Polymers are different - PowerPoint PPT Presentation

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Polymers are different

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  1. While knowledge is most readily assimilated by the prepared mind, that same mind can be refractory to penetration if it is "prepared" to believe that it cannot be taught. 
 – Prof. Leigh Palmer, in a letter to Feynman

  2. Polymers are different

  3. Polymers are different • Normal crystalline materials • Either crystalline (~100 %, neglecting defects ) or amorphous at a particular temperature • Melt at a sharp, well-defined temperature • Crystallizable polymers • Never 100% Crystalline • Melt over a Range of Temperatures

  4. Model for crystalline regions in polymers

  5. Chain folding - polyethylene

  6. Or maybe this So crystalline region looks like this

  7. Image of spherulites in polarized light microscope In many polymers, crystalline regions grow to form spherulites

  8. Get alignment of crystalline regions Pull thin polymer rod in tension

  9. One way to make fibers - extrude viscous melt through tiny holes in "spinneret" Polymer fibers have aligned crystalline regions

  10. Kevlar is highly aligned Polymer fibers have aligned crystalline regions - alignment gives greater strength to fiber

  11. Breaking strength of polymer fibers (tenacity) measure denier (wt. in grams of 9000 meters of fiber) run tensile test

  12. Tenacity also increases w/ chain length - fewer crystal defects

  13. Molecular wt. Glass transition temperature (Tg)

  14. Glass transition temperature (Tg) Chemical structure

  15. Glass transition temperature (Tg) Chain stiffness

  16. Glass transition temperature (Tg) Chain stiffness

  17. Glass transition temperature (Tg) Bulky side groups

  18. Assignment: Review today's classnotes a. crystallinity in polymers b. chain folding c. spherulites d. tenacity e. glass transition temperature