ﬁlms composite Fluorescence quenching method for monitoring oxygen diffusion intoPS/CNT Progress in Organic Coatings

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ContentslistsavailableatScienceDirect

Progress

in

Organic

Coatings

j o ur na l h o me pa g 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

Fluorescence

quenching

method

for

monitoring

oxygen

diffusion

into

PS/CNT

composite

ﬁlms

Ö.

Yargı

a,∗

_,

_S¸

_.

_U˘gur

b

_,

_Ö.

_Pekcan

c

a_Department_of_Physics_Yildiz_Technical_University,_Esenler_34210,_Istanbul,_Turkey b_Department_of_Physics_Istanbul_Technical_University,_Maslak_34469,_Istanbul,_Turkey c_Kadir_Has_University,_Cibali_34320,_Istanbul,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Available online 31 July 2013 Keywords: PS MWNT Fluorescence Oxygen Quenching

a

b

s

t

r

a

c

t

Oxygenpermeabilitiesofnanocompositeﬁlmsconsistingofmultiwallcarbonnanotubes(MWNT)and

polystyrene(PS)weredeterminedtoinvestigatetheoxygendiffusiondependingonMWNTand

tem-perature.Amethodwhichisbasedonquenchingofanexcitedphosphorescentbyoxygenwasapplied

forthemeasurements.Thecompositeﬁlmswerepreparedfrommixturesof(MWNT)and

surfactant-freepyrene(P)-labeled(PS)latexesofvariouscompositionsatroomtemperature.Theseﬁlmswerethen

annealedat170◦_C_which_is_well_above_the_glass_transition_(T_g₎_temperature_of_polystyrene,_for₁₀_min.

DiffusionexperimentswereperformedforeightﬁlmswithdifferentMWNTcontent(0,1.5,3,5,10,15,

25and40wt%)toevaluatetheeffectofMWNTcontentonoxygendiffusion.Diffusioncoefﬁcientswere

foundtoincreasefrom1.1×10−12_to₄₁_×₁₀−12_cm2_s−1_with_increasing_MWNT_content._On_the_other

hand,toexaminetheeffectoftemperatureonoxygendiffusion,diffusionmeasurementswereperformed

overatemperaturerangeof24–70◦_C_for_three_different_MWNT_contents_(3,_15,_and₄₀_wt%)_within_the

ﬁlms.TheresultsindicatedthatthevaluesofthediffusioncoefﬁcientDarestronglydependentonboth

temperatureandMWNTcontentintheﬁlm.Itwasalsoobservedthatthediffusioncoefﬁcientsobey

Arrheniusbehavior,fromwhichdiffusionenergiesweredetermined,whichincreasedwithincreaseof

MWNTcontentandtemperature.

1. Introduction

In the last few years, the most desirable property of poly-merfilmsistheirresistancetothegasdiffusion.Becausesingle componentpolymerfilmshavepoormechanicalandgasbarrier properties, there has been growing interest in producing new materialsby filling polymerswith inorganic natural (minerals) and/orsynthetic(carbonblackandsilica)compounds[1–5].They giveimprovedmechanicalproperties,gasbarrierproperties,and decreasedflammabilityrelativetothepurepolymers[6].In2000, thepotentialforCNTsasgassensorswasfirstreportedbasedonan increaseintheconductivitybyseveralordersofmagnitudeupon exposureof CNTstooxygen[7].Oxygenistheoneof themost importantreactantstobeconsideredinthediffusionphenomenon. Severalspectroscopictechniquesthatutilizeoxygenquenchingto determinetherateofoxygendiffusionthroughpolymerfilmshave beenreported.Cox [8]and Dunn[9], andMacCallum and Rud-kin[10] measuredoxygendiffusioncoefficientsbyfluorescence quenchinginplanarsheetsofpoly(dimethylsiloxane)[8],filled

∗ Correspondingauthor.

E-mailaddress:oyargi@yildiz.edu.tr(Ö.Yargı).

poly(dimethylsiloxane)samples[9],and polystyrene[10].They monitoredoxygenquenchingofafluorophoreasafunctionoftime byassumingthatfluorophoresdispersedhomogeneouslywithin thefilm.ThemathematicaldeterminationofDvaried,butasingle underlyingassumptioninallcaseswasthatthetime-dependent emissionintensitywasmeasuredduringtheexperiment.Insome cases,theintensityversustimecurvewasconvertedtoa concentra-tionversustimecurveusingtheStern–Volmerrelationship[8,9]. Luetal.[11–13]haveusedtime-scanexperimentstomeasurethe decayofluminescenceintensityasoxygendiffusesintopolymer filmsunderconstantilluminationandthegrowthofintensityas oxygendiffusesoutofthefilm.Theyinterpretedtheirdatawiththe aidoftheoreticalexpressionsbasedonStern–Volmerquenching kineticswithFick’slawsofdiffusion

Inthepresentworkweusedamethodfordeterminingdiffusion coefficients ofoxygenin CNT/PScompositesbasedon lumines-cencequenching.Theluminescenceofsomefluorophoresquench inthepresenceofoxygen.Initially,pyrene(P)-labeledpolystyrene and multi-wall carbon nanotube composite film is equilibrated ataparticularoxygenconcentrationandthenafterdisplacement of nitrogen atmosphere over the sample by oxygen, the film beginstoexposetoloweroxygenconcentration.Forpyrene,the intensity of quenching is proportional to the concentration of

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oxygen.Theaverageoxygenconcentrationchangeinthesample wasmonitoredbystudyingtheaverageintensitychangeofthe pyreneusingaspectrofluorometer.Weassumethequenchingis accuratelydescribedbyalinearStern–Volmerequation[14]and thattheopticaldensityislowenoughthatthesampleisuniformly excited.Byfittingtheresultantintensityversustimeprofiletoan appropriatediffusionmodelforthefilmconfiguration,ameasure ofthediffusioncoefficientsforthecompositefilmwereobtained.

2. Experımental

2.1. PSLatex

PyrenelabeledPSparticleswasproducedvia surfactantfree emulsion polymerizationprocess. The polymerizationwas per-formedbatch-wiselyusingathermostatedreactorequippedwith acondenser,thermocouple,mechanicalstirringpaddleand nitro-geninlet.Theagitationratewas400rpmandthepolymerization temperaturewascontrolledat70◦C.Water(100ml)andstyrene (5g)wereﬁrstmixedinpolymerizationreactorwherethe tem-peraturewaskeptconstant(at70◦C).Potassiumperoxodisulfate (KPS)initiator(0.1g) dissolvedin smallamountofwater(2ml) wasthenintroducedinorder toinducestyrenepolymerization. Thepolymerizationwasconductedduring18h.Thepolymerhasa highglasstransitiontemperature(Tg=105◦C).Thelatexdispersion

hasanaverageparticlesizeof400nm.Fig.1showsSEMimageof PSlatexproducedforthisstudy.

2.2. MWNTcarbonnanotube

Commerciallyavailable MWNTs(CheapTubesInc.,VT, USA,) wereusedassuppliedinblackpowderformwithoutfurther puriﬁ-cation. TheMWNT are10–30␮m long, averageinner diameter 5–10nm,outerdiameter20–30nm,thedensityisapproximately 2.1g/cm3_and_purity_higher_than₉₅_wt%._A_stock_solution_of_MWNTs

waspreparedfollowingthemanifacturersregulations:nanotubes weredispersedindeionized(DI)waterwiththeaidofPolyvinyl Pyrolidone(PVP)intheproportionsof10partsMWNTs;1–2parts PVP;2.000partsDIwaterbybathsonicationfor3h.PVPisagood stabilizing agent for dispersions of carbon nanotubes, enabling preparationofpolystyrenecompositesfromdispersionsofMWNT inpolystyrenesolution.Fig.2showstheTEMimageofMWNTsused inthisstudy(www.cheaptubesinc.com).

2.3. PreparationofPS/MWNTcompositeﬁlms

Eightdifferentmixtureswere preparedwith0,1.5, 3,5, 10, 15,25,and40wt%MWNT.Eachmixturewasstirredfor1h fol-lowedbysonicationfor30minatroomtemperature.Byplacingthe

Fig.1.SEMpictureofpurePSlatex.

Fig.2. TEMpictureofMWNT.

samenumberofdropsonaglassplateswithsimilarsurfaceareas (0.8cm×2.5cm)andallowingthewatertoevaporateat60◦Cin theoven,dryﬁlmswereobtained.Afterdrying,sampleswere sep-aratelyannealedaboveTgofPSfor10minattemperature170◦C.

Afterannealingstep,ﬁlmswereremovedfromtheovenandcooled downtoroomtemperature.Thethicknessoftheﬁlmswas deter-minedfromtheweightandthedensityofsamplesandrangesfrom 2to5␮m.Scanningelectronmicroscope(SEM)imagesweretaken byusingLEOSupraVP35FESEM

2.4. Theroticalconsiderations

2.4.1. Fluorescencequenchingbyoxygen

Data generated from oxygen quenching studies on small moleculesinhomogeneoussolutionareusuallyanalyzedusingthe Stern–Volmerrelation(Eq.(1)),providedthattheoxygen concen-tration[O2]isnottoohigh[14].

I0

I =1+kq0[O2] (1)

Inthis equation,Iand I0 aretheﬂuorescenceintensitiesinthe

presenceandabsenceofoxygen,respectively,kqisthe

bimolec-ularquenchingrateconstantand0istheﬂuorescencelifetimein

theabsenceofO2.

Diffusioncoefﬁcientsrelatedtothequenchingeventscanbe cal-culatedfromthetime-independentSmoluchowski–Einstein[14] equation,

kq=pko= 4NA(DP+Dq)pR

1000 (2)

whereDPandDqarediffusioncoefﬁcientsoftheexcitedprobeand

quencher,respectively,pisthequenchingprobabilitypercollision, Risthesumofthecollisionradii(RP+Rq),andNAisAvagadro’s

num-ber.Eqs.(1)and(2)canalsobeappliedtothecaseofquenchingof polymer-boundexcitedstatesinglassaslongastheﬂuorescence decayisexponentialandkqissingle-value.Simplifyingfor

interpre-tationofkqistheassumptionthatDPDqwhentheﬂuorescence

probeiscovalentlyattachedtoapolymer.

2.4.2. Diffusioninplanesheet

Fick’s second law of diffusion was used to model diffusion phenomena in planesheet. The following equation is obtained by assuming a constant diffusion coefﬁcient, for concentration changesintime[15] C C0 = x d+ 2

∞ n=1 cos n n sin nx d exp

−Dn22t d2

(3)

wheredisthethicknessoftheslab,Disthediffusioncoefﬁcientof thediffusant,andC0andCaretheconcentrationofthediffusantat

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DiffusionTime,t(s) 0 6000 12000 18000 24000 30000 Normalized Intensity 0,50 0,60 0,70 0,80 0,90 1,00 0 1.5 5 15 40

(a)

R (wt/wt) 0 10 20 30 40 50 Diffusion Coefficients, Dx10 -12 (cm 2.s -1) 0 10 20 30 40 50 60

(b)

Fig.3. (a)Thetimebehaviorofpyrene,P,fluorescenceintensity,I,duringoxygendiffusionintothecompositefilmswithdifferentMWNTcontent.Numbersoneachcurve indicatestheMWNTcontent(%)inthefilm.(b)Plotofthediffusioncoefficients,DversusWMWNT(wt%).

timezeroandt,respectively.xcorrespondstothepositionatwhich Cismeasured.Wecanreplacetheconcentrationtermsdirectlywith theamountofdiffusant,Mbyusingthefollowingrelation:

M=

v

CdV (4)

whenEq.(4)isconsideredforavolumeelementintheplanesheet andsubstitutedinEq.(3),thefollowingsolutionisobtained[15]:

Mt M∞ =1− 8 2

∞ n=0 1 (2n+1)2exp

−D(2n+1) 2 2_t d2

(5)

whereMt andM∞ represent theamounts ofdiffusant(oxygen)

enteringtheplanesheetattimetandinﬁnity,respectively.

3. Resultsanddiscussion

InFig.3(a),normalizedpyreneintensity,Ipcurvesarepresented

againstdiffusiontime forfilmshavingdifferentMWNTcontent exposed to oxygen. It is seen that as oxygen diffused through theplanarfilm,theemission intensityofthepyrene decreased accordingtoEq.(1)foreachMWNTcontentfilmand was satu-ratedonceoxygenequilibratedinthefilm.Here,ithastobenoted thatinFig.3(a)thequenchingrateforlowMWNTcontentfilmis lowerthanforhighMWNTcontentfilmpredictingthemorerapid quenchingofexcitedpyrenesbyO2 moleculesdiffused intothe

highMWNTcontentcompositeﬁlms.

Allcurvesbehavealmostinthesamefashion,asoxygendiffused throughandequilibratedintheﬁlm.Itisalsoseenthatthediffusion curvesreachtheirequilibrium valueatshorter timesforhigher MWNTcontentﬁlms.

Inordertointerprettheaboveﬁndings,Eq.(1)canbeusedby expandinginaseriesforlowquenchingefﬁciency,i.e.kq0[O2]1

whichthenproducesthefollowingusefulresult:

I≈I0(1−kq0[O2]) (6)

During O2 diffusion into the latex ﬁlms, P molecules are

quenchedin thevolume whichis occupiedbyO2 moleculesat

time,t.ThenPintensityattimetcanberepresentedbythevolume integrationofEq.(6)as It=

Id

v

d

v

=I0− kq0I0 V

d

v

[O2] (7)

wheredvandVarethedifferentialandtotalvolumeofcomposite ﬁlm.Performingtheintegrationthefollowingrelationisobtained It=I0

1−kq0 VO2(t)

(8) whereO2(t)=

d

v

[O2] istheamountofoxygenmoleculesdiffuse

intotheﬁlmattimet.IfitisassumedthatO2(t)correspondsto

MtthenEq.(5)canbecombinedforoxygenwithEq.(8)andthe

followingusefulrelationisobtainedwhichcanbeusedtointerpret thediffusioncurvesinFig.3(a)

Ip I0 =A+ 8C 2exp

−D2t d2

(9)

wheredisnowpresentastheﬁlmthickness,Distheoxygen dif-fusioncoefﬁcient,C=kqoO2(∞)/VandA=1−C.HereO2(∞)isthe

amountofoxygenmoleculesdiffusedintothefilmattimeinfinity. Asaresult,wecombinedthelinearStern–Volmerequationwith thediffusionmodeltoextractthediffusioncoefficientsfromthe experimentaldata.ThelogarithmicformofEq.(9)canbewritten asfollows: Ln

_Ip I0 −A

=Ln

_8C 2

−D2 d2 t (10)

Thismodelisﬁttedwiththeexperimentaldatausingalinear least-squaresﬁttingmethodtoextractbothkqanddiffusion

coef-ﬁcient(D)values.

InFig.3(b)theincreaseinDvalueswithincreasingcarbon nano-tubecontentmaybeattributedtothepresenceofalargefraction ofmicrovoidsinsidethehigherMWNTcontentﬁlms.Oxygencan diffuseveryrapidlytotheinsideofthecompositeﬁlmsthrough thesevoids.

Normalizedpyreneintensity,Ip,curvesarepresentedinFig.4

asafunctionoftimeforthe40wt%MWNTcontentﬁlmexposed

Diffusiontime,t(s)

0 500 1000 1500 2000 2500 3000

I

p

/I

0 0,0 0,2 0,4 0,6 0,8 1,0

24

0

C

50

0

C

60

0

C

(a)

Fig.4.Thetimebehaviorofthepyrene,P,ﬂuorescenceintensity,I,duringoxygen diffusionintothe40wt%MWNTcontentﬁlmatvarioustemperatures.Numberson eachcurveindicatethetemperature.

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T-1*103(0K-1) 2,8 2,9 3,0 3,1 3,2 3,3 3,4 LnD -25 -24 -23 -22 -21 T(0_C) 20 30 40 50 60 70 80 D( cm 2.s n -1 ) 0e+0 1e-10 2e-10 3e-10 40 15 3 40 15 3 (a) (b)

(a)

(b)

Fig.5.(a)Plotofthediffusioncoefficients,Dversustemperatures,Tforthe3,15 and40wt%MWNTcontentfilms.(b)Ln(D)versus1000/TforthedifferentMWNT fractioncontentfilms,respectively.(ED)valuesareobtainedfromtheslopesof

thestraightlinesforeachMWNTcontentﬁlm.

tooxygenatthreedifferenttemperatures.Itisseenthatas oxy-gendiffusedthroughtheplanarﬁlm,theemissionintensityofthe pyrenedecreasedaccordingtoEq.(1)foreachtemperature.The rateofdecreaseinintensityishigherathighertemperatures pre-dictingthemorerapidquenchingofexcitedpyrenemoleculesby O2moleculesdiffusedintotheﬁlms.Itisworthytonotethatin

Fig.5(a)asexpectedtheDincreaseswithincreaseintemperature forallcompositeﬁlms.Increaseintemperaturenaturallyincreases theBrownianmotionofoxygenmoleculesgiventhemmorechance tomeetthePmoleculesinthecompositeﬁlm.

In Fig. 6 also conﬁrm these results. Before annealing, no deformation in PS particles is observed and PS particles keep theiroriginalsphericalshapesforbothsamples.Afterannealing treatmentat 170◦C, SEM images showthat completesparticle coalescencehasbeenachieved.Itcanbeclearlyseenthatthe

com-positefilmconsistsofanetworkofbundlesofCNT,especiallyin the40wt%MWNTcontentfilm,andindicatessignificantporosity. AsshowninFig.6,carbonnanotubesarenotwelldistributedin thepolymermatrixandvoidsbetweenthecarbonnano-particles andpolymermatrixappearedallowingoxygenmoleculestomove rapidly.Therefore, O2 molecules can easilypass through these

voids.Thus,thepermeabilityofO2gasisincreased,yieldinghigh

diffusioncoefficients.Asaresult,morerapiddiffusionofoxygen [14]intothehigherMWNTcontentfilmsoccursduetothepresence ofalargenumberofmicrovoidsinthesefilms.

These results are consistent with previous studies [16–19]. Generally,enhancementinthegaspermeabilityofpolymersby puttinginorganicfillersintotheorganicpolymerresultedfromthe disturbedpolymerchainpackingbythenanofillers[16]. There-fore,thewelldispersedstateofcarbonnanotubesandtheirgood adherenceeffectivelyincreasesgaspermeabilityastheresultof effectiveinsertionsbetweenthepolymerchainsofthematrix.It wasreportedthatadditionof2wt%ofmodifiedcarbonnanotubes loadingtothepolyethersulfoneresultedinabout19.97%increases inthepermeabilityofCO2,whilethepermeabilityofCH4increased

upto33.79%.However,forsmallgasmolecules,suchasCO2,the

permeabilityincreasedslightlywiththeadditionofcarbon nano-tubesinthepolyethersulfone(PES)hostmatrix.Themainpathways ofgastransportthroughthemixedmatrixmembranesarethrough thedenselayerofthePESmatrix,highlyselectivecarbonnanotubes andnon-selectivegapsorvoidsbetweenthematrixandsieve par-ticles.Itwasobservedthatthemainfactoraffectingtheincrease ofCH4 permeabilitywiththeadditionofcarbonnanotubesinto

thepolymerhostresultedfromtheextremelyrapiddiffusionof gasmoleculesadsorbedinside thecarbon nanotubes.SEMdata alsoshowedthatthecarbonnanotubesarewelldispersedinthe polymermatrixandserveaschannelstotransportgasmolecules [17–19].Itisknownthattheadditionofﬁllerintopolymerﬁlms aboveacriticalpercentage createsvoids [17,18]inthepolymer matrix.Ponomarev andGouterman [17]have reportedthatthe additionofhighamountsoftitaniumoxide(TiO2)inpressure

sen-sitive(PSP)paintscausethepresenceofalargefractionofmicro voidsinsidetheﬁlms.Asaresult,aircandiffuseveryrapidlytothe insideofthecoatingthroughthesevoids.

Whenthepyrenediffusioninthelatexﬁlmisomittedandp=1 istakenthenEq.(2)becomesas

kq= 4NADmR

1000 (11)

Asimplifyingfactor intheinterpretationofkq isthegeneral

assumptionthatDPDqwhentheprobeiscovalentlyattachedtoa

polymer.Forquenchersassmallasmolecularoxygen,this assump-tionisreasonable.HereDmiscalledasmutualdiffusioncoefﬁcient

whichcannowbeassumedtobetheself-diffusioncoefﬁcientof O2inthecompositeﬁlm.SincekqisknownandifRistakenasthe

radiusofpyrene[19]thentheaveragedDmvaluesarefoundand

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Table1

Experimentallyobtainedmutualdiffusion(Dm)coefﬁcientsatvarioustemperatures.

Dm×10−5(cm2s−1) T(0_C) ₃ ₁₅ ₄₀ 24 1.4±0.05 1.1±0.01 1.7±0.10 40 2.2±0.007 0.3±0.008 3.8±0.19 50 0.7±0.02 0.4±0.008 2.5±0.02 60 0.2±0.01 0.3±0.005 2.2±0.01 70 2.9±0.007 0.5±0.014 0.5±0.02 Table2

ActivationenergiesofO2diffusiondependingonMWNTcontent(seeEq.(12)).

MWNT(wt%) 3 15 40

ED(kJmol−1) 68 76 100

listedinTable1forthecompositeﬁlmshavingdifferentMWNT

content.

ItisseeninTable1thatDmisindependentofMWNTcontent

in compositeﬁlmsi.e. onceO2 penetratesinto theﬁlmthen it

movesinshortrangeindependentofthematerialstructure.Dm

valueobtainedinthepresentstudy(10−5cm2_s−1₎_is _one_order

largerthanthatpreviouslyobtained(10−6cm2_s−1₎_by_using_the

sametechnique[19–21].Thisshowsthatthevoidsincomposite ﬁlmshelpstherapidquenchingofexcitedpyrenemoleculesand reducetheirresponsetime.

4. Diffusionenergies

Thediffusionof smallmoleculesthroughmembranescanbe describedasathermallyactivatedprocessthatobeysArrhenius behavior.Thetemperaturedependenceofthediffusioncoefﬁcient, D,canbewrittenasfollows:

D=D0exp

_−E D kBT

(12)

herekBistheBoltzmannconstant,D0ispre-exponantialfactor,ED

istheactivationenergyasassociatedwiththeoxygendiffusion.The activationenergywasdeterminedfromthelogarithmicplotsofthe Dcoefﬁcientagainstthereciprocaloftheabsolutetemperature.

InFig.5(b),Ln(D)wasplottedversus1000/Tforthedifferent MWNTfraction, respectively.Thevalueof theactivationenergy associatedwithoxygendiffusion(ED)fordifferentMWNT

frac-tionswascalculatedfromtheslopeoftheseplotsbyﬁttingthedata inFig.3(b)totheEq.(12)byaleastsquareﬁt.Theresultsaregiven inTable2,whereED increaseswithincreasingMWNTcontent.

TheenergyneedforoxygendiffusioninthehighMWNTmediumis muchhigherthaninalowMWNTenvironment.Mostprobably,the motionofO2moleculesisscreenedbythelargenumberofMWNT

barriersduringtheirjourneyinthehighMWNTcontentmedium, inwhereO2needshigherenergytoovercomethisdifﬁculty.

5. Conclusions

Wehavepresentasimple,fast,andpracticalroutetomeasure thediffusionofoxygenintoPS/MWNTcompositefilmsatelevated temperaturesfordifferentMWNTcontentsusingacombinationof fluorescencequenchingmethodandFickiantransport[22–24].The oxygendiffusioncoefficients(D)and relatedactivationenergies

(ED)inthesecompositeﬁlmsweredeterminedandcompared.

Theresultsshowedthatdiffusionofoxygenwasacceleratedby bothincreaseinMWNTfractionandtemperature.Thehigh diffu-sionrateofoxygeninthecompositeisattributedtotheformation ofvoids(pores)intheﬁlmwhichfacilitatesoxygendiffusion.The increaseintheenergyassociatedwiththeoxygendiffusionprocess (ED)isobservedwithincreaseinMWNTfraction.Inconclusion,

this workhasshownthat simpleseadystatefluorescence(SSF) techniquecanbeusedtomeasurethediffusioncoefficientof oxy-genmoleculesintocompositefilmsquiteaccurately.

References

[1]R.A.Vaia,K.D.Jandt,E.J.Kramer,F.P.Giannelis,Microstructuralevolutionof meltintercalatedpolymer-organicallymodiﬁedlayeredsilicates nanocompos-ites,Chem.Mater.8(1996)2628.

[2]M.W.Noh,D.C.Lee,SynthesisandcharacterizationofPS-claynanocomposite byemulsionpolymerization,Polym.Bull.42(1999)619.

[3]H.Z.Friedlander,C.R.Grink,OrganizedpolymerizationIII.Monomers interca-latedinmontmorillonite,J.Polym.Sci.Polym.Lett.2(1964)475–479.

[4]C.Kato,K.Kuroda,H.Takahara,Preparationandelectrical-propertiesandof quaternaryammoniummontromorillonite-polystrenecomplexes,ClayClay Miner.29(1981)294.

[5]J.G.Doh,I.Cho,Synthesisandpropertiesofpolystyreneorganoammonium montmorillonitehybrid,Polym.Bull.41(1998)511.

[6]Y.Li,B.Zhao,S.B.Xie,S.Zhang,Synthesisandpropertiesofpoly(methyl methacrylate)/montmorillonite(PMMA/MMT)nanocomposites,Polym.Int.52 (2003)892.

[7]A.Goldoni,R.Larciprete,L.Petaccia,S.Lizzit,Single-wallcarbonnanotube inter-actionwithgases:samplecontaminantsandenvironmentalmonitoring,J.Am. Chem.Soc.125(2003)11329–11333.

[8]M.E.Cox,Oxygendiffusioninpoly(dimethylsiloxane)usingﬂuorescence quenching1.Measurementtechniqueandanalysis,J.Polym.Sci.24(1986) 621.

[9]M.E.Cox,B.Dunn,Oxygendiffusioninpoly(dimethylsiloxane)using ﬂuores-cencequenching2.Filledsamples,J.Polym.Sci.24(1986)2395.

[10]J.R.MacCallum,A.L.Ruddin,AnoveltechniqueformeasuringTHdiffusion constantofoxygeninpolymerﬁlms,Eur.Polym.J.14(1978)655.

[11]X.Lu,I.Manners,M.A.Winnik,in:B.Vakuer,J.C.Brochan(Eds.),Fluorescence Spectroscopy;NewTrendsinFluorescenceSpectroscopy,SpringerVerlag,New York,2001,p.229(Chapter12).

[12]C.N.Jayarajah,A.Yekta,I.Manners,M.A.Winnik,Oxygendiffusionand perme-abilityinalkylaminothionylphosphazeneﬁlmsintendedforphosphorescence barometryapplications,Macromolecules33(15)(2000)5693.

[13]R.Ruffolo,C.Evans,X.H.Liu,Y.Ni,Z.Pang,P.Park,A.MacWilliams,X.Gu, X.Lu,A.Yekta,M.A.Winnik,I.Manners,Phosphorescentoxygensensors utilizingsulfur-nitrogen-phosphoruspolymermatrixes:synthesis, character-ization,andevaluationofpoly(thionylphosphazene)bpoly(tetrahydrofuran) blockcopolymers,Anal.Chem.72(2000)1894.

[14]S.A.Rice,in:C.H.Bamford,C.F.H.Tipper,R.G.Compton(Eds.),Diffusion-Limited ReactionsinComprehensiveChemicalKinetics,Elsevier,Amsterdam,1985.

[15]J.Crank,TheMathematicsofDiffusion,OxfordUniversityPress,London,1970.

[16]D.M.Delozier,K.A.Watson,J.G.Smith,JrT.C.Clancy,J.W.Connel,Investigationof aromatic/aliphaticpolyimidesasdispersantsforsinglewallcarbonnanotubes, Macromolecules39(2006)1731.

[17]S.Ponomarev,M.Gouterman,6thannualpressuresensitivepaintworkshop, Seattle,WA,Act.6–8,1998.

[18]A.K.Kneas,J.N.Demas,B.Nguyen,A.Lockhart,W.Xu,B.A.DeGraff,Method formeasuringoxygendiffusioncoefﬁcientsofpolymerﬁlmsbyluminescence quenching,Anal.Chem.74(2002)1111.

[19]S.Ugur,O.Yargi,O.Pekcan,Oxygendiffusionintopolystyrene-bentoniteﬁlms, Appl.ClaySci.43(2009)447–452.

[20]O. Pekcan, S. Ugur, Oxygendiffusion into latex ﬁlms annealed at vari-oustemperatures:a ﬂuorescencestudy, J.Coll. InterfaceSci.217 (1999) 154–159.

[21]O.Pekcan,S.Ugur,Packingeffectonoxygendiffusioninlatexﬁlms;aphoton transmissionandﬂuorescencestudy,Polymer41(2000)7531–7538.

[22]J.B.Birks,OrganicMolecularPhotophysics,Wiley-Interscience,NewYork,1975.

[23]S.Ugur,O.Yargi,O.Pekcan,Temperaturedependenceofoxygendiffusioninto clay-dopedPSﬁlms,Polym.Compos.31(2010)77–82.

[24]O. Yargi,S. Ugur,O. Pekcan,Diffusion energiesofoxygen diffusing into polystyrene(PS)/poly(N-isopropylacrylamide)composites,PolymAdv. Tech-nol.23(2012)776–782.