1. Molecular weight molecular weight CH4 CH3 - CH2 _ CH3 CH3 - CH3
Gases
Liquids
Increasing
Molecular Weight 16 30 44 58
114
450
420030
CH4
CH3 - CH2 _ CH3
CH3 - CH3
CH3 - CH2 - CH2 - CH3
CH3 - (CH2)6 _ CH3
CH3 - (CH2)30 _ CH3
CH3 - (CH2)30000 _ CH3
"Semi-solid"
Solids
increasing
molecular weight

2. Molecular weight Chains have different molecular weights
There is a distribution

3. Molecular weight Is it important? Tensile Strength Melt Viscosity
Mol. Wt.
Melt Viscosity
Tensile Strength

4. Molecular weight Is it important? Very important in processing
If viscosity too high, polymer difficult to process
If too low, an extruded material won't hold together until it solidifies

5. Blow molding
Step 1: make preform by extrusion or injection molding

6. Blow molding Step 1: make preform by extrusion or injection molding
Step 2: use air pressure to inflate preform inside closed, hollow mold. Polymer expands to take shape of cooled mold & solidifies under pressure

7. Polymers are different

8. Normal crystalline materials
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

11. Chain folding - polyethylene

12. So crystalline region looks like this
Or maybe this

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

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

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

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

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

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

19. Glass transition temperature (Tg)
Molecular wt.

20. Glass transition temperature (Tg)
Chemical structure

21. Glass transition temperature (Tg)
Chain stiffness

22. Glass transition temperature (Tg)
Chain stiffness

23. Glass transition temperature (Tg)
Bulky side groups