Macromolecules；PreparationandSurfacePro；RichardR.Thomas,*,?Dougl；JacksonLaboratory,E.I.du；ReceivedDecember19,1996;；ABSTRACT:Aseriesofnovelp；reactionmassarediscussed；Intro
Macromolecules3-
PreparationandSurfacePropertiesofAcrylicPolymersContainingFluorinatedMonomers
RichardR.Thomas,*,?DouglasR.Anton,?WilliamF.Graham,?MichaelJ.Darmon,?BryanB.Sauer,§KatherineM.Stika,|andDennisG.Swartzfager|
JacksonLaboratory,E.I.duPontdeNemoursandCompany,Deepwater,NewJersey08023,MarshallLaboratory,E.I.duPontdeNemoursandCompany,Philadelphia,Pennsylvania19146,CentralResearchandDevelopment,ExperimentalStation,E.I.duPontdeNemoursandCompany,Wilmington,Delaware,andCorporateCenterforAnalyticalSciences,ExperimentalStation,E.I.duPontdeNemoursandCompany,Wilmington,Delaware
ReceivedDecember19,1996;RevisedManuscriptReceivedMarch3,1997X
ABSTRACT:Aseriesofnovelpolymershavebeenpreparedbyfreeradicalsolutionpolymerizationofavarietyofhydrocarbonmonomerswiththe(perfluoroalkyl)ethylmethacrylatemonomerH2CdC(CH3)-j≈7.7).Polymerspreparedusingtwodifferentmethodsoffeeding1intotheCO2(CH2)2(CF2)nF,1(n
reactionmassarediscussed.Throughajudiciouschoiceofreactionconditions,somecontrolofpolymerarchitecturewasexhibited.Theresultant(perfluoroalkyl)ethylmethacrylate-containingacrylicpolymerswereshowntobequitesurfaceactiveinthesolidstate.Themagnitudeofsurfaceactivitydependsonthemonomer1feedmethod.Thepolymerswereformulatedintothinfilmsandappliedtoavarietyofsurfaces.Withlevelsat1.5wt%of1,waterandoilrepellentsurfaceswerecreated.Watercontactangles(advancing)of～80-115°andhexadecanecontactangles(advancing)of～60-70°wereobservedroutinely.Inaddition,itwasobservedthattheconcentrationprofileoffluorineinthefilmsexhibitedasteepgradientnormaltothesurfacewhenstudiedbyangle-dependentESCAandsecondaryionmassspectroscopicdepthprofiling.
Introduction
Organicpolymerfilmshavefoundwideapplicationforcoatingadiversearrayofmaterials.Thesurfacepropertiesofthesefilmsisofparamountimportanceforsuchapplicationswhereadhesionconcernsarepresent.Forexample,loweringthesurfacetensionofafilmisdesirableforformulatingnonwettablesurfaces.Oneofthemostpopularandsuccessfulstrategiesforloweringthesurfacetensionofafilmisincorporationoffluorineintothepolymermoleculecomprisingthecoating.Thefluorinecanbeincorporatedintothemainpolymerchain.Examplesusingfluorinateddiols1-7andfluori-natedalcohols8topreparepolyurethaneshavebeenpresented.Surfacetensionreductionhasalsobeenstudiedusingblendsoffluoropolymerswithhydrocar-bonpolymers.9,10Severalreportshaveusedthepro-pensityofenthalpicallydrivenchain-endenrichmentatsurfaces11,12inconjunctionwithfluorinationtoaltersurfacetensions.13-15Surfacetensionreductionhasalsobeenreportedforcopolymerspreparedbyincorpo-rationoffluorinatedmonomers16-21andbygraftingperfluoroalkylgroupstopolymerschemically.22
However,thepreviousstudieshavefocusedmainlyonpolymerswhichcontainrelativelylargeweightfractionsoffluorinatedcomponents(～1-8atom%F1～0.2-0.6molefractionfluoromonomersinacrylates16-20).Thecurrentworkwilldemonstratethatlargeweightfractionsoffluorinatedmaterialsarenotnecessarytoachievelowsurfacetensions.Anexampleofachievingsurfacetensionreductionbyblendingsmallamountsofperfluoroalkyl-endedpoly-ethylenesintopolyethylenehasbeenreported.15Fur-*Towhomcorrespondenceshouldbeaddressed.?JacksonLaboratory.?MarshallLaboratory.
§CentralResearchandDevelopment.
||CorporateCenterforAnalyticalSciences.
XAbstractpublishedinAdvanceACSAbstracts,May1,1997.
thermore,thepresentsystemsarecross-linkable,givinghighmolecularweight,durablefilmswithoutdealingwithviscoussolutionsormelts.Itisthesurfaceactivityofthesemoleculeswhichplaystheimportantroleinthemagnitudeofsurfacetensionreduction.Itwillalsobeshownthatjudiciouschoicesofreactionconditionswillallowsomecontrolofpolymerarchitectureandmolec-ularweight.Thesurfacesofthesefilmswillbeexam-inedusingcontactanglemeasurements,X-rayphoto-electronspectroscopy(ESCA),time-of-flightstaticsecondaryionmassspectroscopy(ToF-SIMS),anddy-namicsecondaryionmassspectroscopy(DSIMS).ResultsandDiscussion
SynthesisofAcrylatePolymerSystems.Theconditionsusedtomakethe(perfluoroalkyl)ethylmeth-acrylate-containingacrylicpolymersaretypicalofthoseusedtoprepareacrylicpolymers.Thestandardpolymersystemispreparedbythefreeradicalpolymerization(usingtert-butylperoxyacetateasinitiator)ofbutylmethacrylate,styrene,butylacrylate,andhydroxypro-pylacrylateinmethylamylketonesolventusingsemi-batch,monomer-starvedconditions.Thecompositionofthistypicalacrylicpolymer,2,isshowninFigure1.Inanefforttomakecomparisonsbetweenstandardandfluorine-containingpolymersequitable,fluorinatedver-sionsofthepolymerswerepreparedbysubstitutionofsmallamountsofH2CdC(CH3)CO2(CH2)2(CF2)nF,1,forbutylmethacrylate.Avarietyoffluorinatedacrylatemonomershavebeenshowntoprovidesu23however,thisworkwillfocusononeparticularvariant,3,whichincorporatesthefluorinatedmonomer1.ThepurepolymersystemswillhaveaTg≈0°Csocross-linkingisnecessarytoprovidethedesiredmechanicalproperties.Thecross-linkerem-ployedinthecurrentstudyistheisocyanuratetrimerofhexamethylenediisocyanateandisshowninFigure2.Synthetically,theparametersvariedincludedreac-?1997AmericanChemicalSociety
S)01868-2CCC:$14.00
2884Thomasetal.Figure1.Typesofacrylicmonomersandpolymersexamined.
Figure2.Structureoftheisocyanuratetrimerofhexameth-ylenediisocyanate.
tiontime,additionmethodoffluorinatedacrylatemonomer,andamountoffluorinatedmethacrylatemonomer.
Previousattemptsatformulatinglowsurfacetension,fluorine-containingacrylatecoatingsfailedbecauseofinefficientandineffectiveuseoffluorinewhichneces-sitatedtheuseoflargeamountsoffluorinatedacrylateormethacrylatemonomers.Furthermore,thelargeamountofaddedfluorineoftendecreasesvaluablebulkproperties.Fromthedata,videinfra,itcanbeseenthattheamountoffluorinatedmonomerandespeciallythemethodofadditionaffectedthefinalsurfaceproper-tiesofthepolymermost.Theadditiondwelltimeofthefluorinatedmonomer(forexample,shotvscontinu-ousfeed)hasprovidedthesinglegreatestvariationinthesepolymers.
Theresultsofthisworkwillfocusontwotypesofpolymerspreparedusingidenticalamountsoffluori-natedmonomer1.Oneispreparedbythecontinuousfeedadditionof1tothereactionmass.Theotherbytheadditionofashotof1tothereactiondesignedtoperturbsteady-statepolymerizationconditions.Asanexample,polymers3a-3cwerepreparedbytheaddi-tionoftheentireamountof1asrapidlyaspossible(i.e.,asashot),undercurrentexperimentalconditions,10minpriortotheendofthepolymerizationreaction.Severalvariationsoftheshotfeedmethodwereexam-inedandthepolymersobtainedfoundtobesurfaceactive.Inaddition,thesurfacepropertiesofpolymerssimilarto3werepreparedusingavarietyoffluorinatedacrylicmonomersanddifferentfeedmethods.Theresultsarecomparabletotheseriesbasedon3.
Acomparisonofthetypesofpolymerspreparedusingthesetwotypesofsyntheticprotocolsisworthnoting.Undertheconditionsusedtoprepare3cand3d(continuousfeedof1),thefluorinatedmonomerisaddedunderstarved-feedconditions.Thisresultsinasteady-
Macromolecules,Vol.30,No.10,1997
stateconcentrationof1(andallmonomersaswell)inthereactionmass.Thesteady-stateconcentrationconditionensuresthatallpolymerchainswillhaveidenticalcompositionsandmolecularweights.Theshotfeedadditionof1isaperturbationofthesteady-stateconditionsemployedinproducing3a-3c.Notonlydoestheshotadditionaffecttheinstantaneouscompositionofthepolymer,italtersthemolecularweightdistribu-tionduringadditionof1byincreasingthemolecularweight(substitutionoflighterhydrocarbonmonomersby1)anddegreeofpolymerization(additionofmonomerwithoutadditionofinitiator).Inessence,theshotfeedmethodresemblesblendingofafluorinatedsystemwithanonfluorinatedone.Moreover,thepreparation,insitu,offluorinatedpolymerchainsseemstoensurethecompatibilityofthefluorinatedpolymerswiththeunmodifiedone.
Thecompositionofthecontinuousfeedpolymers3dand3eisidenticalthroughoutthecourseofpolymeri-zationwitheachpolymerchaincontainingonlyasmallfractionof1.Statistically,thispolymerchainwillcontainafewblocksof(perfluoroalkyl)ethylmethacryl-atepolymerofanyappreciablelengthandisnotexpectedtoresultinaparticularlyeffectiveorefficientsurfaceactivespecies.Contrastthistothetypeofpolymersystempreparedwhen1isaddedasashot10minfromtheendofthereaction.Untiltheshotadditionof1,thepolymerchainscontainnoincorpo-rated(perfluoroalkyl)ethylmethacrylatemonomer.As1isadded,theinstantaneouscompositionofthepoly-merisricherinfluorocarbonmonomerattheexpenseofthehydrocarbonmonomers.Statistically,thereisagreaterchanceoflongerblocksof(perfluoroalkyl)ethylmethacrylateinthepolymerchain.Thisshouldresultinapolymerwhichisamuchmoreeffectiveandefficientsurfaceactivespecies.
SurfaceTensionsofAcrylatePolymers.Asdis-cussedpreviously,thecommercialapplicationofafluorine-containingacrylatepolymersystemdependsheavilyontheeffectiveandefficientuseoffluorine.Thetermseffectivenessandefficiencyareborrowedfromthelanguageofsurfactantscience.Thetermeffectivenessreferstotheamountthattheadditionoffluorinelowersthesurfacetensionrelativetotheparentmaterial.Theefficiencytermreferstotheminimumconcentrationoffluorinenecessarytoreachtheminimumsurfaceten-sion.TheoriginoftheseeffectscanbeseenfromtheGibbsadsorptionequation24
∑Γiμσi(1)
whereγissurfacetension,Γissurfaceexcessofiinmolesperunitareaofsurfaceσ,μσandfisiisthesurfacechem-icaltialoftheithspeciesσtheHelmholtzfreeenergyperunitareaofsurfaceσ.Foratwo-componentsystematconstanttemperaturetheequationisstatedbelow.25
dγ)-Γdμσσ
11-Γ2dμ2
Thesubscripts1and2refertothetwodifferentspecies
inthemixture.Considerthecaseofabinarypolymermixture,oneofthembeingahydrocarbonacrylatepolymerandtheotherablockcopolymerof(perfluoro-alkyl)ethylmethacrylateandhydrocarbonacrylate.RearrangementoftheGibbsadsorptionequationleadstoanexpressionrelatingsurfaceexcessandsurface
Macromolecules,Vol.30,No.10,1997
Table1.ContactAngleDatafor(Perfluoroalkyl)Ethyl
Methacrylate-ModifiedAcrylicPolymers
contactangle(deg)water
hexadecanepolymera
wt%1feedbadvrecadvrec2c.5Sdd1.5Cbe3.0See3.0Ccf
6htotalreactiontime.bS≡singleshotaddition10minfromtheendofthereaction.C≡continuousfeed.cStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate(15/30/17/38wt%).dStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate/1(15/28.5/17/38/1.5wt%).eStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate/1(15/27/17/38/3wt%).fStyrene/butylmethacrylate/butylacrylate/hydroxypropylacryl-ate/1(15/24/17/38/6wt%).
whereCistheconcentrationofspecies1,Ristheidealgasconstant,andTistemperature.Hence,itcanbeseenthatifthetermdγ/dlnC&0,thentherewillbeasurfaceexcessofspecies1.Stateddifferently,ifspecies1hasalowersurfacetensionthanspecies2,therewillbeanenthalpicdrivingforceforsurfacesegregationof1totheair/polymerinterface.Ifthe(perfluoroalkyl)ethylmethacrylate/hydrocarbonacrylatecopolymerhasalowersurfacetensionthanthehydro-carbonacrylatepolymer,thenitwillbeinexcessattheair/polymerinterface.
Thedifferenceinsurfacetensionbetweenthecom-ponentsandinteractionparameterbetweentheseg-mentsoftwopolymerswilldeterminethemagnitudeofsurfaceexcess.26Obviously,thepresentsystemismuchmorecomplicahowever,theaforementionedprinciplesstillapply.ShowninTable1arewaterandhexadecaneadvancing,θreceding,θcontactanglesonsurfacesofavarietyadv,andrec,ofpolymerformulations.Ofimportancetonoteisacomparisonofdataforformulationscontainingdifferingamountsof(perfluoroalkyl)ethylmethacrylateanddif-ferentfeedconditions.Thedataforvaryingamountsoffluorineinthepolymer(1.5-6wt%)donotshowanyobviousadvantageuponincreasingtheweightpercentof1beyond6intheformulation.Thedifferentfeedconditionsexhibitthelargesteffectsoncontactangles.Atagivenweightpercentof(perfluoroalkyl)ethylmeth-acrylateinthepolymer,thesingleshotfeedproducesthehigherwaterandhexadecanecontactangles.Thiscanbeexplainedbyexaminationofthetypesofpoly-mersexpectedfromthetwosystems.Thecopolymerwhichresultsfromthesingleshotadditionwillhavestatistically-longercontiguousblocksof(perfluoroalkyl)-ethylmethacrylatethanthatcopolymerproducedbycontinuousfeed.Thehighhexadecaneθonthesurfacesof3c,foradvandlowhysteresisobtainedexample,wouldargueforclose-packingof-CF3groups,videinfra,fromtheperfluoroalkylchains.Fromeq3,itcanbeseenthat,
dγ/dlnCsingleshot&dγ/dlnCcontinuous
andΓsingleshot&Γfromseveralpolymercontinouschainsshot.Theperfluoroalkylgroupsarethenabletoform
AcrylicPolymersContainingFluorinatedMonomers2885
domainsofappreciablesurfaceareafractions.Sincethepolymerisnotfluorinatedcompletely,thisimpliesthatdomainsofhydrocarbon-richspecies(highersurfacefreeenergy)willalsobepresentonthesurface.Incontrast,considerthepolymermadebycontinuousfeedof1.Onaverage,eachpolymerchainwillhavelessfluorinethanthesingleshotfeed-producedpolymer.
Anyfluorine-richdomainspresentatthesurfacewillbesmallerandmoreinterspersedwithregionswhicharerichinhydrocarbonmoieties.Thepresenceofdomainsorchemicalheterogeneityresultsinaphe-nomenonknownascontactanglehysteresis(equaltothedifferencebetweenadvancingandrθadv-θrec).27Otherexplanationsforhysteresisarepossible(surfaceroughness),buttheyarenotthoughttobeapplicableinthepresentcase.28,29Filmsof3awerespincoatedonsiliconwafersandexaminedbyatomicforcemicroscopy.Surfaceroughnesswasonascalesmallerthanthatwhichwouldproducesubstan-tialhysteresis,yetthosesamplesdidshowappreciablehysteresiswhensubjectedtocontactanglemeasure-ments(Figure7).Sizeofdomainsandspatialorienta-tionwilldeterminethemagnitudeofhysteresisob-served.Allpolymersprepared(Table1)exhibitedvaryingdegreesofhysteresis,indicatingchemicalhet-erogeneity.Caremustbetakenintheinterpretationofadvancingcontactanglesalone.DettreandJohnsoncalculatedadvancingandrecedingcontactanglesonmodelsurfacescontainingvariousdegreesofchemicalheterogeneities.27Theyconcludedthatrelativelysmalldomainsoflowsurfacefreeenergyspecieswillyieldrelativelylargeadvancingcontactanglesaccompaniedbyrelativelylowrecedingcontactangles.Thevalueoftherecedingcontactangledoesnotapproachtheadvancingangleuntillargeareafractionsofthesurfaceareoccupiedbythelowsurfacefreeenergyspecies.Atpresent,insufficientdataexisttomakeanyfirmestimationofpolymerarchitectureandorientationattheair/polymerinterfacebasedoncontactangleorhysteresisdata.However,thedatadosuggestthattheperfluoroalkylchainsareorientedwiththe-CFattheoutermostsurface.Onthebasisofcontact3groupsanglemeasurements,itcanbeestimatedthatthesurfacetensionsofpolymers3a-3carelowerthanthatofPTFE(～24mN/m).30Waterandhexadecaneadvancingcon-tactanglesmeasuredonself-assembledperfluoroalkylacid≈110monolayers-120°(aclose-packed-CF31,323surface)areand80°,33respectively,withlittlehys-teresis.WaterandhexadecaneadvancingcontactanglesonPTFEsurfaceshavebeenmeasuredtobe109°and46°,respectively.15Therelativelylowsurfacetensionsaffordedbypolymersofthetype3servetodemonstratethefactthatasurfacedominatedby-CFlowersurfacetensionthanonedominated3groupswillhaveaby-CF,surface2-.33,34Theparenthydrocarbonacryliccopolymer,2showslittlerepellencytowater(θ&90°)andnorepellencytohexadecane(θ)0°).
ESCASpectroscopyofPolymerSurfaces.ThesurfacesoftheacrylatepolymerswereexaminedbyESCA.Inthissurfacespectroscopicexperiment,thesampleofinterestisirradiatedwithMgKRX-raysasthekineticenergyoftheemittedphotoelectronisrecorded.Sincethemeanfreepathoftheejectedphotoelectroncontrolsanalysisdepth,anondestructivedepthprofileanalysisoftheouter～100?ofthepolymericsurfacecanbeobtainedbychangingtheangleofthesamplesurfacerelativetothecollectionapertureofthespectrometer.Severalfluoromethacrylatepoly-
2886Thomasetal.Macromolecules,Vol.30,No.10,1997
Table2.ESCADataforVariousAcrylatePolymerFormulations
experimental
N6.21.83.43.04.53.04.62.64.1
F00.580.581.001.00
C74.874.974.974.674.6
O23.120.220.220.120.1
N4.24.34.34.34.3
2c3ad3dd3be3ee
wt%101.51.53.03.0
exitangle(deg)
6htotalreactiontime.bS≡singleshotaddition10minfromendofreaction.C≡continuousaddition.cStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate(15/30/17/38wt%).dStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate/1(15/28.5/17/38/1.5wt%).eStyrene/butylmethacrylate/butylacrylate/hydroxypropylacrylate/1(15/27/17/38/3wt
Figure3.High-resolutionESCAspectrumoftheC1sregionofanacrylicpolymer,3a,preparedusing1.5wt%of1addedviatheshotfeedconditions.
Figure4.ToF-SIMSnegativeionspectrumofacrylicpolymer3apreparedusing1.5wt%of1viashotfeedaddition.
mersystemswereexaminedusingthisapproachandcomparedtotheparentacryliccopolymer.TheresultsareshowninTable2.Fortypicalpolymersystems,a90°exitanglecorrespondstoanintegrateddepthsensitivityof～100?.Thereexistsasin (where istheexitangle)rsoa30°exitanglesamplestoaboutone-halfthedepthor～50?.High-resolutionspectrawerecollectedineachatomicregiontoidentifyuniquefunctionalgroupsthatdescribethesurfacechemistry.Thetheoreticalatomiccompositionswerecalculatedfromaknowledgeoftheacrylicpolymerrecipeandcomposition-correctedfortheamountofisocyanatecross-linkeradded.Theparentacrylicco-polymer,cross-linkedwithisocyanate,exhibitsaspec-trumwhichistypicalofmostorganicpolymershavingarelativeatomiccomposition:76%C,17%O,and6.2%N.Thepolymerscontaining1allshowaremarkableexcessoffluorineatthesurface.Ahigh-resolutioncarbon1sspectrum,typicalofoneofthefluorine-containingpolymers,3a,isshowninFigure3.DiscretepeaksassignabletoCF3andCF2functionalgroupsshowclearlythefluorotelomerendgroup,-(CF2)nF,attheair/polymerinterface.
Semiquantitativemeasurementsofthesurfacefluo-rinefortheseparticularsamplesshowedthesurfaceexcessoffluorineranges～20-70-foldabovebulklevels.Forapolymersystem,thismagnitudeofsurfaceexcessisrelativelyrare.Grazingangle(exitangle&12°)datahintsatevenhigher(≈55atom%)concentrationsoffluorineatthesurface.Theangle-dependentESCAdatadoindicate,asanticipated,thattheconcentrationoffluorinebeginstodecreasedeeperintothefilm.ToF-SIMSSpectroscopyofPolymerSurfaces.Inordertoelucidatethestructureofthepolymeratthe
air/polymerinterface,ToF-SIMSwasemployed.Inthestaticmode,theprimarybeamfluxismaintainedatavalue&1012ions/cm2.Inthisfashion,theverytopsurfaceofthepolymercanbeexamined.Cursoryinvestigationsoftheacryliccopolymersmodifiedbyadditionof1revealedthattheprimaryfragmentationpathwayisquitedifferentfromtypicalhydrocarbonacrylatepolymers.35Unlikehydrocarbonacrylatepoly-mers,thepositiveionspectraofthe(perfluoroalkyl)-ethylmethacrylate-modifiedpolymerscontainednoinformationaboutthefluorotelomerendgroups.There-fore,thediscussionwillconcentrateonnegativeionspectra.Amassspectrumobtainedfromapolymerwith1.5wt%1(shotaddition)isshowninFigure4.Thelow-massregiondisplayedinFigure4showsthosepeakstypicalofallhighlyfluorinatedpolymers(m/z19(F)),31(CF),69(CF3),and93(C3F3).Thehigh-massregionhasalargepeakatm/z119(CF2CF3)andaseriesofpeaksatm/z)93+100n(n)1,23,...,8)whichareclearlyduetothefluorotelomerendgroup.Thesefragmentionshavethegeneralstructure-CtC-(CF2CF2)nCF3.Alowerintensityseriesatm/z)55+100nisalsodiscernibleandhasthegeneralstructure-CtCCFdCF(CF2CF2)n-1CF3.Thechainlengthdis-tributionofthefluorotelomerendgroupcanbeinferredfromthefragmentiondistributionifacommonfrag-mentationmechanismisassumed.Shownbelowisthegeneralstructureofthefluorotelomerattachedtothepolymerviaanesterlinkage.
-C(Od)OC1H2C2H2C3F2C4F2...CxF2F
Thesyntheticpathusedtoproducethisparticular
Macromolecules,Vol.30,No.10,1997Figure5.Integratedangle-dependentESCAatompercentvaluesforacrylicpolymer3aincorporating1.5wt%1viasingleshotfeed.Dataforcarbon(9),fluorine(b),oxygen(2),andnitrogen(1)are
fluorotelomermoietydictatesthatxmustbeanevennumber.Inordertoformtheacetylide-typefragmentionswhichareobserved,bondcleavagemustbefollowedbythelossoftwomoleculesof(i)HandC1,(ii)HFifcleavage2ifcleavageisbetweenOoccursbetweenC1andC2,(iii)F2ifcleavageisbetweenC2andC3,and(iv)F2ifcleavageoccursbetweenCxandCx+1(x&2).Thefourthpoitshouldproduceafragmentiondistributionwitha50amu-(CFthanthe100amu-(CF2)-repeatintervalratherwhichwasobserved.Thefirstandthird2CFoptions2)-intervalwouldproduceadistributionwitha100amurepeatintervalbutoffsetfromtheobserveddistributionby50amu(i.e.,peaksatm/z)43+100nratherthantheobservedm/z)93+100n).Onlythesecondfragmen-tationmechanism(cleavagebetweenC1andC2followedbythelossoftwoHFmolecules)canproducetheobservedfragmentiondistributions.
DSIMSandESCADepthProfiling.Surfaceanaly-sisoftheacrylatepolymerscontaining1demonstratesclearlythatthesurfaceisenrichedinfluorinewellabovebulklevels.Thisfactimpliesthatagradientoffluorinemustexistnormaltothesurfaceintothebulkofthepolymerfilm.Themagnitudeofthisgradientanditsspatialdependencewillhaveimportanteffectsoncoat-ingpropertiessuchasrepellencyanddurability.Thefluorinedistributionwasexaminedbyavarietyofspectroscopictechniques.ShowninFigure5aretheangle-dependentESCAatomicpercentdataforoneofthefluorinatedacrylatefilms,3a.Notethatthedataareplottedwithrelativedepthontheabscissa.Theabsolutedepthdependsonthemeanfreepathoftheejectedphotoelectronandvariesbetweensamples.Therelativedepthof1,however,canbeestimatedatseveralhundredangstroms.Theatomicpercentageoffluorinedropsfromitshighlevelatthesurfacetobulkconcen-trationwitharathersharpprofilethatextendsonlyoverasmalldistancenormaltothesurface.Workisinprogresstolookattheeffectsofdifferentfeedconditionsonthisdistribution.Thedistributiondeter-minedbythistypeofexperimentisonlyapproximatesincethesignalisanintegratedone.
Inanefforttoobtainmorequantitativedata,DSIMSdepthprofilingwasperformed.ShowninFigure6aretheDSIMSdepthprofilingdataforanacrylicpolymerfilm,3a,containing1.5wt%1addedviaasingleshotfeedforafilmwhichis370?thick.Thedataindicatethatthefluorinelevelatthesurfaceofthefilmis
AcrylicPolymersContainingFluorinatedMonomers2887
Figure6.DSIMSdepthprofileforfluorine-modifiedacrylicpolymer3a(1.5wt%1;singleshot).Thepolymerfilmwasmeasuredtobe370?thickonasiliconwafer.Shownaredataforfluorine(9),carbon(2),andnitrogen(×5;b).
increasedbyafactor≈10comparedtobulklevels.Thedataalsoindicatethatagradientoffluorineexistsnormaltothesurfaceandextendsdown～100-200?fromtheair/polymerinterface(neglectinganyinstru-mentalbroadening).Similarspectrahavebeenob-servedwithotherpolymercompositions.Suchastepprofilehasbeenobservedandpredictedduringothersurfacesegregationstudiesonmisciblepolymermixtures.22-39
Itisthemagnitude(excess)offluorineatthesurfaceandgradientnormaltothesurfacewhichwilldeterminetheultimaterepellencyanddurabilitypropertiesofthese(perfluoroalkyl)ethylmethacrylate-modifiedpoly-mers.ItisimportanttonotethedifferencesinfluorineatomconcentrationdifferencesprovidedbytheESCAdata(～20-70-foldexcess)comparedtotheDSIMSdepthprofilingdata(～10-foldexcess).Theapparentdifferencecanhaveitsoriginfromtwosources.Oneisinstrumental,wherebythefluorinesignalisdistributedoveraninterfacialwidthwhichisbroadenedartificially.Ifthetotalintegratedfluorinesignalwassummedintoanarrowerinterfacialdepthprofile,theexcessoffluorinewouldappearlarger.Theotherisduetothesamplegeometry.TheGibbsadsorptionequation(eq1)hasbeenusedtoexplainthesurfaceexcessofthefluorine-richcomponenttotheair/polymerinterface.Thisformalismisbasedonamodelwhichisusedtodescribeasurfacestateandhasseveralunderlyingassumptions.Oneoftheseisthatthesurfacelayerisinfinitelythinandadividingsurfacecanbefixedaccuratelybetweenthesurfaceandthetwobulkphases.Anotheristhatthecompositionofthebulkphasesishomogeneousuptothesurfacelayer.Thepresentcaseisnotsosimpleandmaynotfollowthismodelexactly.Thesurfaceregioninthecurrentstudycanbeportrayedasonerichinfluorinatedpolymerchains,butmaynotexistasaphaseseparatefromthebulk.
Thesurfacetensiontermineq1isderivedfromconsiderationofthechemicalpotentialofspeciesi,μσi,ontheinterfacealone.Inreality,thesituationismorecomplex.TheHelmholtzfreeenergyperunitareaofsurfaceσ,fσ,dependsonvariableswhichdeterminethesurfacestateaswellasthosewhichdeterminethestateofthebulkphases.Thetotaldifferentialforfσcanbewrittenasfollows.24
dfσ)-SσdT+
∑μσidΓi+∑ ′idc′i+∑ ′′idc′′i(4)
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