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Published byIsabel York Modified over 6 years ago
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Molecular weight molecular weight CH4 CH3 - CH2 _ CH3 CH3 - CH3
Gases Liquids Increasing Molecular Weight 16 30 44 58 114 450 420030 CH4 CH3 - CH2 _ CH3 CH3 - CH3 CH3 - CH2 - CH2 - CH3 CH3 - (CH2)6 _ CH3 CH3 - (CH2)30 _ CH3 CH3 - (CH2)30000 _ CH3 "Semi-solid" Solids increasing molecular weight
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Molecular weight Chains have different molecular weights
There is a distribution
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Molecular weight Is it important? Tensile Strength Melt Viscosity
Mol. Wt. Melt Viscosity Tensile Strength
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Molecular weight Is it important? Very important in processing
If viscosity too high, polymer difficult to process If too low, an extruded material won't hold together until it solidifies
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Blow molding Step 1: make preform by extrusion or injection molding
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Blow molding Step 1: make preform by extrusion or injection molding
Step 2: use air pressure to inflate preform inside closed, hollow mold. Polymer expands to take shape of cooled mold & solidifies under pressure
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Polymers are different
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Normal crystalline materials
Polymers are different Normal crystalline materials Either crystalline (~100%, neglecting defects) or amorphous at a particular temperature Melt at a sharp, well-defined temperature Crystallizable polymers Never 100% crystalline Melt over a range of temperatures
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Chain folding - polyethylene
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So crystalline region looks like this
Or maybe this
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In many polymers, crystalline regions grow to form spherulites
Image of spherulites in polarized light microscope
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Pull thin polymer rod in tension
Get alignment of crystalline regions
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Polymer fibers have aligned crystalline regions
One way to make fibers - extrude viscous melt through tiny holes in "spinneret"
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Polymer fibers have aligned crystalline regions
- alignment gives greater strength to fiber Kevlar is highly aligned
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Breaking strength of polymer fibers (tenacity)
measure denier (wt. in grams of 9000 meters of fiber) run tensile test
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Tenacity also increases w/ chain length - fewer crystal defects
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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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