İSTANBUL TECHNICAL UNIVERSITY « INSTITUTE OF SCIENCE AND TECHNOLOGY M.Sc. Thesis by Bülent ERİMAN Department : Polymer Science and Technology Programme: Polymer Science and Technology May 2008 EFFECT OF THE POLYETHYLENE GRAFT COPOLYMER TO STRUCTURES AND MECHANICAL PROPERTIES OF POLYETHYLENE/CLAY NANOCOMPOSITES
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İSTANBUL TECHNICAL UNIVERSITY « INSTITUTE OF SCIENCE AND TECHNOLOGY
M.Sc. Thesis by Bülent ERİMAN
Department : Polymer Science and Technology
Programme: Polymer Science and Technology
May 2008
EFFECT OF THE POLYETHYLENE GRAFT COPOLYMER TO STRUCTURES AND
MECHANICAL PROPERTIES OF POLYETHYLENE/CLAY NANOCOMPOSITES
II
ACKNOWLEDGEMENT
First and foremost I would like to express my indebtedness to my advisor Prof. Dr.
Nurseli Uyanık who has supported and encouraged me from the very beginning of
this study and shared her deep knowledge and experience.
I would like to thank to Prof.Dr. Mine Yurtsever and her Ph.D. Student Erol Yıldırım
during the study of our project.
I would like to thank to Prof. Dr. Hulusi Özkul to allow us to use tensile testing
device.
I would like to thank to Gülnur Başer and my other lab. collaborator for their
contribution during the study.
The authors wish to thank Süd-Chemie Inc. for supplying Na MMT, Nanofil 757
used. This research was supported from The Scientific and Technological Research
Council of Turkey (Grant No. 105M049).
Finally, I would like to offer the most gratitude to my brother, my parents and my
fiancee who have always supported me during my whole life.
4.3. Melt Flow Index Test Results……………………………………….. 50
5.CONCLUSION…………………………………………………………….. 52
REFERENCES………………………………………………………………. 54
APPENDIX…………………………………………………………………... 58
BIOGRAPHY………………………………………………………………... 73
VI
LIST OF TABLES
Table 2.1: The most commonly observed coordination plyhedra for the common elements in silicate structures…………………………..……………..…...
5
Table 2.2: Operating Conditions of LLDPE Processes……………............................. 14
Table 2.3: The relationship between density and the various properties of the polymers……………………………………………………………….........................
17
Table 2.4: Changes in polymer properties with melt index………………………….. 18
Table 2.5: Miciblity and immicibility of compatibilizers………….............................. 19
Table 3.1: Test concitions and standarts according sampe type……………………… 42
Table 3.2: Average MFI values with used weight usen in cylinder and measure time…………………………………………………………………………………….
42
Table 6.1: G.D. of LDPE with IA and monoesters and % grafting. ([DBPO]=0.75 g/100g LDPE, T=1400C, MW power=100 W)…………...………………..
57
Table 6.2: G.D. of LLDPE with IA and monoesters and % grafting. ([DBPO]=0.75 g/100g LLDPE, T=1400C, MW power=100 W)……..................................
57
Table 6.3: Sample descriptions and contents of samples which contain LDPE……… 57
Table 6.4: Sample description and contents of samples which contain LLDPE …….. 59
Polymers have become one of the most important materials in our daily life.
Increasing demand for using them forced the scientists to improve their properties.
Therefore, in recent years, inorganic nanoparticle filled polymer composites have
received increasing research interest, mainly due to their ability to improve
properties of polymers.
In general, when composites are formed two or more physically and chemically
distinct phases (usually polymer matrix and reinforcing element) are joined and the
properties of the resulting product differ from and are superior to those of the
individual components. The structures and properties of the composite materials are
greatly influenced by the component phase morphologies and interfacial properties.
Nanocomposites are based on the same principle and are formed when phase mixing
occurs at a nanometer dimensional scale. As a result, nanocomposites show superior
properties over their micro counterparts or conventionally filled polymers. Polymer
nanocomposites are a class of reinforced polymers with low quantities of
nanometric sized clay particles (generally), which give them improved properties.
The reinforcing effect of nanoparticles is related to the aspect ratio (p) (ratio of the
length or thickness to that of the diameter) and the particle-matrix interactions.
Independent of the actual dimensions, for p > 500 the reinforcing effects are the
same as those of any infinitely large particles. Because of the small size, the
nanoparticles are invisible to the naked eye, so nanocomposite are transparent.
Polyolefins (PO) are the most widely used polymers in preparation of polymer
nanocomposites (PNC) and it is more difficult than that of any polymer, which
contains polar groups in its backbone [1,2]. Since low-density polyethylene (LDPE)
and linear low-density polyethylene (LLDPE) are non polar polymers, homogeneous
dispersion of polar clay can not be realized due to lack of PE miscibility with it or
with organically-modified clay (organoclay) with the enhancement of the clay
dispersion, the aspect ratio of the particle increases and the reinforcement effect
improves. Strong interaction between a non-polar polymer and polar organoclay
might be achieved with addition of a compatibilizer [2,4-7]. During melt-blending
olefinic oligomers with polar functionality or PO grafted with polar group are
2
intercalated into clay galleries, facilitating dispersion of silicates into PO. Itaconic
acid (IA) and its monoesters and they can be grafted onto PO [8].
Homogeneous dispersion of nano-sized fillers in the matrix provides a large
interfacial area; otherwise the loosely agglomerated nanoparticles would easily
result in failure of the composites when they are subjected to force. A homogeneous
product, incorporation of any additives requires a serious mixing in molten state,
which is primarily provided by melt blending process by means of extrusion.
In this study, nanocomposites were produced by means of a corotating twin screw
extruder in single step melt mixing method. This study was carried out to determine
the effects of compatibilizer on the properties of PE-based PNC. In order to prepare
the compatibilizers, LDPE and LLDPE were grafted with itaconic acid (IA),
monomethyl itaconate (MMI) and monobutyl itaconate (MBI) by in a microwave
assisting system. Organically modified Na-MMT was used as the nanofiller.
Modifications of clays were done by using alkyl ammonium salts as the surface
active modification agents (dodecyl amine (DDA), hexadecyl amine (HDA) and
octadecyl amine (ODA)).
Dispersion of clay was characterized by using XRD tests, mechanical
characterization of the samples was done with stress-strain measurements data, and
the processabilities of PNCs were investigated by MFI measurements techniques.
3
2. THEORETICAL PART
In general, a composite is defined as two or more components differing in form or
composition on a macroscale, with two or more distinct phases having recognisable
interfaces between them. Nanocomposites (NCs) are materials that comprise a
dispersion of nano meter (10-9) size particles in a single or multicomponent matrix. [1]
The matrix may be metallic, ceramic or polymeric. Depending on the matrix nature
NCs may be assigned into these three categories. The nano particles are classified as; 1)
lamellar, 2) fibrillar, 3) tubular, 4) spherical, and 5) others. For the enhancement of
mechanical and barrier properties of NCs lamellar particles are preferred. For rigidity
and strength enhancement, fibrillar, for optical and electrical conductivity enhancement,
spherical or other particles have been used. In polymer nanocomposites (PNC), matrix
is a single or multicomponent polymer. In this work, LDPE and LLDPE with their
grafted copolymers were used as multi component matrix and the organoclays as nano
particle additives. To understand the PNC structure these main components will be
discussed: clay minerals and polymer matrices.
2.1. Clay Minerals
The terms "clay" and "clay mineral" are used in various subjects. A common
explanation of a clay substance is a material whose particles are very small. This is
general engineering usage. The term "clay" now refers to any material which exhibits a
plastic behavior when mixed with water, while "clay mineral" refers to materials which
have a layer structure. Clay minerals typically form at low temperatures, at low
pressures in the presence of much water, in nature. Under these conditions, perfection in
the organization of the crystal structure is unlikely. The details of the crystal structure
of these materials are of great importance in understanding the physical and chemical
properties of clays. This also is true of the highly disordered or amorphous materials
where there still exists short range order.
4
2.1.1. Silicate Mineral Structures
Silicate minerals are oxides of silicon and a small number of elements from the first
three columns of the periodic table and the transition elements. As such they closely
mirror the most of the elements in the crust of the Earth (Table 2.1). Since the number
of different elements which play a major role in the structure of silicate minerals is
small, it is not surprising that the fundamental building blocks of these minerals, and
many other non-silicate minerals, are few. The basic building blocks are simple platonic
polyhedra, largely tetrahedra and octahedra, which represent the placement of oxygen
atoms and the smaller cations. The number of oxygen atoms arranged about a cation is
termed the coordination number (CN); the smaller the cation, the smaller the CN.
Figure 2.1 shows a tetrahedron and an octahedron in both aspects, i.e., as a polyhedron
and as the arrangements of oxygen coordinating the central cation.
While these drawings show perfect polyhedra, in real mineral structures, these are
rarely perfectly regular. For example, that the edges of the polyhedra are almost always
of slightly different lengths and the central cation may be displaced from the geometric
center. For the purposes of this discussion such deviations are not important. Such a
view of crystal structures leads to a simplistic but nonetheless very useful concept of a
silicate mineral, that is, a mineral is an arrangement of boxes in space (the coordination
polyhedra), and we construct such a mineral by filling the boxes with appropriate
cations.
In a simple structure there might be only two kinds of boxes, representing tetrahedra
and octahedra, appropriately linked together. Thus, we can change the chemical
composition of a mineral by replacing all or part of the cations in one type of box by
another kind of cation, such that size and valence considerations are not violated. Two
divalent cations which are not very different in ionic radius, magnesium and iron, for
example, can readily substitute for each other in an octahedral site.[9]
5
Table 2.1: The most commonly observed coordination polyhedra for the common elements in silicate structures in order of decreasing amount in the Earth’s crust, omiting which is the most abundant element.
Element C.N. Polyhedron Ionic Charge Ionic Radius (Å)
Slicon 4 Tetrahedron +4 0,26
Aluminum 4
6
Tetrahedron
Tetrahedron
+3
+3
0,39
0,54
Iron 6
6
Octahedron
Octahedron
+2
+3
0,78
0,65
Calcium 6
8 Octahedron cube
+2
+2
1,00
1,12
Sodium 6
8 Octahedron cube
+1
+1
1,02
1,10
Pottasium 6
8 Octahedron cube
+1
+1
1,38
1,51
Magnesium 6
8 Octahedron cube
+2
+2
0,72
0,89
Figure 2.1: Polyhedra and the corresponding atomic arrangement for the two principal polyhedra of silicate mineral structures, i.e. octahedra (left) and tetrahedra (right). [9]
2.1.2. Classification of Clay Minerals
There are 4 types of clay minerals which are classified by their chemical formula;
Caolinite, Smectide, Illite and Clorite.
6
2.1.2.1. Caolinit group
This group contains caolinit, dicit and nacrit. The general formula of the caolinit group
is Al2O3·2SiO2·2H2O. There is no pure caolinit source in nature and generally they
contain iron oxide, silica, silica types components. They are used as filler in ceramics
paint, plastics and rubber and they are widely used in paper industry to product bright
paper.
2.1.2.2. Illit group
These groups differ from smectite group clays by including potassium and can called as
mica group. They are water included microscobic muscovit minerals and they are
formation minerals which can be seperated to layers. The general formula of illit group
is (K, H) Al2 (Si, Al)4 O10 (OH)2·xH2O. The stucture of this group is the same with
slicate layered montmorillonite group. It can be used as filler material and in driling
mud.
2.1.2.3. Clorit group
Clorit group clays have slim grain structure and green colour. This group clay includes
a great deal of magnesium, Fe (II), Fe (III) and alumina. Clorit group minerals are
generally known as fillosilicate group and they are not acceppted as one of clay group.
This group has got a lot of members like amesite, nimite, dafnite, panantite and
peninite. General formula of Clorit group is X4·6 Y4O10 (OH, O)8. In this formula, X
shows Al, Fe, Li, Mg, Mn, Ni, Zn and rarely Cr elements, and Y shows Al, Si, B, Fe
elements. They are not used in industry.[10,11]
2.1.2.4. Smectite group
The smectite minerals are classified according to the nature of the octahedral sheet
(dioctahedral versus trioctahedral), by the chemistry of the layer and by the site of the
charge (tetrahedral versus octahedral). The smectite minerals are very complex group,
frequently having both octahedral and tetrahedral substitutions each contributing to the
overall layer charge. The following formula are based on a layer of 0.33, with the value
varying between approximately 0.2 and 0.6 and sodium is indicated as the interlayer
7
cation. There are three important dioctahedral smectites; two are aluminous and the
third is iron rich. With a predominantly octahedral charge, the mineral is
montmorillonite, Na0.33nH2O(Al1.67Mg0.33)Si4O10(OH)2 where, as before, the n indicates
a variable amount of interlayer water coordinating (or not) the interlayer cation. The
interlayer cation needs not be sodium, but this is the common occurrence. The term
"montmorillonite" was frequently used as a group name for any swelling 2:1 clay
mineral as well as the name of a specific mineral, clearly not a good situation. Presently
smectite is the group name and montmorillonite is restricted to a mineral name
belonging to that group. If the charge is predominantly tetrahedral and aluminous the
mineral is beidellite with an ideal composition of Na0.33nH2O Al2(Al0.33Si3.67)O10(OH)2.
Finally, if ferric iron substitutes for aluminum in the octahedral sites and the charge is
tetrahedral, one has montmorillonite, Na0.33nH2O Fe2(Al0.33Si3.67)O10(OH)2. The
trioctahedral equivalent of montmorillonite is the mineral hectorite, ideally Na0.33nH2O
Mg3(Al0.33Si3.67)O10(OH)2. It should be noted that in contrast to montmorillonite,
hectorites have lithium (1+) in some octahedral sites (not shown in the above formula)
adding to the total layer charge. For saponite, aluminum substitutes for magnesium in
the octahedral sites generating a positive contribution to the layer charge which reduces
the negative contribution from the tetrahedral sites. The ideal formula without the
aluminum substitution is Na0.33nH2O Mg3(Al0.33Si3.67)O10(OH)2. The interactions
between adjacent smectite layers are not very strong and the interlayer material,
hydrated cations, water, organics, are disordered, so that there is little coherence from
one layer to the next. As a consequence, it is normally not possible to speak of a crystal
of a smectite. There are some exceptions, saponite being one, where there is a greater
degree of stacking regularity; it shows a degree of disorder. [9]
Montmorillonite is the mineral with the general formula of Na0,2 Ca0,1 Al2 Si4 O10 (OH)2
(H2O)10. Montmorillonite is a fine powder which has monoclinic-pyrismatic crystal
structure, a colour from white to brown-green and yellow, average density of
2.35 g/cm3, molecular weight of 549.07 g/mol and hardness of 1.5–2 (Figure 2.2).
Single montmorillonite crystals are quite fine, granulated and they got random outher
lines. In general a montmorillonite crystal consists of 15–20 silicate units. This property
8
is so usefull for engineering projects. There are two different swelling types of
montmorillonite according to expansion size of the basal space as crystallized and
osmotic swelling. Crystillized swelling occurs when the water molecules enter in to the
unit layers. First layer of the water molecules which are adsorbed occurs when they
bind with hydrogen bonds to hexagonal oxygen atoms. Montmorillonitles whose
cations are exchangable hydrates as Na+, Li+ can swell to 30–40 Å. Moreover,
sometimes this swelling level increases up to hundred. This type ditance is called as
osmotic swelling. Montmorillonits do not swell much when they got high valanced
cations as exchangable cations.[12-16]
Figure 2.2: Shematic represantation of 2:1 clay mineral structure (red Al, small ones O, light violet Ca, light purple Si).
The reason of this sittuation is that gravitational forces between silicate and cation
layers are higher than ion hidration thurst force [17]. Montmorillonites enable polar or
ionic organic molecules to penetrate between the layers. Adsorption of organical
9
mixtures causes to formation of organo-complex montmorillonites. Penetrating of big
molecules in to layers of clay mineral could be determined by using XRD
measurements. Montmorillonits have 2:1 type layered structure. Crystal like structure of
the montmorillonite occures from, silicon-oxygen (Si-O) tetrahedral layer with (Al-O-
OH) oktahedral layer which is between two Si-O layers. Silicon atoms are bonded with
4 oxygen atoms in (Si-O) layers. Oxygen atoms are placed regularly as one in centre of
silicon atom and the other 4 atoms are on the corners of the tetrahedron (Figure 2.3).
Layers are divided between every thirth neighbour tehrahedral layer structure from 4
oxygen atoms of tetrahedron layer. All of the fourth oxygen atom of the tetrahedron has
condition as oriented to lower side of structure which can be seen in Figure 2.4 and
they are at the same plane with the -OH groups of alumina octahedral layers. [18,19]
Figure 2.3: Structure of 2:1 phyllosilicates.
2.1.3. Cation Exchange Capacity
Clay minerals get ability of pulling some ions and push them back again. In this case
ions could replace each other. In the tetrahedron layer of montmorilonite Si+4 with Al+3
and in the octahedron layer of montmorilonite Al+3 with Mg+2, Fe+2, Zn+2 and Li+1 can
10
replace with each other. In tetrahedron this cation exchange capacity is low, despite of
this it is significiantly high in octahedron. At the end of cation exchange, positive and
negative charges occure. Two layered clays have natural surfaces according to their
electrical charges but three layered clays have charged surfaces. Positive charge
deficiency can be overcome by bonding of Na+, K+, Li+ or Ca+2 ions to crystal cage
from their water layer of unit area.[12]
Despite of these conditions units can give these cations back, naturally. The ions
captured by clay minerals in exchangable position are called as “exchangable ions”.
Because these ions are mostly cations and their ion exchange ability or cation echange
capacity is higher then certain values, these ions show properties such as clay minerals
grade of swelling, gelation etc. Cation exhange capacity is defined as (meq.) Na2O in
100 gr clay. This charge is not locally constant, but varies from layer to layer, and must
be considered as an average value over the whole crystal. Layered silicates have two
types of structure: tetrahedrally substituted and octahedrally substituted. In the case of
tetrahedrally substituted layered silicates, the negative charge is located on the surface
of silicate layers and, hence, the polymer matrices can interact more readily with these
than with octahedrally substituted material. The exchangable cations in clay minerals
are H+, Na+, K+, Ca+2 and Mg+2. The exworks shows us cation exchange capacity of
montmorillonite is between 80–150 meq.[10]
The general cation exchange capacity of natural or synthetic clay minerals is between
50–200 meq/100 gr. Because of the cation exchange capacity is higher than 200, the
forces between layers prevent seperation of clay layers. On the other hand, clay
minerals, which cation exchange capacity lower from 50 meq/100 gr clay, could not
seperate the clay layers. Due to these reasons, montmorillonite which cation exchange
capacity between 50–150 meq/100 g clay, are used as swelling agent in NCs.[12]
2.1.4. Inter Layer Formation
Several phyllosilicate minerals, either naturally or as the result of chemical treatment,
have molecular species inserted between the silicate layers. Water is the most common
interlayer species in nature, and water is normally found in smectites, vermiculites and
11
hydrated halloysites. The quantity of interlayer water is a function of relative humidity
and the type of interlayer cation, in the case of smectites and vermiculites. There is a
great interest in the nature of the interface between water and silicate minerals. Much of
the chemical activity in soils, sediments and porous rocks occurs at such an interface.
Experimentally, it is very difficult to examine this interface because it is such a small
part of the liquid-solid system. Hydrated smectites and vermiculites have water between
all of the silicate layers and therefore the percentage of the sample which is interface is
enormously larger than the interface between a grain of quartz in contact and liquid
water. Another way to look at this is that the surface are of a quartz sand is probably
much less than 1 m2/gram while a typical smectite has a surface area of as much as 800
m2/gram.[9]
The surfaces of clay minerals present a number of potential sites at which organic
molecules could attach themselves. These sites include the exchangeable cations. The
oxygen atoms can occupy the surface of the silicate layer and at the edge of these
layers. On the other hand, hydrogen atoms take part of surface hydroxyl groups. So
there is a wide variety of interaction possible between the heterogeneous clay surface
and the different functionalities of organic materials. If any organic material existing on
external particle surfaces, intercalation of layers can occur.[9]
One can categorize the types of organic-clay interactions based on the bonding
mechanisms between the organic and the clay surface/inter layer (inorganic) cations.
• Cationic bonding: These involve organic cations such as the alkyl ammonium cations
or amines and carbonyl groups which have become protonated, depending on the pH.
• Ion-dipole and coordination bonding: This is particularly common for organic
molecules having a permanent dipole, e.g., acetone.
• Hydrogen bonding: The organic molecule can be either the donor or the acceptor or
both, depending on the nature of the clay surface and the organic molecule.
• Tetrahedral tetrahedral (TT) bonding: Molecules such as benzene can interact via their
valance electrons with, for example, Cu2+ interlayer cations. Hydrogen bonding and TT
bonding are both examples of Lewis electrondonor/ electron-acceptor interactions.
12
2.2. Polyolefins
A polyolefin, whose equivalent term is polyalkene, is a polymer produced from a
simple olefin (also called an alkene) as a monomer. Their main members are
polyethylenes and polypropylenes. Industrial production of polyolefins cover low
density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (i-PP),
and together with some copolymers. [22-24]
2.2.1. Polyethylenes
Polyethylene is classified into several different categories based mostly on its density
and branching (Figure 2.4). The mechanical properties of PE depend significantly on
variables such as the extent and the type of branching, the percent crystalinity, and the
molecular weight.
The types of polyethylene mostly consumed are LDPE (low density PE), LLDPE
(linear low density PE), HDPE (high density PE), HMWPE (high molecular weight
poly ethylene), UHMWPE (ultra high molecular weight polyethylene), HDXLPE (high
density cross-linked PE), PEX (cross-linked PE), MDPE (medium density PE), and
VLDPE (very low density PE).
2.2.1.1. Low Density Polyethylene (LDPE)
LDPE is defined by a density range of 0.910 - 0.940 g/cm3. LDPE has a high degree of
short and long chain branching, which means that the chains do not pack into the crystal
structure as well. This results in a lower tensile strength and increased ductility. LDPE
is created by free radical polymerization. The high degree of branches with long chains
gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid
containers and plastic film applications such as plastic bags and film wrap.[28]
LDPE is produced by a free-radical initiated reaction using oxygen or other free radical
initiators such as organic peroxides or azo compounds. Synthesis conditions are usually
250–300 °C outlet temperature, 3000 atm. pressure. Nominal reactor residence times
are about 10–50 seconds.
13
Figure 2.4: Molecules of LDPE, LLDPE and HDPE
Heat of polymerization is about 800 KCal/gm, which must be removed during the short
residence time available. Only a small part of this heat can be removed through the
reactor walls because of their comparatively limited area and necessary thickness. In
addition, the polymer tends to deposit on cool surfaces. In practice, heat is removed by
recirculating excess cool monomer and the system operates essentially adiabatically.
Therefore, production rates vary directly with the ethylene recirculation rate and the
allowable temperature rises through the reactor. Heat balance limits conversion to 15–
20% on each pass. Reactors are of two general types, autoclaves and high pressure
tubes. Each of these types produces slightly different polymers, primarily because of
differing temperature profiles through the reactors. [25-27]
2.2.1.2. Linear Low Density Polyethylene (LLDPE)
LLDPE is defined by a density range of 0.915 - 0.925 g/cm3 is a substantially linear
polymer, with 4-6 carbon containing short branches (short-chain alpha-olefins) with
approximately, every 100 carbons on the main chain. LLDPE has higher tensile
strength, higher impact and puncture resistance than LDPE. Lower thickness films can
be blown compared to LDPE, with better environmental stress cracking resistance
compared to LDPE but is not easy to process in packaging. Cable covering, toys, lids,
buckets and containers, pipe are some of the products can be made by LDPE. While
LDPE
HDPE
LLDPE
14
other applications are available, LLDPE is used predominantly in film applications due
to its toughness, flexibility, and relative transparency.[22,23]
Linear low density polyethylenes are made with transition metals catalysts/initiator
under 100-130 °C and up to 20 atm.(Table 2.2) Butene-1 is the usual comonomer, but
either hexene-1, octene-1, or 4 methylpenetene-1 is employed to give enhanced
physical and optical properties with higher production cost.
Table 2.2: Operating Conditions of LLDPE Processes
Conditions Slurry Fluidized
Temperature 80-120 80-120
Pressure (Mpa) 4-7 2-3
Residence Time (hours) 0,75-1,05 4-7
Convension/pass (%) 95 1-4
2.2.1.3. Other Polyolefins
i-PP and HDPE which are stereospecific polymers can be produced under the same
method given above for LLDPE. In the slurry process, MgCl2 supported TiCl4 is used as
catalyst and Al(C2H5)3 as co-catalyst in the n-heptane solution for both production,
generally. Atactic PP (a-PP) is the by-product of i-PP production.
2.2.2. Properties of Polyolefins
In today's competitive market place, prime grade commercial polyethylenes must be
both processable and uniform. We use the term “processability” to describe the ease or
difficulty with which an olefin can be handled during its fabrication into film, molded
items, pipe, etc. Polyethylene with good processability is one which possesses the
properties necessary to make it easy to convert the polyethylene pellets or powders into
the desired products. The main characteristics or properties which determine an olefin's
processability are molecular structure, uniformity, and additive content. However,
processability is a property which is a result of the basic properties mentioned above.
These characteristics include hot-melt extensibility, sensitivity to pressure and
15
temperature, smoking and odor, product stability during withdraw, and flow rate (which
is an operating condition). Uniformity is a characteristic of critical importance. It is
obtained only through rigorously controlled synthesis, densification, stabilization,
blending, and handling, so that lot-to-lot variation is minimized. The customers expect
to be able to process polymers during extended runs with, at worst, minor adjustments
to their machinery between lots of material. It’s also expected a polymer to be free of
contamination, dirt, discolored material, and other foreign matter and to be of light,
uniform color, unless pigmented. Polyethylene must also be uniformly granulated to
flow through the customers' handling and feeding systems. This means accurate and
uniform pellet size with freedom from excessive fines or oversize particles, and no
strings of agglomerated pellets or streamers. The polymer must also be free of excessive
moisture.
The main structural factors that determine PE properties are the degree of short and of
long chain branching, the average MW and the polydispersity. One of the most
important characteristics that determine in the highest degree the properties and the
behavior of different grades of PE is their branching. Branches prevent the polymer
chains from packing together regularly and closely and have a predominant effect on
the density of PE. The density can be considered a first indication of the degree of
branching: the lower the density the higher the degree of branching. The presence of
branches interferes with the ability of the polymer to crystallize. The degree of
crystallinity of LDPE is usually of the order of 55-70% compared with that of HDPE
which is 75-90%.
Other properties depending on crystallinity, such as stiffness, hardness, tear strength,
yield point, Young’s modulus in tension and chemical resistance, increase with
increasing degree of crystallinity (HDPE) whereas permeability to liquids and gases,
flexibility and toughness decrease under the same conditions
Since PE is crystalline nonpolar hydrocarbon polymer it has no solvents at room
temperature and dissolution takes place only on heating in solvents of similar solubility
parameter such as hydrocarbons and halogenated hydrocarbons. The higher the degree
16
of crystallinity results the higher the dissolution temperature. LDPE dissolves at 60°C
compared to 80-90°C for high density, more crystalline polymers.
The effect of branching also depends on the size of side chain branches. While short
branches have a predominant influence on the degree of crystallinity and therefore on
the density of the polymer, long branches affect more pronouncedly the polydispersity.
The side chains may be as long as the main chain and like it may have a wide
distribution of lengths. The higher the MW of the resulting polymer the wider the
MWD, as chain transfer reactions may occur as well on side chains. Such a polymer
may be made up of short chains grafted onto short chains, long chains onto long chains
and a vast range of intermediate cases. Long chain branches also affect the flow
properties. Long branched molecules are more compact and tend to entangle less with
other molecules resulting in lower solution and melt viscosities as compared with
unbranched polymers.[29,30]
2.2.2.1. Mechanical Properties of Polyolefins
Another factor that influences the properties of the melt, as well as those properties that
involve large deformations, is the weight-average MW. Ultimate tensile strength, tear
strength, low temperature toughness, softening temperature, impact strength and
environmental stress cracking increase as the MW increases; on the contrary, the
fluidity of the melt and the coefficient of friction decrease.
2.2.2.2. Dielectric
The electrical insulating properties of polyethylenes are excellent. The dielectric con-
stant increases linearly with increasing density. As it is a non-polar material, dielectric
constant and the power factor are almost independent of temperature and frequency.
2.2.2.3. Density
Polymer density is a rough measure of crystallinity and, therefore, of the physical and
optical properties that are dependent on the degree of crystallinity. The relationship
between density and the various properties of the polymers is illustrated in Table 2.3
LDPEs and LLDPEs of the same density have somewhat dissimilar properties. This
17
difference is largely because LDPE, being free radical initiated, contains a range of both
long and short side chains attached to the main polymer backbone.
Table 2.3: The relationship between density and the various properties of the polymers.
LLDPE, on the other hand, contains only short branch lengths, those of the comonomer.
Although the degree of crystallization is nearly the same, the morphology of the crystal
is dissimilar. LLDPE possesses many improved solid properties, such as strength,
toughness, and draw-down, but LDPE in general is easier to process, is softer, and
yields films with better optical properties.[28-30]
2.2.2.4. Melt Flow Index
The melt index (MI) is a rough measure of average molecular weight and melt
viscosity. It indicates how readily molten polymer will flow in processing machinery.
18
Table 2.4: Changes in polymer properties with melt index.
Because the melt index is measured at a single temperature at low shear, and because
melt viscosities are highly non-Newtonian, the melt index alone does not adequately
predict how processable a given polymer will be under higher shear conditions in
commercial processing equipment. However, the melt index is often an adequate
discriminant within a given group of resins produced under substantially similar
19
conditions. The various physical properties of the polymers will generally vary as the
melt index varies, as illustrated in Table 2.4.
2.3. Compatibilizer
Polymeric compatibilizers serve as their name indicates to make compatible the
different kinds of materials such as multi component structures. Before discussing the
compatibilization of polymer pairs in multi component structures, the compatibility of
polymer blends and the compatibilizers will be described.
2.3.1. Compatibility and Compatibilizers
When blending two polymers, the resulting behavior falls into three categories (Table
2.5). Either they are miscible and compatible or immiscible and incompatible, or they
behave somewhere in between these two extremes.
Table 2.5: Misciblity and immiscibility of compatibilizers.
20
The requirements are similar polarity and structure, and the result is a single-phase
mixture. The materials have different polarities and structures, and the result is a two-
phase mixture with poor properties, an undesirable state.
Rarer still is immiscibility and compatibility at which a mixture's constituents have
different properties, but show some interaction, because of reactive groups, surface
active agents, or compatibilizers.
Immiscibility and compatibility are not necessarily bad. Actually, the entire technology
of toughened polymers is based on this approach, because it synergistically combines
the properties of completely different polymers to form a blend with properties superior
to those of the individual blend components.[34,35] The theoretical explanation for why
such cases are observed has been extensively treated in the literature.[36]
Figure 2.5: Schematic representation of an AB block copolymer compatibilizing a blend of polymers A and B.
Compatibilizers that compatibilize two polymers, A and B, consist of two parts
(Figure 2.5). One part interacts with polymer A and the other part with polymer B, but
to do so effectively, they must be concentrated at the interface between the two
polymers.
The result is a better dispersion of the polymer blend as shown in Figure 2.6. Infinite
dispersion, however, is not necessarily desirable since a minimum particle diameter
exists for each system below which there will be no synergistic improvement in
properties (e.g., fracture toughness)
21
Figure 2.6: Schematic representation of a compatibilization reaction
2.3.2. Classification According to Properties of Compatibilizers
Compatibilizers can be classified as follows: non-reactive, reactive compatibilizers, and
random, graft, and block copolymers.
Non-reactive compatibilizers, which compatibilize two polymers, A and B, consist of
two parts: the first is soluble in polymer A, and the second is soluble in polymer B. The
compatibilizer's effect is derived from their solubility. Therefore, the solubility
parameters of both parts should be as close as possible to the solubility parameters of
the polymer components in the polymer mixture.[37-39]
A block copolymer contains blocks of the polymer pairs. These blocks can be reactive
or non-reactive polymers.[40]
In random copolymers, the components, the base polymer B and a comonomer A, are
distributed randomly along the polymer chain. Random copolymers are usually
produced in a high-pressure radical polymerization process.[41,42] Random
copolymers only work well as compatibilizers when the comonomer A is reactive.
In graft copolymers, either monomers or polymers are grafted onto each other. If only
monomers are grafted to the backbone, the monomer should be reactive. An example is
PP grafted with maleic anhydride. The exposure of the reactive monomers on the
usually non-reactive base polymer backbone makes them more accessible to an attack
by other polymers, transforming them into effective compatibilizers.
22
2.3.3. Works on IA/MMI/MAH Grafting Polyolefins in Recent Years
S.S.Pesetskii and his coworkers investigated grafting of itaconic acid on low density
polyethylene in molten state via reactive extrusion many times in recently years.
Pesetskii investigated firstly, initiator and stabilizator efficiency on grafting degree of
LDPE with IA. It was shown that initiator solubility affects the grafting degree. The
initiators which can be dissolve easily in LDPE, increases the grafting efficiency and
the closer the thermodynamic affinity between the peroxide and the monomer, and
decreases efficiency of grafting. The stabilizers (e.g.,1,4-dihydroxybenzene) with
increased affinity toward the monomer reduce the grafting yield and inhibit
crosslinking.[43,44] In another work, it was shown that thermomechanical and
rheological properties of LDPE was changed. According to results, while unmodified
PE exhibits two glass transition temperatures, modified PE with IA exhibits three glass
transition temperatures and with increasing of grafting degree melting temperature
increases 1-2 0C and melt flow rate (MFR) values decrease, it means that viscosity of
polymer increases.[45] Pesetskii worked on thermal and photo oxidation of grafted
LDPE and the functionalization of LDPE by grafting of itaconic acid to the
macromolecules was found to accelerate its thermal and photo-oxidation in water.[46]
Pesetskii made his last IA-g-LDPE work in presence of neutralizing agents. When the
grafting takes place in the presence of neutralizing agents, the efficiency of the itaconic
acid grafting onto macromolecules is found to increase. Neutralization of the grafted
itaconic acid contributes to an increase in the mechanical and impact strengths of blends
composed of functionalized low-density polyethylene and polyamide-6.[47]
M. Yazdani and his coworkers worked with monoesters of IA for grafting of PE and
PP. Firstly, to improve the compatibility and properties of blends based on high-density
polyethylene (HDPE) and the ethylene-propylene copolymer (EPR), the
functionalization of both through grafting with an itaconic acid derivative, monomethyl
itaconate (MMI), was investigated. The results show that the grafting reaction increases
the toughness and elongation at break of all tested blends and they retained their
strength and stiffness. Moreover, the grafted polymers behaved as nucleating agents,
accelerating the HDPE crystallization.[48] In another work, Yazdani synthesized
23
functionalized polypropylene by radical melt grafting either with monomethyl itaconate
and dimethyl itaconate to improve its compability of PP with PET. The use of PP
grafted with MMI as compatibilizer resulted in even a better dispersion of PP as the
minor phase increasing the components interface and there after to an improvement of
the adhesion between the two phases. The noncompatibilized blend in this case also
showed an even more pronounced two phase behavior as compared with PP/PET
blends. The impact resistance of PET in noncompatibilized blend was hardly affected
by incorporation of PP. However, when functionalized PP with either MMI or DMI was
used as blend compatibilizers, there was an increase of the impact resistance of PET.
This probably is due to spesific interactions and/or chemical reaction
(transesterification) between the functional groups of the compatibilizer with the blend
constituents resulting in a finer dispersion of the minor phase leading to improved
interfacial adhesion.[49]
The first grafting reaction of LDPE in a solution medium was worked by Yu-Zhong
Wang in recently year. The grafting reactions of MAH were carried out in microwave
assisting system. The reaction of maleic anhydride (MAH) grafted onto low density
polyethylene (LDPE) in xylene solvents in the presence of benzoyl peroxide (BPO) as
an initiator by microwave irradiation has been investigated. The influence of reaction
conditions such as initiator content, monomer content and irradiation time have been
examined. IR spectra of PE and PE-g-MAH show that MAH is really grafted on the PE
in a xylene solution by means of microwave. Moreover, the melting temperature of PE-
g-MAH is lower than that of PE, but the melting enthalpy of PE-g-MAH higher than
that of PE. [50]
2.4. Polymer Nanocomposites
The structures and properties of the composite materials are greatly influenced by the
component phase morphologies and interfacial properties. Nanocomposites are based
on the same principle and are formed when phase mixing occurs at a nanometer
dimensional scale. As a result, nanocomposites show superior properties over their
micro counterparts or conventionally filled polymers.
24
2.4.1. Polymer Nanocomposite Synthesis Methods
Not all physical mixtures of polymer and silicate will form a nanocomposite. The
compatibility between the two phases is important. Nanocomposites are synthesized
from various polymers; nylon 6, polyimide, epoxy resin, polystyrene, polycaprolactone
and acrylic. The exfoliated and homogeneous dispersion of the silicate layers, however,
could be achieved only in few cases, such as polymers containing polar functional
groups such as amides and imides. This is due to the fact that silicate layers of clay have
polar hydroxy groups and are compatible with polymers containing polar functional
groups.[48] Silicate clay layers are bound together by a layer of Na+ or K+ ions and are
naturally hydrophilic.
Ion exchange reactions with cationic surfactants including primary, tertiary and
quaternary ammonium ions render the normally hydrophilic silicate surface
organophilic, which makes intercalation of many engineering polymers possible. The
role of the alkyl ammonium cations in the organosilicates is to lower the surface energy
of the inorganic host and improve the wetting characteristics and, therefore, miscibility
with the polymer.[52]
Nanocomposites can be formed in one of three ways:
• Melt blending synthesis.
• Solvent based synthesis.
• In-situ polymerisation.
2.4.1.1. Melt Blending Synthesis
The melt blending process involves mixing the layered silicate under shear, with the
polymer while heating the mixture above the softening point of the polymer. During
this process, the polymer chains diffuse from the bulk polymer melt into the galleries
between the silicate layers.
In some cases the polymer–silicate mixture can be extruded by using (a) static melt
intercalation: by mixing and grinding dried powders of polymer and organic silicate in a
pestle and mortar and then heating the mixture in vacuum, and (b) extrusion melt
25
intercalation: by extruding the mixture with twin screw extruder to produce a polymer
nanocomposite from the polymer and modified clay.[54,71, 72]
2.4.1.2 Solvent Based Synthesis
The solvent based synthesis involves mixing a preformed polymer solution with clay. A
polystyrene–clay hybrid can be prepared by mixing a polystyrene-toluene solution and
silicate to yield a suspension and then evaporating the solvent. Polyimide–clay hybrids
can be prepared by dissolving clay in dimethylacetamide (DMAC) and mixing with
precursor solution of polyimide and then removing the solvent.[73]
2.4.1.3. In-situ Polymerisation
The clay/organoclay is dispersed in the monomer and the polymerisation reaction is
carried out (Figure 2.7). Polystyrene clay nanocomposites can be prepared by the
polymerisation of styrene in the presence of clay; chemical grafting of polystyrene onto
montmorillonite interlayers have achieved by addition polymerisation reactions.
Thermoset PNCs are prepared by using this method.[78]
Most thermoplastic polymer nanocomposites are produced by either of the first two
methods.[74]
Figure 2.7: Method for creating intercalated polymer-clay architectures via direct polymer contact and via insitu polymerization of pre intercalated polymers.
26
2.4.2. The structure of nanocomposites
Often, there are occasions where retention of the layered nature of a polymer–clay
nanocomposite is the desired outcome. Such regular nanoassemblies have the following
unique characteristics and applications:
1. There are a wide variety of both host materials (clay and nonclay) and polymers.
2. Anisotropic arrangements of polymers in two-dimensional microenvironments occur.
3. The variable gallery spacing is adaptable to polymer size.
4. Microenvironments can induce spatial chemistry and host surface chemistry effects.
5. Rigid host layers provide enhancements to structural, chemical, and thermal
stabilities to more fragile guest polymers.
Two primary methods are utilized to prepare intercalated polymer–clay materials. The
former route is limited because the types of polymers that can be intercalated directly
are limited. The latter route, while more universal, results in a loss of control over the
molecular weight of the final polymer.
Some of the PCN systems are created in a unique way where in the clay layers are
crystallized from a silicate sol-gel in direct contact with a polymer solution. In the
majority of intercalated PCN cases, the linear macromolecules are in nearly fully
extended conformation. Another way to intercalate polymers directly is through a melt
method, where the polymers are heated with a preexfoliated (in most cases) clay. In this
way, a conventional polymer extrusion process can often be utilized. The formation of
PCNs via melt intercalation depends on the thermodynamic interaction between
polymer chains and host layers, and also on the diffusion of polymer chains from bulk
to interlayers. For more hydrophobic polymers, however, the clays must be rendered
more organophilic to enhance their compatibility.
Probably the most successful route to creating PCNs in general has been the in situ
process, wherein monomers are polymerized in the presence of clay mineral layers. In
terms of well-ordered polymer–clay intercalates, using in situ polymerization have
included polyacrylamide, nylon, polyaniline, polycaprolactone, polyimide, PMMA,
27
polystyrene, polyurethane, polyethynylpyridine, and epoxy resins. If conditions are
varied, many of these polymers can also be induced to form exfoliated PCN
architectures.
The dispersion of mineral reinforcing components to a polymeric matrix has been
utilized for many decades. Inactive fillers or extenders act to simply reduce costs, and
their chemistry is less important than factors such as particle size, shape, morphology,
distribution, and, of course, cost. Active fillers are reinforcing materials and require at
least some compatibility between polymer and inorganic, and they must often undergo a
surface modification process to insure this. In contrast to conventionally scaled
composites on the micrometer level, nanocomposites exhibit changes in composition
and structure over the nanometer length scale. Individual clay layers fall into this realm
because they have a thickness on the order of 1 nm. Clays such as kaolinite, mica, and
talc have been important plate-shaped conventional fillers in the past. However,
preparation of a true nanocomposit requires complete dispersion, or exfoliation, of the
elementary clay layers within the polymer matrix, without any aggregation into larger
units such as tactoids or intercalation products. This is a serious challenge that has been
addressed well since the first pioneering research was published in 1987 from Toyota
workers. There are very few successful reports of making fully exfoliated PCNs from
dissolved polymer solutions. This is because even when dispersions of fully exfoliated
clays are exposed to macromolecular solutions, the strong interactions between
macromolecules and silicate layers often just reaggregate the layers. There is a report
that polysulfone exfoliates organoclays via a “solution dispersion” technique. More
success for making the better PCNs has occurred using the melt mixing method
concerning superior composite properties, including more exfoliation. In a study of
PCN synthesis, the PNC structure can also be investigated in the differences in melt
rheology and in the crystalline morphology. As in the case for intercalated materials,
the clays need to be premodified by reaction with alkylammonium ions in order to make
them more compatible with the hydrophobic polymers. The most successful process for
making exfoliated PCNs has been through the polymerization of monomers that are in
the presence of clay minerals. Conditions must be optimized to promote a
28
polymerization that causes the uniform dispersion of silicate layers within the polymer
matrix. The heat of reaction evolved (enthalpy) during polymerization provides an
essential component to the exfoliation. Therefore, exfoliation is enhanced with
increasing amounts of intercalated monomers and with decreasing layer charge on the
clay surface. in situ polymerizations (emulsion, thermal, photo, free radical, etc.) that
employ organoclays and lead to truly exfoliated PCNs.
The main reason for these improved properties in nanocomposites is the interfacial
interaction between the matrix and layered silicate, as opposed to conventional
composites [1]. Layered silicates have layer thickness onthe order of 1 nm, and very
high aspect ratios (e.g. 10∼1,000). A few weight percent of layered silicate particles that
are properly dispersed throughout the matrix can thus create a much larger surface area
for polymer filler interactions than do conventional composites.
Figure 2.8: Shematic representation of the various PNC architectures [10]
On the basis of the strength of the polymer/layered silicate interfacial interaction, three
structurally different types of composites are achievable (Figure 2.8): (1) phase-
29
separated composite, when polymer matrix has no interaction with layered silicate, (2)
intercalated nanocoposites, where insertion of polymer chains into the silicate structure
occurs in a crystallographically regular fashion, regardless of the polymer-to-layered
silicateratio, and a repeat distance of few nanometers, and (3) exfoliated
nanocomposites, in which the individual silicate layers are separated in the polymer
matrix by average distances that totally depend on the layered silicate loading.
2.4.3. Structural Characterization of PNCs
The most commonly used techniques for structural characterisation of nanocomposites
are X-ray diffraction (XRD), SAXS, SEM, transmission electron microscopy (TEM)
and WAXD analysis.
X-ray diffraction allows the determination of the spaces between structural layers of
silicate utilising Bragg’s law: sinθ = nλ /2d. Intercalation and delamination change the
dimensions of the gaps between the silicate layers, so an increase in layer distance
indicates that a nanocomposite has formed. A reduction in the diffraction angle
corresponds to an increase in the silicate layer distance. Generally, diffraction peaks
observed in the low angle region (2θ = 3–9°) indicate the d-spacing (basal spacing, d001)
of ordered intercalated and ordered-delaminated structures. If the nanocomposites are
disordered, no peaks are observed in the XRD due to loss of structure of the layers, the
large d-spacings (>10nm), or both. In general, the following relationship between the
composite and the X-ray diffraction pattern holds [52,71].
Because of its easiness and availability WAXD is used to characterize the
nanocomposite structure and sometimes to study the kinetics of the polymer melt
intercalation. Monitoring the position, shape, and intensity of the basal reflections from
the distributed silicate layers, the nanocomposite structure (intercalated or exfoliated)
can be identified. In an exfoliated nanocomposite, the extensive layer separation
callobrated with the delamination of the original silicate layers in the polymer matrix
results in the final disappearance of any coherent X-ray diffraction from the distributed
silicate layers.
30
Figure 2.9: Wide-angle and small-angle X-ray diffraction of polymer samples.
On the other hand, for intercalated nanocomposites, the finite layer expansion
associated with the polymer intercalation results in the appearance of a new basal
reflection corresponding to the larger gallery height. Although WAXD offers a useful
method to determine the interlayer spacing of the silicate layers in the original layered
silicates and in the intercalated nanocomposites (within 1–4 nm), little can be said about
the spatial distribution of the silicate layers or any structural nonhomogeneities in
nanocomposites.
Additionally, some layered silicates initially do not exhibit well-defined basal
reflections. Thus, peak broadening and intensity decreases are very difficult to study
systematically. Therefore, conclusions concerning the mechanism of nanocomposites
formation and their structure based on WAXD patterns are only temporary. On the
other hand, TEM allows a qualitative understanding of the internal structure, spatial
distribution of the various phases, and views of the defect structure through direct
visualization. However, special care must be exercised to guarantee a representative
cross-section of the sample.[75]
Information about the macrostructure of a polymer such as the dimensions and packing
of crystallites, spherulites, lamellae, separated phases, and voids; particle size and shape
in solution or colloids; and information on branched polymers and the deformation and
31
annealing of polymers can be obtained from small-angle Xray scattering (SAXS). The
reflections that lie very close to the beam stop are necessary for this information. To
obtain sufficient clarity synchrotron radiation is often used. If the sample is reactive
under high intensity X-radiation, this method is unsuitable.
The morphological properties of PNC samples were investigated by the scanning
electron microscope (SEM) which is a type of electron microscope that images the
sample surface by scanning it with a high-energy beam of electrons in a raster scan
pattern. The electrons interact with the atoms that make up the sample producing
signals that contain information about the sample's surface topography, composition
and other properties such as electrical conductivity.
The types of signals made by an SEM can include secondary electrons, back scattered
electrons, characteristic x-rays and light (cathodoluminescence). These signals come
from the beam of electrons striking the surface of the specimen and interacting with the
sample at or near its surface. In its primary detection mode, secondary electron imaging,
the SEM can produce very high-resolution images of a sample surface, revealing details
about 1 to 5 nm in size. Due to the way these images are created, SEM micrographs
have a very large depth of focus yielding a characteristic three-dimensional appearance
useful for understanding the surface structure of a sample. This great depth of field and
the wide range of magnifications (commonly from about 25 times to 250,000 times) are
available in the most common imaging mode for specimens in the SEM, secondary
electron imaging, such as the micrograph taken of pollen shown to the right.
Characteristic x-rays are the second most common imaging mode for an SEM. X-rays
are emitted when the electron beam removes an inner shell electron from the sample,
causing a higher energy electron to fill the shell and give off energy. These
characteristic x-rays are used to identify the elemental composition of the sample.
Back-scattered electrons (BSE) that come from the sample may also be used to form an
image. BSE images are often used in analytical SEM along with the spectra made from
the characteristic x-rays as clues to the elemental composition of the sample.
32
2.4.4. Works on PNC including MAH/EVA Grafting Polyolefins in Recent Years
Polymer-layered silicates are the commonest group of nanocomposites. Although first
reported by Blumstein in 1961, the real exploitation of this technology started in the
1990s.[51] Because of their nanometer size dispersions, nanocomposites exhibit
superior properties in comparison with pure polymer constituents or conventionally
filled polymers. The main advantages are light weight, high modulus and strength,
decreased gas permeability, increased solvent resistance and increased thermal stability.
Their mechanical properties are superior to unidirectional fibre-reinforced polymers
because reinforcement from the inorganic layers will occur in two rather than in one
dimension.[52] Because of the length scale involved that minimises scattering,
nanocomposites are usually transparent.[53] They also exhibit significant increases in
thermal stability as well as a selfextinguishing character. In polymer-layered silicates,
composite properties are achieved at a much lower volume fraction of reinforcement in
comparison with conventional fibre or mineral-reinforced polymers. They can be
processed by such techniques as extrusion and casting common to polymers which are
superior to the costly and cumbersome techniques used for conventional fibre and
mineral-reinforced composites and furthermore are adaptable to films, fibres and
monoliths. Most of the work in this area is at present at the experimental stage, although
some commercial explonation has been reported. For example, the Toyota Motor
Company is using an automotive timing-belt cover made from a nylon-layered silicate
nanocomposite. Potential applications are barrier films for food packaging, aeroplane
interiors, fuel tanks and components in electrical or electronic parts, brakes and
tyres.[54]
Wang and his co-workers prepared maleated polyethylene/silicate nanocomposites with
a different aspect ratio of silicate and maleated polyethylene/SiO2 composite by melt
intercalation. The nanocomposites with a high aspect ratio silicate (montmorillonite)
showed a faster decrease in the terminal slope of the storage modulus and a steeper
increase in complex viscosity than those with a low aspect ratio silicate (laponite) and
SiO2. The addition of montmorillonite increases the crystallization and the melting
temperature of maleated polyethylene but decreases above 3 vol % of the silicate
33
content because of the increased viscosity. The nanocomposite with montmorillonite
showed the highest yield strength and secant modulus among the composites because of
the highest aspect ratio of the filler. It also revealed strong interfacial adhesion with the
matrix and orientation during tensile deformation.[55]
In one study Gopakumar and his co-workers used to the melt compounding to prepare
conventional composites of montmorillonite clay and polyethylene (PE) as well as
nanocomposites of exfoliated montmorillonite platelets dispersed in a maleated
polyethylene matrix. PE/clay composites behaved in a similar manner as conventional
macrocomposites, exhibiting modest increases in their rheological properties and
Young's modulus. Conversely, the nanoscale dimensions of the dispersed clay platelets
in the nanocomposites led to significantly increased viscous and elastic properties and
improved stiffness. This was attributed to the high surface area between the polymer
matrix and the exfoliated clay, which resulted in enhanced phase adhesion.[56]
In study of Mishra and his co-workers, a thermoplastic polyolefin (TPO)/organoclay
nanocomposite was prepared by using maleic anhydride modified polypropylene as a
compatibilizer in melted state. It was shown that the nanocomposite exhibited
remarkable improvement of tensile and storage modulus over its pristine
counterpart.[57]
In a study on PNC, three cationic surfactants (hexadecyltrimethylammonium chloride,
hexadecyldimethylbenzylammonium chloride, and octadecyltrimethylammonium
chloride) were used to modify montmorillonite and polyethylene (PE)/maleic anhydride