HydrophobicallyModifiedAssociativePolymerSolutionsRheologyandMicrostr - PDF document

1 HydrophobicallyModifiedAssociativePolyme
HydrophobicallyModifiedAssociativePolymerSolutions:RheologyandMicrostructureinthePresenceofNonionicSurfactantsRobertJ.English,JonathanH.Laurer,RichardJ.Spontak,andSaadA.Khan*CentreforWaterSolublePolymers,NorthEastWalesInstitute,WrexhamLL112AW,U.K.,andDepartmentsofMaterialsScience&EngineeringandChemicalEngineering,NorthCarolinaStateUniversity,Raleigh,NorthCarolina27695Wereportontherheologyandmorphologyofahydrophobicallymodifiedalkali-swellableemulsion(HASE)polymersolubilizedinalkalinemediacontainingnonionicsurfactants.TheHASEpolymerconsistsofcomplexalkylarylhydrophobescomposedofoligomericnonylphenolcondensatesattachedtoapoly(ethylacrylate--methacrylicacid)backbone.Thecomplexlinearviscoelasticresponseofthepolymerinalkalinesolutionsuggestsanunentanglednetworkwithanappreciablefractionofmicrogel.Theconcentrationandhydrophilelipophilebalance(HLB)ofnonionicsurfactantsprofoundlyaffectthesolutionrheology.AsurfactantofhighHLBinhibitsthedynamicnetworkconnectivityoftheHASEpolymer,asdemonstratedbyreductionsofboththesteady-shearviscosityandthedynamicstoragemodulus.Theshear-inducedstructuringpreviouslyreportedforthispolymerisalsoprogressivelydiminishedasthesurfactantconcentrationisincreased.Incontrast,theadditionofalow-HLBsurfactantpromotessystemstructuring,asevidencedby(i)increasesintheshearviscosityandthehigh-frequencyplateaumodulusand(ii)retentionoftheabilitytoundergoshear-inducedstructuring.Wealsoemploycryofracture-replicationtransmissionelectronmicroscopyforthefirsttimewithregardtoHASEassociativepolymerstoexaminethemorphologicalcharacteristicsofselectedsystems.ThemorphologyoftheHASEpolymerinbothlatexandsolubilizedformappearsmorecomplexthanpreviouslyanticipated,andareasonableinterpretationofthesenewdataisprovided.Hydrophobicallymodifiedalkali-swellableemulsion(HASE)polymersarecomplexterpolymersderivedfromthecopolymerizationofethylacrylate,methacrylicacid,andanethoxylatedmacromerofanonionicsurfactant.Thus,HASEpolymersarestructurallysimilartoªpoly-soaps,ºmacromoleculesbearingmanygraftedam-phiphilicsegments.AlthoughHASEpolymersarecommerciallyimportantasrheologymodifiersinaque-ouspaintsandcoatings,theyarecomplexintermsoftheirmicrostructuralandcompositionalheterogeneity.Asaconsequence,anunderstandingofthedynamicsandmorphologyofHASEpolymernetworksislesswelldevelopedthanthatofassociatingpolymerspossessingsimplerarchitectures,e.g.,telechelicpolymerscomposedofpoly(ethyleneglycol)withterminalhydrophobes.ThisdifferencereflectsthemorecomplexarchitectureofHASEsystemsandtheircontrolledsynthesisbyemulsionpolymerizationinacomplexmultiphaseInparticular,thepreciseassignmentofessentialparameters,suchasthemeannumberofhydrophobesresidinginajunctiondomain(i.e.,theaggregationnumber),constitutesaformidablechal-RecentexperimentalstudiesofHASEpolymershaveaddressedseveralrelevantissues:theeffectsofhydrophobesizeandside-chainlengthonsolutionflowbehaviorinparallelsuperposedsteady/dynamicshear,physicochemicalchangesdur-ingsolubilization,interactionswithsurfactants,andeffectsofaddedelectrolytesonrheologicalbehaviorinthediluteregime.Inthepresentstudy,weexploretherheologicalandmorphologicalcharacteristicsofHASEpolymersinsolutionscontainingnonionicsur-Becausemostwater-solublepolymericrheologymodi-fiersareemployedinformulationscontainingsurfac-tantsorotheramphiphilicspecies,polymerinteractionsareofconsiderabletechnologicalandtheo-reticalinterest.surfactantinteractionsaremanifestedmacroscopicallyinthesolutionrheologyandcaninvolveconformationaltransitionsduetosurfactantadsorption,microphaseseparationbecauseofcomplex-ation,ornetworkmodificationthroughtheformationofmixedmicelles.Mixed-micelleformationisofpar-ticularrelevanceregardingHASEpolymers,asthenetworkjunctionsinvolvedynamicassociationofhy-drophobicgroups.Inthecaseofnonionicamphiphilesexhibitinglowwatersolubility,themicrostructureofaformulationcanbeintrinsicallycomplexandcanbefurthermodifiedbytheinclusionofapolymericspe-Severalstudieshaveexaminedtheimpactofsurfactantphasebehavioronboththerheologyofamphiphilesystemsandthedevelopmentofsystemscapableofthermoreversiblegelation.Forex-ample,Panmaietal.haveexaminedtherheologicalbehaviorofhydrophobicallymodifiedhydroxyethylcel-luloseandpoly(acrylamide)inthepresenceofarangeofsurfactants.Underconditionsfavoringsphericalsurfactantmicelles,interchainhydrophobicinteractions *Correspondingauthor:Phone,919-515-4519;Fax,919-515-3465;e-mail,khan@eos.ncsu.edu.NorthEastWalesInstitute.DepartmentofMaterialsScience&Engineering,NorthCarolinaStateUniversity.Presentaddress:LexmarkInternationalInc.,Lexington,KY40550.DepartmentofChemicalEngineering,NorthCarolinaStateUniversity.Ind.Eng.Chem.Res.10.1021/ie020409sCCC:$22.002002AmericanChemicalSocietyPublishedonWeb11/16/2002 areeffectivelyscreenedathighsurfactantconcentra-

2 tions.Theviscosity,however,remainsunchan
tions.Theviscosity,however,remainsunchangedunderconditionsfavoringworm-orrodlikemicelles.Kaczmarskietal.haveexploredtheinteractionofseveralassociatingpolymersinthepresenceofbothsodiumdodecylsulfate(SDS)andanoctylphenolethoxy-late.Thenonionicsurfactantpromotesstructuringofthesysteminthepresenceofhydrophobicallymodifiedethoxylateurethane(HEUR)polymerswithlargeter-minalhydrophobes.WhileDeguchietal.haveexam-inedthegelationofcholesterol-modifiedpullulaninthepresenceofSDS,Loyenetal.haveinvestigatedthethermoreversiblegelationofhydrophobicallymodifiedpoly(acrylicacid)sinthepresenceoflinearalcoholethoxylates.InteractionsofaHASEpolymerwithnonionicsurfactantsisalsothesubjectofthepresentstudy,inwhichalkylphenolethoxylatesofdifferentlipophilebalance(HLB)andvastlydifferentdegreesofaqueoussolubilityareemployed.Insimilarfashion,Candauandco-workershaverecentlystudiedtheinteractionofhydrophobicallymodifiedpoly(acry-lamide)swithsurfactantsandreportthattheam-phiphile-inducedchangeinlowshearviscosityisat-tributabletomodifieddynamicsofthetransientnetwork.Thismodification,inturn,reflectstheformationofmixedjunctiondomainsthatincorporatethesurfactantandpolymer-boundhydrophobes.Inthiswork,weseektoelucidatetheeffectofsurfactantphasebehavioronthemechanismbywhichahydrophobicallymodifiedpolymerofrelativelycomplexmoleculararchitectureinteractswithsurfactantmolecules.Thedatareportedherearediscussedinlightofrecenttheoreticalconsid-erationsofassociatingpolymersystems,andsuitablecomparisonswithotherclassesofwater-bornepolymersexhibitingtransientassociationsareprovided.ExperimentalSectionA.Materials.ThehydrophobesoftheHASEpolymerexaminedhere(depictedinFigure1)consistedofoligomericcondensatesofnonylphenol,withthemajorhydrophobiccomponentbeingthebis-nonylphenylmoi-ety.Themeandegreeofethoxylationinthemacromer80,andtheweightfractionsofethylacrylate,methacrylicacid,andmacromeremployedinthepoly-mersynthesiswere0.4,0.4,and0.2,respectively.Fulldetailsoftheemulsionpolymerizationusedtosynthe-sizethispolymerwerepreviouslydetailed.TheHASElatexwaspreparedat30wt%solidsunderªstarvedfeedºconditions.RecentanalysisofHASEpolymerspreparedinthismannerbyhigh-field(400MHz)NMRspectroscopyrevealedthatthefractionsofmonomericresiduesinthepolymerwerecomparabletotheindividualmonomerfractionsinthemonomerfeedmixture.FractionationofHASEpolymersinaseparatehowever,indicatedtheexistenceofconsiderablecompositionalheterogeneity,asevidencedbyanunequaldistributionofmacromerthroughoutthechainpopula-MolecularweightdeterminationofHASEpolymerswasperformedbyconventionalGPCanalysis.Nonaque-ousGPCofHASEpolymersinpolarsolventssuchasdimethylformamidewasriddledwithproblemsarisingfromnonstericexclusionofthepolymerfromtheporesofthestationaryphase,whereasaqueousGPCwasadverselyaffectedbysupramolecularchainaggrega-Toovercomethesecomplications,theester/urethanelinkagesoftheHASEpolymerweresubjectedtoalkalinehydrolysis,whichyieldsacopolymerofsodiumacrylate/methacrylatethatisamenabletochar-acterizationbyaqueousGPC.PrioranalysisofHASEpolymerspreparedinsimilarfashiontotheoneinves-tigatedinthepresentstudysuggestedthatisabout250000.BecausethepolymerformulashowninFigure1correspondstoamacromermolefractionofthemeannumberofhydrophobes(ªstickersº)perchainwasestimatedtobe15.Dilutesolutionviscometryrevealedthattheintrinsicviscosity,[],oftheHASEpolymerwas4.8dLgat25ÉCin0.05MNaClatpH9.TheHASEpolymerlatexwaspurifiedbydialysis(usingaSpectropore7cellulosicmembranewithacutoff50000)toremoveserumelectrolyteandexcessanionicstabilizer.ThenonionicsurfactantsemployedhereweresuppliedbyUnionCarbide(nowDow):TergitolNP6,anonylphenolethoxylatewithameandegreeofethoxylationof6[NP6],andTergitolNP10,anonylphenolethoxylatewithameandegreeofethoxy-lationof10[NP10].Bothsurfactantswereusedwithoutfurtherpurification.B.Formulations.Thepolymerlatexwasdissolvedinaqueous2-amino-2-methyl-1-propanol(AMP)ataconcentrationof0.006molofamine/gofpolymer,yieldingapHof9.Sampleswerepreparedataconstantionicstrength(0.05MNaCl)bycombiningtherequisiteamountsofpurifiedlatex;distilled/deionizedwater;0.5MNaCl;1.0MAMP;and,whereappropriate,nonionicsurfactant.TherheologyoftheHASEpolymerintheabsenceofsurfactantwasinvestigatedatpolymerconcentrationsbetween0.4and1.2gdL.ThepolymerconcentrationinsolutionsconsistingofNP6andNP10washeldconstantat0.6and1.0gdL,respectively.TheconcentrationofNP6wasvariedfrom0.05to1.5g,andtheconcentrationofNP10rangedfrom0.1to1.1gdL.Allsampleswerecentrifuged(2500rpmfor5min)toremoveentrainedairandsubsequentlyallowedtositforseveraldayspriortorheologicalC.Rheology.RheologicaltestswereperformedinsteadyanddynamicshearusingaRheometricsScien-tificDSRcontrolled-stressrheometerequippedwithappropriatecone-and-plateandconcentriccylinderge-ometries.Steady-sheardataatlowstrainrates()werederivedfroms

3 equentialcreepexperiments,thusensuringth
equentialcreepexperiments,thusensuringthatthedurationofeachtestwassufficienttoattainasteadystrainrate.High-frequency)dynamicdata,expressedintermsofthedynamicstorageandlossmoduli(,respectively),were Figure1.IdealizedchemicalstructureoftheHASEpolymerexaminedinthepresentstudy.Thehydrophobesconsistofamixtureofoligomericcondensatesofnonylphenol,withthemajorcomponentbeingthebis-nonylphenylmoiety.Ind.Eng.Chem.Res.,Vol.41,No.25,2002 obtainedusingaconi-cylindergeometryonaRheomet-ricsScientificRMS800controlled-strainrheometer.Allexperimentswereconductedat250.1ÉC.Thesteady-shearresponseofthepolymer/surfactantsystemswassensitivetoshearhistory,therebynecessitatingtheuseofpresheartoobtainreproducibledata.Applicationofashearstresssufficienttogenerateastrainrateofeffectivelyeliminatedshearhistoryeffects.Pre-shearingwasperformedfor180s,followedbyarestperiodof120s,priortothestartofeachsteady-shearexperiment.Solventevaporationwaspreventedbycoatingtheexposededgesofatestsolutionwithathinlayeroflow-viscositypoly(dimethylsiloxane)fluid(DowCorningDC200,10cS).D.Microscopy.Cryofracture-replicationtransmis-sionelectronmicroscopy(TEM),alsoknownasfreezefractureTEM(althoughcareisexercisedtoavoidcooling-inducedcrystallization),wasperformedtovisualizethemicrostructureoftheHASE/surfactantsolutionspreparedhere.Asmalldropofsolutionwasgentlyplacedinthewellofabrasssampleholdersothatthemeniscusextendedbeyondthetopofthewell.Theassemblywasrapidlyplungedintoliquidethanecooledbyliquidnitrogentoinducesamplevitrification,andthentransferredunderliquidnitrogentoaprevi-ouslycooledbrassbase,whichwassubsequentlyin-sertedintothespecimenchamberofaJEOLJFD-9000Ccryofracture-replicationunitmaintainedat170ÉCandca.10Torr.Oncethermalequilibrationwasachieved,thespecimenwasfracturedwithamicrotomeblade,andthefreshfracturesurfacewasfirstlightlyetched,shadowedwithPt/Catanangleof45ÉandthenstabilizedwithCdepositedat90Éoffthehorizontal.ResultantreplicaswerecleanedinsuccessivewaterbathsandimagedonaZeissEM902electronspectro-scopicmicroscopeoperatedat80kVinzero-lossmode0eV).Negativesweredigitizedat600dpiforpresentationpurposes.ResultsandDiscussionA.LinearViscoelasticResponseoftheHASEThelinearviscoelasticresponseoftheHASEpolymersolutionsintheabsenceofaddedsurfactantistypifiedbythedataprovidedinFigure2,wherethefrequencyspectraofthecomplexviscosity(*)anddynamicmoduli()aredisplayed.Thesedataarereminiscentofareversiblenetworkwhereininter-chaininteractionsaretransient,inwhichcasethesystemdoesnotbehaveasaphysicalgel.Thatis,atransitionoccursfromanelastic-dominatedresponse()athighfrequencies(1rads)toaviscous-dominatedresponseatlowerfrequencies.Examplesofsystemsexhibitingsimilarviscoelasticbehaviorincludesolutionsofahydrophobicallymodifiedgalactomannanandpoly(vinylalco-Thedynamicsofsuchatransientnetworkcanbedescribedintermsofaterminalrelaxationtime,,thatisgovernedbyboththekineticsofhydro-phobicdisengagementandthemeannumberofhydro-phobesonachainforcomb(graft)copolymers.procedureshavebeenestablishedtoascertainfromthefrequencydependenceofthedynamicInthepresentcase,canbeconve-nientlyestimatedfromthereciprocalofwiththemaximuminthelossmodulus,,thatoccursattheonsetoftheelasticplateauregion.Thecon-nectivityofthenetworkcanbeexpressedasaplateauorpseudoequilibriummodulus,,givenbythevaluecorrespondingto.Alternatively,canbecalculatedbyintegratingacrossthemaximumasfollows:Theupperlimitofthisintegrationmustbesettoexcludetheonsetofthetransitionzoneforconcentratedpolymersystemssuchasmelts.Althoughthecalculatedfromtheaboveapproachesdifferslightly,theyexhibitsimilardependenceonpolymerconcentra-Atexperimentallyaccessibletimescalesshorterthan,interchainconnectivityismaintainedasaconsequenceofhydrophobeassociationinmicellardomains.Incertaintypesofhydrophobicallyassociatingpolymers,connectivityarisesexclusivelyfrominterchainassociationofthehydrophobes.Inothersystems,however,chainentanglementsalsocontributetothenetworkstructure.Recenttheoreticaltreatmentsofassociatingpolymersconsiderbothscenarios.thecaseofunentangledchains,thelineardynamicscanbeconsideredRouse-like,modifiedbythepresenceofassociativegroupsorstickers.Inanalogousfashion,thedynamicsofentanglednetworksofassociatingpolymerscanbedescribedbyasimilarmodificationofreptationtheory.ThecriticalquestionofwhetherchainentanglementsaresignificantinsolutionsofHASEpolymersisconsideredinmoredetailbelow.Attimescalesmuchlongerthan,thedynamicmoduliareseeninFigure2todecreaseaccordingtoapower-lawrelation.Aninterestingfeatureofthesedataisthatafullyterminalresponseisnotachievedevenatthelowestexperimentallyaccessiblefrequencies.Specifically,theexponentsdescribingthefrequencydependenceofthemoduliaremuchlessthanthevaluesof1expectedfor)and2expectedfor).Notealsothat)doesnotattainalimitingvalueatlow,inmarkedcontrasttot

4 hebehaviorofhydrophobicallyassociatingpo
hebehaviorofhydrophobicallyassociatingpolymersforwhichcompletestressrelax-ationoccursatcomparabletimescales.Anexampleofsuchapolymerishydrophobicallymodifiedpoly(acry-lamide)preparedbymicellarpolymerization.apparentweakpower-lawdependenceof)and)inFigure2persistsforalmost3decadesinfrequencyandstronglysuggeststhesuperpositionofa Figure2.Frequencydependenceofthesmalldeformationmaterialfunctionsfora0.8gdLsolutionoftheHASEpolymersolubilizedinthepresenceofAMPin0.05MNaCl. dlnInd.Eng.Chem.Res.,Vol.41,No.25,2002 slowmodeofstressrelaxationinadditiontothatarisingfromthereversiblenetwork.ItisinterestingtonoteatthisjuncturethatasimilarsuperpositionofasingleMaxwellmodeoverapower-lawspectrumofrelaxationtimeshasbeenpreviouslyforsolutionsofatelechelicassociativepoly-mercontaininglatexparticles.Inthiscase,power-lawbehaviordominatesatlongtimescalesandhasbeenattributedtocolloidalprocesseslinkedtodiffusionofthelatexparticles.InapriorrheologicalstudyofHASEpolymers,wehavealsonotedaweakpower-lawdependenceof)and),whichweattributedtoamicrostructurecomposedofclose-packedswollenmicrogelparticleswhosecompressibilitydictatesthemagnitudeoftheelasticmodulus.Areasonableexpla-nationoftheseunexpectedobservationsclearlyrequiresanimprovedunderstandingofthemicrostructureinsystemscontainingcarboxy-functionallatexparticlesswolleninthepresenceofalkali.Thescalingbehaviorofthezero-shearviscosity,andplateaumodulus,,asfunctionsoftheeffectivepolymervolumefractionispresentedinFigure3.Examiningthepolymerconcentrationonavolumetricbasisisusefulinthatitpermitsassessmentoftheextentofcoiloverlapandprovidesinsightintowhetherchainentanglementsarelikelytoimpactviscosification.RubinsteinandSemenovhaverecentlyprovidedathoroughtheoreticaltreatmentofthelinearrheologyofcomb-typeassociatingpolymers.Inanearlierwork,Leibleretal.haveconsideredthecaseofentanglednetworksinwhichthechainscontainnumerousas-sociatinggroups.Bothtreatmentsassumethatthestickersonthechainsassociatepairwisetosimplifythedescriptionofthenetworkstructureintermsofthefractionofstickersthatarefree(unassociated)orinvolvedineitherintra-orintermolecularassociation.Incontrasttothesemodels,thepolymerconsideredinthepresentstudymostlikelyformsmicellarjunctionsinafashionanalogoustononionicsurfactants.Withthisdifferencenotwithstanding,itisnonethelessusefultocomparethepredictionsoftheserecenttheo-rieswiththeexperimentalresultsreportedhere.Inparticular,theformalismproposedbyRubinsteinandpredictsseveraldistinctconcentrationre-gimes(forbothunentangledandentangledsystems)thatreflectchangesinnetworktopologyduetoanincreaseintheconcentrationofassociatingpolymer.Atverylowpolymerconcentrations,singlechainsremainmolecularlydispersedinsolution.Athigherconcentrations,thesechainsaggregateintoclustersthatarerelativelylong-lived,i.e.,thelifetimeofatypicalclusterislongrelativetoitshydrodynamicrelaxationtime.Uponincreasingthepolymerconcentrationfur-ther,apolydisperseensembleoflargerªdynamicºclustersensues.Thelifetimeofaclusterismuchshorterthanitsrelaxationtime,inwhichcasethestructureoftheclusterfluctuates,constantlybreakingandreform-ing.Thisregimeofdynamicclusterspersiststhroughthepercolationconcentration,*,whereasample-spanningnetworkforms.Atyethigherconcentrations,allthechainsareincorporatedintothenetwork,buttheconnectivityremainslow,withonlyafewcriticalbondssupportinganimposedstress.Underthesecondi-tions,manyofthechainsarenotmultiply-connectedtothenetwork,andassociationoccurspredominantlybetweenstickerslocatedonthesamechain.Onlyatveryhighconcentrationsdomostchainsbecomemultiply-connectedtothenetwork.EachoftheconcentrationregimesdescribedabovecanbeidentifiedbyadifferentviscosityscalingrelationshipofthesameformatasthedatashowninFigure3.Atpolymerconcentrationsinslightexcessof*,apronouncedchangeintheconcentrationdependenceofthezero-shearviscosityispredicted.Inthisunen-tangledregime,thischangereflectsatransitionfrompredominantlyintramoleculartointermolecularas-sociationasthepolymerconcentrationisincreased.Suchatrendisanalogoustotheloopbridgetransitionpresumedtooccurinsolutionsoftelechelicassociatingpolymerswithalkylendcaps.Atransitionfromintramoleculartointermolecularassociationwithin-creasingpolymerconcentrationhasbeenpreviouslyforthepolymerconsideredinthepresentstudyonthebasisoftheextentofshear-inducedstructuringachievedduringnonlineardeformation.Iftheconcentrationofpolymerinthesystemisincreasedtoalevelsuchthatthestrandsbetweenstickersoverlap,thenmostoftheassociationinterac-tionsshouldoccurbetweenstickersondifferentchains.AnetworkofthisstructureresultsatavolumefractionInthiscase,amuchreducedconcentration(dependenceofthezero-shearviscosityisexpected:intheregionofconversionofintramoleculartointermolecularassociation,andoncestickersassociateexclusivelybetweendifferentchains.Suchbeha

5 viorhasbeenobservedexperimentallyforsolu
viorhasbeenobservedexperimentallyforsolu-tionsofgraftcopolymerscomposedofpoly(ethylene-poly(propyleneoxide)--poly(ethyleneoxide)withacrylicacid.Inthecaseofsolutionsconsistingofentangledassociatingpolymers,asimilarchangeinthescalingbehaviorofbecomesevidentoncethestrandsbetweenintermolecularlyassociatedstickersoverlap.Inthisregime,furtherviscosificationaccompaniesincreas-ingpolymerconcentrationbecauseofanincreaseinentanglementdensity,andascalingexponentofThisscalingbehaviorhasrecentlybeeninthecaseofhydrophobicallymodifiedpoly-(acrylamide)spossessingblockyhydrophobicsequences.Atpolymerconcentrationsbelowthatrequiredforstrandoverlapinentangledsystems,asharplypro-nouncedchangeintheconcentrationdependenceofthezero-shearviscosityispredicted,ItisinterestingtonotethatthescalingexponentsderivedfromFigure3,7.9and2.7,arereasonablyclosetothosepredictedforsolutionsofentangledassociatingpolymersbelowandabovethepolymerconcentrationforstrandoverlap():8.5and3.75,respectively. Figure3.Scalingofthezero-shearviscosityandplateaumodulusasfunctionsofpolymerconcentration.Ind.Eng.Chem.Res.,Vol.41,No.25,2002 Carefulconsiderationofthedata,however,precludesentanglementasamechanismforviscosification.Mostnotably,theeffectivevolumefractionofthepolymerpolymerè]intheconcentrationrangeexamined(0.4gdL)doesnotexceed2.5.Inthecaseofsolvatedisameasureofcoiloverlapandcanbeinterpretedasthenumberofneighboringchainswhosecenterofmassfallswithinthevolumeoccupiedbyagivencoil.Thus,inthisconcentrationrange,thepoly-mercoilsareonlyslightlyinterpenetrated,anden-tanglementsarenotexpectedtodeveloptoasignificantFurtherverificationofunentangledHASEchainsisobtainedfromthemagnitudeof.Thequantityisrelatedtothechaincontourconcentration,measureofthenumberdensityofelasticallyactivestrandsinthenetworkor,equivalently,themechanicalinteractiondensity.Theplateaumodulusisalsorelatedtothenumberofstepsintheprimitivepathofthechain),thatis,thenumberoftubesegments,bythefollowingexpression:isthechaincontourlength,isadimensionontheorderofthemeshsizeofthenetwork,andisthenumberdensityofchains.Theconceptofaconfiningtubeisvalidonlyforvaluesof1.Atthehighestconcentrationsstudied,isestimatedtobeabout0.1fortheHASEpolymer.CurrentlyunreportedresultsforHASEpolymerspreparedundersimilarconditionssuggestthattheofthesepolymerscantypicallybeDa.Ifthiswerethecase,wouldbe1atmost,whichwouldstillbetoolowavalueforreptationtooccur.Ifthesystemwereentangled,however,itwouldbeimpossibletodistinguishrheologicallybetweenhydrophobicassociationsandtopologicalentanglements,asboththesetypesofinteractionwouldcontributetoOuraboveestimatesofcoiloccupancyandmechanicalinteractiondensityappeartoprecludethepossibilityofentanglementsinthesystemunderstudy,incontrasttoourpreviousinterpretationofthebehaviorofthisInthisrespect,thechangeinviscosityscalingbehavior(Figure3)cannotbeattributedtotheoverlapofstrandsbetweenhydrophobes.ItisalsousefultonotethatEA/MAAcopolymersforwhichissimilartothatofthepolymerofthepresentstudyexhibitaviscosityscalingrelationshipoftheformsimilarlevelsofcoiloccupancy.Thiswouldagainprecludetheexistenceofchainentanglements.Polymerswithouthydrophobeswouldbeexpectedtoshowascalingexponentclosetothepredictedvalueoftheentangledregime,theonsetofwhichshouldoccuratavalueofintherange510.FurtherworkisthereforerequiredtoelucidatethechangesinnetworkstructureunderpinningtheobservedchangeinviscosityB.NonlinearViscoelasticResponseoftheHASEThenonlinearresponseoftheHASEpolymersolutionsisillustratedinFigure4.Aseriesofsequentialtransitionsfromalinearresponsethroughweakshearthinningtoshear-inducedstructuringisobservedwithincreasingstrainrate.Theoriginofshear-inducedstructuringinHASEpolymershaspreviouslybeenconsideredindetailelsewhere.AnoteworthyfeatureofFigure4isevidentathighpolymerconcentrations(0.7gdL).Inthisregime,theflowcurveexhibitsamarkeddiscontinuity.Becausethesedatawereacquiredwithacontrolled-stressinstrument,suchadiscontinuitydemonstratesthatflowoccursatconstantstress.Thisbehaviorimpliestheonsetofstresssaturationwherein.Thesetestshavebeenrepeatedonacontrolled-straininstrument(RMS800),inwhichcaseatimedelayof45swasappliedateachstrainratepriortomeasure-mentoftheshearstress(torque).ConsistentwiththedatashowninFigure4,themeasuredshearstresssaturatestoaconstantplateauvalue,,atpolymerconcentrationsabove0.6gdL.Thisbehaviorisstrikinglysimilartothatofwormlikesurfactantmi-celles,forwhichastressplateauisoftenobservedintheflowcurve.Pronouncedshearthinningathighstrainratesinassociatingpolymersolutionshasbeenattributedtoseveralphenomena.Previousstudieshaveinterpretedthischaracteristicintermsofnonaffinenetworkdefor-Alternatively,ithasbeenrelatedtothepointatwhichthetimescaleoftheimposeddeformationbecomesshorterthantherateofassociationformation,suggestingthatthenumberofhydrophobica

6 ssociationsbecomesdepleted.Inthisregard,
ssociationsbecomesdepleted.Inthisregard,itisagaininterestingtoconsiderthesimilaritiesbetweentherheologicalresponseofHASEpolymersolutionsandthatofworm-likesurfactantmicelles.Catastrophicshearthin-ningorstresssaturationobservedinthesesystemshasbeenattributedtothedevelopmentofshear-bandingflowinstabilities.Thus,intheconstant-stressplateau,thesystembecomesheterogeneous,phase-separatingintozoneswithdifferentvelocitygradients.Thisphe-nomenonistheoreticallypredictedforapurelymechan-icalflowinstabilitybyMcLeishandco-workers,proposethattheplateaushearstress,,andtheplateaumodulusmeasuredinoscillatoryshear,,aregenerallyrelatedbyEquation3hasbeenverifiedexperimentally.Whereastheplateaustressisproportionaltothesteady-shearplateaumodulusfortheHASEpolymersolutionsinFigure5,therelationshipestablishedhereis,whichdeviatesconsiderablyfromthepredictedrelationshipabove.Thereasonforthisdifferenceispresentlyunclear,butrheo-opticalstudiesofwormlikemicelleshavedetectedadditionalflowinstabilitiesduetotheformationofelongatedmicrostructuralelements Figure4.NonlinearresponseofsolutionsoftheHASEpolymerexpressedintermsoftheshearstressasafunctionoftherateof Ind.Eng.Chem.Res.,Vol.41,No.25,2002 thatalignwiththeflowfield.Theseorientedelementsarepresumedtoformwhenthecharacteristicflowtimeisshorterthanthemicellarbreakingtime.AlthoughitisdifficulttoprovideadetailedmechanisticexplanationforshearthinninginHASEpolymersystems,possibleconsiderationsinclude(i)thepersistenceofhydrophobicassociationsathighstrainrates,(ii)mechanicaland/ormicrostructure-inducedflowinstabilities,(iii)nonaffinenetworkdeformation.Itisalsointerest-ingtonoteherethatstresssaturationhaslikewisebeenobservedinsolutionsofpoly(vinylalcohol)andborate,anotherpolymersystemthatexhibitstransientassocia-Flowinstabilitieshavealsobeenreportedinoil-in-watermicroemulsionscomposedofatelechelicasso-ciatingpolymer,aswellasincubicphasesofblockcopolymermicelles.C.ViscoelasticResponseoftheHASEPolymerwithNonionicSurfactants.RheologicaldataforsolutionsoftheHASEpolymerinthepresenceofthewater-solublesurfactantNP10arepresentedinFigures6and7,whereasdatacollectedfromcomparablesolu-tionscontainingNP6,whichexhibitslimitedwatersolubility,areprovidedinFigures810.AnincreaseintheconcentrationofthehydrophilicNP10surfactantisaccompaniedbyapronouncedreductioninthemagnitudeofthezero-shearviscosity,whichisseeninFigure6todecreaseby3ordersofmagnitudeovertherangeofsurfactantconcentrationexplored.Previousofhydrophobicallyassociatingpolymersinthepresenceofanionicandnonionicsurfactantsatconcentrationswellabovetheircriticalmicelleconcen-tration(cmc)alsoreportsubstantialviscosityreduction.Aboveitscmc,NP10isanticipatedtoformsmall,sphericalmicelles.Itimmediatelyfollowsthatpolymer-boundhydrophobesmightbecomeincorporatedinthesemicelles,formingmixedjunctiondomains.quently,astheconcentrationofsurfactantmicellesinthesystemisincreased,theoverallfunctionalityofthejunctiondomains(i.e.,themeannumberofpolymer-boundhydrophobespresentinagivenjunctiondomain)isconcurrentlyreduced.Thislossinconnectivityisreflectedmacroscopicallyasdecreasesinthemagni-tudesofAnotherapparenteffectthatoccursuponincreasingtheconcentrationofNP10isamarkedreductioninshear-inducedstructuring(seeFigure6).Thisobserva-tionisagainconsistentwiththeformationofmixedjunctiondomains,asdescribedabove.Lowconcentra-tionsofsurfactantinsolutionsofcomblikeassociativepolymerscan,insomecases,induceanentropicallydrivenchangeinnetworktopology,yieldinganincreaseinintermolecularhydrophobicassociationsattheex-penseofintramolecularones.Thisscenarioisex-pectedtopromoteanoverallincreaseintheconnectivityofthedynamicnetworkandacorrespondingincrease,asreportedbyAubryandMoanforahydropho-bicallymodifiedgalactomannanetherwithnonionicsurfactant.OurpreviousphotophysicalstudythatthecmcofNP10iswellbelowtherangeofconcentrationsemployedinthecurrentstudy.Infact,theconcentrationofNP10isover2ordersofmagnitudehigherthantheconcentrationofpolymer-boundhydro-phobesinthesystem,whichresultsinthedepletednetworkconnectivitydiscussedabove.Thus,asurfactant-mediatedreductioninintramolecularpolymerassociationsisconsistentwiththedatainFigure6andtheapparenteliminationofshear-inducedstructuring.Insimilarfashion,highconcentra-tionsoftheNP10amphiphileeffectivelyreduceinter-molecularassociations.Thisaccountsfor(i)thesteady-shearresponsebecomingmorelinearand(ii)thedisappearanceofthesurfactant-freeshearthinning/stresssaturationregimesapparentinFigure7.There-fore,interactionsoftheHASEpolymerwiththeNP10surfactantrevealaprofoundeffectofhydrophobicinteractionsonnetworkconnectivityandmoleculardynamicsinassociativepolymersystems.Wenoteherethat,athighNP10concentrations,thesteady-shearresponseofHASE/NP10solutionsissurprisinglyanalo- Figure5.Variationoftheplateaustress(measuredinther

7 egionofstresssaturationattainedathighstr
egionofstresssaturationattainedathighstrainrates)withtheplateaumodulusdeterminedunderconditionsoflinearity. Figure6.MerzplotsforsolutionsoftheHASEpolymercontainingvariousconcentrationsoftheNP10surfactant.Solidsymbolscorrespondtothesteady-shearviscosity(),whereasopensymbolscorrespondtothecomplexviscosity( Figure7.EffectoftheadditionofNP10onthenonlinearresponseoftheHASEpolymer.Ind.Eng.Chem.Res.,Vol.41,No.25,2002 goustothatofalkali-swellablepoly(ethylacrylate-methacrylicacid)latexpreparedwithoutmacromer.Selectedsteady-sheardatafortheHASEpolymerwithandwithouttheNP6surfactantarecomparedinFigure8anddemonstratethatNP6affectstheHASEsolutiondifferentlythanitsNP10counterpart:increased,andshear-inducedstructuringispreserved.AthigherconcentrationsofNP6showninFigure8(1.5gdL),thesystembecomesprogressivelymorestruc-turedandgel-like.Undertheseconditions,thepossibil-ityofshearfractureandthegenerationofflowinsta-bilitiesprecludereliablecharacterizationinsteadyshear,inwhichcasewenowturnourattentiontodynamicexperimentsasameansofprovidingadditionalmicrostructuralinformation.TheNP6-inducedbehaviorseeninFigure8islikewiseevidentinthefrequencyspectraofthedynamicmodulishowninFigure9.ValuesofderivedfromFigure9areincludedinFigure10andconfirmthatanincreaseinNP6concen-trationpromotesasystematicincreaseinand,hence,networkconnectivity.AnincreaseinNP6contentisalsoaccompaniedbyaconcurrentincreaseinthecharac-teristictime,,whichreflectsashiftoftheterminalregiontoprogressivelylowerfrequencies.AccordingtothedatadisplayedinFigure10,increasesnearly2-foldastheconcentrationofNP6isincreasedfrom0.1to1.5gdL.Moreimportantly,isobservedtoincreasebyabout2ordersofmagnitudeoverthesameconcentrationregime.Thissurfactant-inducedchangeappearstobeprincipallyresponsiblefortheincreaseinviscosityachieveduponadditionofthissurfactant.Thesetrendsmight,how-ever,beunderpinnedbymorecomplexphasechangesinducedbyincorporationofanamphiphilicspeciesofrelativelylowaqueoussolubility.SurfactantphasebehaviorhasbeenpreviouslyconsideredmicrostructuraldevelopmentinHASEpolymersolu-tionscontainingaprimaryalcoholexthoxylateoflowHLB(C).Nonionicsurfactantswithlittleethoxy-lationcanexhibitarichvarietyofcomplexmicrostruc-turesinthepresenceofwater-solublepolymers.Specif-ically,disperseduni/multilamellarvesicles(L)orªspongeº-type(L)bicontinuousmicrostructurescanbeThethermodynamicallystablemicro-structurewillbeinfluencedbysuchdesignconsider-ationsasthepolymerandsurfactantconcentrations,aswellasthesurfactantHLB.D.MicrostructuralCharacteristicsoftheHASEPolymerwithNonionicSurfactants.Figure11aisaTEMimageofdilutedHASElatexparticlespriortodissolutioninthepresenceofalkali.Thediscreteparticlesappearnonspherical,exhibitinganirregularªraspberry-likeºmorphology.Becauseonlysmallnum-bersofparticlesareevidentinthisandcomplementaryimages,theparticlesizedistributioncannotbeascer-tained.Rather,wenotethatseveralparticlesexhibitdiametersof300400nm,whereassmallerparticlestypicallymeasureca.100nmacross.Thisdifferenceinsizescalemight,however,reflectdifferencesinthelocationofthefracturefrontasitpropagatedaroundtheparticlesduringspecimenpreparation.Thefineinternalstructureoftheseparticlesappearscomplexandsuggeststhattheparticlesgrowbyacoagulativemechanisminwhichlargeparticlesformbyaggregationofnumeroussmallerparticlesmeasuringabout30nmindiameter.Insimilarfashion,theinfluenceofparticlecoagulationonthegrowthoflatexparticleshasrecentlybeenexploredinthecopolymerizationofethylacrylateandmethacrylicacid.Coagulationinanemulsionpolymerizationisaug-mentedbytheuseofpersulfatesaltsasfree-radicalinitiators.Iftheinitiatorfeediscontinuousthroughoutthepolymerization,thetendencyforparticlecoagulationincreasesastheionicstrengthoftheaqueousmediumincreases,becausetheelectrostaticdoublelayerssur-roundingthegrowingparticlesbecomecompressed.SuchinstabilityinthepresentworkiscorroboratedbytheobservationthattheconcentratedHASElatex Figure8.MerzplotsforsolutionsoftheHASEpolymerwithandwithouttheadditionoftheNP6surfactant.Solidsymbolscorrespondtothesteady-shearviscosity(),whereasopensymbolscorrespondtothecomplexviscosity( Figure9.Frequencyspectraofthedynamicmoduli[)]measuredfortheHASEpolymersolutionswithandwithouttheadditionofNP6. Figure10.InfluenceoftheadditionofNP6ontheplateaumodulusandnetworkrelaxationtimeofHASEpolymersolutions.Ind.Eng.Chem.Res.,Vol.41,No.25,2002 undergoessedimentationuponstorage.Samplesinwhichexcessserumelectrolyteisremovedbydialysisexhibitnosuchinstability.WhilethepresentHASEpolymerwaspreparedatasolidscontentofwhichiscommoninindustrialpractice,polymerswithincreasedcolloidalstabilityandsmallerparticlediam-eters(70100nm)canbepreparedbyreducingthesolidscontenttoCoagulativeparticlegrowthcanbeconfirmedbyrecognizingthatclustersofthistypeexhibitadistincttendencytodegenerate

8 intosmallerparticlesuponalkalinization.B
intosmallerparticlesuponalkalinization.Byincreasingthedegreeofneutralization,,ofthebackbonecarboxylgroupsuponadditionofalkali,thehydrodynamicradius)oftheparticlesincreasestoamaximumofnmat0.4.Athigher0.60),theswollenparticlesdisintegrateintonumeroussmalleroneswith90nm.Incontrast,longerreactiontimesandincreasedsolidslevelshinderparticledissolution,pre-sumablyduetoincreasedcross-linking,whichcanariseviachain-transferreactions.Figure11bdisplaysthemicrostructureofa1.0gdLsolutioncomposedofsolubilizedHASEpolymerinthepresenceof0.5gdLNP10.Thisimageprovidesinformationregardingthemicrostructureofthesolu-bilizedpolymerandconfirmsthatthepolymerandsurfactantdonotphase-separate.Thisobservationisconsistentwiththerheologicaldatapresentedearlier,inwhichNP10issurmisedtoreducemolecularcon-nectivityviatheformationofmixedjunctiondomainswiththepolymerhydrophobes.TheimageshowninFigure11brevealsthatconsiderableheterogeneityexistsinsystemsconsistingofsolubilizedHASEpoly-mers,asarelativelylargenumberofcompactobjectsmeasuringabout30nmacrossareclearlyvisible.Aschematicdiagramofthemicrostructureisshownin Figure11.Cryofracture-replicationTEMimagesofHASEpoly-mersolutionswithandwithoutsurfactant:(a)HASElatexpriortosolubilization,(b)1.0gdLsolutionofthesolubilizedHASEpolymerwith0.5gdLNP10,(c)0.6gdLsolutionofthesolubilizedHASEpolymerwith0.8gdL Figure12.SchematicillustrationsrelatingtotheTEMmicro-graphsdisplayedinFigure11bandc:(a)HASEpolymerwithNP10and(b)HASEpolymerwithNP6.In(a),aggregatesmeasuringabout30nmacrossandcomposedofanEA/macromer-richpolymerfractiondevelopinamatrixofasolubilizedMAA-richpolymerfraction.Inthiscase,NP10ismiscibleandformsmixedjunctiondomainswiththepolymerhydrophobes.In(b),thepresenceofpolymerpromotesdepletionflocculationofNP6micellaraggregates,inwhichcasethesystemsegregatesintopolymer-richandsurfactant-richphases.Ind.Eng.Chem.Res.,Vol.41,No.25,2002 Figure12a.RecallthatthecontrastinthisimageisprovidedbyshadowingafreshfracturesurfacewithevaporatedPt.Toenhancecontrastpriortoshadowing,thespecimenhasbeenbrieflyetchedat100ÉCtoremove(bysublimation)thetoplayerofvitreousicefromthefracturesurface,consequentlyexposingmi-crostructuralfeaturesofinterest.ToassignthefeaturesevidentinFigure11b,itisinstructivetoconsiderpriormorphologicalstudiesofsolubilizedHASEpolymerconductedbycomplementarytechniques,suchasdy-namiclightscatteringandpulsedgradientspin(PGSE)NMRspectroscopy.SeveraldynamiclightscatteringstudiesofHASEpolymersindicatetheexistenceoftwopopulationsofparticlesdifferingintheirtranslationaldiffusioncoef-ficients(Theªfastºparticlesarecharacterizedbyvalueofabout1.2,whichcorrespondsto20nm.Theslowerpopulationexhibitsahydrodynamicrelaxationtimeatleastanorderofmagnitudesmaller,withdecreasingto(2increasingto80100nm.ThesepopulationshavebeenascribedtosingleHASEpolymerchainsandclusterswithupto5chains,respectively.Thelatterresultisderivedfromstaticanddynamiclightscatteringontheassumptionthattherefractiveindexgradientsofthetwoparticlepopulationsareidentical.SupportingstudiesofHASEpolymersystemsbyPGSENMRspectroscopyconfirmthepres-enceoftwoparticlepopulationsexhibitingtranslationaldiffusioncharacteristicssimilartothoselistedabove.However,NMRdataalsoyieldinformationaboutchemi-calconstitution.AlthoughthemeancompositionofpurifiedHASEpolymersdeterminedbyHNMRspec-troscopyiscomparabletothatofthemonomerfeedemployedinthepolymerization,thefasterparticlesareenrichedinbothmacromerandethylacrylate.Theslowparticles,ontheotherhand,tendtoberichinmethacrylicacidanddepletedinmacromer.Suchcompositionalheterogeneityhasbeenverifiedbysamplefractionationviaprecipitationfromtetrahy-drofuran(THF)/hydrochloricacid.Fractionssimilarincompositiontothefast-andslow-diffusingparticlepopulationsexhibitdiffusioncharacteristicssimilartothoseidentifiedabove.Thus,itisreasonabletocon-cludethatthefastspeciesmostlikelyrepresentshydrophobicchainsexhibitingacompactconformation,whichisconsistentwithheightenedlevelsofmacromerandethylacrylate.Moreover,thehydrodynamicsizeofthefast-diffusingspeciesisinfavorableagreementwiththeca.30-nmobjectsvisibleintheTEMimageofthesolubilizedHASEpolymer(seeFigure11b).Suchcom-pact,highlyhydrophobicdispersionsareanticipatedtowithstandfractureandsubsequentsurfaceetching.Iftheslowparticlesconsistofhighlyswollenhydrophobicchainsthatarerichinmethacrylicacidanddepletedinmacromer,itislesslikelythattheseobjectswouldbeasclearlydiscernible(ifdetectableatall)asthemorecompactparticles.ThisexpectationissupportedbyFigure11b,inwhichlargeparticlespossessingahydrodynamicsizeof200nmarenotvisible,aswellasrecentattemptstoimagethemesoscopicstructureinaqueoussolutionsofguargalactomannan.Inthiscase,theguargalactomannansolutions,whichexhibitrheologicalcharacteristicscomparabletothoseofthepresentHASEpolymersolutions(includingcomp

9 lexmodesofstressrelaxationthatpersistove
lexmodesofstressrelaxationthatpersistoverlongtime),appearhomogeneouswhenimagedbycryof-racture-replicationTEM.Thecompositionalheterogene-itydeemedresponsiblefordifferentparticlepopulationsreflectsthesynthesisofHASEpolymersinacomplexmultiphaseenvironment.Inadditiontogeneratingfractionsdifferinginoverallcomposition,itisplausiblethatunexpectedsequencedistributionsdevelopuponmacroradicaltransferfromtheaqueousphase.photophysicalstudies,forinstance,suggestthatthedistributionofethylacrylateinHASEpolymersisTheTEMimageprovidedinFigure11cillustratesthemicrostructureofasolutionofsolubilizedHASEpoly-merinthepresenceof0.8gdLNP6.Inthisimage,atexturedsemicontinuous,ratherthandispersed,mor-phologyisevident.Phaseseparationofmixturescom-posedofwater-solublepolymersandsurfactantsiswellasaconsequenceofdepletion-typeinteractions.Depletionflocculationandphasesepara-tioninthecaseofsurfactantmicellesoccurviaamechanismthatissimilartothatresponsiblefortheflocculationofcolloidalparticles.Thus,thelargeglobu-larfeaturesmeasuringuptoca.1macrossarebelievedtoconstituteasurfactant-richphase,whoseprecisemicrostructureisill-defined.Althoughbilayereddispersions(vesicles,designatedL)mightdevelopwhennonionicsurfactantsinteractwithlinearalkylhydro-phobesatrelativelyhighconcentrations,thesurfac-tant-richphaseseeninFigure11cmostlikelyconsistsofflocculatedmicelles.Micelleflocculationhasbeenobservedtooccuratlowconcentrationsofethoxylatednonionicsurfactantsattemperaturesabovethecloudpoint.Unfortunately,nocompletephasediagramforNP6currentlyexiststothebestofourknowledge.However,atlowconcentra-tions(asinthepresentstudy),thepolymer-freemicro-structureofCconsistsoflargemicellaraggregates(Wphase)coexistingwithsphericalmicelles(Laccordingtocryo-TEM.Ingeneral,theadditionofpolymericspeciestoaqueoussolutionsofnonionicsurfactantsservestolowerthecloudpoint,butthemicrostructureofthesurfactant-richphaseformeduponsuchmixingisusuallysimilartothatofthepureAssociativepolymers,whichcanbindsurfactantswithintheirjunctiondomains,arelessintrusiveintermsofdepletionflocculation.SincethecloudpointofNP6inwaterismostlikelywellbelowthetemperatureatwhichthepresentanalysiswasconducted(20ÉC),weconcludethattheadditionofHASEpolymerpromotesextensiveflocculationofthemicellaraggregatesandthatviscosificationensuesthroughphasesegregationintopolymer-andsurfactant-richphases(seeFigure11c).AschematicdiagramillustratingtheproposedmicrostructureofHASE/NP6derivedfromcombinedrheologicalandmorphologicalconsiderationsisprovidedinFigure12b.ThepresentstudyillustratesthecomplexnatureofHASEpolymers,aloneandincombinationwithnonionicsurfactants.Regardingthepolymersolutionsalone,suchbehaviormostlikelyreflectsacomplexinterplaybetweencompositionalheterogeneitywithinthepolymerandthemechanismofparticlegrowthduringsynthesis.Compositionalheterogeneityinpolymersofthistypeisconsistentwithdifferencesinthewatersolubilityofmethacrylicacidandethylacrylate,whichisfurthercomplicatedbycoagulativeparticlegrowth.PropagationInd.Eng.Chem.Res.,Vol.41,No.25,2002 ofmacroradicalswithinparticlesgeneratesarelativelyhydrophobicfractionenrichedinethylacrylate,whichpromotestheformationofcompactaggregatesuponsolubilizationinanalkalinemedium.ExistenceofslowmodesofstressrelaxationimpliestheexistenceofHASEpolymeraggregates,whichcanbevisualizedbyTEM.Theconcentration-dependentlinearrheologicalresponseofHASEpolymersolutionscanbecompared,atleastqualitatively,topredictionsfromcontemporarytheoreticaltreatments.Ofparticularinterestisthescalingbehaviorofthezero-shearviscosity,whichisdescribedbytworegimesacrosstheconcentrationrangeexamined.Thefirstismostlikelyattributedtotheconversionofintramoleculartointermolecularassocia-tions,whereasthephysicaloriginofthesecondregimeisnotyetfirmlyestablished.Uponadditionofanonionicsurfactant,therheologicalandmorphologicalcharacteristicsoftheHASEpolymersolutionsaresubstantiallyaltered,dependingonthecompositionsofthesurfactantandsolution.Inthiswork,wehaveexaminedtheeffectsoftwoethoxylatedsurfactantsthatexhibitdifferentdegreesofsolubilityinwater.Additionofthesurfactantthatispartiallywater-solubletoaHASEsolutionresultsinnotonlyanincreaseinshearstresswithincreasingsurfactantconcentrationandshearrate,butalsoanincreaseintheshearraterangeoverwhichshear-inducedstructur-ingandstresssaturationoccur.Inmarkedcontrast,thecompletelysolublesurfactantsystematicallyreducestheshearstressofthesolutionwithincreasingsurfactantconcentrationandshearrate.Moreover,athighsur-factantconcentrations,evidenceforshear-inducedstruc-turingandstresssaturationiscompletelyeliminated.TheseunexpectedresultsareinterpretedintermsofdifferencesinmixedmicellizationoftheHASEpolymerandsurfactant.ExaminationoftheseHASE/surfactantsolutionsbyTEMrevealsthattheHASEparticlesaggregateintodicreteparticlesinthep

10 resenceofthefullysolublesurfactantbutfor
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