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Langmuir1-423；StrategytoIntroduceaPend；Poly(N-isopropylacrylami；Xiao-DingXu,?Xian-ZhengZ；Ya-QunHuang?；KeyLaboratoryofBiomedica；Chemistry,WuhanUniVersit；ReceiVedDecember1
Langmuir1-StrategytoIntroduceaPendentMicellarStructureintoPoly(N-isopropylacrylamide)HydrogelsXiao-DingXu,?Xian-ZhengZhang,*,?JieYang,?Si-XueCheng,?Ren-XiZhuo,?andYa-QunHuang?KeyLaboratoryofBiomedicalPolymersofMinistryofEducationandDepartmentofChemistry,WuhanUniVersity,Wuhan430072,P.R.China,andColloidandInterfaceScienceLaboratoryandDepartmentofChemistry,WuhanUniVersity,Wuhan430072,P.R.ChinaReceiVedDecember1,2006.InFinalForm:February2,2007Anovelclassoffunctionalpoly(N-isopropylacrylamide)(PNIPAAm)hydrogelswithpendentmicellarstructureresultingfromthependingamphiphilicpolymerswasdesignedandprepared.TheinfluenceofthependentmicellarstructureonthepropertiesoftheresultedPNIPAAmhydrogelswasexaminedintermsofmorphologyobservedbyscanningelectronmicroscopy,thermalresponsethroughdifferentialscanningcalorimetry,anddeswelling/reswellingkineticsuponexternaltemperaturechanges.Incomparisonwiththeconventionalones,thenovelPNIPAAmhydrogelswithpendentmicellarstructurepresentedimprovedtemperature-sensitiveproperties,i.e.,enlargedwatercontainingcapabilityatroomtemperature,aswellasimproveddeswellingrateuponheating.IntroductionPolymerichydrogelshavebeenwidelyexploredforbiomedicalapplicationsduetothesimilaritybetweenthehighlyhydratedthree-dimensionalnetworksandthehydratedbodytissues,aswellasfavorablebiocompatibility.1,2Inparticular,greatinteresthasbeenfocusedontheintelligentones,whichcanchangetheirvolumesasaresultofaslightvariationofexternalstimuli,suchastemperature,3light,4chemicalenvironment,5electricfield,6antigen,7etc.Inordertomeetvariousapplications,severalstrategieswereproposedtoimprovethepropertiesofhydrogels,suchasdesignofmicroporousstructure,8,9macroporousstruc-ture,10combtypenetworkstructure,11,12etc.Recently,tailoringhydrogelarchitectureisoneofthevigorousresearchsubjectsinhydrogelfields.13-16Severalkindsofhydrogelswithparticularstructuresexhibituniquepropertiesandspecificfunctions.Forexample,hydrogelswithsupermo-lecularstructure,e.g.,atopologicalhydrogelwithfigure-eight*Towhomcorrespondenceshouldbeaddressed.Tel:86-27-.Fax:86-27-.E-mail:xz-zhang@chem..?KeyLaboratoryofBiomedicalPolymersofMinistryofEducationandDepartmentofChemistry.?ColloidandInterfaceScienceLaboratoryandDepartmentofChemistry.(1)Hoffman,A.S.AdV.DrugDeliV.ReV.-12.(2)Hirano,Y.;Mooney,D.J.AdV.Mater.-25.(3)Hoffman,A.S.J.Control.Rel.-305.(4)Suzuki,A.;Tanaka,T.Nature-347.(5)Torres-Lugo,M.;Peppas,N.A.Macromolecules6-6651.(6)Tanaka,T.;Nishio,I.;Sun,S.T.;Ueno-Nishio,S.Science-469.(7)Miyata,T.;Asami,N.;Uragami,T.Nature-769.(8)Antionietti,M.;Caruso,R.A.;Goltner,C.G.;Weissenberger,M.C.Macromolecules3-1389.(9)Takeoka,Y.;Watanabe,M.Langmuir7-5980.(10)Zhang,X.Z.;Yang,Y.Y.;Chung,T.S.;Ma,K.X.Langmuir4-6099.(11)Kaneko,Y.;Sakai,K.;Kikuchi,A.;Yoshida,R.;Sakurai,Y.;Okano,T.Macromolecules7-7723.(12)Yoshida,R.;Uchida,K.;Kaneko,Y.;Sakai,K.;Kikuchi,A.;Sakurai,Y.;Okano,T.Nature-242.(13)Okumura,Y.;Ito,K.AdV.Mater.-487.(14)Gong,J.P.;Katsuyama,Y.;Kurokawa,T.;Osada,Y.AdV.Mater.5-1158.(15)Kumashiro,Y.;Lee,W.K.;Ooya,T.;Yui,N.Macromol.RapidCommun.-410.(16)Nakamura,K.;Murray,R.J.;Joseph,J.I.;Peppas,N.A.;Morishita,M.;Lowman,A.M.J.Control.Rel.-599.cross-links13andadouble-networkhydrogel,14werefabricatedtoimprovemechanicaloropticalproperties.Graftedhydrogelswithcombtypestructureweredesignedtogenerateafastresponserate12ortocontrolthedegradationbehavior,15aswellasthedrugreleasepattern.16Hybridhydrogelswithinterpenetratingpolymernetworkstructure(IPN)werealsopreparedtoincreasethedeswellingkinetics17orfordrugdelivery.18Theabilitytotailorthechemicalandmechanicalpropertiesofhydrogelsatthemolecularlevelisofimportanceinvariousbiotechnologyapplications,includingtheengineeringofcomplextissues,thedevelopmentofbiosensors,andtheelucidationofcell-cellandcell-materialinteractions.19Inthisregard,asafundamentaltechnology,theself-assemblymechanismwasalsowidelyemployedtocontrolthenano-orderedstructureatthemolecularlevel.Apolymericmicelleisatypicalexampleofnano-orderedmolecularaggregates.Itiswellknownthatcopolymersconsistingofhydrophilicsegmentsandhydro-phobicsegmentsareabletoassociatespontaneouslytoformcore-shellmicellarstructuresaboveacertaincriticalconcentra-tion.20,21Theself-assembledmicelleshavebeendevelopedasthecarriersfordrugdelivery.22-25Todate,thereareveryfewreportsdealingwiththemicellesself-assembledintheporousstructureofhydrogels.Furthermore,throughintroducingthemicellesintothehydrogelnetworksviachemicalbondsinsteadofphysicalincorporation,26,27thenetworkarchitectureandpropertywouldbecontrolled.Asaresult,thecorresponding(17)Zhang,X.Z.;Wu,D.Q.;Chu,C.C.Biomaterials3-3805.(18)Gil,E.S.;Hudson,S.M.Biomacromolecules2006,DOI:10.1021/bm060543.(19)Hahn,M.S.;Miller,J.S.;West,J.L.AdV.Mater.9-2684.(20)Inoue,T.;Chen,G.H.;Nakamae,K.;Hoffman,A.S.J.Control.Rel.-229.(21)Chung,J.E.;Yokoyama,M.;Okano,T.J.Control.Rel.-103.(22)Otsuka,H.;Nagasaki,Y.;Kataoka,K.AdV.DrugDeliV.ReV.-419.(23)Wei,H.;Zhang,X.Z.;Zhou,Y.;Cheng,S.X.;Zhuo,R.X.Biomaterials8-2034.(24)Luo,L.;Eisenberg,A.Langmuir4-6811.(25)Wu,J.;Eisenberg,A.J.Am.Chem.Soc.0-2884.(26)Huang,G.;Gao,J.;Hu,Z.B.;John.J.V.S.;Ponder,B.C.;Moro,D.J.Control.Rel.-311.(27)Ramanan,R.M.K.;Chellamuthu,P.;Tang,L.P.;Nguyen,K.T.Biotechnol.Prog.-125.10.1021/la063485zCCC:$37.00?2007AmericanChemicalSocietyPublishedonWeb03/10/20074232Langmuir,Vol.23,No.8,2007hydrogelsmighthavesuperiorproperties,includingfastresponsetostimuli,favorabledrugreleaseprofiles,etc.Inthispaper,wedemonstrateanovelconcepttofunctionalizehydrogelsbyintroducingtheamphiphilicpolymersintohydrogelstoformthenetworkswithpendentmicellarstructure.AthermosensitivehydrogelwasfabricatedbycopolymerizingN-isopropylacrylamide(NIPAAm)andanovelamphiphilicmacromonomer,P(NIPAAm-co-10-undecenoicacid)-NHCOCHdCH2.Duetotheself-assemblybehaviorofthependingam-phiphilicchainsinthenetwork,thependentmicellarstructurewasintroducedintothePNIPAAmhydrogelnetworkbycovalentbonds.Theinfluenceofthependentmicellarstructureonthepropertiesofresultedhydrogelswasexaminedintermsofmorphologyviascanningelectronmicroscopy(SEM),thermalresponsethroughdifferentialscanningcalorimetry(DSC),anddeswelling/reswellingkineticsuponexternaltemperaturechanges.ExperimentalSectionMaterials.N-Isopropylacrylamide(NIPAAm),2-aminoethaneth-iolhydrochloride(AET?HCl),2,2′-dimethoxy-2-phenylacetophe-none(DMPAP),andN-acryloxysuccinimide(NAS)werepurchasedfromACROSandusedasreceived.N-Methylpyrrolidone(NMP),10-undecenoicacid(UA),andN,N′-dimethylformamide(DMF)wereobtainedfromShanghaiReagentChemicalCo.andusedafterdistillationunderreducedpressure.N,N′-Azobisisobutyronitrile(AIBN)andN,N′-methylenebisacrylamide(BIS)wereprovidedbyShanghaiReagentChemicalCo.andrecrystallizedfromethanolandDMF,respectively.Allotherreagentsandsolventswereofanalyticalgradeandusedwithoutfurtherpurification.SynthesisofAmino-TerminatedPoly(N-isopropylacrylamide-co-10-Undecenoicacid)(P(NIPAAm-co-UA)-NH2).P(NIPAAm-co-UA)-NH2waspreparedbyradicalpolymerizationusingAET?HClasachaintransferagent.28NIPAAm(50mmol,5.65g),UA(5mmol,0.94g),AET?HCl(2.2mmol,0.25g),andAIBN(0.24mmol,39.5mg)weredissolvedin20mLofDMF.Thesolutionwasdegassedbybubblingwithnitrogenfor30min.Themixturereactedat70°Cfor24hundernitrogen.Then,theproductwasprecipitatedbytheadditionofchilleddiethylether.Finally,P(NIPAAm-co-UA)-NH2(5.28g)wasobtainedafterprecipitationanddriedinvacuumfor24h.SynthesisofAmphiphilicP(NIPAAm-co-UA)-NHCOCHdCH2.Theamphiphilicmacromonomer,P(NIPAAm-co-UA)-NHCOCHdCH2wassynthesizedbythenucleophilicsubstitutionreactionbetweenNASandP(NIPAAm-co-UA)-NH2.Inbrief,NAS(1.72mmol,0.29g)andP(NIPAAm-co-UA)-NH2(1.51g)weredissolvedin10mLofDMF.Thereactionwascarriedoutat4°Cfor2days.Thentheproductwasprecipitatedbytheadditionofchilleddiethylether.Thefinalproductwasdriedinvacuumfor24htoobtainthecolorlessmacromonomer,P(NIPAAm-co-UA)-NHCOCHdCH2(1.37g).GPCMeasurements.Number-andweight-averagemolecularweight(MnandMw,respectively)ofthemacromonomerweredeterminedbygelpermeationchromatographic(GPC)systemequippedwithWaters2690Dseparationsmodule,Waters2410refractiveindexdetector.THFwasusedastheeluentataflowrateof0.3mL/min.Watersmillenniummodulesoftwarewasusedtocalculatemolecularweightonthebasisofauniversalcalibrationcurvegeneratedbyapolystyrenestandardofanarrowmolecularweightdistribution.FT-IRMeasurements.ThemacromonomersamplewasanalyzedbyFT-IR(Perkin-ElmerSpectrumOne)spectrophotometer.Beforethemeasurement,thesamplewaspressedintopotassiumbromide(KBr)pellet.1HNMRMeasurement.The1HNMRspectrumofP(NIPAAm-co-UA)-NHCOCHdCH2wasrecordedonaMercuryVX-300318(28)-320.Soppimath,K.S.;Tan,D.C.W.;Yang,Y.Y.AdV.Mater.2005,17,Xuetal.Table1.FeedCompositionsofthePNIPAAmHydrogelssampleIDCGelGel20Gel30Gel40NIPAAm(mg)macromonomer(mg)0203040BIS(mg)photoinitiator(mg)O(mL)1111NMP(mL)1.21.21.21.2conversion(%)a85.678.369.462.8aWeightpercentageoftheresultedhydrogelfromthemonomers.spectrometerat300MHz(Varian)byusingCDCl3asasolventandTMSasaninternalstandard.PreparationofHydrogels.ThepolymerizationofthenovelPNIPAAmhydrogelwasinitiatedbythephotoinitiator(DMPAP).ParticularamountsofNIPAAm,P(NIPAAm-co-UA)-NHCOCHdCH2,andthecrosslinker,BIS,weredissolvedinmixedsolvent(distilledwater/NMP)toobtainaclearsolution.Thefeedcomposi-tionsofthemonomersandotherreactantsarelistedinTable1.Themonomersolutionwasirradiatedbyaportablelong-wavelengthUVlamp(356nm,16W)atroomtemperature(22°C)for24h.TheresultedPNIPAAmhydrogelwastakenoutandimmersedindistilledwatertoleachouttheunreactedchemicalsfor3days.Duringthisperiod,thedistilledwaterwasreplacedwithfreshwaterevery4h.ThehydrogelspreparedwerelabeledasGelx,wherexmeanstheamountofthemacromonomerP(NIPAAm-co-UA)-NHCOCHdCH2.Herein,theconventionalPNIPAAmhydrogelwasusedasacontrolanddesignatedasCGel,whichwasfabricatedunderthesameconditionwithoutthemacromonomer.Thehydrogelswerecutintodiskswitha10mmdiameteranda8mmthicknessforthefollowingcharacterizations.InteriorMorphology.Theswollenhydrogelsamples,afterreachingequilibriumswellingratiosinthesolvent(distilledwaterorDMSO)atroomtemperature,werequicklyfrozeninliquidnitrogenand-45then°Cforfreeze-dried3days.Theinafreeze-driedfreezedrier(Labconco)sampleswereunderthenvacuumfracturedatcarefullyinliquidnitrogen,andtheinteriormorphologyofthehydrogelsampleswasstudiedbyascanningelectronmicroscope(SEM,FEI-QUANTA200).BeforeSEMobservation,thehydrogelspecimenswerecoatedwithgoldfor7min.Brunauer-Emmett-Teller(BET)SurfaceArea.Thesurfaceareaofthehydrogelswasinvestigatedvianitrogenadsorptionisothermsat77KonaMicromeriticsASAP2020volumetricadsorptionanalyzer.Allthefreeze-driedbulkhydrogelsamples(～10mg)weredegassedundervacuumfor6hat313Kpriortomeasurements.TheBETsurfaceareawascalculatedusingdatainarelativepressurerangefrom0.05to0.25bytheBETmethod.LCSTBehavior.LCST(lowercriticalsolutiontemperature)behaviorofthehydrogelwasdeterminedbyusingDSC(DSC822e,METTLER).Eachsamplewasimmersedindistilledwateratroomtemperatureandallowedtoreachtheequilibriumstate.Then,theswollensamplewasplacedinahermeticsamplepanandsealed.Thethermalanalysiswasperformedataheatingrateof3°C/minontheswollensampleunderadrynitrogenatmospherewithaflowrateof50mL/min.TemperatureDependenceofSwellingRatios.Thegravimetricmethodwasemployedtostudythetemperaturedependenceofswellingratioofthehydrogels.Thesampleswereequilibratedindistilledwateratatemperaturerangingfrom22to45°C.Thesampleswereallowedtoswellinthedistilledwaterforatleast24hateachpredeterminedtemperaturebyathermostatedwaterbath.After24himmersioninthedistilledwater,thesampleswereweighedafterwipingofftheexcesswateronthesurfacesbymoistenedfilterpaper.Eachsamplewasmeasuredthreetimes,andtheaveragevalueofthreemeasurementswastaken.Afterthisweightmeasurement,thesampleswerere-equilibratedindistilledwateratanotherpredeterminedtemperature,andthentheirwetweightsweredeterminedasabove.Thedryweightofeachsamplewasobtainedafterdriedinvacuumat45°Cfor24h.TheswellingratioateachPendentMicellarStructuresinHydrogelsLangmuir,Vol.23,No.8,[Wateruptake]t)[(Wt-Wd)/Ws]×100(3)whereWtistheweightofthewethydrogelattimetatroomtemperatureandtheothersymbolsarethesameasdefinedabove.ResultsandDiscussionSynthesisoftheMacromonomer.DuringthesynthesisofamphiphilicP(NIPAAm-co-UA)-NHCOCHdCH2,NIPAAmandUAwererandomlycopolymerizedtoobtainthemac-romonomer(Mn)13200g/mol,Mn/Mw)1.8byGPCmeasurement).Thechemicalstructureoftheresultedmac-romonomerwascharacterizedbyFT-IR(Figure1)and1HNMR(Figure2)spectroscopy,respectively.AsshowninFigure1,thetypicalamideIandIIbandsofNIPAAmunitswereobviousat1648and1547cm-1.Besides,thestretchingvariationabsorbanceofCdOincarboxylicgroupsofUAunitsexistedat1712cm-1.FromtheFT-IRspectrum,wecanconcludethattheresultedmacromonomercontainedNIPAAmandUAunits.Herein,inordertoexactlydeterminethemolarratioofUAunitstoNIPAAmunitsinthemacromonomer,the1HNMRmeasurementwascarriedout.Thecorresponding1HNMRspectrumisexhibitedinFigure2.Fromthespectrum,theamideprotons(-NH,signalf)ofNIPAAmunitsandtheprotonsofcarboxylicgroups(-COOH,signalh)oftheUAunitswereat6.4-7.2and8.6-8.8ppm,respectively.Thechemicalshiftsat6.3(signale)and8ppm(signalg)weremainlyassociatedwiththeterminalprotonsof-CHdCH2and-NH,respectively.Otherpeaksandtheirchemicalshiftswereconsistentwiththeliteraturereport.29Onthebasisoftheabove1HNMRspectrumanalysis,theactualmolarratioofUAunitstoNIPAAmunitsintheresultedmacromonomercouldbecalculatedfromthepeakintensitiesofsignalh(UAunits)andd(NIPAAmunits).Thepeakintensitiesofsignalhanddwere0.41and4.33,respectively.Thatis,themolarratioofUAtoNIPAAmwas1:10.5inthepolymericchainoftheresultantmacromonomer,whichnearlycoincidedwiththefeedcomposition(1:10).PreparationofHydrogels.Ingeneral,blockcopolymersconsistingofhydrophilicNIPAAmunitsandhydrophobicUAunitsareabletoformacore-shellmicellarstructureinaqueoussolutionatthetemperaturebelowtheLCSTofPNIPAAm.Inthisstudy,inordertoavoidtheself-assemblingoftheFigure1.FT-IRspectrumofP(NIPAAm-co-UA)-NHCOCHdCH2.temperaturewascalculatedasfollowsSReq)Ws/Wd(1)whereWsistheweightofwaterintheequilibriumswollenhydrogel(wetweight-dryweight)andWdisthedryweight.DeswellingKineticsat50°C.Thedeswellingkineticsoftheequilibratedswollenhydrogelwasmeasuredgravimetricallyindistilledwaterat50°C.Atpredeterminedtimeintervals,thesamplesweretakenoutfromthehotwater(50°C)andweighedafterremovingtheexcesswateronthesurfaceswithwetfilterpaper.Similarly,eachsamplewasmeasuredthreetimes,andtheaveragevalueofthreemeasurementswasused.Waterretentionwasdefinedasfollows[Waterretention]t)[(Wt-Wd)/Ws]×100(2)whereWtistheweightofthewethydrogelattimetat50°C,Wsistheequilibriumwaterweightatroomtemperature,andtheothersymbolsarethesameasdefinedabove.ReswellingKineticsat22°C.Thedriedsamplewasimmersedindistilledwateratroomtemperatureandremovedfromwateratregulartimeintervals.Afterremovingthewateronthesurfacewithwetfilterpaper,theweightwhichwastheaveragevalueofthreemeasurementswasrecorded.ThewateruptakeattimetwasdefinedasfollowsFigure2.1HNMRspectrumofP(NIPAAm-co-UA)-NHCOCHdCH2.4234Langmuir,Vol.23,No.8,2007Xuetal.Figure3.SynthesisschemeofthenovelPNIPAAmhydrogels.macromonomersbeforethehydrogelformation,themixedsolventofdistilledwaterandNMPwasused.ThesynthesisschemeofthenovelPNIPAAmhydrogelsispresentedinFigure3.InteriorMorphology.Inordertoconfirmthepresenceofpendentmicellarstructureoriginatedfromtheself-assemblybehaviorofthegraftedamphiphilicpolymers,thehydrogelsampleswereimmersedindistilledwater(Figure4a)andDMSO(Figure4b)andthenfreeze-driedforSEMobservation.FromFigure4a,itwasfoundthatdifferentkindsofpendentmicellarstructures(sphereordumbbell)appearedinthenetworksofGel20,Gel30,andGel40.Asmentionedabove,theconventionalamphiphiliccopolymersconsistingofPNIPAAmsegmentsandhydrophobicsegmentscanformcore-shellmicellarstructurebelowtheLCST.Recently,itwasreportedthatrandomcopolymersofNIPAAmandUAcouldself-assembleintocore-shellnanoparticles.28Similarly,inthispaper,thegraftedamphiphilicP(NIPAAm-co-UA)chainswouldself-assembleinwatertoformthependentmicellarstructureintheresultedhydrogelnetworks,whichisdemonstratedinFigure4a.Incontrast,asshowninFigure4b,therestrainednetworkstructureinsteadofthependentmicellarstructureofthehydrogelscanbeobservedwhenimmersedinDMSO.Thisdifferenceinmorphologyofthehydrogelsunderdifferentconditionswasreasonable,andsimilarfindingswerealsoreportedinotherstudies.22,23Thecore-shellstructure,i.e.,pendentmicellarstructure,couldbedestroyedinorganicsolvent,suchasDMSO,toformdispersedpolymericchains.Asaresult,nopendentmicellarstructurewouldexistinthehydrogelnetworksinDMSO.Simultaneously,theinitialporousstructureinwaterwasalsochangedtotheheterogeneousandcollapsedmorphologyinDMSO.ThedisappearanceofthemicellarstructurefurtherconfirmedthatthependentmicellarstructureinFigure4aresultedfromtheself-assemblingoftheamphiphilicpolymericchainsinwater.Importantly,themicellarstructurewasintroducedintothenetworkviachemicalbondinsteadoftraditionalphysicalincorporation.Thechemicalincorporationofthemicellarstructureintothehydrogelnetworkwouldovercometheleakageproblemofthetraditionalcompositehydrogel,wherenanoparticleswereincorporatedphysicallyintobulkhydrogelnetworks.30AmongtheSEMmicropicturesinFigure4a,theaverageporesizeofthehydrogelsincreasedfromCGeltoGel40.Generally,inordertopreparemacroporousnetworks,pore-formingagents,suchascellulose31andPEG,10wereaddedphysicallytothemonomersolution,althoughpore-formingagentsdidnotparticipateinthepolymerizing/cross-linkingreaction.Duetotheexistenceofpore-formingagents,theresultantnetworksexhibitedmacroporousorexpandednetworkstructures.Inthisstudy,duringtheformationofhydrogels,thegraftedamphiphilicpolymerswouldleadtospatialhindranceandthesimilareffectofpore-formingagentstoformmacroporousnetworkstructures.BETSurfaceArea.TheBETsurfaceareasofthenovelPNIPAAmhydrogelsdeterminedvianitrogenadsorptioniso-thermsareshowninFigure5.ItwaseasilyfoundthattheBETsurfaceareaofthehydrogelalmostlinearlydecreasedwiththeincreasingcontentoftheamphiphilicpolymers.Forexample,theBETsurfaceareaofCGelwas4.96m2/g,whilethoseofGel20,Gel30,andGel40were3.08,0.73,and0.34m2/g,respectively.Asweknow,therewasatendencythatthemicroporousmaterialshadthelagerBETsurfacearea.ThedecreasingBETsurfaceareainFigure5indicatedthatthenumberoftheporesreducedfromCGeltoGel40.Ontheotherhand,thisdecreasingBETsurfaceareasimultaneouslydemonstratedthattheporesizeincreasedfromCGeltoGel40,whichwasalsoconfirmedbytheSEMmicropicturesinFigure4a.Inthesameway,theconclusionoftheBETsurfaceareaalsodemonstratedthatthegraftedamphiphilicpolymersactedasapore-formingagentduringthehydrogelformation.LCSTBehavior.TheLCSTsofthenovelPNIPAAmhydrogelsdeterminedfromtheDSCthermogramsareexhibitedinFigure6.AsshowninFigure6,allthehydrogelsregardlessofcontentofthegraftedamphiphilicpolymersshowedrelativelyFigure4.SEMmicropicturesofthenovelPNIPAAmhydrogelsandtheconventionalhydrogelindistilledwater(a)orDMSO(b).PendentMicellarStructuresinHydrogelsFigure5.BETsurfaceareasofthenovelPNIPAAmhydrogelsandtheconventionalhydrogel.Figure6.LCSTsofthenovelPNIPAAmhydrogelsandtheconventionalhydrogel(theonsettemperatureofendothermwasreferredasLCST).similarLCSTsataround33°C(rangingfrom32.7to33.4°C).Infact,fromFigure6,althoughtheLCSTofthehydrogelhadthetinytendencytoincreasewiththeincreasingcontentofgraftedamphiphilicpolymers,noapparentdifferencewasfound.Thatis,thependentmicellarstructurehadnoapparentimpactontheLCST.AwidelyrecognizedmechanismtoexplaintheLCSTbehaviorofPNIPAAmhydrogelsisthatthephasetransitionresultedfromabalancebetweenhydrophilicityandhydropho-bicityinthepolymericbackbone.32,33WhenahydrophilicmoietywascopolymerizedintoPNIPAAmhydrogelnetwork,thehydrophilic/hydrophobicbalanceshiftedtoamorehydrophilicnatureandthecorrespondingLCSTshiftedtoahighertem-perature.Incontrast,ifthehydrophobicmoietywascopolymer-izedintothepolymericchain,itsLCSTbecamelower.Here,theamphiphilicP(NIPAAm-co-UA)-NHCOCHdCH2moietywasincorporatedintoPNIPAAmhydrogelnetworkasthependentchains,insteadofthebackbone.Thus,theinitialhydrophilic/hydrophobicbalanceofthebackbonedidnotchangeapparently.Consequently,theLCSToftheresultedhydrogeldidnotchangeobviously.TemperatureDependenceofSwellingRatios.Thetem-peraturedependenceofswellingratioofthenovelPNIPAAmhydrogelswasexaminedtoevaluatetheirtemperature-sensitiveproperties.Figure7exhibitsthetemperature-dependentswellingX.(29)SmallLi,Y.Y.;Zhang,X.Z.;Kim,G.C.;Cheng,H.;Cheng,S.X.;Zhuo,Mater.(30)Xu,2006X.,D.;2,917Wei,-923.R.H.;Zhang,X.Z.;Cheng,S.X.;Chem.(31)Res.Wu,2006X.S.;,DOI:Hoffman,10.1002/jbm.a.31063.Zhuo,R.X.J.Biomed.A.S.;Yager,P.J.Polym.Sci.,69-(32)1992Vernon,,30,B.;2121Kim,-2129.PartA:Polym.S.W.;Ba,Y.H.J.Biomed.Mater.Res.5(33)79.-6540.Shibayama,M.;Fujikawa,Y.;Nomura,S.Macromolecules1996,29,Langmuir,Vol.23,No.8,Figure7.TemperaturedependenceoftheswellingratioofthenovelPNIPAAmhydrogelsandtheconventionalhydrogeloveratemperaturerangefrom22to45°C.Figure8.DeswellingkineticsofthenovelPNIPAAmhydrogelsandtheconventionalhydrogelat50°C.ratiosoverthetemperaturerange22-45°C.Asshowninthefigure,allthehydrogelsdemonstratedasimilarswellingprofiles,i.e.,theswellingratioofallthehydrogelsdecreasedrapidlyasthetemperatureincreasedtowardtheirLCSTs.Traditionally,intermsofswellingratiochanging,thephase-separationtemperatureorLCSTisregardedasthetemperatureatwhichthephase-separationdegree,swellingratiochangevstemperaturechange(?SR/?T),isthegreatest,orthetemperatureatwhichtheswellingratioofhydrogeldecreasesmostdramatically.FromFigure7,itwasclearthattheLCSTsofthehydrogelswerearound33°C,irrespectiveofpresenceofthependentmicellarstructure,whichwasinagreementwiththeLCSTsdeterminedfromtheDSC.EventhoughtheLCSTsofthehydrogelswerenotvirtuallyaffectedbythependentmicellarstructure,thedatainFigure7showthat,atatemperaturebelowtheLCST,theequilibriumswellingratioofhydrogelsincreasedfromCGeltoGel40.Forexample,atroomtemperaturetheequilibriumswellingratioofCGelwas53.5,whilethoseofGel20,Gel30,andGel40were80,232,and255,respectively.DuetotheincreasingaverageporesizefromCGeltoGel40asindicatedinFigure4a,thecapacityofholdingwaterincreasedcorrespondingly.Asaresult,theswellingratioofhydrogelsimprovedfromCGeltoGel40.Onthebasisofourexperiments,duetotheincreasingofwatercontentfromCGeltoGel40,themacroscopicmechanicalpropertyofresultedhydrogeldecreasedcorrespondingly.Furthermore,itwasinterestingtonote,atthetemperatureabovetheirLCSTs,therewasnoeffectofthependentmicellarstructureontheswellingratioofhydrogels,whichsuggestedthat,withorwithoutthependentmicellarstructure,allthehydrogelswouldcollapseintothesimilarstructureatthetemperatureabovetheirLCSTs.DeswellingKineticsat50°C.Thetemperaturedependenceoftheswellingratiosonlydemonstratedtheequilibriumhydrationstateofhydrogelsatdifferenttemperatures.Inpracticalap-plications,thetemperatureresponsekineticsordeswellingkineticsuponthesuddenlyalteredstimulationismoreimportant.Figure8presentsthedeswellingkineticsofthesenovelPNIPAAm包含各类专业文献、各类资格考试、中学教育、文学作品欣赏、生活休闲娱乐、专业论文、应用写作文书、la等内容。