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Pull thin polymer rod in tension

1. 2. 3. 4. 5. Get alignment of crystalline regions. Pull thin polymer rod in tension. Polymer fibers have aligned crystalline regions - alignment gives greater strength to fiber. Kevlar is highly aligned. Polymer fibers have aligned crystalline regions

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Pull thin polymer rod in tension

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  1. 1 2 3 4 5 Get alignment of crystalline regions Pull thin polymer rod in tension

  2. Polymer fibers have aligned crystalline regions - alignment gives greater strength to fiber

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

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

  5. Tenacity increases w/ chain length - fewer crystal defects

  6. Stress/strain characteristicsof polymers Polymer stiffness, strength and toughness vary over extraordinary range

  7. Stress/strain characteristicsof polymers Polymer stiffness, strength and toughness vary over extraordinary range Due to structure - ranges from purely amorphous states to chain folded semi-crystalline to highly oriented (fibers)

  8. Stress/strain characteristicsof polymers Polymer stiffness, strength and toughness vary over extraordinary range Due to structure - ranges from purely amorphous states to chain folded semi-crystalline to highly oriented (fibers) Polymers plastically deform readily, esp. if temp raised (often less than 1000C )

  9. Stress/strain characteristicsof polymers Glassy polymer or semi-crystalline polymer Stress below Tg x Semi-crystalline polymer ( ) s above Tg x x Rubber Strain ( ) e

  10. Stress Yield Point Strain Yielding in flexible semi-crystalline polymers Flexible semi-crystalline polymers such as polyethylene (Tg of amorphous domains is below rm temp) usually display considerable amount of yielding if not stretched too quickly

  11. Relaxation Yielding due to relaxation Time dependent molecular transition or rearrangement, such as change in conformation of a chain, crystalline slip, chain sliding, usw.

  12. Stress Yield Point Strain Yielding in rigid polymers Rigid polymers usually don't have yield point May yield by crazing

  13. Crazing Microscopic cracks form perpendicular to applied stress

  14. Crazing Microscopic cracks form perpendicular to applied stress Tiny fibrils span cracks - hold material together

  15. Crazing Microscopic cracks form perpendicular to applied stress Tiny fibrils span cracks - hold material together Polymer whitens

  16. Polymers aren’t very stiff

  17. Stiffness dictated by structure

  18. Stiffness depends on crystallinity crosslinking Tg

  19. For fibers, stiffness depends on draw ratio

  20. Tensile strength

  21. Glass transition temperature (Tg)

  22. Molecular wt. Glass transition temperature (Tg)

  23. Glass transition temperature (Tg) Chemical structure

  24. Glass transition temperature (Tg) Chain stiffness

  25. Glass transition temperature (Tg) Chain stiffness

  26. Glass transition temperature (Tg) Bulky side groups

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