Study on crystalline morphology of poly(L-lactide)-poly(ethylene glycol) diblock copolymer
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Studyoncrystallinemorphologyofpoly(L-lactide)-poly(ethyleneglycol)
diblockcopolymer
JingruSun,ZhongkuiHong,LixinYang,ZhaohuiTang,XuesiChen*,XiabinJing
StateKeyLaboratoryofPolymerPhysicsandChemistry,ChangChunInstituteofAppliedChemistry,ChineseAcademyofScience,RenminStreet5625,Changchun,Jilin130022,People’sRepublicofChina
Received2December2003;accepted1June2004
Abstract
Crystallizationbehavior,structuraldevelopmentandmorphologyevolutioninaseriesofdiblockcopolymersofpoly(L-lactide)-block-poly(ethyleneglycol)(PLLA-b-PEG)wereinvestigatedviadifferentialscanningcalorimetry,wide-angleX-raydiffraction,polarizedopticalmicroscopyandatomicforcemicroscopy.Inthesecopolymers,bothblocksarecrystallizableandbiocompatible.ItwasinterestingthatthesePLLA-b-PEGdiblockcopolymerscouldformspheruliteswithbandedtextures,whichwasundercoolingdependent.SinglecrystalswithanabundanceofscrewdislocationswerealsoobservedviaAFM.SuchresultsindicatedthattheseringedspherulitesandsinglecrystalswereformedduringthecrystallizationofthePLLAblocks.q2004ElsevierLtd.Allrightsreserved.
Keywords:Crystallinemorphology;Poly(L-lactide);Poly(ethyleneglycol)
1.Introduction
Recentlycrystallizationandmorphologydevelopmentof
blockcopolymershavebeeninvestigatedwidely[1–4].The
processofcrystallizationisofimportancetotheapplication
ofasemi-crystallinecopolymer.Forexample,thefinal
opticalandmechanicalpropertiesofapolymerare
determinedbythesizeofthespheruliteandoverall
crystallinity,whicharealsoinfluencedbythetypeof
nucleation,thegrowthrate,andthecharacteristicsofthe
individualfoldedchains.Thespherulitesofcertain
semicrystallinepolymersexhibitregularlyspaced,circular
birefringenceextinctionpatternsunderapolarizedoptical
microscope,andthusaretermed‘ring-banded’spherulite.
Thepurepoly(3-caprolactone)(PCL)cangrowbanded
spherulitesatatemperaturehigherthan323Kwithlarge
radicalseparationsintherangeof50–100mm[5].Ring-
bandedspherulitemorphologyofPCLinPCL/ethyl
cellulose(EC)blendshasbeenobserved[6],andFouriertransforminfraredmeasurementsshowthataspecific
interactionexistsbetweenthecarbonylgroupsofPCLand
thehydroxylgroupsofthecellulose.Suchspecific
interactionandthepartialcompatibilitybetweenPCLand
amorphousECareinfavoroftheformationofring-banded
spherulitesinthePCLblends.Furthermore,thereareonlya
fewstudiesonblockcopolymerscomposedoftwocrystal-
lizableblocks.InthefirstsuchstudybySkouliosetal.[7],
thestructureofatriblockcopolymerpoly(3-caprolactone)-
block-poly(ethyleneoxide)-block-poly(3-caprolactone)
(PCL-b-PEO-b-PCL)wasinvestigated.Intheselinear
blockcopolymers,bothblockswereabletocrystallize.In
amorerecentstudy[8],Shiomiobservedaunique
morphologyofspherulitesimilartothatofring-banded
spherulites,namelydoublespheruliteslikeconcentric
circles,forPCL-b-PEO-b-PCLtriblockcopolymer.The
centralandoutersectionsintheconcentriccircleswere
thoseofPCLandPEO,respectively.
KeithandPadden[9,10]proposedthatthemechanism
leadingtotheringpatternisaxialtwistinginlamellaeunder
theinfluenceofsurfacestressandco-operativearrangement
oftwistedlamellae.Thethermodynamicalcompatibility
andtheexistenceofspecificinteractionbetweenthetwo
0032-3861/$-seefrontmatterq2004ElsevierLtd.Allrightsreserved.doi:10.1016/j.polymer.2004.06.026Polymer45(2004)5969–5977www.elsevier.com/locate/polymer
*Correspondingauthor.Tel.:C86-431-5262112;fax:C86-431-5685653.E-mailaddress:xschen@ciac.jl.cn(X.Chen).componentsarethetwoimportantfactorsfortheformation
oftheringedspheruliteinbinaryblends.Youshifumi
advancedthatthelamellaetwistingseemstobecausedbya
desperateattemptofthelamellatoreleasethecompressive
stressarisingfromtheopposinglamellarfoldsurface[11].
Insomehomopolymers[12,13],thespiralpatternatthe
centeroftheconcentricextinctionwasobserved.Thespirals
areinterpretedasaconsequenceofasymmetricfanningof
sheaves,leadingtothe‘twisting’onadimensionlevel,
whichisresponsiblefortheconcentricbanding.Asimilar
spiralgrowthpatternhasbeenobservedinsomesingle
crystalsaswellashomo-polymerlamellaewithaspiral
structure.Theoriginofsuchspiraltypesinglecrystalsor
lamellaehasbeenattributedtoascrewdislocation
mechanism.
Duringthelastdecade,mostresearcheshavebeen
focusedonthecrystallizationbehaviorofblockcopolymers
withcrystalline–amorphousblocks.Onlyafewstudieshave
beenpublishedonthemorphologyandcrystallization
propertiesofblockcopolymersthatconsistoftwocrystal-
lizablebocks.Inthispaper,ourattentionisfocusedonthe
crystallinemorphologyofpoly(L-lactide)-block-poly(ethyl-
eneglycol)(PLLA-b-PEG)diblockcopolymerinwhich
bothblockscancrystallize.PLLAandPEGareknowntobe
biocompatible.Theirblockcopolymersexhibitdrug
permeabilityanddegradabilityandhavebeenproposed
forawiderangeofmedicalapplication.Soobtainingmore
detailedinformationaboutthecrystallinemorphology