CHAPTERS 14/15: POLYMER STRUCTURES, APPLICATIONS, & PROCESSING

1. CHAPTERS 14/15:POLYMER STRUCTURES, APPLICATIONS, & PROCESSING • Naturally occurring polymers: • Wood, rubber (Hevea brasiliensis, or Ficus elastica), cotton, wool, leather, silk, starches, etc etc • Biopolymers RNA, DNA etc etc • All livings things are made out of biopolymers • Our brains are essentially bio-polymer processing units, instead of the silicon ones in our computers… • After WWII, synthetic polymers started taking a hold of our daily lives… • Hydrocarbons, Carbon and Hydrogen backbones 1

2. Hydrocarbons (I) • Saturated hydrocarbons molecules • All single bonds • All are gases at RT

3. Hydrocarbons (II) • Unsaturated hydrocarbons molecules • Double or triple bonds between C atoms • More active

4. Hydrocarbons (III) • RADICALS: • Other organic groups can be involved in polymer molecules. In table adjacent R represents radicals: Organic groups of atoms that remain as a unit and maintain their identity during chemical reactions (e.g. CH3, C2H5, C6H5)

5. Polymers • Polymer molecules are very large, gigantic in size: called sometimes macromolecules • Within these atoms are covalently bonded to each other • Most polymers consist of long and flexible chains with a string of C atoms as a backbone. • Side-bonding of C atoms to H atoms or radicals • Double bonds possible in both chain and side bonds • Repeat unit in a polymer chain (“unit cell”) is a mer • A single mer is called a monomer • Many mers is a polymer

6. Chemistry of Polymerization • Ethylene (C2H4) is a gas at STP (RT and pressure) • Ethylene transform to poly-ethylene (solid) by forming active mer through reaction with initiator or catalytic radical (R.) • (.) denotes unpaired electron (active site) • C-C bond length 0.154 nm At certain T and P with polymerize with the help of the Radicals

7. Polymer Microstructure • C-C bond is not 180º, but 109º • Furthermore, chain rotations act as kinks • Double or triple bonds are rigid

8. POLMER MICROSTRUCTURE • Polymer = many mers Adapted from Fig. 14.2, Callister 6e. • Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.7, Callister 6e. 2

9. More on Polymer Microstructure

10. Molecular Weight (I) • Very large molecules can be found in polymers • Final molecular weight (chain length) is controlled by relative rates of initiation, propagation, termination steps of polymerization • Not all chains will grow to the same size hence, formation of macromolecules during polymerization results in distribution of chain lengths and molecular weights • The average molecular weight can be obtained by averaging the masses with the fraction of times they appear (number-average molecular weight) or with the mass fraction of the molecules (weight-average molecular weight).

11. Molecular Weight (II) • Melting / softening temperatures increase with molecular weight (up to ~ 100,000 g/mol) • At room temperature, short chain polymers (molar weight ~ 100 g/mol) are liquids or gases, intermediate length polymers (~ 1000 g/mol) are waxy solids, solid polymers have molecular weights of 104 - 107 g/mol

12. Molecular Shape • Molecular chains can bend, coil and kink due to single C-C bonds rotating on each other • Neighboring chains may intertwine and entangle • Large elastic extensions of rubbers correspond to unraveling of these coiled chains and getting straighter • Mechanical / thermal characteristics depend on the ability of chain segments to rotate, remember double C=C bonds are stiffer.

13. Molecular Structure • Linear polymers: Weak or Van der Waals bonding betweenchains. Examples are polyethylene, nylon, and PVC. • Branched polymers: Chain packing efficiency isreduced compared to linear polymers - lower density • Cross-linked polymers: Chains are connected bycovalent bonds. Often achieved by adding atoms ormolecules that form covalent links between chains.Many rubbers have this structure. • Network polymers: 3D networks made fromtrifunctional mers, covalent bonds. Have distinct mechanical and thermal properties Examples: epoxies, phenolformaldehyde.

14. Isomerism • Hydrocarbon compounds with same composition can assume different atomic arrangements. • Physical properties may also depend on isomeric state (e.g. boiling temperature of normal butane is -0.5 °C, of isobutane -12.3 °C) • There are two main types of isomerism and each have sub groups underneath: • Stereoisomerism: spatial pattern of atoms or functional groups attaching themselves to the chain link • Isotatic=> all at the same side • Syndiotactic=> alternating sides • Atactic=> random positions

15. Summary Natural rubber Inelastic natural latex -used in early golf ball cores

16. Copolymers • Copolymers, are polymers which has at least two different types ofmers. • They can differ in theway the mers are arranged:

17. Polymer Crystallinity (I) polyethylene Atomic arrangementin polymer crystalsis more complexthan in metals orceramics. The unit cellsare typically very largeandcomplex as molecules or chains replace ions and or atoms in these structures. Think of it as packing of molecular chains in a geometrical array

18. Polymer Crystallinity (II) • Molecular substance such as water, methane etc solidify as crystals and are totally amorphous in liquid phase • Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material. Because, any disorder, kink in the long chains induce an amorphous region. • Degree of crystallinity: Factors effecting crystallinity: • Rate of cooling during solidification: time is necessary for chains to move and align into a crystal structure • Mer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easily • Chain configuration: linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, crosslinked polymers can be both crystalline and amorphous • Isomerism: isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize • Copolymerism: easier to crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graft • More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating

19. Polymer Crystallinity (III)

20. Examples

21. MOLECULAR WEIGHT & CRYSTALLINITY • Molecular weight, Mw: Mass of a mole of chains. • Tensile strength (TS): --often increases with Mw. --Why? Longer chains are entangled (anchored) better. • % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases. Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.) 3

22. TENSILE RESPONSE: BRITTLE & PLASTIC Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.) 4

23. Stress-Strain Behavior of Polymers (I) • Overall similar to metals, but… • The stress-strain behavior can be brittle (A),plastic (B),and highly elastic (C) • Deformation shown by curve C is totally elastic (rubberlikeelasticity). This class of polymers - elastomers Elastic Modulus: same as metals Ductility (%EL): same as metals Yield strength: type B curves, maximum on the curve right after elastic region Tensile Strength: defined as the fracture strength, can be lower than YS; different than metals

24. TENSILE RESPONSE: ELASTOMER CASE Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along elastomer curve (green) adapted from Fig. 15.14, Callister 6e. (Fig. 15.14 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) • Compare to responses of other polymers: --brittle response (aligned, cross linked & networked case) --plastic response (semi-crystalline case) 6

25. Stress-Strain Behavior of Polymers (II) Some points with polymers: • Moduli of elasticity are ~ 10 MPa - 4 GPa (metals ~ 50 - 400 GPa) • Tensile strengths are ~ 10 - 100 MPa (metals ~ 100 MPa to 10 GPa) • Percent elongation can be up to 1000 % in some cases (< 100% for metals) • WHY ? • Mechanical properties of polymers change dramatically with temperature, going from glass-like brittle behavior at low temperatures to a rubber-like behavior at high temperatures. • Polymers are also very sensitive to the rate of deformation (strain rate). Decreasing rate of deformation has the same effect as increasing…

26. Stress-Strain Behavior of Polymers (III) Impact of temperature: • Decrease in elastic modulus • Reduction in tensile strength • Increase in ductility polymethyl methacrylate (PMMA) plexiglass

27. Deformation (I) Elastic deformation: • Basic mechanism of elastic deformation is elongation (straightening) of chain molecules in the direction of the applied stress. Elastic modulus is defined by elastic properties of amorphous and crystalline regions and by the microstructure. Plastic deformation: • Plastic deformation is defined by the interaction between crystalline and amorphous regions, and is partially reversible. Stages of plastic deformation: 1. elongation of amorphous chains 2. tilting of lamellar crystallites towards the tensile axis 3. separation of crystalline block segments 4. stretching of crystallites and amorphous regions along tensile axis IMPORTANT: where are the dislocations ?

28. Deformation (II) • The macroscopic deformation involves necking. Neck region gets stronger since the deformation aligns the chainsand increases local strength in the neck region (up to2-5 times) Neck will expand along the specimen. • What is different from metals is ? The neck region will expand ! Whereas in metals deformation will be limited to the neck region !

29. Factors that influence mechanical properties (I) • Temperature and strain rate • Chain entanglement, strong intermolecular bonding (van der Waals, cross-links) increase strength • Drawing, analog of work hardening in metals, corresponds to the neck extension. Is used in production of fibers and films. Molecular chains become highly oriented: • properties of drawn material are anisotropic • perpendicular to the chain alignment direction strength is reduced • Heat treatment - changes in crystallite size and order • undrawn material: Increasing annealing temperature leads to • increase in elastic modulus • increase in yield/tensile strength • decrease in ductility • drawn material: opposite changes (due to recrystallization and loss of chain orientation) Note that these changes are opposite from metals

30. Factors that influence mechanical properties (II) • Tensile strength increases with molecular weight – effect of entanglement • Higher degree of crystallinity – stronger secondary bonding - stronger and more brittle material

31. SUMMARY • General drawbacks to polymers: -- E, sy, Kc, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. • Thermoplastics (PE, PS, PP, PC): -- Smaller E, sy, Tapplication -- Larger Kc -- Easier to form and recycle • Elastomers (rubber): -- Large reversible strains! • Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication -- Smaller Kc Table 15.3 Callister 6e: Good overview of applications and trade names of polymers. 10

32. ANNOUNCEMENTS Reading: Chapters 14 and 15 Core Problems: Self-help Problems: 0