1-s2.0-S0032386104010584-main

Polymer46(2005)193–201

http://wendang.chazidian.com/locate/polymer

Effectsofrubber/?llerinteractionsondeformationbehaviorofsilica?lled

SBRsystems

N.Suzukia,M.Itoa,*,F.Yatsuyanagib

ab

DepartmentofChemistry,TokyoUniversityofScience,1-3Kagurazaka,Shinjuku-ku,Tokyo162-8601,JapanResearchandDevelopmentCenter,TheYakshamaRubberCo.Ltd.,2-1Oiwake,Kanagawa,254-8601,Japan

Received8July2004;receivedinrevisedform11October2004;accepted13October2004

Availableonline23November2004

Abstract

Effectofrubber/?llerinteractionsonthestress–strainbehaviorforsilica?lledstyrene-butadienerubber(SBR)vulcanizateswerestudiedinrelationtothechainscissionsofrubbermoleculesduringdeformation.Therubber/?llerinteractionswerecontrolledbythemodi?http://wendang.chazidian.comrmationonthechainscissionwasobtainedbytheelectronspinresonance(ESR)measurements.ThecombinationofESRresultsandstress–straindatarevealedthat,atagivenstrain,thetensilestressincreasedwithincreasingtheinterfacialinteractionsbetweenrubbermoleculesandsilicasurface,andsimultaneouslythechainscissionbecameremarkable.Further,thedegreeofchainscissionwascloselyrelatedtothemechanicalenergyappliedtothevulcanizates.Thecyclictestsofstress–strainmeasurementssuggestedthatthechainscissionscontributedtotheMullinseffect.q2004ElsevierLtd.Allrightsreserved.

Keywords:Styrene-butadienerubber;Silica;Interfacialbonding

1.Introduction

Itiswellknownthatthemechanicalpropertiesofcrosslinkedrubbersystemsareenhancedbytheincorpor-ationofparticulate?llers,suchascarbonblackandsilica[1–5].Further,suchreinforcementsarerelatedtothesecondarystructureof?llerparticles(agglomerate)[6–8]andtherubber/?llerinteractions[9–11].Itisgenerallyacceptedthatthetensilestressatarelativelylargestrain(O100%)iscloselyrelatedtotherubber/?llerinteractions.Inthecaseofcarbonblack?lledrubbersystems,therubber/?llerinteractionscanbecontrolledbytheoxidationandgraphitizationofcarbonblacksurface[10,12,13].Thusmanystudieshavebeendevotedtoinvestigatetherubber/?llerinteractionsofcarbonblack?lledrubbersystems.Brennanetal.studiedtheeffectsofgraphitizationofcarbonblackonthestress–strainbehaviorofcarbonblack?lledstyrene-butadienerubber(SBR),andreportedthestressata

*Correspondingauthor.Tel.:C81352288275;fax:C81332352214.E-mailaddress:itma56@rs.kagu.tus.ac.jp(M.Ito).

0032-3861/$-seefrontmatterq2004ElsevierLtd.Allrightsreserved.

doi:10.1016/j.polymer.2004.10.066

largestrainwasenhancedbythestrongchemicalbondingbetweencarbonblacksurfaceandrubbermolecules[12].Further,theysuggestedthatthedegreeofchainscissionofrubbermoleculesincreasedwiththeincreaseofthetensilestress.However,littleinformationhasbeenreportedonthedirectevidenceofchainscissionduringthetensiledeformationofcarbonblack?lledrubbersystems.

Whenrubbermoleculesarestretched,acarbonradical(–C%)isformedbyachainscissionofrubbermolecules[14–16].Electronspinresonance(ESR)isapowerfultechniquetodetectthecarbonradicalformedbythechainscission[17,18].However,carbonblackitselfshowsanESRsignalarisenfromloneelectronsinitsparticle[19,20].Further,theintensityofESRsignalforcarbonblackismuchstrongerthanthatforthecarbonradicalsproducedbythechainscissionofrubbermolecules.Thissuggeststhedif?cultyofquantitativeanalysesofchainscissionscausedbythetensiledeformationofcarbonblack?lledrubbersystems.

Silicaisalsoknownasaneffective?llerofrubberreinforcement.Sincesilicadoesnothaveanyradicaland

194N.Suzukietal./Polymer46(2005)193–201

loneelectrons,itdoesnotshowanyESRsignals.Thussilica?lledrubbersystemsaresuitablefortheinvestigationofchainscissionofrubbermoleculesduringthedeformation.Forsilica?lledrubbersystems,therubber/?llerinteractionscanbecontrolledbytheintroductionofcouplingagent[21–23].

Inthisstudy,silica?lledSBRvulcanizateswithdifferentrubber/?llerinteractionswerepreparedbyusingseveralkindsofcouplingagents.Then,theeffectsofrubber/?llerinteractionsonstress–strainbehaviorwerestudiedinrelationtothechainscissionsofrubbermoleculesbyESRandtensiletests.

2.Experimental2.1.Samples

Therawrubberusedwasastyrene-butadienerubber(SBR:NipolSBR1502,MwZ4.3!105,TgZK528C;NipponZeonCo.,Japan).The?llerusedwasaprecipitatedsilica(NipsilAQ;NipponSilica,Japan).Twokindsofcouplingagentswereusedforthesurfacemodi?cationofsilicaparticles.Theoneisamono-functionaldecyltri-methoxy-silane(DS)andtheotherisabi-functionalbis-(triethoxysilylpropyl)-tetrasul?de(TESPT).ThechemicalstructuresofthemarelistedinTable1.Thereactivityofcouplingagentswithsilanolgroupsonsilicaparticlesdependsonthemoisturecontentofsilicaparticles[21,24].Inthisstudy,as-receivedsilicawasdriedat1208Cunderareducedpressurefor12htocontrolthemoisturecontent.Themoisturecontentofdriedsilicawasabout2.0wt%whichwasdeterminedbythermalgravimetricanalyzer.Wecon?rmedthatthemoisturecontentof2.0wt%wasenoughtoreactthecouplingagentwithsilanolgroups[24].

Variouskindsofcompositeswerepreparedbyamechanicalmixing.ThecompositionsofthemarelistedinTable2.Theconditionsforthemechanicalmixingwereasfollows;SBRwasputintoaBanburymixer(Laboplasto-mill50MR;ToyoSeiki,Japan)andmasticatedunder60rpmat608Cfor1min,thendriedsilicaandcouplingagentweremixedwithmasticatedSBRunder60rpmat1008Cfor1min.Next,zincoxide,stearicacidandanti-oxidantweremixedwithSBR/silicacompositesat1008Cfor2min.Finally,themasterbatchwasmixedwithsulfurandacceleratorunder60rpmat608Cfor3min.The?ll

Table1

ChemicalstructureofcouplingagentDecyltrimethoxysilane

(DS)

factorofthecompositesat?nalstagewassettobe60%.Theunvulcanizedcompositespreparedbythemechanicalmixingwereseatedonarollmillfollowedbyavulcanizationat1608Cfor20minunderapressureof400kg/cm2.

Topreparethesamplesfortransmissionelectronmicroscopy(TEM)observations,thevulcanizedcompositeswerecuredagaininmeltedsulfur(w1258C)for36h.Thehardenedcompoundswereeasilycutintoultrathin?lms(!100nm)atroomtemperaturebymicrotoming.Thethin?lmswerestuckontotheTEMgridsforTEMobservations.TEMobservationswerealsocarriedoutforboundrubbersinunvulcanizedcomposites.Theconditionsforthesamplepreparationwereasfollows;2gofSBRwasdissolvedin150mlofbenzene.Then,thesolutionwasmixedwith1gofthedriedsilicadispersedin100mlofbenzeneandgentlystirredatroomtemperaturefor3h.ThemixtureofSBRandsilicawasdroppedontheTEMgridcoatedbycarbon.ThecompoundwasobtainedbyremovingthesolventfromthemixtureontheTEMgridatroomtemperatureunderareducedpressurefor24h.ThecompoundontheTEMgridwasimmersedinalargeamountoftolueneandextractionwascarriedoutatroomtemperaturefor144h.TheremainsontheTEMgridweretheboundrubberandsilica,whichwereusedforTEMobservations.2.2.Measurements

Thecrosslinkdensitiesofvulcanizedsamples(ne)weredeterminedbytheswellingmethodat258Cbyusingtoluene.

Measurementsofstress-straincurveswerecarriedoutonatensiletester(IM-20ST;Intesco,Japan)atroomtemperature(248C).Thestrainratewas3.0minK1.

TheESRspectrawereobtainedbyusinganESRspectroscopy(ES-FA200,JEOL,Japan)ataresonancefrequencyof9.2GHz.ThesamplesforESRmeasurementswerepreparedbythefollowingprocedure;thevulcanizedsheetswerecutinto1mm!2mm!30mm.Thesampleswerestretchedbyatensiletesteratroomtemperatureuptoadesiredstrain(IS:10–200%)followedbyanimmediateESRmeasurementatK1008C.Theradicalconcentrationinthesampleswasobtainedfromadouble-integrationofthe?rstderivativecurvesofESRsignalsbyusinganaqueoussolutionof4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl

Bis-(3-triethoxysilylpropyl)-tetraslu?de

(TESPT)

N.Suzukietal./Polymer46(2005)193–201

Table2

Compositionsofsilica?lledSBRsystemsSamplecodeSBRSilicaDSTESPTZincoxideStearicacideAnti-oxidant6CaAcceleratorCZbAcceleratorDPGcSulfur

abc

195

UM-110050––32111.51.7

UM-210050––32111.52.9

DM100505–32111.51.7

TM10050–532111.51.7

UF100–––32111.51.7

N-(1,3-dimethylbutyl)-N0-phenyl-p-phenylenediamine.N-cyclohexyl-2-benzothiazyl-sulfenamide.Diphenylguanidine.

radical(TEMPOL)asanexternalstandard.Theg-valueandlinewidthweredeterminedbyusingthethirdandfourthsignalsofMn2Cmarker.

TEMobservationswerecarriedoutbyusingtrans-missionelectronmicroscope(TEM:HitachiH-9000NARType;Hitachi)withanacceleratedvoltageof200kV.

3.Resultsanddiscussion3.1.Thestress–strainbehavior

Itiswellknownthatthestress–straincurvesforsilica?lledrubbersystemsareaffectedbythecrosslinkdensityofrubbermatrix[22,25],thesizeofagglomeratesformedbythesilica[24,26]andrubber/silicainteractions[4,22].Theseeffectscanbecontrolledbythecontentsofcuringagents,thenumberofsilanolgroupsonsilicaparticlesandtheintroductionofcouplingagent.

Thecrosslinkdensities(ne)ofsurfacemodi?edsilica?lledvulcanizates(DMandTM)andunmodi?edsilica?lledvulcanizateswithdifferentsulfurcontent(UM-1andUM-2)areshowninTable3.ThecomparisonofthedatafortheUM-1andUM-2revealedthatthehigherthesulfurcontent,thehigherthenethatcouldbeobtained.Itisalsoseenthatthenewasnotaffectedbytheintroductionofmono-functionalcouplingagent(UM-1vs.DM).ThenefortheTMwasthehighestinthesamples.AsshowninTable1,theTESPTinvolvesfoursulfuratomsinonemoleculewhichreactwithdieneunitofSBR[22,27],leadingtotheincreaseofne.

Fig.1showsthestress–straincurvesoftheUM-1,UM-2,DMandTM.Itisseenthattheinitialslopeofthestress–straincurveswereaffectedbythecrosslinkdensityandthe

Table3

Crosslinkdensitiesofsilica?lledSBRsystems

UM-1

ne!10K4molcmK3

2.41

sortofcouplingagents.Theinitialslopeofstress-straincurvesincreasedwiththeincreaseofthene,inaccordancewithourpreviousreport[22].TheinitialslopedecreasedbytheintroductionofcouplingagentsevenfortheTMwhichhadthehighestne.Whenthecouplingagentswereintroducedonsilicaparticles,ahydrophiliccharacteroftheparticlechangedintoahydrophobicone,whichreducedthe?ller–?llerinteractionsviareducedagglomeratesizeformedbysilicaparticles,leadingtothedecreaseoftensilemoduluswhichisknownasaPayneeffects[24,27].Fig.2showsTEMphotographsofsurfacemodi?edandunmodi-?edsilica?lledvulcanizates(DM,TMandUM-1).Itisclearlyseenthereductionofagglomeratesizebytheintroductionofcouplingagents.Thiscon?rmsthatthedecreaseofinitialslopeofstress–straincurvesfortheDMandTMiscausedbythedecreaseofthesizeofagglomerate.Atabout10%ofstrain,thethreesamples(UM-1,UM-2anTM)showedasimilartensilestressalthoughthesesampleshaddifferentne.However,thestressfortheDMwasquitesmallcomparedwiththeUM-1,UM-2andTM.Wereporttheeffectsofchemicalstructureofcouplingagentonthestress-strainbehaviorofsurfacemodi?edsilica?lledrubbercomposites[24].Inthecaseofmono-functionalcouplingagents,thetensilestressatalargerstraindecreasedwiththeincreaseofthelengthofalkylchainsinthecouplingagents.Theresultssuggestedthatthemono-functionalcouplingagentswithlongalkylunitsworkedasaplasticizerofrubbermolecules.Asimilarexplanationcanmadeforthestress–strainbehavioroftheDM.

Atalargerstrain(O20%),thetensilestressoftheTMsteadilyincreasedwithincreasingthestrain,similartothecaseofcarbonblack?lledvulcanizates[10,12].Ontheotherhand,thestressfortheDMalmostleveledoffatalargerstrain.Aprimaryreactionforthedifferencemightbe

UM-24.34

DM2.36

TM5.66

196N.Suzukietal./Polymer46(2005)193–201

theboundrubberexistswithintheagglomerates[28,29].Ontheotherhand,fortheTM,theboundarywasobscure.Thisisduetotheexistenceofrubbermoleculesonsilicaparticles.AsseeninTable1,ethoxylgroupsofTESPTreactwithsilanolgroupsonsilica[21,22,27].Inaddition,sulfuratomsofTESPTcouplewithdienegroupsofSBR[21,22,27].SuchachemicalstructureofTESPTproducedastrongchemicalbondingbetweenSBRandsilica,whichenhancedthetensilestressatalargerstrain.3.2.ESRresults

Theun?lledvulcanizates(UF)showedabroadESRabsorptioncurve,evenintheunstretchedstate.Thesignalintensitygraduallydecreasedwithincreasingthestoragetimeofthesamplewithoutchangingtheg-valueandthelinewidthatthemaximumslope(DHmsl)andalmostleveledoffatoneweekafterthesamplepreparation.Theg-valueandtheDHmslfortheabsorptioncurvewere2.004and1.5mT,respectively.TheabsorptioncurveforthesampleofwhichstoragetimeisoneweekisshowninFig.4(a).Inun?lledSBRvulcanizatesbysulfur,wecanexpecttwokindsofradicals.Theoneisasulfurradical(–S%),andtheotherisacarbonradical(–C%)withag-valuearound2.004.Theg-valueofsulfurradicalisestimatedtobe2.05[30,31],whichisdifferentfromtheobservedg-value.Thismeansthattheabsorptioncurveobtainedfromtheun?lledvulcanizatescanbeassignedtothecarbonradicals.Inthisstudy,allsampleswerepreparedbyamechanicalmixingusingaBanburymixer.Duringthemechanicalmixing,alargeshearforceappliedtothesamples,leadingtothechainscission[32–34].InSBR,thebondenergybetweensp3carbonofbutadieneunitðZCH–CH2–YCH2–CHZÞisthelowest.Thus,theallylradical(–CaC–C%)islikelytobeformed

by

Fig.1.Stress–straincurvesforsurfacemodi?edandunmodi?edsilica?lled

vulcanizates.

attributedtothedifferenceofne.Inaddition,wemusttaketheinterfacialbondingbetweensilicaandrubbermoleculesintoconsideration,sincetheTMwaspreparedbyusingbi-functionalcouplingagent(TESPT).Itiswellknownthatthestronginterfacialbondingsbetweencarbonblackandrubbermatrixenhancethetensilestressatalargerstrain[10,12,27].TheevidenceoftheinterfacialbondingintheTMwasalsoobtainedbyTEMobservations.ThesamplesforTEMobservationswerepreparedbythesolutionmixingmethodasdescribedinSection2.Fig.3showstheTEMphotographsforboundrubbersfromsurfacemodi?edsilicaparticles.FortheDMaclearboundarywasrecognizedbetweeneachsilicaparticleandagglomerate.Inthiscase,

Fig.2.TEMphotographsofsilica?lledvulcanizates.

N.Suzukietal./Polymer46(2005)193–201197

Fig.3.TEMphotographsofboundrubberfromsurfacemodi?edsilica?lledcompositespreparedbysolutionmixingmethod.

thechainscission.Ifso,theESRsignalfromallylradicalshouldshowaclearhyper?nesplitting[16,35,36].Car-stensenreportedthattheallylradicalsinpolybutadieneandpolyisopreneformedatliquidnitrogentemperaturewereeasilytransformedintopolyenylradical(–(CaC)n–C%)duringheating[35,36].Inaddition,theg-valueandDHmslforpolyenylradicalarereportedtobe2.004and1.6mT[35,37].Fromtheseresults,itisinferthattheabsorptionshowninFig.4(a)isassignedtothepolyenylradicals.Thelifetimeofallylradicalsdependsonthetemperature.TheallylradicalsarequicklytransformedintopolyenylradicalsevenatlowtemperatureofK1008C[35,36].Thus,theallylradicalsformedbymechanicalmixingmightquicklytransformintopolyenylradicalsduringsamplepreparation.TheintensityofESRsignalsforun?lledSBRvulcanizatesincreasedwithincreasingthestrain(initialstrain:IS,(Fig.4(b)and(c))withoutchangingtheg-valueandDHmsl.Thissuggeststhatthechainscissionsoccurduringthestretching,resultingintheformationofpolyenylradicals.Sincethesampleswerestretchedatroomtemperature,allylradicalsformedbythechainscissionswerequicklytransformedintopolyenylradicals.

The?rstderivativeESRcurvesforunmodi?edsilica?lledSBRvulcanizates(UM-1)withdifferentinitialstrainsareshowninFig.5.Thevulcanizatesshowedabroadabsorptioncurvewiththeg-valueof2.004andtheDHmslof1.5mT.Further,thesignalintensityincreasedwithincreasingtheinitialstrain(IS).Theg-valueandDHmslwereindependentoftheIS.Theseresultswerequalitativelysimilartothoseforun?lledvulcanizates(UF)shown

in