thesesurfacemodificationprocesses.Anextensivereview ontherelatedliteraturecanbefoundelsewhere[11,13].
Itisworthpointingoutthatfilmcastingisoftenusedasa methodtoprepareCPCsaftersolutionmixing;thisprocess oftentakesmorethan24h.Comparedwiththe solidifica-tionprocessaftermeltprocessing(whichistypicallyless thanafewminutes),thisprocessoftengivesanadequate amountoftimefortheconductivenetworktore-aggregate inCPCs.Therefore,theelectricalpercolationthresholdthat isobservedinCPCsfromsolutionmixingislessthanthat frommeltcompounding[6].
4. Morphologicalcontrolofconductivenetworksin CPCs
4.1. Characterisationofconductivenetworkformationin CPCs
Becausethemorphologyof theconductivenetworks hasanimportantinfluenceontheelectricalpropertiesof CPCs,itiscrucialtocharacterisethemorphologicaldetails ofthese networks.Researchershave demonstrated that variousdirectandindirectmethodscanbeusedto charac-terisethemorphologiesofnanofillersandtheirconductive networksinCPCs.
A range of microscopic methods, including optical microscopy (OM) [48], scanning electron microscopy (SEM)[43,69–74],transmissionelectronmicroscopy(TEM) [75–77], scanning probe microscopy (SPM) [78,79] and atomicforcemicroscopy(AFM)[78,79],canbeusedforthe directobservationofconductivenetworksin nanocompos-ites.AsshowninFig.3,OM,TEM,SEMandHAADF-STEM havebeen usedby differentresearchgroups to charac-terisethemorphologyofCPCsfromdifferentperspectives.
Thesemethodshavebeenwidelyusedasgeneral micro-scopicmethodstocharacterisethemorphologyofpolymer compositesfromdifferentaspectsorondifferentscales.
OMisoftenusedtostudythemorphologyovertherange ofafewmicronsorabove.Foranythingbelowthisrange, SEM,AFMorTEMisneeded.Itis wellknownthatonly informationnearthesurfaceofspecimensiscapturedby conventionalSEMbecausesecondaryelectronshavea rel-ativelyshallowescapedepth(5–50nm)duetotheirrather lowenergylevels[71].However,itwasrecentlyreported thattheSEMobservationofdeeplyembeddedCNTsand anassessmentoftheoverallCNTdispersionstatuswere possiblebyusingvoltagecontrastimaginginCPCsthatare basedonCNT/polymer[43,70–74,80].Thiscontrast mech-anismwasfirstreportedbyChungetal.[81]andhasbeen adaptedbydifferentresearchgroups[43,70–74,80].
Loosetal.usedconventionalSEMinthecharge con-trast imaging mode to investigate the morphology of networksthatwereconstructedwithsingle-wallcarbon nanotubes(SWNTs)[80]andgraphenesheets[82] embed-dedinpolystyrene(PS)matrices.Theauthorsreportedthat
thechargecontrastimagingoftheconductivenetworks under high acceleration voltages could provide three-dimensionalinformationonthestructureoftheconductive networks.Thismethodhasbeengainingmoreattention recently.Bauhoferetal.[70]conductedaseriesofworks toobtaindetailedinformationonthevisualisationofCNTs ininsulatingmatrices.AsshowninFig.4,factorsthathave importanteffects onthevisibilityofCNTsinCNT/epoxy composites, including theimaging mechanism, imaging depth, parameters, requirement of sample conductivity and SEM detectortype, were investigated [71].Further studyrevealedthatCNTsarestillvisiblebelow1kVand exhibit abright contrasttothedarkepoxymatrix.This resultisinterestingbecauseimagingatalowaccelerating voltageisregardedasabetteralternativetohighvoltage imaging forinvestigating CNTdispersionsintheseCPCs becausethechargingofthespecimenscanbeavoidedat alowvoltage.Finally,thekeytosuchanimagingmethod appearstobe(1)thedetectionofsecondaryelectronsthat areexcitedintheelectronbeamimpactareaand(2)theuse ofanappropriatedetectorthatissensitivetoslightcharges onthesamplesurfaces.
SEM can certainly offer valuable information on the morphologies of nanofillers and their conductive networks. However, the actual size of the nanofiller and the detailed information onthelocal network that isobtainedfromSEMarenotveryaccuratedue tolocal charging of the polymer matrix around the nanofillers.
TEMandAFMcanbeutilisedtocomprehensively inves-tigatetheseissues.High-angleannulardarkfieldscanning transmissionelectronmicroscopy(HAADF-STEM)hasbeen investigatedasausefultoolforobtainingareliable quan-tificationofimagestoenhancethecharacterisationofthe conductivenetworkmorphology[75].Intermsofpolymer materials,STEM hasa number of advantages over con-ventionalTEM(CTEM):theimagesareeasytointerpret duetoalackofphasecontrast,thesignalintensityis lin-ear withthickness variationsanda highsignal-to-noise ratioisobtained.Theseadvantagesaremorepronounced when a high-angleannular darkfield (HAADF)detector that is capable of single-electron counting is used. For carbon-nanoparticle-filledsystems,HAADF-STEMcan cre-ateexcellentcontrastbetweendifferentcomponents,as showninFig.3.Generally,itisthoughtthatHAADF-STEM canbeusedasapowerfultoolforobtaininghigh-resolution imagesofunstainedpolymersystems.
AFMisapowerfulmethodforthecharacterisationofthe topographyandpropertiesofsolidmaterials[83].AFMis equippedwithasharpprobeforscanningacrossthe sam-plesurface.Theprobe-sampleinteractionsarerecordedto generatemapsofthematerialtopographyandthematerial properties,includingthemechanical,electricaland mag-neticproperties.Tkalyaetal.[82]usedconductiveatomic force microscopy (C-AFM)to investigatethe local mor-phologyofgrapheneinCPCs.Moreover,theconductivity distributionandthedetectionofpercolatingpathscanalso beobtainedwithnanometreresolution[84].
Allconventionalmicroscopytechniqueshavetheir spe-cific disadvantagesintermof imagingthedispersionof nanofillerswithina polymermatrix:opticalmicroscopy onlycharacterisesverylargeagglomeratesofnanofillers,
Please cite this article in press as: Deng H, et al. Progress on the morphological control of conductive network Fig.3.Morphologycharacterizedwithdifferentmethods:OMandTEMforPCcontaining0.688vol.%CNTs;SEMinchargecontrastmodeshowsthe distributionofMWNTsinPPmatrix;andHAADF-STEMpicturesshowsindividualCBparticlesandtheirclustersinpolymercomposites:OMandTEM, [214],Copyright2011.ReproducedwithpermissionfromElsevierScienceLtd.;SEM,[123],Copyright2009.ReproducedwithpermissionElsevierScience Ltd;andHAADF-STEM,[75],Copyright2009.ReproducedwithpermissionfromAmericanChemicalSociety.
whilesurface-basedmethods,suchasSEMorAFM,only characterise the surface or cross-section of the three-dimensionalarrangementofnanofillers,anditisdifficult todrawconclusionsonthebulkorganisationofthe com-posite fromtheTEMimages ofthin sections(thickness of∼100nm).Therefore,auxiliarymethodsareneededto providemoreinformationonconductivenetworks,suchas theorientationoftheconductivefillerandtheformation oftheconductivenetworks.Manymeasurements, includ-ing electrical conductivity, wide-angle X-ray diffraction (WAXD) and Raman spectroscopy, can be used to fur-thercharacterisethemorphologyofconductivenetworks [73,85]. From WAXD and Raman spectroscopy,the ori-entationsofthefillerandthepolymermatrixaswellas thepolymer-nanofiller and nanofiller-nanofiller interac-tionscanbeestimated.Thedispersionofthenanofillers andthemorphologyoftheconductivenetworkscanthen bepartiallydeterminedusingthisinformation. Addition-ally,aconductivitymeasurementisoftenusedasabasic methodforevaluatingtheformationofelectrically conduc-tivenetworksandgivesdirectevidenceoftheconductive networkmorphologyinCPCs.Alotofstudieshavebeen conductedtocharacterisetheconductivityofCPCs.Asa generalmethodformonitoringaconductivenetwork dur-ingprocessing,insituelectricalmeasurementshavebeen widelyusedtorecordtheresistivityofCPCs[73,76,86–88].
Thismethod canprovide information ontheformation anddestructionofaconductivenetworkduringdifferent processes,includingextrusion[89,90],thermalannealing [73,88]andshearing[15].Furthermore,rheologyhasalso beenusedtoinvestigatetheformationofanetworkwithin apolymermatrix[91–94].It wasdemonstratedthatthe detectionofapolymer-fillernetworkwithrheologycanbe stronglycorrelatedtothedetectionofafiller-filler conduc-tivenetworkwithanelectricalconductivitymeasurement [93,95].
4.2. Morphologicalcontrolthroughpolymerblends
Polymerblendsarearesearchtopicthathasbeenwidely investigated.Byconstructingpolymerblendwithtwoor morepolymers,theadvantagesofthesepolymerscanbe integrated;thus,balancedand optimisedproperties can beobtainedforthefinalmaterial.Furthermore,thephase morphologyofthepolymerblendsplaysacrucialrolein thefinalproperties;therefore,polymerblendswitharange ofpropertiescanbedesignedand fabricatedby control-lingtheirmorphology.Interms ofpolymer-blend-based CPCs,conductivefillerscanbeselectivelylocatedinoneof thepolymerphasesorattheinterfaces[96–100]; further-more,thevariousphasemorphologiesinpolymerblends canbeusedtoregulatetheconductivenetworkswithin
Please cite this article in press as: Deng H, et al. Progress on the morphological control of conductive network Fig.4.IllustrationofimagingmechanismunderSEMinchargecontrastmodeforCPCsbasedonepoxyandCNTsfromdifferentpreparationmethods:
calandered-compositesandstirred-composites.[71],Copyright2011.ReproducedwithpermissionfromElsevierScienceLtd.
theseCPCs[101–103].Therefore,polymerblendscanbe usedtocontroltheconductivenetworkmorphologyand electricalpropertiesofCPCs[103].
Double percolation has been widely investigated to reducethepercolationthresholdofCPCs[104].Inthistype ofpolymerblend,atleastoneofthepolymerphasesis con-tinuous,andthefillerislocatedinthiscontinuousphase.
ComparedwithCPCsthatarebasedonasinglepolymer, thepercolationthresholdoftheseCPCscanbesignificantly reducedduetotheselectivelocalisationoftheconductive networks.Therefore,theelectricalpropertyislargely influ-encedbythelocationoftheconductivefilleraswellasthe
morphologyofthepolymerblends.Thelocalisationofthe conductivefillerdependsonthebalanceoftheinterfacial energiesandcanbepredictedbycalculatingthewetting parameter,ωaa,whichisdefinedinEq.(4)[96].
ωa=filler-polymer1−filler-polymer2
polymer1,2 (4)
Inthisequation,xdenotethedifferentinterfacial ten-sions betweenthefillerand polymers1and 2, andthe interfacialtensionbetweenthetwoblendphasesisinthe denominator.Thefillertendstobelocatedinpolymer1 ifthewettingcoefficientislessthan−1andtendstobe
Please cite this article in press as: Deng H, et al. Progress on the morphological control of conductive network locatedinpolymer2ifthewettingcoefficientisgreater
than1.Furthermore,ifthewettingcoefficientisbetween
−1and1,thefillerismorelikelylocatedattheinterface betweenthetwopolymers.Thiscoefficientcanbeusedas anindicatorofthethermodynamictendencyofthefillerto localiseinaparticularareainanimmiscibleblend.Sucha tendencycorrespondstotheminimisationoftherelevant interfacialtensions.
ElectricaldoublepercolationinCPCsthatarebasedon polymerblendscanberealisedviaaselectivelylocalised conductivefillerinoneofthepolymerphases.The thermo-dynamicfactorsthatwerediscussedaboveoftendetermine thedistributionoftheconductivefillers. Severalgroups [104–106] have shown that the choice of the polymer matrixcansignificantlyinfluencethedistributionofthe conductive fillerinpolymer blendsdue tothedifferent interactionsbetweenthesepolymersandthefiller. Con-ductivefillersareoftenlocatedinthepolymerphasethat theyhavemoreinteractionwith.Thereareother meth-odsthatcanbeusedtomodifythelocationofthesefillers.
Lietal.[107]showedthatthefunctionalisationofafiller canchangethedistributionofthefillerinpolymerblends.
Moreover,Potschke et al.[100]used a reactive compo-nenttotunetheinteractionbetweenSANandMWNTsin apolymerblendthatconsistedofPCandSAN.Theauthors observedthatMWNTsaretransferredfromthePCphaseto theSANphaseaftertheadditionofthereactivecomponent duetoanenhancedinteraction.
Otherthanthermodynamicissues,kineticfactors,such asthemixingproceduresorsequence,blendingtimeand shearstrength,alsoplayimportantrolesinthe localisa-tionoftheconductivefiller[96,98,106].Ithasbeenshown thatthelocalisationoftheconductivefillercanbechanged bycontrollingkineticissues,suchasthemixingsequence [96,98,108,109]. Moreover,the aspectratio and surface roughnessoftheconductivefillerandtheviscosityofthe polymermatrixarealsoinfluentialfactors[99,110].
Toreduce thepercolation thresholdin CPCsthat are basedonpolymerblends,thelocalisationofthe conduc-tivefilleratthecontinuousinterfaceispreferredbecause oftheirlowvolumefraction.Manystudieshavefocused onachieving this difficulttask. For CPCsthat utilise CB asaconductivefiller,severalstudieshavebeenreported [109,111,112]. Bailly and co-workers [96,97] conducted a series of studies to localise MWNTs at the interface ofpolyamide(PA)/ethylene-methylacrylate(EA)random copolymerblends.Theauthorsobservedirreversible poly-merabsorptionontheMWNTsurfacesduringthetransfer fromonephasetotheother.TheseMWNTsaretrapped at the interface due to this absorption [96]. Recently, Potschkeetal.[99]illustratedthatthetransferofafiller from one phase to theother wasdriven by thermody-namicfactorsandwasinfluencedbytheshapeofthefiller.
Theauthorscompared CBwithMWNTsina blend con-tainingpoly(styreneacrylonitrile)andpolycarbonate(PC).
Thehigh-aspect-ratiofiller,MWNTs,transferredfromone phasetotheothereasierthanCBbecausethe low-aspect-ratiofillerappearedtohavemoreresistanceduringtransfer andwasthereforetrappedattheinterface.Itwassuggested thatthesphericalshapeandlowaspectratioofCBmade itmorestableattheinterfacethanMWNTs(seeFig.5).It
isratherdifficulttoobtainameasurableconductivityfor CPCsthatcontainalargeaspectratiofiller(suchasCNTs) locatedattheinterface.OnlyrecentlydidWangand co-workers[113]reportaselectivedistributionofCNTsatthe interfaceofimmisciblePC/acrylonitrile-butadiene-styrene (ABS)byusingacombinationofmaleicanhydridegrafted ABS(ABS-g-MA)andanappropriateprocessingprocedure.
Byadjustingthekineticissuesaswellasthe thermody-namicissues,thepreparedCPCsdemonstrateapercolation thresholdof0.05wt%.
4.3. Morphologicalcontrolthroughthermalannealing
ThermalannealinginvolvestheheatingofCPCstoabove their glass transition temperature or melting tempera-ture.Throughsuchaprocess,thepolymermatrixbecomes mobile.Quite often, this mobility can betransferred to the imbedded conductive filler and trigger the forma-tionofconductivenetworks.Thermalannealinghasbeen usedasaneffectivemethodforinducingtheformationof conductivenetworksin CPCsthatcontainvarious types ofconductivefillers[15,32,43,73,88,114–116].Asshown in Fig. 6, thermal annealing can repair CNT conductive networksthatweredestroyedbyshearinginarheometer.
Amorphologicalstudyconfirmedthatthere-aggregation ofCNTscanbetriggeredinapolymermelt,leadingtothe formationofaconductivenetwork.
Thereal-timetracingoftheelectricalresistivityduring isothermalannealingforisotropicCPCshasbeenwidely investigated[73,90,117–120].Apercolationtimeisneeded forthecompositesthatareannealedabovetheirglass tran-sitiontemperaturetotriggertheformationofconductive networks.Itwasconcludedthatpercolationisdelayedby thebulkmobilityofthepolymerlayeraroundthe conduc-tiveparticles.Therefore,theformationoftheconductive networkissignificantlyinfluencedbythethermodynamic interactionbetweenthefillerandthematrix[119,120].
Manystudieshavebeenconductedtounderstandthe dynamicprocessofconductivenetworkformationinCPCs bycombiningexperimentalobservationsandtheoretical modelling [73,118,120–122]. Variousmodels have been proposedtoexplainthedatathatwasobtainedfrominline electrical measurement using modified percolation the-ory[73][122,123]ortheuniversalinterfacialfreeenergy model [120,124]. A relaxation time or percolation time wasobtainedfromtime-dependentconductivity measure-ments,anditsrelationwithtemperaturewasdescribed by an Arrheniusequation withactivation energies that werevery close tothe values that wereobtained from rheologicalexperiments.Thepercolationtimecanbe char-acterisedastheannealingtimeatwhichtheconductivity startstoincreasedrasticallyduringthedynamic percola-tionprocess.Thetheoreticalanalysisandtheexperimental resultsrevealedthatthepercolationtimeisdirectly corre-latedtothezero-shear-rateviscosityofthepolymermatrix, regardlessofthefillerconcentrations[119,124].However, becausetheseanalyseswereperformedforisotropicCPCs, thetruemechanismfortherelaxationordynamic percola-tionofhighlyorientedCPCsisstillnotclear.
Recently, we described a newconcept in which the skin layers in a highly oriented bicomponent structure
Please cite this article in press as: Deng H, et al. Progress on the morphological control of conductive network Fig.5.(a)Fillerwithlowaspectratiolocateattheinterfaceofpolymerblendsduringmeltcompounding.Thefigureshowsthemechanismofdecreasing drivingforce(Fcurvature)duringthetransferbetweentwopolymers.(b)Fillerwithlargeaspectratioatinterfaceofpolymerblends.Theillustrationshows thedrivingforceisnotdecreasingduringsuchatransferprocess.Typicaldistributionstatusforlow(c)andhigh(d)aspectratiofillersthatwerefirstly dispersedinablendphasewithrelativepoorwetting(indicatedasyellow)andsubsequentlymeltcompoundingwascarriedoutwithanotherpolymer whichhasmorepolarnature(blue).[99],Copyright2011.ReproducedwithpermissionfromtheAmericanChemicalSociety.
can be thermally annealed above their melting temperatureduetothesupportofaneatpolymerphase witha highermeltingtemperature[43,73,74].Insucha system,theformation,destructionandreconstructionof theconductivenetworkscanbemonitoredduringsolid statedrawingandannealing[16,73].Thisstudyprovided interestinginformationonchangesinthehighlyoriented conductivenetworkduringrelaxation.AsshowninFig.7, themorphologyoftheconductivenetworkchangedfrom anisotropic toahighlyorientedstatedue tosolid state drawing.Asubsequentannealingprocessledtothe relax-ationofthehighlyorientednetwork;thus,aconductive network consisting of relaxed oriented bundles with
“hairy”localcontactswasobtained.Duringannealing,the resistivityoftheseCPCswassignificantlyreduceddueto theformationoftheseconductivenetworks.Theresistivity ofpolymerfibres/tapesthatwereproducedfromsucha meltprocessing-basedmethodwascomparabletothatof thefibres/tapesthatwereproducedfromsolution-based orother,morecomplicated,methods[73].
4.4. Morphologicalcontrolthroughashearforce
Shearforcesareinvolvedinvariousprocessing meth-ods,includingextrusion,injectionmoulding,spinningor stretching.Theseforcesoftenplayimportantrolesinthe fillermorphologyviathestresstransferbetweenthematrix andthefiller.Thus,themorphologicalcontrolofthe con-ductivenetworksinCPCscanberealisedbyusingashear force[16,18].
To understand the effect of shear on the network morphologyandtheresultingelectricalproperties,many studieshave beenconducted.Shear-inducedorientation isoftenobservedduringinjectionmoulding[125], spin-ning[126]orstretching[16].Anelectricalanisotropycan
To understand the effect of shear on the network morphologyandtheresultingelectricalproperties,many studieshave beenconducted.Shear-inducedorientation isoftenobservedduringinjectionmoulding[125], spin-ning[126]orstretching[16].Anelectricalanisotropycan