大空形态的控制相分离.docx

大空形态的控制相分离.docx

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大空形态的控制相分离

MacroporousMorphologyControlbyPhaseSeparation

INTRODUCTION

Inordinarysol–gelprocessing,startingcompositionsaswellasreactionconditionsareselectedsoastomaintainthemixtureinahomogeneousstatethroughouttheprocessesincludingmixingofstartingcompounds,gelation,aging,dryingandheat-treatment.Highhomogeneityofaprecursorsolutionisespeciallyimportantforthefabricationoffibersandcoatings.Theapparentdisadvantageoftheformationofmicroscopic（sometimesmacroscopic）heterogeneityingels,however,canbeutilizedtocontroltheporestructureofthegels.Macroporeswithpreciselycontrolledsizeandsizedistributionareespeciallyimportantwhenvariousfunctionalizedgelmaterialsaretobeusedincontactwithliquidsolutionsinthattheyincreasethecontactprobabilityofexternalsubstancesontothesurfacesites.

Thepresentchapterdescribesthemethodofmacroporousmorphologycontrolthroughthesol–gelreactionaccompaniedbytheconcurrentphaseseparation.Generalprinciplesofthemethodareexplainedadoptingtypicalexperimentalsystemsasexamples.Theformationofmacroporousmorphologyinasmallconfinedspaceisalsoshownforanintriguingexampleofminiaturizationofthematerial.Anotherimportantfeatureofthegelswithcontrolledmacroporesisthatthemesoporestructurecanbetailoredindependentlyofthemacroporouscharacteristics.Sharplydistributedmesoporesareformedviabasicorhydrothermalagingprocess.Examplesofsupramolecularlytemplatedmesoporesinmonolithicmacroporousgelsbyuseofamphiphilicadditivesarealsopresented（Nakanishi,1997）.

POLYMERIZATION-INDUCEDPHASESEPARATION

Letusfirstconsideratypicalhydrolysis–polycondensationofalkoxysilanesunderacidicconditionswhichgivesrelativelynarrowdistributionofthemolecularweightofthepolymerizingoligomers.Theaveragemolecularweightofthepolymerizingspeciesinasolutionincreaseswithreactiontimebyvirtuallyirreversiblepolycondensationreactionsamongthemonomers/oligomers.Thethermodynamicsofasolutioncontainingpolymerizingspeciestellsusthatmutualsolubilityamongtheconstituentsbecomeslowerastheaveragemolecularweightofthepolymerizingspeciesincreases（Flory,1971）.Thisismainlyduetothelossofentropyofmixingamongtheconstituentswhichleadstotheincreaseofthefreeenergyofmixing,ΔG.

（23-1）

Thereductioninmutualsolubilitycausedbypolymerizationcanbecontrastedwiththatbyphysicalcoolingofthesystem（deGennes,1979;seeFig.23-1）.Inthelattercase,thefreeenergyofmixingisincreasedbyloweringtemperature.Inbothcases,amulti-componentsystembecomeslessstableastheabsolutevalueoftheTΔStermdecreases.Insomecases,changesinthepolarityofoligomerswiththegenerationand/orconsumptionofsilanolgroupsmaycontributetoincreaseinΔHterm,whichalsodestabilizethesystemagainsthomogeneousmixing.Inanycase,whenthesignoffreeenergyofmixingofthesystembecomespositive,thethermodynamicdrivingforceforphaseseparationisgenerated.

Figure23-1.Physicalvs.chemicalcooling.

Inrealexperimentalsystems,poorsolventsoftheoligomers,severalkindsofwater-solublepolymers,andcationicornonionicsurfactantscanbeusedasanadditivecomponenttoinducethephaseseparationinthecourseofasol–gelreaction.Typicalexamplesfollow:

（a）Low-waterhydrolysisoftetraalkoxysilaneoralkyltrialkoxysilane

Whenhydrolyzedwithunderstoichiometricamountofwater（H2O/Si<2and1.5inthecasesoftetraalkoxysilaneandalkyltrialkoxysilane,respectively）,thesiloxaneoligomersretainaconsiderableamountofunreactedalkoxygroups.Theseoligomerswithrelativelylowpolaritytendtophaseseparateagainstahighlypolarsolventmixture.Additionofanextremelyhighconcentrationofmineralacidorformamideispreferabletoinducephaseseparationinthesolutionderivedfromtetraalkoxysilanes（Kajietal.,1993）.Withalkyltrialkoxysilanes,thegeneratedoligomershaveinherenthydrophobicgroupsandthusexhibithigherphaseseparationtendencyevenagainstthemixtureofwaterandalcoholwithadiluteacidcatalyst.

Aseriesofexceptionhasbeenfoundrecentlywithbridgedalkoxysilanes.Bis（trialkoxysilyl）alkaneswithC6orC8bridgingalkylenechainstypicallyphaseseparateagainst50–70foldmolaramountofwaterrelativetosiliconunderacidicconditions（Nakanishietal.,2002）.Relativelylongalkylenechainsburiedinthesiloxanenetworkonlymoderatelycontributetoenhancethephaseseparationtendencyofthepolymerizingoligomers.

（b）High-waterhydrolysisoftetraalkoxysilaneinthepresenceofweakly-interactingadditives

Withasufficientamountofwater,almostallthealkoxygroupsarehydolyzedintosilanolgroups.Thepolarityofresultantsiloxaneoligomersishighenoughtobedissolvedinalcohol–watersolventmixturecontainingioniccatalysts.Anadditionofwater-solublepolymersuchaspoly（acrylicacid）orpoly（sodiump-styrenesulfonate）tothissystemcaninducethephaseseparationmainlybasedontheincompatibilitybetweenthepolymerandsiloxaneoligomers（NakanishiandSoga,1991,1992）.Theaddedpolymerispreferentiallydistributedtothephasecontainingminoramountofsiloxaneoligomers,andthusconstitutesthe“fluidphase”incontrasttothe“gelphase”richinsiloxaneoligomers.Inthiscase,theadditivecomponentjustplaysanassistingroletoinducethephaseseparationtoformmicrometer-rangeheterogeneousstructures.

（c）High-Waterhydrolysisoftetraalkoxysilaneoralkylene-bridgedalkoxysilaneinthepresenceofhydrogen-bondingadditives

Severalsurfactantsandwater-solublepolymersareknowntoexhibitstronghydrogen-bondinginteractionbetweensilanolgroupsonthesurfacesofsilicacolloidsandinsiloxaneoligomers.Amongothers,polyoxyethylenechainsspecificallyformstronghydrogenbondswithsilanolsbytheiretheroxygens.Whenalkoxysilanesundergohydrolysis/polycondensationinthepresenceofthepoly（ethyleneoxide）orsurfactantcontainingpolyoxyethyleneunits,thepolymerorsurfactantformshydrogen-bondedamorphouscomplexassoonassufficientamountsofcontinuoussilanolsitesaregeneratedasaresultofpolycondensationofhydrolyzedalkoxysilanesinthesolution（Fig.23-2）.Inthecasethatsurfactantsandpolymerscoversilanolssostronglythatanyfurtherpolycondensationisinhibitedbytheadsorbedmolecules,onlylowmolecularweightoligomerswillsegregatetoformadispersed,non-gellingphase.ByanappropriatechoiceoftheHLB（hydrophile–lypophilebalance）valueorthemolecularweight,ontheotherhand,thephaseseparationcanbeconcurrentlyinducedwiththehomogeneoussol–geltransitionofthereactionsystem（Nakanishietal.,1994）.Beingdifferentfromthecaseswithweakly-interactingpolymers,mostoftheadditivesurfactantsorpolymersaredistributedtothephasetowhichmajorityofthesiloxaneoligomersarealsodistributed,andformagelphasetogether.Thefluidphaseisthencomposedmainlyofthesolventmixture.

Figure23-2.Hydrogen-bondingofPEOchainsonsurfacesilanols.

Thesystemcontaininghydrogen-bondingadditiveshasanadvantageincontrollingtheporestructureoftheresultantgels.Aswillbeexplainedindetailbelow,thesizeofthepores（tobemoreexact,thesizeofseparatedphasedomains）primarilydependsonthephaseseparationtendencyofthepolymerizingsiloxaneoligomersolution.Theporevolumeisdeterminedmainlybythevolumefractionofthefluidphase,andthusroughlyproportionaltotheconcentrationofwaterandsolventinthestartingcomposition.Theporesizeandtheporevolumeofagelsamplecanbeindependentlycontrolledbyadjustingtheconcentrationsoftheadditiveandthesolvent,respectively.Inthesystem（b）describedabove,thephaseseparationtendencyandthevolumefractionofthepore-formingphaseareinterdependent,whichmakesitdifficulttodesignawidevarietyofporestructure.

（d）Morphologydevelopmentbyspinodaldecomposition

Inaphasediagramwithamiscibilitywindow,thetwo-phaseregionisdividedintotwosub-regions.Oneisthatbetweenbinodalandspinodal,calledmetastableregion.Inthemetastableregion,anyinfinitesimalfluctuationofthecompositionisenergyconsuming,thatis,finiteactivationenergyisrequiredtodevelopphase-separateddomains.Thetypicalphaseseparationmechanisminthisregionisthe“nucleationandgrowth”wheredispersedsmallregionscallednucleigrowaccompaniedbyanadditionofconstituentsdiffusingfromthebulk（notyetseparated）regionsofthesystem.Thenaturalconsequenceofthismechanismisamorphologywith“dispersedA”and“matrixB”phasedomains（Fig.23-3）.Theotherregionisthatwithinaspinodalline,calledunstableregion.Intheunstableregion,anyinfinitesimalfluctuationgainsenergysothatthefluctuationspontaneouslydevelopswithtimewithoutrequiringtheactivationenergy.Dependingmainlyonthedepthofquench（thedifferencebetweenthecriticaltemperatureandtheactuallyquenchedtemperature）andthemobilityoftheconstituents（moreprecisely,thatofdiffusingunits）,onlyasingleFouriercomponentamongthevariousfluctuationwavelengthssurvivesanddominatesthecharacteristicsizeofthedomains.Acleardifferencecanbeseenbetweenthenucleationandgrowthmechanismthatthephasedomainshavenodistinctinterfaceintheinitialstagesofthephaseseparation.Thecontrastinchemicalcompositiondevelopscontinuouslywithtimeuntiltheequilibriumphasecompositionsarereached.Undercomparablevolumefractionsofconjugatephasedomainswithoutanisotropy,thesponge-likestructurecalledco-conti