Chapter 15

Polymers

Plastics

• They deform readily

• Usually a polymer plus an additive

• Organic molecules

• Three Dimensional

• Monomer -> Polymer

Classes

• Thermoplastics• Soften when heated

• Thermosets• Remain rigid until they decompose

• Elastomers• Deform, then regain their shape

How Does Polymerization Occur

• Addition• Polyethylene

• Condensation• Nylon

n H2 C = C H2 -> -CH2 CH2 CH2 CH2 CH2 CH2 -

Monomer Polymer

Addition Polymer

Condensation Polymer

R – COOH + H2N-R’ -> H2O + R-CO – NH – R’

Monomer 1 Monomer 2 Copolymer

Length of the Chains

• Initiation• A molecule comes along and breaks the double

bond. That leaves a dangling electron, that needs to react.

• The reactive end adds to other monomer molecules

• Termination

H2O2 -> 2OHDisassociation of Hydrogen Peroxide

OH + H2C=CH2 -> HOCH2CH2 .

Initiator

Termination

Linear Polymers

• See the CD

• Bond Rotation• Molecules are always in motion

• The amount of motion possible depends on the constituents attached to the chain

• Its not just chemistry – its physical room!!

Elastomers

• Lightly Crosslinked

• Unkink – which produces large elastic strain

• Vulcanization• Open up double bonds

• Attach sulfur

• Crosslinks the chains

Elastomers

• When you stretch a rubber band, you unkink the polymer, and you break van der Waal bonds – Do it!!

• Check the temperature

• Why did the temperature go up?

• What happened to the temperature when you released the rubber band?

Network Polymers

• Linear polymers are long chains

• If there is more than one attachment point, the chains branch

• The result is a network – Lots of crosslinking

• Usually formed by condensation

• Bakelite is an example – Used in electrical fixtures

• Epoxy Resins are network polymers

Properties

• Polymers can crystalize, but its hard (Why?)

• More often they are amorphous, and so they have properties more like glass

• Glass transition temperature

• Modulus of Elasticity changes at the glass transition temperature, as do other properties

• Often room temperature is above the glass transition temperature

Properties

• Strain Hardening• “Spagetti” Experiment• What is crazing?

• When the polymers align they move closer together, leaving voids

• The material eventually becomes a series of fibrils

• Finally break

Stress Relaxation

• All materials “creep” at high temperature• For most polymers RT is a high temperature• Molecules slide past each other, given

enough time, when a moderate load is applied

• If the load is applied quickly, the molecules break

• You can watch this happen in Silly Putty

Branching

• Isotactic• All on one side• Greatest Regularity - Stronger

• Syndiotactic• Alternating

• Atactic• Random

• Try the quiz

Friday

• Experiments• Shyla – Strain hardening of “Glad” Wrap

• Dan – Weibull modulus

• Holly – Silly Putty

Monday

• Review

• Final is Monday, May 7th

• Your final homework assignment is to complete the “Sample Final”, which you’ll find on line.

• Bring it with you to the final.