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 Point where
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
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
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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- Spring '08
- Polymer, Viscoelasticity, polymer chains