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1 polymer chains become increasingly tangled hinders

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Unformatted text preview: ingly tangled → hinders slippage past one another 2. Effect of physical (VdW) bonding increases higher overall strength in the material 6 TP Strength, Stiffness Slow Increase Rapid Increase Point where mechanical properties develop Viscosity of the Melt So, How Does MW Affect Properties? Lecture 20-21 Molecular Weight I Molecular Weight Why does viscosity follow this trend? Region I: Low η, molecules flow as single entities - stirring small molecules Region II: Higher η, molecules are entangled - stirring long entangled chains II What Effects a TP Properties? 2. Temperature Lecture 20-21 (a) Importance of the Tg in TP (and elastomers) ! - above Tg → pliable and ductile - below Tg → brittle (b) Affects elongation at failure and strength plexiglass PMMA Tg=105oC - As increase temperature, VdW bonds between polymer chains decrease → ↓ E, ↓ σ, ↑ ductility -Greater ductility b/c chains can slide easier with respect to each other 8 TP MSE 230 - Trice Lecture 20-21 How Amorphous TP Polymers Plastically Deform A. Elastic Region - Stretching of covalent (in chain) and VdW bonds (between chains) EPE ~ 1 GPa ESiC ~ 207 GPa E B. Drawing Region (Literally “plastic” deformation”) Mechanisms of Plastic Deformation Comparison i. Metals: slip by movement of dislocations ii. Polymers (Amorphous and Crystalline): no slip, but observe drawing of polymer as amorphous chains and crystalline regions orient themselves parallel to loading direction TP How Amorphous TP Polymers Plastically Deform B. Drawing Region Cont. Lecture 20-21 - alignment of polymer chains in direction of applied stress Neck Random chain entanglement In a metal, would see fracture at Not true for polymers; this is the neck strong region aligned strong region due to covalent bonding Neck formation C. Region 3 -See increase in strength at end of test because load is carried by the stronger covalent bonds Drawing region Region 3 10 TP MSE 230 - Trice after the stress is released. This phenomenon is demonstrated in Figure 15.5...
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