Polymers - Chapter 15 Polymers Plastics • • • • •...

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Unformatted text preview: 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 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 • 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 H2O2 -> 2OH Disassociation of Hydrogen Peroxide 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 • 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. ...
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This note was uploaded on 01/22/2012 for the course ME 2733 taught by Professor Meng during the Fall '10 term at LSU.

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