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Macromolecules2005,38,6979-69876979StabilizationandEarlyDegradationofUV-IrradiatedHeterophasicPropylene-EthyleneCopolymersBasedonESR,ESRImaging,UV-Vis,andDSC:EffectofEthyleneContentandUVWavelengthKrzysztofKruczala,†WasimAris,andShulamithSchlick*DepartmentofChemistryandBiochemistry,UniversityofDetroitMercy,Detroit,Michigan48221ReceivedFebruary1,2005;RevisedManuscriptReceivedJune7,2005ABSTRACT:Heterophasicpropylene-ethylenecopolymers(HPEC)containing1wt%bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate(Tinuvin770)asahinderedaminestabilizer(HAS)wereagedbyexposureof≈3mmthickplaquestoUVAandUVBsourcesinweatheringchambersat318K.Twotypesofcopolymers,HPEC1andHPEC2,wereexamined,whichcontained25and10wt%ethylene(E),respectively,asethylene/propylenerubber(EPR).Spatialandtemporaleffectsoftheagingprocesswerestudiedbyelectronspinresonance(ESR)andESRimaging(ESRI)ofHAS-derivednitroxideradicals(HAS-NO),bycomparisonofthermaltransitionsinoriginalandagedsamplesusingDSC,andbymeasuringtheUV-visabsorptionofHPECfilmsasafunctionoftheirthickness.ESRspectraindicatedthepresenceofnitroxideradicalsintwoamorphoussites,fast(F)andslow(S).ThespatialdistributionofHAS-NOwasobtainedby1DESRI.Nondestructive(“virtual”)slicingofthe2Dspectral-spatialESRimagesresultedinaseriesofESRspectrathatreflectedthevariationoftheF/Sintensityratiowithsampledepth.HAS-NOwasdetectedonthenonirradiatedsideduetothesmall,butnotnegligible,intensityofUVlighttransmittedthroughtheplaque;onthissidethenitroxideconcentrationremainshighbecauseitisnotconsumedinstabilizationprocesses,asthelocalrateofdegradationislow.BothESRIandDSCexperimentsindicatedahigherdegradationrateinHPEC2,whichcontainedmorepolypropylene(PP),90wt%;inthesesamplesthedegradationwasaccompaniedbyanincreaseintheextremeseparationintheESRspectra,indicatingslowerdynamicsasaresultoftheincreaseddegreeofcrystallinity.ThedegradationwasalsomoreadvancedinsamplesexposedtoUVBradiationcomparedtoUVAduetothehigherUVabsorptioncoefficientintheshorterλrange.Thediffe
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