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ME 260: Introduction to Engineering Materials CHAPTER 14 Polymer Structures Chapter 15. Polymer Structures 14.1 ME 260: Introduction to Engineering Materials INTRODUCTION Natural Polymers Leather, wood, rubber, cellulose, cotton, wool. Synthetic polymers Synthesized from small organic molecules. CHEMISTRY OF POLYMER MOLECULES Hydrocarbons (many organic materials)= composed of hydrogen and carbon Covalent Bonding H H-C-H Similarly H Methane Ethylene Chapter 15. Polymer Structures 14.2 ME 260: Introduction to Engineering Materials POLYMERISATION Mers = small structural entities making up the ‘poly’mer. Ethylene(C2H4) gas CAN BE CONVERTED TO POLYETHYLENE(PE) Solid Catalyst bonding unsatisfied for this C atom bonding satisfied for C atom Chapter 15. Polymer Structures 14.3 ME 260: Introduction to Engineering Materials RESULT Angle =109o Chapter 15. Polymer Structures 14.4 ME 260: Introduction to Engineering Materials Similarly Unpaired electron Methyl group Chapter 15. Polymer Structures 14.5 ME 260: Introduction to Engineering Materials Chapter 15. Polymer Structures 14.6 ME 260: Introduction to Engineering Materials MOLECULAR WEIGHT Number average Not all chains same length Weight-average Chapter 15. Polymer Structures 14.7 ME 260: Introduction to Engineering Materials For copolymers Degree of polymerization Molecular weight of mer j Chapter 15. Polymer Structures 14.8 ME 260: Introduction to Engineering Materials Linear e.g. Polyethylene, Nylon Branched Lower density Crosslinked Network e.g. Rubber e.g. Epoxy Chapter 15. Polymer Structures 14.9 ME 260: Introduction to Engineering Materials HOMOPOLYMERS COPOLYMERS Polymers composed of two or more different ‘mer’ units Bifunctional, trifunctional mers Chapter 15. Polymer Structures 14.10 ME 260: Introduction to Engineering Materials Styrene-butadiene rubber (SBR) random copolymer Used in automobile tires. Chapter 15. Polymer Structures 14.11 ME 260: Introduction to Engineering Materials POLYMER CRYSTALLINITY = Packing of molecular chains so as to produce an ordered atomic array. Small molecules (Methane, H2O) either either totally crystalline (as solids) or amorphous (as liquids) However POLYMERS only partially crystalline (semi-crystalline) Max. crystallinity ~ 95%. Chapter 15. Polymer Structures 14.12 ME 260: Introduction to Engineering Materials Properties greatly affected by degree of crystallinity (crystalline = stronger and more resistant to softening by heat. important to quantify degree crystallinity Crystalline polymer = higher density Chapter 15. Polymer Structures 14.13 ME 260: Introduction to Engineering Materials Slower cooling rates from the melt favor higher crytallinity (giving time for ordered configurations) Linear Polymers Crystallization easily accomplished, since no restriction to chain alignment Branched polymers NEVER highly crystalline Side branches interfere with crystallization Network Polymers AMORPHOUS COPOLYMERS More Irregular and random mer less crystallinity Alternating and block copolymers likelihood of crystallization. Random and Graft copolymers Normally Amorphous Chapter 15. Polymer Structures 14.14