Combined XPS and contact angle studies of ethylene vinyl acetate and polyvinyl acetate blends

ContentslistsavailableatScienceDirect

Applied

Surface

Science

jo u rn a l h om epa g e :w w w . e l s e v i e r . c o m / l o ca t e / a p s u s c

Combined

XPS

and

contact

angle

studies

of

ethylene

vinyl

acetate

and

polyvinyl

acetate

blends

I.O.

Ucar

a

_,

_M.D.

_Doganci

a

_,

_C.E.

_Cansoy

a

_,

_H.Y.

_Erbil

a,∗

_,

_I.

_Avramova

b

_,

_S.

_Suzer

b

a_{DepartmentofChemicalEngineering,GebzeInstituteofTechnology,41400Kocaeli,Turkey}

b_{DepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received1January2011

Receivedinrevisedform21March2011

Accepted13June2011

Available online 21 June 2011 Keywords:

Ethylenevinylacetatecopolymers

Polymerblending

Surfacefreeenergy

XPS

Contactangle

Polyolefin

a

b

s

t

r

a

c

t

Inthisstudy,wepreparedthinfilmsbyblendingethylenevinylacetatecopolymers(EVA)containing 12–33(wt.%)vinylacetate(VA)withpolyvinylacetate(PVAc)andhighdensitypolyethylene homopoly-mers.Largeareamicropatternshavingcontrolledprotrusionsizeswereobtainedbyphase-separation especiallyforthePVAc/EVA-33blendsusingdipcoating.ThesesurfaceswerecharacterizedbyXPSand contactanglemeasurements.AreasonablylinearrelationwasfoundbetweentheVAcontentonthe surface(wt.%)obtainedfromXPSanalysisandtheVAcontentinbulkespeciallyforPVAc/EVA-33blend surfaces.PEsegmentsweremoreenrichedonthesurfacethanthatofthebulkforpureEVAcopolymer surfacessimilartopreviousreportsandVAenrichmentwasfoundontheEVA/HDPEblendsurfacesdue tohighmolecularweightofHDPE.WateredecreasedwiththeincreaseintheVAcontentontheblend

surfaceduetothepolarityofVA.Agoodagreementwasobtainedbetweens−andatomicoxygensurface

concentrationwiththeincreaseofVAcontent.TheapplicabilityofCassie–Baxterequationwastested andfoundthatitgaveconsistentresultswiththeexperimentalwatercontactanglesforthecasewhere VAcontentwaslowerthan55wt.%inthebulkcomposition.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polymerblendingis acheapsurface modificationmethodto obtaindesiredsurfacepropertiesofthinpolymercoatingsrather thancomparativelyexpensivemethodssuchasplasmatreatment, surfacegrafting,filmdepositionundervacuumetc.[1].When poly-mersareblended,thepreferentialenrichmentofsomefunctional groupsonthesurfaceaffectsthefinalpropertiesandapplications ofthesefilms.Phase-separatedroughorcomparativelyflat sur-facescanbe obtainedbychoosing convenientpolymer–solvent blending systems such as homopolymer–homopolymer, homopolymer–statisticalcopolymer,homopolymer–block copoly-mer, statistical copolymer–statistical copolymer [1–5]. Surface freeenergy,miscibility,viscosityattheprocesstemperature,and solubilityofeachpolymerinthechosensolventoftheblend com-ponentsarethemostimportantfactorswhichaffecttheblending process and theresultantfilms [1–5]. Themolecularweight of thesepolymers,filmthicknessandthesolventevaporationrateare theotherimportantparameters[1,4].Thispaperisabout prepara-tionandsurfacecharacterizationofPVAchomopolymer/EVA-33 copolymerblendshavingdifferentVAcontents inbulksolution. Wecoatedglassslideswiththepolymerblendsbyapplyingdip

∗ Correspondingauthor.Tel.:+902626052114;fax:+902626052105.

E-mailaddress:yerbil@gyte.edu.tr(H.Y.Erbil).

coating into polymer blend solutions and determined both the wettabilityandthesurfaceenrichmentofPEandVAcontentsby phase-segregationontheseblendsurfacesafterdryinginrelation tothebulkVAcontentoftheblendsolution.

In a phase-segregation process, the surface free energy dif-ferences ofthe involved polymersare thedriving force[2,3,5]. However,someresearchersrejectedthisviewandattributedthe surfacesegregationwiththeconformationalentropydifferences betweenthesurfaceandbulk[6,7].Accordingtothisgroup con-formational entropy in the bulk is higher than in the surface andwhenthenumberaveragemolecularweight(Mn)decreases, the conformational entropy of a chain at thefilm surface also decreases. Consequently, macromoleculehaving lower molecu-lar weight will be at the blend surface in order to minimize the conformationalentropy. Thisview canbe disputed sothat whena volatilesolventis usedincastingofthepolymerblend films,thesolventevaporatesrapidlyfromthesubstrateandthus thesystemcannotbeconsideredasanequilibriumprocess.For such non-equilibrium processes, polymer surface tensions and polymer–solvent interactions play much more important roles. This situation was explained by spreading coefficient concept for thepolymerblends[1,8].Lietal. [8]studiedtheformation ofpolystyreneandpolymethylmethacrylateblendfilmsandlow surface tensionpolystyrenewasfoundtolocate over the poly-methylmethacrylatelayerandspreading coefficientcalculations supportedthisresult.

0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

Polyethylenevinylacetatecopolymer(EVA),whichisawidely usedthermoplasticresin,hasbeenconsideredtobeagood candi-datetobeusedasabiomedicalmaterialduetoitsgoodphysical properties,easeofhandlingandprocessing,andmoderate biocom-patibility[9].EVA wasrecentlyusedtotest theremoval ofthe sporelingsofthegreenalgaUlvaformarinefoulingapplications [10].Ethylenevinylacetatecopolymersareproducedbyrandom copolymerizationofethyleneandvinylacetatemonomers,which aremainlyrecognizedfortheirflexibility,toughness(evenatlow temperatures)andadhesioncharacteristics[11].PropertiesofEVA copolymerschangemostlyduetothevariationoftheVAcontent. WhenpolarVAcontentisincreased,therelativequantityof amor-phousphaseincreasesandthedegreeofcrystallinitythatcomes frompolyethylenedecreases.IncreasingtheVAcontentchanges thefinal copolymer from modified polyethylene torubber-like productsand someofthepropertiessuchasflexibility, elonga-tion,adhesionandsolubilityinorganicsolventsimprove[11,12]. ItispossibletomodifyEVAcopolymersurfacesbyblendingwith polyethylene(PE)andpolyvinylacetate(PVAc)homopolymers.

Contactangle measurements and surface free energy calcu-lations are useful techniques not only for homopolymer and copolymersurfaces,butalsoforpolymerblendsurfacesto charac-terizefilmsurfacesatthetoplayer.Surfacefreeenergyanalysisof LDPE/EVAblendswerepreviouslystudiedbyChattopadhyayetal. [3].Contactanglemeasurementsand surfacefreeenergy calcu-lationsforLDPE/EVAblendswerealsoevaluatedbyAli[5]who concludedthatthemodificationofthesurfacepolarityoccurred whentheVAcontentofEVAcopolymerincreased.Asaresultofthis increase,contactanglesforwaterandreferenceliquidsdecreased andcalculatedsurfacefreeenergyvaluesraised[5].Matsunagaand Tamai[13]andlaterErbil[14]determinedsurfacefreeenergy val-uesofEVAcopolymersbyapplyingcontactanglemethod.Thesame methodwasalsoappliedtopolyethylenehomopolymerbyDann [15]andParketal.[16].

vanOss etal. [17] developed a successful approach to esti-matethesurfacefreeenergyofpolymers.Accordingtothistheory, Lifshitz–van der Waals interactions (indicated by superscript LW) include dispersion, polar–polar, and induction interac-tions, and acid base interactions (indicated by superscript AB) includehydrogen-bonding interactions,in otherwordselectron donor–acceptorinteractions.Totalsurfacefreeenergyisthesumof theseLifshitz–vanderWaalsandacid–baseinteractions[17]. Sur-facefreeenergydeterminationofEVAcopolymersbyapplyingvan Oss–Good–ChaudhurymethodwasstudiedbyGrundkeetal.[18]. Similarly,Michalskiet al.[2]appliedvanOss–Good–Chaudhury methodtodeterminethesurfacefreeenergyofEVA,PVCandtheir blends.

X-rayphotoelectronspectroscopy(XPS)wasappliedto deter-minethesurfacecompositionsoftheEVAcopolymersanditsblends whichhavevaryingVAcontents[19–21].Chihanietal.[19]used XPScharacterizationoftheEVAsurfacesobtainedbytheinjection moldingmethodandfoundthatsurfaceconcentrationofVAgroups washigherthan thatof thebulkwhen perfluorinatedethylene propylene(FEP)wasusedasthemould.Galuska[20]studiedEVA copolymerandEVA/LDPEblendsurfacesbyusingXPSandobtained alinearrelationbetweensurfaceandbulkVAcontentaccordingto oxygenconcentration.SurfacepropertiesofEVAcopolymerswere modifiedbytreatmentwithlowpressureRFplasmas[22],UV radia-tion[23]andthechangeofitsadhesionpropertiesweredetermined bycontactanglemeasurementsandXPS.

Inapreviousstudy,weinvestigatedthesurfacechemical struc-tureandwettingpropertiesofbothflatandroughEVAcopolymer filmsbyvaryingtheconcentrationandtemperatureofthedip coat-ingsolution[24].Asolutionconcentrationof40mg/mlwasused fortheflatcoatingsandupto100mg/mlfortheroughcoatings andthetemperatureschangedfromroomtemperatureto125◦C.

XPSanalysisat0◦ and60◦ take-offangles(approximately10nm and5nmdepths,respectively)wasappliedandcontactangle mea-surementswerecarriedoutbyincreasingtheVAcontentofthe bulkEVAcopolymer.XPSresultsshowthathydrophobicPE com-ponentwasenrichedonEVAsurfacesaround5nmdepthforallthe samples,whereashydrophilicVAcomponentwasenrichedonthe surfaceswhenVA<18%foronlyaround10nmdepth.Hydrophobic PEcomponentwasfoundtoenrichinthenear-surfaceregionfor allflatandroughEVAsamplesforadepthofaround5nm.The dif-ferencebetweentheXPSresultsoftheflatandroughsurfaceswas notsignificantforEVAsamplesexceptEVA-33surfacewherethe atomicoxygencontentdecreased15%for10nmand20%for5nm depthduetoitsverylowmolecularweight[24].

In the present study, we applieddip coating of glass slides in polymer blend solutions of EVA-33 copolymer with PVAc homopolymerforthefirsttimeanddeterminedboththe wetta-bilityof driedblend surfacesand thesurface enrichmentofPE andVAcontentsbyphase-segregationinrelationtotheVA con-tentof theblendsolutionin bulk.In addition,wealsoblended EVAcopolymers(EVA-12,EVA-18,EVA-28andEVA-33)withHDPE homopolymerforcomparison.Contactangle,surfacefreeenergy analysisandXPSmeasurementsweredoneinordertoinvestigate thewettabilitypropertiesandsurfacecompositionsoftheseblend surfaces.ThecorrelationofsurfacefreeenergywiththeXPSresults wasdiscussedandtheapplicabilityoftheCassie–Baxterequation [25],whichwasderivedforthechemicallyheterogeneoussurfaces; wasalsoinvestigatedfortheblendsurfaces.

2. Experimental 2.1. Materials

Polyvinyl acetate and high density polyethylene (HDPE) homopolymers and ethylene-vinyl acetate copolymers with varyingVAcontents(EVA-12,EVA-18,EVA-28-05,EVA-28-40, EVA-28-150, EVA-33and EVA-40)wereused for the preparation of blendsurfaces.ThenamesofEVAcopolymersareself-descriptive, forexamplethatEVA-28-40hasaVAcontentof28wt.%,witha meltflowindexof40.Thenamesofmanufacturers,vinylacetate (VA)contentsandalsoexperimentallydeterminedmeltflowindex values(MFI)ofthepolymersaregiveninTable1.All homopoly-mersandcopolymerswereusedasreceived.Standardglassslides (76mm×26mm,ISOLAB,Turkey)wereusedintheexperiments. Atwo-componentpolyepoxidelayer(404Chemicals,Turkey)was appliedastheprimercoatingontheglassslidesforthefilmstobe usedforcontactanglemeasurements.MERCKspectroscopicgrade water,methyleneiodide,ethyleneglycolandformamideliquids wereusedinstaticanddynamiccontactanglemeasurements. 2.2. Preparationofpolymericcoatings

Glassslides wereusedassubstratesand cleanedinchromic acid,rinsedwithdistilledwaterand driedina vacuumovenat 100◦C.Apolyepoxidelayer(404adhesive)wasdepositedonglass slidesbyapplyingdipcoatingfromitschloroformsolutionasthe primercoatingtocompensatefortheweakadherenceofpolymers ontoglassslides.Polyepoxideprimerwasonlyappliedforsamples, whichwereusedinthecontactanglemeasurements.Thinfilms fromblendsofEVAcopolymerscontaining12–33wt.%VAcontents withPVAc and HDPE homopolymerswere preparedfrom their xylene(mixtureofo-,m-,p-isomers,m-predominating)solutions athightemperaturesbydipcoatingtechnique.Theconcentration ofallthepolymersolutionswas20mg/ml.Cleanglassslideswere dippedintothepolymersolutionsbyusingaprecisehome-made mechanicaldipperat130◦Candwithdrawnfromthepolymer

solu-Table1

Characteristicsofpolymers.

Polymer VAcontentin

bulk(wt.%)

MFIa_{(g/10min)ASTM}

D1238(2.16kg,190◦C)

MFI(g/10min)

experimental(2.16kg,

190◦_C)

Manufacturer Commercialname

HDPE 0 N/A 0.16 LyondellBasell HOSTALENGM8255

EVA-12 12 2.5 2.2 DuPontInc. ELVAX660

EVA-18 18 1.8 1.8 AsiaPolymerCorp. EV101

EVA-28-05 28 5–8 5 ArkemaLtd. EVATANE

EVA-28-40 28 35–45 33 ArkemaLtd. EVATANE

EVA-28-150 28 135–175 124 ArkemaLtd. EVATANE

EVA-33 33 350–450 375 ArkemaLtd. EVATANE

EVA-40b _{40 57 N/A Aldrich –}

PVAcb _{100 N/A 105 Aldrich –}

a_{Quotedfromsuppliers’catalogues.}

b_{MolecularweightsofEVA-40andPVAcare42.000g/moland100.000g/molrespectively[32].}

tionsatspecificrateof320mm/min.Hightemperaturesandlow depositionrateswereusedtoachievecomparativelyflatcoatings. Coatedglassslidesweredriedinavacuumovenovernightat25◦C andkeptinadesiccator.

2.3. Staticanddynamiccontactanglemeasurements

KSV-CAM200-Finlandcontactanglemeterwasusedtomeasure thestaticcontactanglesoftheliquidsunderair.Equilibrium(e) contactanglesofwater,methyleneiodide,ethyleneglycoland for-mamideweremeasuredbyusing5␮ldropletvolumestoneglect thegravity flatteningeffect. Theneedle wasremovedfromthe dropletduringtheemeasurementhoweveritwaskeptwithinthe liquiddropletsduringtheadvancing(a)andreceding(r)contact anglemeasurements.Firstadropletof3_␮lvolumewasformedand itsvolumewasincreasedto8␮lduringtheameasurement.An ini-tialdropvolumeof8␮lwasdecreasedto2␮lwhilemeasuringthe r.Contactanglemeasurementsweretakenover3differentareas foreachpolymersample.Averageandstandarddeviationof val-ueswerecalculatedaslessthan±2.Waterdynamiccontactangle measurementswerecarriedoutusingaKSVSigma700Dynamic Tensiometerapparatusatroomtemperature,usingthepolymer coatedglassslidesasWilhelmyplatesdippinginpurewater. 2.4. Opticalmicroscopy

Surfacetopographyofallthecoatedsampleswereinvestigated byusingaNIKONECLIPSELV100OpticalMicroscopewith×500 magnification.

2.5. X-rayphotoelectronspectroscopy

XPS investigations were carried out by means of a Kratos 800spectrometerwithMg K␣ (unmonochromatized) sourceat 1253.6eVwithatotalinstrumentalresolutionof∼1eV,undera basepressureof10−8mbar.TheC1sandO1sphotoelectronlines wererecordedandcalibratedtotheC1slineat285.0eV.XPSPEAK 4.0fittingprogramwasusedfordeconvolutionofthephotoelectron peaks.Theatomicsensitivityfactorhasbeenevaluatedasgivenin [26].Alldatawererecordedat90◦take-offangle,correspondingto maximumsamplingdepthofapproximately8nm.

3. Resultsanddiscussions 3.1. Opticalmicroscopyimages

OpticalmicroscopeimagesofPVAc/EVA-33blendswithvarying VAcontentsat×500magnificationaregiveninFig.1.Largearea patternshavingspecificprotrusionsizeswereobtainedasseenin

thisfigure,wherethesizeofprotrusionswasdecreasedwiththe increaseofVAcontentinthebulkEVAcopolymer.Itcanbe specu-latedthattheprotrusionscorrespondtoPEregionssincetheirtotal areaonthesurfacedecreaseswiththeincreaseofVAcontent. 3.2. XPSresults

X-rayphotoelectronlinesofC1sandO1shavebeenrecorded forthepolymerslistedinTable2,andweredeconvolutedfor bet-terevaluationofsurface(O/C)ratio.TheC1speaksarecomplex andcanbecurve-fittedtothreepeaksassigned tohydrocarbon (C–H),etheric(C–O)andcarbonyl(C O)groupsonthesurfaceat around285.0eV,286.5eVand289.1eVrespectively.TheO1speaks arecurve-fittedtotwopeaks,whichareassociatedwith(C–O)and (C 0)groups.X-rayphotoelectronlineofC1sandO1speaksare showninFig.2aandbforthePVAchomopolymersurfaceasan indicativefigure.ThemainelementsonthesurfaceofpurePVAc areoxygenandcarbon.ThefunctionalcompositionofpurePVAc filmcanbedeterminedbycurvefittingofC1speak.Three differ-entcarboncomponentswereconsidered:hydrocarbon(C–H/C–C) at285.0eV;alcoholorether(C–OH/C–O–C)at286.4eVandester (O–C O)at288.8eV.TheO1speakofpurePVAcfilmconsistedof twooxygenfunctionalities:ester(C–O–C O)at534.6eVand car-bonyl(O–C O)at533.2eV[27].Blendratios,bulkandsurfaceVA contentsofEVAcopolymersaregiveninTable2.Oxygento car-bonratios(O/C)andatomicoxygenconcentrationsarealsogiven inthistable.Thesurfaceoxygenatomicconcentrationsmeasured at90◦ take-offangleforadepthof8nm,were1–19%lowerthan thetheoreticalvaluescalculatedfromthebulkcopolymer compo-sitionforallthepureEVAcopolymers.Thisisinagreementwith thepreviousreportsindicatingthatPEsegmentsaremoreenriched atthesurfacethanVAsegmentsforEVAcopolymersbydiporspin coating[20,24].

ThechangeofVAcontentonthePVAc/EVA-33blendsurface versustheVAcontentinthebulkisgiveninTable2andFig.3a. AsseeninthisfigurethechangeofVAcontentonthesurfacefora depthof8nm(at90◦take-offangle)wasnotsignificantwhenall thedatapointswereconsideredindicatingthatneitherPEnorVA enrichmentoccurred.Wealsodeterminedthatsimilartothe previ-ousfindings[20,24],PEsegmentsweremoreenrichedatthesurface foradepthof8nmforpureEVAcopolymersasshowninFig.3b wherethesurfaceatomicoxygenconcentrationswere1–19%lower thanthetheoreticalvaluescalculated fromthebulkcopolymer. However,anoppositebehaviorwasseenforalloftheEVA/HDPE blendsasseen fromthedatapointsof (50/50)compositionsof EVA-12,EVA-18EVA-28,EVA-33withHDPEasgiveninFig.3band atomicOconcentrationsmeasuredat90◦take-offanglewerefound tobe37–62%largerthanthetheoreticalvaluesforEVA/HDPEblend surfacesindicatingVAenrichmentattheseblendsurfaces.

Natu-Table2

TheoreticalandexperimentalresultsofXPS.

Theoretical 90◦Take-offangle

Polymer VA%bulk O/C %molatomicO VA%surf. O/C %molatomicO

HDPE 0 0.000 0.00 1.22 0.004 0.40 EVA-12/HDPE(50/50) 6 0.020 1.96 8.93 0.030 2.91 EVA-18/HDPE(50/50) 9 0.030 2.94 14.58 0.050 4.76 EVA-12 12 0.041 3.92 11.78 0.040 3.85 EVA-28/HDPE(50/50) 14 0.048 4.58 22.63 0.080 7.41 EVA-33/HDPE(50/50) 16.5 0.057 5.40 22.63 0.080 7.41 EVA-18 18 0.063 5.89 17.32 0.060 5.66 EVA-28-05 28 0.101 9.19 22.63 0.080 7.41 EVA-28-40 28 0.101 9.19 22.63 0.080 7.41 EVA-28-150 28 0.101 9.19 27.74 0.100 9.09 EVA-33 33 0.122 10.84 27.74 0.100 9.09 EVA-40 40 0.152 13.16 32.66 0.120 10.71 PVAc/EVA-33(20/80) 46.4 0.180 15.29 48.49 0.190 15.97 PVAc/EVA-33(30/70) 53.1 0.212 17.52 52.65 0.210 17.36 PVAc/EVA-33(50/50) 66.5 0.282 22.01 69.73 0.300 23.08 PVAc/EVA-33(65/35) 76.6 0.340 25.39 81.38 0.370 27.01 PVAc/EVA-33(80/20) 86.6 0.404 28.78 82.94 0.380 27.54 PVAc/EVA-33(85/15) 90 0.427 29.92 84.48 0.390 28.06 PVAc 100 0.500 33.33 94.64 0.460 31.51

(a)

(b)

538 536 534 532 530 Binding Energy, eV O1s 292 290 288 286 284 282 280 Binding Energy, eV C1s

Fig.2. X-rayphotoelectronlinesof(a)C1sand(b)O1speaksforPVAchomopolymer.

rally,PEenrichmentisexpectedforallEVA/HDPEblendsurfaces whencomparedwiththeirbulkcompositionbecausePE compo-nenthavingthelowersurfacefreeenergyshouldmigratetothe solid–airinterfaceinablendingprocessinordertominimizethe interfacialtensioninmostofthecases.

Thus,theenrichmentofVA contentontheEVA/HDPEblend surfacewasanexceptionandneedsanexplanation:Sincea phase-separationoccursduringtheformationofEVA-polyolefinblends, it creates regions where VA or PE weremore concentrated on theblendsurfacedependingontheVAcontent[28],densityand molecularweightoftheusedpolymers.TheVAenrichmentonthe EVA/HDPEblendsurfacemaybeattributedtothelowerMFIvalueof HDPEthanalloftheEVAcopolymers,whichallowstheEVAcontent havinglowerMwthanHDPEtogouptothenearsurface.The maxi-mumVAenrichmentwasseenforthe(50/50)EVA-28/HDPEblend composition.TheincreaseintheVAcontentofEVAcopolymerin bulkalsoincreasestheVAcontentontheEVA/HDPEblendsurface (40–67%asO/Cratio),exceptforEVA-33/HDPEblendbecauseof thelowMwofEVA-33copolymerhavingaveryhighMFIvalueas giveninTable1.

Nevertheless,thesurfaceVA compositionsobtainedfromthe XPSmeasurementsgenerallyfittedwiththecorrespondingbulk compositionswithinathinbandasseeninFig.3aandb,although minordeviationsoccurred.Thus,PVAc/EVA-33blendsurfacescan beusedaspracticaltest surfaceswheretheVA contentsofthe blendsonthesurfacecanbecalculated byadding theVA frac-tionofthePVAchomopolymerandEVA-33copolymerinthebulk composition. y = 0,9604x + 1,4468 R2 _{= 0,9844} 0 20 40 60 80 100 20 0 40 60 80 100 VA content in bulk (wt. %) VA content on surface (wt. %)

(a)

y = 0,8816x + 2,6156 R2 = 0,9651 0 20 40 60 80 100 20 0 40 60 80 100 VA content in bulk (wt. %) VA content on surface (wt. %)

(b)

Fig.3.DependenceofVAcontentonsurface(wt.%)versustheVAcontentinbulk

for:(a)PVAc/EVA-33blends,(b)EVA/HDPEblendsandEVAcopolymers.

3.3. Contactangleandsurfacefreeenergyresults

Staticadvancing,a,equilibrium,e,andrecedingcontactangle, r measurementresultsobtainedbyKSV-CAM200-Finland con-tactanglemeteranddynamica,rresultsofwaterdropsobtained byKSVSigma700DynamicTensiometeronallsample surfaces aregiveninTable3.Contactanglehysteresis(),whichisthe differencebetweenadvancingandrecedingwatercontactangles, ( =a−r),indicateseitherthechemicalheterogeneityforflat surfacesorsurfaceroughnessofchemicallyhomogeneoussurfaces [4].Staticanddynamicresultsofallsamplesarealsogivenin Table3.Staticwatereresultsofthepolymersdecreasedfrom102◦ to60◦withtheincreaseofpolarhydrophilicVAcontent.Thesame decreaseofaandrresultswiththeincreaseofVAwasalsoseenin Table3.StaticwatereresultswiththechangeinVAcontentinbulk (wt.%)forallofthesamplesandalsotheliteraturedataaregiven inFig.4a.TheincreaseofpolarVAcontentonpolymersurfaces resultedinadecreaseofthewaterequilibriumcontactanglesin agreementwiththepreviousreports[2,14,29,30].Weplottedboth thestaticanddynamicadvancingcontactangleswiththechangein VAcontentinbulk(wt.%)forallofthesamplesinFig.4bfor compar-ison.Asseeninthisfigure,agoodagreementexistsbetweenstatic anddynamicadvancingcontactangleresultsforthesamples

con-Table3

Staticanddynamicwatercontactangleresultsofhomopolymersandpolymer

blends. Static Dynamic Polymer a e r  a r  HDPE 109 102 90 19 107 88 19 EVA-12/HDPE(50/50) 99 94 76 23 98 78 20 EVA-18/HDPE(50/50) 93 87 80 13 93 77 16 EVA-12 93 84 79 14 100 80 20 EVA-28/HDPE(50/50) 98 90 77 21 96 72 24 EVA-33/HDPE(50/50) 98 87 66 32 96 63 33 EVA-18 92 82 75 17 93 70 23 EVA-28-05 88 79 67 21 93 67 26 EVA-28-40 92 81 63 29 92 66 26 EVA-28-150 93 80 64 29 92 62 30 EVA-33 93 78 48 45 94 48 46 EVA-40 94 77 47 47 96 46 50 PVAc/EVA-33(20/80) 76 76 50 26 82 46 36 PVAc/EVA-33(30/70) 75 73 53 22 84 44 40 PVAc/EVA-33(50/50) 72 62 51 21 83 47 36 PVAc/EVA-33(65/35) 71 61 50 21 79 40 39 PVAc/EVA-33(80/20) 72 61 52 20 80 40 40 PVAc/EVA-33(85/15) 71 61 53 18 80 38 42 PVAc 80 60 34 46 78 34 44

taininglessthan40wt.%VAwhereasthedynamicaangleresults

werearound10◦higherthanthestaticonesafter40wt.%VA con-tentinbulk,forthePVAc/EVA-33blendsurfaces.Thisshowsthat thedynamiccontactanglemeasurementismoresensitivetothe surfaceroughnessandchemicalheterogeneitythanthestatic con-tactanglemethod.Ontheotherhand,lowerstaticvalueswere

50 60 70 80 90 100 110 20 0 40 60 80 100 VA content in bulk (wt. %) VA content in bulk (wt. %) e Erbil 1987 Devallencourt 2002 du Toit1995 Michalski 1998 This work

(a)

50 60 70 80 90 100 110 120 20 0 40 60 80 100 a static dynamic

(b)

Fig.4.Dependenceof(a)waterequilibriumstaticcontactangle(experimentaland literaturedata),(b)waterstaticanddynamicadvancingcontactanglewiththe changeinVAcontentinbulk(wt.%)forallofthepolymers.

Table4

Equilibriumcontactangleresultsoftestliquidsonpolymers.

Polymer MeI2 Formamide EG

HDPE 53 85 72 EVA-12/HDPE(50/50) 47 74 69 EVA-18/HDPE(50/50) 46 70 67 EVA-12 49 77 71 EVA-28/HDPE(50/50) 47 81 71 EVA-33/HDPE(50/50) 42 70 70 EVA-18 46 74 70 EVA-28-05 45 72 68 EVA-28-40 43 77 69 EVA-28-150 49 81 72 EVA-33 43 73 74 EVA-40 42 83 73 PVAc/EVA-33(20/80) 47 65 58 PVAc/EVA-33(30/70) 49 70 61 PVAc/EVA-33(50/50) 41 68 65 PVAc/EVA-33(65/35) 45 53 53 PVAc/EVA-33(80/20) 45 66 53 PVAc/EVA-33(85/15) 45 62 52 PVAc 41 43 54

obtainedforPVAc/EVA-33blendsurfacesthanthatofthepurePVAc andEVA-33surfaces,althoughthereisnotanydirectrelationship betweenthecontactanglehysteresisandtheVAcontent.Wemay attributethedecreaseintothedecreaseofsurfaceroughness duringblendingPVAcandEVA-33.Itwasfoundthatourresultsof pureEVAwereclosetothereportedavaluesgivenin[19,22,23].

SurfaceshavinghigherVAcontentswerealsostudiedinthe liter-aturebyusingEVAcopolymerswithhighVAcontent[29]orEVA blends[2].Michalskietal.[2]reportedwatereofEVA-70(70wt.% VAcontent)copolymeras67.1◦,whichisclosetoourvalueof62◦ forthePVAc/EVA-33(50/50)blendsurfacewhichhas66.5wt.%VA contentinbulk.

Surfacefreeenergyofasolidcanbedeterminedbye measure-mentsofdifferenttestliquiddropsonthesolidsurface[4,17].We appliedvanOss[17]methodforthesurfacefreeenergy calcula-tions. LV(1+cos)=2



_SLWLLW+



_S+L−+



_S−L+



(1) wheresubscriptSissolid,Lisliquid,Visvapor,superscriptLW denotesthe“Lifshitz–vanderWaalsinteractions”andABdenotes the“acid–baseinteractions”,and_i+ istheLewisacid,and_i−is theLewisbaseparameterofsurfacefreeenergy,(AB

i =2



_i+_i−). Both the solid surface and liquid drop consistsof two surface freeenergycomponentterms,oneisLW_{comprising“dispersion”,} “dipolar”,and“induction”interactionsandtheothertermisAB comprisingalltheelectrondonor–acceptorinteractions,suchas hydrogen-bonding.Theirsumgivesthetotalsurfacefreeenergy (Tot

i =iLW+iAB).WeneedasetofvaluesofLLW,L+andL−for thereferenceliquidssuchasmethyleneiodide,␣-bromo naphtha-lene,ethyleneglycol,glycerolandformamide,whichwassupplied byvanOss–Goodbyusingarbitraryrelation,_W+ =_W− forwater [4,17],inorder toapplyEq.(1)tothee data.Ingeneral,three formsofEq.(1)aresimultaneouslysolvedbyusingtheedataof threedifferentliquidswithtwoofthembeingpolarand hydrogen-bonding.

Wecalculated_S−,_S+,AB

S ,andStotvaluesofthepolymersby usingEq.(1)accordingtovanOss–Good–Chaudhurymethodafter determiningthee valuesofthemethyleneiodide(MeI2), ethy-leneglycol(EG),andformamide(F)testliquids,whicharegivenin Table4.Thecalculatedsurfacefreeenergyresultsofallthe sam-plesarereportedinTable5.Weplottedthevariationofatomic oxygensurfaceconcentrationfor90◦ take-offangleandelectron donorparameter,_S−withtheincreaseoftheVAcontentinbulk (wt.%)inFig.5andverygoodagreementwasobtainedbetween_S−

0 5 10 15 20 25 30 35 20 0 40 60 80 100 VA content in bulk (wt.%) 0 5 10 15 20 25 30 35 Atomic O (XPS-90 o ) Atomic O (XPS-90 )°

Fig.5.PlotoftheatomicoxygensurfaceconcentrationobtainedbyXPS

measure-mentsat90оtake-offangleandelectrondonorparameter,_S−withtheincreaseof

VAcontentinbulk(wt.%).

andatomicoxygenconcentrationsimilartoanotherrecentreport showingthestrength of thevanOss–Good–Chaudhurymethod [26]. Theincrease inVA contentresulted ina small riseinthe totalsurfacefreeenergycomponent,tot

S asseeninTable5 how-evertherewasnodirectrelationshipbetweentot

S andVAcontent especiallyforblendsprobablyduetotheintroductionofsurface roughnessbyphase-separationontheseblendcoatings.

3.4. ApplicabilityofCassie–Baxterequation

In 1944, Cassie–Baxter [25] derived an equation for two-component composite solid surfaces with varying degrees of heterogeneitiesanddefinedtheequilibriumCassie–Baxtercontact angle,CB.

cosCB=f1 cos1−f2cos2 (2)

f1andf2aretheliquid/solidcontactareafractionsofsolid com-ponents1and2onthesurfaceand1and2indicatethecontact angleswhicharemeasuredonflat1and2surfacesrespectively.Eq. (2)indicatesthatthecontactanglemeasuredonaheterogeneous surfacecanbecalculatediftheareafractionsofthepolymer com-ponentsareknown.Cassie–Baxterequationwasfoundtobeuseful Table5

SurfacefreeenergyresultsofpolymersurfacescalculatedbyusingvanOss–Good

equation(mJ/m2_). Polymer LW S +S S−  AB S  tot S HDPE 32.6 0.0 0.2 0.0 32.6 EVA-12/HDPE(50/50) 35.9 0.0 1.4 0.0 35.9 EVA-18/HDPE(50/50) 36.5 0.0 4.0 0.0 36.5 EVA12 34.8 0.0 6.3 0.0 34.8 EVA-28/HDPE(50/50) 35.9 0.0 2.8 0.0 35.9 EVA-33/HDPE(50/50) 38.6 0.0 3.4 0.0 38.6 EVA18 36.5 0.0 6.9 0.0 36.5 EVA28-05 37.0 0.0 8.8 0.0 37.0 EVA28-40 38.1 0.0 6.9 0.0 38.1 EVA28-150 34.8 0.0 9.0 0.0 34.8 EVA33-400 38.1 0.0 9.0 0.0 38.1 EVA40 38.6 0.0 9.5 0.0 38.6 PVAc/EVA-33(20/80) 35.9 0.0 11.6 0.0 35.9 PVAc/EVA-33(30/70) 34.8 0.0 14.9 0.0 34.8 PVAc/EVA-33(50/50) 39.1 0.0 23.1 0.0 39.1 PVAc/EVA-33(65/35) 37.0 0.01 25.2 1.0 38.0 PVAc/EVA-33(80/20) 37.0 0.0 25.8 0.0 37.0 PVAc/EVA-33(85/15) 37.0 0.0 25.8 0.0 37.0 PVAc 39.1 0.2 22.1 4.2 43.3 50 60 70 80 90 20 0 40 60 80 100 CB theoretical experimental VA content in bulk (wt.%)

Fig.6. TheoreticalCassie–Baxterand experimentallymeasuredcontactangles versustheVAcontentinbulk(wt.%)byusingweightfractioncalculation.

intheanalysisofchemicallyheterogeneousflatsurfaces,andalso airpocketcontainingroughsurfacesalthoughitcannotexplainthe corrugationofthethree-phasecontactlinebetweenthedropand solid[31].

We tested theapplicabilityof theCassie–Baxterequationto thechemicallyheterogeneous PVAc/EVA-33blendsurfaces: We assumed thatthesolid areafractionsf1 and f2 areequal tothe weight fractions on the surface and calculated them for PVAc homopolymer and EVA-33copolymerseparately. We measured watere onflatPVAcandEVA-33as1 and2.Thenwesolved Eq.(2) forthePVAc/EVA-33blendsand calculatedthe theoreti-calCassie-Baxtercontactangle,CB_{.Fig.6showsthevariationof} thetheoreticalCassie–Baxterandexperimentallymeasured con-tactangleswiththeincreaseofVAcontentinbulkbyusingthe weightfractionresults.Asseeninthisfigure,Cassie–Baxtertheory givesgoodagreementwiththeexperimentalresultsbelow55wt.% totalVAcontentinbulkwhichcanbeattributedtothepresence ofthehigherconcentrationofthemorehydrophobicEVAregions onthesurface.However,theoreticalCassie–Baxtercontactangles andexperimentalonesdidnotfitwitheachotherfortheVA con-tentswhichwerehigherthan55wt.%probablyduetheincreasein hydrophilicityarisesfromtheVAgroup.Inthisregion,eresultsof theblendswereveryclosetotheresultsofPVAchomopolymeras giveninTable3.ThisshowsthattheCassie–Baxterequationgives betterresultsforthecaseswherehydrophobicregionsdominate onthesurface.

4. Conclusions

Large areapatterns having controlledprotrusion sizes were obtainedforPVAc/EVA-33blendsbyapplyinganinexpensivedip coatingmethod.Areasonablylinearrelationwasfoundbetween theVAcontentonthesurface(wt.%)obtainedfromXPSanalysisand theVAcontentinbulkespeciallyforPVAc/EVA-33blendsurfaces. ForpureEVAcopolymersurfaces,PEsegmentsaremoreenriched onthesurfacethanthatofthebulk similartopreviousreports. However,wedeterminedVAenrichmentontheEVA/HDPEblend surfaces,whichmaybeattributedtothehighmolecularweightof HDPE.

The increase in polar and hydrogen-bonding VA content on polymersurface resultedinadecrease e values ofwater drop. The relation between surface free energy and XPS results was investigatedandagoodagreementwasobtainedbetweenbasic surface free energycomponent, s−,and atomic oxygensurface concentrationwiththeincreaseofVAcontent.Wealsotestedthe applicabilityoftheCassie–Baxtertheoryandagoodagreementwas

foundwiththeexperimentalwatereresultsforsurfaceshaving below55wt.%totalVAcontent.However,whenVAcontentswere higherthan55wt.%,thentherewasapooragreementwiththis theoryandexperimentalresultsprobablyduetotheincreasein hydrophilicregionsonthesurfacecontainingVAgroups.In con-clusion,Cassie–Baxterequationfitstheexperimentalresultsbetter forthecaseswherehydrophobicregionsdominateonthesurface. References

[1]D.R. Paul, S. Newman, Polymer Blends, Academic Press, New York,

1978.

[2] M.C.Michalski,J.Hardy,B.J.H.Saramago,J.ColloidInterfaceSci.208(1998)

319–328.

[3] S.Chattopadyay,R.N.Ghosh,T.K.Chaki,A.K.Bhowmick,J.Adhes.Sci.Technol.

15(2001)303–320.

[4] H.Y.Erbil,SurfaceChemistryofSolidandLiquidInterfaces,Blackwell

Publish-ing,Oxford,UK,2006.

[5]Z.I.Ali,J.Appl.Polym.Sci.104(2007)2886–2895.

[6] E.Helfand,Z.R.Wasserman,Macromolecules9(1976)879–888.

[7]S.K.Kumar,T.P.Russell,Macromolecules24(1991)3816–3820.

[8]Y.Li,Y.Yang,F.Yu,L.Dong,J.Polym.Sci.PartB:Polym.Phys.44(2006)

9–21.

[9] M.Yin,Y.Yuan,C.Liu,J.Wang,Biomaterials30(2009)2764–2773.

[10]I.O.Ucar,C.E.Cansoy,H.Y.Erbil,M.E.Pettitt,M.E.Callow,J.A.Callow,

Biointer-phases5(2010)75–84.

[11]A.M.Henderson,IEEEElec.Insul.Mag.9(1993)30–38.

[12]H.Y.Erbil,VinylAcetateEmulsionPolymerizationandCopolymerizationwith

AcrylicMonomers,CRCPress,USA,2000.

[13]T.Matsunaga,Y.Tamai,J.Appl.Polym.Sci.22(1978)3525–3530.

[14]H.Y.Erbil,J.Appl.Polym.Sci.33(1987)1397–1412.

[15]J.R.Dann,J.ColloidInterfaceSci.32(1970)302–320.

[16]S.J.Park,H.C.Kim,H.Y.Kim,J.ColloidInterfaceSci.255(2002)145–149.

[17]C.J.vanOss,M.K.Chaudhury,R.J.Good,Chem.Rev.88(1988)927–941.

[18] K.Grundke,H.J.Jacobasch,F.Simon,S.T.Schneider,J.Adhes.Sci.Technol.9

(1995)327–350.

[19] T.Chihani,P.Bergmark,P.Flodin,JAdhes.Sci.Technol.9(1995)843–857.

[20]A.A.Galuska,Surf.InterfaceAnal.21(1994)703–710.

[21] R.L.McEvoy,S.Krause,P.Wu,Polymer39(1998)5223–5239.

[22]C.M.Cepeda-Jimenez,R.Torregrosa-Macia,J.M.Martin-Martinez,J.Adhes.Sci.

Technol.17(2003)1145–1159.

[23]M.D. Landete-Ruiz, J.M.Martin-Martinez, Int. J. Adhes.Adhes. 25(2005)

139–145.

[24]M.D.Doganci,C.E.Cansoy,I.O.Ucar,H.Y.Erbil,E.Mielczarski,J.A.Mielczarski,

J.Appl.Poly.Sci.,inpreparation.

[25]A.B.D.Cassie,S.Baxter,Trans.Faraday.Soc.40(1944)546–551.

[26] H.Y.Erbil,B.Yas¸ar,S.Suzer,B.M.Baysal,Langmuir13(1997)5484–5493.

[27] G.Beamson,D.Briggs,HighResolutionXPSofOrganicPolymers:TheScienta

ESCA300Database,Wiley,Chichester,1992.

[28]B.Na,Q.Zhang,Q.Fu,G.Zhang,K.Shen,Polymer43(2002)7367–7376.

[29]C.Devallencourt,S.Marais,J.M.Saiter,M.Labbe,M.Métayer,Polym.Test.21

(2002)253–262.

[30]F.J.duToit,R.D.Sanderson,W.J.Engelbrecht,J.B.Wagener,J.FluorineChem.74

(1995)43–48.

[31]H.Y.Erbil,C.E.Cansoy,Langmuir25(2009)14135–14145.

[32]W.L.Lee,M.Hong,E.Wijdada,S.C.J.Loo,Macromol.RapidCommun.31(2010)