R. ML, Nature, Lond. 178, 305, 1956. CHARLESBY, A. and DAVISON, W. H. T., Chem. and Ind. (Rev.) 232, 1957.

CHAPTER 26

REACTIONS OF IRRADIATED MOLECULES

PREVIOUS chapters have discussed the properties of a number of irradiated polymers and have shown that these can usually be related to changes in their chemical structure. A simple overall relation can often be derived between the radiation energy (and sometimes intensity) absorbed in such a system, and the amount of chemical change produced. For a fuller understanding of the radiation process it is however necessary to study each of the intervening steps and here the situation becomes far less clear.

The original absorption of energy from the incident radiation can be viewed as a purely physical process, and its distribution throughout the irradiated material occurs at random, independent of chemical structure. Details of the process by which this energy is deposited in the individual molecules have been discussed in Chapter 3 as an introduction to the chapters dealing with radiation sources. On the other hand most of the final reactions are very specific to individual chemical groups and are directly comparable with thermochemical reactions; many of the rate-constants, for example, are identical. The main gap in our knowledge lies between these two processes.

Among the problems which arise are the role played by ionization, as compared with excitation; the mechanism by which energy, initially absorbed at random, is transferred or converted to give specific reactions, mainly confined to only a few of the different chemical bonds; and the essential differences between radiation chemistry, photochemistry and thermochemistry. These and similar questions constitute the basic problems of radiation chemistry. The study of irradiated polymers under varying conditions can provide evidence on these matters ; relevant infor-mation can also be obtained from other fields, such as mass spectroscopy, photochemistry, electrical conductivity and quantum theory, as well as from radiation studies on simple molecular compounds. Mass spectro-scopy, for example, is confined to the study of breakdown patterns of ionized molecules under conditions of very low pressure which greatly reduces the probability of any intermolecular reaction. In photochemistry solids and liquids may be investigated, but the energies available are usually too low to permit ionizations, and the reactions are those of excited molecules. Electrical conductivity measurements may provide data on the fate of the ejected electrons, while theoretical analyses of electron orbitals may investigate the stability of some simple ionized or excited systems. The study of model compounds irradiated in the presence of additives capable of reacting with radicals may also be expected to provide infor-mation on some of the intermediate products.

441


In spite of the considerable wealth of information available from this wide range of scientific techniques, no convincing explanation can yet be given of the processes intervening between the primary acts of ionization and excitation, and the final chemical change. Experimental work at present proceeding may be expected to provide a much clearer picture within the next few years; in the meantime it is considered necessary to limit oneself to describing the type of information which can be gathered from each source, its limitations, and any tentative hypotheses which can assist those working in the field. The present chapter deals with the reactions immediately following the initial ionization and excitation, which eventually lead to the breakdown of the molecule to give stable products, or reactive products such as radicals. Chapter 27 discusses the formation of radicals from these primary reactions; this information is directly relevant to such reactions as vinyl polymerization. Unfortunately little quantitative information of a parallel character has been derived for the formation of ionic species in solid or liquid organic compounds.

A number of theories have been proposed to account for crosslinking or degradation in polymers. At the present stage these must necessarily be considered as purely tentative in character; they are summarized and discussed in Chapter 28. Finally, Chapter 29 deals with radiation protec-tion effects in polymers—processes by which small changes in chemical structure within a molecule or in its neighbourhood can exert a dispro-portionately large influence on the overall chemical reactions.

BEHAVIOUR OF THE EXCITED MOLECULE

In an excited molecule the energy is increased by the higher energy level of one or more electrons. Under certain conditions this energy may be transferred to vibrational energy, and cause a rearrangement of the molecular structure, or result in its dissociation. In photochemistry these conditions have been studied in some detail (see, for example, Franck and Platzman, 1954). Where electron excitation occurs by exposure to high energy radiation the range of excitation energies may be considerably greater, and the number of possible reactions can be greatly increased.

In studying the conditions for this transfer of energy, an important principle—the Franck Condon principle—must be considered. This prin-ciple is based on the great difference in mass between an electron and a nucleus. When an electron transition occurs, either as a result of an absorption of a photon or by interaction with a charged particle of high energy, the time taken is very much smaller than is needed for any appre-ciable change in the distance between nuclei. In Fig. 26.1 the energy levels for the ground state (I) and for an excited state (II) of a diatomic molecule are shown in terms of the distance between nuclei. A molecule initially in a ground state (a) with a small amount of vibrational energy will, on excitation, be raised to a higher level in the excited state with the same internuclear distance and may hence acquire sufficient potential energy to allow subsequent decomposition, even though the excited state may itself have a stable level (b). Under these conditions dissociation may take

REACTIONS OF IRRADIATED MOLECULES 443

place extremely rapidly—in a time of the order of one atomic vibration or 10-13sec. '

A second possibility, termed predissodation, may arise when the excited level (II) would itself give a stable molecule, but the level (II) intersects a

Internuciear distance

FIG. 26.1. Transition from a ground state (I) to an excited state (II), followed by dissociation.

Internuciear distance

FIG. 26.2. Transition from a ground state (I) to an excited level (II), followed by transfer to (III) allowing dissociation.

level (III) which allows dissociation. Certain quantum mechanical con-ditions must be fulfilled for the transition from (II) to (III) to take place, and the time necessary to meet these may be considerably longer than for direct dissociation from (II). Under these circumstances the time for dis-sociation may be very protracted, and other possible reactions, such as emission of radiation or quenching by collision, may remove the excess energy and prevent dissociation.

When polyatomic molecules are considered, the situation becomes far


more complex, owing to the great variety of possible levels for electronic, vibrational and rotational energy. This will increase the probability of changes in the subsequent distribution of energy between these states by intersection of the corresponding energy levels, and the absorbed energy may be reallocated over a number of vibrational states. Owing to the large number of degrees of freedom a considerable time may then elapse before conditions are suitable for decomposition to take place. In poly-atomic molecules predissociation, and a further process termed internal conversion, are favoured. Both these processes involve a transfer from an excited state (II) to a third state (III), but whereas in predissociation this transfer results in decomposition of the molecule, in internal conversion the excess energy is utilized in increasing the vibrational energy of the molecule. This vibrational energy may then be dissipated amongst a number of degrees of freedom, giving the equivalent of a molecule at high temperature. Thus predissociation leads to the decomposition of a mole-cule into atoms or radicals, whereas internal conversion degrades the electronic energy into heat. The high vibrational energy arising from internal conversion may in itself eventually cause decomposition in a manner similar to pyrolysis.

The removal of an electron from its ground state may allow new con-figurations of the nuclei and the molecule can eventually end in a different equilibrium configuration such as an isomer. Changes in isomerism resulting from high energy radiation may be explained on these lines.

BEHAVIOUR OF THE IONIZED MOLECULE

Ion molecules can be produced either directly, by stripping off an electron, or indirectly, from a highly excited molecule. The ionization potential for an individual atom is the minimum energy required to detach an electron and ranges from 3-9 eV for caesium to 24-6 eV for helium. Multiple ionization requires higher energies, but with high energy radiation this is not necessarily a very rare occurrence (Platzman, 1952). According to Varley (1956) doubly ionized atoms may occur at about 1-10 per cent of the frequency of singly-ionized atoms. In hydrocarbon gases at low pressure, however, multiple ionization accounts for less than 1 per cent of the ions observed, probably due to rapid decomposition by mutual repulsion in the absence of external restraints.

An excited molecule may have more energy than is required to cause ionization, if the electron involved has been removed from an inner level. The excess energy may be lost in several ways, e.g. directly by emission of fluorescence, or indirectly by collision, or it may be transferred inter-nally to another electron energy level to give rise eventually to an ion. The situation is somewhat similar to that described above for excited states leading to predissociation, and is termed pre-ionization. In an excited polyatomic molecule the time elapsing before the molecular ion is formed allows for some selectivity in the course of the reaction. Although any electron may be excited to a high level by high energy radiation, certain forms of ionized structure may therefore be favoured.


Ions are usually formed with considerable excess of vibrational energy; they may dissociate either in a single thermal vibration (if simple in structure) or over longer periods in more complex systems where the vibrational energy is distributed over a number of degrees of freedom. An alternative process is for the positive ion to capture an electron giving a highly excited molecule which then dissociates most frequently into two radicals, one or both of which may be highly excited.

The energy required to form a molecular ion is usually higher than can conveniently be obtained from ultraviolet light, and far less information is provided by photochemistry on ionized than on excited molecules. Considerable data have been obtained by subjecting molecules to bom-bardment by low energy electrons, and studying the potential at which each particular type of ion with its characteristic form of dissociation is first produced. Values of this "appearance potential" are given in the book by Massey and Burhop (1952). However, mass spectroscopy data has shown that the breakdown pattern of hydrocarbon molecules varies but little with the ionizing potential applied in the range 50-175 volts (Table 26.1). Since these energies are typical of δ electrons, no marked

Table 26.1. Distribution of Ions from Ethane in Mass Spectrometer

Ion

Appearance potential (eV)

Electron energy (V)

30 50 60 70

100 175

C2H6+

116

1 C.H.+

12-7

C2H4+

121

C2H3+

15-2

C2H2+

150

Q H +

27

CH 3+

14-2

Percentage of all ions formed

15-7 12-4 13-6 12-4 13-3 13*5

11-3 101 10-5 101 10-5 10-6

54-5 47 48 47 48-2 48-5

11 10-6 14-7 10-6 15-2 15

4-92 101 10-2 101 10 9-5

1-9 1-6 1-25 1-9 1-58 1-4

115 2 1 1-2 2-2 1-55 1-35

(From Hippie, 1938; Magat and Viallard, 1951.)

differences need be expected due to variations in radiation energy. Further-more the amount of any ion produced does not appear to be related to its appearance potential, the fracture pattern of a molecule in the mass spectrometer being determined primarily by its chemical structure.

Ionization of hydrocarbons in the mass spectrometer may result in the formation of a relatively stable parent ion (which is detected in the spectrometer), of ions formed by fracture of C—C or C—H bonds, or more complex rearrangements. A number of general rules relating to the


fracture pattern have been summarized by Magat and Viallard (1951); the following appears most relevant here.

(i) Usually only one of the products formed is an ion. (ii) The number of parent ions detected decreases rapidly with increas-

ing chain length (Table 26.2). Thus in methane 47 per cent of the

Table 26.2. Ions Formed (a) by simple loss of electron (parent ions) (b) by simple loss of H.

Figures are percentage of total ions

CH4 C2H6 C3H8 «C4H16 «C5H12 /?C6H14 «C7H16 «C8H18

/zC9H2o λ7(-"1θΓΐ22

rtCnH24 ^ ^ 1 2 ^ 2 6

(a)

46-6 120 9-2 4 0 2-8 2-6 2-8 1-8 1-54 1-34 0-82 0-87

(b)

84-0 22

2-5 0-33 0 1 0015

C2H4 C3H6 «C4H8 "C5H1 0 / /CßH]^ flC7H14

#C 8 H i e "C9H1 8

#CioH2o

(a)

401 19 12-2 8-8 4-5 2-6 1-7 106 0-8

(b)

55 63-7 115 1-2 0-25 0037

(From Magat and Viallard, 1951.)

ions formed are CH4+; in propane the figure decreases to 9 per

cent, and m decane to 1-3 per cent. The same is true for olefins and acetylenes.

(iii) The yield of hydrogen ions is small, and decreases rapidly with increasing chain length. In general the fracture of C—H bonds occurs most readily in tertiary carbon atoms (Stevenson, 1951).

(iv) In long-chain «-paraffins C—C fracture occurs most readily in the C3—C4 position. In this respect decomposition following ionization differs from that following pyrolysis, where the Cx—C2 bond is most vulnerable.

(v) Branching may have a considerable effect on the fracture pattern. In linear paraffins simultaneous fracture of several C—C bonds is a rare occurrence but in the branched series the products are more complex, and can only be explained in terms of an internal re-arrangement prior to fracture. Branching has little effect on the proportion of parent ions (except when two branches are carried by a single C, when it becomes negligible). However, branching increases the probability of C—C and C—H fractures.

Very rapid rearrangement of ionized molecules can often be deduced from the nature of the ions observed in the mass spectrometer. Slower


dissociations lasting about the time of flight of the ion (10 - 6 sec) can be traced more directly. Hippie (1948) gives as an example the transition

C4H10+ -* C3H7+ + CH3-

where the parent ion has a mass of 58, the daughter ion 43, while in the mass spectrometer a diffuse peak is also observed at 31-9. This value is derived from the sharing of the momentum acquired in the spectrometer gun, between the propyl ion and the methyl radical, so that the propyl ion, which is alone observed, apparently has on the average 43/58 of its true mass.

Although suggestive of certain tendencies observed in the irradiation of organic molecules in the liquid or solid phases, information obtained from mass spectrometry cannot be readily applied to the condensed phase. For example, only information on ionized molecules is provided—excited molecules or radicals are not detected. In the low-pressure gases to which the technique is limited, the ion does not usually recapture an electron, and the primary ion is allowed to fracture or decompose in isolation. Energy transfer, and deactivation by collision cannot take place, and reactions between molecules or ions are not usually possible. In the solid or liquid phase the number of possible reactions is greatly increased, and the more rapid reactions will therefore be favoured. The cage effect, which operates in the condensed phase to restrict the evolution of higher mole-cular weight products, does not occur in the mass spectrometer. Hydrogen evolution is thereby favoured in liquid or solids ; in irradiated long chain paraffins it accounts for over 95 per cent of the gases produced, whereas in the mass spectrometer removal of hydrogen decreases rapidly with increasing chain length, and for pentane is already less than 1 per cent. Even the energy needed to form an ion pair may be modified by the dielectric constant of the medium. In spite of these differences Davison (1958) has drawn some interesting analogies in the fracture pattern of lower «-paraffins.

FATE OF THE FREE ELECTRON

In gases the electron liberated by ionization may have sufficient energy to cause further ionization, but it will eventually either recombine with a positive ion, or be trapped to give a negatively charged ion which can be readily detected, or even be collected as a free electron. In solids or liquids secondary ionization also occurs but no simple technique exists to discover the eventual fate of the electron, and several views have been advanced. In one view proposed by Samuel and Magee (1953) for water, the electron never leaves the field of its parent ion, and is eventually recaptured to give a highly excited molecule. All such radiation-induced reactions are primarily those of excited species. An alternative suggestion is that the electron does become free of this field, and, after losing most of its excess energy in further excitation or ionization or by collision, is trapped by another positive ion, or by a molecule of high electron affinity such as oxygen or a halide which may subsequently dissociate. Most


organic molecules have a low electron affinity (Table 26.3), so that when irradiation is carried out in air, many electrons may be captured by oxygen, possibly giving rise to some of the peroxidic structures often observed :

02 + e -> O r RH + 02- -> ROOH + e, etc.

However, peroxides may also be formed by the combination of an oxygen molecule with a radical produced by radiation,

R· + 02 -> ROO.

Magee and Burton (1951) have discussed the formation of negative ions by electron capture. For many molecules the negative ion immediately dissociates. The C—C bond is likely to fracture under these conditions while the C=C bond is able to resist rupture.

Table 26.3. Electron Affinity of Some Atoms

Atom

H C O F Cl Br I

eV

0-75 (07) 1-45 3-5 3-8 3-5 3-2

kcal/mole

1-74 (16) 33 82 87 82 75

(See Baughan, 1959.)

Subexcitation Electrons Weiss (1954) and Platzman (1955) have pointed out that after several

collisions an electron left with insufficient energy to cause excitation of most of the molecules in the system, may yet be capable of reacting with additives or impurities if these have lower energy levels than the major constituent. According to Platzman the amount of energy available in the subexcitation electron may still be quite appreciable (about 20 per cent of the total energy absorbed) and this could cause large changes in a minor component without causing any loss of the energy utilized by the major component.

The phenomena of fluorescence and phosphorescence, observed in some irradiated polymers, may be explained by the temporary trapping of the electron emitted by radiation (Wilson, 1948, Winogradoff, 1950). If these electrons are held in a shallow energy trap in the pure material or by an impurity they will give rise to an induced conductivity which varies with temperature and which proceeds for some time after radiation (see Chapter 30). If polymethyl methacrylate is irradiated at low temperature it will show a sudden burst of fluorescence on heating as the electrons


become mobile. The trapping of an electron in the neighbourhood of its parent ion gives rise to an exciton and may then show some of the charac-teristics of a chemical radical; its presence may be revealed by para-magnetic resonance and by colour changes.

Although paramagnetic resonance appears to offer an excellent means of investigation the nature of the intermediate structures present in irradiated solids, it has so far proved difficult to interpret the results obtained in irradiated polymers (see, for example, Abraham and Whiffen, 1958). Considerable advances in our knowledge may be expected to follow present work using this technique.

ENERGY TRANSFER One of the major problems in radiation chemistry is that of energy

transfer, which allows the energy captured at random throughout the system to be directed into those chemical bonds, or in the case of mixtures, those compounds which are found to be most susceptible to radiation damage. The term energy transfer is to some extent misleading as it is not necessarily the energy itself which is mobile. Energy transfer as at present used is a generic term, to denote any form of transfer whether by ioniza-tion, excitation, electron or proton transfer, or emission and absorption of radiation. Sometimes it is even used to include selective radical attack. This widespread use of the term is a confession of ignorance of the precise mechanism in any specific case. A more suitable expression might be reactivity transfer.

A considerable amount of information on energy transfer in excited systems is provided by the field of photochemistry. Far less data are available on the reactions of ionized molecules.

Transfer of energy between an excited molecule and its neighbours (or with a free electron) can take place in alternative ways. Thus, an excited molecule A* can transfer its energy to a neutral molecule or atom B, causing excitation in the latter, provided that the excited state of B requires the same or less excitation energy than does A :

A* -f B-*A + B*. If not quickly deactivated or converted to vibrational energy, B* may radiate its own characteristic fluorescent spectrum (sensitized fluorescence) even though the energy was initially absorbed by A.

The excitation energy can also be shared between A and B, giving a lower excitation level to A :

A* + B -> A'* + B* or it may be sufficient to ionize B, if the ionization energy of B is suffi-ciently low:

A* + B -> A + B+ + e. Further reactions described by Franck and Platzman (1954) include chemical changes such as :

A* + BC -* AB + C* A* + B -> AB


In this last case, a third body must be present to carry off excess energy and possible momentum. Reactions of this character might account for the incorporation of an additive into the structure of a polymer molecule.

Molecules excited in the triplet state should be found in abundance because of their relatively low excitation potential and great multiplicity. Such states are of relatively long half-life as the electron cannot revert to the ground state because of spin considerations. Such molecules can provide radicals of long half-life, but little has been published on their possible effect on radiation induced changes.

In a perfect crystal of identical molecules, energy transfer may occur by a process termed exciton migration. The resonance between adjacent molecules in a sense delocalizes the excitation energy, which may be transferred over long distances to react with the small amount of additive present. In organic systems with their relatively low melting point, thermal vibrations will reduce the degree of regularity and render such exciton migration of low probability. Protection effects should then show an inverse temperature relationship, being most effective at low tem-peratures and low degrees of vibrational energy, but so far no such evidence has been published.

In an ionized molecule, the electron vacancy need not be considered as localized, but rather as spread throughout the molecule, with a varying probability depending on its structure. A paraffin molecule, for example, if ionized may to some extent be considered as a vacancy conductor, which can accept an electron at any point. In such an ionized structure the bond valency rules obeyed by the neutral structure need not apply and changes in configuration may readily occur. To some extent the same consideration may apply to a radical molecule which although neutral has an extra energy level available, permitting some electron mobility. The role of catalysts in promoting such reactions may be considered as offering a method of extracting or adding an electron temporarily while a change in molecular configuration takes place.

The concept of a mobile electron vacancy gives one possible explanation for energy transfer within an ionized molecule. Information on charge (or energy) transfer between molecules has been obtained largely from the irradiation of mixtures of gases, with different ionization potentials. In many such mixtures the total efficiency of the process seems to be related primarily to the total amount of ionization produced in the mixture, although the changes produced are confined to only one component.

Charge transfer of the type

A+ + B -> A + B+

can occur with high efficiency if the reaction is exothermic. Depending on the potential curves of A and B it may be associated with the emission of radiation, or with dissociation du© to the change in positions of the nuclei in the ion and the molecule (Magee, 1952).

Except in special systems, charge transfer of an electron

A" + B -> A + B-


is far less likely, as the capture of the free electron is a very selective process, and will usually occur by the molecule with the highest electron affinity. Exceptions may arise, however, when the higher electron affinity of B is compensated for by the much higher concentration of A. Capture of free electrons by A may then be the fast process, followed by a more leisurely redistribution of electrons to B (Lind, 1929).

REACTION OF IONS AND MOLECULES

Recent results based on mass spectroscopy give evidence of reactions between an ion molecule and a neutral molecule which may eventually explain certain radiation-induced reactions. Tal'rose and Lyubimova (1952) report H abstraction by ions, C3H7

+ being formed from C3H6 and C4H9

+ from C4H8. Stevenson and Schissler (1955) observed hydrogen abstraction by ions, even of rare gases :

D2+ + D2 = D3+ -f D

A+ + H2 = AH+ + H

The rate of the reaction is very high and occurs at every collision. Sub-sequently this work was extended to other systems (Schissler and Stevenson, 1956). A number of examples of the reaction

X+ + YH = XH+ + Y

were obtained, in which X is a rare gas, N2, H2, D2, CO, HC1, HBr or CH3OH. Reactions between hydrocarbon ions and molecules have also been studied.

combination plus elimination of H2 = abstraction of a méthylène group

abstraction of H2

abstraction of H

abstraction of méthylène

Other reactions with propylene ions lead to the formation of higher ions such as C5Hio+, C5H8

+, etc. For these reactions, the rate decreases rapidly with increasing ion velocity. Other things being equal, they should there-fore be favoured in the solid. Meisels et al. (1956) give further examples of the same type of reaction for the irradiation of methane in an excess of argon or krypton ; at the same time they studied the radical products by incorporating a small amount of iodine to act as a radical trap.

These observations indicate (at least in the gaseous phase) a form of reaction between a paraffin ion and a paraffin molecule, occurring with a high degree of probability, which leads to a higher molecular weight.

GG

CH3+ + CH4

CD3+ + CD4

C2H3+ 4~ C2H4

C3H6+ + C3H6

C3H6+ + C3H6

C3H5+ -j- C3H6

C2H3+ -f- C2H6

C2H5+ C2D5+

C2H5+ C3H7+

C4H8+ C4H,

+

C3H5+

+ H2 + D2

+ C2H2

+ C3H5

+ C2H4

+ C2H4

+ CH4


Their implications for solids and liquids is less clear, but they emphasize the importance of ionic as well as radical reactions (Schüler, 1957).

Cage Effect Reactions in the solid or liquid will differ from these in gaseous systems

by the greater ease of reactions involving energy transfer, deactivation by collision, or where a third body is required to remove excess energy and momentum. In the condensed phase slow reactions will tend to be reduced by competition with these other processes, so that (somewhat surprisingly) the simpler immediate reactions will be favoured.

An important restriction on the products observed when specimens are irradiated in the solid or liquid phase arises from the Franck-Rabinowitch "cage" effect (1934) which favours the liberation of smaller molecules. Larger entities formed by radiation unless highly energetic are restricted by the surrounding molecules, and have a smaller probability of leaving the "cage", before reacting further, whereas smaller fragments of a dissociation, particularly hydrogen, can more readily diffuse away. All these factors tend to simplify the nature and number of the final products obtained from specimens irradiated in the solid or liquid state.

Sponge Effect In the case of benzene or other aromatic structures containing phenyl

groups, the yield both of radicals or of hydrogen is far lower than in aliphatic molecules. This is ascribed to the so-called "sponge effect". The resonant structure of the phenyl group has a large number of energy levels, making it easy for absorbed energy to be rapidly dissipated through-out the molecule without causing permanent changes. Another manner of considering the matter is to compare the benzene molecule with a sub-microscopic metallic crystal within which some electrons are substantially free, so that the loss of any one electron can be partly compensated by a rearrangement of the remainder, without immediate disruption of the molecule.

On irradiation, benzene gives off very little hydrogen ; the G value of 0Ό37 is about one hundredth of that in the paraffins. However, other changes such as formation of polymeric products may be reduced to a lesser extent; Patrick and Burton (1954) observed the production of a low molecular weight polymer, which was non-homogeneous in character, the disappearance of benzene occurring with a G value of 0-75. As in the aliphatic hydrocarbons, the introduction of /?-terphenyl or anthracene (which greatly increase fluorescence; Burton and Patrick, 1954) or of iodine (Schüler, 1956), which act as radical traps, does not affect the yield of hydrogen except at high concentrations. This may be taken to indicate that hydrogen molecules are not formed by a slow radical process. The effect of such radical traps on the formation of the polymeric product does not appear to have been studied, but the polymerization reaction may well be of a non-radical character, involving a transfer of an electron.

The radiation resistance of the benzene ring can be extended to neighbouring molecules, presumably by some form of energy transfer


mechanism. Manion and Burton (1952) irradiated mixtures of benzene and cyc/tfhexane and observed that the changes produced in the cyc/öhexane are reduced by the presence of neighbouring benzene molecules. One method of explaining this phenomenon is to assume that the energy is transferred in the form of excitation energy from one molecule to another or alternatively that the benzene molecule can furnish an extra electron to the cyc/tfhexane if this becomes ionized, remaining in an ionized form until an electron is recaptured.

The stability of the benzene ring is also shown in more complex mole-cules (including polymers) of which it forms part. Work has been carried out on the structure of such molecules, e.g. terphenyls, with a view to improving their radiation resistance, and enabling them to be used in reactors both as heat transfer materials and as moderators.

Table 26.4. G Values of Gas Evolution from Mixtures of Benzene and Cyclohexane

Benzene %

0 16 50 84

100

cyc/oHexane /o

100 84 50 16 0

G (total gas)

Observed

60 1-45 0-5 015 006

Calculated

50 30 10

Reduction factor

0-29 017 01

Reduction factor is the ratio of gas produced by cyc/ohexane in the presence and absence of benzene, allowing for energy directly absorbed in benzene, and of gas

produced thereby. (From Manion and Burton, 1952.)

REFERENCES BAUGHAN, E. C , Trans. Faraday Soc. (in the press). BURTON, M., MAGEE, J. L. and SAMUEL, A. H., J. Chem. Phys. 20, 760, 1952. BURTON, M. and MAGEE, J. L., J. Phys. Chem. 56, 842, 1952. BURTON, M. and PATRICK, W. N., / . Chem. Phys. 22, 1150, 1954; / . Phys. Chem.

58,421, 1954. CHARLESBY, A., Rad. Res. 2, 96, 1955. DAVISON, W. H. T., Chem. Soc. Sp. Pub. 9, 151, 1958; Chem. & Ind. 662, 1957. DELFOSSE, J. and HIPPLE, J. A., Phys. Rev. 54, 1060, 1038. FRANCK, J. and RABINOWITCH, E., Trans. Faraday Soc. 30, 120, 1934. FRANCK, J. and PLATZMAN, R., High Energy Radiation, Part 1 (Hollaender),

p. 191, McGraw-Hill, New York, 1954. FÜRST, M. and KALLMANN, H., Phys. Rev. 85, 816, 1952. HIPPLE, J. A., Phys. Rev. 53, 530, 1938; J. Phys. Chem. 52, 456, 1948. LIND, S. C , Chemical Effects of Alpha Particles and Electrons, Chemical Cata-

logue Company, New York, 1929. MAGAT, M. and VIALLARD, R., / . Chim. Phys. 48, 385, 1951.


MAGEE, J. L., / . Amer. Chem. Soc. 73, 3270, 1951; Disc. Faraday Soc. 12, 433, 1952; / . Phys. Chem. 56, 555, 1952; Ann. Rev. Nucl. Sei. 3, 171, 1953.

MAGEE, J. L. and BURTON, M , / . Amer. Chem. Soc. 72, 1965, 1950; 73, 523, 1951.

MANION, J. P. and BURTON, M., / . Phys. Chem. 56, 560, 1952. MASSEY, H. S. W., Rep. Progr. Phys. 12, 248, 1949; 1, 395, 1952. MASSEY, H. S. W. and BURHOP, E. H. S., Electronic and Ionic Impact Phenomena,

Clarendon Press, Oxford, 1952. MEISELS, G. G., HAMILL, W. H. and WILLIAMS, R. R., / . Chem. Phys. 25, 790,

1956. PATRICK, W. N. and BURTON, M., / . Amer. Chem. Soc. 76, 2626, 1954. PLATZMAN, R. L., Basic Aspects of Radiation Effects on Living Systems, Wiley,

1952; Basic Mechanism in Radiobiology NRC 305, 1953; Rad. Res. 2(1), 1 1955.

SAMUEL, A. H. and MAGEE, J. L., / . Chem. Phys. 21(6), 1080, 1953. SCHISSLER, D. O. and STEVENSON, D. P., J. Chem. Phys. 24, 926, 1956. SCHÜLER, R. H., J. Phys. Chem. 60, 381, 1956; J. Chem. Phys. 26, 425, 1957. STEVENSON, D. P., / . Chem. Phys. 17, 101, 1949; Disc. Faraday Soc. 10, 35, 1951. STEVENSON, D. P. and SCHISSLER, D. O., / . Chem. Phys. 23, 1353, 1955. TAL'ROSE, V. L. and LYUBIMOVA, A. K., Dokl Akad. Nauk. S.S.S.R. 86,909,1952. VARLEY, J. H. O., Progress in Nuclear Energy, Ser. 1, Vol. 2, p. 672, Pergamon

Press, 1956. WEISS, J., Nature, Lond. 174, 78, 1954. WILSON, C. W., Nature, Lond. 161, 520, 1948. WINOGRADOFF, N. N., Nature, Lond. 165, 123, 1950.

CHAPTER 2 7

RADICAL AND ION YIELD THE reactions of irradiated molecules can proceed either via an ionic mechanism or through the radicals into which the excited or ionized molecules and free electrons produced by radiation may eventually degenerate. Evidence has accumulated that many of the chemical reactions following irradiation proceed via a free radical mechanism, i.e. that the molecules reacting are uncharged but contain an unpaired number of electrons and therefore have free valencies available for chemical com-bination. More recent investigations have, however, re-emphasized the earlier importance attached to ionic reactions so that the tendency to explain radiation reactions in polymers purely in terms of interactions between radicals may have to be revised.

Evidence for radical reactions arises particularly in the study of radiation-induced polymerization :

(1) The reaction may be inhibited by additives such as benzoquinone which have no effect on ionic polymerization. Benzoquinone remains effective as an inhibitor even in the presence of solvents such as nitro-benzene which favour ionic polymerization.

(2) The reaction is sensitive to the presence of oxygen, which reacts with the free radicals to produce peroxides.

(3) Lewis et al. (1948), Walling et al (1950) and Landler (1950) have shown that in the polymerization of an equimolar mixture of methyl methacrylate and styrene the amount of styrene entering the polymer depends on the form of polymerization (50 per cent for radical initiation, 99 per cent for cationic and 1 per cent for anionic polymerization).

When high energy radiation is used to initiate the reaction the per-centage of styrene in the copolymer is close to 50 per cent. This has been confirmed by Schmitz and Lawton (1951), Ballantine and Manowitz (1953), Seitzei et al. (1953) and Lindsey et al. (1954), using γ- and ß-radiation from fission products and monochromatic electrons from an accelerator.

(4) In aqueous solution, polymerization of acrylonitrile can be initiated by x- or γ-radiation, although under these conditions it cannot be initiated by ions (Dainton, 1947, 1948).

(5) The rate of polymerization is often proportional to the root of the radiation intensity; this dependence on initiating radical concentration is typical of many radical polymerizations, where the termination step occurs by the combination or disproportionation of two growing chains.

Although these arguments indicate clearly that these polymerizations take place by a radical mechanism, they do not exclude the possibility that certain further reactions proceed via an ionic mechanism. In the

455


solid or liquid state, ions may be expected to have a very short life before reacting or decomposing (Table 27.1) whereas radicals can exist for a very much longer period, so that they may be more readily observed.

Table 21 A. Approximate Time Scale {seconds) 10~18 High energy electron or γ-ray traverses small molecule 10~17 High energy α-particle traverses small molecule 10~16 Secondary (δ) electron traverses molecule 10-15 Thermal (0025 eV) electron traverses molecule 10~13 Molecular vibration; fast molecular dissociation

Electron retrapped by parent ion (Samuel and Magee, 1953) 10"12 Radical movement of one jump, during diffusion 10-10 Dipole relaxation 10~9 Collision time, molecules in gas at 1 atm 10~8 Reaction between radicals in same spur (Samuel and Magee, 1953)

Time to thermalize a δ-electron in gas at 1 atm Lifetime for radiation of excited single state (if allowed)

10~7 Thermal electron captured in gas 10~6 Slow dissociation of some polyatomic ions 10-2 Lifetime for radiation of triplet state

For assumptions made, see Magee, 1953.

Recent experiments on the polymerization of /^butylène and other monomers at low temperatures (Davison et al., 1957; Charlesby and Worrall, 1958) have given strong evidence for an ionic reaction, /^butylène is normally polymerized at a low temperature by a catalyst such as BF3; with radiation the reaction proceeds in the absence of this catalyst, the yield increasing as the temperature is lowered. The reaction is substan-tially intensity-independent over an intensity range of from 105 to 1. It is suppressed by the presence of other polymers such as polyethylene (in the form of a fine powder) which on irradiation give radicals and therefore would be expected to form grafted materials. The polymerization is not affected by the addition of carbon tetrachloride, which would modify a radical reaction by the formation of additional radicals, but is increased by the presence of inorganic catalysts. Differences in the effect of additives on radiation-induced ionic and radical polymerization are reported by Pinner and Worrall (1959).

MEASUREMENT OF RADICAL YIELD Several methods have been used to measure the G value for radical

production in a number of simple organic structures, usually liquids or gases. In the few cases where comparison is possible these values lie fairly close to those obtained for reactions in long chain polymers irra-diated in a solid state, thereby indicating that for these reactions the physical state of the irradiated specimen is probably not of major impor-tance. Among the techniques used the following have received most attention. (1) The measurement of polymerization rates, each radical formed by radiation initiating the growth of a polymer chain. By irra-diating a monomer in a solvent the number of radicals produced in a solvent (which can also initiate polymerization) can be deduced. (Chapiro

RADICAL AND ION YIELD 457

et al, 1949, 1951, Seitzer and Tobolsky, 1955). (2) The disappearance of a stable free radical such as diphenyl picryl hydrazyl (DPPH)* by com-bination with the radicals produced by radiation (Prevost-Bernas et a!., 1952; Wild, 1952.) (3) Reactions with iodine, giving rise to radical iodides by the reaction R. + I , -> Ri + I·.

All these techniques have a number of limitations, and their degree of reliability is best assessed by comparing the G values for radical production obtained by each of these methods. In measurements involving poly-merization rates corrections must be applied to allow for the loss of radicals in chain termination by combination or disproportionation, and for chain transfer reactions. Some of the limitations of the DPPH method have been discussed by Wild (1952), Magat (1953) and Bouby et ai (1955). For example, it cannot be used with molecules containing a mobile hydrogen, or with double vinyl bonds. The iodine technique has been open to possible objections due to electron capture or charge transfer processes, although this objection is probably invalid, since the hydrogen yield is unaffected by the presence of small amounts of iodine.

The use of these radical traps in low concentration can only yield information on the concentration of thermalized radicals which can diffuse over long distances before meeting an iodine or DPPH molecule. No information is obtained on the products resulting from direct primary decomposition, nor on ionic or "hot" radical processes which occur far too rapidly. Furthermore when the primary radical can react with the surrounding medium (e.g. by carrying out a hydrogen abstraction reaction) the radicals trapped by the additive will be the secondary products. The observation that hydrogen production from irradiated hydrocarbons is unaffected by the presence of iodine in low concentrations, confirms that hydrogen is not produced by a slow radical process. Furthermore the appearance of lower hydrocarbon fragments in paraffins irradiated in the presence of iodine, can be explained in terms of a "hot" radical reaction (Davison, 1958).

EFFECT OF CONCENTRATION AND INTENSITY Earlier work (Forsyth et a/., 1954) has indicated that the rate of iodine

absorption in irradiated «-heptane is independent of iodine concentration (Fig. 27.1.). This behaviour is to be expected when all the thermalized radicals react with the iodine, acting as a radical trap. At high radiation intensities, or at very low concentration of the radical traps, this will not be the case, and the G value for loss of the radical trap will diminish, as the pro-bability of combination of two radicals increases. Krenz (1955) has found experimental evidence of this variation with concentration. Recently the dependence on both radiation intensity and concentration of radical traps has been confirmed quantitatively in the case of cyc/ohexane, using anthra-cene as a radical trap (Charlesby and Lloyd, 1958). The loss of anthracene

* Chapiro et al. (1953) have given details of the omeasurement of DPPH concen-tration by means of its optical absorption at 5100A.


200 400 600 Relative irradiation

FIG. 27.1. Absorption of iodine by //-heptane subjected to x-radiation. The constant slope corresponds to a constant G(—72) value.

(Forsyth et al., 1954.)

\KJ

1

0-1

0-01

p--h

// rs

yb

f-

j /l

11 JP' \\ /<

ί

-Δ

/t a-I i

I \r^&

+

sS r

f cx tjà

τη 1—ι—ι—n Wf—S ' ■ < < > ? ■

|δ| G( M ~R ma>

-

0 Ί

■ ^ , Ο η Ι Ο -vr

10 1 mole ..1, I sec 2 rod 2)

100

FIG. 27.2. Effect of additive concentration and radiation intensity on <7(-A) (anthra-cene in cyclohexane). Co 60 γ: x l l rad/s, O 21 rad/s, D 49 rad/s, Δ 53 rad/s. 2 MeV electrons: + assuming average effective intensity of 0-5 megarads/m. Full

and broken lines show theoretical curves, assuming G(-A)max is 5 or 6. (From Charlesby and Lloyd, 1958.)


by combination with a radical is proportional to (A) (R·), where (A) and (R·) are the concentrations of anthracene and thermalized radicals; the combination of radicals with each other varies as (R*)2. The rate of formation of radicals varies as the radiation intensity /. It can then be shown that under steady state conditions the G value for loss of anthracene is a function of (A)/V7, tending to a constant value when (A)/Vl is large, as in earlier experiments. Fig. 27.2. shows the experimental and calculated dependence. By comparing the values of (A)/V/ at which the intensity or additive concentration first becomes important, the reactivity of various additives, used as radical traps, may be compared.

The extension of this work to polymers is still in its infancy. Certain of the protection effects discussed in Chapter 29 may be ascribed to the reaction of polymer radicals with the additive, acting as a radical trap. In the case of long chain dimethyl siloxane polymers the dependence of protection on additive concentration has been studied (Charlesby et al.9

1959). However, very little information is available on polymers in which, because of the presence of crystallinity or the rigidity of the molecular chains, the mobility of the polymer molecules or of the additive is severely restricted.

EXPERIMENTAL G VALUES FOR RADICAL PRODUCTION

Some experimental G values are shown in Table 27.2. Column 2 refers to polymerization results obtained by the school of Magat and Chapiro, subsequently corrected to a G value for Fe+++ of 15-5 as against the value used by the authors (20-8). Two sets of values are given using different constants for the polymerization kinetics. Figures in brackets are more doubtful. Column 3 gives the results given by the same school using the DPPH technique, while column 4 gives G values based on the polymeri-zation data of Seitzer and Tobolsky (1955). Column 5 are values quoted by Medvedev (1957) derived from DPPH or polymerization data. The results of Magat and Chapiro are based on reactions induced by γ-radiation, while those of Seitzer and Tobolsky made use of ß-radiation from Sr90 radioisotopes, but this would not account for the much lower yields observed by the latter, which may be due in part to dosimetry, and to different data assumed for the polymerization kinetics.

Of particular interest are the low radical yields for the radiation resistant aromatics—benzene, toluene and styrene; and the high yields for the halogenated aliphatics—carbon tetrachloride, chloroform, bromoform, etc. For halogenated aromatics these two tendencies compensate one another, and the radical yield compares with that of aliphatic hydrocarbons.

The G values for radical yield obtained by the iodine method can be compared with those based on polymerization or DPPH disappearance. G values for iodine disappearance in the presence of various hydro-carbons (linear, branched and cyclic) subjected to x-rays, are shown in Table 12.4 (Weber et al. 1955). The iodine concentration used was suf-ficiently high to eliminate any substantial loss of radicals by recombination. These values have been standardized with respect to heptane and the


Table 21.2. G Values for Radical Production

Aliphatic s Acrylonitrile w-Pentane /raPentane ^-Heptane «-Octane /z-Nonane cycloHexane Methanol Propanol Ether Methyl acrylate Methyl methacrylate Methyl acetate Ethyl acetate Vinyl acetate Acetone Dioxane Vinyl butyl ether Aromatics Benzene

Toluene m-Xylene ö-Xylene Ethyl benzene Nitrobenzene Styrene Halogenated aliphatics Carbon tetrachloride Chloroform Méthylène dichloride Methyl chloride cycloHexy] chloride 1-2 Dichloroethane Ethyl bromide Bromoform Ethyl iodide Halogenated aromatics Chlorobenzene o-Dichlorobenzene

Polymeri-| zation

(â)

(0-8M1-9)

(1-3H3-1)

(2·6)-(6·3)

5 2-12-6

12-16

(33)-(8)

0-59

0-9-2-16 2-28-5-5

3-3-8-3

0-52-1-27

(67M160) (22-5)-(54)

9-8-23-5

61-14-6

DPPH

(ä) Air

6 1 7

8-7 14-9 18-4 15-2

12-4 19-5

30-5 12-3

0-6, 0-9, 071 1 95

2-8

24-6 37-5 27 16-5 20 18-4 17-2 44-7

10-7 18-5

Vacuum

6-8

18-3 24-5

Polymeri-zation

(b)

0-96

314

2-38 201-4-65

0-76-0-34

0-40

0-22

10-2 6-49

1-54

2-72

Polymeri-zation and

DPPH

(c)

6-4

8

4-8

0-75

0-4-0-6

22

(a) Bouby et al. (1955) based on G(Fe+++) of 20-8 recalculated for G(Fe+++) of 15-5. The two values correspond to different polymerization constants.

(b) Seitzer and Tobolsky, 1955. (c) Values quoted by Medvedev, 1958, including those of Krongauz and Bagdasarian.


variations for any given hydrocarbon is about ±0-2. The G values of between 2-6-3-3 given for cyc/ohexane compare with 3-4 obtained by Williams and Hamill (1954), 2-8 by Schüler (1957) and 2-4 by Swallow (1958). Krenz (1955) obtained a maximum G value of 3-6 for the disappearance of anthracene used as a radical trap in /2-hexane; if each anthracene molecule can trap two radicals this compares with the G value for disappearance of iodine of 3-8 shown in Table 12.4.

For irradiated heptane the G value for iodine disappearance was 3-4, leading to a radical yield of 6-8 (two radicals trapped per iodine molecule). This compares with the DPPH value of 6-1. In either case the evolution of H a is unaffected by these additives so that the reaction is mainly with the larger radicals formed by irradiation. For other aliphatics (linear, branched and cyclic) radical yields deduced from iodine disappearance vary but little, the extreme values being: cyc/öhexane G (—72) = 3Ό; 2:3 dimethyl pentane G(—72)=4-3 and 2:3 dimethyl hexane G(—72)=4-45. The radical yields therefore lie between 6-0 and 8-9 in good agreement with the DPPPH values of 6-1 to 8-7 (Table 27.2). The individual radicals formed have been studied in greater detail by Gevantman and Williams (1952).

In aromatic molecules the radical yields as deduced from I2 dis-appearance are 0-66 (benzene), 2-4 (toluene), 2-5 (oxylene), 2-82 (ethyl benzene), again in fair agreement with the DPPH and polymerization figures.

From the point of view of crosslinking polymers, particularly poly-ethylene, these results are of importance in several respects. They indicate that the number of radicals formed does not vary greatly with degree of branching or molecular weight, although Weber et al. state that increased branching results in a slightly greater yield of radicals (10-20 per cent). These G values do not of course include hydrogen, the yield of which is markedly decreased by increases of branching. (Schoepfle and Fellows, 1931; Schubert and Schüler, 1954.) As these radical yields are of the same order as those deduced for the crosslinking of polyethylene (if each crosslink is assumed to involve two radicals) they may be taken to indicate that crosslinking is largely independent of the degree of branching, and possibly of the physical state (solid or liquid).

Several compounds such as CC14 have a high G value for radical pro-duction, and may be used to enhance the polymer yield of a monomer with low G value dissolved in them (Chapiro, 1950). Each radical formed in a solvent can initiate chain growth and by working in a solution in methyl alcohol, acetone, cyc/tfhexane or w-heptane, it is claimed that the yield of polyethylene can be increased some 40-50 times (Medvedev, 1957), although this increase may also be caused by a reduction in the termination rate. When ethylene is dissolved in solvents such as CC14 low molecular weight polymers such as C1(C2H4)«CC13 are formed, possibly due to the high concentration of radicals. Chlorination of poly-ethylene has also been achieved with a very high yield, but in this case a chain reaction is certainly involved (Charlesby and Cox, 1959).


COMPARISON WITH PHOTOCHEMISTRY

Many of the reactions induced by high energy radiation can also be initiated by exposure to ultraviolet light which can only produce radicals but not ions. Where similar reactions are observed there is strong reason to believe that the radical reaction occurs via excited states; the ions pro-duced may recapture primary electrons to give further excited species. For example, polymerization of solutions of styrene or vinyl butyl ether in carbon tetrachloride can be initiated by exposure to high energy radiation and to ultraviolet light. The rate of polymerization varies with monomer concentration in the same way with both these methods of initiation (Nikitina and Bagdasarian, 1955). However, excited states can arise in radiation work which are optically forbidden and the yield can then be considerably greater. Medvedev (1958) has given values for the quantum efficiency of initiation with 310 ιημ ultraviolet: 10~3 for styrene, 0-12 for methyl methacrylate and 0Ό03 for vinyl acetate. Expressed in equiva-lent G values (chain initiation/lOOeV of energy absorbed), these values are increased by a factor of 24 but are still considerably lower than those obtained with high energy radiation. Oster (1956) has produced cross-linking polyethylene comparable to that obtained by radiation, by exposing specimens to ultraviolet radiation of 2540 A which can only produce excited molecules. To absorb the radiation, sensitizers such as diphenylamine must be added. Although this also shows that crosslinking of polyethylene can proceed via a free radical mechanism, it does not prove that this is necessarily the case with high energy radiation.

DISTRIBUTION OF RADICALS

Most of the analyses of radiation-induced changes assume that, although the chemical nature of the radicals produced is fairly specific, they are located at random throughout the specimen. This considerable simplification is theoretically only justifiable in certain cases. Along the track of a fast electron, ionization and excitation will occur in small clusters, associated with the production of δ electrons, and these will give rise to radicals in close proximity. Furthermore, radicals formed along the same primary track are produced simultaneously, whereas the overlap of separate radicals from two primary tracks will usually occur at much longer time intervals (depending on the radiation intensity). Reactions between radicals formed in the same track will therefore be favoured, and this will itself tend to produce effects which are largely independent of radiation intensity. Experimentally, however, it is usually found satis-factory to assume a random distribution of the radicals. A more detailed study of the effects of radiation intensity and ion density shows how far this assumption is justified, and provides information of the radical lifetime τ.

Each primary track may be considered as a cylinder, within which the radicals have diffused at random. If during the radical lifetime τ, a number of primary tracks have overlapped, interactions between radicals from different tracks are highly probable, and a random distribution is a


reasonable description. This situation is favoured by high intensity radiation, and (for a given intensity) by sparsely ionizing radiation such as fast electrons or gammas. Conversely, if the radical lifetime τ is small compared with the time between the passage of successive primary tracks through the same volume element (determined by the effective diameter of the track), radical reactions will be confined primarily to each separate track. Where a chain reaction is involved (as in polymerization) it is the lifetime of the growing chain which must be considered.

Magee (1951) has extended this approach by considering that a certain proportion of the radicals formed in a single track react with each other; the remainder diffuse away to give a general background, which can then react at random with radicals produced in succeeding tracks. If additives such as iodine or DPPH (which act as scavengers for radicals) are present in low concentration, they will react primarily with the background radicals, but will be unable to compete effectively with radical-radical combination in the dense portions of the track. To account for the formation by a radical process of so-called molecular products (such as hydrogen in irradiated paraffins) which are independent of additive, it would be necessary to explain why the intermediate radicals (if formed) do not last long enough to provide a background.

In chain reactions where termination of the reactions occurs by a bimolecular combination of two radicals, the reaction rate will proceed at a rate depending on the concentration and distribution of the radicals. If this distribution is completely random, the reaction rate is proportional to the root of the intensity (/°'5), whereas if the reaction is initiated and terminated by radicals in the same track, the rate should vary as the intensity /. In between these two limiting cases it varies as 7a where a lies between 0-5 and 1. For a number of chain reactions such as the decom-position of hydrogen peroxide, the chlorination of aromatic hydrocarbons, or the polymerization of certain vinyl monomers, it varies as 705 down to very low intensities. This justifies experimentally the assumption of a random distribution of radicals. Chapiro (1953, 1957) has studied the reaction of DPPH in solutions of chloroform or methyl acetate, and has observed evidence of the track effect at very low gamma radiation inten-sities. For highly ionizing radiation such as a-particles, the track effect should be observable up to much higher intensities.

Direct measurements of radical lifetime can be achieved by comparing the reaction rates of a suitable reaction exposed either to a continuous radiation flux, or to an intermittent series of exposures. This technique has been frequently used in photochemistry (e.g. Noyes and Leighton, 1937) but is far less advanced in radiation work. The method relies on the fact that the instantaneous recombination rate varies as (R)2, and for a stated total dose will therefore be different depending on whether the radical lifetime τ is much less than, equal to, or much greater than, the exposure time. Experiments have been carried out with cobalt 60 radia-tion, using a rotating lead shield with an aperture (Hart and Matheson, 1952; Hummel et al., 1954) in the case of some simple organic systems. A comparison of uniform cobalt radiation and intermittent (Van de Graaff)


electron radiation has been made by Charlesby and Pinner (1957) in the case of grafted polymers, but no reliable data are yet available in this field. Promise of interesting developments can be expected in studies using the pulsed nature of the electron beam from a Van de Graaff accelerator or from a linear accelerator, modified to give variable fre-quencies of exposure. The lifetime of radicals trapped in solid polymers can also be deduced by paramagnetic resonance methods, and by their subsequent reactivity. By photolysis or radiation, trapped radicals can be produced in hydrocarbons in the solid state. These radicals may be detected subsequently by spectroscopy (Norman and Porter, 1955) or by their ability to initiate polymerization on heating. Similar reactions will certainly occur in irradiated polymers, and the presence of radicals trapped in this way can be detected for many days or weeks.

MEASUREMENT OF IONIC YIELD No general method exists for measuring the ionic yield in a solid or

liquid. Values of the energy absorbed per ion-pair formed in gases are given in Table 2.2. (Chapter 2) and often correspond to G values of about 3. Part of the absorbed energy is of course lost in other ways, e.g. by excita-tion, and the total energy absorbed per ion-pair is considerably greater than the minimum needed to extract an electron. Information on the ionization energy can be obtained from mass spectrometry data, and from the appearance potential of ionized species subjected to bombardment by low energy electrons. Excitation energies can also be investigated by bombardment of the gas with electrons of varying energy, sudden changes in the velocity of the incident electrons being observed at the appropriate energy level.

In the ranges of particle or photon energy considered in this work, the absorption of energy depends primarily on interaction with the orbital electrons in each molecule. It varies relatively little with chemical struc-ture; one would therefore expect the same primary reaction to take place whatever the physical state, and the ionization in the gas may therefore provide at least a rough estimate of the primary energy transfer in the solid or liquid. Subsequent changes may, however, be very different ; the electron may be recaptured by the primary ion to give a highly excited species, or the electron may be trapped within the material, processes which are not possible in the gaseous state; further reactions are sum-marized in the previous chapter. No firm conclusions can therefore be reached as yet as to the number of ions or excited molecules produced by irradiation of solids or liquids. One possible technique may involve the study of ionic polymerization kinetics.

REFERENCES BAGDASARTAN, C. S., Collected Papers on Radiochemistry (Acad. Sei. U.S.S.R.,

1955), Zh. Fiz. Khim. SSSR 18, 294, 1944. BALLANTINE, D. S., COLOMBO, P., GLINES, A. and MANOWITZ, B., Chem. Engng

Progr. Symp. 50Π1), 267, 1954. BALLANTINE, D. S. and MANOWITZ, B., B.N.L. (T35), 229, 1953. BOUBY, L. and CHAPIRO, A., / . Chim. Phys. 52, 645, 1955.


BOUBY, L., CHAPIRO, A., MAGAT, M., MIGIRDICYAN, E., PREVOT-BERNAS, A., REINISCH, L. and SEBBAN, J., Int. Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1955, 7, 526.

BURTON, M., MAGEE, J. L. and SAMUEL, A. H., J. Chem. Phys. 20, 760, 1952. CHAPIRO, A., J. Chim. Phys. 41, 747, 764, 1950; C.R. Acad. Sei., Paris 233, 792,

1951; 237, 247, 1953; J. Chim. Phys. 51, 165, 1954; Rad. Res. 6, 11, 1957. CHAPIRO, A., BOAG, J. W., EBERT, M. and GRAY, L. H., J. Chim. Phys. 50, 468,

1953. CHAPIRO, A., COUSIN, C , LANDLER, Y. and MAGAT, M., Rec. Trav. Chim. Pays

Bas. 68, 1037, 1949. CHAPIRO, A., DURUP, J. and GROSMANGIN, J., / . Chim. Phys. 50, 482, 1953. CHAPIRO, A., MAGAT, M., PREVOT-BERNAS, A. and SEBBAN, J., / . Chim. Phys.

52, 689, 1955. CHARLESBY, A. and Cox, R. (to be published). CHARLESBY, A. and LLOYD, D., Proc. Roy. Soc. A249, 51, 1958; and to be pub-

lished (1959). CHARLESBY, A., LLOYD, D. and VON ARNIM, E., (to be published). CHARLESBY, A. and PINNER, S. H., Indust. Plast. Mod. 9(9), 30, 1951; 9(10), 43,

1957. CHARLESBY, A. and PINNER, S. H. (to be published). DAINTON, F. S., Nature, Lond. 160, 268, 1947; / . Phys. Chem. 52, 490, 1948. DAVISON, W. H. T., Chem. and Ind. (Rev.), 25 May, 662, 1957; Chem. Soc.

Special Publ. 9, 151, 1958. DAVISON, W. H.T. , PINNER, H. and WORRALL, R., Chem. and Ind. (Rev.), 1274,

1957. FORSYTH, P. F., WEBER, E. N. and SCHÜLER, R. H., / . Chem. Phys. 22, 66, 1954. GEVANTMAN, L. H. and WILLIAMS, R. R., J. Phys. Chem. 56, 569, 1952. HAMILL, W. H. and WILLIAMS, R. R., J. Amer. Chem. Soc. 72, 1857, 1950. HART, E. J. and MATHESON, M. S., Disc. Faraday Soc. 12, 169, 1952. HENGLEIN, A., Z. Naturforsch 106, 616, 1955. HENGLEIN, A. and BOYSEN, M., Makromol. Chem. 20, 83, 1956. HENGLEIN, A., Makromol. Chem. 14, 128, 1954. HUMMEL, R. W., FREEMAN, G. R., VAN CLEAVE, A. B. and SPINKS, J. W. T.,

Science 119, 3083, 1954. KRENZ, F. Η., Nature, Lond. 176, 1113, 1955. KRONGAUZ, V. A. and BAGDASARIAN, C. S. (in press). LANDLER, Y., C.R. Acad. Sei., Paris 230, 539, 1950. LEWIS, F. M., WALLING, C , CUMMINGS, W., BRIGGS, E. R. and MAYO, F . R.,

/ . Amer. Chem. Soc. 70, 1519, 1948. LINDSEY, H., BROWN, D. E. and PLETCHER, D. W., Bull. Amer. Phys. Soc. 29(3),

14, 1954. MAGAT, M., Transformazioni Radiochimiche, Rome, 1953. MAGEE, J. L., / . Amer. Chem. Soc. 73, 3270, 1951; Ann. Rev. Nucl. Sei. 3, 171,

1953. MEDVEDEV, S. S., / . Chim. Phys. 52, 677, 1955; / . Appl. Rad. Isotopes, 2, 186,

1957; International Conference on Radioisotopes, Pergamon Press, 1958. NIKITINA, T. S. and BAGDASARÎAN, C. S., Collected Papers on Radiochemistry

(Acad. Sei., U.S.S.R., 1955), Zh. Fiz. Khim SSSR (in press). NORMAN, I. and PORTER, G., Proc. Roy. Soc. A230, 399, 1955. NOYÉS, W. A. and LEIGHTON, P. A., The Photochemistry of Gases, Reinhold,

1937. OSTER, G., / . Polymer Sei. 22, 185, 1956. PINNER, S. H. and WORRALL, R. (in the press).


PREVOST-BERNAS, A., CHAPIRO, A., COUSIN, C , LANDLER, Y. and M A G AT, M., Disc. Faraday Soc. 12, 98, 1952.

SAMUEL, A. H. and MAGEE, J. L., / . Chem. Phys. 21, 1080, 1953. SCHMITZ, J. V. and LAWTON, E. J., Congress I.V.P.A.C., New York, 1951. SCHOEPFLE, C. S. and FELLOWS, C. H., Industr. Engng Chem. 23, 1396, 1931. SCHÜLER, R. H., / . Phys. Chem. 60, 381, 1956; J. Amer. Chem. Soc. 79, 273, 1957. SCHÜLER, R. H. and HAMILL, W. H., J. Amer. Chem. Soc. 74, 6171, 1952. SCHUBERT, C. C. and SCHÜLER, R. H., / . Chem. Phys. 20, 518, 1952. SEITZER, W. H., GOECKERMANN, R. H. and TOBOLSKY, A. V., / . Amer. Chem.

Soc. 75, 755, 1953. SEITZER, W. H. and TOBOLSKY, A. V., / . Amer. Chem. Soc. 77, 2687, 1955. WALL, L. A., ONR Symposium ACR-2, p. 139, December 14-15, 1954; Soc.

Plastics Engrs / . , p. 17, March 1956. WALLING, C , BRIGGS, E. R., CUMMINGS, W. and MAYO, F. R., / . Amer. Chem.

Soc. 72, 48, 1950. WEBER, E. N., FORSYTH, P. F. and SCHÜLER, R. H., Rad. Res. 3, 68, 1955. W I L D , W., Disc. Faraday Soc. 12, 127, 1952. WILLIAMS, R. R. and HAMILL, W. H., Disc. Faraday Soc. 12, 169, 1952; Rad.

Res. 1, 158, 1954. WORRALL, R. and CHARLESBY, A., / . Appl. Rad. and Isotopes 4, 84, 1958.

CHAPTER 28

MECHANISM OF CROSSLINKING AND DEGRADATION

IN spite of the complex sequence of events which occur in irradiated polymers between the initial event (ionization or excitation) and the final crosslinked or degraded product, the overall reactions are surprisingly simple. It was observed in early work (Charlesby, 1952; Lawton et al., 1953) that both the degree of crosslinking and degree of degradation are directly proportional to the radiation dose and are independent of its intensity. In polymers which crosslink, this initial observation has since been confirmed over an extremely wide range of doses, e.g. by gel measure-ments on polydimethyl siloxanes, by elastic measurements on rubber and polyethylene above its melting point, by swelling measurements on a number of polymers, by infusibility measurements on long chain paraffins. For polymers which degrade, the corresponding evidence is based primarily on measurements of the intrinsic viscosity. This proportionality to dose at low conversions is also observed in the evolution of hydrogen and other gases both in polymers and in many hydrocarbons. The lack of any dependence on the radiation intensity has been confirmed over an even wider range; no very marked differences in either the density of crosslinking or of degradation have been observed when similar materials were sub-jected to electron radiation or to gamma radiation when the intensities differ by a factor of 105 or more.*

Most of the polymers studied appear to fall into two distinct classes —those which crosslink and those which degrade. A useful rule sug-gested by Miller et al. (1954) states that vinyl polymers in which there are two side chains attached to a single carbon (e.g. — CH2 — CRiR2 —) degrade while those with a single side chain or no side chain (e.g. — CH2 — CRxH or — CH2 — CH2 —) crosslink. In this connexion H is not taken as a side chain whereas Cl, F are so considered. Although there is some evidence that this rule is not universally valid (in the case of methacrylates with long side chains some crosslinking may occur) it does offer a useful approximation to many of the observed effects in polymers with a carbon backbone. For other polymers no general rule has been given; cellulose degrades while silicones (analogous to polywöbutylene) crosslink.

Any theory of the mechanism of crosslinking and degradation must account for these basic observations, as well as for other phenomena such

* Evidence of a minor intensity effect has been reported by Dewhurst and Winslow (1957) for Az-hexane, and a small effect in silicones has been noted by Charlesby and Lloyd.

467 HH


Table 28.1. Behaviour of Solid Polymers under Radiation (in the absence of oxygen)

Polymers which degrade Poly/jobutylene Polytetrafluorethylene (PTFE) Polymonochlorotrifluorethylene

(Kel F) Poly-a-methyl styrene Polyvinylidene chloride

Polymethacrylonitrile Polymethylacrylic acid and esters

(polymethyl methacrylate) Poly methacryla mide

Polymers which crosslink Polymethylene Polyethylene Polypropylene

Polystyrene Poly vinyl chloride ( ?) Polyvinyl alkyl ether Polyvinyl methyl ketone Chlorinated polyethylene Chlorosulphonated polyethylene Polyvinyl acetate Polyacrylonitrile Polyacrylic acid and esters

(polymethyl acrylate) Polyacrylamide Rubber Polybutadiene Polychloroprene (Neoprene) Polyamides

nylon polycaprolactam

Copolymers styrene-butadiene butadiene-acrylonitrile styrene-acry loni trile vinyl chloride-vinylidene chloride

Polydimethyl siloxane Polyphenyl siloxane Polyethylene oxide

as the temperature dependence of crosslinking and degradation, and for the reduction in these effects when certain additives are incorporated. In the case of degradation, the lack of an intensity dependence offers no serious difficulty, but for crosslinking, where each crosslink involves two polymer molecules, it imposes certain restrictions on the type of reaction which can be envisaged. The probability of two activated entities being produced independently and in sufficiently close proximity to form a link varies as the square of the radiation dose, while if competing reactions are possible (such as reaction with additives) crosslinking should also show a marked dependence on intensity. Since neither of these has been observed, one must adopt one of the two possible lines of argument :

(1) Each crosslink between polymer molecules involves a single ioniza-tion or excitation event. The molecule activated by radiation reacts directly or indirectly with a neighbouring neutral molecule to give rise to a crosslink.

(2) Each crosslink requires two separate and independent radiation events, but the products, e.g. radicals, are sufficiently mobile to move

Cellulose cellulose derivatives : acetate, nitrate, etc.

Copolymers butyl rubber

MECHANISM OF CROSSLINKING AND DEGRADATION 469

along polymer chains and from one chain to another until they are in close proximity when they can interact with each other to form a link.

A number of reaction mechanisms have been proposed to account for crosslinking, for degradation, and for the difference in radiation behaviour of polymers in terms of their structure. Some relate to a single polymer only, others attempt a more general approach. In the opinion of the author none of these theories can be accepted without serious reservations. The primary need is for more experimental data of a basic character before firm conclusions can be reached as to radiation mechanisms. The following discussion of published theories is intended merely to show the various lines of argument pursued, and to present the strong objections to which many of them are subject.

Although recent evidence from mass spectroscopy has re-emphasized the importance of ion reactions between simple hydrocarbon molecule, most of the existing theories on crosslinking or degradation assume some form of radical reaction. Sun (1954), for example, has listed a series of reactions which can be expected to occur in irradiated long chain paraffin molecules, and which may serve as models for polyethylene and possibly for other polymers. These comprise :

(1) ionization and excitation, (2) decomposition of ionized or excited molecules to give radicals and

possibly ions, (3) recapture of an electron by an ion, giving a highly excited molecule, (4) molecular rearrangement within a molecule, (5) migration of a radical along a molecular chain, and transfer between

chains, (6) combination of two radicals to give a crosslink Rf + R2· -> Ri R2. (7) abstraction of hydrogen, etc., from a radical molecule by means of

a hydrogen, giving rise to unsaturation,

RiCH — CH2R2 ~l· H —>* RiCH = CHR2 ~f" H 2*

(8) elimination of a double bond, leaving a radical,

RXCH = CHR2 + H -» RxCH«, — CHR2,

(9) direct formation of unsaturation by removal of molecular hydrogen,

RxCH» — CH2R2 -> R ^ H = CHR2 + H2.

Reactions (1) and (2) are well-established phenomena, while (3) appears a likely reaction in solids, and is discussed in Chapter 26. The evidence for radical mobility (5) is perhaps less strong, and would require quantitative analysis in the case of polymeric solids; such mobility is assumed in certain theories of crosslinking by reaction (6). Reactions (7), (8) and (9) occur in other suggested mechanisms of crosslinking. It will be noticed that all these processes involve radical reactions, the only postulated chemical change due to ionization being decomposition as in mass spectroscopy, when no reaction between molecules is possible.


THEORIES OF REACTION MECHANISMS IN POLYMERS

Various theories have been published, attempting to explain one or more of the aspects of the observed reactions such as crosslinking or degradation, the effect of temperature, or to relate the observed behaviour under radiation with those observed in thermal polymerization or depoly-merization. As they cannot readily be subdivided into subject matter these theories, which are mainly qualitative in character, are outlined below in chronological order, and are followed by a general discussion.

Reaction with a Neutral Molecule

In the first paper to discuss the crosslinking mechanism in polyethylene, Charlesby (1952) suggested that the reaction arises from the liberation of a hydrogen atom by radiation. The remaining highly energetic radical mole-cule can then react with a neighbouring neutral molecule, ejecting a hydrogen atom, and forming a crosslink. Thus, each crosslink would correspond to a single ionization or excitation and the observed dose and lack of intensity dependence can be readily understood.

RXH > Rx· + H Rx- + R2H -* ΚλΚ2 + H H + H ■-► H2

Objections can be raised to this theory on the grounds that a radical mole-cule is unable to eject a hydrogen from a neighbouring molecule to form a crosslink. It may be, however, that in the case of radiation, highly excited states occur which permit reactions which cannot be initiated by thermal radicals. An alternative possibility is that in the primary act a proton or electron is ejected and the ionized molecule remaining then reacts with a neighbouring neutral molecule, subsequently recapturing an electron.

RXH > RxH+ + e RXH+ + R2H -> RXR2 + H2

+

H2+ + e >H2

Mobility of Active Sites

Dole and Keeling (1953) put forward the concept of the mobility of radicals formed by radiation and used this concept to explain the rapid disappearance of vinylidene groups (RXR2 = CH2) in irradiated poly-ethylene. Although the incident energy is captured at random and the concentration of vinylidene groups in the polyethylene studied was very low (0*043 groups/100 carbon atoms) the reactivity of the latter, as judged from the rate of disappearance of vinylidene unsaturation, was found to be about one-third the rate of hydrogen evolution (with 200 H atoms/100 carbons). To account for this preferential effect, Dole and Keeling suggested that the active centres formed by radiation are mobile and


attack the vinylidene groups which are of lower energy.* Although the exact character of these mobile groups was not specified, Dole and Keeling suggested that they may be free radicals. A similar mechanism involving the mobility of active entities (probably radicals) might therefore account for the formation of a crosslink between two active centres (formed independently) when they come into register. To account for the cross-linking observed at very low doses, it would be essential to assume that such centres can move not only along polymer chains but from one polymer chain to another.

In a subsequent note Dole (1954) considers the reaction of these mobile radicals with a vinylidene group to give crosslinks of the type :

OH2 — C/ — CH2 O H = C — CH2 I I

CH2 C H 2

I I OH2 — C H — CH2 CH2 — C H — CH2

Stabilization of Fractured Chains

In their study of the degradation of polymethyl methacrylate Alexander et al. (1954) pointed out that in the solid state the cage effect is likely to prevent direct decomposition of an excited polymer molecule into two radical chains. They therefore suggested that following the loss of a hydrogen atom a slow re-arrangement of the molecule takes place to give a stable structure. One such re-arrangement is as follows:

CH2 CH3 II I

CH2 — C -1- CH3 — C I I C00CH3 C00CH3

(The breakdown of the carboxyl side chain was ascribed to a separate ionization or excitation event.) Where no such stable re-arrangement can be obtained by a disproportionation reaction following loss of a hydrogen atom, crosslinking is assumed to occur. In the paper no corresponding details of the mechanism for crosslinking are given, however, A similar explanation was advanced to account for the degradation of poly/so-butylene (Alexander et al, 1955).

* This reaction is an example of the non-random pattern of radiation-induced chemical reactions resulting from energy captured at random, as shown, for example, by the preferential decomposition of the end groups of long chain alcohols and fatty acids, but the extent of mobility required here is far greater. However, the preferential destruction of vinylidene unsaturation does not require the concept of mobile radicals since it can be accounted for by other means such as energy transfer, electron mobility or hydrogen addition to unsaturation.


Resonance Stabilization

Miller et al. (1954) brought forward an alternative suggestion to explain the difference between crosslinking and degrading polymers. They assume that in a polymer molecule containing one side group per monomer (— CH2 — CHR —) the removal of an a-hydrogen atom by radiation leaves a radical molecule which is resonance stabilized with the side group. The hydrogen atom itself abstracts a similar α-hydrogen from a neighbouring molecule giving a second resonance stabilized molecule. These two being in close proximity can interact very readily to form a crosslink. In molecules with a tertiary carbon (—CH2 — CRiR2 —)M the abstraction of a hydrogen from the side group Ri or R2 does not produce any such resonance stabilization, and in this case the molecule fractures giving an unsaturated molecule and a polymer radical :

CH2

Cr l 2 — C H 2 — C —>■ CH2 4~ CH2 = O

I I R R

This mechanism for crosslinking differs from that suggested by Charlesby (1952) in that the second (neutral) molecule is attacked by the H atom rather than by the radical or ion molecule. While it explains the absence of any dose rate effect it does not account for the crosslinking of polyethy-lene where no resonance stabilization by a side chain can occur (Chapiro, 1956); the fate of the radical trapped within a degrading polymer is also not described.

Comparison with Polymerization Characteristics

Charlesby and Bevington (1954, 1955) attempted to relate the cross-linking or degrading properties of an irradiated polymer to its termination mechanism during thermal polymerization. Growing polymer chains may terminate either by disproportionation or by combination, in the former case giving rise to two separate polymer molecules, in the latter to a single one. In the polymerization of methyl methacrylate, for example, dispro-portionation occurs to a marked extent and when corresponding radical chains are produced by radiation these may likewise be expected to dis-proportionate, leaving two stable molecules, one of which will contain an unsaturated end group. On the other hand, during the polymerization of styrene, chain termination takes place by combination; when two radical chains are formed by main chain fracture under radiation, these may equally be expected to recombine to give a single molecule. No degra-dation will then be observed. In the polymerization of methyl metha-crylate, the ratio of disproportionation to combination rise with tem-perature. The observed temperature effect with irradiated polymethyl methacrylate would therefore be explained by the increased probability

M E C H A N I S M OF C R O S S L I N K I N G AND D E G R A D A T I O N 4 7 3

of disproportionation at the higher temperature. No corresponding infor-mation is available for poly/sobutylene which is polymerized by an ionic mechanism.

On this view, the degradation of certain polymers arises from their ability to stabilize by disproportionation. (This is very similar to the view previously put forward by Alexander et al.) On the other hand, irradiated polymers for which combination is the usual termination mechanism for polymerization can readily recombine if fractured, and will not suffer permanent main chain fracture. The difference between polymers which disproportionate and those in which combination is the usual termination step in polymerization may itself be related to the steric effects of the side groups. Chapiro (1956) has, however, pointed out that the chain rupture of polymethyl methacrylate gives radicals whose structure is probably different from that encountered during polymerization.

Depolymerization Characteristics

A more detailed comparison of radiation and thermal behaviour of polymers was made by Wall (1954). Table 28.2 shows polymers which crosslink or degrade, their heat of polymerization, and their behaviour during thermal pyrolysis. The heat of polymerization depends to some extent on the steric hindrance between side chains, low heats of poly-merization representing considerable hindrance. The degree of strain present in polymers with large side chains therefore tends to increase the probability of permanent fracture when the main chain is broken.

Table 28.2. Polymerization and Pyrolysis Characteristics of Polymers

Polymer of

Ethylene Propylene Methyl acrylate Acrylic acid Styrene Methacrylic acid ùoButylene Methyl methacrylate a-Methyl styrene

Radiation behaviour

Crosslinks Crosslinks Crosslinks Crosslinks Crosslinks Degrades Degrades Degrades Degrades

Heat of polymerization

(kcal/mole)

22 >16·5

19 18-5 17 15-8 13 13 9

Monomer yield on pyrolysis (weight %)

0025 2 2

— 40 — 20

100 100

Source: Wall, 1955.

During pyrolysis an α-hydrogen favours transfer reactions, and an α-methyl depropagation (Wall et al., 1955). Similarly under radiation, α-hydrogen favours crosslinking and α-methyl degradation. This analogy does not apply in the case of polyisobutylène (with a low monomer yield


on pyrolysis) nor to PTFE*, which degrades very rapidly under radiation and yet is extremely stable thermally.

The theory put forward by Wall for crosslinking is similar to that pro-posed earlier by Miller^/ al. (1954), except that Wall does not postulate any resonance stabilization. He assumes that hydrogen liberated by radiation always abstracts hydrogen from a neighbouring molecule; this second hydrogen can either be an a-hydrogen or a hydrogen from α-methyl. In the first case, crosslinking takes place between two molecules as pre-viously suggested, whereas in the second case steric hindrance prevents crosslinking, and the radical molecules decompose to give unsaturated end-groups and fractured molecules. In his theory for degradation, Wall assumes that the hydrogen abstracted comes from the α-methyl; Slovokh-tova and Karpov (1955) and Chapiro (1956) have suggested that a secon-dary hydrogen is more likely to be removed from the main chain to give

CH3 CH3

I „ I — c — CH — c — I I

CH3 CH3 in which case steric hindrance by methyl groups prevents recombination. Shielding by Methyl Groups

Shultz et al. (1956) have discussed the shielding effect of methyl groups in a series of aery late polymers. Whereas most aery late polymers require about 80-107 eV per crosslinked unit, poly-ter/.-butyl acrylate is far more radiation resistant, and requires 300 eV. In the series studied, all the other acrylates have a hydrogen atom in the a-position relative to the alcoholic oxygen of the ester group, and this hydrogen is known to be particularly susceptible to removal by ionizing radiation. Shultz con-cludes that approximately 80 per cent of the crosslinking events may

* In the exceptional case of PTFE which degrades very rapidly under radiation in spite of its thermal stability, Wall (1954) considers the relative bond energies. The F—F bond has a low energy (37 kcal/mole) as compared with a high bond energy for F—C (120 kcal/mole). Wall concludes that the lack of fluorine in the product arises from the subsequent reaction between the fluorine radicals produced by radiation and the fractured chain. The fluorine is re-captured by the radical molecule, which is highly excited and fractures. Disproportionation then follows:

F f F 1 F F — C· + F" -> < — CF \ -> — C* + FC*

F t F J F F F F

-> — C = C + CF4 F

This mechanism may perhaps be considered in the light of the observation that PTFE when irradiated in the presence of water gives rise to fluoride ions, the number being approximately proportional to the radiation dose, r whereas if no water is present, the fluorine ions can react with the polymer and cause further decomposition, the rate of which might well vary as r2 (see page 349). The concentration of trapped radicals which persist for long periods after radiation may be shown by paramagnetic resonance techniques.


involve the removal of this hydrogen. In tert.-butyl acrylate no a-hydrogen is present, and the polymer is far more resistant to crosslinking.

The concept of steric hindrance to crosslinking (due to the presence of methyl groups) is rejected since poly-tf<?0pentyl acrylate

CH2 — CH —

O CH2

Me C Me Me

also has three methyl groups on the side chain, but crosslinks as readily as the other acrylates. It is significant that it also has a hydrogen a to the alcoholic oxygen.

The energy absorbed per crosslink in the acrylate series is perhaps six times as great as in a straight polyethylene chain; this is attributed by Shultz and Bovey (1956) to an energy transfer from the main chain to the ester group. The high energy absorption per crosslink may be attributable to other reactions which result in the destruction of the side chain; the analogous effect is observed in polymethyl methacrylate, where 60 eV is absorbed per main chain fracture, and the side chain decomposes, as against about 15 eV per main chain fracture in isobutylene where no such decomposition occurs.

Multiple Fracture

An alternative explanation of the degradation of poly/sobutylene studied by Alexander et al. (1955) has been proposed by Chapiro (1956). A new feature of this explanation is the assumption that as a result of a single ionization or excitation, the polymer molecule may break up into several fragments, one of which is a multiple radical. Three such reactions are:

CH3

I CH2 — C —CH 2 > — CH2 — C + 2 C H 3 + CH2

I CH3

CH3

I CH2 — C — CH

I CH3

CH3

I CH2 — C —CH2 > — CH2 — C + C H 3 + C H 3 —CH:

I CH3

CH3

—> — CH2 — O *CH3 -f- "CH2


To account for the production of hydrogen, Chapiro further assumes that secondary hydrogen atoms may be removed as a separate process :

CH3 1

1 C —CH 2

CH3 1 1

— C — - * -

CH3 1

- C — CH -

CH3 1 1

- C — H I I I I CH3 CH3 CH3 CH3

These various radicals may then react with each other to produce stable molecules of shorter length or molecules of the same average length as the initial material but which contain unsaturation. As suggested by Bevington and Charlesby (1954) the temperature effect would arise from competition between reactions involving recombination of radicals (with no reduction in average molecular size) and degradation which increases the number of polymer molecules and reduces their average length. The evidence for multiple fracture following ionization is based on the analysis by Magat and Viallard (1951) of the fracture pattern of branched paraffins in the mass spectrometer (which gives no information on radicals, multiple or single). It is not certain that the same ionic reactions occur in solids or liquids. Furthermore, the reaction postulated by Chapiro would also allow the formation of branched molecules and even of some crosslinking, but in irradiated poly/söbutylene or polymethyl methacrylate this is cer-tainly not the case; the ratio of weight average to viscosity average mole-cular weight corresponds to that for linear molecules only (Shultz et al, 1956).

Network Formation by Main Chain Fracture

Existing theories deduce the presence of crosslinking from the formation of an insoluble, three-dimensional network. Each crosslink is formed by the formation of a lateral bond between two polymer molecules, this bond requiring the removal of at least two side groups or atoms. Charlesby (1955) has shown theoretically that an insoluble network can also be formed by main chain fracture, if it is assumed that each fractured end can link itself to a neighbouring neutral molecule. The junction points formed in this way are trifunctional (i), as compared with the tetra-functional junction points (ii) involved in crosslinking (page 174). This

(0

process is termed endlinking. For an initially random molecular weight distribution, the relationship between sol fraction and crosslinking density would be very similar to that observed in conventional crosslinking, and with the existing data it would not be easy to differentiate between cross-linking and endlinking. In low molecular weight paraffins, to account for


the intermediate products a process similar to endlinking must be assumed to take place, but the same process does not appear to have been considered in polymeric reactions.

Main Chain and Side Chain Fracture Ballantine (1956) has pointed out that crosslinking may result from

main chain fracture if hydrogen mobility along the chain is permitted. H H H H H H H H C — C — C — C C — C — C — C — CH H H H H H H H H H

Main chain fracture of a polyethylene molecule leaves two radicals with free valences. Hydrogen mobility along the chain can saturate these ends leaving radical molecules which differs in no wise from those formed directly by the breaking of C — H bonds. This provides a possible mechanism for crosslinking by main chain fracture, but the statistical treat-ment differs from conventional crosslinking, in that the average molecular weight is simultaneously being reduced by radiation. Each crosslink requires the combination of two radicals, produced one main chain fracture, so that in equation (11.8) p0 = 2q0 and sm = 1. The polymer still remains completely soluble. However a small amount of additional conventional crosslinking would result in gel formation, with a sol/dose relationship different from that observed in the same polymer when only crosslinking by side chain fracture is considered.

Crosslinking via Unsaturation Pearson (1956, 1957) proposed a theory of crosslinking of polyethylene

based on the reactions of unsaturated groups, either present in the original polymer, or formed subsequently by radiation. In this theory mobile radicals are still essential, to allow for the rapid disappearance of initial vinylidene groups, as observed by Dole et al. (1954), and to permit the transfer of a radical to the neighbourhood of an unsaturated bond, with which it may interact:

— CH2 — C H — CH2 — — CH2 — C H — CH2 — I .

— CH2 — C H — C H — C H 2 — — C H 2 — C H — C H — C H 2 — Thus each crosslink corresponds to the destruction of an unsaturated bond, but the number of radicals is unaltered.

In the initial stages, when the amount of radiation-produced trans-vinylene groups is small, crosslinking may occur via the vinylidene groups giving a méthylène crosslink, and leaving a radical available for transfer to another chain. The latter suggestion is similar to that proposed by Dole (1954). Kinetic calculations lead to the conclusions that the vinylene unsaturation tends to a limiting value, while the vinylidene groups dis-appear rapidly. Both these predictions are observed in practice. Further-more the rate of crosslinking is not constant, since it depends on the concentration of unsaturation. No such variation has been reported but Pearson claims that the experimental results so far obtained are insuffi-


ciently accurate to show the calculated variation of G (crosslinking) with dose.

The theory as suggested applies primarily to polyethylene. It does not account for the relatively small difference in G values for crosslinking in unsaturated and saturated hydrocarbons, the low G values for cross-linking of rubber which is highly unsaturated, and for crosslinking of silicones which carry no unsaturation. Furthermore crosslinking occurs at least as readily in low density as in high density polyethylene, although the latter often has a considerably higher degree of initial unsaturation. Unlike the formation of crosslinks, unsaturation is produced with a con-stant G value over a wide range of temperature. At low temperature, when radical mobility is lowered one would expect unsaturation to be greater, since it is not being destroyed by reaction with radicals, whereas experimentally it is found to be unaffected. The assumption of radical mobility is in itself sufficient to account for the formation of crosslinks (by a direct combination of two radicals moving into register). This same assumption is implicit in the mechanism suggested, but the latter requires in addition the presence of unsaturation for crosslink formation, an assumption for which there is no experimental evidence.

Okamoto and Isihara (1956) have analysed quantitatively the conditions for network formation, unsaturation and hydrogen evolution assuming either that the free radicals produced by radiation can or cannot migrate, and that the H atoms can diffuse out of the polymer. The dose for gelation is then found to depend on the diffusion rate, i.e. on the ratio of surface to mass of the specimens. No experimental evidence has yet shown such a dependence in the absence of oxygen.

Simha and Wall (1957) have analysed theoretically the kinetics of pro-duction of unsaturation, volatile products, H atoms, radical sites, cross-links and scissions, assuming only radical reactions. Many of the steps involved are similar to those assumed by Okamoto and Isihara, but iso-topic effects are also considered. It is concluded that G(H2) and G(C=C) decrease with dose, although G (crosslinking) is constant. However, dis-crepancies arise in that an isotopic effect is observed in polystyrene, but not in polyethylene.

The relationship between crosslinking and the formation of unsaturation has been investigated by Snow and Moyer (1957). Specimens of «-paraffin wax, mainly C26-C27 were subjected to the radiation from spent fuel rods, and the dimer separated by three different methods. It was confirmed that the disappearance of the initial paraffin was linear with dose. Measure-ments of the iodine number showed that unsaturation was more likely to occur in the dimer fraction, and on extrapolation to low doses, the lower limit in the dimer was 0-36 -C=C- per C27. These results show a definite relationship between crosslinking and unsaturation, but unlike Pearson's theory indicate an increase in unsaturation associated with crosslinking. Snow and Moyer suggest the formation of an allylic type of free radical by hydrogen abstraction from an earlier formed olefin,

R r - CH = CH—CH—R2


It may be anticipated that radicals, if mobile, will tend to congregate near unsaturation sites. This would explain not only the correlation of crosslinking and unsaturation, but equally the high rate of formation of conjugated double bond systems, which can account for the colour changes in many irradiated polymers.

Ionic Mechanism

Collyns and Weiss (1957) reverted to the earlier view that a single ionization gives rise to a crosslink, or to unsaturation, but assume that the location of ionization is mobile since an electron can move through an irradiated plastic quite readily. They suggest that crosslinking takes place only when the ion (or electron deficiency) is approached closely by a neutral carbon

CH2 —CH2+ —CH 2 CH2 — C H —CH 2

-* I + H 2+

CH2 — CH2 —CH 2 CH2— C H — CH2

A somewhat similar explanation can account for the formation of un-saturation

CH2 — CH2+ — CH2 -* CH2 — CH = CH +H2+

followed by neutralization of the H2 ion

H2+ + e -> H2.

This theory differs primarily from earlier theories involving a single ionization or excitation (e.g. Charlesby, 1952) in stressing the need for C atoms to be close together for a link to form. With increasing tem-perature it should therefore become easier for crosslinking to occur in competition with other processes, and such an increase is in fact observed. In a subsequent note Pearson (1957) argues against this theory, on the grounds that it predicts a strict proportionality between dose and cross-linking density. Hydrogen is produced both by crosslinking, and by increased unsaturation. In the equation

G(H2) = G (crosslinks) -f G (unsaturation)

the G value for hydrogen evolution is independent of dose, whereas G (unsaturation) is observed to decrease at high doses; the equation there-fore requires G (crosslinks) to increase with dose, in contradiction to the Collyns theory. However recent measurements of G(H2), G(crosslinks) and G(unsaturation) do not appear to confirm this equation, so that the objection made by Pearson no longer applies.

Highly Excited Radicals

A theory of network formation based on an endlinking process (Charlesby et a/., to be published) can be proposed, based on the work of Dewhurst (1956) and Davison (1957) on irradiated «-paraffins of low


molecular weight. Dewhurst and Davison find that in irradiating liquid paraffins of low molecular weight, Cn H2n+2, not only lower products CP H2j)+2 (p<n) and dimer (C2n H2w+2) but also intermediate products Cn+P U2n+2P+2 are formed. The lower products arise from C — C fracture, the dimer from crosslinking, but the intermediate products most probably arises from a combination of a fractured molecule (e.g. a radical CP H2p+1) with the parent molecule, or more probably a radical Cn H2n+i. This gives a branched molecule formally analogous to that assumed in the endlinking process analysed by Charlesby (1955), the junction point being trifunctional as distinct from the tetrafunctional junction points involved in cross-linking.

In the theory proposed by Davison (1957) for hexane, both thermal and highly excited radicals may be produced. The latter may arise from the decomposition of an ion, followed by recapture of an electron by an ionized fragment. Because of its higher energy, a highly excited (or hot) radical (denoted by Rf) can readily carry out hydrogen abstraction from a neighbouring molecule.

If R« H or RP R 3 represents the initial molecules the reaction can be written: RPRg -> RPRÇ+ + e -> RP + Rq

+ + e -> RP + Rfft Ret -f- RM H —> Rg H + Rn

and RA -f- Rp -> Rn+P.

In this process trifunctional links are formed with no hydrogen evolution. Applying this theory to polyethylene leads to the following reactions :

H H H+ H C — C -> C — C + e H H H H

H+ H H H -> C + C + e -> Cf + C'

H H H H

Hydrogen abstraction may then occur from an adjacent neutral molecule

H H H H H Ct + C· CH + C CH + CH2 H H H H H I

H H H H I C — C C — C C — C H H H H H H

where the combination of the two thermal radicals leads to an endlink. It should be noted that the formation of an endlink in this manner occurs without any hydrogen evolution. Furthermore temperature changes can be expected to modify the rate of recombination of two primary radicals in competition with endlinking. This process would therefore explain the apparent temperature dependence of crosslinking, with no corresponding change in hydrogen evolution or unsaturation (Charlesby and Davison,


1957). Furthermore it requires no assumption of radical mobility along or between polymer chains. Superimposed on endlinking, a process of conventional crosslinking may also be envisaged, possibly at a rate which is not temperature dependent.

Cyclization The formation of cyclic structures in irradiated polyethylene has been

investigated theoretically (Saito, 1958) as an extension of the analysis described in Chapter 9, which ignores internal crosslinks. The importance of such internal crosslinks (which increase the gelation dose) cannot be readily assessed until further information is obtained on polymer chain flexibility.

Dole, Milner and Williams (1957) have presented evidence for the existence of "square" links, derived from unsaturation

— CH = CH > — CH — CH — I I

— CH = CH — — CH — CH — Each square link results in the elimination of five unsaturated bands, but from the physical point of view only one effective crosslink is formed. A further reaction postulated by Dole et al. (1958) leads to the formation of endlinks from terminal unsaturation.

— CH - CH2 -> — CH — CH2 CH3

I I I I — ÇH — CH2 + CH2 -> — CH2 — CH2 — CH or — CH — CH

The large number of theories propounded in the last few years to account for the reactions observed in irradiated polymers are an indication of the uncertainty which prevails in the subject. While the quantitative informa-tion already available concerning the effects of radiation on polymers is adequate for many practical purposes, it is not sufficiently accurate for many theoretical studies. For example, there is some tentative evidence of a small dependence of crosslinking on radiation intensity and this type of information, if confirmed, might well require a revision of many of our views. Detailed information is now becoming available on the effect of additives, temperature and physical state, while data on trapped radical concentration in irradiated polymers can assist in the selection of suitable mechanisms. In the present state of knowledge, the writer is hesitant to advance any firm conclusions as to radiation mechanism and in this dis-cussion can only hope to indicate some current trends of thought.

Information obtained from the irradiation of suitable model compounds is clearly relevant to the reaction processes occurring in polymers but cannot always be considered as a reliable guide. Most of the reactions studied in radiation chemistry relate to simple organic molecules in the liquid or gaseous phases, in which molecular mobility is high. This is not the case for polymers. In radiation chemistry attention is directed to the

DISCUSSION


large range of chemical changes which take place, whereas in polymer work, attention is largely confined to dimerization (crosslinking), main chain fracture (degradation) and, to a lesser extent, changes in unsatura-tion. Other reactions such as gas evolution and changes in chemical structure have received far less notice. To a large extent this distinction arises from the great sensitivity in the physical properties of polymers to radiation, which has largely determined the type of measurements carried out. Further investigations may be expected to reveal far more complex chemical changes than the simple ones which have so far been adequate to explain the modified physical properties.

One of the most surprising features of this work concerns the apparent simplicity of the effects produced. Many of the mechanisms postulated assume competition between crosslinking and degradation. Such com-petition would be expected to be temperature dependent, and to lead to varying degrees of branching in polymers which degrade. So far, however, this has not been observed, and comparative measurements of viscosity and weight averages for irradiated polymers have shown that an initially linear polymer such as polymethyl methacrylate remains essentially linear even when highly degraded. Only in a few polymers (polyethylene, poly-propylene) is there any indication that degradation as well as cross-linking occur simultaneously. In others the evidence is not conclusive; oxygen effects or the presence of some branching (which is particularly sensitive to radiation) may complicate the issue.

One of the main difficulties in obtaining reliable experimental data arises in assessing accurately the degree of crosslinking. Measurements of solubility, swelling and elastic modulus are often suspect, as they require an accurate knowledge of average molecular weight or molecular weight distribution to enable appropriate corrections to be applied for the end-effects. Furthermore these measurements often involve some treatment of the irradiated specimen (e.g. by solution, swelling, or heating) in the course of which a great increase in chain mobility is possible. This may permit reactions to occur between active groups in the polymer, previously trapped. The measured specimen is therefore not identical with that obtained by radiation alone.

The sharp distinction between crosslinking and degrading polymers often appears to be related to the properties they display during poly-merization, and may be partly due to steric hindrance between side chains. The rule according to which polymers with two side chains degrade, whereas those with one or none crosslink, may represent one aspect of this steric effect, but it does not provide a complete picture. Both poly/^butylene and polydimethyl siloxane carry two methyl side chains but the former degrades whereas the latter crosslinks. Poly/sobutylene suffers considerable distortion of the C — C backbone which prevents it from crystallizing at room temperature and from taking up the usual trans configuration typical of paraffins. In dimethyl siloxane polymer this steric hindrance is far less marked owing to the large valency angles and long bond lengths and presumably as a result it crosslinks very readily.


Many of the existing theories of reaction mechanism in irradiated polymers assume complete mobility of the molecule, and draw no distinc-tion between largely crystalline polymers such as PTFE and polyethylene, glass-like polymers as polystyrene or polymethyl methacrylate, and rubber-like materials such as silicones or rubber. In the former two, this assump-tion is obviously unjustifiable; in the last some degree of mobility between junction points is present, but is certainly inadequate to allow chains carrying a very small number of active groups to move into register. A detailed analysis of polydimethyl siloxanes, either as fluids of widely-differing viscosity, or as lightly crosslinked networks, has been under-taken as a means of studying the influence of bodily molecular motion on the reaction. The effect of changes in crystallinity can be most readily investigated using linear polyethylene below and above its melting point, or by comparing linear and branched material.

The influence of chain length also merits further study. In mass spectro-metry, for example, the distribution of the ions formed from «-paraffins changes as the chain length increases, and in long chain molecules the influence of the end effects becomes very minor. This will itself tend to simplify the reactions occurring; furthermore the presence of neigh-bouring molecules can effect a further simplification, as the faster reactions will become increasingly predominant.

Degradation The direct proportionality between dose r and fracture density p can be

readily accounted for if each ionization and/or excitation gives rise to one main chain fracture. One can assume that only a proportion of the ionizations or excitations result in permanent main chain fracture, the remainder being lost by recombination of fragments and the dissipation of the absorbed energy in other ways. The proportion of excited or ionized molecules which eventually degrade or recombine may well vary with temperature, particularly if the process is a slow (thermal) one com-parable with pyrolysis. The number of main chain fractures is generally measured by viscometric methods, which do not distinguish between several adjacent fractures in the main chain, but count them as a single fracture. Thus, each excitation or ionization can only give rise to a maximum of one observable main chain fracture even if a more complex rearrangement of chemical groups takes place in the immediate neigh-bourhood. The number of observed main chain fractures can therefore at the most be equal to the number of excitations or ionizations. In poly/stfbutylene, the number of such fractures increases with temperature and a G value of 11 has already been observed. By extending these measurements to even higher temperatures it should therefore be possible to evaluate the maximum number of excitations and/or ionizations capable of producing degradation. If, however, the observed G value for degradation as measured by viscosity is found to increase indefinitely with temperature, it may be necessary to abandon the basic assumption that each single ionization or excitation can cause degradation in only one polymer chain.

II


One possible explanation of an indefinite increase in G values for main chain fracture would involve a chain reaction in which radicals formed by fracture abstract hydrogen from neighbouring chains

fl) CH3 CHS C — CH2 — C — CH2 CH3 CH3

CH3 CH3 • v w — c — CH — c — CH2 — + H

CH3 CH3

CH3 CH3 -> C — CH = C 4- CH2

CH3 CH3

followed by hydrogen abstraction from a neighbouring molecule (II)

(II) CH3 CH3 C — CH2 — C + CH2 CH3 CH3

CH3 CH3 -> — c — CH — c — + CH3 —

CH3 CH3

The second molecule can then degrade and continue the reaction. The addition of radical traps should then reduce the effect of radiation, by reacting with, and stabilizing these polymer radicals (a form of protection). The degree of unsaturation would also be proportional to fracture density. This type of chain reaction would occur between adjacent molecules (whereas in the decomposition during pyrolysis it occurs primarily within each fractured molecule). It is therefore surprising that the effect of tem-perature on the degradation rate of poly/^butylene and of polymethyl methacrylate is so similar, in spite of the great difference in their molecular mobility at room temperature. Unsaturation

Many of the explanations advanced for the production of unsaturation by radiation assume a mechanism of hydrogen abstraction. The hydrogen atom liberated by radiation from one molecule is assumed to be capable of abstracting a second hydrogen from the same or another molecule, giving rise to unsaturation in the first case and to a radical molecule in the second. Recent experience has thrown doubt on this concept and evidence is accumulating that unsaturation is largely due to a molecular process.

In the experiments of Dorfman (1956) on mixtures of ethane and deutroethane (C2He + C2D6) and on mixtures of methane and deutro-methane (CH4 + CD4), the hydrogen evolved in the atomic state should give rise to the products H2, HD and D2 in a statistically random ratio.


However, the observed amounts of H2 and D2 are larger than, and those of HD smaller than those calculated, indicating that the hydrogen or deuterium is evolved largely in the molecular state from each molecule, at least 60 per cent of the gas being so produced. The preferential forma-tion of molecular hydrogen or deuterium was also found by Charlesby in the irradiation of mixtures of polyethylene or paraffin wax and deuterated paraffins. Delfosse and Hippie (1938) and Williams and Essex (1949) have found the same tendency in paraffins in the mass spectrometer.

A further indication of a molecular process is shown by the measure-ments of Charlesby and Davison (1957) on polyethylene irradiated at various temperatures down to liquid nitrogen. These measurements show little significant difference in the G value for unsaturation over a very extensive range of temperatures from —196° to +180°. Moreover, specimens irradiated at liquid nitrogen temperature and examined before they could warm up gave the same G value for unsaturation as did speci-mens irradiated at much higher temperatures, when conditions for hydrogen abstraction should be far more favourable. Again, the G(C = C) values for unsaturation produced in /2-paraffins irradiated in the liquid state and in solid polyethylene show no large differences. In view of the partly crystalline nature of polyethylene, this would be difficult to explain in terms of hydrogen abstraction. The limiting degree of trans unsaturation (at about 200^00 megarads) found in both polyethylene and paraffins, may be accounted for by hydrogénation of the unsaturation formed, by hydrogen produced as a result of radiation. This is shown by the work of Dewhurst (1957) who observed an equal decrease in hydrogen evolution and in cycfohexane unsaturation when the latter was irradiated in the presence of excess w-hexene. Schumacher (1958) also observed a reduction in the G value for unsaturation in polyethylene when H2 was initially present.

One of the characteristic features of unsaturation is that (unlike peroxidation, for example) it is independent of film thickness. If hydrogen evolved as a result of radiation can cause hydrogénation, it can only be expected to do so for very limited diffusion distance, before becoming inactive (since otherwise portions near the surface of a thick specimen, being subject to a greater flow of hydrogen, would be more affected). A closer study of the variation of G(C = C) with dose under various experi-mental conditions, e.g. temperature seems desirable.

Several theories assume that crosslinks are formed via unsaturation. This view is difficult to sustain in view of the different characteristics of crosslinking and formation of unsaturation, the former being far more sensitive to temperature, and to the presence of additive. Other objections are given on page 480.

One aspect which has received comparatively little attention is the yellow coloration observed in many irradiated polymers. This colour in polyethylene and polystyrene is due to a different cause to that observed in poly methyl methacrylate. In the latter it fades readily in the presence of oxygen, or on heating, and may be due to trapped electrons or radicals. In the former it is far more permanent in character, and persists even when


the irradiated polymer is swollen in a solvent. If it is due to a system of conjugated double bonds, a series of about 4-6 would be necessary to account for the absorption pattern. Without a considerable degree of radical mobility within the system it appears difficult to account for such a high local concentration of unsaturation.

Crosslinking

The mechanisms proposed to account for crosslinking can be considered as falling largely into three groups ; those based on an ionic reaction, those assuming a combination of two mobile radicals, produced independently, and those in which two adjacent radicals are formed directly or indirectly as a result of a single ionization or excitation.

The ionic reaction is somewhat analogous to that observed in mass spectrometry, where it occurs with a high degree of probability in a very short time. The variation of crosslinking effect with temperature is not a vital objection since a temperature variation of ionic yield is observed in the mass spectrometer (Fox and Hippie, 1947). On the other hand cross-linking in linear polyethylene has been produced by ultraviolet radiation at 254 ηιμ, equivalent to a photon energy of 4-9 eV (Oster, 1956) whereas the minimum energy needed for ionization of a series of w-paraffin of increasing length extrapolates to 10 eV (Lennard-Jones and Hall, 1951). Ultraviolet photons of greater wavelength (300 ιημ or 4Ί eV) have little effect. This indicates that crosslinking can be produced by excitation. An objection to the ionic theory is the ability of very small amounts of certain additive to reduce the G value of crosslinking. Additives such as benzoquinone can act as radical traps, so that the total number of radicals required to form a network is increased (the remainder being destroyed by reaction with the additive). Measurements of the increased dose needed to produce a given degree of crosslinking agree approximately with that calculated assuming that the number of radicals lost is equal to the number of additive molecules absorbed (Charlesby et al., 1958). A further objection to the ionic theory is the presence of radicals, detected by paramagnetic resonance in measurements of irradiated polymers. Although none of these objections is insurmountable, taken together they do argue strongly against an ionic mechanism of crosslinking.

Theories which assume that a pair of radicals are eventually formed by a single radiation event (e.g. abstraction of an H atom by another pro-duced in the neighbourhood) account equally well for the proportionality of crosslinking and dose, as well as for the lack of any dependence on radiation intensity. However, such mechanisms cannot readily explain the considerable effect produced by small amounts of additive, since only rarely will an additive molecule happen to be close to the site of the reaction. That additive molecules can migrate through the polymer network is shown by the behaviour of many antioxidants which, when present in excess, can diffuse to the surface (blooming). To achieve protection, the additive must move into the neighbourhood of the reaction before com-bination of two radicals takes place. This is likely to be very rapid so


that the time available is probably too short. The same objection may arise as far as the effect of oxygen is concerned.

It has been suggested that the period available for interaction is lengthened by resonance stabilization. This appears unlikely in poly-ethylene, where the radical

Cri2 OH. C-112

has a very low resonance energy. Experiments on acrylate polymers indicate that crosslinking may be bound up with the ease of removal of hydrogen, and is not greatly affected by shielding, e.g. by methyl groups. Crosslinking via vinyl or vinylene groups have also been envisaged. Since the number of such groups is usually very small, the probability of their reacting with a radical is remote, unless the radical is mobile.

The assumption of random ionization is itself an oversimplification, and along the track of a δ electron, for example, several ionizations may occur in close proximity. It appears unlikely that these are sufficiently close frequently enough to account for the crosslinking observed, and comparable data on crosslinking by densely ionizing radiation such as α-particles would provide a useful comparison.

In any mechanism of crosslinking involving a random motion of radicals throughout a specimen, additive concentration and radiation intensity become of prime importance. In the absence of any alternative method of removing radicals, they will eventually combine to give cross-links, but unless the frequency of transfer from one site to another is extremely short the time needed to reach equilibrium can be considerably longer than that of the radiation exposure. This may account for the trapped radicals observed in paramagnetic resonance, and for slight variations in degree of crosslinking reported at varying periods after exposure.

When additives which can act as radical traps are present, competitive means of removing radicals from circulation are available, and the degree of crosslinking should vary with radiation intensity and with additive concentration. Increasing the former raises the probability of radical-radical combination, and therefore reduces the degree of protection afforded. The exact form of the kinetics depends on whether steady state conditions can be assumed, i.e. on whether the average lifetime of the radical is much shorter than the radiation exposure time.

If this assumption is valid, then the equation for steady concentration of radicals can be written

k,I= k2(R-y + M R " ) (A)

where / is the radiation intensity, (R·) the radical concentration, and (A) that of the additive. ku k2, k3 are suitable constants for the reaction. The rate of crosslinking is &2(R·)2, which from the above can be written


This expression shows that the rate of crosslinking is reduced by the presence of additive in the ratio

fr2(R·)2 = _ A:32(A)2 J / 4kjk2y*

k2(R-)\=0 2k2kJ \ [ + k~5\Ay)

which can be written in the form

G (crosslinking) G (crosslinking)

= \+x-(x2 -f 2xf A = 0

where x = (k32/2k2ki) (A)2/1. This relation shows that the degree of

protection offered depends on the ratio of additive concentration (A) to the root of the intensity, and is therefore lowered by high radiation intensity, or low additive concentration.*

A direct relationship can be derived for the reduction of additive with dose, in terms of the ratio (A)2// (Charlesby and Lloyd, 1959). This rests on the assumption that the additive, acting as a radical trap, is itself destroyed by reacting with the radicals. The G(—A) value for loss of additive can be written

G ( - A ) = G(-A)m a x [(x2 + 2*)*-x]

with the above value for x. Figure 27.2 shows the shape of this function, which is followed by

anthracene in hexane (Charlesby, Davison and Lloyd, 1959) and in cyclo-hexane (Charlesby and Lloyd, 1958). However, with silicone fluids the type of dépendance on additive concentration is somewhat different, and little dépendance on radiation intensity is observed (Charlesby and Lloyd, 1959). The observations agree with the assumption that radicals combine directly or indirectly with a neutral environment, as well as with anthracene additive. In the steady state

kj = k2>(R-) + k*(R-)(A)

so that G(-A) = G(-A)mmx UA)

2 + k3(A)

The difference in intensity dépendance of protection by anthracene may be taken to imply different fundamental reactions. The two systems are not necessarily comparable, since in liquid paraffins radical mobility can be achieved by movement of the individual molecule, which is not possible in most polymers. In several of the theories proposed for crosslinking, this distinction has been ignored, and a treatment applicable to liquids or gases has been applied to solids.

Apart from the motion of individual molecules, radical mobility may be achieved, e.g. by a series of hydrogen abstraction reactions. The

* The expression derived above can be extended to take into account the reduction of inhibitor concentration with dose, but the essential features remain.


activation energy for H abstraction by H atoms is about 0-3 eV, and therefore greater than that required to account for the temperature dependence of crosslinking.

If the major effect of mobile radicals is to form crosslinks, the reduction in crosslinking due to the presence of small amounts of additives, acting as radical traps, should also be shown by an increased rate of loss of additive. Thus

— AG(crosslinking) ^ AG(loss of additive).

Experiments on several polymers, including polyethylene and poly-dimethyl siloxanes, have given tentative evidence of this relationship, which deserves fuller study.

The theory of endlinking, first suggested on mathematical grounds, has received relatively little study from the chemical side. This may be in part due to the objection that a radical molecule cannot attack, and link itself to, a neutral molecule. The theory as proposed does not require this assumption, however, and such a link could be formed via two radicals one terminal, one lateral, in the same manner as a crosslink is produced by a combination of two lateral radicals. It can also result from a com-bination of unsaturation and a neutral molecule.

One of the advantages of introducing the concept of endlinking as a possible method of network formation is that it overcomes certain stringent restrictions imposed by crosslinking, such as the direct relationship between density of crosslinking and hydrogen evolution. If each fractured molecule provides one molecule with a trifunctional (end) link, and one shorter molecule :

CH2 — CH2 C H 3 C H 2

+ 1 CH2 — CH2 CH2 — CH.

no hydrogen is evolved but the molecular weight distribution, and con-sequently the conditions for gel formation may be greatly affected. Many of the experimental results can be more readily interpreted in terms of a combination of both crosslinking and endlinking, each with its own temperature dependence and rate constants. It may also be necessary to take into account the presence of trapped radicals, whose presence is ignored in the conventional chemical balance.

Temperature Effects The density of crosslinking and of degradation has been found to

vary with temperature. Experiments carried out on the degradation of polymethyl methacrylate (Alexander, et al., 1954; Wall and Brown, 1956) and of poly/söbutylene (Alexander, et al., 1955) and the cross-linking of linear polyethylene (Black, 1956) linear, and branched polyethylene (Charlesby and Davison, 1957) and dimethyl silicones (Charlesby, Lloyd and von Arnim, 1959), show the same general features: a slight temperature variation at low temperature (corresponding to an activation of about 0-1 kcal/mole or 4 x 10~3eV) and a temperature


dependence, greater by a factor of 10, at higher temperatures. This tem-perature dependence is also shown in some biological effects of radiation, and would appear to be a characteristic feature of certain types of radiation-induced reactions. Other radiation-induced reactions such as the forma-tion of trans-v'mylene unsaturation do not show this feature, however. This distinction may serve as one method of classifying chemical reactions in terms of fast and slow (or "hot" and thermalized) radicals. Crystallinity

The molecular configuration and mobility of lengths of polymer chain is very different in crystalline and in amorphous regions—the latter may in some respects be considered as limited regions of liquid structure. Attempts have been made to discover whether there are corresponding differences in the ability to crosslink or degrade. Lawton et al. (1958), using paramagnetic resonance techniques, detected a difference in trapped radical concentration, and in the resultant mechanical properties. How-ever, measurements of the solubility, swelling and elastic modulus of poly-ethylenes of different degrees of branching and hence crystallinity show little difference in the G value for crosslinking (Epstein, 1957; Charlesby et ah, 1958; Waddington, 1958; Schumacher, 1958). Absence of a crystal-linity effect may perhaps be ascribed to the considerable amount of energy released locally at a site of ionization or excitation. Data on the cross-linking of crystalline polymers or paraffins slightly below and above their melting point appear desirable.

Experimental evidence is accumulating to show that crosslinking is a more complex process than was at first assumed. It may well be that crosslinking, and possible degradation, occurs by the superposition of two independent processes, only one of which is temperature dependent. This chapter has summarized many of the theories proposed to account for crosslinking and for degradation, but it is hardly necessary to empha-size their tentative character, and many of the crucial experiments suggested by these theories still remain to be done.

REFERENCES

ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. A232, 31, 1955.

ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. BALLANTINE, D. S., Plastics, November, 1954; Soc. Plastics EngrsJ. 12(7), 1956. BEVINGTON, J. C. and CHARLESBY, A., La Ricerca Scientifica, Milan, Turin, 1955. CHAPIRO, A., / . Chem. Phys. 52, 246, 295, 1955; / . Chim. Phys. 53, 306, 1956. CHARLESBY, A., Proc. Roy. Soc. A215, 187, 1952; Nature, Lond. 171, 167, 1953. CHARLESBY, A., Proc. Roy. Soc. A231, 521, 1955. CHARLESBY, A. and BEVINGTON, J. C , AERE, M/M 85, 1954. CHARLESBY, A., CALLAGHAN, A. and VON ARNIM, E., / . Appl. Rad. and Isotopes

3, 226, 1958. CHARLESBY, A., DAVISON, W. H. T. and LLOYD, D. G., to be published. CHARLESBY, A. and DAVISON, W. H. T., Chem. and Ind. (Rev.) 232, February 23,

1957. CHARLESBY, A., LLOYD, D. G. and VON ARNIM, E., to be published, 1959.


CHARLESBY, A. and LLOYD, D., Proc. Roy. Soc. A249, 51, 1958; to be published, 1959.

COLLYNS, B. G., FOWLER, J. F. and WEISS, J., Chem. and Ind. (Rev.), (3), 74,1957. DAVISON, W. H. T., Chem. and Ind. (Rev.) 652, 1957. DEWHURST, H. A., / . Chem. Phys. 24, 1254, 1956. DEWHURST, H. A. and WINSLOW, E. H., / . Chem. Phys. 26, 969, 1957. DELFOSSE, J. and HIPPLE, J. A., Phys. Rev. 54, 1060, 1938. DOLE, M., Chem. Engng News 32, 1342, 1954. DOLE, M. and KEELING, C. D., / . Amer. Chem. Soc. 75, 6082, 1953. DOLE, M., KEELING, C. D. and ROSE, D. G., / . Amer. Chem. Soc. 76, 4304, 1954. DOLE, M., MILNER, D. C. and WILLIAMS, T. F., / . Amer. Chem. Soc. 79, 4809,

1957; 80, 1580, 1958. DORFMAN, L. M., / . Phys. Chem. 60, 826, 1956. EPSTEIN, L. H., / . Polymer Sei. 26, 399, 1957. Fox, R. E. and HIPPLE, J. A., / . Chem. Phys. 15, 208, 1947. INOKUTI, M. and WATANABE, T., / . Phys. Soc. Japan 13, 221, 1958. LAWTON, E. J., BALWIT, J. S. and POWELL, R. S., / . Polymer Sei. 32, 257, 277,

1958. LAWTON, E. J., BUECHE, A. M. and BALWIT, J. S., Nature, Lond. 172, 76, 1953. LENNARD-JONES, J., and HALL, G. G., Disc. Faraday Soc. 10, 18, 1951. MAGAT, M. and VIALLARD, R., / . Chim. Phys. 48, 385, 1951. MILLER, A. A., LAWTON, E. J. and BALWIT, J. S., / . Polymer Sei. 14, 503, 1954. OKAMOTO, H. and ISIHARA, A., J. Polymer Sei. 20, 115, 1956. OSTER, G., / . Polymer Sei. 22, 185, 1956. PEARSON, R. W., Chem. and Ind. (Rev.), 903, 1956; 209, 1957; J. Polymer Sei. 25,

189, 1957. SAITO, O., J. Phys. Soc. Japan 13, 198, 1958. SCHÜLER, R. H., / . Chem. Phys. 26, 425, 1957. SHULTZ, A. R. and BOVEY, F., / . Polymer Sei. 22, 485, 1956. SHULTZ, A. R., ROTH, P. I. and RATHMANN, G. B., / . Polymer Sei. 22, 495, 1956. SCHUMACHER, K., Kolloid 2 157(1), 16, 1958. SIMHA, R. and WALL, L. A., / . Phys. Chem. 61, 425, 1957. SIMHA, R., WALL, L. A. and BLACK, R. H., / . Appl. Chem. 8, 1958. SLOVOKHOTOVA, N. A. and KARPOV, V. L., Radiation Chemistry, Akad. Nauk

SSSR, p . 196, 1955. SNOW, A. I. and MOYER, H. C , / . Chem. Phys. 27, 1222, 1957. STRACIE, E. W. R., Atomic and Free Radical React ions No\. 1, 516, Reinhold, 1954. SUN, K. H., Mod. Plast. 32, 141, Ser. 1, September 1954. WADDINGTON, F. B., / . Polymer Sei. 31, 221, 1958. WALL, L. A., ONR Symp. Report ACR 2, 139, December 1954; / . Polymer Sei.

17, 141, 1955; Soc. Plastics Engrs J. (3), 17, 1956. WALL, L. A. and BROWN, D. W., / . Phys, Chem. 61, 129, 1957. WALL, L. A., BROWN, D. W. and HART, V. E., / . Polymer Sei. 15, 157, 1955. WATANABE, T., / . Phys. Soc. Japan 13, 1063, 1958. WEISS, J., / . Polymer Sei. 29, 425, 1958. WILLIAMS, N. T. and ESSEX H., / . Chem. Phys. 17, 995, 1949.

CHAPTER 2 9

RADIATION PROTECTION CHEMICAL groups differ widely in their sensitivity to radiation; this may be seen, for example, in the different G values for radical production in simple organic compounds (Chapter 27) and in the wide range of G values for crosslinking and degrading polymers. Bopp and Sisman (1955) have listed such groups in the order of their radiation stability, from the phenyl group in polystyrene (with the highest resistance to radiation) to the methacrylate or — C F 2 — groups (which decompose most readily) (Fig. 31.1). The order chosen was based on their mechanical resistance to radiation; a different order would be obtained using a different criterion, e.g. resistance to chemical change.

The sensitivity of a chemical group to radiation is, however, a purely relative term and it may be greatly modified by the presence of other groups either in the same or in adjacent molecules. Where the effect is to reduce the reactivity of a major component of the system by the presence of another component or additive, one refers to radiation protection.* This is a generic term to indicate the effect observed experimentally and may arise through a number of possible mechanisms such as energy transfer, radical-radical reactions or repair processes. As our knowledge of the subject advances, it should become increasingly possible to use more specific terms to denote the various processes by which the reactivity can be reduced.

Although radiation protection has been found to occur in irradiated polymers in the solid or liquid state and in solution, the amount of quantitative data available in this field is as yet very scant. The problem of radiation protection has however received a considerable amount of attention from radiobiologists who found that the effect of radiation on biological systems could be greatly modified by small amounts of additives. (for references see Dale et al., Bacq and Alexander, 1955).

An accurate analysis of the reactions involved is rendered difficult by the complex nature of such biological systems and by indirect effects arising from the presence of water. However, a relationship may be traced between protection in biological systems and protection in long chain polymers. Alexander et al. (1955) compared the effectiveness of some hundred additives in the protection against radiation of mice and of

* The converse of radiation protection may be termed radiation sensitizaiion, a process by which the reactivity of a component is increased by the presence of other groups or additives. It is perhaps advisable to restrict the terms "protection" and "sensitization" to the effects observed on the major component, when these effects are widely out of proportion to the primary distribution of absorbed energy.

492

RADIATION PROTECTION 493

polymers in dilute aqueous solution and found a high degree of correlation. The possibility of radiation protection by additives is not confined to any specific biological system; a study of the mechanisms of radiation protec-tion in polymers would therefore appear to be directly relevant to the effects observed in radiobiology with the added advantages of a much wider range of initial conditions and greatly simplified possibilities of analysis and interpretation.

Irradiation protection may also be observed in polymers irradiated in the solid state where the mobility of molecules is very low and the number of possible mechanisms of radiation protection is correspondingly reduced. Investigations of radiation protection in such systems may therefore pro-vide valuable information on the nature of chemical reactions occurring in the solid state. In the following sections, the existing data on radiation protection of polymers are considered separately under the headings of protection effects within a polymer molecule {internal protection) and pro-tection of irradiated polymer molecules by additives external to those molecules (external protection). One reason for this distinction is that within a single molecule energy transfer may be expected to occur more readily, whereas in external protection radical-radical reactions and a greater degree of relative mobility of active chemical groups may be envisaged. In view of the very scant amount of reliable quantitative data published to date on this subject, it is not usually possible to reach firm conclusions as to the precise method by which protection takes place in each case, particularly since the basic mechanisms of crosslinking and degradation are themselves uncertain.

INTERNAL RADIATION PROTECTION Resonance effects in low molecular weight compounds such as aromatic

derivatives are known to result in considerably increased radiation stability. The same effect is found in certain polymers, notably polystyrene. Poly-styrene can be crosslinked by radiation but the yield is extremely small, the G value for crosslinking being of the order of 005 . Gas evolution is also very low. In other respects, crosslinking takes place in very much the same way as in other polymers. It therefore appears that in polystyrene, the benzene ring greatly reduces the possibility of chemical reactions taking place, only a few per cent of the absorbed energy being utilized in this manner. The high radiation stability reported for polyethylene terephthalate may also arise from the same cause.

The extent of protection offered against degradation by the presence of benzene rings in the polymer chain has been estimated by comparing the radiation stability of copolymers of styrene and /sobutylene in varying pro-portions (Alexander and Charlesby, 1955). The course of the reaction was traced both from the intrinsic viscosity of the irradiated copolymer and from the changes in solubility. In the absence of styrene, the homo-polymer poly/sobutylene degrades very readily at a known rate (Chapter 18). As the amount of styrene in the copolymer is increased, the rate of degradation drops far more rapidly than can be explained by the reduced proportion of /^butylène present. The G values deduced for degradation


of the WO butylène units, allowing for the presence of styrene, are shown in Table 29.1. The results are consistent with the assumption that the benzene ring protects not only its parent styrene unit but also about one or two neighbouring isobutylene units on either side, i.e. a total protection range of about four or six atoms. Similar protection effects have been observed in other copolymers involving styrene (Charlesby and Groves, 1954; Bauman and Glantz, 1957).

Table 29.1. G Value for Main Chain Fracture lsobutylene-Styrene Copolymer

Styrene content ( %)

0 20 50 80

G (fracture)

5-9 3 1-8 1

Estimates have also been obtained of the extent of protection against crosslinking provided by aromatic groups in the molecule. Alexander and Charlesby (1954) irradiated linear paraffins of known length to which were attached napthyl groups at various positions along the alkyl chain. The irradiation doses needed to form an infusible network or gel were compared with those required for a similar paraffin in the absence of the napthyl group. The degree of protection (deduced from the increased radiation dose for network formation) was found to depend on the position of the naphthyl group along the paraffin chain, being least when the naphthyl was present at one end of the paraffin and could therefore only protect the alkyl chain in one direction. Fig. 29.1 shows the energy required per crosslink for varying positions of the naphthyl group. That the effect is due to the aromatic character of the naphthyl group was further demonstrated by comparing the gelation doses for naphthyl-6-dodecane and for decalyl-6-dodecane where the steric factors are not very different but only the former is resonance stabilized. As compared with the energy absorbed per crosslink in normal dodecane, the energy required for gelation is increased by about 150 per cent for the naphthyl derivative but only by 35 per cent for the decalyl derivative. Here again, the data indicate an approximate range of protection against crosslinking of about 4-5 carbon atoms.

Dole and Keeling (1953) have drawn attention to the rapid disappearance of vinylidene groups in irradiated polyethylene. Although the initial con-centration of these groups is very low (1/2300 carbon atoms in the poly-ethylene studied) their rate of disappearance corresponds to a G value (based on their concentration) approximately 1000 times greater than the G value for hydrogen production. One possibility is that the energy initially absorbed at random within the molecule is transferred to the vinylidene groups which react preferentially. Alternative explanations such as attack on the vinylidene groups by the hydrogen evolved or radical


Substance Energy per crosslink (eV)

CrV(Cri2)10-Cr13 dodecane 20

CH2-(CH2)10-CH3 naphthyl-1- dodecane 32

CH3-(Cr12)2-Cr1 '(CH2)7'CH3 naphthyl-4-dodecane 46

Ch3-(CH2)4«CH •(CHgJç/CHj naphthyl-6-dodecane 49

CH3-(CH2)^-CH · (CH2)5-CH3 cyclo-decalyl-6-dodecane 37

FIG. 29.1. Influence of an aromatic group substituted in different portions along the chain of the straight-chain hydrocarbon dodecane on the energy from ionizing

radiations which has to be absorbed to produce one crosslink.

mobility along the molecular chain must also be considered. By their greater radiation sensitivity, vinylidene groups may effectively protect other components of the system, being themselves destroyed in the process.

Ryan (1954) compared the radiation stability of several chemical groups by studying the gases evolved from copolymers containing these groups. In a copolymer containing equal molar amounts of phenyl and méthylène, the ratio of méthylène to phenyl groups decomposed was over 200: 1. Similarly, a polymer containing carbonyl and méthylène groups showed a decomposition ratio of approximately 1000: 1 when expressed on an equimolar basis. Ryan therefore concluded that the radiation stability was in the order

O H H > —c— > c —o — H II

o


This conclusion may however be invalidated by the possibility of energy transfer or radiation protection of one group by its neighbour. The radiation stability of a repeating chemical group within a polymer must be clearly distinguished from its stability in the presence of other groups in the same molecule.

Energy transfer between optically excited species can be readily traced by measurements of fluorescence emitted by one species, arising from energy absorbed by the other (Franck and Livingston, 1949). Forster (1948, 1949) has proposed a mechanism for long-range energy transfer, but Moodie and Reid (1951, 1952, 1954) have shown that the transfer of energy occurs most readily when one species is absorbed on the surface of the other, while in the case of anthracene and polystyrene, Krenz (1955) found that the efficiency is improved when the two species form part of the same molecule. However, it does not follow that this process of energy transfer causes any corresponding change in the chemical reactions induced by high energy radiation. Thus Burton and Patrick (1954) found no change in the G values for the decomposition of benzene, when anthracene or terphenyl are present although these show strong fluorescence.

EXTERNAL PROTECTION Marked changes in sensitivity of a polymer to radiation may be pro-

duced by the use of additives which do not form part of the polymer chain itself. Where such additives reduce the effect of radiation on the polymer itself, they are often referred to as protectors. The amounts added may be quite low, of the order of a few per cent, and often considerably less. Although the energy absorbed directly by these additives is correspond-ingly small, their presence may modify the response of the polymer to radiation by a factor considerably greater than the proportion of additive present.

Radiation protection by additives can be considered to operate through several alternative mechanisms :

(i) Some form of energy transfer may take place from polymer to pro-tective additive, the polymer being chemically unaltered. The additive may either dissipate the energy without suffering any permanent chemical change or it may itself be modified and cease to be active. In the latter case, the protection will cease when all the additive has been destroyed or modified.

(ii) The protecting additive may repair the damage caused by radiation. Here again, the additive may or may not itself suffer as a consequence. In many polymers, the major reaction is a loss of hydrogen leaving a polymer radical R · . Protection against further reactions can occur if the protector (AH) can itself furnish a hydrogen atom and remain as a radical of low activity.

RH w > R + H AH + R· -> A· + RH

Again, if an electron is ejected from a polymer molecule by ionization, the additive may furnish the molecule with a replacement electron and


itself be sufficiently stable to remain unaffected until it is able to recapture another electron.

Radical-radical reactions may be considered to fall under this same heading when they prevent further reaction by radicals produced on a polymer. For example, if crosslinking is assumed to take place by the interaction of two radicals

Ç C

additive molecules may combine with these radicals to form stable side chains or less reactive radicals, e.g.

C Ç A,

+ 2A-^ C A2

C

In the case of degradation by main chain fracture, a protective additive may link the two polymer radical chains together and thereby heal a radiation-induced fracture. There will be no significant change in average molecular weight.

· + ' + A -> A'

In many of these instances, protection is not offered against radiation-induced chemical changes as such but the changes produced are converted from those being studied to others which are not observed under the experimental conditions used.

(iii) The additive may react with a radical formed elsewhere by radiation before this radical can attack and modify the polymer. Thus, in polymers irradiated in an aqueous solution, the additive can combine with radicals such as OH, H, H 0 2 formed in the irradiated water and largely suppress their effect on dissolved polymer. In this case, protection is only offered against the indirect effect although the additional possibility remains of some form of repair protection also being present. Again, when polymers are irradiated in the presence of oxygen, the additive can react with the oxygen to prevent the formation of unstable peroxides on the polymer molecule, which may otherwise result in degradation.

These three broad classifications may be considered as representing respectively removal of the absorbed energy before chemical changes occur, inactivation of the chemical entities, e.g. radicals formed by radia-tion, and protection of a polymer molecule against reactive entities pro-duced elsewhere. In cases where the additive molecule is itself modified, the presence of surrounding polymer in considerable excess may be considered as a sensitizer of the additive molecule to radiation.

Attention must also be drawn to the possible role of subexcitation electrons, i.e. electrons with insufficient energy to cause ionization or


excitation of the major constituent but which can nevertheless react with an additive of lower ionization or excitation potential. Oxygen and halides which have a strong affinity for electrons are known to increase and modify some of the effects of radiation on polymers and may, in some respects, be considered as the converse of protecting additives. Reactions between these and the subexcitation electrons do not appear to have received adequate attention.

RADIATION PROTECTION AGAINST DEGRADATION Alexander, Charlesby and Ross (1954) studied the degradation of poly-

methyl methacrylate in the solid state by measuring the decrease in intrinsic viscosity after irradiation in the nuclear pile or by γ-rays from radioactive cobalt. The number of chain breaks induced by radiation was found to be proportional to the radiation dose and independent of its intensity, so that the system is a simple one in which to study protection effects quanti-tatively. Thin films of the polymer containing 10 per cent of additive were cast from solution and subjected to a radiation dose of 0-135 BEPO pile units. Table 29.2 shows the change in molecular weight in the absence and presence of various additives.

Table 29.2. Degradation of Solid Poly me thy I Methacrylate (10 per cent of additive)

Additive

Nil Allyl thiourea Di-m-tolyl thiourea Aniline 8-Hydroxyquinoline Benzoquinone

106/Μ*

5-4 2-57 1-725 2-44 1-96 1-82

Protection coefficient

l — r0lrp

0 0-57 0-72 0-59 0-68 0-70

Energy capture factor

61 7-5 6-3 7-1 7-3

If r0 is the radiation dose needed to achieve a certain molecular change and rP the corresponding dose in the presence of a protector, the reduction in radiation effect is r0jrv and the proportion of energy lost by some means or another due to the presence of additive is 1 —r0lrP. This is referred to as the protection coefficent. The last column in the table gives the energy capture factor-, this represents the proportion of energy diverted to the additive divided by its concentration. Thus, for benzoquinone, only 30 per cent of the energy incident on each polymer molecule causes degra-dation as compared with the effect observed in the absence of additive. As there is now only 90 per cent of polymer by weight, the energy required to produce the observed degradation is only 0-9 x 30 per cent or 27 per cent and 73 per cent is diverted (in one form or another) by the additive. Since there is only 10 per cent of the additive present by weight and energy is initially captured at random, protection has increased the effective


uptake by the additive molecule by a factor of 7-3. This argument does not necessarily imply that the energy is in fact transferred as energy of ionization or excitation. In the paper, it was suggested that energy transfer occurs either directly or by the contribution of an electron from the additive, since no mass motion of the individual molecules is possible at the temperature of radiation.*

As yet, no quantitative information is available on the fate of the additive in this system, although there is some qualitative evidence that the additive is itself altered while still in the solid state (Alexander and Charlesby, 1954). Bevington and Charlesby (1955) confirmed protection effects in polymethyl methacrylate and found, by using a radioactive additive, that this becomes incorporated into the polymer molecule. Although this points to a repair mechanism of protection (the additive joining two fractured radical chains together) the evidence is not con-clusive and it may well be that the additive becomes attached to some other part of the chain. At present, the evidence for protection against degradation can only be considered as being of a qualitative nature. Further data on the effect of concentration of additive (particularly at low concentrations) and on the chemical changes produced in it are needed for a fuller interpretation of the mechanisms involved.

PROTECTION AGAINST CROSSLINKING Alexander and Toms (1956) have studied the protection effect against

crosslinking in thin films of polyethylene containing 10 per cent phenol or allyl thiourea. The specimens were subjected to pile radiation and the soluble fraction of the irradiated product was determined. In the absence of air, no significant difference in solubility was observed whether or not additive was present in the polymer; in the presence of air specimens containing additive showed a reduced solubility. These experiments indicate that additives offer no protection against crosslinking per se, but that they reduce the oxygen effect (Table 29.3). From the slope of the

Table 29.3. Effect of Additives on Crosslinking of Polyethylene (Pile radiation at 70-80°C)

Additive

Nil Allyl thiourea (10%) Phenol (10%)

Percentage soluble

Vacuum irradiation

28 27 25

Irradiation in air

61 50 55

* It can be objected to this argument that trapped radicals may be formed during irradiation but the process of degradation or protection only occurs during the sub-sequent solution of the irradiated material to determine its intrinsic viscosity. This objection cannot be raised in other systems where protection has been measured while the polymer is still in the solid form.

KK


sol-dose curve, Alexander and Toms deduced that the effect of oxygen is supplementary to the crosslinking occurring in vacuum and results in main chain fracture additional to crosslinking. According to these experi-ments, therefore, the role of the additive is primarily to remove the oxygen ions resulting from irradiation and prevent them from reacting with the polyethylene. Arguing from some data on hexane (Dewhurst, 1958) St. Pierre and Dewhurst concluded that oxygen does intervene in the primary crosslinking process.

Although these experiments show no evidence of protection against crosslinking, this may have been in part due to the loss of additive by volatilization in vacuum. Positive evidence of protection against cross-linking in polyethylene has been obtained more recently for radiation doses both below and above the gel point. Lloyd (1958) irradiated thin sheets of polyethylene, both branched and linear, incorporating small amounts of antioxidants many of which show protective properties. After small radiation doses, insufficient to cause gelation, the viscosity of these materials at 190°C was measured by the Davenport grader, a method in widespread use for the commercial determination of grade number. In

0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0-9 Dose of 2 MeV electrons, megarad

FIG. 29.2. Protective effect of antioxidants on crosslinking of polyethylene. 2 MeV electron irradiation at 20°C; melt index (MI) measured at 190°C using Davenport

grader. O Alkathene 2. £> Alkathene 7. Δ Alkathene 2 -f 0-2 per cent Q Alkathene 7 + 02 per cent

D.N.B. Ethyl thiourad. 3 Alkathene 2 + 0-5 per cent

Nonox CI. © Alkathene 2 -f 02 per cent

Ionol.


this instrument specimens are extruded under known pressure, the amount extruded in a given time being a measure of the grade number. These measurements are not true indications of the viscosity since above its melting point polyethylene is not a Newtonian liquid and the viscosity depends on shear rate. Nevertheless, these measurements offer a con-venient indication of changes in crosslinking density.

Some of the results obtained by Lloyd are shown in Fig. 29.2 where the amount of irradiated polyethylene extruded in a given time is compared with the amount of unirradiated polyethylene extruded under identical conditions. This ratio may be taken as a reciprocal of relative viscosity and the plot used in the figure is convenient as giving an approximately linear relationship. Although the amount of antioxidant present in the specimens is small (0-2 per cent) it is seen to have a marked effect on the viscosity of the product. The increased dose needed to obtain this same change in viscosity or grade number is a measure of the effectiveness of protection. The energy capture factor (which equals the proportion of the incident energy diverted by the additive, divided by its concentration) may reach large values of the order of 100. Fig. 29.3 shows that the protection offered by one of these antioxidants is proportional to its concentration and the energy capture factor is therefore approximately constant. Furthermore, linear slopes are obtained for samples both with and without

ι·υ

0-9

0-8

Ιθ·7 >

X <υ

?0 ·5 £0-4 H ^ 0 - 3 ce

0-2

0-1

0

^ \ \

V \ \ > \ \ \ \ > v \

\ \ \ \ \ \ \

• N

Δ

V

·. ^ s ^

0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0 9 Dose, megarad

FIG. 29.3. Effect of additive concentration on protection. Concentratior

Nonox CI 0

0-2 per cen 0-4 per cen 0-5 per cen

l O f

t t t

Pei pr

rcenta otecti

0 29 44 65

on En ergy

fac

14 11 13

captu tor

5 0 0

re S}

O Δ D •

S} Symbol


additive, indicating that the degree of protection is independent of dose within the range considered. That the additive is itself affected and suffers chemical change is shown by prolonged extraction of the irradiated specimens. Whereas in unirradiated polyethylene the additive can be readily removed, after irradiation a high proportion remains attached to the polyethylene. The doses used in these experiments are inadequate to cause most of the additive to react. At much higher doses it may well be that the additive is destroyed, and no further protection is offered.

Radiation protection against crosslinking of polyethylene has also been studied by Charlesby, Davison and von Arnim (1959) at the higher radia-tion doses required to form a network. The additives considered consist of various fatty acids and some of their derivatives, which were milled into the sheets of polyethylene prior to irradiation. The density of cross-linking in the polyethylene was deduced from elastic measurements carried out at 150°C, when all crystallinity has disappeared and the elastic pro-perties depend primarily on the density of crosslinking. In addition to determinations of the crosslinking density of polyethylene, measurements on the changes produced in the additive were made by means of infra-red spectroscopy prior to heating, thereby ensuring that the measured chemical changes in the additive were those effectively occurring during radiation and not those following heating, when the increased mobility may permit further reaction.

In the experiments, it was confirmed that the relationship between stress and strain for a crosslinked polyethylene network follow the theoretical equation 946 (Fig. 29.4). A correction was then applied to allow for the

1 l-lo CA,~U———

0-05 0-1 °0-2 0-3 0-4

5-0

4-0 CM

1-0

0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0-9

FIG. 29.4. Load-extension curves in filled polyethylene (100 megarads).

'-»4-i)a-a=»«4-i)H-a) O Alkathene 2. + 90 per cent Alkathene 2, 10 per cent stearic acid. X 90 per cent Alkathene 2, 10 per cent paraffin wax.


finite initial molecular weight of the polymer and from the modulus-dose curves the value of G for crosslinking was deduced. The values of G obtained in this way had to be further corrected to allow for the dilution effect of the additive when this was present in amounts as high as 10 per cent. Various methods of applying this correction are possible; in the case of paraffin wax additive, where, due to the similarity in chemical structure, no net energy transfer is to be expected the corrected G values were not significantly different from those obtained for polyethylene without additive.

The results obtained with additives of paraffin wax, stéarates and stearic and other acids are shown in Table 29.4. No protection was observed

Table 29.4. Effects of Additives on G {Crosslinking) of Polyethylene {Alkathene 2)

Additive

Nil 10% Paraffin wax 10% Stearic acid 2 % Stearic acid 1 % Stearic acid

10% Ethyl stéarate 10% Sodium stéarate 10% Unsaturated fatty acid 10% Erucic acid 10% Behenic acid

G

1-44 1-22 0-89 102 112 1-27 1-37 0-92 0-83 0-86

Gc

1-44 1-50 109 106 114 1-56 1-69 114 102 106

Protection effect*

(%)

27 19 24

24 32 29

G deduced from modulus, after correction for end effects. Gc is the value of G corrected for dilution effect of additive.

* Protection effect taking an average value for G of 1-5 in the absence of additive.

with paraffin wax, stearic esters or sodium stéarate, whereas with stearic and other acids as additive, the Gc values were decreased by about one quarter.

The effect of concentration of additive is important. At low concentra-tions of the order of 1 per cent of stearic acid, protection is nearly as com-plete as at higher concentrations. This may be ascribed to the inability to obtain dispersion on a molecular scale for these higher concentrations. The degree of protection offered by a 1 per cent addition of stearic acid is about 24 per cent and the energy capture is therefore approximately 24. Lower concentrations of stearic acid gave reduced degree of protection, although the energy capture factors were somewhat similar.

The effect on the stearic acid additive was measured directly by infra-red prior to heating. At low concentrations, the destruction of carbonyl in the irradiated stearic was about 25 times greater than when the stearic acid was irradiated alone. This indicates clearly that the reaction occurs in the solid state during or immediately after irradiation and is not due to


the mobility of stearic acid in the molten polymer. Some form of energy transfer is therefore involved, possibly by transfer of an electron or proton from the acid to the polymer, the additive being itself destroyed in the process. In this sense, the polyethylene acts as a sensitizer for stearic acid. The observation that protection is offered by a range of fatty acids saturated or unsaturated but not by their esters or metallic salts, indicates the importance of the labile hydrogen atom in the system.

A comparison of the degree of protection offered against crosslinking of polyethylene, using a number of techniques of measuring these changes, is shown in Table 29.5.

Table 29.5. Protection in Polyethylene (Various doses in Alkathene)

Additive

Di-ß-naphthyl /7-Phenylene diamine

2, 6-Di-ter/.-butyl-4-methyl phenol

(In Marlex 50) Stearic acid

Concentration (%)

0-5

0-4 0-2

0-2 01 20

Protection (%)

65 59 51 46 61 43 29

75 26 25 29


130 118 102 92

122 108 145

375 260

12-5 14-5

Method

Grader Modulus Swelling Sol fraction Elastic modulus Grader Grader

Grader Grader Elastic modulus Grader

The effect of additives on the crosslinking of polydimethyl siloxanes has been studied by Charlesby, Lloyd and von Arnim (1959). The use of these polymers avoids the difficulties introduced by the presence of crystalline regions in polyethylene, and enables a direct comparison to be made with the effect of these additives in low molecular compounds such as cyc/öhexane. In the latter case it was found that the additive (anthra-cene) is destroyed by reacting with the radicals formed on the hydrocarbon by radiation (Charlesby and Lloyd, 1958). The kinetics of the reaction show competition between radical-radical combination, and radical-additive reactions, the ratio of the two depending on the relative concen-tration of additive (A) and radicals, i.e. on (A)2// where / is the radiation intensity. This dependence on both (A) and / implies that protection occurs by a reaction such as

R· + A->RA*

this reaction competing with the crosslinking reaction

R· + R - > R R

rather than by an energy transfer reaction with no chemical change.


In the case of the silicone polymer protection effects were also observed (Table 29.6) although no intensity dependence was found (Charlesby and Lloyd, 1959).

Table 29.6. Effect of Additives on Gelation Dose of Dimethyl Siloxane Polymer

(M«, ~ 28000; 2 MeV electron radiation at 20°C and at an average intensity of 0-2 megarad/min)

Additive

Silicone only 01 % 8-Hydroxyquinoline 01 % Benzoquinone 0-86%Ionol* 0-7% Phenol 004% Anthracene

Dose to gel (megarads)

12 5 15 5 14-5 190 190 150

Protection (%)

19 14 34 34 17


190 140 40 49

425

G value f for additive

removal

2-2 4-5 5-8

110 (0-9)

* 2,6-Di-fer/.-butyl 4-methyl phenol. tAssuming all additive removed by increased dose.

From the increased dose needed for gelation, G(—A) values for the additive may be deduced on the assumption that all the extra radiation is required to destroy or render ineffective all the additive. These are mini-mum values, since not all the additive is destroyed in a competitive reaction. The values are rendered less reliable by the small differences in gelation dose (due to low additive concentration), and by the possibility of other reactions such as the formation of RAR or RAAR crosslinks.

PROTECTION IN SOLUTIONS A number of papers deal with radiation protection of dilute aqueous

solutions of polymers; in this case the major effect on the polymer is indirect, i.e. it occurs by the reaction of radicals (such as H, OH, H02) produced in the water, with the polymer molecules. Protection may then be considered as a means of removing these active radicals before they can modify the polymer molecule (type (iii) protection above), although some repair mechanism (type (ii) protection) may also intervene.

Alexander and Fox (1952, 1953) showed that the viscosity of dilute solu-tions of polymethacrylic acid (0Ό25 per cent) fell considerably when irradiated in the presence of oxygen, but that this reduction was greatly decreased by the presence of low concentrations (8 x 10~4M) of additives such as potassium cyanide and thiourea during irradiation. At the low doses used (1000 r of x-rays) the direct effect of radiation on the polymer is negligible, as it would only produce one main chain break in a total molecular weight of about 109, and protection is therefore only needed against the radicals produced in water, irradiated in the presence of air. From a comparable study in which polystyrene was degraded in carbon tetrachloride solution Wall and Magat (1953) concluded that protection


occurs in a second stage; the primary radical in the solvent reacts with the polymer molecule to give a radical, and the effect of the protective agent is to combine with this radical, and prevent it from reacting further with dissolved oxygen.

Protection against the indirect effect of radiation has also been investi-gated by Alexander and Fox (1953) and Prevot (1951, 1953) who studied the action of a protective additive on the polymerization of a soluble monomer, such as methacrylic acid in water, acrylonitrile in water and methanol, and styrene in methanol. In this case the additive may react with the radicals formed in the solvent, before these can initiate chain growth, or it may serve to terminate the growing chains. In the presence of oxygen the additive may even increase the yield by reacting with oxygen, or with the radicals formed in the solution.

Radiation protection against crosslinking has been observed by Alexander and Charlesby (1955, 1957) in the case of solutions of polyvinyl alcohol and polyvinyl pyrollidone, but the data were insufficient to deter-mine whether this protection operated by preventing the solvent radical from attacking the polymer molecule, or whether it operated at a sub-sequent stage on the polymer molecule itself. Fig. 29.5 shows the increased

Concentration of thiourea, %

Ο Λ 0-004 0-008 0-012 0-016 0-02

14

12 ■σ σ σ 10 σ> Φ Ε

8 0;

Û 6

4

2

0-2 0-4 0-6 0-8 1-0 q thiourea/lOOa of polymer

FIG. 29.5. Effect of thiourea additive on gel formation in a 2 per cent aqueous solution of polyvinyl alcohol subjected to γ irradiation.

dose required to gel a dilute (2 per cent concentration) aqueous solution of polyvinyl alcohol, when small amounts thiourea are present in the

I 1

^ 1 ! i

1 - .. L—/

y Ύ '

i / /

/

— —

JL


solution. The gelation dose is increased from about 0-7 to 19 megarads by 0Ό2 per cent of thiourea in the solution.

Fox claims (1953, 1954) to have observed a post-irradiation effect in the degradation of polystyrene in chloroform, the viscosity of the solution continuing to drop for a considerable time after irradiation has ceased. He distinguishes two groups of protecting compounds; those containing mobile H atoms (including some phenols) and those with groupings such

as S — C or / N <

S = C (e.g. thiourea, dithiocarbamate com-

pounds). Protection against the post-radiation effect is only found in the case of compounds in the second group (many of which are also of interest due to their antioxidant behaviour, e.g. in rubber) and the effect may be due to protection against an oxygen-induced chain reaction. On the other hand protection during radiation may be due to a reaction with the radicals formed in the water, or on the polymer.

PROTECTION IN CURING OF UNSATURATED POLYESTERS The effects observed during the radiation cure of unsaturated polyester

molecules have been shown to arise from a chain reaction initiated by irradiation (Charlesby, Wycherley and Greenwood, 1958). The reaction occurs in two stages, during the first of which the viscosity of the liquid

0-4 0 -6 0-5 Dose , megarad

F-G. 29.6. Effect of additives as inhibitors in the curing of polyesters. Cobalt 60 irradiation

Additives eV/molecule O No additive — V Allyl thiourea 100 p.p.m. 1-8 X Nitrobenzene 2000 p.p.m. 14 □ 8-Hydroxy quinoline 1000 p.p.m. 3-5 Δ Anthracene 1000 p.p.m. 9-9


polyester increases until a network starts to form. Beyond this gelation point, the solubility of the system decreases rapidly with radiation dose.

By incorporating various additives in the polyester before radiation, their effect on each of these two stages of cure can be assessed. The experimental results indicate that the initial rise in viscosity may be con-siderably inhibited by the presence of the additive with a corresponding increase in the radiation dose for gelation. However, once the gel point has been reached, the subsequent decrease in solubility is unaffected by these additives (after allowing for the increased dose needed to reach the initial gelation point).

The changes in viscosity as a function of radiation dose are shown in Fig. 29.6 for various additives; the curves indicate that the degree of protection initially offered is high but then decreases rapidly. This behaviour would be expected if protection is only available until the additive is itself destroyed. The extreme case is shown in Fig. 29.7 where

o 4-0 ΪΌ 2-0 3-0 Dose , megarad

FIG. 29.7. Effect of benzoquinone on the radiation cure of polyesters. Cobalt 60 irradiation

benzoquinone p.p.m. A 0 B 300 C 600 D 1000 E 2000

eV/molecule quinone

— 16 4 13-4 18-8 18-3

the additive is benzoquinone. In this case, no increase in viscosity is observed for a dose which is approximately proportional to the amount of the additive. Beyond this point, the viscosity increases at the same rate as for the non-protected polyester. The ratio of the additional energy absorbed to the amount of the additive initially present can serve as a measure of the degree of protection. In the system studied, this amounts to about 16 eV per benzoquinone molecule (G = 6).


The reduction in the effect of radiation may be due to some form of energy transfer prior to any chemical change, or to subsequent reactions between additive and radicals (either initiated directly by radiation or resulting from the subsequent chain reaction). The polyesters studied by Charlesby et al. could also be cured thermally by the addition of a catalyst, known to produce radicals on heating. It was confirmed that the number of radicals produced thermally was proportional to the catalyst concen-tration and under the conditions used, to the time of heating. By incor-porating additives in the polyester, their effect on thermally initiated radicals could therefore be deduced from the increased heating time needed to gel or cure the polyester. By comparing the time or dose needed for thermal and for radiation treatment in the presence of a given additive, its relative effect on the two methods can be compared. Fig. 29.8 com-pares the dose and the heating time needed for gel formation in the

1-4

3 1-21

| 1-0

0-8

OT 0-6

0-4

0-2

Δ I

O-^^L- l

10 20 30 40 50 60 70 80 90 100 110 Time to gel, min

FIG. 29.8. Comparison of thermal and radiation cure of unsaturated polyester. Horizontal scale: thermal cure with catalyst. Vertical scale: dose to cure.

O No additive. Δ Anthracene 1000 p.p.m. □ 8-Hydroxyquinoline 1000 p.p.m. X Benzoquinone 300 p.p.m.

(Initiator: 1 per cent benzoyl peroxide at 90 °C.)

presence or absence of several additives. The straight line represents the equivalence of thermal and radiation induced curing. The figure indicates that benzoquinone and 8-hydroxyquinoline are equally effective as inhibitors for thermal and for radiation initiated cure, showing that for these additives protection can be attributed entirely to radical-radical reactions. In the case of anthracene, however, the protection offered to radiation cure is greater than that obtained with a thermal cure, showing that in this case protection is due partly to a radical-radical reaction and partly to some form of reaction (energy transfer) not possible in the thermally-formed radicals.

MECHANISMS OF RADIATION PROTECTION In describing the experimental work on protection, a distinction was

drawn between protection within a polymer molecule and protection by


other molecules. An alternative approach would consider protection in which no permanent chemical change is produced, protection when only the additive suffers damage and protection in which the polymer is itself changed (as well possibly as the additive) but in a different and less important manner.

That protection can occur with little permanent chemical change is shown not only by many aromatic compounds irradiated in the liquid state, but also by polymers such as polystyrene in the glasslike state. Complete protection does not appear to be possible in organic systems but reductions in radiation sensitivity by factors of at least 100 times may be obtained, in spite of the similar number of initial ionization and excitation events. Protection must therefore be offered against the effects of both excitation and ionization. A suitable model to consider is one involving electron mobility. To allow a chemical reaction to proceed in an organic molecule, a binding electron must be removed from its orbit. Where a resonant structure is present, the electron removed by ionization or excitation may be replaced before the nuclei can move to new equili-brium positions. The experiments involving protection by aromatic groups in polymers provide some estimate of the range over which this electron transfer can operate.

Protection against radiation damage has also been found to occur by the combination of radiation-induced radicals with small amounts of additive acting as a radical trap. In this case protection is a misleading term, since chemical changes are produced, but these are different in character from those occurring in the absence of the protective molecule or group. These reactions have been mainly studied in liquid systems where considerable mobility of the individual molecules is possible. Com-petitive reactions occur, and the degree of protection varies with concen-tration and radiation intensity. It may also be expected to vary with radiation temperature, although as yet this does not appear to have been confirmed.

In polymers the degree of mobility is considerably smaller, and there is as yet only limited evidence of the same type of protection. The clearest evidence for radical combination with additive is given in the curing of polyesters, and to a lesser extent in the crosslinking of viscous silicone fluids. A much fuller analysis seems called for, particularly in polymers such as polyethylene with a partially crystalline structure, and using various radiation intensities. If the formation of a network polymer occurs by several different crosslinking or endlinking mechanisms, some evidence of this should appear from data on protection at various temperatures, when the relative mobilities of the reacting groups is altered. A dependence on radiation intensity should also be observed, particularly at high intensities.

In between the two extremes of energy or electron transfer, and slow radical reactions similar to conventional chemical reactions, lie a number of other potential mechanisms of protection which are as yet largely unexplored. Clarification of the two simplest processes will delimit this


field more closely. Further information could also be derived from model compounds, irradiated both in the solid and the liquid state.

The rate of oxygen requires special mention. When present during irradiation it often causes considerable changes in the character of the reaction, but nevertheless cannot be considered as a protector. In cross-linking polymers, for example, its presence may increase the dose for gelation, but this is not due primarily to protection but to the resulting degradation of the polymer. The average molecular weight is reduced, and a higher density of crosslinking is needed for gel formation; the total amount of chemical change produced is, if anything, increased. The lack of protection by oxygen is best shown when an antioxidant capable of acting as a protector is introduced into the polymer. In the presence of oxygen the net effect is a reduction in the degree of protection, whereas the simultaneous presence of two protectors should increase the degree of protection offered against radiation.

Although some clear instances of protection by some form of energy transfer and by radical-radical reaction have been established, the extremely wide range of compounds found to show some protection effects and the limited amount of quantitative data make it difficult to classify these additives in terms of their mechanism of protection. In view of the importance of radiation protection and of the oxygen effect in radio-biology, in radiation chemistry and in solid-state physics the subject of radiation protection merits much further attention.

REFERENCES ALEXANDER, P., Brit. J. Radiol. 26, 413, 1953. ALEXANDER, P., BACQ, Z. M., COUSENS, S. F., Fox, M., HERVÉ, A. and LAZER, J.,

Rad. Res. 2, 392, 1955. ALEXANDER, P., BLACK, R. M. and CHARLESBY, A., Proc. Roy. Soc. A232, 31,

1955. ALEXANDER, P. and CHARLESBY, A., Nature. Lond. 173, 578, 1954; Radiobiology

Symposium Liege, p. 49. 1954, Butterworths, 1955; Proc. Roy. Soc. A230, 136, 1955; J. Polymer Sei. 23, 355, 1957.

ALEXANDER, P., CHARLESBY, A. and Ross, M., Proc. Roy. Soc. A223, 392, 1954. ALEXANDER, P. and Fox, M., Nature, Lond. 170, 1022, 1952; J. Chim. Phys. 50

415, 1953; 52, 710, 1955. ALEXANDER, P. and TOMS, D., / . Polymer Sei. 22, 343, 1956. BACQ, Z. M. and ALEXANDER, P., Fundamentals of Radiobiology, Chap. 14,

Butterworths, 1955. BACQ, Z. M. and HERVE, A., Bull. Acad. Roy. Med. Belg. 17, 13, 1952. BAUMAN, R. and GLANTZ, J., J. Polymer Sei. 26, 397, 1957. BEVINGTON, J. C. and CHARLESBY, A., Simposio Inter, di Chim. Macrom., La

Ricerca Scientifica, 1955. BOPP, C. D. and SISMAN, O., Nucleonics 13(10), 51, 1955. BOWEN, E. J., Quart. Rev., Lond. 1, 1, 1947. BURTON, M. and PATRICK, J., / . Chem. Phys. 22, 1150, 1954. CHARLESBY, A. and ALEXANDER, P., J. Chim. Phys. 52, 699, 1955. CHARLESBY, A., DAVISON, W. H. T. and von ARNIM, E. (to be published). CHARLESBY, A. and GROVES, D., Proc. Third Rubber Tech. Con f., Lond. 317,

1954.


CHARLESBY, A. and LLOYD, D. G., Proc. Roy. Soc. A249, 51, 1958; to be pub-lished, 1959.

CHARLESBY, A., LLOYD, D. G. and VON ARNIM, E., (to be published). CHARLESBY, A., WYCHERLEY, V. and GREENWOOD, T. T., Proc. Roy. Soc, A244,

54, 1958. COLICHMAN, E. L. and FISH, R. F., NAA-SR-1287, 1288, 1955. DALE, W. M., Biochem.J., 34, 1367, 1940; 36, 80, 1942; Brit. J. Radiol. 16, 171,

1943;Suppl. 1,49, 1947. DALE, W. M., DAVIES, J. V. and MEREDITH, W. J., Brit. J. Cancer 3, 31, 1949. DEWHURST, H. A., / . Phys. Chem. 62, 15, 1958. DOLE, M. and KEELING, C. D., / . Amer. Chem. Soc. 75, 6082, 1953. FÖRSTER, T H . , Ann. Phys. 2, 55, 1948; Fluoreszenz Organischer Verbindungen,

Vandenhoeck and Ruprecht, Göttingen, 1951; Z. Elektrochem. 53, 93, 1949; Z. Naturforsch. 4a, 321, 1949.

Fox, M., C.R. Acad. Sei., Paris 237,1682, 1953; Radiobiology Symposium, Liège, p. 61, 1954, Butterworths.

FRANCK, J. and LIVINGSTON, R., Rev. Mod. Phys. 21, 505, 1949. KRENZ, F. H., Trans. Faraday Soc. 51, 172, 1955; Nature, Lond. 176, 1113, 1955. LLOYD, D. G., 1958, private communication. MOODIE, M. M. and REID, C , / . Chem. Phys. 19, 986, 1951; 20, 1212, 1510,

1952; 22, 1126, 1954. PATT, H. M., Physiol. Rev. 33, 35, 1953. PREVOT, A., C.R. Acad. Sei., Paris 233, 366, 1951 ; / . Chim. Phys. 50, 445, 1953. REID, C , Phys. Rev. 88, 422, 1952. RYAN, J. W., Soc. Plastics Engrs J. 10(4), 1954. ST. PIERRE, L. E. and DEWHURST, H. A., J. Chem. Phys. 29, 241, 1958. WALL, L. A. and MAGAT, M., J. Chim. Phys. 50, 308, 1953; Mod. Plast. I l l ,

July, 1953.

CHAPTER 30

CHANGES IN ELECTRICAL CONDUCTIVITY MANY polymers are very good insulators, but their conductivity is greatly increased by exposure to high energy radiation. This increase is observed even at very low intensities, insufficient to cause appreciable chemical changes. Moreover the current decreases again when no further radiation falls on the specimen, showing that the effect is not due to any permanent change. Many of the published results relate to temporary changes in conductivity resulting from exposure to intensities of the order of 1-102

r/min of ß- or γ-radiation, and therefore do not appear to fall directly within the scope of this work. However, the fate of the ejected electrons may be very relevant to studies into the mechanism of radiation-induced chemical changes, and a brief summary of the present position may therefore be justified.

In the absence of radiation a low-intensity current (leakage, static or dark current) is observed, which is directly proportional to the applied voltage. On irradiation the current increases rapidly, but not immediately, to a higher value, the difference being termed the induced current (/*). As in Ohm's law, ix is proportional to the applied voltage so that one can give a value for the conductivity under given radiation conditions. * The dependence of the induced current ix% or the conductivity σ*, on the intensity / of the incident radiation is more complex. It can be written in the form

ix α / Δ

where Δ has values of between 0-5 and 1 characteristic of the material. This formula holds for a variation in / of several decades.

Both the static current and the induced current rise with temperature, though at different rates (see Fig. 30.3). This indicates that they relate to different phenomena. Per 10°C temperature increase, the static conduc-tivity rises by a factor of 4-8 times, while the induced conductivity increase is only about two-fold, and less for materials with Δ ^ Ι . The initial rise of induced current to a steady value is more rapid than its subsequent fall after cessation of irradiation. The characteristic time factors for these changes appear to be related to the Δ value (Fowler, 1956), being most rapid for materials with Δ^~Ί.

* Pigg et al. (1956) have found different conductivities, depending on the direction of the applied field; furthermore the current-voltage characteristic is not linear at low voltages, but behaves in some respects as that of a crystal rectifier. At higher exposure doses the insulation is essentially ohmic.

513


X-rays ' 20°C

lh j*r**

_1_ _L

240

160

80

-10 0 20 40 60 FIG. 30.1a. Polyethylene at 20°C.

Ί

3r

•>t-

11 1

ί Ζ Τ 80-5°C

Π A \ / \

/ - -

/ -1»* » J 1

-10 0 20 40 60

80

60

H40

20

0

FIG. 30.1b. Polyethylene at 80-5°C, units of current 2x 10-14A. Typical curves of current against time showing initial build-up of induced current ix during 10 min of exposure to x-rays, and subsequent decay after cessation of irradiation. Linear rising curve is reciprocal of induced current, and shows hyperbolic

nature of its decay. Horizontal scale: time in minutes from cessation of irradiation. Vertical scale: (left hand) log ix · ; (right hand) reciprocal of induced current (x)

in units of 2 x 10x14 A.

Typical values of the static and induced conductivity for a series of polymers are given in Table 30.1 (Fowler and Farmer, 1956). The recovery times quoted in the table vary by little more than 10 per cent for a ten-fold change in dose rate. Table 30.2 compares the results obtained by different experimenters for polyethylene and polystyrene reduced to standard radiation conditions.

No general agreement has been reached as to the mechanism of induced

CHANGES IN ELECTRICAL CONDUCTIVITY 515

conductivity; a number of authors (Mayburg and Lawrence, 1952; Warner, Müller and Nordlin, 1954; Feng and Kennedy, 1955) favour the view that conductivity is due to ion mobility, while others (Fowler and

logio**

oC

f>-U

1 1 ' I »·

-1 0 +1 logiol

FIG. 30.2. Variation of induced conductivity ix with radiation intensity / at various temperatures. The slope Δ equals 0-81 ± 003.

30 3-2 103/T°K

FIG. 30.3. Temperature dependence of activation energy EQ for static conductivity, and Ex for induced conductivity in polyethylene.

LL


Table 30.1

Material

Polyethylene terephthalate

Moulded amber Natural amber Polystyrene Unplasticized Perspex Plasticized Perspex Red "400" Perspex PTFE Polyethylene

Static con-ductivity

(ohm cm)r1

1 0 - 2 3

5xl0-2 2

io-21

1 0 - 2 2

1 0 - 2 2

1 0 - 2 0

1 0 - 2 0

2xl0-2 0

5xl0- 2 1

Equilibrium induced conduc-

tivity 8 r/min

7xl0-2 0

io-17

io-18

7xl0~19

4xl0~19

3xl0- 1 8

8xl0~17

8xl0- 1 7

3xl0- 1 6

Time taken to recover from equilibrium

induced conductivity to the following values

io-19

(ohm cm)-1

0

12 min 30 min 30 min 70 hr 3hr

1400 hr 3000 hr 330 hr

io-20

(ohm cm)-1

3hr

2hr 5-5 hr 500 hr 8000 hr

33 hr 15,400 hr

♦

3330 hr

Δ

0-83

1 1 0-60 0-55 1 0-93 0-63 081

* The static conductivity is > IO-20 (ohm cm)-1. Source: Fowler and Farmer, 1956.

Table 30.2(a). Conductivity of Irradiated Polyethylene (Reduced to 10 r/min, 20nC)

Reference

(a) MAYBURG and LAWRENCE (1952) (b) COLEMAN (1955) (c) KEEL et al. (1953) (d) FOWLER (1956): cable

film cyl. condenser

Δ

0-75 0-75 0-58

0-81+003 0-82+003 0-79 + 0-05

Induced conductivity at equilibrium

(lO-îohmcm)"1)

— 10 60 1-2 1-4 10

Table 30.2(b). Conductivity of Irradiated Polystyrene (Reduced to 8 r/min, 20°C)

Reference Conductivity in 10~18/ohm cm

ARMISTEAD, PENNOCK and MEAD (1949) COLEMAN (1955) FOWLER (1956)

IO"18

3xl0- 1 8

1 xl0-1 8 ,2xl0"1 8

Farmer, 1954; Fowler, 1956; Bohn, 1951) believe that it arises from the motion of electrons. Trapping of these electrons in impurities has been


advanced as an explanation of the colour changes observed in polymethyl methacrylate (Day and Stein, 1951).

To account for the different values of Δ, Fowler (1956) investigated theoretically the influence on electron mobility of the energy distribution of electron traps, some of which may be present on the surface of crystal-lites in partly crystalline polymers such as polyethylene and PTFE. If the distribution of traps is uniform Δ=0·97 (^Ί), while for an exponential distribution of electron traps Δ has a lower value, down to 0-5. In the former case the temperature coefficient of the induced conductivity should be very small, while with an exponential distribution for which Δ is close to 0-5, a rapid increase with temperature is predicted. The "equivalent activation energy" Ex would vary from about 0Ό5 (for Δ=1) to about 0-3 eV (for Δ=0·5). Fowler also showed that the decay in induced current after irradiation should be exactly hyperbolic in the latter case, and approximately so in the former. The decay rate should also be far more rapid in the case of a uniform distribution of electron traps, for which Δ=1. The experimental results (Table 30.3) generally favour these predictions.

From this theory Fowler was able to deduce a number of parameters of the electron current, such as number, recombination cross-section and mean diffusion distance. The value n0 given in Table 30.3 relates to the number of current carriers under equilibrium radiation conditions, on the assumption of an electron mobility of 10~3 cm2/sec V. The diffusion distance between electron traps is deduced as lying between 3000 Â (plasticized polymethyl methacrylate) and 500 Â (polyethylene), while the total diffusion distance of an electron in these polymers is given as 7000 and 17,000 Â respectively. The number of traps per cm3 is estimated to ie between 1015 and about 1020.

Table 30.3

Material

Moulded amber Natural amber PMMA plasticized Mica PMMA red "400" Polyethylene

terephthalate Polyethylene* Polystyrene

(U.S.A. sample) Polystyrene PTFE PMMA unplasticized

Δ

10 10 10 0-95 0-93

0-83 081

0-75 0-65 0-63 055

£*(eV)

006 0-22 01 018 007

018 0-35

018 0-44 0-5 0-5

a*at20°C 8 r/min

(ohm cm)-1

lxlO-1 7

1 x 10-18

3xl0"1 8

lx lO-1 7

2xl0"1 7

6 x 10-20

9xl0"1 7

lx lO- 1 8

2xl0" 1 8

8xl0"1 7

2xl0"1 8

«0/cm3

6x l0 4

6 x l 0 3

2x l0 4

6x l0 4

1-3 xlO5

4x l0 2

6x l0 5

6x l0 3

1-2 xlO4

5x l0 5

1-3 xlO4

1 /decay slope

at 20°C

0-5 sec 40 sec 45 sec 10 sec

90 min

8-5 hr 7-5 min

30 min 13 hr 19 hr 24 hr

*Mayburg and Lawrence (1952) obtain an activation energy of 013 eV.


The temperature variation in the conductivity of polyethylene and PTFE during irradiation has also been studied by Meyer (1954). Fig. 30.4

^ 5

~i 1 1 r SOURCE C O 6 0

INTENSITY 49.3R MIN Δ TEFLON O POLYETHYLENE

-ΘΟ -60 -40 -20 TEMPERATURE (°C )

FIG. 30.4. Conductivity of irradiated polyethylene and PTFE at various tem-peratures : + 560, —560V are applied voltage.

Source: Meyer, 1954.

shows some of the data obtained which correspond to an activation energy of about 0-05 eV below 0°C and 0-26 eV above this temperature. This change is somewhat reminiscent of the temperature variation observed in the crosslinking of polyethylene. However, the activation energy (Fig. 30.3) obtained by Fowler (1956) is appreciably greater than that in crosslinking polyethylene in the same temperature range.

Most of the fundamental work reported here refers to radiation inten-sities of the order of 1-100 r/min. It would be of interest to extend this work to higher radiation intensities, and to materials with known amounts of impurities to act as electron traps. Gemant (1949) has reported a reduced conductivity when myristic acid is added to paraffin wax; in general dipolar mixtures appear to reduce the radiation effect, possibly by the formation of additional electron traps.


Several authors (Linder, 1953; Coleman et al, 1953) have envisaged the direct conversion of radiation energy to electrical power; electrons emitted from a radioactive source such as strontium 90 are collected on an adjacent plate which becomes charged to a high potential. To avoid leakage through the ionized air a thin plastic insulator is used. The conductivity of this insulator limits the voltage build-up, while the extensive exposure time may produce a total dose sufficient to cause appreciable chemical changes in the insulator. Coleman and Böhm (1953) have reported an increased resistance following prolonged exposure of polystyrene, poly-ethylene and other polymers, this increase being permanent. In their comprehensive set of data on pile-irradiated plastics Sisman and Bopp (1951) reported a few cases of a change in conductivity after radiation, although the doses used are sufficient to cause considerable chemical change. Somewhat similar experiments were also described by Pigg et al. (1956), measurements being made on the conductivity of conductors both during and after irradiation. The resistivity is decreased by a factor of about 10 during the actual operation of the reactor, as compared with the resistivity of the same conductor after exposure, when the reactor was not operating. Pigg et al. also report on a photo voltaic effect, which increases with radiation dose. From their results they conclude that the initial conductivity arises from photoelectrons, but at high doses conduction is by the electron deficiency in ionized atoms.

REFERENCES

ARMISTEAD, F. C , PENNOCK, J. C. and MEAD, L. W., Phys. Rev. 76, 860, 1949. BLACK, R. M. and CHARLESBY, A., Progress in Semiconductors, 3, to be published. BOHN, D., RRC Technical Report 3 July 1951. BURR, J. G. and GARRISON, W. M., AECD, 2078. COLEMAN, J. H., Nat. Acad. Sei., Washington 23rd Annual Meeting, 1955;

Nucleonics 11(12), 42, 1953. COLEMAN, J. H. and BOHN, D., / . Appl. Phys. 24, 497, 1953. DAY, M. J. and STEIN, G., Nature, Lond. 168, 644, 19M. FARMER, F. T., Nature, Lond. 150, 521, 1942; Brit. J. Radiol. 18, 148, 1945;

19, 27, 1946. FENG, P. Y. and KENNEDY, J. W., / . Amer. Chem. Soc. 77, 847, 1955. FOWLER, J. F., Proc. Roy. Soc. A236, 464, 1956. FOWLER, J. F. and DAY, F. T., Nucleonics 13(12), 52, 1955. FOWLER, J. F. and FARMER, F. T., Nature, Lond. 171, 1020, 1953; 173, 317,

1954; 174, 136, 800, 1954; 175, 516, 590, 648, 1955; Brit. J. Radiol·, 29, 338, 1956.

GEMANT, A., Phys. Rev. 58, 904, 1940; / . Appl. Phys. 20, 887, 1949. KALLMAN, H. and KRAMER, B., Phys. Rev. 87, 91, 1952. KEEL, D. K., KOGANOFF, S., MAYHEW, C. H. and NORDLIN, H. G., USAEC

NYO 4518, 1953. LINDER, E. G. and RAPPAPORT, P., Phys. Rev. 91, 202, 1953. LrvERSAGE, W. E., Brit. J. Radiol. 25, 434, 1952. MAYBURG, S. and LAWRENCE, W. L., / . Appl. Phys. 23, 1006, 1952. MEYER, R. A., ONR Symposium Report ACR 2, p. 93, Washington, 1954. MEYER, R. A., BOUQUET, F. L. and ALGER, R. S., / . Appl. Phys. 27, 1012, 1956.


PIGG, J. C , BOPP, C. D., SISMAN, O. and ROBINSON, C. C , Communication and

Electronics, January 1956. RAMSEY, K , Nature, Lond. Ill, 214, 1953. RAPPAPORT, P. and LINDER, E. G., J. Appl. Phys. 24, 1110, 1953; Phys. Rev. 91,

202, 1953. ROZMAN, I. M. and TSIMMER, K. G., Zh. Tekh. Fiz. (in Russian), 1681, 1956. STARK, H. H. and GARTON, C. G., Nature, Lond. 1225, 1955. SISMAN, O. and BOPP, C. D. , ORNL 928, 1951. WARNER, A. J., MULLER, F. A. and NORDLIN, H.G., / . Appl. Phys. 25, 131, 1954.

CHAPTER 31

USE OF POLYMERS IN NUCLEAR REACTORS

A NUMBER of reports have been issued comparing the resistance to radiation of plastics of potential use in certain reactor components. The data presented in these reports are of a practical character; little or no attempt is made to study the cause of failure, and the specimens examined are often commercial samples containing plasticizers, fillers, etc., in un-specified amount. Most of the measurements listed comprise information on the mechanical and electrical properties at room temperature, and therefore may not be always completely satisfactory for the purpose in view, since the mechanical damage may be more serious at higher operating temperatures, while the electrical properties such as resistance during irradiation may be very different from those a few hours after the exposure.

Among the early workers Burr and Garrison (1948) studied the mechanical properties (tensile, hardness and elasticity at room temperature) as well as electrical resistance and in a few cases dielectric constant and power. The specimens studied included PTFE, polyvinyl chloride, Neo-prene, polymethyl methacrylate, polystyrene, rubber and a copolymer of vinyl and vinylidene chloride. Both 2-5 MeV fast electrons from a Van de Graaff accelerator, and γ-radiation produced by electrons impinging on a target were used, in the latter case at levels of up to 106 r/min. Burr and Garrison concluded that the electron radiation was at least as damaging as the γ-radiation, and ascribed the difference to a dose-rate dependence arising from the temperature rise at high dose rates.

Further data of the same character accumulated during the period 1942-46 have been compiled by Miller and Steel (1948). In addition to the above polymers, measurements were carried out on the electrical resistance of wire coatings and power leads. This paper also summarizes earlier work, dating to 1943, on the change in electrical resistance on irradiation and the recovery after standing.

A very comprehensive set of measurements were published in 1951 by Sisman and Bopp. The plastics studied were mainly of commercial origin and the data relates entirely to their properties at room temperature. On the basis of the results observed, Sisman and Bopp were able to classify the plastics in terms of their radiation resistance (Table 31.1) although the physical cause of the changes produced by radiation was not examined. The properties measured comprise: stress-strain curve, tensile strength, elongation at break, elastic modulus, shear strength, impact strength, Rockwell hardness, changes in weight, specific gravity, water absorption, light transmission, volume resistivity, dielectric strength and arc resistance.

521


Table 31.1. Radiation Resistance of Plastics

Plastics Exposure 1018nvt Change in properties

(1) Mineral-filled furan and mineral-filled phenolics: Dura-Ion, Haveg 41, asbestos-fibre Bakelite, asbestos-fabric Bake-lite and Karbate

(2) Styrene polymers: Amphenol and Styron411C

(3) Modified styrene polymer: Sty-ron 475

(4) Aniline formaldehyde (Ciba-nite) and polyvinyl carbazole (Polectron)

(5) Polyethylene and nylon

(6) Mineral-filled polyester: Plas-kon alkyd

(7) Unfilled polyesters: Selectron 5038 and CR-39

(8) Phenolics with cellulosic fillers: paper - base Bakelite, linen-fabric Bakelite and Micarta

(9) Melamine and urea: Melmac, Beetle, Plaskon urea and Plas-kon Melamine

(10) Unfilled phenolic: Catalin

(11) Vinylidene chloride (Saran fi-ll 5) and vinyl chloride acetate (Vinylite)

(12) Casein (Ameroid), methyl methacrylate (Lucite), Teflon, Fluorothene and the cellulosics : cellulose nitrate (Pyralin), cellu-lose acetate (Plastacele), cellu-lose acetate butyrate (Tenite II), cellulose propionate (Forti-cel), and ethyl cellulose (Etho-cel R-2)

10

10

10

10

10

10

0-5

01

Little change except for darken-ing in colour.

Little change except for darken-ing in colour. Impact strength and elongation decrease until the same as un-modified styrene polymers. Tensile strength decreases a little.

Impact strength decreases but tensile strength increases. These plastics become so brittle that the corners of the specimens chip off. Tensile strength and impact strength are decreased about 50 per cent. Develop small cracks. Tensile strength and impact strength decrease. Become brittle, swell and de-crease in tensile and impact strength. Tensile strength and impact strength are decreased about 50 per cent. Tensile strength and impact strength are decreased about 50 per cent. Soften, blacken, evolve HC1 and decrease in tensile strength.

Tensile strength and impact strength are decreased about 50 per cent.

5

3

2

1

USE OF POLYMERS IN NUCLEAR REACTORS 523

Most plastics could be considered as falling into two main groups: (i) those in which elastic modulus, Rockwell hardness, tensile strength and shear strength increase, but impact strength and elongation fall with increasing dose, and (ii) those in which these properties behave in the opposite way.

In a subsequent report, Bopp and Sisman (1953) extended their work. The radiation resistance of a number of elastomers was compared and special formulations investigated in an effort to improve their radiation resistance. No striking improvement was observed, however. A number of comparative measurements were made with gamma radiation from a cobalt 60 source, and with pile radiation using a cadmium shield (which absorbs slow neutrons and emits gammas by the reaction Cd113 (n, y)Cd114, thereby enhancing the yield). In plastics containing chlorine atoms, a significant amount of change is produced by these slow neutrons but in other plastics where they have little effect, the incorporation of a cadmium shield may increase the changes produced by a factor of up to about 6.

In this compilation, Bopp and Sisman also list changes in specific volume due to pile radiation, and total gas evolved (summarized in Table 31.2). In the latter case, gas evolution may be primarily due to the

Table 31.2(a). Damage to Long Chain Polymers

Monomer

Methyl methacrylate

Tetrafluorethylene Monochlorotri-

fluoroethylene Styrene Styrène a methyl Amide (nylon)

Ethylene terephthalate

(Terylene, Dacron) Ethylene Vinyl butyral Vinyl carbazole Vinyl chloride Vinyl formal Vinyl vinylidene

chloride Vinyl chloride-

acetate

Type

Lucite Teflon

Fluorothene Amphenol

Q817 FM.10001 FM.3003

Mylar Polythene Butacite Polectron Geon 2046 Formvar

Saran

Vinylite

Dose for damage Thresh-

old 25% (megarads)

0-82 0017

1-3 800 4-3

0-86 0-86

30 19

4-7 88 19 16

4-1

1-4

11 0037

20 4000

43 4-7 4-7

120 93 19

4400 110 82

45

2-5

Gas evolution ml/g

megarad x lO 3

30

0-25

20

4 64

90

μΜ/g megarad

1-34

0011

0-9

018 2-85

4


Table 31.2(b). Damage to Formaldehyde Resins

Formal-dehyde

Aniline Melamine

Phenol Phenol

Phenol

Phenol Phenol Phenol

Phenol

Phenol Urea

Filler

Cellulose

— Asbestos

fabric Asbestos

fibre Asbestos Graphite Linen fabric

Paper laminate

Paper Cellulose

pulp

Type

Cibanite Melmac, Plaskon Melamine Catalin Bakelite

Bakelite

Haveg 41 Karbate Bakelite

Micarta

Bakelite Beetle, Plaskon urea

Dose for damage Thresh-

old 2 5 % (megarads)

0-74 7-4

2-7 18

78

390 0-89 0-34

0-34

0-38 8-3

14 110

11 770

890

3900 77 2-8

8-2

26 51

Gas evolution ml/g

megarad x lO 3

6-4 5-4

3-2 < 0 1 4

< 0 1 4

< 0 1 4 < 0 0 3

14

18

17 10

μΜ/g megarad

0-3 0-24

014 < 0 0 0 6

< 0 0 0 6

< 0 0 0 6 < 0 0 1 3

0-63

0-8

0-76 0-45

Table 31.2(c). Damage to Cellulosics

Cellulosic

Acetate Acetate butyrate Nitrate Proprionate Ethvl

'

Type

Plastacele Tenite II Pyralin Forticel Ethocel

Dose for damage

Threshold 25% (megarads)

2-7 0-37 0-63 0-44 1-4

1-9 7-3 5-7 4-4 5-5

Gas ev ml/g

megarad x lO 3

20 30

130 35 31

olution

μΜ/g megarad

0-9 1-34 5-8 1-6 1-4

Table 31.2(d). Damage to Miscellaneous Plastics

Plastic

Allyl diglycol car-bonate

Casein Furan asbestos +

carbon black Polyester

Type

CR39 Ameroid

Duralon Plaskon alkyd

Dose for damage

Threshold 25% (megarads)

1-5 2-8

330 87

88 27

3300 3900

Gas evolution ml/g

megarad x lO 3

57 6

< 0 1 5 3 1

μΜ/g megarad

2-54 0-27

<0006 014


decomposition of the filler when present, as in the case of certain formal-dehyde or phenolic resins. The radiation resistance of aniline formal-dehyde, of the phenolic resins and of styrene compounds is noteworthy and arises in part from the stability conferred by the benzene ring. On the basis of this work, Bopp and Sisman prepared a table of chemical groupings in the order of their resistance to radiation (Fig. 31.1).

H H I I c —c — I I

H

H I

-C —N — I I H / \

H H I I

■c= c-

The repeating unit in the structural formula of polystyrene, which is the most stable of the un-

filled polymers tested.

The repeating unit of aniline formaldehyde polymer. As for poly-styrene, stability is attri-buted to the bulky ben-zene-ring-containing side

groups.

Present in many elasto-mers; since the stability of elastomers appears to be insensitive to the amount of unsaturation, this group is ranked with

next group.

H H I I

— c —c — I I

H H The repeating unit of

polyethylene.

O II

— C—N — I

H Present in nylon, which shows the same order of stability as polyethylene.

CH3 I

— Si — O — I

CH3 The repeating unit of silicone rubber, which shows the same order of stability as most other

elastomers.

OH I H

~ \ _ / —c-!

H The repeating unit of phenol formaldehyde polymer. Presence of benzene ring in main chain is thought to in-crease cleavage, since un-filled phenolic crumbles for exposures that do not decrease strength of poly-ethylene (this contrasts with effect of benzene ring in polystyrene, in which it is in a side

group).

H I

— c — o — I

H

Also taken to be less stable than polyethylene. Polyallyl diglycol carbo-nate, polyvinyl formal and polyvinyl butyral are softened. Selectron-5038 is hardened ; however, this plastic is initially very soft and shows a high rate of crosslinking.

H S I I

— c —s — I

H

Present in Thiokol, for which a balancing of cleavage against cross-linking causes small hard-ness change, but de-creases the u l t ima te

strength.


O II

-c —o —

Present in Dacron. The predominant radiation change is embrittlement.

F I

— c-

H C\ I I

- c — c — I I

H H

Present in polyvinyl chloride. Unplasticized polyvinyl chloride is softened by cleavage, though highly plasticized

forms are hardened.

(F or Cl) I

- c —

H I 1 c -/ l

/ OH - C

l \ H

H \ | c — | CH2

OH 1

1 c l \ H \

C -,/\

/ H O

OH The repeating unit cellulose. brittlement

Rapid

- O -

of em-

of cellulosic plastics shows that structure is sensitive

chain < cleavage.

this Î tO

R

F F The repeating unit of Teflon and Fluorothene, which become brittle and crumble apart at rela-tively short exposure. Resistance to cleavage is

poor.

H I I

— c — c — I I

H R' The repeating unit in polymers with quater-nary carbon atoms : poly-methyl methacrylate , butyl rubber and poly-

alphamethyl styrene.

FIG. 31.1. Polymer groups ranked in order of stability against cleavage. (Bopp and Sisman, 1955). A different order may be obtained if the assessment is based on other

properties such as solvent resistance.

A recent report by Bennett (1954) compares plastics for potential use in radiation fields, mainly on the basis of the work by Bopp and Sisman, but also including data on paint films incorporating some vinyl material. The paper includes technical data on unirradiated plastics.

Harrington (1956) presents an extensive set of data on radiation damage in a number of commercial plastics and elastomer. The full tables give changes in hardness, elasticity, elongation, tensile and weight for exposure of between 5 and 150 megarads of γ-rays from spent fuel rods., Most materials examined showed appreciable changes for a dose of only 5 mega-rads, and in a few cases there was an improvement in properties for the lower doses. Comparisons were also made between cobalt 60 radiation at 0-37 megarad/hr at 25°C, and radiation from spent fuel elements at about 0-1 megarad/hr at 15°C, but it is difficult to predict the effect of radiation over a wider intensity range. This work was subsequently extended to a number of physical and chemical properties of irradiated plastics (Harrington, 1957, 1958; Harrington and Giberson, 1958).

Parkinson and Kirkland (1957) noted an increase in tensile strength of asbestos mat or paper, impregnated with such crossl inking polymers as polyvinyl acetate, polystyrene or silicone. An interesting feature is that


such impregnated materials show improved mechanical properties on ageing over a period of months, this improvement being more marked with the irradiated specimens. The general shape of the tensile and elongation-dose curves is reminiscent of the corresponding curves for rubber contain-ing carbon black. Johnson and Sicilio (1958) reported on the radiation resistance of a number of plastics, fabrics and elastomers, subjected to pile radiation, but were unable to deduce any general pattern in the thermal, mechanical and electrical changes. Leininger (1958) studied the behaviour of fluorine-containing elastomers, and found that the rate of degradation depended on the environment, being reduced by oil or argon.

Reactor Components

Calkins (1954) gives data on the expected lifetime of reactor components subjected to a radiation flux. Gaskets, seals and electric motor windings involving elastomers, leather or organic insulants may often be expected to show 25 per cent threshold damage after some 4-5-7 megarads. The figure wiH, however, vary widely with the type of material, and the definition of damage. Alternative materials are suggested.

Bresee et al (1956) describe the change in properties of a number of plastics, used as gasket material. Polyethylene shows little change in tensile strength or shear strength, but a marked drop in elongation at break (from 325 per cent to 7-6 per cent) and in breaking energy (a few fold decrease) for a dose of 1000 megarads. Approximately the same behaviour is shown by polyethylene with 1 per cent carbon loading. Poly-styrene is far more radiation resistant, showing only a two-fold drop in breaking energy for the same dose. Both Teflon and Kel-F fail at less than one-tenth of this dose. McCarthy (1955) also gives information of the behaviour of irradiated Teflon bellows. The effect of various plasticizers in PVC compositions is discussed by Wells and Williamson (1958).

Electrical Properties

According to Bopp and Sisman, the electric properties such as volume resistivity, dielectric strength or arc resistance show little change until the mechanical properties are modified. Exceptions are polyvinyl chloride and vinyl chloride-acetate polymer, presumably due to the HC1 evolved. Damage by radiation was found to be increased by the presence of water in nylon, polyester and organic filled phenolics. The effect of nuclear pile radiation on the electric properties of some polymers was also summarized by Pigg et al. (1956).

Weeks and Binder (1954) subjected cables with polyethylene and Teflon insulation to nuclear reactor radiation at Oak Ridge. The dose was about 2 x 1018 nvt of mixed radiation, equivalent to about 103 megarads. After this high dose, the dielectric constant was changed by 1*4 ± (0-4) percent while the power factor increased by 9 ( ± 2) per cent, in both cases at 4 Mc/s. In subsequent work Weeks (1954) measured the loss angle and the dielectric constant of polyethylene, polystyrene and Teflon at three


Table 31.3. Effect on Electrical Properties

Polymer

Allyl diglycol carbonate

Cellulose acetate

Cellulose acetate butyrate

Melamine formaldehyde (filled)

Phenol formaldehyde (asbestos-fabric filled)

(asbestos-fibre filled)

Polyalphamethyl styrene

Nylon

Terylene

Polystyrene

Polyvinyl chloride

Vinyl-vinylidene chloride

Urea formaldehyde

Vinyl chloride-acetate

Type

CR39

Plastacele

Tenite II

Melmac Plaskon Melamine Bakelite

Bakelite

Experim. Type Q.817 FMI

Mylar

Amphenol

Geon 2046

Sapan

Beetle

Plaskon urea Vinylite

Dose (megarads)

0 500 900

0 34 68

170 180

0 26 95

190 0

1500 2200

0 2100 3100

0 3100

0 430

0 2800 3900

0 210 360

0 3600 1

0 5300

0 27 39 61

140 220

0 600

1800 0

29 460 740

Volume resistivity (ohm-cm)

1014

6xl01 3

2xl01 2

5 x 1012

2 x 1012

2x lO u

2x lO n

— 1014

5xl01 3

2xl01 2

— 1 x 1011

— 1 x 1011

2x l0 9

1 x 1010

— 2xl01 0

3 x 1010

lxlO1 5

5xl0 9

lxlO1 3

— 4xl0 1 3

lxlO1 5

lxlO1 5

1 x 1012

1 x 1014

1 x 1014

lxlO1 3

1x10e

2xl01 4

2xl01 3

7xl01 2

3x l0 9 ! 3 x 107

— 2xl01 3

— 1 x 1011

1014

1014

10e

—

Dielectric strength (V/mil)

850

600 750 — — — 750

1000

— 700 200 200 — 80

— 100 160 200 — — 860 860 — — — —

1600 1300 — — 660 — — — — 230 230 230 —

1100 — — 600

Source: Collins and Calkins (1956), Pigg et al (1956).


frequencies—1, 3 and 8-6 kMc/s, after exposures of up to 8x l0 1 8 nv t . Teflon showed the major change in power factor, which increased by a factor of about 7-3 at 1 and 3 kMc/s, and only 2-1 at 8-6 kMc/s, for an exposure of 2 x 1018 nvt. In polyethylene the corresponding changes were an increase by a factor of 2-5 at 1 and 3 kMc/s for an exposure of 8xl01 8 nvt, although no significant changes were observed at 8-6 kMc/s. Polystyrene, which is known to be very radiation-resistant as far as mechanical pro-perties are concerned, showed no significant change in loss angle of up to 2 x 1018 nvt. Changes in the dielectric constant were only observed in the case of Teflon, these changes (of about 2 per cent) being expected in view of the great sensitivity of Teflon to radiation damage. Some anomalous results might be associated with the reaction of Teflon with water leaking into the container. The difference in behaviour of Teflon when irradiated in air and in water has already been mentioned (Chapter 20).

In connexion with electrical insulation in the Submarine Intermediate Reactor, Mannal (1954) measured the radiation resistance of mica or glass bound with a silicon resin based varnish. No change at voltage breakdown was observed for doses up to 12,000 megarads, nor was there any marked change in resistivity up to at least 800 megarads. Studies were also made of resistance to abrasion, and gas evolution under radiation. The latter is sufficiently high to warrant special attention being paid to specimens irradiated in a sealed container.

In a continuation of this work Klein and Mannal (1956) measured the breakdown voltage in thin films of irradiated polyethylene, poly vinyl chloride and cellulose acetate. The effect of radiation was to cause an increased scatter in the values of the breakdown voltage. In polyvinyl chloride the breakdown voltage is reduced by about 10 per cent and 20 per cent by radiation doses of 10 and 100 megarads. In polyethylene tape the average breakdown voltage is hardly affected, although the worst 5 per cent of the irradiated polyethylene specimens (10-100 megarads) had a breakdown voltage some 10-20 per cent below the worst 5 per cent of the unirradiated specimens. Cellulose acetate if anything showed a slight improvement after the same doses. A polyvinyl formal-insulated wire showed no effect after 100 megarads if annealed after radiation.

Ryan (1954) subjected some representative insulating materials to nuclear pile radiation both at Brookhaven and at Oak Ridge for 30 days, equivalent to some 6-3 x 1018 and 2-5 x 1018 nvt. The materials studied included an oil-modified phenolic varnish, silicone resin, silicon-glass or phenolic-glass laminates, and the tests included colour changes, crazing and cracking, embrittlement, deformation and creep, abrasion resistance, dielectric strength and gas evolution. Of the specimens tested a silicone impregnated coil showed least damage as far as elongation and creep are concerned, but a substantial decrease in dielectric strength, and in abrasion resistance. A number of papers study the effect of atomic radiation on electronic components including some involving plastics: Green et al. (1956), Javitz (1955), McClinton (1953), Pigg et al. (1956), Robinson (1956), Skelton and Kenney (1955, 1956).


Organic Moderators

Callinan (1955) studied the use of additives as a means of increasing the radiation resistance of various oils. Collins and Calkins (1956) sum-marized a considerable amount of technical data on radiation damage to elastomers, plastomers and organic liquids. They showed that gas evolu-tion from several organic compounds is proportional to dose, and indepen-dent of dose rate, in line with the results on the crosslinking of polymers. Viscosity changes in a number of irradiated organic materials are also given in terms of the radiation dose. The rapid viscosity rise with increasing dose may be ascribed to crosslinking (page 4), although in at least one case (w-butyl benzene) the rise was much less marked when the irradiation was carried out in the absence of air. Hexadecane, for example, showed a ten-fold rise in viscosity for a dose of 1200 megarads, and an infinité viscosity might be expected for 2000-3000 megarads, correspond-ing to a G value for crosslinking of a hydrocarbon chain of about 2. For these paraffins the observed increase in viscosity is in good agreement with these predicted for the crosslinking of polyethylene.

Colichman et al. (1956, 1957) have studied the radiation resistance of terphenyls, employed as organic moderators and coolants in atomic reactors. These compounds have high radiation stability owing to their highly resonant structure.

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PARKINSON, W. W. and KIRKLAND, W. K., ORNL, 2413, August 1957. PFAFF, E. R. and SKELTON, R. D., Air Force Report AD 11763, 1955. PIGG, J. C , BOPP, C. D., SISMAN, O. and ROBINSON, C. C , Commun, and Electro-

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1956. RYAN, J. W., GEL 57, 1952; Nucleonics 71(8), 13, 1953; Mod. Plast. (April) 148,

1954. SISMAN, O. and BOPP, C. D., ORNL, 928, 1951. SISMAN, O. and WILSON, J. C , Nucleonics 14(9), 58, 1956. SKELTON, R. D., Air Force Report AD 82829, 1956. SKELTON, R. D. and KENNEY, J. G., Nucleonics 14(9), 66, 1956. WEEKS, R. A., ORNL, 1762, 1954. WEEKS, R. A. and BINDER, D., ORNL, 1700,1954; ORNL, 2413,1957; Nucleonics

(to be published). WELLS, H. and WILLIAMSON, I., (AERE) E/R2518; Br. Plastics 31, 310, 1958.

MM

APPENDIX THEORY OF NETWORK FORMATION BY RANDOM

CROSSLINKING

Polymer Molecules of Uniform Size

The simplest case to consider is one where the polymer molecules are initially all of the same size (wi units). The notation given in Chapter 8 will be used. For a given crosslinking density, the number m of the initial molecules carrying 0, 1, 2, . . . / crosslinked units is

«o = A(l-<?)W l

«! = Λ(1-<7)*1_1<7"ι in general

m = A0 (1 -q)u*-*qW-l /! O i ~ 0 !

where A0 is the initial number of molecules. For a long chain polymer in which the number of crosslinked units is

a small proportion of the total units, q<\, i<uu

n0 = A0 e-v nx = A0 ye-v

Ύ2

n* = AQ — e~y

Y* m = AQ— e~v

i\

where γ = qux. The proportion /(l), t(2) . . . *(/) of all crosslinked units on molecules

carrying a total of 1, 2 . . . i crosslinked units is as follows :

f(l) = n1/qA1 = erf i(2) = 2n2/qA1 = γ^-ν

γ ί - 1 t(i) = ιΐΐι/ςτ^! = (y3yy| *~τ

where m« is the total number of crosslinked units on the m molecules with / crosslinked units each, and qAl or yA0 is the total number of crosslinked units in the specimen.

The number of polymer molecules which, not having been crosslinked by radiation, and therefore conserving their initial size ux is

n(u0 = A0e~t.

532

APPENDIX 533

Polymer molecules with 2ux units formed by linking together two of the initial molecules (each carrying one crosslinked unit) number

n(2«0 = J A ye-y · /(l) = ^ e~*r.

Polymer molecules with 3 ux units are formed by linking together a mole-cule with two linked units, and two with one linked unit each.

w(3«i) = n2. /(I). /(l)/2

Flory (1941) derived a general expression for the weight fraction Wz of polymer molecules, of weight zui, as a function of z and γ. By définition

„ . ZWÜ! n(zu^ z w(zwi) w Αχ A0

Flory finds as a general expression

zl

In the notation used in this work this becomes

zz-z n(zih) = A0—ΓΎ*-Ι<Γ*Υ.

Polymers with an Arbitrary Initial Distribution

The initial molecular weight distribution, which may take on any values consistent with physical limitations, is represented by a function n{u) (or n(M)) or alternatively by the set of parameters A0, Au A2 . . . At as defined in Chapter 8. It is required to determine the change in this distri-bution, and the formation of an insoluble fraction, when the specimen is subjected to a density of random crosslinking q.

The method of analysis consists in studying the probability of a given molecule A remaining soluble either because it carries no crosslinking units, or alternatively because all crosslinking units which it carries bind it to molecules B, C, etc., but that B, C, etc., are not themselves bound to the gel fraction of the polymer, in this latter case the crosslinked units on A are termed sterile. The complete calculation is given by Charlesby (1954).

The number of molecules with u monomer units, of which c are cross-linked, is

nc{u) = n(u) {\-q)u~cqcu\j{u—c)\c\ ^ n(u) exp (—qu)q cu c/c I {c<u).


The proportion of all crosslinked units carried by molecules with c cross-linked units is

t(c) = H*cnc{u)\qAx = cLn(ü) cxp{—qü)qc~1uclc\A1.

If S is the probability that a given crosslinked unit on molecule A is sterile, i.e. its link to B does not tie it via other linked units on B to the gel, then

S = î(l) + Stil) + S21(3) + . . . + S*-1 t(c) + ...

Here /(I) represents the possibility of B carrying one crosslinked unit, so that a link to B is necessarily sterile. Similarly f(3) is the probability of the link occurring with a molecule B carrying a total of three crosslinked units; S2 is the probability of the other two crosslinked units (additional to that with A) being sterile. By substituting values for t(c)

S = Σ 5 0 - 1 cpi(w) e x p ( - r t e - 1 « c / c ^ i .

Inverting the order of summation and adding for all values of c

S = Σ n(u) u exp(-qu) exp(quS)/Ai

= — Σ n(u) u exp Iqu S— 1 L

This gives a relation for the sterile coefficient S in terms of the initial molecular weight, and the crosslinking density q.

The soluble fraction s is obtained by summing the weights of molecules all of whose crosslinked units are sterile, and dividing by the total weight Αχ\ν.

s = Σ Σ nc(ü) uw SciAiW u c

= Σ n{u)u exp Iqu S— 1 \lAx = S

so that the soluble fraction s is equal to the proportion S of crosslinked units which are sterile.

The gel fraction g = 1— s g = 1 - Σ n(u) u exp(—0j*ff)//4i.

By expanding the exponential, and replacing the summation in terms of the distribution parameters A0, Ax...

g = ^(A.qg-A.q^lll + A.q^ßl- . . .) Ax

= u2qg-u3u2q2g2/2\ -f uxuzu2q*gzIV.— . . .

or in terms of the crosslinking coefficient δ ( = qu2)

2! w2 3! «2

as given previously (equation 9.21).

APPENDIX 535

Shape of Gelation Curve

For an initially uniform distribution u2 = u3 = ué = . . .

g = l—sxp(—qug) = l - e x p ( - y g ) and s = 1 — g = exp(l — δ 1 —s).

For an initially random distribution u2\2 — w3/3 = z/4/4 — . . .

^ = 1-1/(1+T^)2

which can be written in the form

s + V7= 2/8.

For a pseudo-random distribution u2 = w3/2 = w4/3 = . . .

^ = δ ^ - δ ^ 2 + S y - . . . = ^ / (1 + Äg) or s= 1/S.

In the general case, no simple analytical expression can be derived, but the values of g (and hence of δ) can be readily calculated for a series of values of δ#, when the initial distribution is known in terms of u2, u3, w4, etc. The number average degree of polymerization ux is irrelevant to gel formation.

Gel Point

For very small values of g, higher orders of Sg may be ignored :

2! «2

or 8- l = I-38'g. 2! wa

As g tends to zero δ tends to 1. Thus all polymers whatever their initial molecular weight distribution, first form an incipient gel when δ = 1.

Initial Slope

The slope of the sol/δ curve near the gel point,

as _ ag __ 2u2

d8 d8 w3

( = —2 for a uniform distribution, —1*33 for a random distribution, —1 for a pseudo-random distribution).

The slope of the curve for higher values of δ depends on the higher degrees of polymerization ui9 w 5 , . . . etc.

Weight Average up to the Gel Point

The weight average molecular weight of a polymer, subjected to an irradiation dose r insufficient to cause gelation can be derived from the


condition that at the gel point 8 = 1 . If the initial weight average degree of polymerization is u2, and after a dose r it is u2\ then

U2 <7o ' 'gel = 1 and u2 go(rge\—r) = 1

since the dose needed for gelation of the initial polymer can be given in two stages, r and rgei—r. Then

tfoi/gei—r) u2q0rëe\-u2q0r 1 —δ'

A similar method can be used to derive u3\ « / , etc., in terms of δ. The number of crosslinked units in the sol is ΣΣηε(ιι) cSc. Replacing

tic(u) by its value in terms of n{u) (as given above) and summing gives AtfS2, or since S—s, Axqs2. The number of monomer units in the sol is Axs, and the density of crosslinked units in the sol is

qs = Arfst/Atf = qs.

The number of crosslinked units in the gel is Axq—Axqs2 or Axq{\— J 2 ) , distributed over Ax{\— s) units. The density of crosslinking in the gel

qg = A^l-sÂ^l-s) = q(l+s).

Other parameters of the molecular weight distribution can be derived from the quantities

ΣΛ(#) exp (—qug), X«(w)w2 exp(-qug)

which are analogues to the expression for s used above :

r = Σ«(ί/) exp(—qug)fën(ti) s = Σ«(ί/) u Qxp(—qug)^n(ü)u t = E«(w)w2 exp(—qug)/%n(u)uz.

The fraction of initial molecules left in the sol, for example, is r (Charlesby, 1954). For a random distribution r = Vs. The number of monomer units left in the sol is Axs, while the number of molecules is A0r or A0Vs, so that the number average degree of polymerization is Ais/A0Vs or uiVs. This is the initial value of the degree of polymerization of molecules eventually left in the sol. The number of crosslinked units in the sol is Aisqs

or Axqs2, and these reduce the number of separate molecules by A^s2/!, so that the number average degree of polymerization of molecules in the sol is A^KAoVs^-Arfs2/!) or Ι / Λ / J / O — έγ$8/2). It should be recalled that these last expressions apply only to an initially random distribution.

REFERENCES

FLORY, P. J., / . Amer. Chem. Soc. 63, 3096, 1941. STOCKMAYER, W. H., / . Chem. Phys. 12, 125, 1944. CHARLESBY, A., Proc. Roy. Soc. A222, 542, 1954

NAME INDEX ABRAHAM, R. J., 223, 255, 292, 296,

336, 341, 342, 346, 348, 356, 358, 366, 448

ADICOFF, A., 383, 392 AITKEN, P. B., 74 ALEXANDER, P., 15, 35, 58, 73, 107,

109, 238, 239, 256, 290, 296, 326, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 341, 343, 346, 355, 358, 428, 429, 430, 432, 436, 437, 438, 440, 471, 473, 475, 489, 490, 492, 493, 494, 498, 499, 500, 505, 506,511

ALGER, R. S., 519 ALLEN, A. O., 14, 74, 98, 101, 111,

187, 196, 197, 530 ALLISON, S. K., 50 ALLSOP, C.B., 15 AMPHLETT, C. B., 73 ANDER, P., 256, 257, 393 ANDERSON, L. C , 57, 196, 197, 354,

369, 392 ANDREWS, H. L., 1, 101, 109 ANEMIYA, A., 257 ANGIER, D. J., 411 ARAND, L., 296 A R D , —., 348, 355, 356, 358 ARMISTEAD, F. C , 516, 519 ARNIM, — . , 223 ARNOLD, E. D., 73 AUERBACH, I., 272, 278, 283

BACCAREDDA, M., 241, 246, 257, 295, 296

BACH, N., 194, 197 BACK, M. H., 110 BAGDASARIAN, C. S., 374, 382, 392,

393, 394, 460, 462, 464, 465 BACQ, Z. M., 15, 35,492, 511 BAIN, T., 257, 272, 277, 283 BALL, R. M., 74 BALESTIO, F., 106, 110 BALLANTINE, D. S., 231, 236, 237,

248, 256, 257, 374, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387,

389, 392, 393, 394, 395, 399, 400, 409, 410, 411, 418, 419, 420, 425, 455, 464, 477, 490

BALSHISER, R. E., 73 BALWIT, J. S., 204, 229, 231, 241, 244,

245, 247, 252, 255, 256, 257, 275, 311, 323, 326, 334, 335, 347, 384, 393, 491

BAMBAUER, H. U., 394 BAMFORD, C. H., 377, 392 BAUGHAN, E. C , 448, 453 BAUMAN, R., 280, 283, 296, 494, 511 BAUMAN, R. G., 72, 74, 95, 105, 106,

110 BARB, W. G., 377, 392 BARDWELL, D. G., 2, 194, 197, 212,

255, 393 BARELKO, E. V., 195, 197 BARR, N. F., I l l BARRY, A. J., 298, 299, 310 BASKETT, A. C , 181, 212, 255, 256 BAYSAL, B. J., 394 BAXENDALE, J. H., 430, 440 BEATTIE, J. W., I l l BEARD, D. S., 74 BECQUEREL, — . , 1 BEHR, J., 392,411 BELLAMY, W. D., 363, 367 BENDER, A., 296 BENHOUGH, W. I., 392 BENNETT, J. F., 526, 530 BENSASSON, R., 377, 393 BENT, H. A., 255, 257 BERHOWITCH, J., 439, 440 BERNAS, A., 377, 380, 381, 393 BERNSTEIN, I. A., 377, 392 BERNSTEIN, W., 96, 110, 111 BERRY, K. L., 349, 358 BERTHELOT, — . , 1 BETHE, H. A., 50 BEVINGTON, J. C., 346, 472, 476, 490.

499, 511 BERZELIUS, — . , 1 BlLLINGTON, D . S., 13 BINDER, D., 527, 531 BISCHOFF, — . , 336, 346

537

538 NAME I N D E X

BLACK, R. M., 58, 73, 95, 109, 231, 232, 233, 236, 238, 256, 257, 323, 326, 329, 330, 331, 332, 333, 334, 440, 511, 519

BLACK, R. H., 489, 490, 491 BLAIR, G. E., 104, 105, 110 BLOMGREN, R. A., 74 BLOMQUIST, G., 169 BOAG, J. W., 49, 50, 97, 110, 465 BÖHM, D., 516, 519 BOLZ, F., 169 BONNAUD, M., 110 BOPP, C. D., 56, 60, 73, 109, 110, 251,

257, 266, 267, 268, 280, 281, 283, 294, 296, 310, 313, 319, 321, 323, 324, 326, 334, 335, 343, 344, 346, 347, 349, 356, 357, 358, 364, 365, 366, 367, 492, 511, 519, 520, 521, 523, 525, 526, 527, 530, 531

BORDON, P. G., 257, 296 BOUBY, L., 58, 192, 196, 392, 409,

411,457,460,464,465 BOUQUET, F. L., 519 BOVEY, F. A., 317, 318, 324 BOVEY, F., 475, 491 BOWEN, E. J., 511 BOWERS, — . , 207, 209 BOWERS, G. H., 231, 255 BOYSEN, M., 440, 465 BRASCH, A., 95, 392 BRAY, Β. G., 196 BREGER, I. A., 193, 196, 197, 255 BREITENBACH, J. W., 394 BRESEE, J. C , 257, 527, 530 BRETTON, R. H., 369, 392 BRIGGS, E. R., 465, 466 BRILL, R., 296 BROOKS, H., 14 BRODY, — . , 1 BROWN, D. E., 283, 287, 288, 291,

292, 296, 393, 465, 489, 491 BROWN, D. W., 336, 338, 339, 341,

342, 343, 346, 347 BROWNELL, G. L., 110 BROWNELL, L. E., 64, 73, 74, 392 BRYANT, M. P., 367 BURHOP, E. H. S., 445, 454 BUECHNER, W. W.. 95 BUECHE, A. M., 241, 244, 245, 247,

252, 255, 257, 275, 283, 311, 323, 326, 334, 335, 347, 491

BUECHE, F., 299, 300, 304, 305, 307, 310

BUNN, C. W., 348, 358 BURLANT, W. J., 319, 323, 389, 392,

399,411 BURLART, — . , 383 BURNETT, G. M., 377, 392 BURR, J. G., 313, 323, 346, 519, 521,

530 BURROWS, J., 272, 277, 283 BURTON, M., 14, 30, 50, 57, 70, 73,

74, 111, 197, 448, 452, 453, 454, 465,496,511

BURTON, V. L,, 187, 193, 196, 197, 229, 256

BURTON, — . , 187 BUSSE, — . , 207, 209 BUSSE, W. F., 231, 255 BUTLER, J. A. V., 440 BUTTA, E., 257, 296 BYRNE, J., 314, 315, 316, 323, 357,

358

CALKINS, V. P., 54, 57, 58, 60, 73, 527, 528, 530

CALLAGHAN, — . , 210, 490 CALLAGHAN, L., 218, 220, 222, 223,

241,244,245,255,256,408 CALLINAN, T. D., 74, 384, 388, 392,

414, 425, 530 CAMERON, — . , 1 CHAD WICK, J., 51 CHAPIRO, A., 58, 73, 74, 101, 110,

196, 197, 236, 238, 251, 255, 256, 257, 312, 314, 316, 323, 334, 336, 345, 346, 365, 366, 371, 374, 376, 380, 386, 392, 393, 397, 398, 399, 406, 407, 408, 410, 411, 428, 432, 440, 456, 459, 461, 463, 464, 465, 466, 472, 473, 474, 475, 476, 490

CHARLESBY, A., 58, 73, 74, 95, 101, 104, 108, 109, 110, 111, 141, 157, 160, 161, 169, 171, 174, 176, 178, 180, 181, 190, 191, 196, 204, 207, 208, 210, 211, 213, 214, 218, 220, 222, 223, 224, 227, 228, 231, 232, 233, 234, 235, 236, 238, 239, 240, 241, 244, 245, 255, 256, 257, 261, 263, 264, 265, 270, 272, 277, 279, 280, 283, 285, 287, 288, 296, 299, 300, 301, 303, 304, 305, 306, 311, 312, 313, 320, 321, 323, 326, 329, 330, 331, 332, 333, 334, 335, 336,

NAME I N D E X 539

337, 338, 339, 340, 341, 342, 343, 346, 347, 350, 351, 354, 355, 358, 360, 361, 366, 389, 391, 393, 394, 398, 404, 405, 406, 408, 409, 410, 411, 414, 415, 416, 417, 418, 422, 424, 425, 428, 429, 430, 432, 436, 437, 438, 440, 453, 456, 457, 458, 459, 461, 464, 465, 467, 470, 472, 476, 479, 480, 485, 486, 488, 489, 490, 491, 493, 494, 498, 499, 502, 504, 505, 506, 507, 509, 511, 512, 519, 533, 536

CHARLETON, E. E., 95 CHEN, W. K. W., 401, 402, 411 CHICK, D. R., 95 CLARK, G. L., 283 CLARKE, A. M., I l l CLELAND, J. W., 14 COCKBAIN, E. G., 411 COCKCROFT, J. D., 74, 95 COLEMAN, J. H., 358, 516, 519 COLICHMAN, E. L., 295,296, 393,416,

425, 512, 530 COLLINS, C. G., 54, 60, 73, 528, 530 COLLINSON, E., 14, 182, 196, 377 COLLYNS, B. G., 479, 491 COLOMBO, P., 384, 392, 411, 464 COMBRISSON, J., 323, 341, 342, 347,

366 COMPTON, A. H., 50 CON WAY, B. E., 440 COOLIDGE, — . , 2 COOLIDGE, E., 369, 393 COOLIDGE, W. D., 393 COPPERMAN, A., 296 COREY, V. B., 530 CORMACK, D. V., 110 CORTH, R., 296 CORVALL, M., 73, 109, 110 COSTIKYAN, T. W., 323, 357, 358 COTTIN, M., 74 COTTRELL, A. H., 13 COTTRELL, T. L., 19 COUSENS, S. F., 511 COUSIN, C , 73, 110, 197, 465, 466 Cox, R. A., 196, 440, 461, 465 CRAGGS, J. D., 77, 78, 95 CRAWFORD, J. H., 14 CUMMINGS, W., 465, 466 CURIE, — . , 1 CURRIE, C. C , 311

DAINTON, F. S., 14, 377, 393, 465 DALE, W. ML, 492, 512 DANIELS, F., 74 DANIELS, M., 440 DANNO, A., 257, 319, 323, 439, 440 DAVID, V. W., 57, 73, 74, 110 DAVIDSON, W. C , 74 DAVIDSON, W. L., 283, 326, 334 D A VIES, J. V., 512 DAVISON, W. H. T., 101, 108, 110,

184, 187, 196, 222, 226, 232, 234, 235, 256, 259, 355, 358, 389, 390, 393, 440, 447, 453, 456, 457, 465, 479, 480, 485, 488, 489, 490, 491, 502,511

DAY, P., 14 DAY, M. J., 99, 101, 103, 104, 346,

347, 517, 519 DEELEY, C. W., 257, 320, 323 DELFOSSE, J., 453, 485, 491 DEMPSTER, L. E., 95 DESREUX, V., 336, 346, 440 DEWEY, R. D., 95 DEWHURST, H. A., 100,110, 111, 184,

186, 197, 226, 235, 238, 256, 257, 300, 311, 467, 479, 480, 485, 491, 500, 512

DICKINSON, R. W., 74 DIENES, J. C , 13 DIENES, C. J., 14 DIENES, G. J., 256, 257, 393, 394 D O L E , — . , 2 1 5 , 217 DOLE, N„ 219, 226, 227, 231, 232,

233, 235, 236, 252, 256, 257, 470, 471, 477, 480, 481, 491, 494, 512

DOMEN, S., I l l DONALDSON, H. A., 197 DONDES, S., 101, 110 DORFMAN, L. ML, 355, 358, 484, 491 DRIGO, A., 393 DRAGANIC, 58 DUDLEY, R. A., 108, 110 DUFFEY, D., 73, 282, 283 DUGDALE, R. A., 13 DURFEE, W. H., 160, 169 DURR, W., 169 DURUP, J., 161, 168, 169, 296, 376,

377, 393, 432, 440, 465 DYNE, P. J., 74

EBERT, ML, 50, 110,465 EHRENBERG, N., 110

540 NAME I N D E X

EIDUS, Y., 393 EINSTEIN, — . , 308 EISLER, S. L., 283 ELLIS, C. D., 51 EMERY, E. W., 110 EPSTEIN, L. H., 490, 491 EPSTEIN, L. M., 222, 255 ESSEX, H., 485, 491

FAILLA, G., 50 FAIRBAIRN, H. W., 14 FARIS, F. E., 530 FARMER, E. C , 392 FARMER, F. T., 110, 348, 358, 514,

516, 519 FARO, U., 50 FELLOWS, C. H., 188, 189, 192, 197,

461, 466 FENG, P. Y., 287, 288, 296, 348, 358,

515, 519 FIQUET, F., 377, 393 FISCHER, F., 187, 197 FISH, R. F., 295, 296, 393, 530 FISH, R., 512 FIXMAN, M., 296 FLANARY, J. R., 257 FLETCHER, J. M., 67 FLETCHER, D. W., 283 FLINT, O., 74 FLORY, P. J., 139, 150, 152, 156, 157,

160, 169, 181, 536 FLORY, P. T., 244, 298, 299, 311, 325,

326, 334 FLOWERS, B. H., 110 FORSYTH, P. F., 188, 192, 197, 457,

458, 465, 466 FORSTER, T H . , 496, 512 FORTESCUE, R. L., 95 FOSSEY, E. B., 110 FOSTER, F. L., 95 FOWLER, J. F., 103, 104, 110, 346,

347, 348, 358, 491, 513, 514, 515, 516,517,518,519

Fox, M , 325, 326, 334,428,430,432, 440

Fox, R. E., 486, 491, 505, 506, 509, 511, 512

Fox, T. G., 298, 299, 311 FRANCK, J., 442, 453, 496, 512 FREDERICK, C. L., 530 FREEMAN, G. R., 74, 465 FRIEDLANDER, G., 50

FREUDENBERG, Κ., 169 FRICKE, H., 110 FRITH, E. M., 156, 158, 181 FRY, D. W., 95 FUCHS, L. A., 14 FUKADA, E., 416, 417, 435 FÜRST, M., 453 FUSCHILLO, N., 257 F U T AGAMI, T., 14

GALE, A. J., 95 GARRISON, W. M , 14, 313, 323, 393,

519, 521, 530 GARTON, C. G., 253, 254, 257, 520 GEE, G., 283 GEHMAN, S. D., 270, 272, 278, 283 GEIB, I. G., 259, 283, 326, 334 GEMANT, A., 518, 519 GERCKE, R. H. J., 530 GENNA, S-, 111 GEVANTMAN, L. H., 461, 465 GHORMLEY, J. A., 69, 70, 74, 97, 98,

110 GIBERSON, — . , 526 GIESEL, — . , 1 GILFILLAN, E. C , 365, 366 GLANTZ, J., 280, 283, 296, 494 GLASSER, O., 95 GLEGG, R. E., 366 GLEN, J. W., 13 GLINES, A., 384, 392, 411,464 GLOCKER, R., 110 GOECKERMANN, R. H., 394, 411, 466 GOLD, O., 155, 158, 278, 308, 311 GOLDBLITH, S. A., 101, 110, 111 GOLUB, M. A., 265, 280, 283 GOMBERG, H . J., 74 GOODE, J. H., 257 GOODMAN, C , 197, 296, 358 GORDAM, — . , 425 GORDON, M., 425 GORDY, W., 356, 358 GOULD, S. E., 74 GRAY, L. H., 49, 50, 110,465 GREEN, D. H., 392, 399, 411, 529, 530 GREENFIELD, M. A., 74 GREENWOOD, T. T., 425, 507, 512 GRIEVESON, B. M. 425 GROSMANGIN, J., 465 GROVES, D., 263, 280, 283, 494, 511 GRUBB, T. C , 311, 384,393 GRÜN, F., 158

NAME I N D E X 541

GUERON, J., 14 GURNEY, R. W., 14 GUTH, E., 150, 151, 154, 155, 158,

278,308,311

HASS, L. L., I l l HAISSINSKY, M., 15, 49, 50, 74 HALL, E., 367 HALL, G. G., 486, 491 HALE, D., 270, 271, 272, 283 HAMILL, W. H., 197, 454, 461, 465,

466 HAMMERLE, O. A., 110 HANCOCK, N. H., 239, 240, 241, 257 HANFORD, C. B., 323, 357, 358 HANNAN, R. S., 15, 110 H ARD WICK, T. J., 98, 100, 110 HARLEN, F. 226, 227, 228, 256 HARMER, D. E., 196, 197 HARMON, D. J., 283 HARRINGTON, R., 251, 257, 526, 530 HART, E. J., 14, 74, 111, 463, 465 HART, V. E., 491 HARTECK, P., 101, 110 HARVEY, R. A., I l l HAYBITTLE, J. L., 98, 99, 110 HAYWARD, J. C , 369, 392 HEINE, K.,411 HEISING, G. B., 369, 393 HEITLER, W., 50 HENGLEIN, A., 383, 384, 393, 394,

402, 411, 428, 433, 434, 440, 465 HENDERSON, W. J., I l l HENLEY, — . , 106, 107 HENLEY, E. J., 316, 323 HERMANS, — . , 215 HERMANS, P. H., 256, 367 HERVE, A., 511 HERZBERG, G., 17 HETTINGTON, W. R., 256 HlGASHIMURA, 394 HINE, G. J., 108, 110 HIPPLE, J. A., 445, 447, 453,485, 486,

491 HOBBS, L. M., 270, 283 HOCHANADEL, C. J., 14, 69, 70, 74,

97,98, 100, 110 HOLLAENDER, A., 15, 50 HONIG, R. E., 17, 95, 197, 212, 255 HOPWOOD, F. L., 370, 393 HORIKX, M. M., 171, 181 HORNBECK, R. F., 256, 296, 323, 347

HOTALING, G., 95 HOWARD, W. H., 217, 219, 252, 256 HOWARTH, J. L., 95 HOWELLS, E. R., 348, 358 HUBBARD, M. S., 95 HUBER, W., 392 HUGGINS, M. L., 158, 323 HUMMEL, R. W., 74, 110,465 HUNTER, —.,215 HUNTER, E., 256 HUNTER, M. J., 311 HUNGATE, R. E., 367 HURLEY, P. M., 14 HUSEMANN, E., 160, 169 HYDE, J. F., 311 HYDE, J. R., 311

ICHIMYA, T., 358 INOKUTT, M., 491 IRETON, H. J., 197 IRVING, R., 57, 73,74, 110 ISIHARA, A., 181, 478, 491

JACKSON, W. W., 270, 271, 272, 283 JAMES, H. M., 150, 151, 158 JAVITZ, A. E., 529, 530 JECH, C , 290, 296 JEFFERSON, S., 74 JELLINEK, H. H. G., 160, 169 JENKINS, A. D., 377, 392 JENNINGS, W. A., 91, 94, 95 JOHNS, H. E., 110 JOHNSON, D. L., 323, 358 JOHNSON, E., 527, 530 JOHNSON, G. R. A., 110 JOHNSON, D. A., 393 JOLIOT, F., 370, 393 JONES, D., I l l JONES, J. C , 95, 108, 109 JONES, S. S., 283

KAILAND, 1 KALLMAN, H., 453, 519 KANTOR, — . , 307 KARPOV, V. L., 256, 257, 281, 282,

283, 312, 323, 334, 347, 491, KEEL, D. K., 591 KEELING, C. D., 233, 256, 257, 470,

471, 491, 494, 512 KELLIHER, M. G., 95

542 NAME I N D E X

KENNEDY, J. W., 50, 287, 285, 296, 348, 358, 515

KENNEY, J. G., 529, 531 KERN, W., 394 KERTESZ, Z. I., 160, 169, 366 KHENOKH, M. A., 363, 367, 440 KHOMINKOVSKI, P. M., 394 KINCHIN, G. H., 13 KING, — . , 343 KIPPING, — . , 297 KIRKLAND, W. K., 257, 296, 346,

526, 531 KLAGES, F., 160, 169 KLEIN, O., 50 KLEIN, P. H., 310, 311, 529, 530 KLEINEST, T., 169 KLEINMAN, D. A., 14 KLICK, C. C , 111 KLINE, D. E., 257 KNOWLTON, J. A., 84, 95 KOCH, W., 393 KOEHLER, S., 5, 13 KOGANOFF, S., 519 KONOBEEVSKY, S. T., 13 KRAMER, B., 519 KRASNIKOV, A. L., 283 KREIDELL, N. J., 14, 104, 105, 110 KRENZ, C , 296 KRENZ, F. H., 186, 197, 457, 461,

465, 496, 512 KRIMM, S., 215, 256 KRONGAUZ, V. A., 382, 393, 460, 465 KUHL, O. A., 74 K U H N , W., 150, 154, 158, 160 KUHN, H., 158, 169 KUPER, J. B. R , 111 KUPRIYANOV, S. E., 197 KUTAITSEV, V. I., 13 KUZMINSKII, A. S., 281, 282, 283, 530

LANDLER, Y., 58, 73, 197, 290, 374, 393, 455, 465, 466

LANGER, A., 17 LARK-HOROVITZ, Κ., 14 LARSON, N. R., 256 LASETTRE, E. N., 160, 169 LAUGHLIN, J. S., 97, 111 LAWRENCE, W. L., 514, 517, 519 LAWSON, J. D., 110 LAWTON, E. J., 160, 169, 204, 207,

223, 227, 229, 231, 235, 241, 244, 245, 247, 249, 252, 255, 256, 257,

283, 311, 312, 320, 323, 326, 334, 335, 347, 363, 367, 383, 384, 393, 394, 395, 411, 413, 425, 440, 455, 466, 467, 490, 491

LAZER, J., 511 LAZO, R. M., 97, 98, 111 LEA, D. E., 15, 29, 30, 51 LECLERC, P., 110 LEESER, D. O., 13 LE FORT, M., 58, 73 LEIGHTON, P. A., 463, 465 LEININGER, R., 527, 530 LEMMON, — . , 226, 275 LEMMON, R. M., 182, 186, 196 LENNARD-JONES, L, 486, 491 LEVY, P. W., 14 LEWIS, E. E., 348, 358 LEWIS, F. M., 455, 465 LEWIS, J. G., 392 LIND, S. C , 1, 2, 7, 13, 194, 196, 197,

255, 369, 393, 451, 453 LINDEN, L., 365, 366 LINDER, E. G., 95, 519, 520 LINSEY, H., 393, 395, 411, 455, 465 LITTLE, K., 320, 322, 323 LIUZZI, A., I l l LIVERSAGE, W. E., 384, 358, 519 LIVINGSTON, R., 496, 512 LLOYD, — . , 106, 107 LLOYD, D., 457, 458, 465, 467, 488,

489, 490, 491, 500, 501, 504, 505, 512

LOBO, A. H., 73 LOEWE, S., 393 LONG, F. R., 13 LOVELL, C. M., 256 LUY, H., 231, 257 LYONS, B. J., 323 LYUBIMOVA, A. K., 451, 454

MAGAT, M., 73, 74, 196, 197, 288, 290, 296, 324, 341, 347, 376, 377, 380, 392, 393, 411, 428, 432, 440, 445, 446, 453, 457, 459, 465, 466, 476,491,505, 512

MAGEE, J. L., 14, 17, 30, 50, 447, 448, 453, 454, 456, 463, 465, 466

MAHN, G. R., 84, 95 MAJURY, J. G., 320, 323, 375, 383,

393 MAMHAL, C , 529, 530

NAME I N D E X 543

MANDELL, —.,216 MANDEL, E. R., 252, 256 MANION, J. P., 197, 453, 454 MANN, W. L., 323, 357, 358 MANN, W. B., 95 MANNAL, C , 310, 311 MANOWITZ, B., 64, 66, 73, 74, 256,

257, 384, 392, 411, 418, 419, 420, 425, 455, 464

MARINELLI, L. D., I l l DE MARIO, L., 393 MARK, H., 158, 160, 169 MARKHEIM, L. S., 74 MARTIN, J. J., 196, 197, 392 MASSEY, H. S. W., 445, 454 MATHESON, M. S., 463, 365 MATHEWS, J. L., 256 MATSUMAE, K., 349, 358 MATSUO, H., 256 MATTHES, A., 160, 169 MAYBURG, S., 515, 517, 519 MAYER, G., 14, 465, 466 MAYHEW, G. H., 519 MAYO, F. R., 465, 466 MAYNEORD, W. V., 51 MCCARTHY, P. B., 527, 530 MCCLINTON, A. T., 529, 530 MCDONEL, W. R., I l l , 194, 197 MCELHINNEY, J., 97, 111 MCLENNAN, J. C , 197 MCMILLAN, I. D., 425 MEAD, L. W., 516, 519 MEDVEDEV, S. S., 369, 374, 382, 393,

394,459,460,461,462,465 MEEK, M., 77, 78, 95 MEISELS, G. G., 451,454 MELVILLE, H. W., 346 MEREDITH, W. J., 512 MESROBIAN, R. B., 256, 257, 383, 392,

393, 394,411 MESSNER, D., 110 METZ, D. J., 392,411 MEYER, J. A., 257 MEYER, R. H., 518, 519 MEYERHOFF, F., 336, 347 MIGIRDICYAN, E., 196, 374, 393, 411,

465 MILLER, — . , 203, 204, 212 MILLER, A. A., 467, 472, 474, 491 MILLER, A. J., 521, 530 MILLER, C W., 76, 86, 95, 111, 226,

227,231,232,233,255,257 MILLER, H„ 95

MILLER, N „ 98, 99, 110, 111, 197 MILLETT, M., 160, 169 MILLETT, M. A., 367 MILNER, D. C , 232, 233, 256, 481,

491 MINCHER, E. L., 530 MOELWYN-HUGHES, E. A., 19 MOMIGNY, J., 376, 394 MONTROLL, E. W., 160, 169 MOREHEAD, E. F., 74 MOROWETZ, H., 394 MORSE, S., 110 MOSSMER, V., 169 Μοττ, N. F., 14 MOVDIE, M. M., 496, 512 MOYER, J. G., 257, 478, 491 MULLER, F. A., 515, 520 M U N D , W., 2, 13, 312, 323, 369, 374,

393, 394 MUSSA, C , 256, 257

NEFF, H. F., 283 NEHEMIAS, J. V., 74 NELMS, A. T., 51 NEWMEYANOV, A. N., 14 NEWTON, A. S., 194, 197 NETKA, H. F., 108, 109, 111 NEWTON, U. S. P., 259, 283 NIKITINA, T. S., 281, 282, 283, 374

392, 394, 462, 465 NISHICKA, A., 349, 358 NISHINA, 7, 50 NORDLIN, H. G., 515, 519, 520 NORMAN, I., 464, 465 NORRISH, R. J. W., 392 NOYÉS, W. A., 463, 465 NYGARD, J. C , 95

OAKES, —.,215 OAKES, W. G., 256, 257 OBRYOKI, R. F., 74 OFFENBACH, J., 394 OKAMOTA, H., 181,478,491 OKAMURA, S., 160, 166, 169, 378, 394 OSTER, G., 462, 465, 486, 491 OSTHOFF, R. C., 309, 311 OVADIA, J., I l l OVENALL, D. W., 346 OVEREND, W. G., 440 OWAKI, M., 358

544 NAME I N D E X

PARKINSON, — . , 346 PARKINSON, W. W., 227, 256, 257,

290,291,296,323,526,531 PATERSON, — . , 349 PATRICK, J., 496, 511 PATRICK, W. L., 197 PATRICK, W. N., 452, 453, 454 PATT, H. M., 512 PEACOCKE, A. R., 440 PEARLSTEIN, E. A., 14 PEARSON, R. W., 233, 257, 477, 478,

479, 491 PEASE, R. S., 13 PEISER, H. S., 256 PELLON, J. J., 377, 394 PENDLE, T. D., 411 PENNOCK, J. C , 516, 519 PERRIN, M. W., 197 PERRY, J. H., 194, 197, 393 PETERS, K., 187, 197 PETERSON, J. H., 358 PFAFF, E. R., 531 PHILLIPS, J. T., 370, 393 PICCOLI, W. A., 311 PIGG, J. C , 513, 519, 520, 527, 528,

529, 531 PILLING, F. D., 110 PINNER, 313 PINNER, S. H., 108, 111, 255, 257,

316, 320, 323, 349, 390, 398, 404, 405, 406, 409, 411, 456, 464, 465

PLATZMAN, R., 442, 444, 448, 453, 454

PLATZMAN, R. L., 14, 30 PLETCHER, D. W., 393, 411, 465 POMEROY, G., 61 PORTER, G., 464, 465 POWELL, R. S., 255, 491 PARVDYUK, N. F., 13 PRESTON, B. N., 440 PREVOST-BERNAS, A., 196, 197, 457,

465, 466 PREVOT-BERNAS, A., 73, 108, 377,

392, 393,394,411 PREVOT, —.,410 PREVOT, A., I l l , 506, 512 PRICE, F. P., 363, 367 PRICE, W. C , 17 PRIMAK, W., 14 PROCTOR, Β. E., 110, 111 PROSKURNIN, 195 PUCHEAULT, J., 58, 73, 109, 111 PUROHIT, S. N., 73

PUZITSKII, K. V., 393

QUIMBEY, E. H., 95

RABIN, H., I l l RABINOWITCH, E., 453 RAINE, H. C , 215, 286 RAMSAY, — . , 1 RAMSEY, M., 520 RANFTH, J. W., 84, 95 RAPPAPORT, P., 95, 520 RASETTI, F., 51 RATHMAN, G. B., 296, 338, 347, 491 REDING, F. P., 286 REHNER, J., 157 REID, C , 496, 512 REINISCH, L., 465 RENFREW, M. M., 348, 358 RENNER, G., 394 RESTAINO, A. J., 383, 384, 385, 392,

394, 397,411 REXER, E., 370, 394 RHEINISCH, L., 411 RHENISH, L., 196 RICHARDS, —.,215 RICHARDS, R. B., 256, 257 RICHARDSON, D. M., 73, 109, 111,

530 RICHMAN, R., 106, 107, 110 RIGBY, J. W., 348, 358 ROBINSON, C. C , 520, 529, 531 ROSINGER, S., 110 ROSE, D. G., 256, 257, 491 Ross, —.,217 Ross, M., 73, 104, 109, 111, 218, 252,

256, 334, 335, 336, 337, 338, 339, 340, 341, 343, 346, 347, 355, 358, 440,490,498, 511

ROSSINI, F. D., 19 ROTBLAT, — . , 99, 100 ROTBLAT, J., I l l ROTH, P. I., 296, 338, 347, 491 ROTHSCHILD, W. G., I l l , 392 ROWBOTTOM, J., 14 ROZMANN, I. N., 14, 520 RUGG, F. M., 231,256 RUTHERFORD, E., 51 RUTHERFORD, H. A., 322, 324 RYAN, J. W., 257, 270, 280, 283, 306,

310, 311, 323, 350, 352, 354, 358, 495, 512, 529, 531

RYDER, H., 256

NAME I N D E X 545

SACHS, F., 196 SAEMAN, J. F., 160, 169, 360, 362, 367 ST. PIERRE, L. E., 238, 257, 300, 305,

306,311,512 SAITO, O., 491 SAKURADA, L, 160, 166, 169 SALDICK, — . , 98 SALDICK, J., I l l SALDWICK, J., 4 SALLAND, E., 110 SAMUEL, A., 30, 50 SAMUEL, A. H., 447, 453, 454, 456,

465, 466 SAUER, J. A., 257, 323 SAUNDERS, D. F., 74 SAUNDERS, J. H., I l l SAUNDERS, R. D., 110 SAZMOV, I. S., 14 SAZNOV, L. A., 14 SCARBOROUGH, J. M., 416, 425 SCHINDLER, A., 394 SCHISSLER, D. O., 451, 454 SCHLICHTER, — . , 252 SCHMITZ, J. V., 380, 394, 395, 411,

413, 414, 425, 455, 466 SCHNABEL, W., 402, 411, 440 SCHNEIDER, C , 440 SCHNEIDER, E. C , 110 SCHNEIDER, E. E., 257, 296, 336, 341,

347, 348, 356, 358 SCHOEPFLE, C. S., 188, 189, 192, 197,

461, 466 SCHOLES, G., 440 SCHUBERT, C. C , 461, 466 SCHÜLER, R. H., 96, 98, 101, 110,

111, 187, 188, 192, 197, 452, 454, 461, 465, 466, 491

SCHULMAN, J. H., 104, 111

SCHULTZ, —., 171

SCHULTZ, A. R., 287, 288, 296, 317,

318,324,347,421,425 SCHULTZ, G. V., 160, 169, 336, 338,

339, 347, 394 SCHULZ, R., 383, 384, 393 SCHUMACHER, Κ., 223, 226, 227, 231,

232, 233, 238, 251, 257, 485, 490, 491

SCHWARZ, H. A., 74 SEARS, W. C , 290, 291, 296, 323 SEBBAN, J., 196, 393,411,465 SEBBAN-DANON, J., 374, 377, 393,

394,406,411 SEITZ, F., 5, 13, 14

SEITZER, W. H., 380, 394, 395, 410, 411,455,457,459,460,466

SELLA, — . , 252 SELLA, C , 257 SHEKHTMAN, Y. L., 101, 111 SHEPHERD, C. W., 197, 212 SHERRILL, F. A., 14 SHIELDS, H., 356, 358 SHINOHARA, K., 252 SHOCKLEY, W., 13 SHORE, P. A., 101, 109 SHULTZ, — . , 181 SHULTZ, R., 347, 394 SHULZ, — . , 336, 338, 339 SHULZ, A. R., 474, 475, 491 SICILIO, L, 527, 530 SEIGEL, S., 13 SlLVERMAN, L. B. , 74 SIMHA, R., 160, 169, 478, 491 SIMPSON, W., 256 SISMAN, O., 56, 60, 73, 109, 110, 251,

257, 266, 267, 268, 280, 281, 283, 294, 296, 310, 313, 319, 321, 323, 324, 326, 334, 335, 343, 344, 346, 347, 349, 356, 357, 358, 364, 365, 366, 367, 492, 511, 519, 520, 521, 523, 525, 526, 527, 530, 531

SLATER, J. C , 13, 14, 95 SLICHTER, —.,216 SLICHTER, W. P., 256 SLOVOKHTOVA, N. A., 257, 474, 491 SMITH, F. F., 230, 257 SMOLOCHOWSKI, R., 14 SNOW, A. I., 257, 478, 491 SOBOLEV, L, 255, 257 SOWDEN, J. C , 160, 169 SPALDING, F. F., 392 SPARROW, A. H., 74 SPEAR, F. G., 15 SPIERS, F. W., 51 SPINKS, W. T. J., 74, 110,465 STANNETT, V., 257, 384, 394 STARK, F. O., 311 STARK, H. H., 520 STARK, K. H., 253, 254, 257 STEARLE, E. W. R., 19 STEEL, G., 521, 530 STEIGMAN, J., 291, 296 STEIN, G., 101, 110, 346, 347, 440,

517, 519 STEIN, S., 74 STEINBRUNN, G., 169 STEINWEHR, H. E., 394

546 NAME I N D E X

STEVENSON, D. P., 446, 451, 454 STOCKMAN, C. H., 95, 283 STOCKMAYER, W. H., 158, 171, 181,

288, 296, 536 STRACIE, E. W. R., 491 SUN, K., 14, 25, 53 SUN, K. H., 394, 469, 491 SUTTON, C. R., 13, 99, 100 SUTTON, H. C , 111 SWALLOW, A. J., 14, 182, 196, 316,

323, 393, 461 SWALLOW, S. J., 108, 110, 111 SWORSKI, T. J., 53, 57 SZWARC, M., 257, 384, 394

TAIT, — . , 1 TAJIMA, M., 358 TAL'ROSE, V. L., 451,454 TAYLOR, C. R., 319, 323 TAYLOR, D., 74 TAYLOR, L., I l l TAYLOR, L. S., 95 TESZLER, O., 322, 324 THENARD, P., 1, 394 THOMAS, J. K., 430, 440 THOMAS, W. M., 377, 394 TIKHOMIROV, M. V., 197 TKACHENKO, G. V., 394 TKATCHENKO, — . , 377 TOBOLSK Y. A., 169, 215 TOBOLSKY, A. V., 256, 380, 381, 393,

394, 410, 411, 457, 459, 460, 466 TODD, A., 321, 322, 324 TOLBERT, — . , 226 TOLBERT, B. M., 182, 186, 196 TOMS, D., 238, 239, 256, 290, 296,

499, 500, 511 TRAPP, E. C , 74 TRELOAR, L. R. G., 150, 154, 158 TRESISE, H. C , 73 TRILLET, J. J., 252, 257 TRUMP, J. G., 95, 107, 111 TSIMMER, K. G., 14, 520 TUCKETT, R. F., 156, 158, 181 TUNITSKY, N. N., 197 TURNER, D. T., 266, 283, 399, 411

UEBERSFELD, J., 323, 341, 342, 347, 366

ULLMAN, J. W., 73 USHER, 1

VAN CLEAVE, A. B., 74, 465 VAN DE GRAAFF, R. J., 95, 107, 463 VAN MEERSCHE, M., 376, 394 VARLEY, J. H. O., 9, 13, 444, 454 VEDENEEVA, N. E., 14 VERMEIL, C , 74, 100, 111 VIALLARD, R., 445,446, 453, 476,491 VlCTOREEN, J. A , 51 VON ARNIM, E., 210, 241, 244, 245,

255, 264, 279, 283, 465, 489, 490, 502, 504,511

WADDINGTON, F. B., 223, 256, 323 324, 490, 491

WARL, P., 73, 393 WALBY, P., 392 WALDRON, J. D., 256 WALKINSHAW, W., 95 WALL. F. T.. 150. 158 WALL, L. A., 287, 288, 290, 291, 292,

293, 296, 324, 336, 338, 339, 341, 342, 343, 346, 347, 355, 358, 428, 432, 440, 466, 473, 478, 489, 491, 505, 512

WALLING, C , 455, 465, 466 WALTON, E. T. S., 95 WALTON, G. N., 73, 74 WARNER, A. J., 515, 520 WARTMAN, L. H., 257 WARWICK, E. L., 299, 303, 305, 306,

308,309, 311 WATANABE, T., 358, 491 WATSON, C. D., 257 WATSON, J. S., 257 WATSON, W. F., 160, 169, 171, 181 WEATHERWAX, J. L., 95 WEBER, E. N., 188, 192, 197, 459,

461,465,466 WEECH, M., 73 WEEKS, R. A., 527, 531 WEIDINGER, A., 251, 256 WEISS, J., 14, 98, 100, 110, 111, 196,

440, 448, 454, 479, 491 WELLS, H., 527, 531 WESTENDORP, W. F., 95 WETHERINGTON, J. A., 256 WHIFFEN, D. H., 223, 255, 292, 336,

346, 348, 356, 358, 366, 448 WHITE, G., 160, 169 WHITEHEAD, W. L., 197 WIENER, H., 187, 197 WILD, — . , 192

NAME INDEX 547

WILD, W., 64, 73, 192, 457, 466 WILKINSON, J., I l l WILLARD, J. E., 14, 196 WILLIAMS, E. J., 30, 50 WILLIAMS, N. T., 491 WILLIAMS, R. R., 197, 217, 461, 465,

466,481,484,485 WILLIAMS, T. F., 232, 233, 252, 256,

491 WILLIAMSON, T., 527, 531 WILSON, C. W., 448, 454 WILSON, J. C , 531 WILSON, S., 440 WILSON, T., 110 WINOGRADOFF, N. N., 257, 290, 296,

363, 367, 448, 454 WINSLOW, E. H., 467, 491 WIPPLER, C , 316, 324, 433, 440 WITTELS, M. C , 14 WOLFRAM, M. L., 160, 169 WOLLASTON, 1 WOODLEY, 350, 358

WOODWARD, A. E., 217, 252, 257, 323

WOODWARD, A. S., 13 WORRALL, R., 58, 73, 109, 352, 353,

354, 358, 389, 390, 391, 393, 456, 465, 466

WOURTZEL, E. , 1 WRIGHT, J., 58, 64, 73, 109, 111, 347 WRIGHT, K. A., I l l WRIGHT, W. W„ 377, 392 WYCHERLEY, V., 414, 415, 418, 421,

425, 507, 512

YAMASHITA, T., 394

ZEITLIN, H. R., 73 ZEMANY, P. D., 229, 231, 256 ZENDEL, B., I l l ZIMM, B. H., 296 ZSULA, J., I l l ZVEREV, B. L, 256

NN

SUBJECT INDEX (Page numbers printed in bold type

concerned are discussed at leng

ACETYLATION 132 acetylene 183, 190 acrylamide 384, 385, 387, 388 acrylates 385, 387, 388 acrylonitrile 376, 387, 398, 400,

401,409,455 addition polymerization 114 additive, effect of concentration 420,

457, 486, 488, 489, effect on gelation of polyesters 420 in rubber 265 in solid polymerization 392 protection by 426, 492, 496

additives as radical traps 182, 457, 468, 486

aggregation in solution 429 alcohols, gaseous yield 183,193,194,

195 alkyl benzene 57 alpha radiation 1, 2, 3, 25, 26, 42,

53, 99, 259, 290, 369 aliphatic molecules, effects of irra-

diation 183, 184 amorphous polymers 122 anthracene 455, 457, 459, 488, 496,

505, 507, 509 antioxidant 268 aqueous solutions of polymers 428 aromatic 192 (see also low mole-

cular weight molecules, benzene) atomic radiation 5 average molecular weight 114, 128,

139, 204 (number average M n , weight average Ml0i viscosity average Mv) between crosslinks (Mc) 137, 147

BACKSCATTER 28 benzene 101, 183, 192, 453

radiation resistance 452, 453, 459, 495

benzoquinone 420,455, 498, 505, 508 BEPOpile 56,57,59

indicate pages on which the subjects h rather than merely mentioned)

beta, radiation (see electron radia-tion) rays 1, 3, 99

betatron 52, 53, 09 biological molecules 427, 492 block copolymers 123, 395 bond energy 19, 306 branching 116, 117, 188, 199, 200,

226, 300, 461 Bremsstrahlung 28, 97 bulk viscosity, poly/söbutylene 299,

324 polystyrene 299 silicones 298, 299, 302

butadiene 387 butyl rubber 60, 259, 325, 326

CAESIUM 21, 65, 67 cage effect 185,452 calorimetric method of dosimetry 97 carbon black 268, 271, 273, 281, 282,

309, 325 polymerization 392

carbonyl in polyethylene 237 carboxylic acids 183, 193, 195 casein 60 catalyst 369, 378, 386, 391, 412, 456 cellophane, dyed, as dosimeter 106 cellulose 359

derivatives 60, 359, 522, 523 eerie sulphate 100 cerium 67 chain reaction 12, 177, 368

in polyester cure 412 chain transfer 116, 120, 450 chemical changes

cellulose 362 polyethylene 223 polystyrene 290 poly vinyl chloride 315 PTFE 350 rubber 265 silicones 306

549

550 SUBJECT INDEX

chlorine 59 cobalt 21 ,68 ,96 ,99

sources 69 colour changes 1, 9, 104, 316, 346 Compton scattering 35, 37, 41, 59 concentration dependence in solu-

tions 426,428,430 condensation polymerization 113,

114, 115, 116, 170 conductivity of irradiated polymers

513, 527, 528 configuration 119 conversion of energy 76 copolymer 112, 123, 284, 395 costs of irradiation

fission products 64 nuclear reactors 61

covalent lattice 8, 9 creep 248 crosslinked unit 134 crosslinking 6, 13, 134, 442, 467,

486, 493, 494, 532 crosslinking and degradation

in solution 437 polyacrylates 317 polystyrene 285 poly vinyl chloride 312 simultaneous 171,211,338

crosslinking, coefficient (S) 137, 140, 148, 172, 201 density (q) 135, 201, 245, 265 during polymerization 378 effect on molecular weight 139 in hydrocarbons 189, 190 index (Y) 137,201 in polymers (see individual poly-

mer) in solution 316, 431, 435 non-random 177 random 122, 127, 136, 142, 171,

184, 201, 244, 260, 264 sol and gel 145

crystal size 112, 223 crystallinity 112, 120, 122, 198, 200,

201, 215, 383, 490 effect on grafting 406

curie 21, 64 curing, of rubber 258, 277

of unsaturated polyesters 412 cyclization 185, 481 cyclotron 52, 53, 90

DARK current 513 Dacron 321 (see Mylar and poly-

ethylene terephthalate) deamination 182 decarboxylation 182 decomposition 1 (see also degrada-

tion) degradation 6, 13, 138, 159, 171,

288, 442, 467, 483, 493 cellulose 360 in solution 428, 430, 432 poly/sobutylene 107, 326, 328 poly methyl methacrylate 107, 335,

337 PTFE 348

degree of polymerization 129 delta electron 30, 43 density of crosslinks 135, 222 deposited polymer 396 depolymerization 159 deuteron 48 Dextran 363 dimerization 12

of low molecular weight com-pounds 189, 190 (see also cross-linking)

dimethyl siloxane polymer 297 (see silicones)

diphenyl picryl hydrolzyl (DPPH) 101, 182, 457, 459, 461

direct effect in solutions 427 displacement of atoms 5 disproportionation 118, 120 distribution, molecular weight 115 dose 56, 58

rate 54, 60, 68, 78, 79, 82, 93 dosimeter 96, 102 dosimetry 96, 109

in reactors 109 Dowtherm 57

EFFECT of temperature polyethylene 233 poly/sobutylene 333 polymethyl methacrylate 343 polystyrene 292

elastic properties 148, 239, 245, 263, 270, 295, 304

elastomer 258 electrical, accelerators 3, 75

conductivity 513, 517, 527, 348, 441

SUBJECT INDEX 551

properties of miscellaneous plas-tics 528 of irradiated polyethylene 253

sources 3, 75 electron affinity 449

radiation 2, 3, 22, 24, 26, 27, 43 vacancy 450 volt(eV) 7, 16, 18

electrons 26, 53, 98, 99 electrostatic generator 78 elongation at break 249 emulsion polymerization 385 end effect 152, 160,243 endlinking 135, 174, 184, 472

in solution 435 energy, absorption coefficient 38

deposition 18, 40, 45, 56, 57 equivalents 16, 18, 23 for crosslinking (see individual

polymers, G values) for main chain fracture

poly/sobutylene 327 poly methyl methacrylate 337 transfer 11, 296,449,496

entanglements 153 ethylene 369, 382, 386, 387 (see

olefins) excitation 5, 6, 10, 11, 20, 28, 29,

182, 441, 442, 469 excited molecules 442, 444 exciton migration 450 exotherm 414 exponential distribution 114,131 external protection 493, 496

F CENTRES 10 ferrous sulphate 58, 98, 460 filler action 155 fillers 268, 272, 278, 307, 526 filter for x-rays 91 fission products 52, 53, 64 fluorescence 1, 290, 296 fluorine evolution from PTFE 352 fluorinated monomers 387 Fluorothene (see polymonochloro-

trifluorethylene) 356 formaldehyde 522, 523, 525, 528 Franck-Condon principle 182, 442 free electrons 182, 447 fuel rods 52, 53, 64, 270 fusibility 208

G VALUE 20, 136, 187, 188, 191, 209, 235, 244, 264, 287, 293, 294, 300, 305, 318, 327, 334, 337, 338, 339, 343, 357, 362, 380, 381, 382, 387, 388, 391, 392, 410, 433, 434, 438, 459, 460, 479, 485, 494, 504

gamma rays 1, 2, 3, 4, 7, 20, 21, 22, 24, 26, 34, 53, 55, 98, 99

gas evolution miscellaneous plastics 523 simple organic compounds 101,

186, 187, 189, 194,484,485 polyethylene 201, 224, 478, 523 poly/sobutylene 332 polymethyl methacrylate 338 polymonochlorotrifluorethylene

360 poly vinyl chloride 315 silicones 307

gaseous fission products 63 gel, (g) 145

effect in polymerization 377 formation in polyesters 418, 419,

421, 422 point 140, 142, 172, 179, 203, 206,

288, 300, 302, 535 glass, as dosimeter 104

coloration 1, 104, 105 graft copolymers 124, 398 grafted membranes 403 grafting rate

effect of intensity 404, 405, 409 effect of monomer 410

HALIDES 194 halogen evolution from polymono-

chlorotrifluorethylene 357 halogenation 182, 195 high elongation 154 high energy radiation 5 highly excited radicals 471 high voltage transformer 52, 53 homopolymer 112 Hycar 280 hydrocarbons radiation effects in 2 hydrogen evolution 101 (from poly-

ethylene) hydroxyquinoline 498, 505, 507, 509 Hypalon 60, 281

IMPULSE generator 52, 53, 77

552 SUBJECT INDEX

indirect effect in solutions 427 induced current 513 infra-red spectrum,

poly/sobutylene 332 polystyrene 291

inhibitor 117, 420 initiator 117 intensity (reactors) 56 intensity dépendance in polymeri-

zation 370, 384, 455 internal, conversion 444

linking 439 radiation protection 493

intrinsic viscosity (η) 107, 130, 167, 261, 287, 298, 325, 336, 338, 360

iodine as a radical trap 182,188,452, 457, 458, 461

ion 5, 446 density 26, 96 distribution 26, 47, 445 pair 7, 18

ionic lattice 8, 9 polymerization 368, 389, 455 yield 453,464

ionization 5, 6, 7, 10, 11, 20, 28, 29, 33, 182,441,444,451,469,479 chamber 97 density 32 distribution 47 effect of wall 34 potential 16, 17

ionized molecules 444, 445, 446, 450, 451

ionizing radiation 5 wobutylene 389, 456, 493, 494 isomerism 182, 259, 265 isoprene 259, 325 isotatic 119, 124, 284 isotopes 3, 52, 65 isotope effect, polystyrene 294

JUNCTION point 134

KINETICS of network formation 417 krypton 64

low molecular weight compounds, effect of irradiation 182

luminescence 1

MACHLETT tube 93 main chain fracture 6, 159, 477 (see

degradation) cellulose 360 poly/sobutylene 325 polymethyl methacrylate 337 PTFE 348

mass absorption coefficient 40 mechanical properties,

of irradiated polymers 522 cellulose and derivatives 364 nylon 319, 320 polyesters 414 polyethylene 239 polymethyl methacrylate 335, 343 polymonochloroticfluorethylene

356 polystyrene 294 poly vinyl chloride 313 PTFE 349 rubber, 27, 266, silicones 304, 309

mechanism of degradation in PTFE 355

melting point of paraffins 208, 209 memory effect 253 metals 8 methyl methacrylate 342, 374, 375,

379,381,387,388,455 methyl methacrylate,

grafts 397,400,405,410 modified polyesters 418, 421

méthylène blue 101 molecular structures 8, 10, 117, 123 molecular weight,

distribution 127, 128, 533 cellulose 362 poly/sobutylene 325 polymethyl methacrylate 335 rubber 262 silicones 298, 299

monomers 2 multiple fracture 475 Mylar 321, 523, 526

LINEAR accelerator 85, 90 linear energy transfer (LET) 47, 49, NEOPRENE 60, 281, 521

98 network copolymer 396

SUBJECT INDEX 553

network formation (see crosslinking, endlinking)

network formation by endlinking 177, 476

network polymer 112, 150, 412 neutrons 2, 7, 25, 44, 53, 54, 55, 56,

59 nitration 182 nitrous oxide 101 non-random distribution 115, 328 nuclear, displacement 5, 8,#9

reactor 2, 52, 53, 53, 61 ' sources 3

number average molecular weight (Mn) 129, 141, 146, 161, 327

nvt 55, 522 nylon 60, 113,319,528

OAK RIDGE 54, 56, 57, 59 olefin and acetylene, radiation effects

183, 190, 191, 205, 370 organic moderators 529 orientation 121

in poly vinyl chloride 314 in rubber 264

oxidation 182, 195, 201, 235 oxygen,

effect in polyethylene 235 in polymerization 454 in solution 428,429,430

PAIR production 35, 38, 41 paraffins 56, 174, 183, 184, 203, 208,

476, 480 gaseous yield 186, 187, 189

paramagnetic resonance 294, 449, 486 polymethyl methacrylate 336,341, 342 PTFE 348, 357

penetration 25, 32, 53, 83 permeability 255 peroxide 397, 398, 455 phenol 498 photoelectric effect 35, 36, 41 photographic methods of dosimetry

108 physical properties of cured poly-

esters 414, 415 pile radiation,

effects in PTFE 354

pile radiation (continued) in polyisobutylène 330

equivalent 58, 60, 331, 337, 354 plastic, dyed 101 polyacrylamide 468

grafts 398, 406 solution 429

polyacrylates 280, 317, 468, 473, 475 polyacrylic acid in solution 429 polyacrylonitrile 280, 281, 319, 377,

468 polyamide 113, 319, 468 polybutadiene 60, 280, 468 polycaprolactum 320, 468 polychloroprene 280, 468 polydichlorostyrene grafts 400 polydimethyl siloxanes 297, 468, 504

siloxane grafts 398 polyester 113, 412, 522 polyesters, protection 507 polyethylene 48, 56, 113, 121, 198,

468, 473, 500, 516, 527 conductivity changes 516,518 grafts 398, 400, 401, 403, 406, 407,

409 terephthalate 321, 468, 516, 517,

528 poly/sobutylene 58, 280, 300,

325, 427, 468, 473, 475 polymerization 113,368,455

acrylamide 383, 384, 385, 387 acrylates and methacrylates 383,

385, 387 addition 114, 368 and crosslinking 472 as measure of dose 108 butadiene 387 condensation 114,170,281 early work 371

effect of radiation intensity 372 ethylene 369, 382, 386, 388 fluorinated compounds 387 in emulsion 385 in solids 382 in solution 380, 456 kinetics 371, 412 zsöbutylene 389 methyl methyacrylate 342, 370,

374, 375, 378, 379, 380, 381, 386, 388

non-steady state 375 rate 370

554 SUBJECT INDEX

styrene 370, 374, 378, 379, 380, 381, 386, 388

styrene, methyl methacrylate 389, 395

vinyl acetate 374, 387 pyrollidone 382 stéarate 385,

yield 386, 391 polymethacrylic acid in solution 428,

429, 434, 504 poly methyl methacrylate 60, 101,

103, 113, 335, 4+8, 468, 473, 498, 516, 517, 521, 522 grafts 398,399,400

polymethylene 121, 468 polymethyl styrene 468, 473, 523,

528 polymonochlorotrifluorethylene

358, 468 polypropylene 119, 323, 468, 473 polystyrene 103, 284, 432, 468, 473,

516, 517, 521, 522, 523, 527, 528 sulphonate 429

poly-ter/-butyl methacrylate 336 polytetrafluorethylene (PTFE) 348,

400, 402, 468, 474, 517, 518, 521, 523, 527

polyvinyl alcohol in solution 429, 431, 506 chloride 60, 113, 312, 468, 521,

523, 528 chloride, acetate 60, 523 chloride grafts 398 chloride in solution 433 pyrollidone 429, 430, 436, 506

polyvinylidene chloride 523 post-effect 432 predissociation 443 pre-ionization 444 promethium 67 protection against crosslinking 295,

496, 499 against degradation 333, 494, 498 by benzene 11, 296

protection, coefficient 498 in polyesters 507 in solution 495, 505 radiation 11,295,492

proton 25, 26, 42, 48, 53, 59 pseudo-random distribution 131 PTFE 58, 59, 60, 348, 468, 474 pyrolysis 473

RAD 97 rad, définition 22 radiation, as initiator of polymeri-

zation 368 damage 61, 521 protection, polymethyl methacry-

late 346 sources 52 stability of chemical groups 524 stability of polystyrene 295

radical distribution 462, 487 lifetime 463 polymerization 368, 371, 455 products of irradiation 188, 451,

479 yield 456,459,460,461 yield in aromatics 191, 192

radicals 17, 182, 336, 341, 348, 469, 497

radioactive isotopes 52, 53 radioactivity induced by gammas 35 radon 1 random copolymers 123. 124

distribution 114, 131, 163, 167, 285, 532

range, effect of wall 34 of radiations 25, 29, 43, 53

reactor, components, radiation damage 527 dosimetry 109

reduction 182 reinforcement, of rubber 278

ofsilicones 308 relative biological efficiency (RBE)

23 rep, definition 22 resonance stabilization 472 resonant transformer 52, 53, 84 roentgen 97

definition 22 rubber 60, 258, 400, 521

SIDE chain fracture (see crosslinking) 6

silicon fluoride 350 silicones (see dimethyl siloxane poly-

mers) 280, 297 sol(s) 145, 172,211,238,532 solid state polymerization 376, 382 solubility (see also sol) 143, 147, 179,

180, 210, 212, 260, 285, 299, 302, 317, 321, 532

SUBJECT INDEX 555 of polyesters 421

solution, irradiation of polymers in 426

specific volume, of filters in rubber 272 of polyethylene 210

sponge effect 11, 452 stopping power 31 strontium 65, 67 styrene 58, 370, 374, 378, 455, 493

grafts 397, 400, 401, 403, 405 modified polyesters 416 protection by 494

subexcitation electron 31, 448 surface-mass effect PTFE 353 swelling 155, 213, 261, 286, 303

corrections to formula 286 rate of grafts 406

synchrotron 90 syndiotactic 119, 124 synthetic rubbers 280

TANTALUM 68, 73 technetium 67 Teflon 58, 59, 60 (see PTFE)

conductivity changes 518 grafts 400, 401

temperature, dépendance in poly-merization 378, 384 effect 58, 233, 307, 333, 342, 490,

in ionic polymerization 390 in solutions 427 of crosslinking 485 on polyester cure 414, 416, 419 on properties of polyesters 416 poly/sobutylene 333 polystyrene 292 silicones 307

tensile strength 154, 247, 249, 270, 272, 276

terphenyl 452, 496, 530 Terylene 60, 113, 321 tetrafunctional junction point 134,

476 theories of crosslinking 470 theories of polymer behaviour in

solution 434 thiourea 498 transformer 52, 53 transition temperature 416 trapped electrons 448

radicals 397

trifunctional junction point 135 triplet state 450 Trommsdorf effect 377, 399

ULTRAVIOLET, light 5, 20, 46 spectrum, polymethyl methacry-

late 103, 345 polystyrene 103 polyvinyl chloride 315

uniform distribution 114, 131, 168, 285, 532

unsaturation 108, 117, 185, 191, 201. 228, 234, 265, 331, 355, 477, 478, 484, 485, 494 as measure of dose 108 poly/sobutylene 331

VAN DE GRAAF 52, 53, 78 vinyl, effect of irradiation 183, 184 vinyl acetate 374, 387

grafts 405 modified polyesters 418

vinyl carbazol grafts 401 vinylidene chloride 405 vinyl, pyrollidone 382

stéarate 383, 385 virtual degradation 162 viscosity 189, 205

average molecular weight (Mv) 128, 130, 289, 325 average molecules weight,

cellulose 360 polyethylene 205, 206 polymethyl methacrylate 336

changes in glycerine and toluene 4 organic liquids 61,529 of polyesters 422, 423 of polymer solutions 428

voltage doubler circuit 76 vulcanization 64, 258, 259, 277 Vulcollan 280

W 7, 18, 22, 97 WATER, decomposition 1 weight, average molecular (Mw) 129,

142, 289, 338, 429 PTFE 350, 354 random distribution 131

556 SUBJECT

X-RAY, pattern of, polyethylene 216. 284 poly/sobutylene 325 poly methyl methacrylate 335 PTFE 348

production 76, 89 sources 90

x-rays 2, 3, 4, 7, 9, 22, 34, 53, 75, 99

INDEX

xenon 64

YTTRIUM 67

z AVERAGE molecular weight 130 zinc oxide 391 zirconium 67 Zoe 58

Atomic Radiation and Polymers

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