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