ACRYLICESTERPOLYMERS

ACRYLICESTERPOLYMERSIntroductionTheusageofacrylicestersasbuildingblocksforpolymersofindustrialimpor-tancebeganinearnestwiththeresearchofOttoR¨ohm(1).Thefirstrecordedpreparationofthebasicbuildingblockforacrylicesterpolymers,acrylicacid,tookplacein1843;thissynthesisreliedontheairoxidationofacrolein(2,3).Thefirstacrylicacidderivativestobemadeweremethylacrylateandethylacry-late.Althoughthesetwomonomersweresynthesizedin1873,theirutilityinthepolymerareawasnotdiscovereduntil1880whenKahlbaumpolymerizedmethylacrylateandtesteditsthermalstability.Tohissurprise,thepolymerizedmethylacrylatedidnotdepolymerizeattemperaturesupto320◦C(4).Despitethisfind-ingofincrediblyhighthermalstability,theindustrialproductionofacrylicesterpolymerswasnotrealizedforalmostanother50years.ThecommercialdiscoveryofacrylicesterpolymerstookplacewhileOttoR¨ohmwasconductinghisdoctoralresearchin1901.R¨ohmobtainedaU.S.patentin1912coveringthevulcanizationofacrylateswithsulfur(5).Commercialpro-ductionofacrylicesterpolymersbytheRohmandHaasCo.ofDarmstadt,Germany,commencedin1927(6).PropertiesThestructureofacrylicestermonomersisrepresentedbythefollowing:TheRestergroupsdeterminethepropertiesofthepolymersformed.ThisRside-chaingroupconveyssuchawiderangeofpropertiesthatacrylicesterpoly-mersareusedinapplicationsvaryingfrompaintstoadhesives,concretemodi-fiersandthickeners.Theglass-transitionrangeforapolymerdescribesthetem-peraturerangebelowwhichsegmentalpinningtakesplaceandthepolymertakesonastiff,rigid,inflexiblenature.Thisrangecanvarywidelyamongtheacrylices-terpolymersfrom–65◦Cfor2-ethylhexylacrylate(R=C4H9)to103◦Cforacrylicacid(R=H).Filmpropertiesaredramaticallyinfluencedbythischangingofthepolymerflexibility.1EncyclopediaofPolymerScienceandTechnology.Copyrightc
2010JohnWiley&Sons,Inc.Allrightsreserved.2ACRYLICESTERPOLYMERSWhencopolymerized,theacrylicestermonomerstypicallyrandomlyin-corporatethemselvesintothepolymerchainsaccordingtothepercentageconcentrationofeachmonomerinthereactorinitialcharge.Alternatively,acrylicestermonomerscanbecopolymerizedwithstyrene,methacrylicestermonomers,acrylonitrile,andvinylacetatetoproducecommerciallysignificantpolymers.Acrylicestermonomersaretypicallysynthesizedfromthecombinationofacrylicacidandanalcohol.Thepropertiesofthepolymerstheyformarecon-trolledbythenatureoftheestersidechainaswellasthemolecularweightoftheproduct.Acrylicesterpolymersaresimilartoothersinthattheyshowanimprovementinpropertiesasafunctionofmolecularweightuntilacer-tainthresholdbeyondwhichnofurtherimprovementisobserved.Thisthresholdistypicallyreachedatamolecularweightvalueof100,000–200,000foracrylicpolymers.Glass-TransitionTemperature.TheGlass-transitiontemperature(Tg)describestheapproximatetemperaturebelowwhichsegmentalrigidity(ie,lossofrotationalandtranslationalmotion)setsin.Althoughasinglevalueisoftencited,inrealityapolymerfilmundergoesthetransitionoverarangeoftemper-atures.Thereasonforthisrangeoftemperaturesfortheglasstransitionisthatsegmentalmobilityisafunctionofboththeexperimentalmethodused[dynamicmechanicalanalysis(DMA)vsdifferentialscanningcalorimetry(DSC)]aswellastheexperimentalconditions.FactorssuchashydroplasticizationinvaryingdegreesofhumiditycanalterTgresults.Mostpolymersexperienceanincreaseinthespecificvolume,coefficientofexpansion,compressibility,specificheat,andrefractiveindex.TheTgistypicallymeasuredasthemidpointoftherangeoverwhichthediscontinuityofthesepropertiestakesplace.CareshouldbetakenwhenanalyzingTgdata,however,assomeexperimenterscitetheonsetofthediscontinuityastheTgvalue.TherigidityuponcoolingbelowTgisobservedasanembrittlementofthepolymertothepointwherefilmsareglass-likeandincapableofhandlingsignif-icantmechanicalstresswithoutcracking.If,ontheotherhand,oneraisesthetemperatureofapolymerabovetheglass-transitionrange,thepolymerfilmbe-comesstretchable,soft,andelastic.Foramorphousacrylicpolymers,manyphys-icalpropertiesshowdramaticchangesafterpassingthroughtheglass-transitiontemperaturerange.Thephysicalpropertiesalteredarediffusion,chemicalreac-tivity,mechanicalanddielectricrelaxation,viscousflow,load-bearingcapacity,hardness,tack,heatcapacity,refractiveindex,thermalexpansivity,creep,andcrystallization.Themostcommonthermalanalysesusedtodeterminetheglass-transitiontemperatureareDMAandDSC.MoreinformationonthesetechniquesandhowtointerprettheresultsarecontainedinReferences(7–9).TheTgvaluesforthemostcommonhomopolymersofacrylicestersarelistedinTable1.Themostcommonwayoftailoringacrylicesterpolymerpropertiesistocopolymerizetwoormoremonomers.Inthisfashion,thebalanceofhard(highTg)andsoft(lowTg)monomersusedtomakeuptheoverallcompositionwilldeterminetheoverallhardnessandsoftnessofthepolymerfilm.AnestimateoftheTg,andthereforethefilmhardness,canbecalculatedusingtheFoxequationACRYLICESTERPOLYMERS3Table1.PhysicalPropertiesofAcrylicPolymersMonomermolecularCASregistryTg,Density,RefractivePolymerformulanumber◦Cag/cm3bindex,nDPoly(acrylicacid)C3H4O2[9003-01-4]1061.081.496Poly(methylacrylate)C4H6O2[9003-21-8]91.221.479Poly(ethylacrylate)C5H8O2[9003-32-1]−241.121.464Poly(propylacryl