MSE200 Lecture 16 CH 10 1 10 2 10 4 1 10 4 4 Polymer Materials Instructor Yuntian Zhu Objectives outcomes You will learn the following Polymerization reactions The degree of polymerization Addition polymerization and copolymers Structure of partly crystalline polymers Stereoisomerism Linear and network polymers http www mse ncsu edu zhu Introduction to Polymers Polymers many parts Poly mers many mers Polymers e g rubber Plastics Thermoplastics Can be reheated and formed into new materials Elastomers Can be recycled Can melt Thermosetting Plastics Cannot be reformed by reheating Set by chemical reaction Can not be recycled Decompose http www mse ncsu edu zhu Plastics Advantages Advantages Minimum finishing Good insulation Light weight Noise Reduction http www mse ncsu edu zhu Disadvantages Low strength Polymerization http video google com videosearch hl en q plaastics ndsp 18 ie UTF 8 sa N tab iv q polymerization hl en emb 0 Chain growth polymerization Monomers link together to form long chains polymers Example Ethylene Ethylene molecule unsaturated H H mer H H C C H H n n degree of Polymerization DP range 3500 25000 Heat n C C H H Pressure Catalyst Double covalent bond DP n Single covalent bond saturated Functionality Number of active bonds in a monomer http www mse ncsu edu zhu Chain Polymerization 3 Steps Step 1 Initiation A Radical is needed Example H2O2 In General R is a chemical group One of free radicals react with ethylene molecule to form new longer chain free radical http www mse ncsu edu zhu Chain Polymerization Steps cont Step 2 Propagation Process of extending polymer chain by addition of monomers R CH2 CH2 CH2 CH2 R CH2 CH2 CH2 CH2 Energy of system is lowered by polymerization Step 3 Termination By addition of termination free radical Combining of two chains Impurities R CH2 CH2 m R CH2 CH2 n R CH2 CH2 m R CH2 CH2 n R Coupling of two chains http www mse ncsu edu zhu Average Molecular Weight Average molecular weight Mm fi Mi Example M m average molecular weight of thermoplastics Mi Mean molecular weight of each molecular range selected fi Weight fraction of the material having Molecular weights of a selected molecular Weight range http www mse ncsu edu zhu Structure of Noncrystalline Linear Polymers Zig Zag configuration in polyethylene due to 109o angle between carbon covalent bonds Chains are randomly entangled Entanglement increases tensile strength http www mse ncsu edu zhu Vinyl and Vinylidene Polymers Vinyl polymers One of the hydrogen atom is replaced by another atom or group of atoms Vinylidene Polymers Both hydrogen on a carbon are replaced by another atom or group of atoms http www mse ncsu edu zhu Homopolymer and Copolymers Homopolymers Polymer chain is made up of single repeating units mers Example AAAAAAAA Copolymers Polymer chains made up of two or more repeating units Random copolymers Different monomers randomly arranged in chains ABBABABBAAAAABA Alternating copolymers Definite ordered alterations of monomers ABABABABABAB Block copolymers Different monomers arranged in long blocks AAAAA BBBBBBBB Graft copolymers One type of monomer grafted to long chain of another AAAAAAAAAAAAAAAAAAA B B B B B B http www mse ncsu edu zhu Other Methods of Polymerization Stepwise Polymerization Monomers chemically react with each other to produce linear polymers and a small molecule of byproduct Network polymerization Chemical reaction takes place in more than two reaction sites 3D network http www mse ncsu edu zhu Solidification of Thermoplastics There is no sudden change in specific volume on cooling in noncrystalline thermoplastics Tg glass transition temperature Glass brittle below Tg above Rubbery Tg for polyethylene is 1100C For PVC it is 820C In crystalline thermoplastics sudden decrease in specific volume occurs due to more efficient packing of polymer chains http www mse ncsu edu zhu Structure of Partly Crystalline Thermoplastics Longest dimension of crystalline region is 5 50 nm Fringed micelle model Long polymer chains of 5000 nm wandering successively through a series of disordered and ordered region Folded chain model sections of molecular chains folding on themselves Polyethylene folded chain http www mse ncsu edu zhu Stereoisomerism in Thermoplastics Stereoisomer Same chemical composition but different structural arrangements Atactic stereoisomer Pendent methyl group of polypropylene is randomly arranged on either side of main carbon chain Isotactic stereoisomer The pendent methyl group is always on same side of the carbon chain Syndiotactic stereoisomer The pendant group regularly alternates from one side of the chain to the other side Polychlorotrifluroethylene PCTFE http www mse ncsu edu zhu Homework Example problems 10 1 10 2 10 3 10 4 10 5 HW download from the website Reading assignment 10 9 10 11 http www mse ncsu edu zhu