Fluorescence ﬁlm formation study of from PS/Al O nanocomposites Progress in Organic Coatings

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ContentslistsavailableatScienceDirect

Progress

in

Organic

Coatings

jou rn a l h om ep a 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

Fluorescence

study

of

ﬁlm

formation

from

PS/Al

2

O

3

nanocomposites

Ö.

Pekcan

a,∗

_,

_S¸.

_U˘gur

b

_,

_M.S.

_Sunay

c

a_Kadir_Has_University,_Cibali,₃₄₃₂₀_Istanbul,_Turkey

b_Istanbul_Technical_University,_Department_of_Physics,_Maslak,₃₄₄₆₉_Istanbul,_Turkey c_Piri_Reis_University,_Faculty_of_Science_and_Letters,_Tuzla,₃₄₉₄₀_Istanbul,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Availableonline22February2014 Keywords: Nanocomposites Polystyrene Annealing Films Fluorescence

a

b

s

t

r

a

c

t

Steadystateﬂuorescence(SSF)andUV–vis(UVV)techniqueswereusedtostudytheﬁlmformation behaviorofpyrene(P)labeledpolystyrene(PS)latexandAl2O3(PS/Al2O3)compositesdependingonPS

particlessizeandAl2O3content.Theclose-packedarraysofPSspheres(SmPS:203nm;LgPS:382nm)

templatesoncleanglasssubstrateswerecoveredwithvariouslayersofAl2O3bydip-coatingmethod.

Twodifferentﬁlmseries(SmPS/Al2O3andLgPS/Al2O3)werepreparedinvariousAl2O3layercontent.The

ﬁlmformationbehaviorofthesecompositeswerestudiedbyannealingthematatemperaturerange of100–250◦Candmonitoringthescatteredlightintensity(Isc),ﬂuorescenceintensity(IP)fromPand

transmittedlightintensity(Itr)throughtheﬁlmsaftereachannealingstep.Opticalresultsindicatethat

classicallatexfilmformationwasoccurredforallAl2O3contentfilmsandfilmformationprocesswas

unaffectedbytheAl2O3contentforbothﬁlmseries.ExtractionofPStemplateproducedhighlyordered

porousstructuresforhighAl2O3contentinbothﬁlmseries.SEMimagesshowedthattheporesizeand

porositycouldbeeasilytailoredbyvaryingthePSparticlesizeandtheAl2O3content.

1. Introduction

Asaresultofworldwidetheoreticalandexperimentalefforts,a verygoodunderstandingofthemechanismsoflatexﬁlm forma-tionhasbeenachieved[1–4].Filmformationfromsoft(low-Tg)

andhard(high-Tg)latexdispersionscanoccurinseveralstages.

Inbothcases,thefirststagecorrespondstothewetinitialstage. Evaporationofsolventleadstosecondstageinwhichtheparticles formaclosepackedarray,hereiftheparticlesaresofttheyare deformedtopolyhedrons.Hardlatexhoweverstaysundeformed atthis stage.Annealingof soft particlescausesdiffusionacross particle–particleboundarieswhichleadstoahomogeneous con-tinuousmaterial.Intheannealingofhardlatexsystem,however, deformationofparticlesfirstleadstovoidclosure[1–4]andthen afterthevoidsdisappeardiffusionacrossparticle–particle bound-ariesstarts,i.e.themechanicalpropertiesofhardlatexfilmsevolve duringannealing;afterallsolventhasevaporatedandallvoidshave disappeared.

This understanding of latex ﬁlm formation can now be exploited tounderpin theprocessing of newtypes of coatings anddevelopmentofnewmaterials.Processingandmicrostructure developmentofceramicandpolymercoatingpreparedby deposit-ingasolutionordispersionhavebeenofinterestinlastfewyears

∗ Correspondingauthor.Tel.:+902125336532x1330;fax:+902125332286. E-mailaddress:pekcan@khas.edu.tr(Ö.Pekcan).

[5,6].Colloidalceramics,sol–gelderived ceramicsandpolymers havebeenstudiedascoatingsystems.Organizationof monodis-persedcolloidalparticles likelatexandsilica microspheresinto higher-ordermicrostructuresisattractinggrowinginterest[7,8], sinceitprovidesuniquestructuressuitableforvariousadvanced devices and functional materials such as photonic crystals [9] andporouspolymers[10].Colloidalcrystalsconsistingof three-dimensionalorderedarraysofmonodispersedspheres,represent noveltemplatesforthepreparationofhighlyordered macropo-rous inorganicsolids, exhibiting precisely controlled pore sizes and highly ordered three-dimensional porousstructures. These features are requirements for new photonic crystals, and can bebeneﬁcial incatalysisorlarge-moleculeseparationprocesses by potentially improvingmass transfer processes and efﬁcien-cies.Orderedarraysofpolymer(e.g.polystyreneorpoly(methyl methacrylate))orsilicananosphereshavebeenextensivelystudied inrecentyearsforphotoniccrystalapplications[11,12].Recently, theyhaveattractedrenewedinterest,mainlybecausetheyprovide amuchsimpler,fasterandcheaperapproachthancomplex semi-conductor nanolithography techniques to create 3D photonic crystalsworkingintheopticalwavelengthrange[13,14].Photonic crystals (i.e.spatially periodicstructures of dielectric materials withdifferentrefractiveindices)have beenextensively investi-gatedworldwide.Becausethelatticeconstantofphotoniccrystals isinthevisibleorinfraredwavelengthrange,theycancontrolthe propagationofphotonsinawaysimilartothewayasemiconductor doesforelectrons.

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Aluminaisaveryimportantinorganicmaterialowningtoits thermal, chemical, and mechanical stability, and the hierarchi-callyporousaluminashouldbeanattractivematerialwhichcan bepotentiallyusedascatalystsupports,adsorbents,ion-exchange materials,membranesubstrates,etc[15].Severalreportsonporous aluminapreparationcanbefound[16].Al2O3 coatingshavealso

becomemorepopularfortheirhighdielectricstrength,exceptional stability,durabilityagainsthostileenvironmentsandhigh trans-parencydownto250nm.DuringthelastfewyearsAl2O3coatings

have beenwidely used for theirpractical applications,suchas refractorymaterials,antireﬂectioncoatings,highlyanticorrosive materials[17],microelectronicdevices[18],capacitancehumidity sensors[19]andalsoinheatsinksinIC’sandpassivationofmetal surfaces[20].Theseﬁlmshavebeenpreparedbyvarioustechniques suchasSpraypyrolysis[21],thermalevaporation[22],sputtering [23]etc.

In thepresent work,we reportthe preparation and charac-terization of PS/Al2O3 films. The film formation of these films

wasstudieddependingonPSparticlesizeandAl2O3content.The

resultsindicatethatLgPS/Al2O3 ﬁlmsshowedcompleteﬁlm

for-mation independent of Al2O3 content while no ﬁlm formation

occurredaboveacertainAl2O3contentforSmPS/Al2O3ﬁlms.The

ﬁlmformationstagesweremodeledandrelatedactivationenergies were determined. After completion of ﬁlm formation, PS tem-plateswereextractedwithtoluene. Theresultantstructurewas thereplicaofthePSparticlesandthematerialsgeneratedfrom thisprocessexhibitremarkableorderingoftheporeswithdifferent size.

2. Experimental

2.1. Materials

2.1.1. Polystyrene(latex)spheres

In this study, we used two types of PS latex with differ-ent diameters. The latex samples are composed of pyrene (P) labeled polystyrene. Fluorescent PS latexes were produced via emulsion polymerizationprocess [24]. The polymerizationwas performedbatch-wiselyusingathermostattedreactorequipped withacondenser,thermocouple,mechanicalstirringpaddleand nitrogeninlet.Water(50ml),Styrenemonomer(3g;99%purefrom Janssen)andthe0.014gofﬂuorescent1-pyrenylmethyl methacry-late(PolyFluor®394)wereﬁrstmixedinthepolymerizationreactor wherethetemperaturewaskeptconstant(at70◦C).Thewater soluble radicalinitiatorpotassium persulfate(KPS)(1.6%wt/wt overstyrene)dissolvedinsmallamountofwater(2ml)wasthen introducedin ordertoinducestyrenepolymerization. Different surfactantsodiumdodecylsulfate(SDS)concentrations(0.03%and

0.12%wt/vol)wereaddedinthepolymerizationrecipetochange the particlesize keeping all other experimental conditions the same. Thepolymerizationwasconducted under400rpm agita-tionduring12hundernitrogenatmosphereat70◦C.Theparticle sizewasmeasuredusingMalvenInstrumentNanoZS.Themean diameteroftheseparticlesis203nm(SmPS)and382nm(LgPS). Theweight-averagemolecularweights(Mw)ofindividualPSchain

(Mw)weremeasuredby gelpermeationchromatography (GPC)

andfoundas90_×103_g_mol−1_for_both₂₀₃_nm_(SmPS)_and₃₈₂_nm

(LgPS), respectively. The particle size of the polystyrene latex wasdecreased withincreasingtheconcentrationof SDSbutits molecularweightremainedalmostunchangedwithincreasingSDS concentration.Glasstransitiontemperature(Tg)ofthePSlatexes

weredeterminedusingdifferentialscanningcalorimeter(DSC)and foundtobearound105◦C.Fig.1showstheSEMimagesofSmPS andLgPSlatexparticlesproducedforthisstudy.

2.1.2. Al2O3solution

Al2O3 solwaspreparedinthefollowingway:Atotalof2ml

aluminum-tri-sec-butoxide(Aldrich;97%)wasdissolvedin45cm3

waterat70◦C.Thesolutionwasstirredfor30min.Asmallamount ofacidicacidwascontinuouslyaddedascatalyst,untilthesolution becametransparentandstirredforanother2h.Oxidenetworks areformeduponhydrolyticcondensationofalkoloxideprecursors. Finally,auniformandtransparentAl2O3solwasobtainedforﬁlm

fabrication.

2.2. PreparationofPS/Al2O3ﬁlms

Firstly,LgPSandSmPSaqueoussuspensionsweredroppedon cleanglasssubstratesanddriedatroomtemperature.Uponslow dryingatroomtemperature,powderLgPSandSmPSﬁlmswere produced. In order tostudy theparticle size effect of PSlatex andAl2O3 contentonﬁlmformationbehaviorofPS/Al2O3

com-posites,wepreparedtwoseriesofﬁlms:Series1:LgPSandAl2O3

(LgPS/Al2O3)andSeries2:SmPSandAl2O3(SmPS/Al2O3).

Al2O3solwasﬁlledintothePStemplatesbydip-coatingmethod.

HeretheAl2O3contentintheﬁlmswasadjustedbyconsecutive

dippingcycle.Sevendifferentfilmsforeachseriesoffilmswere producedwith0,1,3,5, 8,10,and15 layers(dippingcycle)of Al2O3.InordertostudythefilmformationbehaviorofPS/Al2O3

composites,theproducedﬁlmswereseparatelyannealedaboveTg

ofPS,attemperaturesrangingfrom100to250◦C.Thetemperature wasmaintainedwithin±2◦_C_during_annealing._After_each

anneal-ingstep,ﬁlmswereremovedfromtheovenandcooleddownto roomtemperature.

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2.3. Methods

2.3.1. Fluorescencemeasurements

Afterannealing,each samplewasplacedin thesolid surface accessoryofaPerkin-ElmerModelLS-50ﬂuorescence spectrom-eter.Pyrenewasexcitedat345nmandscatteringandﬂuorescence emissionspectraweredetectedbetween300and500nm.All mea-surements were carried out in the front-face positionat room temperature.Slitwidthswerekeptat8nmduringallSSF measure-ments.

2.3.2. Photontransmissionmeasurements

Photontransmissionexperimentswerecarriedoutusing Carry-100BioUV–vis(UVV)scanningspectrometer.Thetransmittances oftheﬁlmsweredetectedat500nm.Aglassplatewasusedasa standardforallUVVexperiments,andmeasurementswerecarried outatroomtemperatureaftereachannealingprocesses.

2.3.3. Scanningelectronmicroscopy(SEM)measurements

ScanningelectronmicrographsofthePS/Al2O3ﬁlmsweretaken

at10–20kVinaJEOL6335Fmicroscope.Athinﬁlmofgold(10nm) wassputteredontothesurfaceofsamplesusingaHummer-600 sputteringsystemtohelpimagethePS/Al2O3ﬁlmsagainsttheglass

background.

3. Resultsanddiscussions

Figs.2and 3showtransmitted(Itr), scattered(Isc)and

ﬂuo-rescence(IP)light intensitiesversusannealingtemperaturesfor

bothSmPS/Al2O3 and LgPS/Al2O3 composite ﬁlmseries,

respec-tively.Uponannealingthetransmittedlightintensity,Itr,started

toincreaseaboveacertainonsettemperature,calledtheminimum ﬁlmformationtemperatureT0,forallﬁlmsamples.Scatteredlight

intensityshowedasharpincreaseatthesingletemperaturenamed asthevoidclosuretemperature,Tv.Fluorescenceintensity,IPofall

ﬁlmsamplesﬁrstincrease,reachamaximum,andthendecrease withincreasingannealingtemperature[25,26].Thetemperature whereIPreachesthemaximumiscalledthehealingtemperature,

Th.Minimumﬁlm formation(T0),void closure(Tv)and healing

(Th)temperaturesareimportantcharacteristicrelatedtotheﬁlm

formationpropertiesof latexes.T0 isoftenused toindicatethe

lowestpossibletemperatureforparticledeformationsufficientto decreaseinterstitialvoiddiameterstosizeswellbelowthe wave-lengthoflight[27].Belowthiscriticaltemperature,thedrylatexis opaqueandpowdery.However,atand/orabovethistemperature, alatexcastfilmbecomescontinuousandclearfilm[28].HereTvis

thelowesttemperatureatwhichIscbecomehighest.Thehealing

temperature(Th)istheminimumtemperatureatwhichthelatex

ﬁlmbecomescontinuousandfreeofvoids.Thehealingpoint indi-catestheonsetoftheparticle–particleadhesion[28].Therefore, theincreaseinItraboveT0canbeexplainedbyevaluationofthe

transparencyofthecompositeﬁlmsuponannealing.Most proba-bly,increasedItrcorrespondstothevoidclosureprocess[29];i.e.

polystyrenestarttoﬂowuponannealingandvoidsbetween parti-clescanbeﬁlled.SincehigherItrcorrespondstohigherclarityofthe

composite,thenincreaseinItrpredictsthatmicrostructureofthese

filmschangeconsiderablybyannealingthem,i.e.thetransparency ofthesefilmsevolveuponannealing.PSstartstoflowdueto anneal-ing,andvoidsbetweenparticlescanbefilledduetotheviscous flow.Furtherannealingathighertemperaturecauseshealingand interdiffusionprocesses[26,29],resultinginamoretransparent film.

ThesharpincreaseinIscoccursatTv,whichoverlapsthe

inﬂec-tionpoint ontheItr curve.Below Tv,light scattersisotropically

becauseoftheroughsurfaceofthePSfilms.Annealingofthefilm atTv createsaflatsurfaceonthefilm,which actslikea mirror.

Asa result,light is reﬂected tothephotomultiplier detectorof the spectrometer. Further annealing makes the PS ﬁlm totally transparenttolight andIscdropstoitsminimum.On theother

hand, the increase in IP above T0 presumably corresponds to

Fig.2.PlotofItr,IscandIPintensitiesversusannealingtemperature,TforSmPS/Al2O3compositeﬁlmswith0(pure),1,5and10layersofAl2O3.Numbersoneachcurve

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thevoid closure process up tothe Th point where the healing

processtakesplace.DecreaseinIPaboveThcanbeunderstoodby

interdiffusionprocessesbetweenpolymerchains[30,31]. Inordertodeterminetheextentoffilmformation,filmswere soakedintoluenefor24htocompletelydissolvethePStemplate afterthefilmformationiscompleted.AfterextractionofPS,the morphologyofSmPS/Al2O3filmswithoneandfivelayersofAl2O3

(Fig.4a)donotchangesigniﬁcantly.Intheseimages,SmPSspheres highlycoatedwithAl2O3areclearlyseen.However,somespherical

poreswhichmustbelongtotheAl2O3encapsulatedreplicaofSmPS

latexesarealsoobserved.Theseporeshaveawell-pronounced cir-cularshape andareisolatedfromeachother.SEMimageofthe ﬁlmpreparedwith10layersofAl2O3 inFig.4cshowsan

inter-connectedandopenporositywithaverageporesizediameterof 203nm,correspondingtoapproximatelySmPStemplatediameter. Inallimages,besidestheporestherearealsovoidsthatmightbe leftbytheinterconnectedSmPSaggregatedspheres.Itis under-stoodthathigherAl2O3contentandsmallPSsizecreatedaporous,

disorderedmaterialafterextractionoftemplate.

Ontheotherhand,SEMimagesofLgPS/Al2O3compositesgiven

inFig.4bpresenthighly porousstructuresafterextraction pro-cess.Theporesareuniformlydistributedinspace,butrandompore morphologyintheﬁlmswith1and5layersofAl2O3(seeFig.4b)

destroytheirsphericalshape.Theseﬁlmsshowapoorlyordered porestructureandheterogeneouspore-sizedistribution.Forthese lowAl2O3 contentﬁlms,apartiallybrokenwallframeworkwas

obtainedbecausetheAl2O3particlesmightnotbesufﬁcienttofully

coverthesurfaceoftheLgPStemplate.However,itcouldbeseen fromFig.4bthatﬁlmwith10layersofAl2O3showswell-deﬁned

spherical-orderedpores.Theporesareuniformlydistributedinthe sampleandshowedanorderedconnectedporousstructure.The homogenousdistributionoftheporesintheAl2O3framework

indi-catesthattheporousstructureretainstheperiodicityoftheLgPS template.

Inconclusion,SEMimagesofbothﬁlmseriesshowedthatwell deﬁnedopenstructureandinterconnectedporositywereobtained

whenAl2O3contentwasincreased.Thisbehaviorcanbeexplained

by removal of PSfrom the surface of the Al2O3 covered latex

particlesduringthedissolutionprocess.Inotherwords,theﬁlm formationfromSmPSandLgPSparticleshasoccurredontopofthe Al2O3coveredparticlesduringannealingand,duringdissolution,

PSmaterialiscompletelydissolvedshowingthemicrostructureof PSparticlescoveredbyAl2O3layer.Thispictureisnowdepictedin

Fig.5wherethebehaviorofSmPS/Al2O3andLgPS/Al2O3

compos-itefilmsduringannealingarepresented[26].InFig.5a,filmposses manyvoids,whichresultsinshortmean-freeandopticalpathsof aphotonyieldingverylowIPandItr.Fig.5bshowsafilminwhich

interparticlevoidsdisappearduetoannealing,whichgivesriseto alongmeanfreeandopticalpathintheﬁlm.Atthisstage,IPand

Itrreachitsmaximumvalues.Finally,Fig.5cpresentsalmost

trans-parentﬁlmwithnovoidsbutsomeAl2O3background.Atthisstage,

ﬁlmhaslowIPbuthighItrbecausethemeanfreepathisverylong

buttheopticalpathisshort. 3.1. Filmformationmechanisms 3.1.1. Voidclosure

InordertoquantifythebehaviorofIPbelowThandItrabove

T0 inFigs.2and 3,aphenomenologicalvoidclosuremodelcan

beintroduced.Latexdeformationandvoidclosurebetween par-ticles canbeinduced by shearingstress which isgenerated by surface tensionof thepolymer,i.e. polymer–airinterfacial ten-sion.Thevoidclosurekineticscandeterminethetimeforoptical transparencyandlatexﬁlmformation[32].Inordertorelatethe shrinkageofsphericalvoidofradius,r,totheviscosityofthe sur-roundingmedium,,anexpressionwasderivedandgivenbythe followingrelation[32]. dr dt =− 2

₁ (r)

(1) whereisthesurfaceenergy,tistimeand(r)istherelative den-sity.Ithastobenotedthatherethesurfaceenergycausesadecrease

Fig.3.PlotofItr,IscandIPintensitiesversusannealingtemperature,TforLgPS/Al2O3compositeﬁlmswith0(pure),1,5and10layersofAl2O3.Numbersoneachcurveshows

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Fig.4. SEMimagesof(a)SmPS/Al2O3and(b)LgPS/Al2O3compositeﬁlmswith1,5and10layersofAl2O3afterextractionofPStemplatewithtoluene.

in void size and the term (r) varies with the microstructural characteristicsofthematerial,suchasthenumberofvoids,the ini-tialparticlesizeandpacking.Eq.(1)issimilartoonethatwasused toexplainthetimedependenceoftheminimumﬁlmformation temperatureduringlatexﬁlmformation[33,34].Iftheviscosityis constantintime,integrationofEq.(1)givestherelationas

t=−2 r

ro (r)dr (2)

wherer0istheinitialvoidradiusattimet=0.Thedependenceofthe

viscosityofpolymermeltontemperatureisaffectedbythe over-comingoftheforcesofmacromolecularinteraction,whichenables thesegmentsofpolymerchaintojumpoverfromone equilibra-tionpositiontoanother.Thisprocesshappensattemperaturesat whichthefreevolume becomeslargeenoughandis connected withtheovercomingofthepotentialbarrier.Frenkel–Eyringtheory producesthefollowingrelationforthetemperaturedependenceof viscosity[35,36] = N0h V exp

_G kT

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where N0 is Avogadro’s number, h is Planck’s constant, V is

molar volume and k is Boltzmann’sconstant. It is known that G=H−TS,soEq.(3)canbewrittenas

=A exp

_H

kT

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whereHistheactivationenergyofviscousflow,i.e.theamount ofheatwhichmustbegiventoonemoleofmaterialtocreatethe actofajumpduringviscousflow;Sistheentropyofactivationof viscousflow.HereArepresentsaconstantfortherelated param-etersthatdonotdependontemperature.CombiningEqs.(2)and (4),thefollowingusefulequationisobtained

t=−2A exp

_H kT

r or (r)dr (5)

Inordertoquantifytheaboveresults,Eq.(5)canbeemployed byassumingthattheinterparticlevoidsareequalinsizeandthe

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Fig.5.CartoonrepresentationofPS/Al2O3ﬁlmsatseveralannealingsteps.(a)Film

possesmanyvoidsthatresultsinverylowIPandItr,(b)interparticlevoidsdisappear

duetoannealing,IPreachesitsmaximumvalue,and(c)transparentﬁlmwithno

voidsbutsomeAl2O3backgroundandhaslowIPbuthighItr.

number of voids stays constant during ﬁlm formation (i.e. (r)≈r−3),ThenintegrationofEq.(5)givestherelation

t= 2AC exp

_H kT

₁ r2− 1 r2 o

(6) whereCisaconstantrelatedtorelativedensity(r).Aswestated before, decreasein void size(r) causesanincrease in IP.Ifthe

assumptionis madethatIP is inverselyproportional tothe6th

powerofvoidradius,r,thenEq.(6)canbewrittenas t= 2AC exp

_H kT

I1/3

₍₇₎

herero−2isomittedfromtherelationsinceitisverysmall

com-paredtor−2valuesaftervoidclosureprocessesarestarted.Eq.(7) canbesolvedforIPandItr(=I)tointerprettheresultsinFigs.2and3

as I(T)=S(t) exp

−3H kT

(8) whereS(t)=(t/2AC)3_._For_a_given_time_the_logarithmic_form_of_Eq.

(8)canbewrittenasfollows ln I(T)=ln S(t)−

3H

kT

(9) Asitwasalreadyarguedabovethat,theincreaseinbothIPand

Itroriginateduetothevoidclosureprocess,thenEq.(9)wasapplied

toItraboveT0andtoIPbelowThforallﬁlmsamplesintwoseries.

Fig.6presentsthelnIPversusT−1andFig.7presentslnItrversus

T−1 plots forSmPS/Al2O3 ﬁlmseriesfromwhichHPand Htr

Fig.6. Theln(IP)versusT−1plotsofthedatainFig.2forSmPS/Al2O3composite

ﬁlmcontain(a)0(pure),(b)1,(c)5and(d)10layersofAl2O3.Theslopeofthe

straightlinesonrightandlefthandsideofthegraphproduceHPandEactivation

energies,respectively.

Fig.7.Theln(Itr)versusT−1plotsofthedatainFig.2forSmPS/Al2O3compositeﬁlm

contains(a)0(pure),(b)1,(c)5and(d)10layersofAl2O3.Theslopeofthestraight

linesproducesHtr.

activationenergieswereobtained.Similarﬁttingswerealsodone forLgPS/Al2O3 ﬁlmseriesandthemeasuredHPandHtr

acti-vationenergiesarelistedinTable1forbothseries.Itisseenthat HPvalues,exceptforpureSmPSandLgPSﬁlms,forbothseries

donotchangemuchbyincreasingtheAl2O3layershowingthatthe

amountofheatthatwasrequiredbyonemoleofpolymericmaterial toaccomplishajumpduringviscousﬂowdoesnotchangeby vary-ingtheAl2O3layersonthelatexﬁlms.Inaddition,Htrvaluesof

bothfilmseriesalsodonotchangemuch.Ithastobenotedthatthe measuredactivationenergiesforviscousflowprocesswerefound tobedifferentindifferenttechniques.Thisdifferencemost proba-blyoriginatesfromdifferenttechniquesandsecondonemeasures thefilmformationfromtheinnerlatexes.Sincepyrenesarelabeled toPSchain,itisbelievedthatHPvaluesaremorerealisticto

inter-prettheviscousﬂow.Ontheotherhand,Htrvalueswereobtained

indirectlycomparedtoHPvalues.Whencomparingthe

activa-tionenergiesofbothseries,itisseenthatHvaluesofLgPS/Al2O3

seriesarelargerthanthoseofSmPS/Al2O3series.Thisimpliesthat

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Table1

ExperimentallyproducedactivationenergiesofSmPS/Al2O3andLgPS/Al2O3ﬁlmseries.

Al2O3layer SmPS/Al2O3 LgPS/Al2O3

HP(kcalmol−1) Htr(kcalmol−1) E(kcalmol−1) HP(kcalmol−1) Htr(kcalmol−1) E(kcalmol−1)

0 2.5 2.2 7.5 2.2 10.6 12.6 1 1.2 0.8 3.2 3.1 8.2 3.6 3 1.7 0.6 3.4 2.2 4.9 5.8 5 2.2 1.0 7.7 3.0 6.1 7.1 8 2.7 0.8 6.0 2.5 7.2 4.6 10 0.9 0.8 10.0 2.2 6.7 8.7 15 1.5 1.1 7.8 2.4 8.3 5.1

size.Withsmallerdiameter(i.e.203nm),theSmPSparticleshave largersurfaceareaorsurfacefreeenergy.Thedrivingforceforfilm formationisproportionaltotheinverseoftheparticlesize, accord-ingtothedescriptionsoffilmformationdrivenbycapillaryforces [30].Thegreatercurvatureandhighersurfaceareaofsmallparticles areexpectedtoencouragefilmformation.Thespecificsurfacearea orthetotalsurfaceenergyofSmPSparticles(diameter203nm)is muchlargerthanthatofLgPSparticles(diameter382nm).Astheir totalsurfaceenergyismuchlessthanthatofSmPSparticles,LgPS particlerequireshigherenergytocompleteviscousflowprocess.

3.1.2. Healingandinterdiffusion

ThedecreaseinIP wasalreadyexplainedinprevioussection,

byinterdiffusion of polymerchains.As theannealing tempera-tureisincreasedabovemaxima,somepartofthepolymerchains maycrossthejunctionsurfaceandparticleboundariesdisappear, asaresultIP decreasesduetotransparencyoftheﬁlm.Inorder

toquantifytheseresults,thePrager–Tirrell(PT)model[37,38]for thechaincrossingdensitycanbeemployed.Theseauthorsusedde Gennes’s“reptation”modeltoexplainconﬁgurationalrelaxationat thepolymer–polymerjunctionwhereeachpolymerchainis con-sideredtobeconﬁnedtoatubeinwhichexecutesarandomback andforthmotion[39]Thetotal“crossingdensity”(t)(chainsper unitarea)atjunctionsurfacethenwascalculatedfromthe con-tributions1(t)duetochainsstillretainingsomeportionoftheir initialtubes,plusaremainder2(t)i.e.contributioncomesfrom chainswhichhaverelaxedatleastonce.Intermsofreducedtime =2 t/N2_the_total_crossing_density_can_be_written_as_[40]

() (∞)=2

−1/21/2 ₍₁₀₎

where andNarethediffusioncoefﬁcientandnumberoffreely jointedsegmentofpolymerchain[37].

Inordertocompareourresultswiththecrossingdensity of thePTmodel,thetemperaturedependenceof()/(∞)canbe modeledbytakingintoaccountthefollowingArrheniusrelation forthelineardiffusioncoefﬁcient

= o exp

−E kT

(11) hereEisdeﬁnedastheactivationenergyforbackbonemotion dependingonthetemperatureinterval.CombiningEqs.(10)and (11)ausefulrelationisobtainedas

() (∞)=Ro exp

−E 2kT

(12) whereRo=(8 ot/N2)1/2isatemperatureindependentcoefﬁcient.

ThedecreaseinIPinFigs.2and3aboveThisalreadyrelatedtothe

disappearanceofparticle–particleinterface.Asannealing tempera-tureincreased,morechainsrelaxedacrossthejunctionsurfaceand asaresultthecrossingdensityincreases.Now,itcanbeassumed

thatIP isinverselyproportionaltothecrossingdensity␴(T)and

thenthephenomenologicalequationcanbewrittenas IP(∞)=R−10 exp

E 2kBT

(13) Theactivationenergyofbackbonemotion,Eisproducedby least-squaresﬁttingthedatainFig.6(thelefthandside)toEq. (13)andarelistedinTable1.TheEvaluesforeachseriesseems almostnottochangewithincreasingAl2O3contentshowingthat

interdiffusionprocessis notaffectedbyAl2O3 content.

Further-more,EvaluesforLgPS/Al2O3seriesareslightlylargerthanthatof

SmPS/Al2O3series.Thepolymerchainscontainmorefreevolume

andlessinteractionbetweensegmentsinSmPSparticlesleading tohigherconformationalenergyandlessinteractionofpolymer chains[31,41].PolymerchainsintheSmPSparticleareinahighly conﬁnedstatebecauseofthespatiallimitationcomparedtothat oftherandom-coilstate[31]inLgPSparticles.Thisisthemajor reasonfortheSmPSparticlesneedlessenergytoaccomplish inter-diffusionprocessincomparisonwithLgPSparticlesincomposite ﬁlms.

4. Conclusions

Inthisstudy,weemployedthesteadystateﬂuorescence(SSF) techniqueinconjugationwithUVVandSEMtechniquestostudy ﬁlmformationprocessofPS/Al2O3nanocompositesand

morpho-logicalchangesdependingonPSparticlesizeandAl2O3content.

Theresultsshowedthatfilmformationprocessofboth compos-itefilmserieswasunaffectedbytheAl2O3contentsincethefilm

formationhasoccurredontopoftheAl2O3coveredparticles

dur-ingannealing. However,activationenergy values ofLgPS/Al2O3

serieswerefoundslightlylargerthanSmPS/Al2O3serieswhichcan

beexplained byPSsizeeffect.ExtractionofPSproducedhighly orderedporousstructuresforhighAl2O3contentinbothﬁlmseries.

ThemeasurementobtainedfromtheSEMshowedthatthepore sizeandporositycouldbeeasilytailoredbyvaryingthePSparticle sizeandtheAl2O3content.Theresultsalsoshowedthatthereisa

goodcorrespondencebetweentheopticaldataandSEMimages. Thesefindings provide insight intotheprinciple mechanism of latexfilmformationininorganicoxide-basedsystems.Thus,our studypresentsusefulinformationsandideasaboutthekineticsof filmformationincompositesystems.Ontheotherhand,similar observationswasreportedpreviouslybyHollandetal.,where tita-nia,zirconia,andaluminawereusedtoconstructhighlyordered periodic3Darraysofmacroporous,usingPSlatexspheresas tem-plates [42,43].In Holland’swork,correspondingmetalalkoxide precursorspermeatethroughPSspheresinroomtemperature,then closepacked,open-porestructureswith320to360nmvoidswere producedaftercalcinationsoftheorganiccomponentat575◦C. Besidesdip-coatingandfilm formationmodeling,themain dif-ferencebetweenourworkandtheirpresentationcomesfromthe dissolutionofPStemplateusingtoluenetoproduceorderedpore structurefromhighAl2O3contentcomposites.Inconclusion,the

(8)

whichcouldhaveapplicationsinareasrangingfromquantum elec-tronicstophotocatalysistobatterymaterials.

Acknowledgment

Dr.SunaywouldliketothanktheLaboratoriesinPhysics Depart-mentofITU,whereshehasdonetheexperimentalworkduringher Ph.D.studies,haveusedtopreparethismanuscript.

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