Chapter 15 Polymers

Plastics They deform readily Usually a polymer plus an additive Organic molecules Three Dimensional Monomer -> Polymer

Classes Thermoplastics Thermosets Elastomers Soften when heated 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 - Addition Polymer n H2 C = C H2 -> -CH2 CH2 CH2 CH2 CH2 CH2 - Polymer Monomer Condensation Polymer R – COOH + H2N-R’ -> H2O + R-CO – NH – R’ Monomer 1 Monomer 2 Copolymer

Length of the Chains Initiation Termination 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

Initiator Disassociation of Hydrogen Peroxide H2O2 -> 2OH OH + H2C=CH2 -> HOCH2CH2 .

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 Syndiotactic Atactic Try the quiz 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.