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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
baDepartmentofChemicalEngineering,GebzeInstituteofTechnology,41400Kocaeli,Turkey
bDepartmentofChemistry,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)andhighdensitypolyethylenehomopoly- 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].Whenpoly- mersareblended,thepreferentialenrichmentofsomefunctional groupsonthesurfaceaffectsthefinalpropertiesandapplications ofthesefilms.Phase-separatedroughorcomparativelyflatsur- facescanbe obtainedbychoosing convenientpolymer–solvent blending systems such as homopolymer–homopolymer, homopolymer–statisticalcopolymer,homopolymer–blockcopoly- mer, statistical copolymer–statistical copolymer [1–5]. Surface freeenergy,miscibility,viscosityattheprocesstemperature,and solubilityofeachpolymerinthechosensolventoftheblendcom- ponentsarethemostimportantfactorswhichaffecttheblending process and theresultantfilms [1–5]. Themolecularweight of thesepolymers,filmthicknessandthesolventevaporationrateare theotherimportantparameters[1,4].Thispaperisaboutprepara- 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].Accordingtothisgroupcon- 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 thepoly- methylmethacrylatelayerandspreading coefficientcalculations supportedthisresult.
0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2011.06.070
Polyethylenevinylacetatecopolymer(EVA),whichisawidely usedthermoplasticresin,hasbeenconsideredtobeagoodcandi- datetobeusedasabiomedicalmaterialduetoitsgoodphysical properties,easeofhandlingandprocessing,andmoderatebiocom- patibility[9].EVA wasrecentlyusedtotest theremoval ofthe sporelingsofthegreenalgaUlvaformarinefoulingapplications [10].Ethylenevinylacetatecopolymersareproducedbyrandom copolymerizationofethyleneandvinylacetatemonomers,which aremainlyrecognizedfortheirflexibility,toughness(evenatlow temperatures)andadhesioncharacteristics[11].PropertiesofEVA copolymerschangemostlyduetothevariationoftheVAcontent.
WhenpolarVAcontentisincreased,therelativequantityofamor- 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,butalsoforpolymerblendsurfacestocharac- terizefilmsurfacesatthetoplayer.Surfacefreeenergyanalysisof LDPE/EVAblendswerepreviouslystudiedbyChattopadhyayetal.
[3].Contactanglemeasurementsand surfacefreeenergycalcu- lationsforLDPE/EVAblendswerealsoevaluatedbyAli[5]who concludedthatthemodificationofthesurfacepolarityoccurred whentheVAcontentofEVAcopolymerincreased.Asaresultofthis increase,contactanglesforwaterandreferenceliquidsdecreased andcalculatedsurfacefreeenergyvaluesraised[5].Matsunagaand Tamai[13]andlaterErbil[14]determinedsurfacefreeenergyval- uesofEVAcopolymersbyapplyingcontactanglemethod.Thesame methodwasalsoappliedtopolyethylenehomopolymerbyDann [15]andParketal.[16].
vanOss etal. [17] developed a successful approach toesti- 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)wasappliedtodeter- 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],UVradia- tion[23]andthechangeofitsadhesionpropertiesweredetermined bycontactanglemeasurementsandXPS.
Inapreviousstudy,weinvestigatedthesurfacechemicalstruc- tureandwettingpropertiesofbothflatandroughEVAcopolymer filmsbyvaryingtheconcentrationandtemperatureofthedipcoat- ingsolution[24].Asolutionconcentrationof40mg/mlwasused fortheflatcoatingsandupto100mg/mlfortheroughcoatings andthetemperatureschangedfromroomtemperatureto125◦C.
XPSanalysisat0◦ and60◦ take-offangles(approximately10nm and5nmdepths,respectively)wasappliedandcontactanglemea- surementswerecarriedoutbyincreasingtheVAcontentofthe bulkEVAcopolymer.XPSresultsshowthathydrophobicPEcom- ponentwasenrichedonEVAsurfacesaround5nmdepthforallthe samples,whereashydrophilicVAcomponentwasenrichedonthe surfaceswhenVA<18%foronlyaround10nmdepth.Hydrophobic PEcomponentwasfoundtoenrichinthenear-surfaceregionfor allflatandroughEVAsamplesforadepthofaround5nm.Thedif- 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 homopolymerforthefirsttimeanddeterminedboththewetta- bilityof driedblend surfacesand thesurface enrichmentofPE andVAcontentsbyphase-segregationinrelationtotheVAcon- 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.Allhomopoly- 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◦Candwithdrawnfromthepolymersolu-
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 –
aQuotedfromsuppliers’catalogues.
bMolecularweightsofEVA-40andPVAcare42.000g/moland100.000g/molrespectively[32].
tionsatspecificrateof320mm/min.Hightemperaturesandlow depositionrateswereusedtoachievecomparativelyflatcoatings.
Coatedglassslidesweredriedinavacuumovenovernightat25◦C andkeptinadesiccator.
2.3. Staticanddynamiccontactanglemeasurements
KSV-CAM200-Finlandcontactanglemeterwasusedtomeasure thestaticcontactanglesoftheliquidsunderair.Equilibrium(e) contactanglesofwater,methyleneiodide,ethyleneglycolandfor- mamideweremeasuredbyusing5ldropletvolumestoneglect thegravity flatteningeffect. Theneedle wasremovedfromthe dropletduringtheemeasurementhoweveritwaskeptwithinthe liquiddropletsduringtheadvancing(a)andreceding(r)contact anglemeasurements.Firstadropletof3lvolumewasformedand itsvolumewasincreasedto8lduringtheameasurement.Anini- tialdropvolumeof8lwasdecreasedto2lwhilemeasuringthe
r.Contactanglemeasurementsweretakenover3differentareas foreachpolymersample.Averageandstandarddeviationofval- 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.Itcanbespecu- latedthattheprotrusionscorrespondtoPEregionssincetheirtotal areaonthesurfacedecreaseswiththeincreaseofVAcontent.
3.2. XPSresults
X-rayphotoelectronlinesofC1sandO1shavebeenrecorded forthepolymerslistedinTable2,andweredeconvolutedforbet- 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.Threediffer- 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)at534.6eVandcar- bonyl(O–C O)at533.2eV[27].Blendratios,bulkandsurfaceVA contentsofEVAcopolymersaregiveninTable2.Oxygentocar- bonratios(O/C)andatomicoxygenconcentrationsarealsogiven inthistable.Thesurfaceoxygenatomicconcentrationsmeasured at90◦ take-offangleforadepthof8nm,were1–19%lowerthan thetheoreticalvaluescalculatedfromthebulkcopolymercompo- sitionforallthepureEVAcopolymers.Thisisinagreementwith thepreviousreportsindicatingthatPEsegmentsaremoreenriched atthesurfacethanVAsegmentsforEVAcopolymersbydiporspin coating[20,24].
ThechangeofVAcontentonthePVAc/EVA-33blendsurface versustheVAcontentinthebulkisgiveninTable2andFig.3a.
AsseeninthisfigurethechangeofVAcontentonthesurfacefora depthof8nm(at90◦take-offangle)wasnotsignificantwhenall thedatapointswereconsideredindicatingthatneitherPEnorVA enrichmentoccurred.Wealsodeterminedthatsimilartotheprevi- 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
Fig.1. OpticalmicroscopeimagesofPVAc/EVA-33blendsatX500magnification(a)46.4,(b)53.1,(c)66.5,(d)76.6,(e)86.6,(f)90wt.%VAcontentinbulk.
(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 whencomparedwiththeirbulkcompositionbecausePEcompo- nenthavingthelowersurfacefreeenergyshouldmigratetothe solid–airinterfaceinablendingprocessinordertominimizethe interfacialtensioninmostofthecases.
Thus,theenrichmentofVA contentontheEVA/HDPEblend surfacewasanexceptionandneedsanexplanation:Sinceaphase- separationoccursduringtheformationofEVA-polyolefinblends, it creates regions where VA or PE weremore concentrated on theblendsurfacedependingontheVAcontent[28],densityand molecularweightoftheusedpolymers.TheVAenrichmentonthe EVA/HDPEblendsurfacemaybeattributedtothelowerMFIvalueof HDPEthanalloftheEVAcopolymers,whichallowstheEVAcontent havinglowerMwthanHDPEtogouptothenearsurface.Themaxi- 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 theVAfrac- 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-Finlandcon- 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.4bforcompar- ison.Asseeninthisfigure,agoodagreementexistsbetweenstatic anddynamicadvancingcontactangleresultsforthesamplescon-
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.%VAcon- tentinbulk,forthePVAc/EVA-33blendsurfaces.Thisshowsthat thedynamiccontactanglemeasurementismoresensitivetothe surfaceroughnessandchemicalheterogeneitythanthestaticcon- 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].
SurfaceshavinghigherVAcontentswerealsostudiedintheliter- aturebyusingEVAcopolymerswithhighVAcontent[29]orEVA blends[2].Michalskietal.[2]reportedwatereofEVA-70(70wt.%
VAcontent)copolymeras67.1◦,whichisclosetoourvalueof62◦ forthePVAc/EVA-33(50/50)blendsurfacewhichhas66.5wt.%VA contentinbulk.
Surfacefreeenergyofasolidcanbedeterminedbyemeasure- mentsofdifferenttestliquiddropsonthesolidsurface[4,17].We appliedvanOss[17]methodforthesurfacefreeenergycalcula- tions.
LV(1+cos)=2
SLWLLW+
S+L−+
S−L+
(1)
wheresubscriptSissolid,Lisliquid,Visvapor,superscriptLW denotesthe“Lifshitz–vanderWaalsinteractions”andABdenotes the“acid–baseinteractions”,andi+ istheLewisacid,andi−is theLewisbaseparameterofsurfacefreeenergy,(iAB=2
i+i−).
Both the solid surface and liquid drop consistsof two surface freeenergycomponentterms,oneisLWcomprising“dispersion”,
“dipolar”,and“induction”interactionsandtheothertermisAB comprisingalltheelectrondonor–acceptorinteractions,suchas hydrogen-bonding.Theirsumgivesthetotalsurfacefreeenergy (iTot=iLW+iAB).WeneedasetofvaluesofLLW,L+andL−for thereferenceliquidssuchasmethyleneiodide,␣-bromonaphtha- lene,ethyleneglycol,glycerolandformamide,whichwassupplied byvanOss–Goodbyusingarbitraryrelation,W+ =W− forwater [4,17],inorder toapplyEq.(1)tothee data.Ingeneral,three formsofEq.(1)aresimultaneouslysolvedbyusingtheedataof threedifferentliquidswithtwoofthembeingpolarandhydrogen- bonding.
WecalculatedS−,S+,SAB,andStotvaluesofthepolymersby usingEq.(1)accordingtovanOss–Good–Chaudhurymethodafter determiningthee valuesofthemethyleneiodide(MeI2), ethy- leneglycol(EG),andformamide(F)testliquids,whicharegivenin Table4.Thecalculatedsurfacefreeenergyresultsofallthesam- plesarereportedinTable5.Weplottedthevariationofatomic oxygensurfaceconcentrationfor90◦ take-offangleandelectron donorparameter,S−withtheincreaseoftheVAcontentinbulk (wt.%)inFig.5andverygoodagreementwasobtainedbetweenS−
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-90o )
Atomic O (XPS-90 )°
Fig.5.PlotoftheatomicoxygensurfaceconcentrationobtainedbyXPSmeasure- mentsat90оtake-offangleandelectrondonorparameter,S−withtheincreaseof VAcontentinbulk(wt.%).
andatomicoxygenconcentrationsimilartoanotherrecentreport showingthestrength of thevanOss–Good–Chaudhurymethod [26]. Theincrease inVA contentresulted ina small riseinthe totalsurfacefreeenergycomponent,StotasseeninTable5how- evertherewasnodirectrelationshipbetweenStotandVAcontent 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/solidcontactareafractionsofsolidcom- ponents1and2onthesurfaceand1and2indicatethecontact angleswhicharemeasuredonflat1and2surfacesrespectively.Eq.
(2)indicatesthatthecontactanglemeasuredonaheterogeneous surfacecanbecalculatediftheareafractionsofthepolymercom- ponentsareknown.Cassie–Baxterequationwasfoundtobeuseful
Table5
SurfacefreeenergyresultsofpolymersurfacescalculatedbyusingvanOss–Good equation(mJ/m2).
Polymer SLW +S S− SAB totS
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 calculatedthetheoreti- calCassie-Baxtercontactangle,CB.Fig.6showsthevariationof thetheoreticalCassie–Baxterandexperimentallymeasuredcon- tactangleswiththeincreaseofVAcontentinbulkbyusingthe weightfractionresults.Asseeninthisfigure,Cassie–Baxtertheory givesgoodagreementwiththeexperimentalresultsbelow55wt.%
totalVAcontentinbulkwhichcanbeattributedtothepresence ofthehigherconcentrationofthemorehydrophobicEVAregions onthesurface.However,theoreticalCassie–Baxtercontactangles andexperimentalonesdidnotfitwitheachotherfortheVAcon- 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.Incon- clusion,Cassie–Baxterequationfitstheexperimentalresultsbetter forthecaseswherehydrophobicregionsdominateonthesurface.
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