ChE541_Basic Concepts and Definitions

HISTORICAL OVERVIEW Age of Polymers

1839 Vulcanization of natural rubber 1868 Celluloid 1909 Bakelite

References: 1. Flory, P.J., "Principles of Polymer Chemistry",

Cornell University Press, Ithaca, NY(1953). 2. Brydson, J.A., "Plastics Materials", 3rd edition,

Butterworths, London (1975). 3. Stahl, G.A., "A Short History of Polymer

Science", ACS Symposium Series 175, American Chemical Society, Washington D.C. (1981).

4. Mark, H.F., "Polymer Chemistry: The Past 100

Years", C&EN, p.176 (April 6, 1976) 5. Bolton, E.K., I/EC, 34(1), 53-58 (Jan. 1942)

History and uses of nylon.

BASIC CONCEPTS AND DEFINITIONS

Polymer: Many parts

1920 Staudinger

Large molecule (macromolecule) made up of smaller repeating units

Polymers have molecular weights varying from

5x103 to several millions Examples:

Natural polymers: Cellulose: polysacharide Proteins: polyaminoacids DNA: polynucleotide

Synthetic polymers:

Polyethylene (PE) Polyvinylchloride (PVC)

Cl Cl Cl Cl Cl

Monomer: Only one part

A small molecule which reacts with other molecules of the same or different type to form a polymer

Example:

Styrene Polystyrene (PS)

Oligomer: A few parts

Low molecular weight ( < 5000 )

Repeat unit: Segment of the chain or macromolecule which repeats at regular intervals

Examples: Polymethylmethacrylate Polypropylene Polyvinylchloride

Functionality # of sites on monomer molecule available for bonding to other molecules under polymerization conditions

Degree of polymerization (DP)

Number of repeat units in the polymer molecule

( MW )chain = DP * ( MW )monomer

Homopolymer: Polymer derived from one type of monomer

Copolymer: Contains structural units of two or

more different monomers Random - AABABABBBABABABABB - Alternating- ABABABABABABABABAB - Block - AAABBBBBBAAAABBBBB - Graft - AAAAAAAAAAAAAAAAAA -

B B B B

NOMENCLATURE OF POLYMERS Polymer nomenclature has been largely a matter of custom without any system being universally accepted. It is not unusual for a polymer to have several different commonly used names due to different nomenclature systems.

IUPAC nomenclature systems are currently under consideration

Based on source: applicable to polymers

synthesized from a single monomer

"Poly"monomer Examples: Polyacrylonitrile

Poly(carbonate)

Based on structure: applicable to polymers synthesized from two monomers

"Poly" (name of structural grouping attached to the parent compound)

The parent compound is the particular member of the class of the polymer - the particular ester, amide, urethane and so on.

Examples Poly(ethylene terephthalate) Poly(hexamethylene adipamide)

TRADE NAMES: Dacron, teflon, nylon

MOLECULAR ARCHITECTURE

Linear polymers: Each repeating unit is linked only to two polymers

-- CH2 - CH2 - CH2 --

Branched polymers: Same repeating units can be linked to more than two others, and form branches which are not present in the monomers

-- CH2 - CH - CH2 -- CH2 CH2

Network polymers: Inter-connected polymers of

extremely high MW

They can be formed: a) directly from monomers

e.g. polyester b) by cross-linking linear or

branched polymers (e.g. vulcanization of rubber)

CHAIN STRUCTURE Basic chain properties:

Molecular weight and its distribution Conformation Configuration

Examples: Configurations Head-to-tail

Head-to-head

Conformations (trans-gauche) Bond rotational angle

Conformations Random flight or freely jointed chain For a random walk process Then:

∑=r

rPrr )(22

22

222

23

)exp()(

nlb

rbArrP

=

−=

r−

22 nlr =

• Root-mean-square of end-to-end distance

• Real chains

lnr 2/12/12 =

22 Cnlr =

STEREOCHEMISTRY OF REPEATING UNITS Chiral centers Chiral

centre

Enantiomorphs

Tacticity in polymers

Monosubstituted ethylene polymers

Pseudochiral centers (no optical activity)

Tacticity

Different spatial arrangements

CLASSIFICATION OF POLYMERS 1. Method of formation

Step-growth or condensation

- Monomers with reactive groups (e.g. carboxyls) - Slow reaction

Chain or addition

- Vinyl monomers (e.g. monomers with C=C ) - Fast reaction

2. Morphology

Amorphous: Disordered state; chains in random coil arrangement; no sharp melting point

Examples: PS / PVAc

Crystalline: Ordered state; chains exist in well-defined shapes; sharp melting point; chemical and geometric regularity required

Examples: Linear PE / Nylon

Polymers can be found in two states reflecting the shapes (conformations) of polymeric chains

Amorphous Crystalline

In the amorphous state polymer chains have random conformations associated with the flexibility of chemical bonds along the chains

An important concept associated with the amorphous state is that of physical entanglements When more than one polymer molecules with random conformation coexist, entanglements can result Chain entanglements are largely responsible for the transfer of forces between chains in spite of the absence of bonds

force force

Chain at rest Entanglement

Some polymeric chains are capable of crystallizing Crystallinity implies a regular ordering of the repeat units into unit cells on a very small scale, lamellae on a larger scale and spherulites on an even larger scale

lamellae

spherulites

In the crystallization process, solid material starts forming at discrete points called nuclei The solid material then grows as spherulites until the whole melt has turned into a solid consisting of adjacent fully grown spherulites

Depending on the number of nuclei involved, the crystallized material may feature many small spherulites or few large ones

fine coarse

Polymers are never perfectly crystalline materials

A semi-crystalline polymer can be viewed as a combination of crystals (lamellae) and amorphous material in-between

Degree of crystallinity (DC)

Properties depend on DC and structure Examples: Transparency / Toughness

3. Mechanical behaviour

Why do polymers form useful materials?

i) Large molecules ii) High intermolecular forces iii) Entanglements

Elastomers: Recover instantly from extensive stretching (up to 100 %) Low initial modulus (up to 1000 psi) Modulus increase with decreasing temperature

Fibres: High initial modulus (106 psi)

Low extension at break Deformation only partially recoverable

Plastics: Intermediate modulus (104 - 105 psi)

Breaking strain varies from a few percent (e.g. PS) to 400% (PE) Elastic recovery generally very small

Elastomers Flexible chains with weak intermolecular

forces, do not crystallize. Recoverability due to cross-linking

Fibres Stiff chains with strong

intermolecular forces, highly crystallizable

Plastics Variable chain stiffness, generally weak

intermolecular forces, usually crystallize (due to regular, symmetric chain structures)

4. Processing characteristics

Thermoplastic: Can be heated and softened to a viscous fluid which can be extruded or moulded; hardens on cooling and retains the shape imposed; temperature limit, degradation; can be heated and reformed repeatedly; linear or branched not cross-linked materials; will dissolve.

Examples: PE, PP, PVC, PTFE, PS

Thermosetting: Reactive materials; polymerization and processing usually done in the same step; 2 components, prepolymer & curing agent; once shaped can not be reformed via heating; can not be dissolved.

Examples: Epoxies (adhesives), phenol-

formaldehyde resins

Commodity Resins Olefinics Vinylics Styrenics Acrylonitrilics Acrylics Misc. Copolymers & Polyalloys Cellulosics Engineering Resins Polyamides Polyesters Polycarbonates Sulfone Polymers Imide Polymers Ether-oxide polymers Ketone Polymers Fluoropolymers Misc. Heterochain polymers

THERMAL TRANSITIONS Change of state with temperature

Glass transition temperature (Tg)

Amorphous polymers

Virtual cessation of local molecular motions

Chains become stiff; material becomes glass-like

Macroscopic effects: discontinuity of temperature derivatives of enthalpy, entropy & specific volume

Effect of molecular weight

Effect of chemical structure

Effect of chain topology Copolymerization

Branching & X-linking

Melting point (Tm)

Crystalline polymers; crystal lattice destroyed above Tm

Effect of chain flexibility and other steric factors

Effect of copolymerization

POLYMER STRUCTURE AND PHYSICAL PROPERTIES

Properties Small deformations Large deformations e.g. electrical e.g. tensile strength optical melt properties yield stress impact strength

Properties involving large deformations depend primarily on the long-chain nature of polymers and the configuration; MW, MWD, branching

Properties involving small deformations are

influenced by factors determining the manner in which chain atoms interact at small distances; symmetry, steric effects, ability to crystallize, polar groups