1. ….when you are young you only want to go as fast, as far, and as deep as you can in one subject - all the others are neglected as being relatively uninteresting. But, later on, when you get older, you find that nearly everything is interesting if you go into it deeply enough.

 – Feynman

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

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

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

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

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

7. Stress/strain characteristics of polymers
Polymer stiffness, strength and toughness vary over extraordinary range

8. Stress/strain characteristics of 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)

9. Stress/strain characteristics of 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 )

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

11. semi-crystalline polymers
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
Stress
Yield
Point
Strain

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

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

14. Crazing
Microscopic cracks form perpendicular to applied stress

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

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

17. Polymers aren’t very stiff

18. Stiffness dictated by structure

19. Stiffness depends on
crystallinity
crosslinking Tg

20. For fibers,
stiffness
depends on
draw ratio

21. Tensile strength

22. Glass transition temperature (Tg)

23. Glass transition temperature (Tg)
Molecular wt.

24. Glass transition temperature (Tg)
Chemical structure

25. Glass transition temperature (Tg)
Chain stiffness

26. Glass transition temperature (Tg)
Chain stiffness

27. Glass transition temperature (Tg)
Bulky side groups

28. Assignment:
Review today's classnotes
a. fiber strength
b. tenacity
c. stiffness
d. yielding, relaxation, crazing
e. tensile strength
f. Tg