cm062693+

聚合物纳米复合材料的综述 Polymer Nanocomposites with Prescribed Morphology: Going beyond Nanoparticle-Filled Polymers

2736Chem.Mater.2007,19,2736-2751

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PolymerNanocompositeswithPrescribedMorphology:

GoingbeyondNanoparticle-FilledPolymers

RichardA.Vaia*andJohnF.Maguire

MaterialsandManufacturingDirectorate,AirForceResearchLaboratory,Wright-Patterson

AirForceBase,Dayton,Ohio45433-7750

ReceiVedNoVember12,2006.ReVisedManuscriptReceiVedFebruary16,2007

Polymernanocomposites(PNCs),i.e.nanoparticles(spheres,rods,andplates)dispersedinapolymermatrix,havegarneredsubstantialacademicandindustrialinterestsincetheirinception,ca.1990.Withrespecttotheneatmatrix,nanoparticledispersionhasbeenshowntoenhancephysical(e.g.,barrier,erosionresistance,andreducedflammability),thermomechanical(e.g.,heatdistortiontemperature,thermalexpansioncoefficient,andstiffness),andprocessing(e.g.,surfacefinishandmeltstrength)characteristics.Beyondmaximizationofthenanoparticledispersion,however,themorphologyofthesematerialsismanytimesuncontrolled,yieldingisotropicnanofilledsystems,notnecessarilyspatially“engineered,designedandtailored”materials.Toimpacthigh-technologyapplicationsrequiringuniqueelectrical,thermal,andopticalproperties,manufacturingtechniquesenablingcontrolofthenanoparticlearrangementanddistributionmustbedeveloped.Thispaperwillexaminethestatusofapproachesfordirectingthehierarchicalmorphologyofnanoparticledispersionsinthreedimensions,andbeyonduniaxialalignment,usingexamplesfromtheliteraturetohighlightthepotentialandissues.Ultimately,twogeneralapproachestothischallengeareemerging,namely,external-in(directedpatterningofnanoparticledispersions)andinternal-out(mesophaseassemblyofnanoparticles).

Introduction

Thelargevarietyofplasticsavailableonthemarkettodayistheresultofblending,thatis,combiningvariouspolymersoraddingmicrometer-scaleorlargerfillers,suchasminerals,ceramics,andmetals(orevenair).Overthepastdecade,theutilityofinorganicnanoparticlesasadditivestoenhancepolymerperformancehasbeenestablishedandnowprovidesadditionalopportunitiesformanydiversecommercialap-plications.Low-volumeadditions(1-10%)ofisotropicnanoparticles,suchastitania,alumina,andsilver,andaniostropicnanoparticles,suchaslayeredsilicates(nano-clays)orcarbonnanotubes,providepropertyenhancementswithrespecttotheneatresinthatarecomparabletothatachievedbyconventionalloadings(15-40%)oftraditionalmicrometer-scaleinorganicfillers.Thelowerloadingsfacilitateprocessingandreducecomponentweight.Mostimportantthoughistheuniquevalue-addedpropertiesandpropertycombinationsthatarenotnormallypossiblewithtraditionalfillers,suchasreducedpermeability,opticalclarity,self-passivation,andflammability,oxidation,andablationresistance.Beyondmaximizingnanoparticledisper-sion,however,themorphologyofthesematerialsismanytimesuncontrolled,yieldingisotropicnanofilledsystems,notnecessarilyspatially“engineered,designedandtailored”compositematerials.

*Correspondingauthor.E-mail:richard.vaia@wpafb.af.mil.

Thisreviewarticlewillendeavorto“look-over-the-horizon”atthechallengesandopportunitiesinprovidingthetoolboxtodirectpolymernanocompositemorphologyinthebulk,thatis,deliver“nanocomposites-by-design”.Amongthemanychallengesaspolymernanocomposites(PNCs)movebeyondcommodityplasticapplications,precisemorphologycontrolisparamount.Randomarrange-mentsofnanoparticleswillnotprovideoptimizedelectrical,thermal,oropticalperformanceformanypotentialhigh-technologyapplications,suchasdielectricunderfillsforelectronicpackaging,printedflexibleelectronics,engi-neeredaerospacestructuralcomponents,reconfigurableconductiveadhesives,andopticalgratingstojustmentionafew.

Specifically,wewillfocusonthemethodologiestocontrolthearrangementanddistributionofdispersed,preformednanoparticlesbeyonduniaxialalignment(D∞andC∞).AfterabriefsummaryofthecurrentstatusofPNCs(Background)andadiscussionemphasizingtheopportunitiesaffordedbytheabilitytocontrolnanoparticlehierarchy(GoingBeyondFilledSystems),twogeneralapproachestothischallengeareexplored,namely:external-in(DirectedPatterningofNanoparticleDispersions)andinternal-out(MesophaseAs-semblyofNanoParticles).Selectedexamplesfromtheliteratureareusedtohighlightthepotentialandchallenges;interestedreadersareencouragedtofurtherexploretheliteraturesurroundingtheseexamplesforadditionalinforma-

10.1021/cm062693+CCC:$37.00©2007AmericanChemicalSociety

PublishedonWeb04/21/2007

聚合物纳米复合材料的综述 Polymer Nanocomposites with Prescribed Morphology: Going beyond Nanoparticle-Filled Polymers

ReViewstion.Wewanttoemphasizethattheseexamplesarebynomeansintendedtobeinclusive.Acollectionofeffortsthatdemonstratethepotential(explicitlyorimplicitly)israpidlygrowingattheintersectionofchemistry,physics,materialsscience,andbiomaterials,includingthosehighlightedhere,aswellasothers,suchasbackfillingofsacrificialtemplatesorin-situnanoparticleformationwithinamesophase.

Background

Polymericnanocompositeshavebeenanareaofintenseindustrialandacademicresearchforthepast15years.Nomatterthemeasuresarticles,patents,orresearchanddevel-opmentfundingsworldwideeffortsinPNCshavebeengrowingexponentially.Forexample,thetotalnumberofhitsfor“polymer”and“nanocomposite”onSciFinder(ChemicalAbstractService(CAS)oftheAmericanChemicalSociety)from1988to2005is>9400,wheretheyearlynumberhasapproximatelydoubledevery2yearssince1992.1RecentmarketsurveyshaveestimatedglobalconsumptionofPNCsattensofmillionsofpounds(?$250M),withapotentialannualaveragegrowthrateof24%toalmost100millionpoundsin2011atavalueexceeding$500-800M.2-4Majorrevenuesareforecastfromlargecommercialopportunities,suchasautomobile,coatings,andpackaging,wherelowercost,higherperformanceresinswouldimprovedurabilityanddesignflexibilitywhileloweringunitprice.Inlightofglobalpolymerproduction,whichfromoilaloneexceeds200-450billonpoundsannually,nanoparticleadditionstoplasticsaffordsoneofthecommerciallylargestanddiversenear-termapplicationsofnanotechnology.

Sincethefirstreportsintheearly1990s5-10theterm“polymernanocomposite”hasevolvedtorefertoamulti-componentsystem,wherethemajorconstituentisapolymerorblendthereofandtheminorconstituentexhibitsalengthscalebelow100nm.Assuch,thetermissometimesusedasasynonymforinorganic-organichybrids,molecularcomposites,ortoencompassmaturecommercialproducts,suchasfilledpolymerswithcarbonblackorfumedsilica.Thenumerousreportsoflargepropertychangeswithverysmall(<5vol%)additionofnanoparticleshavefueledtheviewthatnanoparticleadditiontopolymersdelivershugedividends.

Giventheextensivevarietyofnanoparticlesnowcom-merciallyaccessible(clays,carbonnanotubes,quantumdots,metals,silica,titania,zirconia,andvariousoxides,etc.),thepotentialcombinationsofpolymersandnanoparticles,andthusthetailorabilityofthepropertysuite,isessentiallyendless.Thediversityinscientificinvestigation,technologyadvancement,processinginnovations,andproductdevelop-mentisstaggering.Asignificantnumberofexcellentreviewpapers(e.g.,clays11-18andcarbonnanotubes17-21)andbooks22-25areavailablethatchronicleandsummarizethestatusofvariousnanoparticle-polymercombinationsandthebroadscientificandtechnologicalchallengesstilltobeovercome.

Arguably,thegoalforthevastmajorityoftheseinvestiga-tionsistoachieveincreasedthermomechanicalperformancethroughdispersionatthesingle-particlelevel.TheresultingPNCsaretreatedmuchasanisotropic,filledpolymer.Thus,

Chem.Mater.,Vol.19,No.11,20072737

Figure1.Representativepolymernanocompositemorphologiesexhibitingrandomdispersionofspherical(0D),rodlike(1D),andplatelike(2D)nanofillers.(a)Spherical:CdSe/ZnSquantumdotswithcarboxylicacidsurfacefunctionality(EvidentTechnologies)dispersedat5×1015particles/cm3inarecombinantsilk-elastinprotein(MW,70000).26(b)Rod:5wt%carbonnanofibers(AppliedScience,Inc)dispersedinthermoplasticpolyurethane.27and(c)Plate:3wt%organicallymodifiedmontmorillonite(Cloisite30A)inEpon862/Wcuredepoxy.28

fromthehistoricperspective,nanocompositestodayarereallynanoparticle-filledplastics,Figure1.26-28Theuseofthemoniker“composites”,though,invokesstrongparallelstotraditionalcontinuousfiber-reinforcedcomposites(CFRC)andtheabilitytospatiallyengineer,design,andtailormaterialsperformanceforagivenapplication.ThepayoffofCFRCmanufacturingtechnologiesisexemplifiedbytheincrediblematerialadvancementsthatenablecurrentaero-spacesystems,bothmilitaryandcivilian.CurrentprocessingandfabricationapproachesforPNCsfallwellshortofthisfabricationanddesigncapability.

GoingbeyondFilledSystems

Performanceenhancementsofpolymernanocompositescapitalizeonadvantagesaffordedbyuptoa3ordersofmagnitudespatialrefinementofmorphologyrelativetotraditionalmicrometer-scalefilledpolymersandcomposites.Thiscontrastsnanotechnologyinelectronics,optics,anddatastorage,wherethenanoscaleprovidesaccesstonewphysicalprocessesbasedonquantumphenomenon.Many

discussions

聚合物纳米复合材料的综述 Polymer Nanocomposites with Prescribed Morphology: Going beyond Nanoparticle-Filled Polymers

2738Chem.Mater.,Vol.19,No.11,2007consideringtheimplicationsandphysicalmanifestationofthisrefinementofPNCmorphologycanbefoundintheliterature.29-33Forpolymers,manybulkpropertiesarerelatedtothesizeofthepolymerchain,whichischaracterizedbytheradiusofgyration,Rg(?2-20nm).Thedominatelengthscaleofthemorphologybecomescriticallyimportantasthedimensionsofparticleandpolymer,aswellastheinterfacialcurvatureandinterparticledistance,becomecomparable.Atthispoint,thepropensityofinterfaceandcooperativitybetweenparticlesdominatemacroscopicproperties.Strongfundamentalparallelscanbedrawnwitheffortsonthermal-mechanicalcharacteristicsofultrathinfilmsofpolymers34,35oroffluidsconfinedwithinnanopores.36Schadlerandco-workersrecentlyprovidedstrongexperimentalevidencefortheseparallelsthroughthedepressionofglasstransitiontemperatureinsilica-filledpolystyrene.31

Forthefinalmaterialperformance,though,theextent,spatialarrangement,andorderingoftheconstituentsisasimportant,ifnotmoreso,thansimplythedegreeofrefinementofthemorphologyortheroleoftheinterfaceregiononpolymerconformationanddynamics.Forexample,morphologyhierarchyiswell-acknowledgedintheprocess-ing(injectionmolding,casting,etc.)ofsemicrystallinepolymersandpolymerblendswheremacroscopicvariations,suchasskin-coreandfountain-flowpatterns,http://wendang.chazidian.comparableprocessingissuesalsoulti-matelydeterminetheutilityofmanyhigh-performanceliquidcrystallinepolymers.Forcurrentnanoparticle-filledproducts,thehierarchyofmorphologyisevenmoreimportantthancomparablemicrometer-scalefilledpolymersduetotheextremeaspectratioofmanynanoparticles,suchasexfoliatedclaysandcarbonnanotubes.Kojimaandco-workerswereoneofthefirsttodocumentanddiscusstheimpactofprocessingconditionsonthefinestructureofPNCs.37,38Theydemonstratedthattherelativeorientationofmontmorillonitelayersandtheconcomitantimpactonnylon6crystallitesvariedwithdistancefromthesamplesurface.Figure2summarizesX-raydiffractionstudiesofaninjection-moldedbarofNylon6smontmorillonitenanocomopositeshowingthattheorientationofmontmorilloniteandpolymercrys-tallitesreflectsthefillingpatternofthemold.Similarobser-vationsarenowcommoninthePNCliterature.Morerecently,Curlissnotedthesubstantialroleofthefibermatingloballytemplatingthealignmentofmontmorillonitelayersduringvacuum-assistedresin-transfermolding(VARTM).39ComparisonofbulkPNCepoxytoreinforcedcompositewithPNCepoxymatrixindicatedthatthistemplatedlocalalignmentalongthecarbonfiberaxiswasbeneficial.Unanticipatedimprovementsoftransverseandaxialstrengthinthefinalfiber-reinforcedstructurewereattributedtothealignmentofthemontmorillonitealongthecarbon-fibersurface.Recognitionofthecriticialimportanceofquantifyingandascertainingthecontributionofcon-nectivityofNPs,theresultantstructureofthis“NPnetwork,”ifitexhibitsfractalorself-similarfeatures,andtheassociatedpropertiesofthisnetwork,suchascomplaince,relaxationtimes,junctionstrength,andspanninglinkageflexibility,areincreasing.

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Figure2.ExampleoftheimpactofprocessingonmacroscopicdistributionoflocalmorphologiesinPNCs.Small-angleandwide-angleX-rayscattering(SAXSandWAXS)patternsfrominjection-moldednylon6/montmorillonitenanocomposites,reflectingalignmentoflocalstructure.16Theneckregion(Aintheopticalpictureofthesample)containshighlyaligned,well-dispersedmontmorillonitelayers,asindicatedbytheorientedstreakinSAXS.AweakpeakonthemeridianoftheWAXSpatternsarisesfromwell-orientedγ-phasecrystallamellae(seearrows).Withinthebaseregion(regionD),themontmorillonitelayersreflectthefountainflowpatternoftheinjectionmolding.Thepolymercrystallitesarenothighlyaligned,however,asindicatedbythemoreuniformazimuthalscatteringintheWAXSpatternfromregionD.

Modelingeffortstoestablishstructure-performancecor-relationsfurthersupporttheneedformorerefinedprocessingtechniques.Reportspredicthugedividendsinmechanical,barrier,andelectricalperformanceifprocessingcouldprescribeprecisespatialarrangementofnanoparticles.Forexample,GusevandRozmanshowedthatcomparableshearmodulicouldbeobtainedatonlyhalfthevolumefractionofparticlesifaweblikemorphologycouldbegeneratedratherthanrandomorhexagonalarrangement,Figure3.40AdditionalworkbyGusevandco-workersonbarrierproperties41andthecoefficientofthermalexpansion(CTE),http://wendang.chazidian.comingcontinuummechanics,LiuandKumarexaminedtheelasticconstantsofsingle-wallcarbonnanotube(SWNT)ropesandfibersandshowedthatshearmoduliofthefiberdropsprecipitouslyasthewidthoftheuniaxialorientationfunctionoftheSWNTincreases;upwardof2ordersofmagnitudefor10-20%disorderatlargerfiberdiameters.45Thissensitivitytodeviationsfromperfectorder,anditsimplicationstofiberspinning,parallelsthatknownforhigh-performancerigid-rodpolymerfibers.Veryrecently,Forestandco-workersconsideredanisotropicgeometricpercolationofhigh-aspect-ratiorodensemblesdispersedinaviscoussolventandsubjectedtocontrolledrheologicalflows.46Resultsindicatedthatthespanningdimensionofthepercolatingclustercanbecontrolledusingacombinationofvolumefractionandshearrate.Percolationphasediagramsresult,inwhichshearrateproducestransitionsbetween3D,2D,and1Dpercolatingclusters,aswellastheloss

of

聚合物纳米复合材料的综述 Polymer Nanocomposites with Prescribed Morphology: Going beyond Nanoparticle-Filled Polymers

ReViewsFigure3.NumericalresultsobtainedbyGusevandRozman40forhoneycombandweblikepackingarraysofsilicaparticles(E)70GPa,ν)0.2)inanelastomermatrix(K)1GPA,G)0.0005GPa),demonstratingtheimpactofcontrolledmorphologyonenhancingmechanicalpropertiesofacomposite.ThetwosolidlinesgivethepredictionsoftheHashin-Shtrikmanvariationalbounds.Theverticaldashedlinesshowthemaximumpackingdensitythatcanbeachievedassumingweblikeandhoneycombpackingarraysofidenticalcylindricalfibers.Reprintedwithpermissionfromref40.Copyright1999Elsevier.

percolationatagivenvolumefraction,Figure4.Numerousapplications,includingsmartmaterials,piezo-andpyrore-sistivesensorsandactuators,areenabledbytheproductionoffilmswithanisotropicconductivityarisingfromcontrollingthestructureofthepercolationnetwork.

Whatisthestatusthenofprocessingtechniquesthatwillprovideafeedbackto,andademonstrationof,theseinsights?Itseemsclearthatmorethanuniaxialcontrolofrodsandplatesisnecessary.

Ultimately,twogeneralapproachestothischallenge,parallelingnanofabricationconcepts,areemerging,namely,external-in(top-down)andinternal-out(bottom-up),Figure5.Forexternal-in,directedpatterningofnanoparticledisper-sions(DPND)reliesonthecreation,byanexternalmeans,ofamultidimensionalmorphologydirectingpotential,suchasaspatiallyvaryingfieldorsusceptibilitywithinthematerial.Thistransformsarandomdistributiontoapre-scribed,orderedconstruction.Forinternal-out,mesophaseassemblyofnanoparticle(MANP)reliesontheabilitytotailorinterparticleinteractions,bothparticle-particleandparticle-matrix,toresultinthermodynamicallystable(anddefined)mesophases.Thepossibilitiesforhigherorderstructuresarebolsteredbythecontinuingsuccessesinthereproducibleproductionofnanospheres,fibers,plates,andothergeometrieswithmanufacturingtolerancesthatapproachthecurrentnormofmicrometer-scalefibers,colloids,andfilms(<1%).47-52Thesewillprovidenanoscalebuildingblocksapproachingtheprecisionofmolecules,macromol-ecules,andbiomacromoleculesandthusaccesstotheassociatedcomplexphasespace.Notethattheseconceptsarenotmutuallyexclusive,butcanbecombined,e.g.,directedpatterningofmesophaseassembledNPs.

DirectedPatterningofNanoparticleDispersionsTheabilitytouniaxiallyalignnanoelements,bothplatesandtubes,usingexternalforcesandgradientshasbeenexten-

Chem.Mater.,Vol.19,No.11,20072739

sivelydemonstrated.Approachesincludesedimentation,53spincoating,54mechanicaldeformation(fiberspinning,55-58filmblowing,59injectionmolding,60andshear61,62),magneticfields,63,64andelectricalgradients.65One-dimensional,out-of-planeperiodicityhasalsobeenshownbysequentialdepositionapproachessuchaselectrostaticorhydrogenbondmediatedself-assembly.66,67

Muchlessworkhasbeendirectedtowardestablishingrobustprocessingtechniquesthatenablebroadtunabilityoftwo-orthree-dimensionalstructureswithinthebulk.Creationofmultidimensionalstructuresischallengedbyhowtocontrollablygenerateamultidimensional,morphology-direct-ingpotentialwithinthebulkmaterial.Forexample,ifthecontrolinputismechanicaldeformation,morphologyma-nipulationwilldependnotonlyonthemagnitudeandgradientofananisotropicexternaldeformation(allcompo-nentsofthestresstensor)butalsoonthelocaldistributionofthestressfieldandonthecouplingto,andinterplayof,thenanoparticle’sshapeandmechanicalresponse(buckling,fracture,etc.),surfaceenergies,andviscoelasticproperties,etc.Extensiveeffortsontherefinementoftwo-componentpolymerblendsexemplifythecomplexityofthisspecificcase.68

Thefollowingsectionshighlightfourareaswhichpro-videanoverviewofcurrentcapabilities.Theinitialthreesections(MechanicalDeformation,ElectricandMagneticFields,andOpticalFields)discussthecouplingbetweenappliedexternalfieldsandnanoparticleorientation,distribu-tionandinterparticleinteractions.Thelastsection(Multi-componentInterfacialSystems)considersthepossibilitiesaffordedbyinterfacialsegregationofnanoparticlesinanimmiscibleblend,andthustemplatingthenanoparticledistributiontotheinterfacialregionsoftheimmiscibleblend,andsubsequentmacroscopicmanipulationofthestructureviadistortionsandrefinementsoftheunderlyingmultiphasemorphology.

MechanicalDeformation.Formechanicaldeformation,uniaxialorin-planebiaxialarrangementofthenanoparticleandpolymercrystallitesiscommon,asnotedbytheaforementionedapproaches.Multidimensionalcontrolofthemorphologybycomplexstrainfieldsisrarelyreported.However,somereportsofusingcontrolledstraininputstointroducelocaldistortionsofthenanoparticlecanbefound.Forexample,beginningfromquenchedbiaxiallyextrudedfilmsofnylon6/montmorillonitenanocomposites,Parkandco-workersobservedwithinzone-drawnfilmsthatthemontmorillonitelayersbuckleperpendicularallytothedrawdirection,analogoustofailureofauniaxiallystrainedsheetofpaper.69Thefailuremodeappearedtooccurforacollectionofparallelaluminosilicatelayers(2-4)andexhibitedameanspatialfrequency,Figure6.Thisbehaviorhasanalogiestoshearbandinganddeformationofuniaxiallyalignedlamellarandcylindricalblockcopolymes.70-72

Ingeneral,nanoparticlealignment,aswellasmorphologyrefinement,dependsonthetypeofimposedflow,whetherextensional,shear,ormixed.73Thisisanalogoustoothercomplexfluidsincludingpolymers.Fundamentally,thecomplexityofthelocalresponsetoimposedstressorstrainnotonlydependsontheinterfacialstrength,particle

size,

聚合物纳米复合材料的综述 Polymer Nanocomposites with Prescribed Morphology: Going beyond Nanoparticle-Filled Polymers

2740Chem.Mater.,Vol.19,No.11,2007ReViewsFigure4.“Percolationphasediagram”ofanisotropicpercolationthresholdsshowingtherelationbetweentheonsetvolumefractionθp(percolationthreshold)andnormalizedshearratePe,formonodisperserodsofaspectratio50.46Theorientationalprobabilitydistributionfunction(PDF)oftherodswerederivedfromDoi-Hesstheoryforflowingrigid-rodmacromoleculesinaviscoussolvent.RepresentativecriticalspanningpercolationclusteratPe)80correspondingto(top)3Dpercolation(θp)0.0134)withpercolatingpathsspanningthegradient(y),vorticity(z),andflow(x)directionand(bottom)1Dpercolation(θp)0.012)withpercolatingpathonlyspanningtheflow(x)

direction.

Figure5.Schematicrepresentationofthetwogeneralapproachestocontrolnanoparticledistributionandarrangementbeyondrandomorder:external-in(top)andinternal-out(bottom).Forexternal-in,directedpatterningofnanoparticledispersions(DPND)reliesonthecreation,byanexternalmeans,ofamultidimensionalmorphologydirectingpotential(topcenter:gradedbackgroundcontours),suchasaspatiallyvaryingfield(depictedabove)orsusceptibilitywithinthematerial.Imposingthiscomplexfieldonthesample(topcenter)transformsarandomdistributiontoaprescribed,orderedconstructionduetomassflowofnanoparticles(arrows)tominimizethepotentialenergyofthesystemwithintheexternallyappliedfield.Forinternal-out,mesophaseassemblyofnanoparticles(MANP)reliesontheabilitytotailorinterparticleinteractions,bothparticle-particleandparticle-matrix,toresultinathermodynamicallystable(anddefined)mesophase.Thephasebehaviorofthesystemmaybemodulated(bottomcenter)byuniformchangesinthesystemsintrinsic(pressureortemperature)orextrinsic(numberdensityorentropy)thermodynamicparameters.IncontrasttotheexternallypatternedpotentialappliedforDPND,changesinparticleorganizationinMANPoccurinresponsetoauniformchangeinthesystem’senvironment.

andmechanicalcharacteristicsoftheconstituentsbutalso

onelasticinstabilities,suchasbuckling,ofthehighaspect

rationanoparticles,aswellasthecooperativeresponseof

nanoparticlescoupledthroughoverlappinglocalstrainfields.

Theextentofthelatterismanytimesreflectedinalower

volumefractionformechanicalpercolation(??f0)than

electricalpercolation.Systematicexplorationofthiscomplex

processingspacetoproducehierarchicalstructuresby

inducingparticleorientationandparticledeformation,aswellastransferringinsightsfromshearthinning(pseudo-plastic)andshearthickening(dilatant)fluids,isstillinitsinfancy.ElectricandMagneticFields.Themorphologicaloriginsofstiffeningofrheologicalfluids,suchaselectro-(ER)andmagneto-(MR),provideaperspectiveonthepossibilitiesforexternallyapplied,spatiallypatterned,electrical,andmagneticfieldsforcomplexPNCfabrication.Undertheelectricfield,particulateadditives,suchasoxidesormetals,reversiblyformfibrousstructureswithinthe

nonconducting