Natrue-Materials-关于导电水凝胶的热敏性研究

LETTERSPUBLISHEDONLINE:10AUGUST2015|DOI:10.1038/NMAT4363ThermoresponsiveactuationenabledbypermittivityswitchinginanelectrostaticallyanisotropichydrogelYounSooKim1,MingjieLiu2,YasuhiroIshida2*,YasuoEbina3,MinoruOsada3,TakayoshiSasaki3,TakaakiHikima4,MasakiTakata4andTakuzoAida1,2*Electrostaticrepulsion,longusedforattenuatingsurfacefriction1,2,isnottypicallyemployedforthedesignofbulkstructuralmaterials.Werecentlydevelopedahydrogel3withalayeredstructureconsistingofcofaciallyorientedelectrolytenanosheets4.Becausethisunusualgeometryimpartsalargeanisotropicelectrostaticrepulsion5tothehydrogelinterior,thehydrogelresistedcompressionorthogonaltothesheetsbutreadilydeformedalongparallelshear.Buildingonthisconcept,hereweshowahydrogelactuator6–11thatoperatesbymodulatingitsanisotropicelectrostatics12inresponsetochangesofelectrostaticpermittivityassociatedwithalowercriticalsolutiontemperaturetransition13,14.Intheabsenceofsubstantialwateruptakeandrelease,thedistancebetweenthenanosheetsrapidlyexpandsandcontractsonheatingandcooling,respectively,sothatthehydrogellengthensandshortenssignificantly,eveninair.AnL-shapedhydrogelwithanobliquenanosheetconfigurationcanthusactasaunidirectionallyproceedingactuatorthatoperateswithouttheneedforexternalphysicalbiases15–18.Poly(N-isopropylacrylamide)(PNIPA)isaparticulartypeofpolymer,whichiswatersolubleatambienttemperaturesbutcoagulatestobecomeinsolubleonheatingatatemperatureabove32C(ref.6).Thisisbecausedipoledipoleandhydrogen-bondinginteractions,responsibleforthehydrationofPNIPAinwater,aredecreasedabovethistemperature7.Thistransitionisfullythermoreversibleandknownasthelowercriticalsolutiontemperature(LCST)phenomenon611.AsignificantbutoftenignoredaspectassociatedwiththeLCSTtransitionisthatwaterreversiblychangesitsapparentelectrostaticpermittivity,whichdecreasesonthehydrationofPNIPAandrecoversonitsdehydration13,14.Recently,wedevelopedahydrogeladoptingalayeredstructurewithcofaciallyorientedunilamellarelectrolytetitanate(IV)nanosheets(TiNSs;Fig.1a)4initsmatrix,whosemechanicalpropertiesaredominatedbytheanisotropicelectrostaticrepulsionbetweenthosenanosheets3.WeconjecturedthattheLCSTbehaviourmightreversiblymodulatetheanisotropicelectrostaticrepulsioninthehydrogelmatrix12ifthegelnetworkisconstructedwithPNIPAtothermallyswitchtheinternalelectrostaticpermittivity13,14.Therefore,comparedwithconventionalPNIPA-basedhydrogelsthatsimplyshrinkandexpandinvolume,respectively,onheatingandcooling611,thethermalbehaviourofourhydrogel,containingcofacialTiNSs,couldnotonlybeoppositebutalsoanisotropic.Namely,onheatingandcooling,itlengthensandshortens,respectively,asaconsequenceoftheexpansionandcontractionofthecofacialTiNSdistance(Fig.1d).Thissimpleconcepthasneverbeenconsideredforthedesignofthermoresponsivehydrogels,butindeedworksasdescribedinthepresentpaper.Wealsodiscoveredthat,incontrastwithconventionalPNIPA-basedhydrogels,thethermalmotionofourhydrogeloccurswithoutsubstantialwateruptakeandrelease,sothattheactuationcanberealizedeveninnon-aqueousmedia,includinganopen-airenvironment.Finally,wedesignedabipedalwalkingobjectmadeofanL-shapedhydrogelthatproceedsunidirectionallyunderthermalcyclingwithoutrequiringanyexternalphysicalbias(suchasaratchet-patternedbase).Asreportedpreviously4,whenanaqueousdispersionofTiNSs(Fig.1a)isplacedinamagneticfield,TiNSsalignorthogo-naltotheappliedfluxlines3,incontrasttoothermetaloxidenanosheets,whichalignparalleltothefluxlines19,20.Inthisori-entation,TiNSsthemselvesarearrangedcofaciallytooneanother,sothemaximalelectrostaticrepulsionemergesbetweenthem5.Tominimizethethermodynamicinstabilitycausedbyfluctuationsinthenanosheetdistribution,theaqueousdispersionadoptsaquasi-crystallinestructuralorder2123,inwhichauniformlylargeplane-to-planeseparationdevelopsuptoamacroscopiclengthscale.Thismagneto-inducedtemporalstructuralorderingcanbefixed24byinsituradicalpolymerizationofwater-solublevinylmonomerssuchasN,N-dimethylacrylamide(DMA;Fig.1b)3.Consequently,amechanicallyanisotropichydrogelforms.Inthepresentwork,aPNIPA-basedhydrogelcontainingcofaciallyori-entedTiNSs(PNIPA/TiNSk)waslikewiseprepared(Fig.1c).Typi-cally,anaqueousdispersionofTiNSs(1.6wt%)washydrogelatedinasuperconductingmagnet(10T)byinsitucrosslinkingpolymer-izationofN-isopropylacrylamide(NIPA;8.0wt%)inthepresenceofN,N0-methylenebis(acrylamide)(BIS;0.48wt%)asacrosslinker(Fig.1b).TheresultantPNIPA/TiNSkhydrogelwasself-standingand,asconfirmedbytwo-dimensionalsmall-angleX-rayscattering(2DSAXS)analysis,itsinteriorconsistedofaquasi-crystallinestructuralarrangementofcofaciallyorientedTiNSs(SupplementaryFig.1).Dierentialscanningcalorimetry(DSC)revealedthatthishydrogelexhibitsLCSTbehaviourat32C,justasitoccursingenuinePNIPA(refs611;SupplementaryFig.2).1DepartmentofChemistryandBiotechnology,SchoolofEngineering,TheUniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo113-8656,Japan.2RIKENCenterforEmergentMatterScience,2-1Hirosawa,Wako,Saitama351-0198,Japan.3NationalInstituteforMaterialsScience,InternationalCenterforMaterialsNanoarchitectonics,1-1Namiki,Tsuk