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1、EXPERIMENT28IntrinsicViscosity:ChainLinkageinPolyvinylAlcoholChapterXIIMacromoleculesWhilethebasicchemicalstructureofasyntheticpolymerisusuallywellunderstood,manyphysicalpropertiesdependonsuchcharacteristicsaschainlength,degreeofchainbranching,andmolecularweight,whicharenoteas
2、ytospecifyexactlyintermsofamolecularformula.Moreover,themacromoleculesinagivensampleareseldomuniforminchainlengthormolecularweight(whichforalinearpolymerisproportionaltochainlength);thusthenatureofthedistributionofmolecularweightsisanotherimportantcharacteristic.Apolymerwhosem
3、oleculesareallofthesamemolecularweightissaidtobemonodisperse;apolymerinwhichthemolecularweightsvaryfrommoleculetomoleculeisaidtobepolydisperse.Specimensthatareapproximatelymonodispersecanbepreparedinsomecasebyfractionatingapolydispersepolymer;thisfractionationisfrequentlydoneo
4、nthebasisofsolubilityinvarioussolventmixtures.Theexperimentisconcernedwiththelinearpolymerpolyvinylalcohol(PVOH),-(CH2-CHOH)-n,whichispreparedbymethanolysisofthepolyvinylacetate(PVAc)obtainedfromthedirectpolymerizationofthemonomervinylacetate,CH2=CH-OOCCH3.Aordinarilyprepared,
5、polyvinylalcoholshowsanegligibleamountofbranchingofthechains.Itissomewhatunusualamongsyntheticpolymersinthatitissolubleinwater.Thismakepolyvinylalcoholcommerciallyimportantasathickener,asacomponentingums,andasafoamingagentindetergents.AcharacteristicofinterestinconnectionwithP
6、VOHandPVAcistheconsistencyorientationofmonomerunitsalongthechain.Intheformulagivenaboveitisassumedthatallmonomerunitsgotogether“headtotail."However,occasionallyamonomerunitwilljoinontothechainina"head-to-head"fashion,yieldingachainoftheformReversedmonomerunitHead-to-headlinkag
7、ewhereXisAcorOH.Thefrequencyofhead-to-headlinkagedependontherelativeratesofthenormalgrowth-stepreactionα(1a),andtheabnormalreactionβ,(2a)(whereR•isthegrowingpolymerradical,thearrowrepresentingthepredominantmonomerorientation).Therates,inturn,mustdependontheactivationenergies:(
8、2)whereΔEα=Eαβ−Eααistheadditionalthermalactivationenergyneede
