PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering1Chapter Outline: Characteristics, Applications, and Processing of PolymersMechanical propertiesStress-Strain BehaviorDeformation of Semicrystalline PolymersCrystallization, Melting, Glass TransitionThermoplastic and Thermosetting PolymersViscoelasticity Deformation and ElastomersFracture of PolymersPolymerizationElastomersOptional reading: 15.6, 15.8, 15.14-15.15, 15.17-15.19, 15.21-15.24Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering2Stress – Strain Behavior (I)Stress-strain behavior can be brittle (A), plastic (B), and highly elastic (C) Curve C is totally elastic (rubber-like elasticity). This class of polymers - elastomersA: Brittle Polymer B: Plastic Polymer C: ElastomerIntroduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering3Stress – Strain Behavior (II)Elastic Modulus – defined as for metalsDuctility (%EL) – defined as for metalsYield strength – For plastic polymers (B) maximum on curve just after the elastic region (different from metals)Tensile strength is defined at the fracture point and can be lower than the yield strength (different from metals)Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering4Stress – Strain Behavior (III)•Moduli of elasticity•Polymers: ~ 10 MPa - 4 GPa •Metals: ~ 50 - 400 GPa•Tensile strengths•Polymers: ~ 10 - 100 MPa •Metals: 100’s - 1000’s MPa •Elongation•Polymers: up to 1000 % in some cases •Metals: < 100%Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering5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. Polymers are also very sensitive to the rate of deformation (strain rate). Decreasing rate of deformation has the same effect as increasing T.Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering6Stress – Strain Behavior (IV)Temperature increase leads to:Decrease in elastic modulusReduction in tensile strengthIncrease in ductilitypolymethyl methacrylate (PMMA)Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering7Deformation of Semicrystalline Polymers Elastic deformation:Elongation (straightening) of chain molecules in direction of applied stress.Elastic modulus: determined by elastic properties of amorphous and crystalline regions and the microstructure.Semi-crystalline polymers: crystalline regions separated by amorphous materialIntroduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering8Plastic Deformation of Semicrystalline Polymers Plastic deformation: defined by interaction between crystalline and amorphous regions and is partially reversible.Stages of plastic deformation:1. elongation of amorphous tie chains2. tilting of lamellar crystallites towards the tensile axis3. separation of crystalline block segments4. stretching of crystallites and amorphous regions along tensile axis1 2 3 4Initial structureIntroduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering9Plastic Deformation of Semicrystalline Polymers Necking. Neck gets stronger since deformation aligns the chains and increases local strength in the neck region (up to 2-5 times)  neck expandsElongation by extension of neckChains in neck align along elongation direction: strengtheningDifferent from ductile metals where the deformation is confined in the initial neck region.Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering10Factors that Influence Mechanical properties (I)Temperature and strain rateChain entanglement, strong intermolecular bonding (van der Waals, cross-links) increase strengthDrawing, analog of work hardening in metals, corresponds to neck extension. Is used in production of fibers and films. Molecular chains become highly oriented  properties of drawn material are anisotropic (perpendicular to the chain alignment direction strength is reduced)Heat treatment: changes crystal size and order•undrawn material: Increasing annealing temperature leads to increase in elastic modulusincrease in yield/tensile strengthdecrease in ductility.(changes are opposite to metals)•drawn material: opposite changes (due to recrystallization and loss of chain orientation)Introduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering11Factors that Influence Mechanical properties (II)Tensile strength increases with molecular weight – effect of entanglementHigher degree of crystallinity – stronger secondary bonding - stronger and more brittleIntroduction to Materials Science, Chapter 15, Applications and Processing of PolymersUniversity of Virginia, Dept. of Materials Science and Engineering12Crystallization, Melting, Glass Transition (I)Crystallization: nuclei form and grow, chains align and order. Rates can be defined from the same type of S-curves we saw in the case of metals. Nucleation becomes slower at higher temperatures.Glass transition: polymers become rigid solids (viscosity increasing) upon cooling yet retain disordered molecular