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Polymers aren’t very stiff

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Presentation on theme: "Polymers aren’t very stiff"— Presentation transcript:

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 Glass transition temperature (Tg)
Molecular wt.

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

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

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

15 Creep Relaxation

16 Creep of cellulose acetate

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

18 Creep & recovery

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

20 Some specific polymers
Polyvinyl chloride (PVC) {-C-C-}n Cl H 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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