Multiple.docx
- 文档编号:5268067
- 上传时间:2022-12-14
- 格式:DOCX
- 页数:11
- 大小:50.94KB
Multiple.docx
《Multiple.docx》由会员分享,可在线阅读,更多相关《Multiple.docx(11页珍藏版)》请在冰豆网上搜索。
Multiple
Multiple
14
characterization
applications
Thischapterisintendedasareview,showinghowthevarioustechniquesdescribedinpreviouschapterscanbecombinedtogethertoinvestigateparticularproblems,providingpowerfulcomplementaryinformation.Theexampleschosenarenotmeanttobecomprehensive,butillustratehowcharacterizationstudiescanbeofvalueinthedevelopmentofadeeperandbroaderunderstandingofpolymericmaterialsontheonehand,orcanprovideinvaluableinformationthatmaybeusedtoremedyproblemsofapracticalnatureontheother.
14.1THESTRUCTUREOFASPHERULITE
Muchoftheunderstandingofpolymercrystalscamefromstudiesofsolutiongrownsinglecrystals,withtheTEMastheprincipaltoolforcharacterization.Thecrystalorientationwasdeterminedusingelectrondiffraction,andanapproximatemeasurementofthecrystalthicknesswasobtainedfromtheshadowlengthinshadowedsamples.Itwasquicklyrealizedthatthethicknessismuchlessthanthemolecularlengthandthisledtotheideaofchain-foldedcrystals[1-3].Althoughthisdiscoverydidnotprovethatsuchstructuresexistinbulkcrystallizedmaterial,itdidprovidecrucialinsightintopolymercrystalhabitsandisofgreathistoricalimportance.
Manypolymerswereknowntobesemicrystallinelongbeforetheconceptofspheruliticmorphologyevolved.X-raydiffractionshowedcrystalstobepresent,butthehighlevelofscatteringintoregionsfarremovedfromthecrystalpeaksshowsnon-crystallinematerialtobepresenttoo.Densitymeasurementsconfirmthatthematerialmustcontainamixtureofcrystallineandnon-crystallineregions.Thefringedmicellemodel[1,2]wouldappeartobeadequatetoaccountfortheseexperimentalobservations,anditrequiredmoredetailedmicroscopicalexaminationtodeducethetruearrangementofthecrystals.
Polarizedlightmicroscopestudyofthinmeltcrystallizedsamplesshowsthecrystallineregionstohaveradialsymmetry,andthisisdifficulttoreconcilewiththefringedmicellemodel.Thedirectionofbirefringencewithinthespherulitecanbeusedtodeterminethemolecularchainorientationandinmanycommonpolymers,includingpolyethylene,itisfoundtobearrangedtangentially.Thisleadstothemodelofaspheruliteasconsistingofaradialarrayofcrystallinelamellae,withthemoleculeaxeslyingintangentialdirections.
Nexttheconnectionismadebetweentheselamellaeandsolutiongrowncrystals,foritisfoundthatthelamellarthicknessinbulkcrystallizedsamplesliesinthesamerangeasinthinsolution-growncrystals.ThiscanbeconfirmedbyX-raylinebroadeningandSAXDmeasurements.TEMofstainedsolution-castsamplesandstudiesofetchedandstainedsamplesbybothSEMandTEM(ofreplicas)haveprovidedconvincingverificationoftheproposedmodelforspherulitestructure.Thesemethodshavealsorevealedthatthelamellaeoftentwistandthatundercertaincrystallizationconditionsthetwistperiodisveryregular,withalllamellaetwistinginphase.Thisgivesrisetoringsintheimageinthepolarizedlightmicroscope.Inaddition,theworkbyBassettandco-workers[4],usingspecimensstainedbythechlorosulphonationtechniqueandviewedintheCTEM,hasdemonstratedclearlythepresence,insomecases,ofabimodallamellarthicknessdistribution,leadingtostillfurtherrefinementofthemodelforthestructure.ThislastfeaturemightbeinferredfromDSCstudies,buttheTEMprovidesthemostpositivestructuraldetails.
Thusitcanbeseenthatseveraltechniqueshavecontributedtothedevelopmentofourcurrentpictureofaspherulite.Manyofthestudiesmentionedabovewerebasedonspeciallypreparedlaboratoryspecimens,however,andtogetherhelpedtodevelopageneralmodelforthestructureofaspherulite.Itisaquitedifferentmattertodeducethestructureofaparticularsampleofameltcrystallizedpolymer.Forthistaskasuitablestartingpointwouldbetopreparethinmicrotomesectionsforpolarizedlightmicroscopy.Ifthepolymercutssatisfactorilythiswillrevealthesizeofthespherulites.Anestimateoftheaveragespherulitesizeoveralargersamplecouldbemadeusingsmallanglelightscatteringfromthinsections.SAXDcouldthenbeusedtomeasurethelongperiodandwideangleX-raydiffractiontoobtainthelamellarthicknessfromlinebroadeningmeasurements.FordetailedstudystainingfollowedbysectioningforTEMobservationmaybethebestoption.Thechlorosulphonationtechniqueforpolyethylenemaybesuitableforsomeotherpolymer,butotherstainsmayneedtobedeveloped.Similarly,thepermanganateetchingprocedure,followedbyreplicationforTEMstudy,ordirectinspectionintheSEM,isnotsuitableforallspheruliticpolymersandotheretchantsmayneedtobetested.
ItisdisappointingthatverylittlesuccesshasbeenachievedwithdirectdiffractioncontrastimagingofthinsectionsofspherulitesintheTEM.Itwouldbeexpectedthatthistechniqueshouldbecapableofprovidingthemostdetailedandunambiguousinformation,butexperimentalproblemsseemtobedifficulttosurmount.Ifthiscouldbeachievedsuccessfullyitwouldshowwhetheralamellaconsistsofasmallnumberoflongyettwistedanddefectivecrystals,orwhetheritcontainsmanysmallercrystals.
Finally,ifamorecompletecharacterizationofthespherulitesthansimplyamorphologicalassessmentisrequired,thenDSCorDTAcanbeusedtomeasureTg,andthecrystalmeltingtemperature.TMA,IRandNMRareothertechniquesbywhichTgmaybefound.Lightmicroscopycouldbeusedtoinvestigatewhethernucleationhadtakenplaceonaforeignbodywithinthematerial.Anideaofthelamellarpackingcouldbeobtainedbyestimatingfractionalcrystallinity,fromdensityorX-raydiffractionmeasurements.
14.2SKIN-CORECHARACTERIZATIONINMOLDEDPOLYMERS
Injectionmoldedpolymerscontainaskin-coremorphology.Theskinformswhenthepolymermeltcontactsthecoldmouldwalls,freezingveryrapidly,whereasthecorecoolsmuchmoreslowly.Polymermoleculesbecomeorientedduringflow,andtheskinnormallysetstooquicklytopermitmuchrecovery,whereasthecorecoolssufficientlyslowlytopermitsubstantial(sometimescomplete)recoveryandisoftennearlyisotropic.Skinandcorehavequitedifferentmorphologyandpropertiesandtheoverallpropertiesofthemouldingdependonthesepropertiesandonthedivisionofthematerialbetweenthetworegions.Forexample,inasimpletensiletestonpolypropylenetheorientedskinoftenfailsandseparatesfromthecorewhichcontinuestocolddraw.
Againitissensibletostartacharacterizationstudybyexaminingamicrotomesectioninthelightmicroscope.Theskin/coreboundaryisnormallyclearlyvisible,withbestcontrastusuallyinpolarizedlight.Withsemicrystallinepolymersthecoreoftencontainsequiaxedspherulites,whereasthecrystalsintheskinhavepreferredorientationandadifferentmorphology.Sometimestwolayerscanbedistinguishedwithintheskin,andsometimesmorethanthreelayersmaybevisiblebetweenthesurfaceandthecentreofthemoulding.Inthecaseofnon-crystallinepolymerscontrastattheskin/coreboundaryisaconsequenceofaverylargechangeinmolecularorientation.Birefringencemeasurementscanbemadeatthisstagetoinvestigatethestateofmolecularorientationasafunctionoflocationandshouldbeconductedonsectionscutbothtransverseandlongitudinaltotheflowdirection.
Theskinistypicallyoftheorderof0.3mmthickandpartsofitcanberemovedusingamicrotomeforstudyinisolationfromthecore.Alternativelyhighspeedmillingusingasinglepointtoolwithaflycuttingactioncanbeusedtoremovematerialfromaflatmouldedsurface,leavingattheendjusttheskinonthelowerfacemountedonthemachinebed.Samplesofthecorecanbemadebyremovingmaterialfrombothsides.Samplescontainingonlyskinoronlycorecanthenbeusedforarangeofmeasurementsasfollows:
1.Densitymeasurementsinadensitycolumn:
insomenon-crystallinepolymerstheskinisfoundtobesignificantlydenserthanthecore.Thereisevenmorescopeforvariationinsemicrystallinepolymers.
2.DSCmeasurements:
smallyetsignificantdifferencesinTgarefound.Furthermore,enthalpymeasurementsrevealdifferencesthatseemtocorrelatewiththedifferentpressuresprevailingatthedifferentpartsofthemouldingcycleduringwhichthematerialintherespectiveregionssolidified.
3.DMTAmeasurements:
significantdifferencesinYoung'smodulusareobservedbetweenskinandcore,andmeasurementsparallelandperpendiculartoflowrevealanisotropyintheskin.ThedifferenceinTgbetweenskinandcoreisconfirmed.
4.TMAmeasurements:
differencesinorientationleadtodifferencesinlinearthermalexpansioncoefficient:
theskinagainshowsanisotropy,andasignificantdifferenceexistsbetweenskinandcore.Thiswillcauseresidualstressestodevelopwhenthetemperatureofthemouldingchangesinservice,andmayleadtodistortionorwarping.
5.WideangleX-raydiffraction:
thiscanbeusedtorevealdifferencesincrystallinityandcrystalorientation,andwiththosepolymersthatarepolymorphicmayshowthatdifferentcrystalstructuresdevelopinthetwodifferentregions.
6.FTIR:
measurementofthedichroicratiocanbeusedasanindicationofpreferredorientation.
Becauseoftherapidcoolingratethematerialintheinjectionmouldingsolidifiesintoanon-equilibriumstateanditscharacteristicschangegraduallywithtimeaftermoulding('ageing').Thiscanbefollowedbymakingmeasurementsatintervalsusingseveralofthemethodslistedabove.Densitychangescanbemonitoredsimp
- 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:
- Multiple
![提示](https://static.bdocx.com/images/bang_tan.gif)