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?

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

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

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

Chemistry of Polymers Note: polyethylene is just a long HC Adapted from Fig. 14.1, Callister 7e. Polymer- can have various lengths depending on number of repeat units Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity Polymers Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains. Lower M higher M Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes Mw is more sensitive to higher molecular weights Adapted from Fig. 14.4, Callister 7e.

Molecular Weight Calculation Example: average mass of a class

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

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.

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

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

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

Copolymers two or more monomers polymerized together 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

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. 10 nm

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.)

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 strain ca. 10% or less)

Thermoplastics vs. Thermosets -- 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

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 Can be brittle or flexible & linear, branching, etc.

T and Strain Rate: Thermoplastics (MPa) • Decreasing T... -- increases E -- increases TS -- decreases %EL • Increasing strain rate... -- same effects as decreasing T. 20 4 6 8 Data for the 4°C semicrystalline polymer: PMMA 20°C (Plexiglas) 40°C to 1.3 60°C e 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.)

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.

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 Polymers are almost never used as a pure material Migration of plasticizers can be a problem

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

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

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

.357 Magnum, 22 Layers of Kevlar

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

Advanced Polymers Ultrahigh molecular weight polyethylene (UHMWPE) Molecular weight ca. 4 x 106 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.

•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

Poly (p-phenylene-2,6-benzobisoxazole) (PBO) Zylon has 5 Poly (p-phenylene-2,6-benzobisoxazole) (PBO) Zylon has 5.8 GPa of tensile strength,[3] which is 1.6 times that of Kevlar. Like Kevlar, Zylon is used in a number of applications that require very high strength with excellent thermal stability. Tennis racquets, table tennis blades, various medical applications,

Plastics & Environment To replace all the plastic bags being used in the European Union with paper/year, need to cut down an additional 2.2 million trees. This is the same as chopping down 110 Km2 of forest / forest! According to CDA area of Islamabad minus the hills is 906.5 sq km! So a forest as big as Islalmabad will be chopped down every year to make paper bags! 1 year’s supply (EU) = 220 million

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

RETHINK! saves 520 bag/year 1/week

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

25 minutes! OR 6 hours!

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

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.