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Polymers aren’t very stiff
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Stiffness dictated by structure
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Stiffness depends on crystallinity crosslinking Tg
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For fibers, stiffness depends on draw ratio
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Tensile strength
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Glass transition temperature (Tg)
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Glass transition temperature (Tg)
Molecular wt.
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Glass transition temperature (Tg)
Chemical structure
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Glass transition temperature (Tg)
Chain stiffness
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Glass transition temperature (Tg)
Chain stiffness
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Glass transition temperature (Tg)
Bulky side groups
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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
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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)
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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
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Creep Relaxation
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Creep of cellulose acetate
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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
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Creep & recovery
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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"
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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"!
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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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