Molecular weight
description
Transcript of Molecular weight
Molecular weight Molecular weight
Gases
Liquids
IncreasingMolecular 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
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"Semi-solid"
Solids
increasingmolecular weight
Molecular weight Molecular weight
Chains have different molecular weights
There is a distribution
Molecular weight Molecular weight
Is it important?
Mol. Wt.Mol. Wt.
Melt ViscosityMelt ViscosityTensile StrengthTensile Strength
Mol. Wt.Mol. Wt.
Molecular weight Molecular weight
Is it important?Is it important?
Very important in processing
If viscosity too high, polymer difficult to process
Very important in processing
If viscosity too high, polymer difficult to processIf too low, an extruded material won't hold together until it solidifies
If too low, an extruded material won't hold together until it solidifies
Blow molding Blow molding
Step 1: make preform by extrusion or injection molding
Step 1: make preform by extrusion or injection molding
Blow molding Blow molding
Step 1: make preform by extrusion or injection 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
Step 2: use air pressure to inflate preform inside closed, hollow mold. Polymer expands to take shape of cooled mold & solidifies under pressure
Polymers are different Polymers are different
Polymers are different Polymers are different
• Normal crystalline materials– Either crystalline (~100%, neglecting defects) or amorphous at a particular temperature
– Melt at a sharp, well-defined temperature
• 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
• Crystallizable polymers– Never 100% crystalline– Melt over a range of temperatures
Chain folding - polyethyleneChain folding - polyethylene
So crystalline region looks like this
So crystalline region looks like this
Or maybe thisOr maybe this
In many polymers, crystalline regions grow to form spherulites
In many polymers, crystalline regions grow to form spherulites
Image of spherulites in polarized light microscope
Image of spherulites in polarized light microscope
Pull thin polymer rod in tensionPull thin polymer rod in tension
Get alignment of crystalline regionsGet alignment of crystalline regions
Polymer fibers have aligned crystalline regions
Polymer fibers have aligned crystalline regions
One way to make fibers - extrude viscous melt through tiny holes in "spinneret"
One way to make fibers - extrude viscous melt through tiny holes in "spinneret"
Kevlar is highly aligned
Kevlar is highly aligned
Polymer fibers have aligned crystalline regions
- alignment gives greater strength to fiber
Polymer fibers have aligned crystalline regions
- alignment gives greater strength to fiber
Breaking strength of polymer fibers (tenacity)
1. measure denier (wt. in grams of 9000 meters of fiber)
2. run tensile test
Breaking strength of polymer fibers (tenacity)
1. measure denier (wt. in grams of 9000 meters of fiber)
2. run tensile test
Tenacity also increases w/ chain length - fewer crystal defects
Tenacity also increases w/ chain length - fewer crystal defects
Glass transition temperature (Tg)
Glass transition temperature (Tg)
Molecular wt.Molecular wt.
Glass transition temperature (Tg)
Glass transition temperature (Tg)
Chemical structure
Chemical structure
Glass transition temperature (Tg)
Glass transition temperature (Tg)
Chain stiffnessChain stiffness
Glass transition temperature (Tg)
Glass transition temperature (Tg)
Chain stiffnessChain stiffness
Glass transition temperature (Tg)
Glass transition temperature (Tg)
Bulky side groups
Bulky side groups