Presentation on theme: "CHAPTER 14: POLYMER STRUCTURES"— Presentation transcript:
1CHAPTER 14: POLYMER STRUCTURES ISSUES TO ADDRESS...• What are the basic microstructural features?• How are polymer properties effected bymolecular weight?• How do polymeric crystals accommodate thepolymer chain?
2Chapter 14 – Polymers What is a polymer? Poly mer many repeat unit C H Polyethylene (PE)ClCHPolyvinyl chloride (PVC)HPolypropylene (PP)CCH3Adapted from Fig. 14.2, Callister 7e.
3Ancient Polymer History Originally natural polymers were usedWood – RubberCotton – WoolLeather – SilkOldest known usesRubber balls used by IncasNoah used pitch (a natural polymer) for the ark
4Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and CSaturated hydrocarbonsEach carbon bonded to four other atomsCnH2n+2
6Unsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bondsDouble bond – ethylene or ethene - CnH2n4-bonds, but only 3 atoms bound to C’sTriple bond – acetylene or ethyne - CnH2n-2
7IsomerismIsomerismtwo compounds with same chemical formula can have quite different structuresEx: C8H18n-octane2-methyl-4-ethyl pentane (isooctane)
8Chemistry of Polymers Free radical polymerization Initiator: example - benzoyl peroxide
9Chemistry of Polymers Note: polyethylene is just a long HC Adapted from Fig. 14.1, Callister 7e.Polymer- can have various lengths depending on number of repeat unitsNote: polyethylene is just a long HC- paraffin is short polyethylene
10Bulk or Commodity Polymers Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers
13MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains. Lower Mhigher MSimple for small moleculesAll the same sizeNumber of grams/molePolymers – distribution of chain sizesMw is more sensitive to higher molecular weightsAdapted from Fig. 14.4, Callister 7e.
14Molecular Weight Calculation Example: average mass of a class
15Degree of Polymerization, n n = number of repeat units per chainni = 6Chain fractionmol. wt of repeat unit i
16End to End Distance, rAdapted from Fig. 14.6, Callister 7e.
17Molecular Structures • Covalent chain configurations and strength: BranchedCross-LinkedNetworkLinearsecondarybondingDirection of increasing strengthAdapted from Fig. 14.7, Callister 7e.
18Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bondsnote: no bond breaking neededAdapted from Fig. 14.5, Callister 7e.
19Polymers – Molecular Shape Configurations – to change must break bondsStereoisomerismmirror plane
20Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chainsyndiotactic – R groups alternate sidesatactic – R groups random
21cis/trans Isomerism cis trans cis-isoprene (natural rubber) bulky groups on same side of chaintranstrans-isoprene (gutta percha)bulky groups on opposite sides of chain
22Copolymers two or more monomers polymerized together Adapted from Fig. 14.9, Callister 7e.two or more monomers polymerized togetherrandom – A and B randomly vary in chainalternating – A and B alternate in polymer chainblock – large blocks of A alternate with large blocks of Bgraft – chains of B grafted on to A backboneA – B –randomalternatingblockgraft
23Polymer Crystallinity Adapted from Fig , Callister 7e.Ex: polyethylene unit cellCrystals must contain the polymer chains in some wayChain folded structureAdapted from Fig , Callister 7e.10 nm
24Polymer Crystallinity Polymers rarely 100% crystallineToo difficult to get all those chains alignedcrystallineregion• % Crystallinity: % of material that is crystalline.-- TS and E often increasewith % crystallinity.-- Annealing causescrystalline regionsto grow. % crystallinityincreases.amorphousregionAdapted from Fig , Callister 6e.(Fig is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)
25Polymer Crystal Forms Single crystals – only if slow careful growth Adapted from Fig , Callister 7e.
26Polymer Crystal FormsSpherulites – fast growth – forms lamellar (layered) structuresSpherulite surfaceNucleation siteAdapted from Fig , Callister 7e.