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Chapter15c - Chapter Outline Characteristics Applications...

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1 1 MSE 2090: Introduction to Materials Science Chapter 15, Processing of Polymers Chapter Outline: Characteristics, Applications, and Processing of Polymers ¾ Mechanical properties Stress-Strain Behavior Deformation of Semicrystalline Polymers ¾ Crystallization, Melting, Glass Transition ¾ Thermoplastic and Thermosetting Polymers ¾ Viscoelasticity ¾ Deformation and Elastomers ¾ Fracture of Polymers ¾ Polymerization ¾ Elastomers Optional reading: 15.6, 15.17-15.19, 15.21-15.24 “The Graduate” 1967: Mr. McGuire: I want to say one word to you. Just one word . Benjamin: Yes, sir . Mr. McGuire: Are you listening? Benjamin: Yes, I am . Mr. McGuire: Plastics. 2 MSE 2090: Introduction to Materials Science Chapter 15, Processing of Polymers ¾ The description of stress-strain behavior is similar to that of metals Stress – Strain Behavior (I) Polymers can be brittle ( A ), plastic ( B ), or highly elastic ( C ). Deformation shown by curve C is totally elastic (rubber-like elasticity, large recoverable strain at low stress levels). This class of polymers - elastomers A: Brittle Polymer B: Plastic Polymer C: Elastomer 3 MSE 2090: Introduction to Materials Science Chapter 15, Processing of Polymers Stress – Strain Behavior (II) Characteristics of stress-strain behavior: ¾ Modulus of elasticity – defined as for metals ¾ Ductility (%EL) – defined as for metals ¾ Yield strength - For plastic polymers (B), yield strength is defined by the maximum on curve just after the elastic region (different from metals) ¾ Tensile strength is defined at the fracture point and can be lower than the yield strength (different from metals) 4 MSE 2090: Introduction to Materials Science Chapter 15, Processing of Polymers ¾ 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. ¾ Polymers are also very sensitive to the rate of deformation (strain rate). Decreasing rate of deformation has the same effect as increasing T. Stress – Strain Behavior (III) Moduli of elasticity for polymers are ~ 10 MPa - 4 GPa (compare to metals ~ 50 - 400 GPa) Tensile strengths are ~ 10 - 100 MPa (compare to metals, hundreds of MPa to several GPa) Elongation can be up to 1000 % in some cases (< 100% for metals)
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2 5 MSE 2090: Introduction to Materials Science Chapter 15, Processing of Polymers Stress – Strain Behavior (IV) Temperature increase leads to: 9 Decrease in elastic modulus 9 Reduction in tensile strength 9 Increase in ductility polymethyl methacrylate (PMMA) - Plexiglas The glass transition temperature (T g ) of PMMA ranges from 85 to 165 °C – all of the above curves are for temperatures below T g . Viscoelasticity (I) ¾ Amorphous polymer: glass at low temperatures, rubber at intermediate temperatures, viscous liquid at high T.
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