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MaterialsChemistryandPhysics134(2012)958e965

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MaterialsChemistryandPhysics

journalhomepage:http://wendang.chazidian.com/loca

te/matchemphys

PrescribedmorphologyandinterfacecorrelationofMWNTs-EP/PSFhybridnano bersreinforcedandtoughenedepoxymatrix

GangLia,b,*,XiaolongJiab,ZhibinHuangb,BoZhub,PengLib,XiaopingYangb,*,WuguoDaic

a

CollegeofMechanicalandElectricalEngineering,BeijingUniversityofChemicalTechnology,Beijing100029,PRChina

StateKeyLaboratoryofOrganiceInorganicComposites,BeijingUniversityofChemicalTechnology,Beijing100029,PRChinac

NantongXingChenSyntheticMaterialsCo.,ChinaBlueStarCo.,Nantong226017,PRChina

b

articleinfo

Articlehistory:

Received21May2011Receivedinrevisedform30January2012

Accepted23March2012

abstract

MWNTs-EPweresuccessfullypreparedbyfunctionalizationofMWNTswithepoxy-basedgroups,andMWNTs-EP/polysulfone(PSF)hybridnano berswerefabricatedtoobtainex-situdispersionandalign-mentofMWNTs-EPbyelectrospinning.Theprescribedmorphologyandinterfacecorrelationofhybridnano bersreinforcedandtoughenedepoxymatrix(RTEP)wereinvestigated.Thealignmentdegreeofhybridnano berswasenhancedwithincreasingMWNTs-EPloadings,andMWNTs-EPwerefoundtobewelldispersedandalignedalongthenano beraxis.ThedispersionandalignmentstatesofMWNTs-EPduringinhomogeneousphaseseparationofRTEPwereproposedandveri ed.MWNTs-EPdispersedandalignedalongtheorginalnano beraxiswereenveloped,bridgedorpinnedbyPSFspheresarrangedinthenano berdirection.TheinterfacechemicalcorrelationbetweenMWNTs-EPandresinmatrixwasgeneratedduetothefurtherreactionofepoxideringsonthesurfaceofMWNTs-EP,whichresultedinsimultaneousimprovementofmechanicalandthermalpropertiesofRTEP.

Ó2012ElsevierLtd.Allrightsreserved.

Keywords:Interfaces

Nanostructures

ElectronmicroscopyFractureandtoughness

1.Introduction

Sincecarbonnanotubes(CNTs)were rstusedas llerinepoxymatrixinthemid1990s,CNTreinforcedepoxymatrixhavebeenexploredasnewprospectsforsuperiorcompositematerialswithhighstrength,lowweightandmulti-functionalfeatures[1e3].However,theatomicallysmoothnonreactivesurfaces,thedistinc-tivelypoorinterfacialbonding,andthespontaneouslyentangledaggregationpropertieshavelimitedtheeffectivenessofCNTs[4,5].Therefore,thestronginterfacialinteractionbetweenCNTsandmatrixaswellasthehomogenousdispersionofCNTsintheepoxymatrixshouldbeapre-requisiteinordertoobtainhighperfor-mancepolymercomposites[6,7].Forthispurpose,chemicalfunc-tionalizationofthesurfaceofCNTshasbeenproposedasanef cientmethod[8e10]toimprovetheinterfacialstresstransferandpositivelyaffectthedispersibilityofCNTs.Epoxy-basedfunc-tionalizationofCNTswasshowntobethemostfavorableduetotheabilityoffurtherreactingofepoxidegroupsgraftedonthesurfaceofCNTs[11].

*Correspondingauthors.StateKeyLaboratoryofOrganiceInorganicComposites,BeijingUniversityofChemicalTechnology,Beijing100029,PRChina.Tel./fax:þ861064412084.

E-mailaddresses:ligang@http://wendang.chazidian.com(G.Li),yangxp@http://wendang.chazidian.com(X.

Yang).

0254-0584/$eseefrontmatterÓ2012ElsevierLtd.Allrightsreserved.doi:10.1016/j.matchemphys.2012.03.098

Anotherimportantaspecttobeconsideredforthesematerialswasthenano-andmicro-morphologyofreinforcementphaseandreinforcedcomposites[2,12].Vaia[13]hassuggestedtheideaof“nanocomposites-by-design”,andtriedtoestablishstructure-performancecorrelationsofcompositesbycontrollingthedistri-butionandarrangementofdispersed,preformedCNTs.Xie[14]andEsawi[15]alsoconsideredthattheenhanceddispersionandalignmentofCNTsinthepolymermatrixweredirectlyrelatedtothemechanicalandfunctionalpropertiesofCNTs/polymercomposites.TooptimizethepropertiesofCNTs/polymercompos-ites,Raravikar[16]andFeng[17]pointedoutthatthedispersionandalignmentofCNTscouldbeachievedpriortocompositefabricationwherealignednanotubesareincorporatedintothepolymermatrixusinganexternalelectric eld.Therefore,variousprocessingtechniqueshavebeenexploitedtoestablishthecouplingbetweenappliedexternalforcesandthedistributionandalignmentofCNTs[18e20].Atpresent, berspinningisconsideredaseffectivemethodtopossesstheabilitytoinduceex-situdispersionandalignmentofCNTsbymechanicaldeformation.TheelectrospinningprocesshasbeensuccessfullyusedtodisperseandalignCNTsinpolymercompositenano bers,inwhichthestabledispersionandhighalignmentofCNTswereobtained[21,22].

Inourpreviouswork,theinhomogeneousphasestructureshavebeenobservedinpolysulfone(PSF)nano berstoughenedepoxy

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matrix[23,24],andPSFspheresgeneratedfromphaseseparationofnano bersexhibitedrandomalignment.Onthebasisoftheinho-mogeneousphaseseparation,theelectrospinning-induceddisper-sionandalignmentofCNTswereintroducedtofabricateCNTs/PSFhybridnano berswithwelldispersedandalignedCNTs,andtopreparereinforcedandtoughenedepoxymatrix(RTEP)usingsuchhybridnano bers.Duringtheinhomogenenousphaseseparationofhybridnano bers,theCNTswerewelldispersedandalignedinthephasestructureduetotheex-situdispersionandalignmentofCNTsinhybridnano bers.Therefore,theprescribedmorphologyofRTEPwasobtainedthatcouldachievethesynergisticeffectsoftheimprovementofstrengthandtoughness.

Toachievethispurpose,(i)MWNTs-EPwerepreparedbycarboxylationandfurtherattachmentofepoxyterminatedmole-cules,andMWNTs-EP/PSFhybridnano berswithwelldispersedandhighlyalignedMWNTs-EPwerefabricatedbyelectrospinning,(ii)aschematicrepresentationofthedispersionandalignmentstateofMWNTs-EPduringtheinhomogeneousphaseseparationofhybridnano berswasproposed,andcomparedwiththeactualmorphologiesofRTEP,(iii)thechemicalcorrelationbetweenMWNTs-EPandepoxymatrixinterfaceswereinvestigated,andthemechanicalandthermalpropertiesofRTEPwerealsodiscussed.2.Experimental2.1.Materials

Multiwalledcarbonnanotubes(MWNTs,purity!95%,diameter10e20nm,length5e15mm)wereproducedbyShenzhenNano-techPortCo.,Ltd,China.Polysulfone(PSF,Udel1700)wassuppliedbyAmocoCo.DiglycidoletherofbisphenolA(DGEBA,EPON828)wasobtainedfromShellChemicalCo.4,40-diaminodiphenylsulfone(DDS)weremanufacturedbyYinshengChemicalCo.N,N0-dimethylformamide(DMF),triphenylphosphorus(TPP),tetrahydrofuran(THF),N,N0-dimethylacetamide(DMAC)andacetonewerepur-chasedfromBeijingChemicalAgentCo.2.2.PreparationofMWNTs-EP

TheschematicofthepreparationprocedureforepoxygraftedMWNTs(MWNTs-EP)isshowninFig.1.ThecarboxylatedMWNTswaspreparedbyfollowingourpreviouslyreportedprocedure[21].Thecarboxylgroupswere rstintroducedontothesurfaceoftheMWNTsbyacidoxidationofas-receivedMWNTsfor1hwithamixtureofconcentratedH2SO4andHNO3(1:1,volumeratio).ThecarboxylatedMWNTsweredispersedinDMFbybathsonicationfor1h.DGEBAwasdissolvedinDMFinaseparatevessel,afterwhichthetwodissolvedmaterialswerefurthermixedandbath-sonicatedforhalfanhour.Thenthemixturewasslowlystirredandheatedto130 C,afteraddingTPPasacatalystforthereaction.Thesystemwasallowedtoreactfor12h.ThereactedMWNTswerewashedwithTHFand lteredthroughaPTFEwitha2mmporesizetoobtaintheepoxygraftedMWNTs(MWNTs-EP).

2.3.FabricationofMWNTs-EP/PSFhybridnano bers

MWNTs-EPweredispersedin20mlmixtureofDMAC/acetone(7:3volumeratio)byultrasonicagitationfor6h,then4.0gPSFpelletswereaddedtoobtainasuspensionforelectrospinning.Thesuspensionwasplacedin30mlmedicalsyringewith12gaugeneedle,andelectrospunat0.5ml/h owrateunder16kVappliedvoltage.TheamountofMWNTs-EPinhybridnano berswascontrolledto5,10,15wt%ofPSF.Forcomparison,PSFnano berswithoutMWNTs-EPwerepreparedbyelectrospinning.2.4.Preparationofhybridnano bersreinforcedandtoughenedepoxy(RTEP)

Epoxymatrixwaspreparedbydissolving100gofDGEBAand30gofDDSbyvigorousstirringat120 Cfor25min,andtheblendwasdegassedundervacuumtoobtainhomogeneoussolution.PSFnano bersandhybridnano berswithvariousMWNTs-EPloadingswerecutandplacedhorizontallyintoalaboratory-mademold.Theepoxymatrixwasthenpouredintothemoldandcuredatthreestages:120 Cfor2h,160 Cfor2h,and180 Cfor1h.Theamountofthenano berswascontrolledto2wt%ofresinmatrix,thatis,theMWNTs-EPloadingswerecontrolledto0.1,0.2and0.3wt%inRTEP,respectively.2.5.Characterization

ChangesofthefunctionalgroupsonthesurfaceofMWNTsweredetectedwithanFTIRspectrometer(Nicolet670).

MorphologiesofPSFnano bers,MWNTs-EP/PSFhybridnano- bersandthenano bersreinforcedandtoughenedepoxymatrix,wereobservedbyscanningelectronmicroscope(SEM,S4700,HITACHI)andtransmissionelectronmicroscope(TEM,JEM100CX,HITACHI),respectively.

ThealignmentofMWNTs-EPinthehybridnano berswascharacterizedusingRamanspectrometer(TY-HR800)withlaserexcitationat532nm.

Thetensilepropertiesweremeasuredbytensiletestingmachine(INSTRON1121)accordingtoGB2568-1995.Thedumbbellspecimensofdimension100mmÂ6mmÂ2.5mmwaspreparedbythenano berdirectionparalleltothelengthofspecimens,andtestedatarateof1mm/minusinga5kNloadcell.The nalvalueswereaveragesof vemeasurements.

Dynamicmechanicalthermalanalysiswasperformedbythree-pointbendingmode(DMTA-V,RheometricsScienti http://wendang.chazidian.comA).ThedimensionofaspecimenforDMTAtestingwas50mmÂ6mmÂ2mm.Theheatingratewas5 C/minfrom50 Cto300 C,andthe xedfrequencywas1Hz.TheTgwasmeasuredfromthepeakofthetandspectrum.

Thecriticalstressintensityfactor(KIC)wasmeasuredbysingle-edgenotchedfracturetoughnesstestinthree-pointbendingmode,accordingtoASTM5045-1999.Thecrossheadratewas1mm/min,andthe nalvalueswereaveragesof ve

measurements.

Fig.1.SchematicofthepreparationprocedureforMWNTs-EP.

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aromaticringsofMWNTs[11],asshowninFig.1.Theseresultsindicatedthattheepoxide-terminatedgroupsweresuccessfullygraftedontothesurfaceofMWNTs.

3.2.MorphologyofMWNTs-EP/PSFhybridnano bers

Fig.3showstheSEMimagesofMWNTs-EP/PSFhybridnano- berswith0,5,10,and15wt%MWNTs-EP.PSFnano berwithoutMWNTs-EPpossessedcoarserdiameterwithmorerandomdistri-butionthanhybridnano berswithMWNTs-EP.WithincreasingMWNTs-EPloadings,theaveragediameterofnano bersdecreasedalongwiththeenhancementofalignmentdegree,asshowninFig.3bed.Ingeneral,theelectricconductivityofMWNT-EP/PSFsuspensionsforelectrospinningincreasedwithincreasingMWNTs-EPloadings,whichledtotheincreaseofnetchargedensitycarriedbythespinningjet[25].Therefore,theliquiddropletofthesuspensionsweresubjectedtothelargerelectrostaticforces,andthespinningjetunderwentthelargerdrawratiosduringtheelectrospinningprocess[26],whichallowedthedis-solvedpolymertobeorientedbytheelongational owofthechargedjet[25], nallyresultingintheincreaseofthedegreeofalignmentofhybridnano bers.

Fig.4showstheTEMimagesofMWNT-EP/PSFhybridnano- berswith5,10,and15wt%MWNTs-EP.TheMWNTs-EPweresuccessfullyembeddedinthePSFnano bers,andwerewelldispersedandhighlyalignedalongthenano beraxis.ThedegreeofalignmentofMWNTs-EPincreasedwithincreasingMWNTs-EPloadings.WhentheloadingofMWNTs-EPinthenano berwasreached15wt%,theMWNTs-EPwerealmoststretchedintostreamlines,whichwasattributedtotheextremelylargeeffectivedrawratioresultedfrombendinginstabilityintheelectrospinningprocess[20].TheincreaseofthedegreeofalignmentofMWNTs-EPinhybridnano bersmightbefavorabletothedispersionandorientationofMWNTs-EPinepoxymatrix.

ThealignmentofMWNTs-EPinhybridnano berswasfurthercharacterizedwithRamanSpectroscopy.Fig.5showsthepolarizedRamanspectraofMWNTs-EP/PSFhybridnano berswith5,10,and15wt%MWNTs-EP,inwhichtheVVcurverepresented

the

Fig.2.FTIRspectraof(a)as-receivedMWNTs,(b)carboxylatedMWNTs,and(c)

MWNTs-EP.

3.Resultsanddiscussion3.1.FunctionalizationofMWNTs

Fig.2showstheFTIRspectraofas-receivedMWNTs,carboxyl-atedMWNTsandMWNTs-EP.Thepeakat1709cmÀ1inthespectraofcarboxylatedMWNTscorrespondedtothecarboxylofthecarboxylicacid,whichindicatedthechangesoffunctionalgroupsonthesurfaceofMWNTsduetothecarboxylation[5].Thispeakwasnotdetectedontheas-receivedMWNTs.InthespectraofMWNTs-EP,thedisappearanceofthepeakat915cmÀ1,whichcorrespondedtotheepoxideringintheDGEBA[5],indicatedthattheepoxideringsopenedandreacted.Thepeakat1100cmÀ1wasrelatedtotheCeObondwhichresultedfromthereactionbetweenthecarboxylicacidandtheepoxidering.Thepeakat1260cmÀ1wasassociatedwithCeOoftheester,whichwasattachedtothe

Fig.3.SEMimagesofMWNTs-EP/PSFhybridnano berswith(a)0,(b)5,(c)10,and(d)15wt%MWNTs-EP.

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Fig.4.TEMimagesofMWNTs-EP/PSFhybridnano berswith(a)5,(b)10,and(c)15wt%MWNTs-EP.(Thewhitecurvesinpartaandbandthewhitearrowsinpartcrepresentthealignmentdirectionsofthe

MWNTs-EP.)

spectrumwhenthepolarizationoftheincidentlaserwasparalleltothenano beraxis,andtheVHcurverepresentedthespectrumwhenthepolarizationoftheincidentlaserwasnormaltothenano beraxis.Thestrongabsorptionpeakat1582cmÀ1wasassignedastheGbandandassociatedwithtangentialCeCbondstretchingmotionthatoriginatedfromtheE2g2modeinthegraphite.Thepeakat1351cmÀ1wasassignedastheDbandandderivedfromdisorderedgraphitestructures[27].Theshoulderpeakat1607cmÀ1wasusuallydenotedastheD0bandandhasalsobeenattributedtodisorder-inducedfeaturesintheCNTs[28].WithincreasingMWNTs-EPloadings,theup-shiftofGandDbandsindicatedthatMWNTs-EPpossessedasubstantialinteraction[27],whichmightbeattributedtobettercompatibilitybetweenPSFandepoxyonthesurfaceofMWNTs-EP.Inaddition,therewasanobviousdifferenceintheintensityoftheGbandfordifferentpolarizationdirectionswithincreasingMWNTs-EPloadings,whichwasassociatedwiththealignmentofCNTsinthehybridnano bers.Ingeneral,thedegreeofalignmentofCNTscanbeevaluatedby

the

Fig.5.PolarizedRamanspectraofMWNTs-EP/PSFhybridnano berswith(a)5(b)10,and(c)15wt%MWNTs-EP.

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depolarizationfactorR,theratioofthepeakintensitiesoftheGbandinthetwopolarizationdirections.TheRvaluesforhybridnano berswith5,10,and15wt%MWNTs-EPwere0.977,1.070,and1.152,respectively,whichweregreaterthanthatofas-receivedMWNTs(ca.0.81)[28].TheincreaseinRvaluecanbeascribedtothebetteralignmentofMWNTs-EPinthehybridnano berswithincreasingMWNTs-EPloadings.

3.3.Prescribedmorphologyofhybridnano bersreinforcedandtoughenedepoxy

Theex-situdispersionandalignmentofMWNTs-EPwereobtainedbyfabricatingMWNTs/PSFhybridnano bersthroughelectrospinning,whichwasassociatedwiththephaseseparationofMWNTs-EP/PSFhybridnano bersreinforcedandtoughenedepoxymatrix(RTEP).Fig.6showstheschematicrepresentationofthedispersionandalignmentstatesofMWNTs-EPduringreaction-inducedphaseseparationinRTEP.

TheepoxymatrixwascompoundedwithMWNTs-EP/PSFhybridnano bers(a).Duringthecuringprocess,PSFnano bersweredissolvedintoepoxymatrix,whereasMWNTs-EPretainedtheoriginaldistributionasinhybridnano bers,asshownin(b).

Withthecuringreactionproceeding,thephaseseparationofhybridnano bersbegantooccur,andPSFspheresweregeneratedduetophaseseparationofnano bers,shownin(c).Herein,thedistributionandalignmentstatesbetweenMWNTs-EPandPSFspheresweresuggested.The rstpossiblestatewasthattheMWNTs-EPwasenvelopedbymanyPSFspheres,whichwasthemostfrequent.AnotherstatewasthatMWNTs-EPmightbepinnedbyonePSFsphere.Inaddition,theMWNTs-EPmightalsobebridgedbysomePSFspheres.However,whateverstatetheMWNTs-EPlocatedamongPSFspheres,theMWNTs-EPwerealmostdispersedandalignedalongtheoriginalnano berdirec-tion,whichwasattributedtothedispersionandalignmentofMWNTs-EPinhybridnano bers.ThedispersionandalignmentofMWNTs-EPandthegenerationofPSFsphereswouldresultinthesynergisticeffectsofhybridnano bersonreinforcingandtough-eningtheepoxy

matrix.

Fig.6.SchematicrepresentationofthedispersionandalignmentstatesofMWNTs-EPduringreaction-inducedphaseseparationinRTEP.(a)Recombinationofepoxymatrixandhybridnano bers,(b)DissolutionofPSFnano bersintoepoxymatrix,and(c)PSFspheresfromthephaseseparationofnano bersand xationofMWNTs-EPinthephase

structures.

Fig.7.SEMimagesofRTEPwith(a)0,(b)0.1,(c)0.2and(d)0.3wt%MWNTs-EP.