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

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

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

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. 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. Or maybe this So crystalline region looks like this

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

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

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

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

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. Molecular wt. Glass transition temperature (Tg)

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