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Understanding Polymer Microstructure: Key Features and Their Impact on Tensile Response

This chapter explores the fundamental microstructural features of polymers and how they influence tensile response at room temperature. It distinguishes between thermoplastics and thermosets, highlighting their properties such as cross-linking, ductility, and thermal behavior. The correlation between molecular weight, crystallinity, and tensile strength is examined, showcasing how longer chains enhance entanglement and strength. Additionally, the role of annealing in promoting crystallinity and its effects on material performance is discussed, setting the stage for addressing key issues in polymer structures.

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Understanding Polymer Microstructure: Key Features and Their Impact on Tensile Response

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  1. CHAPTER 4:POLYMER STRUCTURES ISSUES TO ADDRESS... • What are the basic microstructural features? • How do these features dictate room T tensile response? • Hardening, anisotropy, and annealing in polymers. 1

  2. POLMER MICROSTRUCTURE • Polymer = many mers Adapted from Fig. 14.2, Callister 6e. • Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.7, Callister 6e. 2

  3. THERMOPLASTICS VS THERMOSETS • Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) • Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) 3

  4. MOLECULAR WEIGHT & CRYSTALLINITY • Molecular weight, Mw: Mass of a mole of chains. • Tensile strength (TS): --often increases with Mw. --Why? Longer chains are entangled (anchored) better. • % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases. 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.) 4

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