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Polymers aren’t very stiff

Polymers aren’t very stiff. Stiffness dictated by structure. Stiffness depends on crystallinity crosslinking T g. For fibers, stiffness depends on draw ratio. Tensile strength. Glass transition temperature (T g ). Molecular wt. Glass transition temperature (T g ).

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Polymers aren’t very stiff

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  1. Polymers aren’t very stiff

  2. Stiffness dictated by structure

  3. Stiffness depends on crystallinity crosslinking Tg

  4. For fibers, stiffness depends on draw ratio

  5. Tensile strength

  6. Glass transition temperature (Tg)

  7. Molecular wt. Glass transition temperature (Tg)

  8. Glass transition temperature (Tg) Chemical structure

  9. Glass transition temperature (Tg) Chain stiffness

  10. Glass transition temperature (Tg) Chain stiffness

  11. Glass transition temperature (Tg) Bulky side groups

  12. d q Describe deformation under shear by angle q d t tan q = = g x y z xy z ˙ t = h g xy xy Viscous flow Occurs only by shear

  13. Log ha (Pa) Zero Shear Rate Viscosity 5 4 3 2 1 . 0 Log g (sec-1) -3 1 -2 -1 0 2 3 4 SHEAR RATES ENCOUNTERED IN PROCESSING Compression Injection Spin Molding Extrusion Molding Drawing Calendering 102 103 104 105 100 101 Strain Rate (sec-1) Viscous flow Most polymer melts are shear-thinning (pseudoplastic) - i.e., become thinner at high shear rates

  14. Poly(di-methylsiloxane) Poly(iso-butylene) Poly(ethylene) Poly(butadiene) Log hm + constant Poly(tetra-methyl p-silphenyl siloxane) Poly(methyl methacrylate) Poly(ethylene glycol) Poly(vinyl acetate) Poly(styrene) 1 2 3 4 5 Log M + constant Viscous flow Viscosity increases with MW Plot is for zero shear rate values

  15. Creep Relaxation

  16. Creep of cellulose acetate

  17. 400C 10 600C 920C 800C 1000C 9 1100C Log E(t), (dynes/cm2) Stress relaxation of PMMA 1120C 8 1200C 1150C 1250C 7 1350C 0.001 0.01 0.1 1 10 100 1000 Time (hours) Relaxation in PMMA

  18. Creep & recovery

  19. Cl H – – Polyvinyl chloride (PVC) {-C-C-}n – – Cl H Some specific polymers Very rigid and strong, Tg = 60-80 C siding, pipe, conduit, usw. Presence of Cl gives rise to solubility in various organic solvents - allows "solvent welding"

  20. Cl H – – Polyvinyl chloride (PVC) {-C-C-}n – – Cl H Some specific polymers Presence of Cl gives rise to solubility in various organic solvents Rigid PVC difficult to form by some techniques (e.g., calendaring)……so add solvent as "plasticizer" PVC sheet then roll-formed onto fabric backing and - voilá - "vinyl"!

  21. Some specific polymers Rigid PVC difficult to form by some techniques (e.g., calendaring)……so add solvent as "plasticizer" PVC sheet then roll-formed onto fabric backing and - voilá - "vinyl"! Problem: solvent slowly evaporates, exp. when (auto) vinyl seats & fascia heated by sun Soln: Armorall! Periodically put solvent back into polymer

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