大空形态的控制相分离.docx
- 文档编号:30550407
- 上传时间:2023-08-16
- 格式:DOCX
- 页数:18
- 大小:1.20MB
大空形态的控制相分离.docx
《大空形态的控制相分离.docx》由会员分享,可在线阅读,更多相关《大空形态的控制相分离.docx(18页珍藏版)》请在冰豆网上搜索。
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
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 形态 控制 分离