DMF in Percolation regions of during glassy photopolymerization of epoxyacrylate Progress in Organic Coatings

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ContentslistsavailableatScienceDirect

Progress

in

Organic

Coatings

jou rn al h om ep ag e :w w w . e l s e v i e r . c o m / l o c a t e / p o r g c o a t

Percolation

of

glassy

regions

during

photopolymerization

of

epoxy

acrylate

in

DMF

Z. Do˘gruyol

a

_,

_N.

_Arsu

b,∗

_,

_S.

_Keskin

_Do˘gruyol

b

_,

_Ö.

_Pekcan

c,∗∗ a_Department_of_Engineering_Science,_Istanbul_University,_Avcılar_Campus,₃₄₈₅₀_Istanbul,_Turkey b_Department_of_Chemistry,_Yıldız_Technical_University,_Davutpas¸_a_Campus,₃₄₂₂₀_Istanbul,_Turkey c_Faculty_of_Arts_and_Sciences,_Kadir_Has_University,_Cibali,₃₄₃₂₀_Istanbul,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received10April2014

Receivedinrevisedform31October2014 Accepted10November2014

Availableonline16December2014 Keywords:

Photopolymerization Glasstransitionpoint Photoinitiator Solventconcentration Photo-DSC

a

b

s

t

r

a

c

t

Thegelationofepoxyacrylate(EA)80%andtripropyleneglycoldiacrylate(TPGDA)20%wasstudied

through theuseofphoto-differential scanningcalorimetric(photo-DSC)technique inthepresence

of athioxanthone basedinitiator. Photo-induced polymerizationreactions were performed under

identicalconditions oftemperature, initiator concentrationand UVlight intensityinvarious

N,N-dimethylformamide(DMF)contents.Photo-DSCtechniqueallowedustomonitorthegelationwithout

disturbingthesystemmechanically,andtotesttheuniversalityofthegelationasafunctionofDMF

content.Duringgelation,itwasobservedthatallconversioncurvespresentedusefulsigmoidalbehavior

whichwaspredictedbyemployingapercolationmodel.Observationsaroundtheglasstransitionpoint,

tg,revealedthatgelfractionexponentˇobeyedthepercolationpicture.Asigniﬁcantsolventeffecton

thephotopolymerizationkineticsofEA/TPGDAwasobservedwithchangesinDMFcontent.DMFisused

assolvent,whichactsasadiluentandprotondonorduringphotogelation.

1. Introduction

Itiswellknownthatsolventshavesomeeffectonfree radi-calhomo-andcopolymerization[1–3],particularlyonpropagation reaction[4].Thepropertiesofthesolventstronglyinﬂuencethe rateoffreeradicalpolymerizationofvinylmonomers[5]. Beuer-mannetal.studiedthesolventeffectonthepropagationkinetics inradicalpolymerization,intermsofchaintransferreactionswith solvents,polarityandpolarizabilityofthemedium[6,7].Theyalso studiedthechangeofmonomerreactivityasregardsthesolvent, inadditiontoreversiblecomplexformationsbetweenradicaland solventmolecules.Severalstudieshavebeenconductedto exam-inewater-basedpolymerizations[6,8,9],anditwasgenerallyfound thattheyenhancethepolymerizationrateinwatermedia[8,10]. Valdebenito and Encinasstudied the solventeffect onthe free radicalpolymerizationof N,N-dimethylacrylamide[10],through studiesofpolymerizationreactionsinseveralsolventsrangingfrom watertolowpolaritysolventssuchasTHF.

Inphotopolymerizationexperiments,mainlytype I(bybond cleavage)andtypeII(byH-abstraction)initiatorsareemployed

∗ Correspondingauthor.Tel.:+902123834186;fax:+902123834134. ∗∗ Correspondingauthor.

E-mailaddresses:[email protected](N.Arsu),[email protected](Ö.Pekcan).

[11]. Although type I photoinitiators are more effective than type II initiators, type II initiators operating in the visible range are advantageous in terms of energy policies. Recently, mercapto derivative [12], carboxylic acid derivatives [13,14], and morpholine-attached [15] TX initiators were used for the polymerizationofacrylatesandmethacrylatesastypeIIone com-ponentphotoinitiators,andanotherapproachwasundertakenand thioxanthone–benzotriazole was synthesized, which presented stabilizerandinitiatorpropertiesinonecomponent[16].

In previous studies we conducted, the efﬁciency of 2-mercaptothioxanthone (TX-SH) was investigated [12]. A thiol derivativeofthioxanthoneTX-SHphotoinitiatorservesasbotha tripletphoto-sensitizer[17]andahydrogendonorforfree radi-calpolymerization.Themechanismofphotoinitiationisbasedon theintermolecularreactionofthetriplet,3_TX-SH*,_with_the_thiol moietyofgroundstateTX-SH.Theresultingthylradicalinitiates polymerization(Scheme1)[12].

SincethesolubilityofTX-SH athighinitiatorconcentrations createssomeproblems,theadditionofa solventwasbeneﬁcial fortheenhancementoftheinitiator’ssolubility.Itiswellknown thatthemostcommercialphotoinitiatorsarenotsolubleinsome ofthemonomersandoligomers.Moreover,asmanyreactionsin industryarecarriedoutinorganicsolventsratherthaninbulkand manycommercialphotoinitiatorsarenotdissolvedinapresented bulk,itisusefultodeterminetheeffectofthesolventcontenton http://dx.doi.org/10.1016/j.porgcoat.2014.11.010

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Scheme1. Photoinitiatedfreeradicalpolymerizationbyusing 2-mercaptothiox-anthone(TX-SH).

photogelationkinetics.Therearesigniﬁcantlyfewstudiesinthe literaturewhichaddresssolventcontent,andthisstudyintendsto ﬁllthatgap.

Bulkfree-radical polymerizationcanusually bedivided into threedifferentstages:alow-conversionstage,ageleffectstage andaglasseffectstagewhichoccursinthelaststageof polymer-ization[18].Monomerconversionﬁrstincreasesveryslightly,but thenacceleratesbecauseofthegeleffectandtherateofreaction reachesamaximum[19,20],whichisawell-knownphenomenon forsomeofthelinearpolymersandthecross-linkedbulkpolymers [21].

Historically,in the latticepercolation model, monomers are thoughttooccupythesitesofaperiodiclattice.Abondbetween theselatticesitesisformedrandomlywithprobabilityp.Fora cer-tainbondconcentrationpc,definedasthepercolationthreshold, theinfiniteclusterisstartedtoform.Thecriticalexponentsforthe sol–geltransitionaredifferentfromthepointoftheuniversality. Consider,forexampletheexponentsandˇfortheweight aver-agedegreeofpolymerization,DPw,andthegelfractionG(average clustersize)nearthegelpoint,wheretheFlory–Stockmayertheory (so-calledtheclassicalormean-fieldtheory)givesˇ==1, inde-pendentofthedimensionality,whilethepercolationstudiesbased oncomputersimulationsgiveandˇaround1.7and0.43in three-dimension.Thesetwouniversalityclassesforgelationproblemare separateddependingonthechainlengthNbetweenthebranch pointsaswellastheconcentrationofthenonreactingsolvent.For example,thevulcanizationoflonglinearpolymerchains(largeN) belongstothemean-fieldclass;howeverthecriticalpercolation (smallN)describesthepolymerizationofsmall multifunctional monomers.

Itisknownthatthegelationphaseis notatransitioninthe thermodynamicsense,butisrathergeometrical.Asthesubjectof criticalphenomenon,itbehaveslikeasecondorderphase transi-tionconstitutingauniversalclassbyitself.Theexactsolutionof gelationwasgivenﬁrstbyFloryandStockmayer[22,23]ona spe-ciallatticecalledtheBethelattice,onwhichtheclosedloopswere ignored.Analternativetothechemical-kinetictheoryisthelattice percolationmodel[24]inwhichmonomersarethoughttooccupy thesitesofaperiodiclatticeandthechemicalbonds correspond-ingtotheedgesrandomlyjointhesesiteswithsomeprobability p,where pistheratiooftheactualnumberofbondsthathave beenformedbetweenthemonomerstothetotalpossiblenumber ofsuchbonds.Thegelpointcanbeidentiﬁedwiththepercolation

thresholdpcwhere,inthethermodynamiclimit,theincipient inﬁ-niteclusterstartstoform,andthesystembehavesinamannerthat isviscoelasticallyrigid[25].

Thepredictionsofthesetwotheoriesaboutthecritical expo-nents for gelation are different from the point of universality. Consider,forexample,theexponentˇforthegelfractionG(the strengthoftheinﬁnitenetworkinpercolationlanguage)nearthe gelpoint,whichisdeﬁnedinEq.(1)

G∝(p−pc)ˇ,p→p+c (1)

where the Flory–Stockmayer theory (the so-called classical or mean-ﬁeldtheory)givesˇ=1whichisindependentof dimension-ality,whilethepercolationstudiesbasedoncomputersimulations give ˇ around 0.43 in three-dimensions [22,23,26]. These two universality classes for the gelation problem are separated by a Ginzburg criterion [27] that depends upon the chain length betweenthebranchpointsaswellastheconcentrationofthe non-reactingsolvent.Criticalpercolationdescribesthepolymerization ofsmallmultifunctionalmonomers[24,25,28].

Critical exponents at the glass transition in free-radical crosslinking copolymerization have been studied for different monomericsystems.TheﬂuorescencelifetimesofPywereused tomonitorthegelation process, where thechanges in the vis-cosityofthepregelsolutionsduetoglassformationdramatically enhancetheﬂuorescentyieldofaromaticmolecules.Thiseffect isusedtostudytheglasstransitionupongelationofthese sys-temsasafunctionoftime,atvarioustemperaturesandcrosslinker concentrations,wheretheresultswereinterpretedintheviewof percolationtheory.Thegelfractionandweightaveragedegreeof polymerizationexponentsarefoundtobeinagreementwith per-colationresults.

Findingsinthisworkaresimilartotheﬁndingsinour previ-ousworks[29,30]wheremethylmethacrylate(MMA)andethyl methacrylate(EMA)andtheirmixtures(MMA–EMA)producedthe resultsaccordancewiththepercolationmodel.Bulklinearpoly methylmethacrylate(PMMA)alsogavethesimilarﬁndings[31] aswasobservedherewithEAandTPGDA.Fromherewepredict andconcludethatacrylateresinsmostprobablyobeythe percola-tionpicturewhereglassyregionspercolateduringgelationasgiven inthecasesofEAandTPGDA.

Recently,ithasbeenshownthatphoto-DSCcanbesuccessfully appliedtomeasurethecriticalexponentsduring photopolymeriza-tionreactionsofepoxyacrylateandtripropyleneglycoldiacrylate mixturesneartheglasstransitionpoint[32,33].

Inthisstudy,ouraimistounderstandtheuniversalbehavior ofgelationandtheinﬂuenceofphotopolymerizationkineticsin thepresenceofvariousamountsofDMF.Thecurecharacteristics ofEA/TPGDAwereanalyzedintermsofsolventcontent.DMFwas chosenfortworeasons:ﬁrstly,ourcomponenttypeIIinitiatoris easilysolubilizedevenathighconcentrationsinDMF,andsecondly, thisapproachmakesitpossibletoseethesolventeffectontherate ofpolymerizationbymeansofcontrollingsuddenincreasesinthe viscosityofthemedium.Inparticular,controllingauto-acceleration mayhelpexplainthepercolationofglassyregions.

Inordertounderstandthephysicalbehaviorofthe polymer-ization processes underlying gelation, one may follow reaction kinetics through experiments utilizing ﬂuorescence [20] and photo-DSC[32–35]techniques,whichdonotmechanicallydisturb thesystem.

2. Experimental

2.1. Materialsandmethod

2-Mercaptothioxanthone [12] was synthesized according to the previously described procedure. Dimethylformamide DMF

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(99+%, Aldrich) was distilled over CaH2 under reduced pres-sure.Epoxyacrylate(EA)andtripropyleneglycoldiacrylate(TPGDA) wereobtainedfromCognisFrance.

The photoinitiated polymerizationof EA 80wt% and TPGDA 20wt%wascarriedoutinvariousamountsofDMF15,20,25,30, 35and50wt%byTA-DSCQ100equippedwithamediumpressure mercuryarclamp.Thisunitemitsradiationpredominantlyinthe 220–400nmrange,andprovidesalightintensityof40mW/cm2 asmeasuredbyaUVradiometercapableofbroadUVrange cov-erage.Themassofthesampleswasapproximately2±0.1mgand themeasurementswerecarriedoutinisothermalmodeatroom temperatureunderanitrogenatmospherewithnitrogenﬂowof 50mL/min.Thesampleswereirradiatedfor100satroom tem-perature.Measurementswererecordedatasamplingintervalof 0.05s/point. Thethicknessofthecured thinﬁlmswas approxi-mately0.20±5mm.

Heatﬂow(W/g)asafunctionofreactiontimewasmonitored usingphoto-DSCunderisothermalconditions,andboththerateof polymerization(s−1)andconversion(%)werethencalculatedfrom theheatﬂowvaluesasafunctionoftime.Theheatofthe reac-tionvalueHtheory

p =86kJ/molwasusedasthetheoreticalheat evolvedforacrylatedoublebonds[36].Ratesof polymerization werecalculatedaccordingtoEq.(2)

Rp=

_dC dt

= dH/dt Htheory 0 (2)

whereHisthereactionheatevolvedattimet,Htheory

0 isthe the-oreticalheatforcompleteconversionandCisthepercentageof conversion.

3. Resultsanddiscussion

Photo-DSCisauniquemethodforobtainingmoreinformation about the photopolymerization system,and photo-DSC experi-mentsarecapableofprovidingkineticsdatainwhichthemeasured heatﬂow can beconverted directlytothe ultimatepercentage conversionandthepolymerizationrateforagivenamountof for-mulation[32].

Inordertorevealtheeffectofsolvent(DMF)contenton pho-topolymerization,amixtureofEA+TPGDAformulationwaschosen tobeinitiatedbyTX-SH.Itiswellknownthatepoxyresinsprovide anexcellentcombinationofproperties,suchashighabrasion resis-tance,verylowshrinkageduringandaftercure,excellentadhesion tomostconstructingmaterials,highmechanical strengthand a widerangeofcureschedules[37].

Photopolymerizationreactionswereperformedunderidentical conditionsoftemperature,initiatorconcentration,massof sam-ples,nitrogenﬂowrateandUVlightintensity.InFig.1therate of polymerizationcurves versus reactiontime for variousDMF contentsarepresented.

DMFcontent in the formulations changed from15% to 50% (w/w).Ascan beseen fromFig.1, thelowest concentrationof DMFformulationledtothehighestrateof polymerization.The initialviscosityofaphotocurablesystemhasastrongimpacton polymerization, and the initialviscosityof the reaction system determinestheinitialpolymerizationrate;thegreaterthe viscos-ity,thehighertherateofpolymerization.Ourresultsconﬁrmed this,andanincrease insolventconcentrationinverselyaffected therateofpolymerization(Figs.1and7).

However,thehighestﬁnalconversionvalue,Cs,wasobtained whenthesolventcontentwasincreasedto25–30%,andnearly50% conversionwasachieved.Thegapbetweenthehighestrateof poly-merization(33_×10−3s−1)andthelowestrateofpolymerization (16×10−3s−1)wasreduced,whiletheCsvaluesdidnotchangeto thesameextent.Csvaluewascalculatedas45%whereformulation

Fig.1. RateofpolymerizationspectraofphotopolymerizationofEA/TPGDAwith variousphotoinitiator(TX-SH)concentrationsirradiatedat25◦_C_by_UV_light_with

anintensityof40mW/cm2_.

Table1

Experimentallyobservedparametersmeasuredby“photo-DSC”andcalculatedvia the“percolationtheory”duringdiacrylate,EA/TPGDA,andphotopolymerization withvariousDMFcontent.

[DMF](%) tg(s) 10−3×Rpmax(s−1) Conv.attg(%) Cs(%) ˇ 15 2.4 33.0 5 45 0.551 20 2.5 28.4 5 49 0.554 25 2.5 26.7 4 52 0.554 30 2.6 25.7 4 52 0.564 35 2.3 23.9 4 47 0.568 50 3.8 15.7 4 41 0.558

consistsof15%DMFwhenDMFcontentwasincreasedto50%for

theformulationCsvaluegraduallydecreasedto41%(Table1).

AsseenfromFig.2,whenthereactioncontinues,theincreased cross-linkinglevelmayeventuallylimitmonomermobilityandthe propagationandterminationreactionmaybecomediffusion con-trolled.Moreover,theﬁnalconversiondecreaseswithincreasing initialviscosity.Thisisbecausetheresinsystemwithahigher sol-ventcontentleadstoamoreburiedmonomerandconsequentlya lowerconversionvalue.

Aspartofourcontinuinginterestinthepercolationmodelof gelation (photopolymerization),we studiedthechangingof the parametersof thephotopolymerization. Interestingly,these sig-moidalcurves(conversion%)aretypicalforthegelationprocess

Fig.2.ConversionspectraofphotopolymerizationofEA/TPGDAwithvarious pho-toinitiator(TX-SH)concentrationsirradiatedat25◦_C_by_UV_light_with_an_intensity

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Fig.3.Doublelogarithmicplotoftheconversionversustimecurvesabovetgfor

variousphotoinitiator(TX-SH)concentrations.

predictedbythepercolationmodel,whichcanbeelaboratedas follows:ingelationtheorytheconversionfactor,p,alone deter-minesthebehaviorofthegelationprocess,thoughpmanydepend ontemperature,concentrationofmonomers,viscosityofmedium andtime.

Thevolumefractionoccupiedbythetotalnumberofmonomers incorporatedintotheglassyregionsasthe‘occupationprobability’, p,ofthesiteshasathreedimensionallattice.Inaglassyregionthe motionofthemonomeriscompletelyrestricteddueto vitrifica-tion.Theseglassyregions,whichmaybeconsideredtheinitiation centersofvitrificationonamicroscopicscale,willgrowintimeas thepolymerizationproceeds.Furthermore,aspercolatepcreaches thecriticalexponent,ˇcanbemeasuredbyusingthedouble loga-rithmicplotsoftheconversionversus(t−tc).Thecriticalexponents wereproducedfromtheslopeofthestraightlinesduringthefitting ofdatainFig.3.

TheconversioncurvesproducedfromFig.1byEq.(2)areshown inFig.2againstthereactiontime.Thesigmoidalconversioncurves inFig.2aretypicalforagelationprocesspredictedbythe percola-tionmodel,whichcanbeelaboratedasfollows:ingelationtheory theconversionfactor,p,alonedeterminesthebehaviorofthe gela-tionprocess,thoughpmaydependontemperature,concentration ofmonomers,andtime.

Iftheparametersthatcanchangethereactionkineticslike tem-perature,concentrationandlightintensityetc.arekeptﬁxed,then pwillbedirectlyproportionaltothereactiontime,t.This propor-tionalityisnotlinearoverthewholerangeofreactiontimebutit canbeassumedthatinthecriticalregion,i.e.,aroundthecritical point,|p−pc|islinearlyproportionalto|t−tc|.Therefore,below thegelpoint,i.e.,fort<tc,conversionmeasurestheweightaverage degreeofpolymerizationoraverageclustersize.Abovetc, how-ever,conversionmeasuressolelythegelfractionG,thefractionof themonomersthatbelongtothemacroscopicnetwork.

Inthissection,wetriedtointerpretourresultsbyconsidering thequasi-staticpropertiesofthegelneartheglasstransitionpoint inthelanguageofpercolation[30].Heretheglassyregionisdefined asaregionofsufficientlyhighviscositysuchthatitinhibitsthe motionofmoleculesonashorttimescale.Thesamestudiesinthe literatureproposethatthe3Dglasstransitioniscontrolledbythe percolationofsmalldomainsofslowdynamics,whichallowsus toexplaintheheterogeneousdynamicsclosetotheglass transi-tion[38,39].Theseauthorshavesuggestedthatdomainsofslow dynamicspercolateatalowertemperatureinthequasi-2Dcaseof thinsuspendedpolymerfilmsandtheycalculatedthe correspond-ingreductionintheglasstransitiontemperatureinquantitative agreementwiththeexperimentalresults.Hereitisassumedthat

Fig.4. Doublelogarithmicplotoftheconversionversustimecurvesabovetgfor (a)0.02%,(b)0.10%and(c)0.50%TX-SHconcentration,respectively.Thevaluesofˇ exponentweredeterminedfromtheslopeofthestraightlines.

thevolumefractionoccupiedbythetotalnumberofmonomers incorporatedintotheglassyregionsarethe‘occupation probabil-ity’,p,ofthesitesofathreedimensionallattice.Inaglassyregion, themotionofamonomeriscompletelyrestricteddueto vitriﬁca-tion.Theseglassyregions,whichmaybeconsideredtheinitiation centersofvitriﬁcationonthemicroscopicscale,willgrowintime aspolymerizationproceeds,andpercolateaspcisreached.

Thecriticalexponent,ˇ,canbemeasuredbyusingthedouble logarithmicplotsoftheconversionversus|t−tc|.Thecritical expo-nentswereproducedfromtheslopeofthestraightlinesduring ﬁttingofthedatainFig.3.

Heretheimportantproblemwastheprecisedeterminationof theglasstransitionpointand thecriticalregion.Inparticular,a smallshiftintcresultsinlargeshiftsinthecriticalexponent.Such alog–logplotrevealsthatthedatashouldbeparticularlyaccurate nearthegelpoint.Usuallythecriticalpointcanthenbedetermined byvaryingtc insuchawayastoobtaingoodscalingbehaviour overthegreatestrangein|t−tc|,iftheexperimentsareperformed againsttime.Here,thetimecorrespondstothemaximumofthe rateofpolymerizationchosenasthecriticaltime,tc,whichmaybe consideredastheglasstransitionpoint,tg,forthegelationunder consideration.Theplotsoflogconversionversuslog|t−tg|above tgforthreedifferentDMFcontentgelsareshowninFig.4a,band c,respectively,wheretheslopesofthestraightlinesproducedthe gelfractionexponent,ˇ.

Theˇvalues produced forgelation arepresentedin Table1 togetherwithtg andRpmaxvalues.Here itshouldbenotedthat theaveragevalue(=0.55ofproducedˇvaluesabovetg)strongly suggeststhattheglassyregionspercolateduringgelformationfor allthesamplesunderconsideration,bypredictingthattheybelong tothesameuniversalityclass.

TheplotsoftgandRpmaxversustheDMFcontentarepresented inFigs. 5and 6,respectively. Itis seenin Fig.5that thereis a longdelayinglasstransitionduringgelationofEA/TPGDA with thehighestamountofDMF,whichalsoresultsinaverylowRpmax valuecomparedtotheothers.Asexpected,itcanbeclearlyseen

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Fig.5.Theeffectofphotoinitiator(TX-SH)concentrationontgvalueforEA/TPGDA.

Fig.6. Theeffectofphotoinitiator(TX-SH)concentrationontherateof photopoly-merizationforEA/TPGDA.

inFig.6thatincreasingthesolventcontentdilutedthemonomer concentration,henceslowingdownthepolymerizationrate[40].

Theseresultsareexpectedbecauseglassyregionshavedifﬁculty percolatinginhighamountsofsolventwhichresultsinalongtg andalowrateofpolymerizationvalue.Hereweshouldpointout thatallformulationsproducedapproximatelythesameconversion valuesattg(seeTable1),indicatingthathightgandlowRpmaxdo notaffecttheproductionofthegelationattg.However,thehighest ﬁnalconversion,Cs,asseeninFig.7withlowandhighDMFcontent

Fig.7.Theeffectofphotoinitiator(TX-SH)concentrationontheﬁnalconversionof photopolymerizationforEA/TPGDA.

gels,presentslowervaluesthanCsinmediumDMFcontentgel.In addition,thelargeamountofDMFcanleadtoaconsiderablylow Csvalue.Itwasalsonotedthatchangingthesolventcontenthas littleinﬂuenceontheconversionvaluesattg,whichisdeﬁnedin theearlyreaction(seeTable1).

This behavior can be explained with the assumption that polymerizationin highandlow viscous mediumresultsin low conversionvaluescompared toa mediumviscousenvironment. Mostprobablyinhighandlowviscousmediums,the polymeriza-tionreactioncannotbecompletedduetokineticdifﬁculties,which resultsinlowconversionvalues.Moreover,whilethesolvent con-tentdemonstratesadominanteffectintermsoftherateoftheearly stageofpolymerization(gelationregion),inthelaststage(glassy region)thiseffectbecomeslesspronounced.

4. Conclusions

Inthisstudy,aphoto-DSCtechniquewasusedtomeasurethe criticalexponent,ˇ,duringgelformationforEA/TPGDAmixtures invariousamountsofDMF.Itshouldbeemphasizedthatˇ val-uesdisplayednovariationduringgelationforallsamplesprepared withvariousDMFcontents.However,itwasobservedthattheother gelationparameterssuchastg,RpmaxandtheﬁnalconversionCs presentedconsiderablevariations,dependingontheDMFcontent. Theaveragedvalueforthecriticalexponentˇwasfoundtoobeythe percolationmodel,predictingthatuniversalbehaviorholdsnear theglasstransitionpointforallgelspreparedwithdifferingDMF content.

Acknowledgements

Authorsthank Yıldız Technical University, TUBITAK and the TurkishStatePlanningOrganization(DPT)fortheirﬁnancial sup-port.

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