CHAPTER 14: POLYMER STRUCTURES

1. CHAPTER 14:POLYMER STRUCTURES ISSUES TO ADDRESS... • What are the basic microstructural features? • How are polymer properties effected by molecular weight? • How do polymeric crystals accommodate the polymer chain? • What are the tensile properties of polymers and how are they affected by basic microstructural features? • How does the elevated temperature mechanical response of polymers compare to ceramics and metals?

2. H H H H H H H H H H H H H H H H H H C C C C C C C C C C C C C C C C C C H H H H H H H H H H CH3 H CH3 H CH3 Cl Cl Cl Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP) Chapter 14 – Polymers What is a polymer? Polymer manyrepeat unit repeat unit repeat unit repeat unit Adapted from Fig. 14.2, Callister 7e.

3. Ancient Polymer History • Originally natural polymers were used • Wood – Rubber • Cotton – Wool • Leather – Silk • Oldest known uses • Rubber balls used by Incas • Noah used pitch (a natural polymer) for the ark

4. Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and C • Saturated hydrocarbons • Each carbon bonded to four other atoms CnH2n+2

6. Chemistry of Polymers Adapted from Fig. 14.1, Callister 7e. Note: polyethylene is just a long HC - paraffin is short polyethylene

7. Bulk or Commodity Polymers

10. • Molecular weight, Mi: Mass of a mole of chains. Lower M higher M Mwis more sensitive to higher molecular weights MOLECULAR WEIGHT Adapted from Fig. 14.4, Callister 7e.

11. Molecular Weight Calculation Example: average mass of a class

12. Degree of Polymerization, n n = number of repeat units per chain ni = 6

13. Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bonds • note: no bond breaking needed Adapted from Fig. 14.5, Callister 7e.

14. End to End Distance, r Adapted from Fig. 14.6, Callister 7e.

15. secondary bonding Linear B ranched Cross-Linked Network Molecular Structures • Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.7, Callister 7e.

16. Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain syndiotactic – R groups alternate sides atactic – R groups random

17. Copolymers Adapted from Fig. 14.9, Callister 7e. two or more monomers polymerized together • random – A and B randomly vary in chain • alternating – A and B alternate in polymer chain • block – large blocks of A alternate with large blocks of B • graft – chains of B grafted on to A backbone A – B – random alternating block graft

18. 10 nm Polymer Crystallinity Adapted from Fig. 14.10, Callister 7e. Ex: polyethylene unit cell • Crystals must contain the polymer chains in some way • Chain folded structure Adapted from Fig. 14.12, Callister 7e.

19. Polymer Crystallinity Polymers are rarely 100% crystalline • Too difficult to get all those chains aligned crystalline region • % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. amorphous region 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.)

20. Mechanical Properties • i.e. stress-strain behavior of polymers brittle polymer FS of polymer ca. 10% that of metals plastic elastomer elastic modulus – less than metal Adapted from Fig. 15.1, Callister 7e. Strains – deformations > 1000% possible (for metals, maximum strainca. 10% or less)

21. Thermoplastics vs. Thermosets • Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene • Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin

22. Processing of Plastics • Thermoplastic – • can be reversibly cooled & reheated, i.e. recycled • heat till soft, shape as desired, then cool • ex: polyethylene, polypropylene, polystyrene, etc. • Thermoset • when heated forms a network • degrades (not melts) when heated • mold the prepolymer then allow further reaction • ex: urethane, epoxy

23. T and Strain Rate: Thermoplastics 8 0 6 0 4 0 20 s (MPa) • Decreasing T... -- increases E -- increases TS -- decreases %EL • Increasing strain rate... -- same effects as decreasing T. Data for the 4°C semicrystalline polymer: PMMA 20°C (Plexiglas) 40°C to 1.3 60°C 0 e 0 0.1 0.2 0.3 Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)

24. Melting vs. Glass Transition Temp. What factors affect Tm and Tg? • Both Tm and Tg increase with increasing chain stiffness • Chain stiffness increased by • Bulky sidegroups • Polar groups or sidegroups • Double bonds or aromatic chain groups • Regularity – effects Tm only Adapted from Fig. 15.18, Callister 7e.

25. Polymer Additives Improve mechanical properties, processability, durability, etc. • Fillers • Added to improve tensile strength & abrasion resistance, toughness & decrease cost • ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. • Plasticizers • Added to reduce the glass transition temperature Tg • commonly added to PVC - otherwise it is brittle

26. Polymer Additives • Stabilizers • Antioxidants • UV protectants • Lubricants • Added to allow easier processing • “slides” through dies easier – ex: Na stearate • Colorants • Dyes or pigments • Flame Retardants • Cl/F & B

27. Polymer Types: Elastomers Elastomers – rubber • Crosslinked materials • Natural rubber • Synthetic rubber and thermoplastic elastomers • SBR- styrene-butadiene rubber styrene butadiene – Silicone rubber

28. Polymer Types: Fibers Fibers - length/diameter >100 • Textiles are main use • Must have high tensile strength • Usually highly crystalline & highly polar • Formed by spinning OR • the fibers are drawn • leads to highly aligned chains- fibrillar structure

29. .357 Magnum, 22 Layers of Kevlar

30. Polymer Types • Coatings – thin film on surface – i.e. paint, varnish • To protect item • Improve appearance • Electrical insulation • Adhesives – produce bond between two adherands • Usually bonded by: • Secondary bonds • Mechanical bonding • Films – blown film extrusion • Foams – gas bubbles in plastic

31. Advanced Polymers • Ultrahigh molecular weight polyethylene (UHMWPE) • Molecular weight ca. 4x106 g/mol • Excellent properties for variety of applications • bullet-proof vest, golf ball covers, hip joints, etc. UHMWPE Adapted from chapter-opening photograph, Chapter 22, Callister 7e.

32. •SWNTs are unique:•Polymers of pure carbon•High aspect ratio (>1000:1)•Unique electron configuration•SWNTs have extraordinary properties•Strength (~100x steel)•Electrical conductivity (~Copper)•Thermal conductivity (~3x Diamond)•Accessible surface area (theoretical limit)•Thermal stability (~500 °C air, ~1400 °C anaerobic)•SWNTs can be customized via organic chemistry•Modification of properties•Compatibilization with other materials~nm~1 nm1000 1000 nm ––10,000 nm10,000 nmA polymerA new backbone polymerThe The most inherently conductive organic moleculemoleculeThe The most thermally conductive materimaterialalThe The strongest, stiffest, toughest molecule bemolecule there will ever beThe emitterThe best field emitterThe The ultimate engineering polymerengineering polymerA

34. Plastics & Environment 1 year’s supply (EU) = 220 million

35. Estimation done by Pacebutler Corporation says that $15,000 worth precious metals can be extracted by recycling 1 ton of discarded cell phones. • Only little more than 12% of the E-Waste is getting recycled.Read more at http://www.inquisitr.com/908167/cell-phone-recycling-facts-and-statistics-for-united-states-2013/#LTDFlCc4vttBwDUT.99

37. RETHINK! saves 520 bag/year 1/week

38. Recycling Economics Sales Contributions = $236 billion annually in the US (EPA) 21.2% = Rs. 271 million (US $4.5 million) annually in Lahore! Gap = Opportunity!

39. 25 minutes! OR 6 hours!

40. 2000 lbs! 1 ton + 1 year! + + + 2 months! 1.8 lbs!

41. 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 • Plastics & Environment -- Practice 4 R’s; Reduce, Reuse, Recycle, Rethink! Table 15.3 Callister 7e: Good overview of applications and trade names of polymers.