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