Chapter 16: Polymers

What are polymers on a molecular level?

Huge, chain-like molecules in which atoms forming the backbone (usually carbon) are linked by covalent bonds

What are the three categories of polymers?

1. Thermoplastic

2. Thermosetting

3. Elastomers

Thermoplastic

• flexible linear chains

• when heated, secondary bonds between chains weaken and chains can glide past each other

• can be reshaped or recycled or softened

• No cross-linking

• ex. PVC, polyethylene

Thermosetting

• rigid 3D network

• heating up = degradation

• cannot be reshaped

• strong cross-linking

• chains can easily slide past each other if not properly linked

• ex. epoxies, resins

Elastomers

• thermoplastics or lightly cross-linked thermosets consist of coil or spring-like molecules

• not as many cross-links

• cross-links prevents the chains from permanently sliding part

• ex. rubber

Polyethylene

• Properties:

  • flexible

  • lightweight

  • chemically resistant

• Application:

  • packing films

  • wire insulation

  • squeeze bottles

  • tubing

  • household items

What are some applications of polypropylene?

• tanks

• carpet fibers

• rope

• packaging

What are some applications of polystyrene?

• packaging and insulation foams

• lighting panels

• appliance components

• egg cartons

What are some applications of polyacrylonitrile?

• textile fibers

• precursor for carbon fibers

• food container

What are some applications of polymethylmethacrylate?

• windows

• windshields

• coatings

• hard contact lenses

• lighted signs

What are some applications of polychlorotrifluoroethylene?

• valve components

• gaskets

• tubing

• electrical insulation

What are some applications of polytetrafluoroethylene?

*Hydrogen is replaced by fluorine

• seals

• valves

• nonstick coatings

Degree of polymerization

# of repeat units in a polymer chain

= average molecular weight of polymer chain / molecular weight of repeat unit

Which is usually bigger than the other: number-average molecular weight (M_n) or weight-average molecular weight (M_w)? Why?

M_w >= M_n

b/c long chains receive extra emphasis in the weight-average calculation

When are M_w and M_n equal?

When all polymer chains have exactly the same molecular weight

Describe the mechanical behaviour of polymers (for a ductile thermoplastic polymer)

Tangled chains → neck formation → chain alignment → fully drawn polymer → fracture

Strong strength-hardening b/c chains cannot untangle anymore, a higher force is needed to continue

At fracture, aligned chains and covalent bonds cannot withstand the load, so fracture occurs

What does relaxation modulus measure in polymers?

How well a polymer maintains stress under constant deformation

What is the behaviour of polystyrene below T_g?

Glassy, stiff, hard, brittle

What is the behaviour of polystyrene below T_g?

Softens and becomes rubbery

What is the meaning of T_g in polymers?

Glass transition temperature

What are the effect of high crosslinking?

Polymer softens after T_g but does not easily flow

What are the effect of low crosslinking?

Polymer softens after T_g and may flow

Why do crystalline polymers stay stiff until T_m?

Crystalline regions act as reinforcing structures

What happens to relaxation modulus as temperature increases?

It decreases, indicating reduced stiffness

Why does a car bumper crack in winter?

Polymer is below T_g and behaves brittle

Why do car bumpers dent in summer?

Polymer is above T_g and behaves rubbery

Which type of polymer retains stiffness until T_m?

Crystalline polymer

What structural feature prevents polymer chains from sliding past each other?

crosslinking