Stereocomplexed PLA nanocomposites_ From in situ polymerization to materials properties

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G.L.Reetal./EuropeanPolymerJournal54(2014)138–150139

weightPLAisobtainedbyring-openingpolymerizationoflactide(LA).Duetothepresenceoftwochiralcenters,LAexistsastwoopticallyactivestereoisomers:D,D-lactide(D-LA)andL,L-lactide(L-LA).The?nalpropertiesofPLAde-pendmoreonthecontentofopticalimpuritiesbetweenbothLAenantiomerswithinPLAchainsratherthanitsmolecularweight.ForcontentinD-lactidehigherthanca.7%,theresultingPLAmaterialsareamorphouswithaglasstransitiontemperature(Tg)ofaround55°C[6,7].Incon-trast,PLAhomopolymerspreparedfromopticallypureL-LAorD-LAaresemi-crystallinepolyesterswithameltingtemperature(Tm)ofabout175°CandaTgofca.55–60°C.Themechanicalpropertiesofsemi-crystallineP(L-LA)aresuitableforshort-timeapplicationslikefood-packaging[8–13].

However,themainissueaboutPLAconcernsitslowcrystallizationrateanddegree,impedingitsindustrialimplementationforlong-lastingapplicationslikeautomo-tiveandelectronics[14,15].Inordertospantherangeofpossibleapplications,severalstrategieshavebeenat-temptedforPLAtoenhanceitscrystallizationrate,namelybyaddingnanosized?llersorviastereocomplexation.PLAstereocomplexescanbereadilyformedbymixinganequivalentratioofPLLAandPDLAchains.DuetothestrongvanderWaalsinteractionswithinthestereocomplexcrys-tallinestructure[16–18],Tmofstereocomplexesreachesmeltingtemperaturearound230°C,whichisapproxi-mately50°ChigherthanTmofeitherPLLAorPDLAhomo-polyesters[19].Forthisreason,stereocomplexationisusedtoenhancethecrystallizationrate,thethermal-resistance,http://wendang.chazidian.comy-eredsilicate/PLAnanocompositesgenerallyobtainedviamelt-blendingarealsoconsideredimprovingthermal,mechanicalandtransportingpropertiesofPLAmatrix[20–26].Duetothenanosizeddimensionsofthelayeredsilicate,theseimprovedpropertiesaregenerallyreachedatlowsilicatecontent(lessthan5wt%).IntercalationofPLAchainsintosilicategalleriesandevenclayexfoliationthroughoutthepolyestermatrixareobtainedusingade-quatelyorganomodi?ednanoclay,speci?callyCloisite30B(Cl30B)[27–29].

Inthepresentwork,bothstrategies,i.e.theadditionoflayeredsilicatenano?llersandstereocomplexationwerehenceinvestigatedtocooperativelyenhancetherigidity,barrierandthermalproperties,andthecrystallinityextentofPLA-basednanocomposites.Inaddition,weinvestigatedCl30Basnano?llerstocarryouta‘‘graftingfrom’’approachfortheenhancementofaf?nitybetweennano?llersandthematrix[27,30,31].Itconsistedtopromotering-openingpolymerization(ROP)ofLandD-lactidefromthetwohydroxylmoietiesofthequaternaryammoniumavailableatthesurfaceoftheorganomodi?edclayforthesynthesisofCl30B-graftednanohybrids.TopromoteROP[32,33]ofL(D)-lactidemonomers,1,8-diazabicyclo[5,4,0]undec-7-ene(DBU)wasemployedasmetal-freecatalyst,affordingasustainablesolutionratherthanusingcurrentorganometalliccatalystsliketin(II)octoate[34–36].Theseresultingnanohydridsweresubsequentlymelt-blendedwithPLAuponstereocomplexationornotinordertoassesstheirthermal,mechanicalandbarrierpropertiesbyDSC,TGA,DMTAandgaspermeationanalyses.

2.Experimentalpart2.1.Materials

Polylactide(PLA)waskindlyprovidedbyFuterroS.A.,Belgium(0.4%molinD-lactidecontent,number-averagemolecularweightofca.100,000g/mol).L-lactideandD-lac-tideweresuppliedfromPurac,respectively,recrystallizedfromdriedtolueneandstoredinaglovebox.1,8-Diazabicy-clo[5,4,0]undec-7-ene(DBU)(Fluka)wasdriedoverBaO,distilledandstoredinaglovebox.CloisiteÒ30B(Cl30B),amontmorilloniteorganomodi?edby23.4wt%ofmethylbis(2-hydroxyethyl)tallowalkylammoniumcationswassuppliedbySouthernClayProducts(USA).AmberlystÒ15,ion-exchangeresin,inhydrogenform,wassuppliedbyFluka,andusedovendriedovernightat60°C.

2.2.Characterization

GelPermeationChromatography(GPC)wasperformedinCHCl3at35°CusingaPolymerLaboratoriesliquidchro-matographequippedwithaPL-DG802degasser,aniso-craticHPLCpumpLC1120(?owrate=1ml/min)refractiveindex(ERMA7517),anautomaticinjector(Poly-merLaboratoriesGPC-RI/UV)andthreecolumns:aPLgel10lmguardcolumnandtwoPLgelMixed-B10lmcol-umns.PSstandardswereusedforcalibration.

Attenuatedtotalre?ectance-Fouriertransforminfrared(ATR-FTIR)spectrawererecordedonpolymersamples,usingaBIO-RADExcaliburspectrometerequippedwithanAttenuatedTotalRe?ectance(ATR)HarrickSplitPeaTM.Unlessspeci?ed,theotheranalyseswerecarriedoutontheinjection-moldedsamples.Themorphologyofthenano-compositeswasanalyzedbyWide-AngleX-rayDiffraction(WAXD)andtransmissionelectronicmicroscopy(TEM).WAXDpatternswererecordedbetween1.65°and30°(bystepsof0.04°)withaSiemensD5000diffractometer(Germany)operatingwithCuKaradiation(k=1.5406Å).ForTEManalyses,thesampleswerecryomicrotomedatÀ100°CwithanUltracutFC4EmicrotomefromReichert–Jung.AtruncatedpyramidwascutinthesampleandultrathinsectionsofthepyramidwereremovedforTEManalyses.TheTEMimageswererecordedwithaPhilipsCM100.

DSCQ2000fromTAinstrumentwasusedforDifferen-tialScanningCalorimetry(DSC)analysis.Samples(weigh-ing$10mg)weresealedinaluminumDSCpans(Tzero)andplacedintotheDSCcell.TheDSCanalyzerwascali-bratedwithindium.SampleswereheatedfromÀ20°Cto230°Cataheatingrateof10°C/min.Thermogravimetricanalyses(TGA)werecarriedoutwithaTGAQ5000appara-tusfromT.A.Instrumentsundernitrogenorair?ow(heat-ingrate:20°C/min).

ForDynamicMechanicalThermalAnalyses(DMTA)injection-molded(35Â12Â2mm)specimensofeitherun?lledPLAorPLA-Cl30Bbasednanocompositeswerepre-paredaccordingly(specimenscollectedafterextrusionwereinjection-moldedintorectangularbarsfor3minat190°C–at220°Cforstereocomplexes),underapressureof900MPaandthenrapidlycooledtoroomtemperature.

Y

GOLONHCETONANRALUCELOMORCAM

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140G.L.Reetal./EuropeanPolymerJournal54(2014)138–150

Y

GOLONHCETONANRALUCELOMORCAMThebarswerecharacterizedforDTMAwithaDMA800fromTAInstrumentfromÀ20°Cto140°Con,accordingtoASTMD5026–06.MeasurementswerecarriedoutinMulti-FrequencyModeusingadualcantileverat1Hzwithdeformationamplitudeof20lm.Theheatingratewasof3°C/min.DSCandDMTAanalyseswereperformedoneweekafterpreparation.

TransportpropertiesexperimentswereperformedusingthegaspermeabilitytesterPERMEÒVAC-VBSpro-videdbyLabthinkÒ(China)forcompression-moldedcircu-lar?lmsusingaCARVER(Wabash,IN)laboratorypress(190°C,5min,thenquenchedat50°C.Forcompression-moldedstereocomplex-containingsamples,thetempera-turewasincreasedto220°Cforthelast2minprocessingtime).Testingisbasedonthedifferentialpressuremethodtodeterminethegastransmissionrateof?lmsaccordingtotheGB/T1038-2000standards.Thetemperaturewascontrolledat23±0.1°Cbyaconstanttemperaturecontroldevice.Beforebeginningthegaspermeationexperiments,thesamplewasexposedtocontrolledroom(temperatureandmoisture)foratleast48h.Themeasurementswerecarriedoutconsideringamonolayermembrane,whosethicknessisoflandthesurfaceA,submittedtoapermeantandQthetotalamountofpermeant,whichtransferthroughthismembraneduringatimet.Inthesteadystate,thequantityofpermeant,whichpassedacrossthemem-braneisJthe?uxdensity,givenby:

J¼Q=ðAÂtÞð1Þ

The?uxcanberelatedtotheaverageconcentration,C,onbothsidesofthemembranethankstokthetotalmasstransfercoef?cientacrossthemembrane.

Q=t¼kÂðC1ÀC2ÞwhereC1>C2ð2Þ

Thetransportpropertiesmeasurementsarebasedontheintegralpermeationmethod,whichwasperformedinaclosedchamber.Theoxygentransmissionratecurvereachesasymptoticallyamaximumvalueafteralongtimeandthenthepermeancecanbedetermineduponthetime-lagmethodfrommanometricdata.Thepermeancewasob-tainedbyinstrumentsoftwareusingcm3/m2dPaunit.Datawereaveragedonthreesamples.Firstmeasurementswereconductedonreference?lmprovidedbyLabthinkÒandtherelativeerrorofpermeancewasfoundtobelessthan10%.

2.3.Insituintercalativering-openingpolymerizationof(L,D)-lactidetosynthesizeCl30B-g-PL(D)LAnanohybrids

PLLAorPDLAweregraftedontheclaysurfacebyROPofL-LAorD-LAinthepresenceofDBUasmetal-freecatalyst.Inaglovebox,ymmolofL(D)-LAweresolubilizediny/2mLofdryCHCl3andkeptundernitrogenatmosphereandtheamountofCl30B(3or6wt%)werethenadded.Then,0.25wt%ofDBUwasequimolarlyaddedinfunctionofthehydroxylcontentpresentwithintheclayatroomtemperature.After60min,AmberlystÒ15ion-exchangeresinwasaddedtoquenchtheLApolymerization.Thenanohybridwasrecoveredafterprecipitationin8volumesofn-heptane,?lteredanddriedovernightat55°C.Formolecularcharacterizations,theresultingnanohybrids

wereextractedbySoxhletextractionfromdichlorometh-aneovernight.Thesolidresidue(fromthimble)wasdriedundervacuumat50°Covernight,whereasasolidfractionwasrecoveredafterevaporatingdichloromethanefromthesupernatantunderreducedpressureovernight.

2.4.PreparationofPLAnanocompositesbymeltintercalationPLAlayeredsilicatenanocompositeswerepreparedbymeltblendingthePL(D)LA-basednanohybrids(orPDLA-basednanohybrids)withPLAusingaDSMtwin-screwmicrocompounderoperatingat190°Cfor10minandatarotationspeedof60rpm.Allnanocompositeswereelab-oratedforaninorganiccontentof3%,correspondingtothebestnanoclaycontentasreportedelsewhere[27,37].Forsakeofcomparison,melt-blendingbetweenbothPLLA-andPDLA-basednanohybridswascarriedout.Forblendingstereocomplexes-basedmaterials,thetemperaturewasin-creasedto220°Cforthelast2minprocessingtime.Thecollectedsampleswereinjection-moldedintorectangularbarsorcompression-molded,dependingonthetypeofcharacterization(seeSection2.2).Forthesakeofcompar-ison,abinaryPLA/PLLAmadeofentirelyL-con?gurationsystemwaspreparedbymelt-mixingthecommerciallyavailablePLAmatrixwithPLLArecoveredafterevaporatingdichloromethanefromthesupernatantoftheSoxhlet.Ter-naryblends,i.e.PLA/PL(D)LA/Cl30Bwiththesamenano-claycontentwerealsoproducedintheDSMtwin-screwmicrocompounderat190°Cfor10min.,atarotationspeedof60rpm,increasingthetemperatureto220°Cforthelast2minprocessingtimeforthePDLA-basedblend,asde-scribedinthecaseofstereocomplex.Table1summarizesthedifferentcompositionsofPLA,therelatedblendwithPLLAandnanocompositescontaining3wt%ofinorganicsincluding.Theclaycontentwithineachcompositewasas-sessedbyTGA.

ItisworthnotingthatfortheblendmadeofCl30B-g-PLLAandCl30B-g-PDLAwepreparedananocompositecontainingonly3wt%inorganiccontent(Cl30B-g-PLLA/Cl30B-g-PDLA(8)).Allnanocompositeswerecarriedoutfromnanohybridscontaining6%ininorganiccontent,ex-ceptforthestereocomplex(8).Thelatterwaspreparedfromthenanohybridscontaining3%ininorganiccontentassummarizedinTable1.3.Resultsanddiscussion

3.1.Insituintercalativering-openingpolymerizationofL(D)-lactide

Well-de?nedCl30B-g-PLLA(6)andCl30B-g-PDLA(7)nanohybridswerepreparedbyrespectiveDBU-catalyzedringopeningpolymerization(ROP)ofL-lactideandD-lac-tideinitiatedfromthehydroxylmoietiesofquaternaryammoniumcationsintercalatedinthegalleriesofCl30B,assketchedinScheme1.Thesenanohybridswereelabo-ratedataninorganiccontentofeither3or6wt%andtheinorganiccontentwascon?rmedbyTGAasshowninFig.1.Bothnanohybrids(PLLAorPDLA)with3%or6%inor-ganiccontenthavecomparablethermalpropertiesas

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G.L.Reetal./EuropeanPolymerJournal54(2014)138–150141

Table1

Summaryofdifferentblendcomposition,includingPLAandPLAstereocomplex.Allnanocompositeswerecarriedoutusingnanohybridscontaining6%ininorganiccontent,exceptforthestereocomplex(8),thatwaspreparedfromthenanohybridscontaining3%ininorganiccontent.

SamplePLA(g)Cl30Ba(g)Cl30B-g-PDLAorPDLAb(g)Cl30B-g-PLLAorPLLAb(g)Targetedinorganiccontentc(%)PLA(1)15––––

PLA/PLLA(2)7.5––7.5c–

PLA/Cl30B(3)14.40.6––3.0

PLA/PLLA/Cl30B(4)7.20.6–7.23.0

PLA/PDLA/Cl30B(5)7.20.67.2–3.0

shownbytheirrespectiveglasstransition(Tg),meltingingthesecondheatingscaninthethermogramswhen(Tm)andcoldcrystallizationtemperatures(Tc,Fig.2).TheDBUwasusedascatalyst.Indeed,ithasbeenreportedthatdoublemeltingpeakobservedinlowmolecularweighttheDBUcatalystcanadverselypromotetransesteri?cationPLAisfairlycommonasascribedtothemeltingandrecrys-reactionsaswellasepimerizationreactionsathightem-tallizationeventsonheating[38,39].Asecondpossibleperature[41].InthisworkDSCanalysescon?rmedtheexplanationofthedoublepeakcouldbeascribedtoapoly-crystallinepropertiesfortheresultingnanohybrids,high-morphismofthetwodifferentcrystalformsofpolymor-lightingtheef?ciencyoftheion-exchangeprocesstore-phicPL(D)LA(a-anda0-formcrystals)[40].moveDBU.

Itisremarkabletopointoutthatnomodi?cationre-Theeffectivenessofthe‘‘graftingfrom’’approachfromlatedwithTmandcrystallinityofPLLAwasobserveddur-Cl30BwasalsoassessedbyATR-FTIRanalyses,carriedYGOLONHCETONANRALUCELOMORCAM

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