化学和树脂干胶的先驱体制备过程.docx

化学和树脂干胶的先驱体制备过程.docx

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化学和树脂干胶的先驱体制备过程

ChemistryandApplicationsofPolymericGelPrecursors

Polymerizablecomplex（PC）methodseemstobetheeasiestandthemostelegantvariationofsol–gelmethods.Thesimplestimplementationofthistechniquerequiresonlyinexpensivechemicals,abeaker,astirrer,ahotplateandafurnace.Itisnotsensitivetothepresenceofwater（fortherareexceptions）;itdoesnotrequireinertatmosphere,andevenwithoutcarefulcontrolofgelprocessingtimeandconditions,oneshouldbestillabletoobtainsamplesofoutstandinghomogeneity.Itisthecombinationofthesefactorsthatexplainsthegrowingpopularityofthepolymerizablecomplexmethodforthesynthesisofferroelectric,superconducting,ferromagnetic,CMR,photocatalytic,fuelcell,electrode,catalyticandothercomplexoxidematerials.

TheoriginofthepolymerizablecomplexmethodisdatedbacktothePechinipatent（Pechini,1967）onfabricationofthinfilmcapacitorsbyutilizingmultifunctionalorganicacidscapableofchelatingmetalionsintothestablecomplexesandadiol,whichservesasasolventduringcomplexformationstepandlaterparticipatesinthepolyesterificationreactiontoformthree-dimensionalpolymernetworkwithincorporatedmetalcomplexesmixed,asassumed,ontheatomicscale.TheessentialstepsofthepolymerizablecomplexmethodarepresentedinFigure4-1.Suitablemetalsaltsareintroducedintotheethyleneglycol（EG）afterdissolutionofcitricacid（CA）,whichisaddedinalargeexcesstoformmetal-citratecomplex.Thenthetemperatureshouldbeincreasedto100–130°Ctospeeduptheformationofpolyesterduetothereactionbetweenfreecitricacidandethyleneglycol.Aftertheformationofplastic-likegel,thetemperatureisincreasedfurthertoremovetheexcessofethyleneglycol.Theobtainedrelativelyhardsubstanceshouldbetreatedat450–600°Ctooxidizeorganiccompounds.Precursorpowdersthusobtainedcontainamixtureofhomogeneouslydistributedmetaloxides,carbonatesorsometimesanintermediatesinglephasecompoundwiththeproperstoichiometryofthemetalions.

Thebasicideaofthisapproachistouseinsitupolymerizationofmonomersspeciallyintroducedintosolutioninadditiontotherequiredmetalcations.ThistechniqueisknownasPechinimethod（Pechini,1967）.Thebackgroundoftheproposedapproachisesterificationreactionbetweenglycolandmultifunctionalcarboxylicacid.Forcontinuousgrowthofthepolymerchaintheexistenceofatleasttwofunctionalgroupsinamonomerisimportant.Theviscosityofthesolutiondrasticallyincreasesduringthepolymerchainaugmentation（Fig.4-2）.AsonecouldnoticeformFigure4-2,constructedfromthereporteddata（Arima,1996;Kakihana,1997）,thenatureofthecationsaffectstherateofviscosityincrease,whichindicatesthatmetalionsplayanimportantroleincross-linkingthepolymerchainsbymeansofcomplexformation.Thisviscosityincreaseisoftenreferredtoasa“gelation”.Probablyitwouldbemoreaccuratetousetheword“inspissation”toreflectthenatureandconditionsofthepolyesterificationreaction;however,inthischapterwewillfollowtheestablishedterminology.Duringthefirststageofthepolymergrowththesolutionprovidesnecessaryenvironmenttopreventcationsegregation,andlater,therelativelyrigidpolymernetworktrapscationsandpreservestheinitialhomogeneityofthesolution.TheoriginalPechinipatentstatestheimportanceofthecarboxylicacid,whichcanformastablechelatecomplexsuchascitricacidandthepolyhydroxyalcohol,whichisliquidbelow100°Candmayservesimultaneouslyasamonomerandasasolvent.Afterthepolymerizationiscompletedandtheexcessofsolventisevaporated,thepolymermatrixofgelistobeoxidizedyieldingaerogelwhichiscomposedofoxidesandcarbonates.

Figure4-1.Schemeofpolymericgelmethod.

Figure4-2.Viscosityincreaseofthemetal-citricacid-ethyleneglycolsystemasafunctionofreactiontime.

Sincethepublicationoftheoriginalpatent,thepolymerizablecomplexmethodwasintensivelystudied,sometimesremarkablymodifiedtomeettheparticularsyntheticneedsortoovercometheobstaclesthatmayariseduringthesynthesis.Inthepastfewyearsitbecamemorecommontorefertopolymerizablecomplexmethodas“Pechinimethod”,“modifiedPechinimethod”,“liquidmixtechnique”,etc.Inthischapterwewillalsofollowthewell-establishedcustomwheneverwewilldiscusssynthesisemployingcitricacidandethyleneglycolasessentialreagents.InadditiontothatwewilldiscussthesynthesistechniquesdevelopedfromPechinimethodthatcanbeclassifiedaspolymerizablecomplexmethod（Kakihana,1996,1999a）sincetheypossesstwodistinctivefeatures:

formationofstablemetalcomplexinthesolutionandpreservingatomicscalehomogeneityachievedinthesolutionbyuseofpolymertohindertheionsmobilityandpreventsegregation.

Chemistry

ComparedtotheknowledgebaseonthepracticaluseofPechinimethodforsynthesisofdifferentoxidematerials,relativelylittleisknownaboutchemistryofthistechnique.Mostofthefundamentalstudiesoncitricacidanditsmetalcomplexeswerecarriedoutin60and70s—eitherbeforePechinipublicationorrightafteritwhenpolymerizablecomplexmethodhasnotdeservedmuchattentionyet.Thereforemostoftheresultsarenotdirectlyrelatedtopolymerizablecomplexmethod.MorerecentstudiesonthechemistryofPechinimethodoftenwerecarriedoutbymaterialscientistswithnecessarychemistrybackgroundandfocusedonveryspecificquestionsornarrowsetofparticularproblems.Wewillattempttosummarizetheavailableinformationandcreateageneral（andhopefullyuseful）pictureofthechemistryinvolvedinthismethod.

Precursors

InthetypicalsynthesisofoxidematerialsbyPechini-typeprocess,solublemetalnitrates,acetates,chlorides,carbonates,isopropoxidesorothersuitablemetalcompoundsaredissolvedintheCA–EGsolution.Citricacidisrelativelystrongmultifunctionalorganicacid.Theacidityofthemiddlecarboxylicgroup（–mCOOH）isenhancedby–OHgroupattachedtoternarycarbonatom.Methylenegroups,ontheotherhand,provideadestabilizingeffect;howeveritisreducedbytheneighboringterminalcarboxylicgroups（–tCOOH）.ThereforemiddlecarboxylicgroupinwatersolutionlosesH+veryeasily（pK1=2.91）（Harris,1976）andpHofcitricacidaqueoussolutionisusuallyintherangeof0–2dependingonconcentration.Theterminalcarboxylicgroupsarelessacidic（pK2=4.36andpK3=5.74）（Harris,1976）andthedissociationbecomessubstantialathigherpH.AtveryhighpHthehydroxygroupofcitricacidmaybecomedeprotonated（pK4=10.96）（Grigor’eva,1975）.Onemayalsonoticethatlocationof–OHgroupisfavorablefortheformationofhydrogenbondsbetweencarboxylicgroupsandhydroxygroup,whichshouldstabilizethecarboxylicion.Whatisimportant,isthatsuchaskeletonisresponsiblefortheformationofstablefive-andsix-memberringsinmetalcitratecomplexes.

Citricacidiswellsolubleinethyleneglycol,whichprovidesawiderangeofCA:

EGratiosforthePechinimethod,andmakesitpossibletotunetheconditionsofsynthesisforeachparticularsystem.Chemicalinteractionbetweencitricacidandethyleneglycoloccursatroomtemperaturewithoutanytreatmentofthesolution.Ofcourse,thisinteractionisnotcompleteandchemicalequilibriumisstronglyshiftedtowardfreecitricacidandethyleneglycol,however13C-NMRspectrumunambiguouslyprovesesterformation（Kakihana,1999a;Fang,2001）.Thequalityofthefinalpowderdependsontheextenttowhichthemolecular-scalehomogeneityachievedinthesolutionmaybepreservedintheprocessofpolymerformationandpyrolysisofthepolymerresin.ForthepurposeofoxidepowderssynthesiscitricacidandethyleneglycolratiointhePechiniprocessseemstobefarfromtheoptimalvalue.Figure4-3summarizestheresultsoftheinvestigationofCA–EGsystembehaviorduringgelation,solventremovalandthermaldecompositionoforganicmatrixconductedbyTaiandLessing（1992a,1992b）.TherangeofCAconcentrationsfrom50to60%seemstobethemostappropriateforthecomplexoxidepowderspreparationsinceitprovidesthemaximumviscosityoftheobtainedgel.Strongfoamingisanadditionalfactorthatpreventssegregationduringthermaldecompositionofthepolymer.Mildburningwouldguaranteerelativelylowtemperatureinsideoftheprecursorand,consequently,relativelyslowgraingrowth.Oneshouldkeepinmind,however,thatesterformationisareversibleprocess.

Theequilibriuminthisreactioncanbeshiftedtowardpolyesterbyeitherincreaseofastartingreagentconcentrationorbyaproductremovalfromthereactionmedium.OnthecitricacidrichsideonewouldberestrictedbytheCAsolubilityinethyleneglycol（Fig.4-3）.Frompracticalviewpoint,highviscosityoftheconcentratedCAsolutionwillslowdownthedissolutionofmetalsaltsand,inaddition,theprecipitationofcitricacidmightoccurwhilepH,temperatureormetalsaltconcentration（ionicstrength）willchangeduringtheprocessing.AnotherreasontouseexcessofEGisconnectedwithaneedtoremovewaterfromthereactionmixture.Inthiscase,thevaporwillcontainethyleneglycolasamajorfractionthatwillbeprogressivelyremovedfromthereaction.However,thecompleteremovaloftheexcessofEGrarelyoccursduringpolyesterificationandtheuseof20%citricacidconcentrationrequiresrelativelylongheattreatmenttoeliminateunreactedglycol.TheboilingpointofEGisthelowestamongdials,sothechoiceofethyleneglycolasasolventandasamonomeristhemostconvenien