and conductivity of polymer films Investigation of PSt-MWCNT concentration on epoxyacrylatephotopolymerization Progress in Organic Coatings

ProgressinOrganicCoatings76 (2013) 944–949

ContentslistsavailableatSciVerseScienceDirect

Progress

in

Organic

Coatings

jo u r n al h om epa ge :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

Investigation

of

PSt-MWCNT

concentration

on

epoxyacrylate

photopolymerization

and

conductivity

of

polymer

films

Zekeriya

Do˘gruyol

_,

_Gokhan

_Temel

_,

_Sevnur

_K.

_Do˘gruyol

_,

_Önder

_Pekcan

_,

_Nergis

_Arsu

a_{IstanbulUniversity,DepartmentofEngineeringScience,34850Istanbul,Turkey} b_{YalovaUniversity,DepartmentofPolymerEngineering,77100Yalova,Turkey} c_{YildizTechnicalUniversity,ChemistryDepartment,34320Istanbul,Turkey} d_{FacultyofArtsandScience,KadirHasUniversity,34320Istanbul,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received15July2012 Accepted20August2012 Available online 21 November 2012 Keywords: ModifiedPSt-MWCNT Nanocomposite Photopolymerizationkinetics Filtereffect

a

b

s

t

r

a

c

t

Photopolymerizationkineticsandconductivitychangesofepoxyacrylatecompositesforvariousloading

modifiedPSt-MWCNTweightfractionschangingfrom0.0025to0.2wt.%wereevaluatedbyperforming

photodifferentialscanningcalorimetry(photo-DSC)andfourpointconductivitymeasurements.0.2%

PSt-MWCNTadditivepolymericfilmshadtheirelectricalconductivityboostedby6%morethannon-additive

polymericfilms.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polymer nanocomposites with carbon nanotube fillers have generatedmuchinterestamongresearchersfortheimprovement oftheirmechanicalpropertiesandelectricalconductivity[1–8].In recentyears,CNTcompositeshavebeenamajorareaofresearch and development,where they are usedas reinforcing particles embeddedin a matrix (polymeric,ceramic or metallic)to con-fertheCNTs’ inherentpropertiestothecompositeswhich then gainenhancedfunctionalities.Theyareexcellentnano-filler mate-rialsfor transformingelectricallynon-conductingpolymersinto conductivematerials,whichhaveshownpotentialapplicationsin electromagneticinterference(EMI)shielding,photovoltaicdevices, andtransparentconductivecoatings[9–17].

AlthoughCNTsexhibitexcellentproperties,suchaslowmass density,electricalconductivityand,mechanically,asa nanocom-posite component, they suffer from self-aggregation and poor solubilityinorganicsolventsand water arisingfromtheirrigid honeycombstructures[18].Inordertoovercomethelatter draw-back, several approaches involving non-covalent and covalent functionalizationmethodswerereportedandCNTswithimproved solubilitywereobtained.Non-covalentfunctionalizationssuchas ␲–␲stackinginteractionsbetweenthesurfaceofCNTsand polynu-cleargroupsofpolymerswerebasedonVanderWaalsforces[19].

∗ Correspondingauthor.

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

Covalent functionalization examplesinclude grafting of macro-moleculesusingboth“graftingonto”[20–22]and“graftingfrom” [23]approaches.The“graftingonto”methodisthemostwidely usedfunctionalizationapproachtopreparemodified CNTswith varioustypesofpolymers.Dependingonthetypeofpolymerused, theresultantcompositesmayexhibithydrophilic[24], hydropho-bic[25]oramphiphilic[26]properties.

Inphotopolymerization experiments,mainlytype I(bybond cleavage)andtype II(byH-abstraction)initiatorsareemployed [27].AlthoughtypeIphotoinitiatorsaremoreeffectivethantypeII initiators,typeIIinitiatorsoperatinginthevisiblerangeare advan-tageousintermsofenergypolicies.Recently,mercaptoderivatives [28], carboxylic acidderivatives [29,30],anthracene derivatives [31,32]andmorpholine-attached[33]TXinitiatorswereusedfor thepolymerizationofacrylatesandmethacrylatesastypeII one-componentphotoinitiators.Anotherapproachwasalsoundertaken andthioxanthone-benzotriazolewassynthesized,whichpresented stabilizerandinitiatorpropertiesinone-componentinitiators[34]. In previous studies we conducted, the efficiency of 2-mercaptothioxanthone (TX-SH) was investigated [28]. A thiol derivativeofthioxanthoneTX-SHphotoinitiatorservesasbotha tripletphoto-sensitizer[35]andahydrogendonorforfree radi-calpolymerization.Themechanismofphotoinitiationisbasedon theintermolecularreactionofthetriplet,3_{TX-SH*,withthethiol}

moietyofgroundstateTX-SH.Theresultingthylradicalinitiates polymerization(Scheme1)[28].

We report here the results, by means of photopolymeriza-tionkineticsandtheconductivityofpolymericfilms,ofaddingof 0300-9440/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Scheme1.PhotoinitiationmechanismofTX-SH. polystyrenegraftedmultiwallcarbonnanotubeMWCNTby

thiol-eneclickchemistryviathethermalinitiationmethod.

2. Experimental

2.1. Materials

2-Mercaptothioxanthone (TX-SH) wassynthesized according tothepreviouslydescribedprocedure[28].Dimethylformamide (DMF, 99+%, Aldrich) was distilled over CaH2 under reduced

pressure. Epoxy diacrylate (EA) and tripropyleneglycoldiacry-late (TPGDA) were obtained from Cognis France. Styrene (St, 99%, Aldrich) was distilled under reduced pressure beforeuse. 2,2-Azobis(isobutyronitrile) (AIBN, 98%, Aldrich) was recrystal-lized from ethanol. Tetrahydrofuran (THF, 99.8%, J.T. Baker) was dried and distilled over benzophenone-Na. N,N,N,N,N -Pentamethyldiethylenetriamine(PMDETA, Aldrich)was distilled overNaOHethyl-2-bromopropionate(>99%,Aldrich)and trimethy-lolpropanetris(2-mercaptoacetate)(technicalgrade,Aldrich)was usedasreceived.Multiwallcarbonnanotube(MWCNT)Baytubes®

C150P(Bayer)andallotherreagentswereusedasreceived. 2.2. Preparationofformulations

Photoinitiator (TX-SH) concentration was held constant (0.5%, w/w) for all formulations; DMF was used to dissolve the initiator easily and various weight percentages of PSt-MWCNT (0:0.0025:0.0050:0.010:0.0250:0.0500:0.1000:0.2000 (%, w/w)) content in CHCl3 and epoxydiacrylate (80%) and

tripropylenegylcoldiacrylate (20%) were added and the for-mulations were sonicated and then stirred for one day at 500rpm.

2.2.1. Generalprocedureforatomtransferradicalpolymerization ToaSchlenktubeequippedwithamagneticstirringbar,the degassedmonomer (St,44mmol), ligand(PMDETA,0.44mmol), catalyst(CuBr,0.44mmol)andinitiator(ethyl-2-bromopropionate, 0.44mmol)wereadded,respectively.Thetubewasdegassedby threefreeze-pump-thawcycles,leftundervacuum,andplacedin a90◦Cthermostatedoilbath.Afterpolymerization,thereaction mixturewasdilutedwithTHFandthenpassedthroughacolumn ofneutral aluminatoremove themetalsalt.Theexcessof THF and unreactedmonomer wereevaporated under reduced pres-sure.ThepolymerwasdissolvedinTHFandprecipitatedin10-fold excessmethanol.Theresultingpolymerwasdriedinavacuumoven at roomtemperature.Molecularweights and molecularweight distributionsofthepolymers(Mn=3000gmol−1,PDI=1.18)were

determinedbyGPC.

2.2.2. Synthesisofthiolend-functionalpolystyrene(PSt-SH) Thethiolend-functionalpolystyrenesweresynthesizedfrom theabove obtainedPSt-Br byorganic substitution reaction fol-lowingtheliteratureprocedure[36].Thus, amixtureof1.0gof polystyrene(PSt-Br),ofthiourea(0.08g,1.05mmol,10equiv.)and 30ml of DMFwas heated at100◦C under flow for 24h. NaOH (0.042g,1.05mmol,10equiv.)which wasdissolvedin0.8mlof water,wasaddedandthemixtureheatedto110◦Cfor24h.Two dropsof95%sulfuricacidin0.5mlofwaterwereaddedandthe

946 Z.Do˘gruyoletal./ProgressinOrganicCoatings76 (2013) 944–949

Scheme3. ModificationofMWCNTwiththiol-eneclickchemistrywithPSt-SH.

mixturewasstirredatroomtemperatureforanadditional5h.The functionalizedpolymerwaspurifiedbysuccessiveprecipitations inmethanol(Scheme2).

1_HNMR(CDCl

3,250MHz):ı=7.25–6.24(m,aromaticprotonsof

PS),3.94(broad,2H, CH2 O C O),3.51ppm(broad,1H, CH S).

2.2.3. ModificationofMWCNTwithPSt-SHbythiol-eneclick chemistry

PSt-SH(150mg)andacatalyticamountofAIBN(1–2%,w/w) weredissolvedin30mlofDMF,and50mgMWCNTwasadded. Themixturewassonicatedfor10mininanultrasonicbathand theresultingsuspensionwasbubbledwithargonfor15min.Then thesuspensionwassonicatedagainfor10minandallowedtostir at80◦Covernight.Attheendoftheperiod,theresultingmixture wascentrifugedtoremovesolventandunreactedpolymer. Mod-ifiedMWCNTwasredispersedinfreshTHFusingmildsonication andthencentrifugedagain.Theredispersionandrecentrifugation processwasthenrepeatedthreetimestoremoveanyfreePSt.The finalproductwasdriedinvacuum.

1_HNMR(CDCl

3,250MHz):ı=7.38–6.25(m,aromaticprotons

ofPS),2.34–1.22(m,aliphaticprotonsofPS)(Scheme3).

2.3. Photodifferentialscanningcalorimeter(photo-DSC)

The heat of the photoinitiated polymerization reaction was measuredbymeansofaphotodifferentialscanningcalorimeter. ThephotoinitiatedpolymerizationofEA/TPGDAwithvarious con-centrationsof PSt-MWCNT in the presence of TX-SH (0.5wt.%) wasperformedinaphoto-DSCsetup(TA-DSCQ100).UVlightwas appliedfromamediumpressuremercurylampataconstant inten-sityof40mW/cm2 _{for3minunderanitrogenflowof50ml/min}

atroomtemperature(isothermalmode).Theweightofthe sam-ples2±0.1mgwasplacedintoanopenaluminumliquidDSCpan. Measurementswere carried outunder identicalconditions and recordedatasamplingintervalof0.05s/point.Thethicknessof curedthinfilmswasabout0.25mmaccordingtothepreviously describedprocedure[37].

Thereaction heat liberatedin thepolymerizationis directly proportionaltothenumberofacrylatesreactedinthesystem.By integratingtheareaundertheexothermicpeak,theconversionof

theacrylategroups(C)ortheextentofthereactionwasdetermined accordingtoEq.(1):

C= Ht Htheory

0

(1)

whereHtisthereactionheatevolvedattimetandH₀theoryis

thetheoreticalheatforcompleteconversion.Areactionheatforan acrylatedoublebondpolymerizationofHtheory

0 =86.25kJ/mol

wasused[38,39].Therateofpolymerization(Rp)isdirectlyrelated

totheheatflow(dH/dt)asinEq.(2): Rp=d_dC_t = (dH/dt)

Htheory 0

(2) Electricalresistivitymeasurement

Theelectricalresistivitiesofthefilmswereconductedwiththe Signatonesystem,incorporatedwithafour-pointcylindricalprobe. Thefour-pointprobehasfourprobesinastraightlinewithanequal inter-probespacingof1.56mm.Theprobeneedleradiusis100␮m. Theelectricalresistanceofthefilms,Rs,canbeobtainedfromEq. (3)

Rs=4.53×V_I (3)

whereIistheconstantcurrentthatpassedthroughthetwoouter probesandVistheoutputvoltagemeasuredacrosstheinnerprobes withtheKeithley2400source-meter.

3. Resultsanddiscussion

3.1. Kineticassessmentofphotopolymerization

Thephotocuringofepoxydiacrylate(80%)dilutedwithTPGDA (20%) in the presence of TX-SH photoinitiator with various PSt-MWCNT loading wasfollowed by photo-DSC.To study the photopolymerizationkineticsofnanocompositesbyphoto-DSCis importantandprovidesheatflowdata.Theheatflowcanbe con-vertedbyEq.(3) totheultimatepercentageconversionandthe polymerizationrateforagivenamountofformulationisgivenin Table1.However,itshouldbepointedoutthatalthoughthe solubil-ityofMWCNTissignificantlyincreasedbythemodification,there stillexistsomedispersionproblemswhichinterferewiththelight

Fig.1.HeatflowversustimecurvesofEA/TPGDAmonomer,photoinitiatedwith TX-SH,whichcontainsvariousamountsofPSt-MWCNT.

transmission,especiallyforthehighestloadingsofPSt-MWCNTs.

TheprofilesofthephotopolymerizationofEA+TPGDAwith

vari-ousPSt-MWCNTloadingsinitiatedbyTX-SHareshowninFigs.1–3

at25◦CbyUVlightwithanintensityof40mW/cm2_.

Addingbetweenarangeof0.0025%and0.2%PSt-MWCNTto theUV-curableformulationsgraduallyledtolowerpolymerization ratesofupto0.5%loadingofPSt-MWCNT.With0.2%ofPSt-MWCNT loading,theconversionpercentagedecreasedatleast20% com-paredtoformulationswhichhadnoPSt-MWCNT.Thecorrelation betweenvariousloadingamountsofPSt-MWCNTandthe maxi-mumofrateofpolymerizationandfinalconversionpercentagesare summarizedinFigs.4and5.IncreasingtheamountofPSt-MWCNT reducesthelightpenetrationandinaddition,theprobabilityof producingofinitiatingthiylradicalsfromTX-SHislowered,which isreflectedinboththeRpandfinalconversionCsvalues.

Formulations,whichareusedforphoto-DSCexperiments,have beencoatedwiththedoctorblademethodinordertoexaminethe

Fig.2.RateofpolymerizationversustimecurvesofEA/TPGDAmonomer, photoini-tiatedwithTX-SH,whichcontainsvariousamountsofPSt-MWCNT.

Fig. 3. Monomer conversion percentages versus time curves of EA/TPGDA monomer,photoinitiatedwithTX-SH,whichcontainsvariousamountsof PSt-MWCNT.

Fig.4.PSt-MWCNTconcentrationversustherateofpolymerization(Rpmax)of

EA/TPGDAmonomer,photoinitiatedwithTX-SH.

UVlightpenetrationontothefilms.Theseformulationswereplaced into1cm×1cmglassesandcuredwith8passesundera Mini-UV-cureunit.Thepolymericfilmsobtained,consistingofthevarious concentrationofPSt-MWCNT,aregiveninFig.6.

Todemonstratethecut-off lightpenetration offormulations containingvariousamountsofPSt-MWCNT,transmissionspectra ofallUVcuredfilmsweretakenandaregiveninFig.7.

The TX-SH photoinitiator has an absorption maximum at 382nm and as seen from Fig. 7, increasing the amount of

Fig. 5.PSt-MWCNT concentrations versuschange of final conversion (Cs)of

948 Z.Do˘gruyoletal./ProgressinOrganicCoatings76 (2013) 944–949

Fig.6.ThepicturesofUVcuredpolymericfilmsconsistingofdifferent concentra-tionsofPSt-MWCNT.

Fig.7.UV–visregiontransparencyspectraofUVcuredthinfilmsthatcontain. PSt-MWCNTindifferentconcentrations.

PSt-MWCNT decreases the transmission percentage of cured filmsandthis diminishestheeffectof thephotoinitiator’slight absorptioncapability.ThiseffectissummarizedinFig.8.

Examiningconductivitypropertiesofepoxyacrylatebasedfilms containingvariousamountsofPSt-MWCNT

Fig.9showsthemorphologyofPSt-MWCNTcontaining pho-tocured nanocomposite films. The figures show no obvious agglomeration of PSt-MWCNT in thematrix and indicate good interactionbetweenfillerandpolymermatrix.Thegooddispersion

Fig.8.Permeabilityintensitiesandmonomerconversationpercentagesof epoxy-acrylatefilmsat382nm,whichcontaindifferentconcentrationsofPSt-MWCNT.

ofPSt-MWCNTwithinthepolymermatrixhelpstoenhancement ofthephysicalpropertiesofnanocompositecoating.

TX-SHphotoinitiator,EA+TPGDAmonomer andPSt-MWCNT atdifferentconcentrationswerecoatedontocoverglassandthen curedbypassingthrougha MiniUV-cureunit.Theconductivity oftheobtainedfilm’ssurfacewasmeasuredbyusingafour-point probe.Withthismethod,surfaceresistancevalueswereobtained usingtwoouterelectrodesandtwoinnerelectrodes.Acurrentwas sentfromthetwoouterelectrodestothetwoinnerelectrodesand thenthecurrentbetweenthetwoinnerelectrodeswasmeasured. Bycalculatingthereverseoftheresistancevalues,conductivities weredetermined.Theseprocedureswereconductedforthinfilms containingdifferentconcentrationsofPSt-MWCNTandtheDC con-ductivityvaluesdependentonlogarithmicconcentrationcanbe observedinFig.10.Allsurfaceresistivityandconductivityvalues havebeensummarizedinTable1.

Table1showssurfaceresistanceandelectricalconductivities ofepoxyacyrylatebasedfilms,photo-curedwithTX-SH, contain-ingdifferentconcentrationsofPSt-MWCNT.Incomparison,0.2% PSt-MWCNTadditivepolymericfilmshada6%boostofelectrical conductivitycomparedwithnon-additivepolymericfilms.

Fig.10.DCconductivityvaluesofpolymericfilmsdependentonPSt-MWCNT con-centrationusingpointmethod.

4. Conclusion

In this work, thiol functional polystyrene was successfully attachedtotheMWCNTwiththermalinitiatedthiol-eneclick strat-egy.DifferentconcentrationsofresultingPSt-MWCNTwereusedas fillerindiacrylatesformulationsandconductivityofthepolymeric filmswerefoundtobe0.610×10−9× (Siemens)inmaximum loadingofthePSt-MWCNT(0.2%).Meanwhile,increaseofthe PSt-MWCNTloadingisinverselyproportionaltopenetrationofthelight intothecuredfilmsandthisnegativelyaffectsthelight absorp-tionbehaviorofphotoinitiatorduringthephotoinducedfreeradical polymerization.

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