MAT_101_Winter08_Lecture_19

MAT_101_Winter08_Lecture_19 - T and Strain Rate:...

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T and Strain Rate: Thermoplastics Adapted from Fig. 15.3, Callister 6e . (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics , American Society for Testing and Materials, Philadelphia, PA, 1944.) 20 40 60 80 0 0 0.1 0.2 0.3 4°C 20°C 40°C 60°C to 1.3 σ (MPa) ε Data for the semicrystalline polymer: PMMA (Plexiglas) • Decreasing T. .. --increases E --increases TS --decreases %EL • Increasing strain rate. .. --same effects as decreasing T.
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Elastic, Viscoleastic and Viscous Behavior Input: Constant Stress Response: Viscoelastic Response: Elastic Response: Viscous
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Time Dependent Deformation Stress relaxation test : • Data: Large drop in E r for T > T g . (amorphous polystyrene) Sample T g (C) values: PE (low M w ) PE (high M w ) PVC PS PC -110 -9 0 + 87 +100 +150 10 3 10 1 10 -1 10 -3 10 5 60 100 140 180 rigid solid (small relax) viscous liquid (large relax) transition region T(°C) T g E r (10s) in MPa Adapted from Fig. 15.7, Callister 6e . (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers , John Wiley and Sons, Inc., 1960.) Selected values Callister 6e . --strain to ε ο and hold. --observe decrease in stress with time. Relaxation modulus : time strain tensile test ε o t σ () E r (t) = σ (t) ε
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More on the Relaxation Modulus Schematic E r (t) for a thermoplastic E r (t) for amorphous polystyrene
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More (again!) on the Relaxation Modulus A: Crystalline isotactic B: Lightly cross-linked atactic C: Amorphous Relaxation Modulus: Polystyrene
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Fracture of Polymers Crazing (thermoplastics) Formation of localized yielding Fibrillar bridges between microvoids Molecular chain reorientation + bridge fibers = increased fracture toughness Crazing in Polyeythylene Oxide
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Fatigue in Polymers Fatigue in polymers Less studied than in metals Strong dependence on testing frequency (why?)
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Melting and the Glass Transition Temperature Melting Temperature T m Transition to a viscous liquid Can occur over a range of T Depends on sample history! Glass Transition Temperature T g Amorphous or semicrystalline polymers Transition Rubbery solid to a rigid solid
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m and T g Melting Temperature T m as chain stiffness e.g., add double bonds on chain T m with bulky side chains or polar side groups e.g., polypropylene: T m = 175 °C; polyethylene: T m = 175 °C e.g., polyvinyl chloride: T m = 175 °C; polyethylene: T m = 175 °C Tm with significant side branching
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MAT_101_Winter08_Lecture_19 - T and Strain Rate:...

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