Chapter 14: Polymer Structures - Iowa State University

1 Chapter 14 -1 ISSUES TO What are the general structural and chemical characteristics of Polymer molecules? What are some of the common polymeric materials, and how do they differ chemically? How is the crystalline State in polymers different from that in metals and ceramics ? Chapter 14: Polymer StructuresChapter 14 -2 What is a Polymer ?Polymermanyrepeat unitAdapted from Fig. , Callister & Rethwisch (PE)ClClClCCCCCCHHHHHHHHHPoly(vinyl chloride) (PVC)HHHHHHP olypropylene (PP)CCCCCCCH3 HHCH3CH3 HrepeatunitrepeatunitrepeatunitChapter 14 -3 Ancient Polymers Originally natural polymers were used Wood Rubber Cotton Wool Leather Silk Oldest known uses Rubber balls used by Incas Noah used pitch (a natural Polymer ) for the arkChapter 14 -4 Polymer CompositionMost polymers are hydrocarbons , made up of H and C Saturated hydrocarbons Each carbon singly bonded to four other atoms Example.

2 Ethane, C2H6 CCHHHHHHC hapter 14 -5 Chapter 14 -6 Unsaturated Hydrocarbons Double & triple bonds somewhat unstable can form new bonds Double bondfound in ethylene or ethene - C2H4 Triple bondfound in acetylene or ethyne - C2H2 CCHHHHCCHHC hapter 14 -7 Chapter 14 -8 Isomerism Isomerism two compounds with same chemical formula can have quite different Structures for example: C8H18 normal-octane 2,4-dimethylhexaneCCCCCCCCHHHHHHHHHHHHHH HHHHH3 CCH2CH2CH2CH2CH2CH2CH3=H3 CCHCH3CH2 CHCH2CH3CH3H3 CCH2CH3()6 Chapter 14 -9 Polymerization and Polymer Chemistry Free radical polymerization Initiator: example - benzoyl peroxideCHHOOCHHCHHO2 CCHHHH monomer(ethylene)R+free radicalRCCHHHH initiationRCCHHHHCCHHHH+RCCHHHHCCHHHH propagationdimerR= 2 Chapter 14 -10 Chemistry and Structure of PolyethyleneAdapted from Fig.

3 , Callister & Rethwisch : polyethylene is a long-chain hydrocarbon- paraffin wax for candles is short polyethyleneChapter 14 -11 Bulk or Commodity PolymersChapter 14 -12 Bulk or Commodity Polymers (cont) Chapter 14 -13 Bulk or Commodity Polymers (cont) Chapter 14 -14 Chapter 14 -VMSE: Polymer Repeat Unit Structures15 Manipulate and rotate Polymer Structures in 3-dimensions Chapter 14 -16 MOLECULAR WEIGHT Molecular weight, M:Mass of a mole of Mhigh MNot all chains in a Polymer are of the same length , there is a distribution of molecular weights Chapter 14 -17xi= number fraction of chains in size range imolecules of #totalpolymer of wttotal nMMOLECULAR WEIGHT DISTRIBUTION Mn xiMiMw wiMiAdapted from Fig. , Callister & Rethwisch weight fraction of chains in size range iMi= mean (middle) molecular weight of size range iChapter 14 -18 Molecular Weight CalculationExample: average mass of a classStudentWeightmass (lb)110421163140414351806182719182209225 10380 What is the averageweight of the students inthis class:a) Based on the number fraction of students in each mass range?

4 B) Based on the weight fraction of students in each mass range? Chapter 14 -19 Molecular Weight Calculation (cont.)Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table:weightnumber ofmeannumberweightrangestudentsweightfra ctionfractionNiWixiwimass (lb)mass (lb) Ni NiWi101881total numbertotal weightCalculate the number and weight fraction of students in each weight range as follows: xi NiNi wi NiWiNiWi For example: for the 81-120 lb range x81 120 210 x 212081 wChapter 14 -20 Molecular Weight Calculation (cont.) Mn xiMi ( x 110 x 142 + x 184 + x 223 + x 380) = 188 lbweightmeannumberweightrangeweightfract ionfractionWixiwimass (lb)mass (lb) Mw wiMi ( x 110 x 142 + x 184 + x 223 + x 380) = 218 lb Mw wiMi 218 lbChapter 14 -21 Degree of Polymerization, DPDP= average number of repeat units per chainiimfmm :follows as calculated is this copolymers forunit repeat of weightmolecular average whereCCCCCCCCHHHHHHHHHHHHHHHHHCCCCHHHHHH HHH()DP= 6mol.

5 Wt of repeat unit iChain fractionmMDPn Chapter 14 -22 Adapted from Fig. , Callister & Rethwisch Structures for PolymersBranchedCross-LinkedNetworkLinea rsecondarybondingChapter 14 -23 Polymers Molecular ShapeMolecular Shape (or Conformation) chain bending and twisting are possible by rotation of carbon atoms around their chain bonds note: not necessary to break chain bonds to alter molecular shapeAdapted from Fig. , Callister & Rethwisch 14 -24 Chain End-to-End Distance, rAdapted from Fig. , Callister & Rethwisch 14 -25 Molecular Configurations for PolymersConfigurations to change must break bonds StereoisomerismEBADCCDABE mirror planeCCRHHHCCHHHRorCCHHHRS tereoisomers are mirrorimages can t superimposewithout breaking a bondChapter 14 -26 TacticityTacticity stereoregularity or spatial arrangement of Runits along chainCCHHHRRHHHCCRHHHCCRHHHCC isotactic all Rgroups on same side of chainCCHHHRCCHHHRCCHHHRRHHHCC syndiotactic Rgroups alternate sidesChapter 14 -27 Tacticity (cont.)

6 Atactic Rgroups randomlypositionedCCHHHRRHHHCCRHHHCCRHHH CCC hapter 14 -28cis/trans IsomerismCCHCH3CH2CH2 CCCH3CH2CH2 Hciscis-isoprene (natural rubber)H atom and CH3group on same side of chaintranstrans-isoprene (gutta percha)H atom and CH3group on opposite sides of chainChapter 14 -VMSE: Stereo and Geometrical Isomers29 Chapter 7 - 19 Manipulate and rotate Polymer Structures in 3-dimensions Chapter 14 -30 Copolymerstwo or more monomers polymerized together random A and B randomly positioned along chain alternating A and B alternate in Polymer chain block large blocks of A units alternate with large blocks of B units graft chains of B units grafted onto A backboneA B randomblockgraftAdapted from Fig. , Callister & Rethwisch 14 -31 Crystallinity in Polymers Ordered atomic arrangements involving molecular chains Crystal Structures in terms of unit cells Example shown polyethylene unit cellAdapted from Fig.

7 , Callister & Rethwisch 14 -32 Polymer Crystallinity Crystalline regions thin platelets with chain folds at faces Chain foldedstructure10 nmAdapted from Fig. , Callister & Rethwisch 14 -33 Polymer Crystallinity (cont.)Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned Degree of crystallinity expressed as % Some physical properties depend on % Heat treating causes crystalline regions to grow and % crystallinity to from Fig. , Callister 6e.(Fig. is from Hayden, Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)crystalline regionamorphousregionChapter 14 -34 Polymer Single Crystals Electron micrograph multilayered single crystals (chain-folded layers) of polyethylene Single crystals only for slow and carefully controlled growth ratesAdapted from Fig.

8 , Callister & Rethwisch 14 -35 Semicrystalline PolymersSpherulite surfaceAdapted from Fig. , Callister & Rethwisch 8e. Some semicrystalline polymers form spherulitestructures Alternating chain-folded crystallites and amorphous regions Spherulite structure for relatively rapid growth ratesChapter 14 -36 Photomicrograph Spherulites in PolyethyleneAdapted from Fig. , Callister & Rethwisch light used -- a maltese crossappears in each spherulit