Modified Polyacrylamide

ShuhuiWu,RobertA.Shanks,GaryBryant

SchoolofAppliedSciences,RMITUniversity,Melbourne,Victoria3001,Australia

Received3March2005;accepted22September2005DOI10.1002/app.23282

Publishedonline8March2006inWileyInterScience(www.interscience.wiley.com).ABSTRACT:Hydrophobicallymodifiedpolyacrylamide(HMPAM),withamolecularweightof104g/mol,wasstudiedusingarangeofrheologicalmethodsanddynamiclightscattering(DLS).DLSmeasurementsindicatethattheassociationofthemodifiedpolymerbeginsatlowconcen-tration.Themodifiedpolymerwithhighsubstitutionformstransientnetworksbelowthecriticalconcentration,butthenetworksaredisruptedbythemicellesformedbythepoly-meritself,andthenetworksdonotcontributetoviscosityenhancement.Themodifiedpolymersexhibitedsurfaceac-tivity,andsotheymayberegardedasnonionicpolymericsurfactantsratherthanthickeners.Ontheotherhand,HM-PAMisshowntointeractwiththesurfactantSDSwhile

PAMisinerttoSDS.Inthehydrophobicdomains,itunder-goesasurfactant-inducedassociationprocess;inthehydro-phobe-surfactanttransitionregions,thesurfactantbindstothepolymerinanoncooperativewayandformsapolymer–surfactantcomplex.Contractedpolymerchainsbegintoex-tendbecauseofelectrostaticrepulsion,whichcanovercometheassociationatsurfactantdomains.TheconformationofHMPAMpolymerchainscouldbecontrolledbyaddingaspecificamountofsurfactant.©2006WileyPeriodicals,Inc.

JApplPolymSci100:4348–4360,2006

Keywords:association;rheology;lightscattering;solutionproperties;surfactants

INTRODUCTION

Associativepolymersarehydrophilicpolymersmod-ifiedwithoneormorehydrophobicgroups.Theyhavewideapplicationinpaints,foods,pharmaceuti-calproducts,andinenhancedoilrecovery.Thehy-drophobicgroupscanassociatetominimizetheirex-posuretowater,eventuallyanetworkisformedandtheviscosityofthesolutioncanincreasemarkedly.Forthesereasons,associativepolymerscanbeusedasthickeners.

Therearetwotypesofassociativepolymers.Onetypeisthegroupoftelechelicpolymers,withhydro-phobeslocatedatthepolymerchainterminalposi-tions.Anexampleofthistypeishydrophobicethoxy-latedurethane(HEUR),whichhasmolecularweightoftheorderof104g/mol,andispreparedbyasub-stitutionmethod.1–3Theothertypeisthecomb-likepolymers,withhydrophobesdistributedrandomlyalongthepolymerbackbone,suchashydrophobicallymodifiedalkalisolubleemulsions(HASE)4–6andhy-drophobicallymodifiedpolyacrylamides(HMPAM).7–9Themolecularweightofthesepolymersisoftheorderof105to106g/mol,andthepolymersarepreparedby

Correspondenceto:R.A.Shanks(Robert.shanks@rmit.edu.au).JournalofAppliedPolymerScience,Vol.100,4348–4360(2006)©2006WileyPeriodicals,Inc.acopolymerizationmethod.Thesetwotypesofpoly-mershavedifferentassociationprocessesandrheo-logicalbehavior.

Astrictlyalternatingmethodwasdevelopedtosyn-thesizecomb-likepolymers.10–12Usingthismethod,comb-likecopolymerscanbepreparedwithabroadrangeofmolecularweights,from104to105g/mol.Thistypeofpolymercanbeusedtostudythediffer-encebetweencomb-likepolymersandtelechelicpoly-mers(withthesamebackboneandmolecularweight),orthedifferencesamongcomb-likepolymerswithdifferentmolecularweights.Therheologicalbehaviorofassociativepolymersisverycomplicated,andisinfluencedbyassociationtype,molecularweight,backbonestructure,hydrophobestructure,andevenspacernumber.13Recentlyanewmodificationmethodhasbeende-velopedinourlaboratory.Usingasimplesubstitutionmethod,polyacrylamide(PAM)hadbeensubstitutedwithoctylandhexylgroupstoformacomb-likehy-drophobicallymodifiedPAM(HMPAM),withoutchangingthebackbone.14,15Themodifiedcomb-likepolymerinthisstudyhasamolecularweightof104g/mol,ofthesameorderastelechelicpolymers.ThemolecularweightissmallwhencomparedwiththatofHMPAMpreparedbycopolymerizationmethods.Thesubstitutedgroups,octylandhexyl,areweakhydro-phobes.Forthisreason,thepropertiesofHMPAMinthisstudyaredifferenttothatofothercomb-like

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4349

associativepolymerspreparedbycopolymerizationmethods.Inthisstudy,acombinationofrheology,dynamiclightscattering,andsurfacechemistrymeth-odswereappliedtocharacterizethedistinctiveprop-ertiesandsolutionstructureofthemodifiedpolymers,withparticularemphasisontherheologicalbehavior.WhenthePAMhydrophilicbackboneismodifiedwithhydrophobicgroups,themoleculesbecomeam-phiphilic,andconsequentlybecomesurfaceactive.Asthepolymersstudiedhavesmallmolecularweightsandastheyareofamphiphilicnature,theycanbeconsideredtobesomewherebetweenapolymerandasurfactant.Thisbehaviorisinvestigatedherebystudyingtheinteractionbetweensodiumdodecylsul-fate(SDS)andbothPAMandHMPAM.Althoughrecently,newmethodologiesandinstrumentationhavedevelopedrapidly(suchaspulsedmagneticfieldgradient(PFG)NMR,16–19lightscattering,20–2424–27fluo-rescence,andneutronandX-rayscattering28),otherclassicalandfundamental17,29methods(suchasphaseequilibria,surfacetension,andviscometry)aremostfrequentlyusedtostudytheinteractionofpolymersandsurfactants,astheseexperimentsarepreciseandreadilyavailable.Inthispaper,surfacetensionmeasurementsandphaseequilibriumareusedtoinvestigatetheinteractionbetweenHMPAMandSDS.

EXPERIMENTAL

Materials

Polyacrylamide(PAM),asa50wt%aqueoussolutionwithweightaveragemolecularweight(Mg/molanddensityof1.1g/mL,wasw)of10,000purchasedfromSigmaAldrich(Australia).Thesolutionwasdi-lutedtoaconcentrationofabout10%,thenlyophilizedtoobtaindryPAMsolid.Sodiumdodecylsulphate(SDS,99%purity)wasusedasreceived.

HydrophobicallymodifiedPAM(HMPAM)wasobtainedbyatransamidation14,15methodthathasbeendescribedelsewhere.Theamidegroupinthepoly-merbackbonehasbeenN-substitutedinwatersolu-tion,wherethesubstitutingamineiswatersoluble,suchashexylamine(C6)andoctylamine(C8).Fortheoctylgroup,substitutionratioswerechosenas1,3,or5%(moleratio)toPAMmonomerunits.ThesymbolsforthemodifiedpolymersusedarelistedinTableI.

Solutionpreparation

SDSadditivesweredissolvedinwaterwithavarietyofconcentrations(g/L).DryPAMorHMPAMwasdissolvedinwaterorSDSaqueoussolutions.Solutionconcentrationsareexpressedasaweightfractionofsolidpolymers.

DesignationTABLEofModifiedI

Polymers

SubstitutionSubstitutiongroup

ratio(%)

PAM––C61Hexyl1C81Octyl1C83Octyl3C85

Octyl

5

Dilutesolutionviscositymeasurements

AUbbelohdeviscometerwasusedtomeasuretherelativeviscosityofthesolutions.30Themethodandtheoryaredescribedelsewhere.Rheologyofconcentratedsolutions

Rheologicalpropertiesof50wt%ofPAM,C81,C83,andC85solutionsweremeasuredusingaFluidsSpectrometerII(RFSII)fromRheometrics,withaparallelplatemeasuringsystem.Siliconeoilwasplacedattheedgeofthetwoplatestopreventsolventevaporation.First,adynamicstrainsweepwasmea-suredat1rad/stodeterminethelinearviscoelasticregion.ThestoragemodulusGЈandlossmodulusGЉweremeasuredoverthefrequencyrange0.1–100rad/s.Phaseangleswerealsorecorded.Astrainam-plitudeof0.2radwasappliedforallsamples.Forcontinuousshearmeasurement,thesteadystatevis-cosity(␩)wasmeasuredovertheshearraterangeof0.1–100sϪ1.

Dynamiclightscattering

Fordynamiclightscattering(DLS)thesamplewasilluminatedbyalaserbeam,andthefluctuationsinthescatteredlight,whicharerelatedtothemotionoftheparticles,weremeasured.Thetheoryisbrieflyoutlinedhere.Thefundamentalquantitymeasuredistheintensityautocorrelationfunction,g(2)(␶),whichisrelatedtothe(1)normalizedelectricfieldautocorrelationfunction,g(␶),by

g͑2͒͑␶͒ϭ1ϩB͉g͑1͒͑␶͉͒2

(1)

whereBisaninstrumentalconstantoforder1.Foradilute,monodispersesuspensionofnoninteractingparticlestheelectricfieldautocorrelationfunctionisdescribedby

2

͉g͑1͒

͑␶͉͒ϭexp͑Ϫ⌫␶͒⌫ϭͩ4␲n␪␭0sinͩ2

ͪͪD(2)

where⌫isthedecayconstantofthefluctuations,Dis

theparticlediffusioncoefficient,nistherefractive

4350indexofthesuspendingliquid,␪isthescatteringangleand␭Forspherical0isthewavelengthofthelaserinvacuum.particlestheStokes–Einsteinrelation-shiprelatesthediffusionconstantDtotheparticleradiusr

Dϭ

kBT6␲␩r

(3)

WhereTistheabsolutetemperature,kconstantand␩istheviscosity.

BistheBolt-zmannDynamiclightscattering(DLS)wasperformedus-inganALV-compactgoniometer.Sampleswereillu-minatedwithahelium-neonlaserwithawavelengthof633nm.Measurementswerecarriedoutat25°C,andthescatteringanglewassetat30°.Autocorrela-tionfunctionsweremeasuredusinganALV-5000cor-relatorcard,andanalysiswascarriedoutusingtheinbuiltsoftware.Forthesamplespresentedhere,multi-exponentialfitswith1,2,or3componentswereused.TherefractiveindexofsolutionswasmeasuredusinganATAGOilluminator,Japan.Therelativevis-cosityofthedilutesolutions(under25wt%)wasmeasuredusinganUbbelohdeviscometerasde-scribedearlier,andtherelativeviscosityratiowasusedtomultiplytheviscosityvalueofwater.Theabsoluteviscosityofconcentrated50wt%solutionswasmeasuredusingtherheometer.Thesampleswerefilteredpriortomeasurementusing0.8␮mMilliporefilters.Highlyconcentrated,50wt%solutionsweredifficulttofilter,andtheseweremeasuredwithoutfiltration.

Surfacetensionmeasurement

Theapparentsurfacetensionwasmeasuredbythesuspendeddropmethod,usingaContactanglesys-tem(OCA20,ParticleandSurfaceSciencePtyLtd.)atroomtemperature.Eachsamplewasmeasured5timesandthemeanwasquotedastheresult.Forphase-separatedsamples,thesupernatantwasmeasured.Phaseequilibria

Theturbidityofpolymer–surfactantsystemswasde-terminedbyvisualobservationofeachsolutionatroomtemperature,withthesimpleclassificationofclear,turbid,orphase-separated.Opticalmicroscopy

ANikonLabophotIImicroscopewasusedtostudytheemulsionandthephaseseparatedsolutions.Afewdropsofliquidweregentlyplacedinthewellofaglassslide,whichwasplacedunderthemicroscopeforobservation.Dyewasmixedwiththeliquidto

WU,SHANKS,ANDBRYANT

TheIntrinsicViscosityTABLE[␩II

k]andHugginsConstant

HofPolymers

[␩](dL/g)

kHPAM0.11010.99C810.11250.92C830.13280.6C85

0.1293

0.66

increaseimagecontrast.ImageswerecapturedusingaSonyvideocameraconnectedtoaMacintoshcom-puterwithIPLabimageanalysissoftware(SpectraAnalyticsCo.).

RESULTSANDDISCUSSION

Dilutesolutionviscosity

TableIIsummariestheresultsofUbbelohdeviscom-etermeasurements.Theintrinsicviscosity[␩](dL/g)ofthemodifiedpolymersisalmostidenticaltothatofPAM.Normallytheintrinsicviscosityofamodifiedpolymerislowerthanthatofitsunmodifiedanaloguebecauseofreducedintramolecularinteractionsindi-luteregions.Theresultsshowsimilarvaluesforthedifferentsamples(theslightlyhighervalueofHM-PAMmayduetoassociationofhydrophobes).TheHMPAMusedhaslowmolecularweight.OtherHM-PAMs,preparedbymicellarcopolymerizationmeth-ods,canhavemolecularweightsupto106g/mol.ThemolecularweightofthemonomerunitofPAMis71g/mol.Forapolymerwith10,000g/molmolecularweight,theaveragepolymerizationdegreeis140.Modifiedpolymerswith1%substitutionhaveoneortwohydrophobesoneachpolymerchain,soonlyintermolecularassociationscanoccur.Forapolymerchainthathas140averagerepeatunits,withsomeside-chainscontaining8carbons,thelength–radiusratioisverylow,themoleculesarequiteelongated.Comparedwithpolymersthathavehighmolecularweight,thelength–radiusratioisveryhigh,thechainsaremorecoiled.Whenthepolymerchainsareshort,modifiedpolymerswithhighsubstitutionratiotendtohavehighelongation,whichmakesitdifficultforthechainstobendtoformintramolecularassociations.Astheintrinsicviscosityisrelatedtotheeffectivehydro-dynamicvolumeofthemoleculesinsolution,theresultsindicatethatanelongatedpolymericbackboneproducesasimilarhydrodynamicvolumetothatoftheunmodifiedpolymer.

TheHugginsconstantsofthemodifiedpolymers(showninTableII)arelowerthanthatofPAM,whichindicatesabetterpolymer–solventinteraction.30Be-causeofthepresenceofhydrophobesinaqueousso-lution,theHugginsconstantofamodifiedpolymerisexpectedtobehigherthanthatofitsunmodified

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4351

Figureconcentration.

1RelativeviscosityratioofPAMversuspolymeranalogue.However,thetrendisreversedhereanditisproposedthatthisisduetotheassociationofthehydro-phobicdomains.Asimilarreversedtrendhasalsobeenobservedinunmodified/modifiedmethacrylicacid–ethylacrylatecopolymersystems.Anexplanationwasthattheblockyethylacrylatesegmentsinthepolymerbackbonewereabletoself-aggregatetoreducethetotalhydrophobicdomainsites.31ThecriticalconcentrationC*ofPAM,calculatedasthereciprocalofintrinsicviscosity,wasbelow10g/dL(about10wt%).Figure1showsthePAMconcentra-tion(wt%)versusrelativeviscosityratio.ThecurveshowsthatC*isbetween15and20wt%,whichishigherthanthecalculatedvalue.Rheologyofconcentratedsolutions

Figure2showsthesteadyviscosityofpolymersolu-tions(50wt%)versustheshearrate.Thefigureshows

Figuretion50%2versusSteadyshearshearrate.

viscosityofpolymersatweightfrac-FigureGЉversus3frequencyDynamicstorageforweightmodulusfractionGЈ50%andpolymer.

lossmodulusthattheviscosityofC81andC83isevenlowerthanPAM,whileC85isslightlyhigherthanthatofPAM.Eventhoughthereareslightdifferences,theviscositiesareofthesameorder.Forotherhydropho-bicallymodifiedsystems,theviscosityofthemodifiedpolymercanbeafewordershigherthanthatoftheunmodifiedpolymerinthesemidiluteregion.

Figure3showsthedynamicshearmodulusversusshearrateforeachofthepolymersataconcentrationof50wt%.TheC81samplehasthelowestGЈandGЉ,whileC83isalmostthesameasthatofPAM,andbothGЈandGЉofC85areslightlyhigherthanthatofPAM.Allthevaluesareofthesameorder,indicatingthatthemodifiedpolymersdonothavesignificantviscosityenhancement.TableIIIsumma-rizestheexponentsofthepowerlawforthefourdifferentHMPAMs.ExponentsofGЈareverysmall,andϾPAMtheϾexponentsC83ϾofC8GЉ1,arecloseintheto2.orderTableofIIIC8indi-5catesthatallfoursolutionsareviscousliquids.Figure4showsthephaseangleofthesamples.C81hasthehighestphaseanglethoughtheotherthreearesimilar.ThedynamicresultiswellcorrelatedwiththesteadystateresultinFigure2.CombiningtheresultsfromFigures2–4,thesolutionofC81showsthemost“liquid-like”propertiesandC85theleast.

Therearetwotypesofhydrophobicmodifiedpoly-mers:telechelic,withhydrophobesat4terminalposi-tions,molecularweightsoforder10g/mol,suchas

ExponentsoftheTABLEPowerIII

LawforPolymers

GЈ

GЉPAM0.1032.2C810.0511.44C830.0721.83C85

0.107

2.35

4352Figurelutions4versusPhasefrequency.

angleofweightfraction50%polymerso-HEUR;andcomb-likepolymers,withthehydro-phobesdistributedrandomlyalongthebackbone,mo-lecularweightsintherange105to106g/mol,suchasHASEandHMPAM(preparedbycopolymerizationmethod).Fortelechelicpolymerswithlowmolecularweightentanglementisusuallyignored,whileaRouse-likerelaxationprocessisobserved,andthechainsrelaxindependently.2Forcomb-likepolymers,whichhavehighmolecularweight,chainsbecomeentangledinthesemidiluteregion,whileahinderedreptationrelaxationprocessisobserved,withchainsdisengagingfromassociationjunctionsfirst,followedbyareptationprocess.Chainentanglementandhy-drophobicassociationarebelievedtoco-contributetoviscosityenhancement.6,9,32,33Themodifiedpolymersstudiedhereareanalogoustocomb-likepolymers.ThedynamicmodulusshowedthatGЈwastwoorderslowerthanGЉ;theeffectofentanglementcanbeig-nored.Thereforeonlyhydrophobicassociationcon-tributestotheviscosity.

GrootandAgterroffuseda“bead-spring”modeltosimulatetheviscoelasticpropertiesofcommoncomb-likepolymers.34Thebeadshadbinaryassociations,eitherfreeorpaired.Becauseofentanglement,disso-ciationofindividualassociationsdidnotbreaktheconnectivityofthenetworks,andthenormalcomb-likeassociativepolymerhadabroadrelaxationtimedistribution.35Onlybridgestructures(hydrophobesinthesamepolymerchainsdistributedindifferentmi-celles)couldsupportstress.2Lackofentanglementofpolymersinthepresentstudyprovidesmanyfeweropportunitiesforhydrophobesinthesamechaintoenterdifferentmicellesandformbridgestructures.Lowmolecularweightscanleadtoabroadrangeofdiluteregions,andpolymerchainscannotinteractsignificantlybelowC*.Itiswellknownthatthickenerbehaviorisobtainedinsemidiluteregions.Themod-ifiedpolymersstudiedheredonotformanetwork

WU,SHANKS,ANDBRYANT

sufficientlytoincreasetheviscosityofthesystemsstudied.

Themodifiedpolymerscanbedissolvedinwaterevenatconcentrationsashighas50wt%.Althoughtheviscosityanddynamicshearmodulusdonotchangesignificantly,themodifiedpolymerscanself-assemblebecauseoftheiramphiphilicstructure(thiswasconfirmedbydynamiclightscatteringandsur-facetensionmeasurement).Anumberofpolymerchainsformsingleflower-likemicelles.Semenow’smodelpredictedatwo-phasesolutioncomposedofclose-packedmicellesandapolymer-leansolventphase.36Theinteractionofmicellescontainstwoterms:oneisabridgingattractionandtheotherisanosmoticrepulsion.Thebridgingattractioncanleadtophaseseparationwhenthepolymercannotinteractproperlywiththesolvent.Ontheotherhand,osmoticrepulsioncanleadtodispersionofpolymersinsolu-tion.Heitzandcoworkersusedanalternativestrategytosynthesizecomb-likeassociativepolymerswiththesamebackboneandmodificationratio,butwithdif-ferentmolecularweights.Theyfoundthatphasesep-arationoccurredforthehighmolecularweightpoly-merswhiletheloweronesdidnotseparate.11,12Theauthorsdebatedthatthebridgingbetweenmicelleswasonlyfavoredwhenthelengthofthepolymerchainwastoolongforonemicelletoaccommodateanentirepolymerchain.Ifapolymerchaincontainedhydrophobesmorethantheaggregationnumberofamicelle,phaseseparationcouldoccur.Theaggrega-tionnumberofmicellesinoursystemswasnotstud-ied,butitmaybeoftheorderof20–50accordingtootherresults.11,24,37–39Thisvalueishigherthantheaveragenumberofhydrophobesinthehighestsubsti-tutedpolymerC85(about7).Freechainendsex-tendingintothesolutionprovideastericbarrierthatpreventsbridgingassociation.Thisexplainswhythemodifiedpolymercanstillbedissolvedatsuchhighconcentration(50wt%),whilemaintainingasimilarviscositytotheunmodifiedPAM.

ForthetelechelicpolymerHEUR,thesituationisdifferent.ViscoelasticpropertiesofHEURaresensi-tivetohydrophobestructureandfunctionalitybutnottopolymerchainlength.Thuressonetal.22usedmix-turesofPEOdiblock(DB)andtriblock(TB)copoly-merstostudyrheologicalproperties.TheDBwasPEOwithahydrophobictailononeend,whichwasanonionicsurfactantwhentheTBwasPEO,withhy-drophobictailsonbothends,thesameasHEUR.TheyfoundthatonlytheTBcontributedtoformationofatransientnetwork,whiletheconnectivitywaspro-videdbybridgingchains.FortheDB,thecopolymerscouldformlargeclustersofvarioussizethatwereslightlyinterconnectedtoeachother.ThefunctionalityofDBwasnotenoughtoformbridgingchains(crosslinkingjunctions).Thesystemsstudiedherearedifferent,withhydrophobesrandomlyattachedtothe

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4353

Figureweightfraction5Autocorrelation50%.

functiong(2)(t)ofpolymersatbackbone.Intramolecularassociation,analogoustotheloopstructureofaunimerinatelechelicpolymer,doesnotcontributetoviscosityimprovement;whileintermolecularassociation,notreallyananaloguetobridgestructure,canleadtoformationofclusterswithlargersize,butthelargeparticlesizedoesnotimplyefficientbridgestructures.Onlywhenthehydro-phobesonthesamechainjoindifferentmicellesitispossibletoformanetwork.Polymerchainentangle-mentcanprovidetheopportunitytoformanefficientnetwork.Meanwhile,thereisabalancebetweenat-tractiveandrepulsiveforces.Whenrepulsionover-comesattraction,theviscosityofthesolutioncanbeincreasedwhilemaintainingpolymerdissolution.Butwhentheattractionislargerthantherepulsion,phaseseparationoccursandtherheologicalbehaviorissus-pension-like.40Therearefarfeweropportunitiesforthelowmolecularweightpolymerstoformbridgestructures,whileviscositycannotbeincreasedsignif-icantly.Incontrast,therepulsionofpolymerclustersislargeenoughtomaintain9polymerdissolution.Re-galadaoandcoworkersstudiedtherheologyofHM-PAM(preparedbycopolymerization),andfoundmo-lecularweightwasaimportantparameteratveryhighconcentration.However,theresultsobtainedheresuggestthat,forcomb-likepolymers,theremaybeacriticalmolecularweightafterwhichviscosityen-hancementcanoccur.

Dynamiclightscattering(DLS)

Figure5istheautocorrelationfunctiong(2)(t)ofPAMandHMPAMat50wt%.PAMdecaysatshortdelaytimes,whilethethreeHMPAMdecayatamuchlongerdelaytime.ThisindicatesthatnetworksareformedintheHMPAMsolution,butnotforthePAM,evenatthishighconcentration.ForPAMwithmolec-

ularweightof104g/mol,theparticlesizeofaunimerisoforderof1–2nm.Forcomparison,theparticlesizeofϫ10anonassociated4HEUR,withmolecularweightof2g/mol,is4.2nm24;andforaHASEunimer,withamolecularweightof2.2ϫ105g/mol,itisabout20nm.23TableIVshowsthehydrodynamicradiiforthemodifiedpolymersatvariousconcentrations.Insomecasestwo-component-fitsgavethebestresults,andinthesecasestheapproximatefractionofsmallerparti-clesisshowninparentheses.Foreachpolymertheparticlesizeisoftheorderof100–200nmat5wt%concentration,whichindicatesthattensofmoleculesassociate.Oneaggregatedoesnotnecessarilycontainonlyonemicelle(thepossibleaggregatenumberisabout20–50,seediscussionmentionedearlier),fewmicellesmayaggregatetogether,butnetworkisnotformed.ForpolymerC81,thesizeoftheparticlesdoesnotchangesignificantlyuntil25wt%,followedbyanincreaseintheapparentsizebyanorderofmagnitudeat50wt%.Inaddition,thereisnosignif-icantcontributionfromanysecondcomponentoverthewholeconcentrationrange.Thesituationforsolu-tionsofC83andC85wasquitedifferent.ForC83theparticlesizeshowedalargeincreaseat15wt%,andtwocomponentswereneededtofitthedata,indicatingabimodalsizedistribution.ForC85therewasasimilarincreaseat10wt%.Theverylargeparticlesizesuggeststhatatransientnetworkisformed,withC85formingthisnetworkatalowerconcentrationandhigherratio.Inbothcases,thecon-centrationswherethesenetworksformedwerebelowC*.Whentheconcentrationwasincreasedbyafurther5wt%,thesizedecreasedtoavaluecomparablewiththatoftheC81polymer.Ataconcentrationof50wt%,allthreepolymersappearedtoformanetwork.

HydrodynamicTABLEIV

PolymerRadii(nm)Concentrations

ofHMPAMatVarious

Weightfraction(%)

C81C83C85

5117

166

220–22650–100(16)10129–142150–2502000–3000

87(16)a15130–1702000–300096(52)20140–240150–170170–22025180–220120–140160–18050

950–1600

680–1100

600–1000

atrationValues(%)inofparenthesesparticlesforindicatethebimodaltheapproximatesamples.

concen-43Above25wt%,inthesemidiluteregion,thepolymerchaininteractionswerewelldevelopedevenwithoutentanglement.

Thevaluesofthelargestparticlesizes(2000–3000nm)arelargerthanthefilterporesize.Theinterpre-tationisthatthereversibleassociationjunctionswerebrokenduringfiltration,23andre-formedafterpassingthroughthefilter.TheDLSanalysisshowsthattheHMPAMinthisstudydoesassociateeventhoughthereisnosignifi-cantincreaseinviscosity.Theparticlesizewasquiteconsistentoverthewholerangeofconcentration.Thequestionsthatneedtobeansweredare:whyistherenosignificantchangeinviscosityeventhoughanet-workisformed?AndwhydotheparticlesizesofC83andC85decreaseagainathighconcentra-tion?AsFigure1showed,thecriticalconcentrationC*wasbetween15and20wt%,andthisvalueishigherthanthatforwhichC83andC85showedanin-creasedparticlesize.BelowC*,thepolymerchainscannotinteractefficiently;thejunctionsofnetworksareveryweakreversibleassociationsinsteadofchem-icalbonds.Inthissituation,thesystemcannotsupportstressanymoreefficientlythantheunmodifiedPAM.RheologyandDLSreflectdifferentaspectsofthesolutionproperties.DLSshowsthatassociationdoesoccurinthemodifiedpolymer,whilerheologyshowsthattheassociationhasnotcontributedtoasignificantviscosityenhancementasforassociativethickeners.Theassociationiscausedbytheamphiphilicnatureofthemolecules.Surfactantsarealsoamphiphilic,andtheirmoleculesareassociatedtoformmicellesinaqueoussolution,buttheydonotleadtoviscosityincreases.PEOdiblockcopolymer(DB)isanonionicsurfactant,andstudiesofDB,triblockcopolymer(TB),andtheirmixtureshowedthatTBpolymerplayedacrucialroleintheestablishment22ofastrongnetworkathighconcentration.Aspreviouslydiscussed,C81polymercontainsonlyoneortwohydrophobesperchain,whichisanalogoustoDB,anditthereforecouldnotcontributetonetworkformationeventhoughitwasassociated.WhileC83andC85areanalogoustoTB,thereareexcesshydrophobestoformbridgestructures.ThedatashowsthatC85ismoreefficientasitformsanetworkatlowerconcentration,com-paredwithC83.Athigherconcentrations,thenet-worksbecomere-dissolvedinthemicellesthemselves,andtheconnectivityisdisrupted.Thevaluesoftheparticlesize(exceptwherenetworkswereforming)wereconsistentoverthewholerangeofconcentra-tions.Aftermodification,thepolymershavesurfac-tantproperties,andthemoleculescanself-assembleintomicelles,withbridgestructuresformingbelowC*inpolymerswithahighsubstitutionratio,butthenre-dissolvinginthemicellesthemselves.

Theviscoelasticpropertiesofassociativepolymersareverysensitivetosurfactants.Theclassicalbehavior

WU,SHANKS,ANDBRYANT

Figurepurewater6Surfacesolution.

tensionversuspolymerconcentrationinofassociativepolymersandsurfactantinteractionsisthatthereisaviscositymaximumatacriticalsurfac-tantconcentration.Belowthisconcentration,theap-parentviscosityoftheassociativepolymer–surfactantsolutionincreasesbecausethesurfactantincreasesthestrengthandnumberofassociations;abovethecriticalconcentration,excesssurfactantwoulddissolvethehydrophobes,andnetworkconnectivitywouldbedis-rupted.Theviscositycandropevenbelowthelevelwhensurfactantisabsent.41–44Thepolymersstudiedherehadbothsurfactantpropertiesandassociativepolymerproperties.So,oncethepolymersassociate,theparticlesizeincreasesandthiscanleadtoenhance-mentofviscosity.Ontheotherhand,asthepolymerbehaveslikeasurfactant,itwillpreventnetworkfor-mation,thusloweringtheviscosity.Thebalancebe-tweenthesecompetingeffectsmayexplainwhythetotalviscosityisofthesameorderasthatofPAM.Surfaceactivity

Afterbeingmodifiedwithhydrophobicgroups,PAMchangesfromapurelyhydrophilicpolymerintoanamphiphilicpolymer,andbecomessurfaceactive.Fig-ure6showsthatPAMhasweaksurfaceactivity.ForC61,thehydrophobesarenothydrophobicenoughtoformmicelles,thoughtheydecreasethesurfacetension.ForoctylmodifiedPAM,thesurfaceactivityincreasedwithhydrophobecontent,andthesurfacetensiondecreasedfrom75mJ/m2toaplateauatabout30mJ/m2athighpolymerconcentrations.Otherevi-denceofsurfaceactivitywastheHLB(hydrophile-lipophilebalance)valueofthemodifiedpolymer.HLBisequaltomol%ofhydrophilicgroupdividedby5,giveaarbitrary45rangebetween20(hydrophilic)and0(lipophilic).TheHLBvalueofHMPAMisabout10,indicatingitsamphiphilicnature.Thus,HMPAMcouldberegardedasapolymericnonionicsurfactant,

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4355

Figuretems(ϫ7100).

ImagesofphaseseparationatHMPAM–SDSsys-wherethehydrophilicgroupsofthesurfactantaretheamidegroupsalongthepolymerbackbone.InteractionwithSDSinhydrophobedomainsAlthoughPAMinteractsweaklywithSDS,HMPAMcaninteractmorestronglywithSDS.Theimportantparameterfortheinteractionistheratiobetweensur-factantandhydrophobes(Nbelow1,theSDSandHMPSDS/Nsolutionh).WhenNislocatedSDS/Ninthehishydrophobedomainregions.Thepolymerconcentra-tionwasfrom5to25wt%,whiletheSDSconcentra-tionwasbetween0.5and2.5g/L,consistentwithexperimentsinotherdomains.

AftermixingwithSDS,thesurfacetensionofsolu-tionsdecreasedrapidlytoabout24–26mJ/m2.Thepurepolymersolutionwasclear,butsomeHMPAM–SDSsolutionsbecameturbid,orphaseseparated.Smallgelparticlesofabout0.2–0.3mminsizeap-

pearedinthesolution,asareshowninFigure7.Phaseseparationhaspreviouslybeenobservedbydilutionofa19,46HMPsystematanintermediateSDSconcentra-tion.Figure8showshowphaseconditionschangedwithNSDS/Nh(below1).Turbidsolutionsvariedfromslightlyturbidtoveryturbid,althoughtheyhadthesameranking.Figure8indicatesthatthesolutionsbecameturbidandphase-separatedgradu-allywhenNAlthoughtheSDS/Nregionshincreasedfrom0.01tocloseto1.overlap,thetrendisclearthatthepolymerchainsbecomecontractedwhenSDSwasadded,untilfinallyprecipitationoccurs.Thehydro-phobesonthepolymerchainsarehydrophobicenoughtosolubilizeindividualsurfactantinthissur-factant-inducedassociationprocess.

Interactiondomains

withSDSintransitionalandsurfactantThetransitionaldomainsrefertowhentheratioofsurfactantandhydrophobesisstoichiometric(ofthesameorder).ThevalueofNSDS/Nhwas1–4.Surfac-tantdomainsrefertoNthepolymerSDS/Nconcentrationhvaluesover4.Fortheseexperiments,waschoseninthediluteregion,1and2wt%,andSDSconcentrationstartedfrom0.5g/L.

Figure9illustratessurfacetensionversusSDScon-centrationsforthepolymersolutions.ItshowsthatthesurfacetensiondecreasesrapidlyonaddingsmallamountsofSDS(seehydrophobedomains),toamin-imum,thenthesurfacetensionbegantoincreaseonfurtheradditionofsurfactant.Therearefoursolutionsinthefigure,1wt%C81(C811%)andC83(C831%)solution,2wt%C83(C832%)andC85(C852%).ThenumberofhydrophobesitesinC831%,C832%,andC852%solutionwere,respectively,3,6,and10timesthatoftheC811%solution.Therangeofminimumsurface

Figuresusvalue8ofPhaseNconditionsofHMPAM–SDSsystemsver-SDS/Nh(Ͻ1).

4356FigureSDSconcentration.

9SurfacetensionofHMPAM–SDSsystemsversustensionwasincreasedwiththeincreaseinthenumberofhydrophobesites.Figure10ranksthephasecondi-tionofsolutionsversusSDSconcentration.Compari-sonofFigure9withFigure10indicatesthatdecreaseinsurfacetensioncorrespondstophaseseparation.Theminimumsurfacetensionregioncorrespondstoaturbidsituation,andthesurfacetensionincreasingregioncorrespondstoaone-phaseregion.Phase-changeprocessesalwaysfollowedfromclear,slightlyturbid,veryturbid,phaseseparation,veryturbid,slightlyturbidtoclear,itwasacontinualtransition.Figures11and12showthedatafromFigures9and10plottedasfunctionsoftheparameterNwhenNSDS/Nh.TheyshowthatthesurfaceSDS/Ntensionhisbelow1(hydro-phobedomains),ofthepolymersolutionsdecreasedrapidly,andthepolymersbegantoprecipitate(seehydrophobedomainssection).WhenNSDS/Nhisbetween1and3,itwaswithintheminimumregionofapparentsurfacetensionandthesolutionsgraduallybecameclear.WhenNSDS/Nhwas

FiguresusSDS10concentration.

PhaseconditionsofHMPAM–SDSsystemsver-WU,SHANKS,ANDBRYANT

Figurevalueof11NSurfacetensionofHMPAM–SDSsystemsversusSDS/Nh(Ͼ1).

above3,thesurfacetensionbegantoincreaseandthesolutionsfinallybecamesingle-phaseclearsolutions.PAMinteractswithSDSveryweakly,andsothephenomenadescribedearlierareduetothehydro-phobes.Therearethreeclassificationsofpolymer–surfactantinteractions.47Thefirstistheinteractionofanionicsurfactantwithahydrophilicpolyelectrolyte.Ifthepolyelectrolytehastheoppositecharge,phaseseparationmayoccurwithonelayerofsurfactantmicelleandonelayeroftheoppositelychargedpoly-mer.Ifthepolyelectrolytehasthesamecharge,theionicsurfactantwillhaveascreeningeffectonit.Forthiscategory,theinteractionispurelyelectrostatic,andnohydrophobicinteractionisinvolved.Asecondcategoryissurfactantinteractionwithslightlyhydro-phobicpolymers,wherethehydrophobicityofthepolymersisnotenoughforthemtoself-assemble.Theimportantparameterforthisinteractionisthecriticalaggregationconcentration(cac).Atconcentrationsbe-lowthecac,thereisnointeraction;atconcentrations

Figuresusvalue12ofPhaseNconditionsofHMPAM–SDSsystemsver-SDS/Nh(Ͼ1).

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4357

abovethecac,thesurfactantmicellesbegintobindtothepolymerchainstoformpolymer–surfactantcom-plexes,aprocesscalledpolymer-inducedmicellariza-tion.Athirdcategoryiswhenthesurfactantinteractswithhydrophobicallymodifiedpolymer(HMP),wherethehydrophobesarestrongenoughtoformmicellesbythemselves,andtheyhavethecapacitytosolubilizetheindividualsurfactantmolecules,result-inginmixedmicellecomplexes.Thiscategoryexhibitsthestrongesthydrophobicinteraction.

Thebindingratioofsurfactanttopolymerisdefinedas␤ϭCsurfactants,b/Ch,whereCandCs,bistheconcentrationofboundhistheconcentrationofhy-drophobes.16,47–50Whenthebindingratioisverysmall(␤ϽϽ1),thereisnopolymer–surfactantinteraction,thereareonlypureHMPmicellesinthesolution,andthepolymerbehaviorisindependentofsurfactantconcentration.When␤ϾϾ1,cooperativebindingoc-curs.Thisoccursforthesecondpolymer–surfactantinteractioncategory,wherethepolymerisslightlyhydrophobic.Atlowsurfactantconcentrationsthereisnobinding,so␤issmall;butastheconcentrationincreasespastthecac,thevalueof␤increasesmark-edly.Aclosedassociationmodelisusedtodescribethecooperativebinding,assumingthattheboundmi-cellehasafixedaggregationnumber.When␤ϳ1,thenumberofboundsurfactanthasthesameorderofthehydrophobes.Forthethirdcategoryofpolymer–sur-factantinteraction,wherepolymercontainsstronglyhydrophobicgroups,thepolymercanbindindividualsurfactants,anditwillsaturatewithsurfactantinthesameorderasthehydrophobicsites,makingthe␤valuecloseto1.Thisprocessisdescribedasanonco-operativeprocess,anditisacontinuousprocess,withtheboundsurfactantbeingalwaysproportionaltothenumberofhydrophobicsites,whichisrepresentedbyaLangmuirisothermmodel.51,52Withthesurfactantconcentrationcontinuouslyin-creasing,theHMP–surfactantsystemmovedfromthehydrophobedomainthroughatransitionregionintothesurfactantdomain.Itfollowsaprocessof“hydro-phobicspecies”inthesystem,startingfromfreehy-drophobeside-chains,purehydrophobeaggregates,mixedaggregates,freesurfactanttopuresurfactantaggregates.Freehydrophobeside-chainsonlyexistatinfinitedilution.Purehydrophobeaggregatesandpuresurfactantaggregatesexistinverylargehydro-phobesorsurfactantdomainregions.Therearediffer-entbindingisothermsinthedifferentregions.Themoststudiedpolymersarehydrophobicallymodifiedethyl(hydroxyethyl)-celluloseethers(HM-EHEC)orhydroxyethylcellulose(HM-HEC).16,20,21,25–27,41,53,UnmodifiedEHECorHEChaveslighthydrophobic-ity,andsothesurfactantcanbindtothepolymerinacooperativewayabovethecac.ThecorrespondingHMPhasmorecomplicatedbindingbehavior,whichisbelievedtoinvolveatwo-stepprocess.Inthehy-drophobedomains,thesurfactantisboundtothepolymerinanoncooperativeway,whilethesystemcontainspurehydrophobeaggregatesandhydro-phobe-dominatedmixedmicelles.Inthetransitionre-gion,thetransitionalmixedmicellesexistinsolution.Abovethecac,thesurfactantstartstobindtothepolymerinacooperativeway,complexesareformed,andthesystemsmoveintothesurfactantdomains.Themixtureofmixedaggregates,freesurfactantandpuresurfactantaggregatescoexistinthissystem.Otherpolymers,suchasnonionicpolyacrylamide(PAM),haveveryweakinteractionwiththesurfactantSDS.Thecorrespondinghydrophobicallymodifiedpolymer,HMPAM,couldabsorbsurfactantathydro-phobe17,55–59sitesinanoncooperativeway,untilitissatu-rated.HydrophobesinteractwithSDSinanoncooperativeway,whichisacontinuousprocessdescribedbytheLangmuirisotherm.Itisproposedthattheabsorptionisproportionaltothenumberofnucleationsites–hydrophobes.Effingetal.56usedsurfactantswithar-omaticringstostudytheinteractionbetweenHM-PAMandanionicsurfactants.TheyfoundthatHM-PAMwassaturatedwithalimitedamountofsurfactant.Belowacertainamountofsurfactant,thepolymer–surfactantsystemwasdescribedbyatwo-sitemodel,composedoffreesurfactantandsurfac-tant-boundpolymer;abovethatamountofsurfactant,thesystemwasathree-sitemodelwithfreesurfactant,freesurfactantmicellesandsurfactant-saturatedco-polymer.OtherHMPAMs,whichwerestudiedbypulsefieldgradientNMRandsurfacetensionmeth-ods,boundonlylowamountsofSDS(about1–3SDSperhydrophobe).Combinedwiththeresultsobtainedinthisstudy,itisproposedthatSDSandHMPformacomplexwithNSDS/Nminimumhfrom1to3.Inthisregion,thesystemexperiencedaapparentsurfaceten-sion,andthepolymerchainscontractedbyhydropho-bicassociation.

PhaseseparationofdiluteEHECandHMEHECatintermediateSDSconcentrationswasreportedbyNil-ssonetal.,19whoalsoobservedsmallgel-likeparti-cles,andthesolubilityofHMEHEC-SDScomplexwasdecreasedinthenoncooperativeregion.AcriticalNSDS/Nhvaluewasresponsibleforsurfactant-medi-atedgelationofHMHEC–surfactantsystems,whichwasattributedtomicelle-likeaggregationbridgedwithmultiplepolymerchains.Howeverifagelformsentirelyfromsolutioninsemidiluteregions,polymerchainsmayformanetwork,andthegelphasewillnotappearassmallparticles.Thereisabalancebetweenthehydrophobicattractiveinteractionsandtheelec-trostaticrepulsiveforces.AddinglowamountsofSDScouldincreasetheattractions,thenaddingmoreSDScouldleadtorepulsiveforcesbeingincreasedtoover-cometheattractions,resultinginthemicellesbeingdissolved.

4358WhenNSDS/Nhwasover3,theapparentsurfacetensionbegantoincreaseandthesolutionbecameclearagain.Afterthepolymerwassaturatedwithsurfactant,thesenonionicHMPAMformedapoly-mer–SDScomplex,whichhadanapparentpolyelec-trolytecharacter.The“necklace”modelhasbeenpro-posedforpolymer–surfactantcomplexes,48consider-ingthepolymerchainasastringandsurfactantclustersasbeads.Theelectrostaticrepulsionexpandsthepolymerchains,andsurfactantclustersareevenlydistributedalongthebackbone,withhydrophobesasnucleationsites.Thismolecularpicturewasrepre-sentedbyBiggsandcoworkers.58Phasebehaviorisanotherimportantaspectofthestudyoftheinteractionofpolymer–surfactantsys-tems.PhasestudieshavebeenreviewedexhaustivelybyPiculelletal.,whopioneeredresearchinthisarea.46,49,50,60Ingeneral,therearetwotypesofphaseseparation:associativeseparation,wherepolymerandsurfactantareenrichedinthesamephase;andsegre-gativeseparation,wherepolymerandsurfactantareseparatedintodifferentphases.61Itisbelievedthatassociativeseparationoccurswhenpolymer–surfac-tantinteractionsdominate,whilesegregativesepara-tionoccurswhenpolymer–solventinteractionsdomi-nate.Compositefractions,polymerandsurfactantcharge,hydrophilicityandtemperatureallinfluencethephasebehavior.TheFlory–Hugginstheoryisex-tensivelyusedtodescribephasebehavior.62,63TheexperimentswerenotcontinuedtoveryhighSDSconcentrations.Someresearchershaveproposedthatphaseseparationcouldoccuragain.50Unlikeinterme-diateSDSconcentrations,whereassociativephaseseparationoccurs(complexformation),thesecondphaseseparationshouldbeasegregativeseparation,whichiscomposedofamixedaggregationphaseandanexcesspuresurfactantaggregationphase.AddingSDScanincreasethepreferenceforinteractioneitherwithpolymerorwithwater,theformerpromotesassociativeinteraction,whilethelaterpromotesseg-regativeseparation.Thegeneralphasebehaviorfornonionicpolymer-anionicsurfactantsconsistsofmonophasicandbiphasicassociation,monophasicandbiphasicsegregationprocesses.

Questionsaboutpolymer–surfactantinteractionThehydrophobedomainsrangefrom5to25wt%,andarelocatedinthedilutetosemidiluteregions,whileintransitionalregionsandsurfactantdomains,theconcentrationofpolymerwaschosenas1and2wt%,totallyinthediluteregion.ThebehaviorofHMP–SDSinhydrophobedomainsmaybedifferentifitwasperformedatthesameconcentrationasintransitionalregionsandsurfactantdomains.Viscositystudieswithhydrophobicallymodified(hydroxypropyl)guarshowedaverycomplicatedbehaviorofpolymer–sur-

WU,SHANKS,ANDBRYANT

factantinteractions.Therewerefourregionsidenti-fied:infinitepolymersolution–surfactantbeforecriti-calconcentration,lowpolymersolution–surfactantbe-forecriticalconcentration,infinitepolymersolution–surfactantafteracriticalconcentration,andlowpolymersolution–surfactantafteracriticalconcentra-tion.Eachwasresponsiblefordifferentbehavior.Inthecurrentstudy,thelowmolecularweightleadstoaverybroaddiluteregion;thehydrophobedomainswerestudiedoverabroadrangefromthedilutere-giontothesemidiluteregion,whileSDSconcentrationwasconsistentwiththatoftransitionalandsurfactantdomains.

Anotherquestioniswhatwasthecompositionoftheprecipitate?Wasitpurepolymer–surfactantcom-plexoranaggregationofafewpolymerchains?Orwasitamixtureofcomplexandaggregation?NMRcouldbeusedtoanalyzethecomposition.

Thethirdquestionisthatafterre-dissolution,itisnotclearwhythesurfacetensionbegantoincrease.Inhydrophobedomains,thesurfactantinducedthepolymertoassociate,thepolymerchainscompacted(indilutesolution)orbridgeattractionsincreased(inthesemidiluteregion),leadingtophaseseparation.ThesurfacetensionattainedaminimumvaluewhenNSDS/Nhwasbetween1and3,correspondingtoacomplexformationratio.Thesmallvalueofsurfacetensionindicatedthesurfaceactivity,thepolymerchainswereverycompact,andmoremoleculeswereassembledattheair–liquidinterface.Figure11showedthatthesurfacetensionofpolymerwithmorehydrophobesiteswasslightlyhigher.Thismaybeduetoarelativelylowerexcessofsurfactant,whichcausedthepolymertohavealowerscreeningeffect,thereforethepolymerchainswereslightlymoreexpanded.Thequestionsposedherecannothoweverbeansweredbysurfacetensionmethods,andsoothertechniquesneedtobeappliedtofurtherstudyHMPAM.

CONCLUSIONS

Fromdilutesolutionviscositymeasurements,itwasfoundthatthevariousHMPAMswithmolecularweightsof10,000g/molhadmolecularsizesthatweresimilaratinfiniteconcentrationbecauseoftherelativeelongationofthebackboneregardlessofsubstitution.RheologicalstudyofPAMandHMPAMconcen-tratedsolutionsshowedthatthemodifiedpolymershadthesameorderofsteadyviscosityanddynamicshearmodulusasthatofPAM.Therewasnosignifi-cantviscosityenhancementofthemodifiedpolymer.BothPAMandHMPAMat50wt%formviscoussolutions.Amongthreemodifiedsamples,C81showedthemostliquid-likebehavior.Itisproposedthatmolecularweightisacriticalfactorforcomb-likepolymerstoprovidethickenerbehavior.

HMPAM–SURFACTANTINTERACTIONINAQUEOUSSOLUTION4359

Dynamiclightscatteringmeasurementshowedthathydrophobicallymodifiedpolymersareassociatedfromlowconcentrations.TheparticlesizeofC81wasconsistentovertherangeof5to25wt%,whichsuggestedthatthemoleculesself-assembleatacertainaggregationnumber.HoweverforHMPAMC83andC85,transientnetworkswereformedatconcen-trationsbelowC*,thoughathigherconcentrationsthesenetworksre-dissolvedintomicelles,formedbythepolymeritselfandwereconsequentlydisrupted.C85couldformbridge-chainseasierthanC83,anditformedanetworkatlowerconcentrationandhigherfractionwhencomparedwithC83.Overthewholerangeofconcentration,anetworkwasnotes-tablishedefficientlyenoughtoresultinviscosityen-hancement.

SurfacetensionmeasurementsandtheemulsifyingbehaviorofmodifiedpolymersshowedthatHMPAMwassurfaceactive,andsoHMPAMcanberegardedasanonionicpolymericsurfactant.

HMPAMcouldinteractwiththesurfactantSDS.Theratiobetweensurfactantandhydrophobes(NNanimportantparameter.SDS/h)was1,thesurfaceInthetensionhydrophobedomains,withNSDS/NhϽofthepolymer–surfactantsystemdecreasedrapidlywithin-creasingNSDS/Nh,andthesolutionchangedfromcleartoturbidthenfinallyphaseseparated,indicatingthatpolymerchainscontractedonadditionofsurfac-tant.Thiswasidentifiedasasurfactant-inducedasso-ciationprocess.

AtNSDS/Nhbetween1and3or4,therewasahydrophobe-surfactanttransitionregion,wherethesurfacetensionofthepolymer–surfactantsystemhadaminimum,thetotalsystemwasseparatedintotwophasesatNtembecameSDS/Nclearathofabout1,andgraduallythesys-NSDS/Nhofabout4.Inthisregion,thesurfactantwasboundtopolymerinanoncooper-ativeway,surfactantandHMPAMformedacomplex,untilthehydrophobesweresaturatedwithsurfactant.ThepolymerchainsweremostcontractedatNexpandedonaddingSDS/Nofabout1,andthechainssur-hfactant.The“necklace”modelwasusedtodescribethecomplex;electrostaticrepulsiveforcefinallyover-cametheassociationandthesolutionbecameclearagain.

AtNSDS/Nthehabove3,thesystembecametotallyclear,andsurfacetensionbegantoincreaseonadditionofsurfactants,thoughthereasonforthisremainsunclear.

Ingeneral,eventhoughHMPAMitselfcanbere-gardedasanonionicsurfactant,itshowedtypicalpolymericbehaviorwhenitinteractedwithsurfactant.Thepolymerchainswerefirstcontractedonaddingsurfactant,andboundwithsurfactanttoformacom-plex.Laterthepolymerchainsbecameextendedbe-causeofelectrostaticrepulsionovercomingassocia-

tion.Addingacertainamountofsurfactantcouldcontroltheconformationofthepolymerchains.

ShuhuiWuacknowledgestheAustralianGovernmentforprovidingaPostgraduateResearchScholarship.

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