1、Film Thickness Dependence of the Surface Structure ofImmiscible Polystyrene/Poly(methyl methacrylate) BlendsKeiji Tanaka, Atsushi Takahara, and Tisato Kajiyama*Department of Chemical Science Revised Manuscript ReceivedDecember 20, 1995XABSTRACT: Thefilmthicknessdependenceofsurfacestructureforimmisci
2、blepolystyrene/poly(methylmethacrylate) (PS/PMMA) films was investigated on the basis of atomic force microscopic observationandX-rayphotoelectronspectroscopicmeasurement. InthecaseofthePS/PMMAfilmof25 mthickness,the air-polymer interfacial region was covered with a PS rich overlayer due to its lowe
3、r surface freeenergycomparedwiththatofPMMAandawell-definedmacroscopicphase-separatedstructurewasformedinthebulkphase. Also,inthecaseofthePS/PMMAthinfilmof100nmthickness,thephase-separatedstructure, in which the PMMA rich domains separated out of the PS rich matrix, formed at the filmsurface. The for
4、mation of the surface structure for the PS/PMMA thin film can be attributed to eitherthechainconformationorchainaggregationstructurebeingfrozenattheair-polymerinterfacialregionbefore the formation of a PS rich overlayer due to the fairly fast evaporation of solvent molecules. Ontheotherhand,thetwo-d
5、imensionalPS/PMMAultrathinfilmof10.2nmthicknessdidnotshowdistinctphase-separatedstructure. Whenthefilmthicknessbecamethinnerthan10.2nm,thetwo-dimensionalPS/PMMA ultrathin film of 6.7 nm thickness showed fine and distinct phase-separated structure withthe domain size of a few hundred nanometers. This
6、 structure can be designated as “mesoscopic phase-separatedstructure”. Thesurfacephasestateforthetwo-dimensionalPS/PMMAultrathinfilmscanbeexplained by the film thickness dependence of both the interaction parameter and the degree ofentanglement among polymer chains.IntroductionInvestigations of surf
7、ace structure for multicompo-nent polymer systems have been extensively done forthe last decade, both experimentally1-8and theoreti-cally,9with respect to associated functional propertiessuchasbloodcompatibility,lubricant,wetting,perme-ability,andsoon. Ithasbeenrevealedthatthesurfacestructure of mul
8、ticomponent polymer systems is fairlydifferentfromthatinthebulk,thatis,alowersurfacefree energy component is generally enriched in thesurface region in order to minimize the air-polymerinterfacial free energy. Also, for the past few years,surfacemolecularmotionsofpolymericsolidshavebeenpaidgreatatte
9、ntionbyseveralgroups10-16duetotheirimportance in practical applications. Both vigorousthermal molecular motion and thermal instability ofpolymericchainsatthesurfacecanbeexplainedbythelarger free volume fraction at the surface region com-pared with that in the bulk due to the preferentialsegregation
10、of chain end groups at the surface re-gion11,13,14and/ortheunsymmetricalenvironmentattheair-polymer interface.13,14Polymeric films of whose thicknesses are less thanabout twice the radius of gyration of an unperturbedchain, 2Rg, can be defined as two-dimensional ultra-thin films.17-19Then, the polym
11、eric blend filmswiththicknesseslessthan2Rgofthehighermolecularweight component can be defined as the two-dimen-sionalultrathinblendfilms.20Aflexiblepolymerchainin an ultrathin film is in a nonequilibrium state, sincethe conformational entropy of an individual chainin a constrained thin region is red
12、uced in comparisonwith that in a three-dimensional solid state.19Sincepolymeric chains at the interface, in general, arethermally unstable, the molecular aggregation struc-ture in the two-dimensional ultrathin film of binarypolymer blend must be greatly different from that inthethickfilm. Theauthors
13、haveinvestigatedthefilmthickness dependence of surface structure for the mis-cible polystyrene/poly(vinyl methyl ether) (PS/PVME)blend films.20Even though the blend system wasmiscible in the bulk region, it was revealed that sur-face phase-separated structure was formed even atthe temperature below
14、the bulk phase separationtemperature in the case of the two-dimensional ul-trathin state. The mechanism of surface phase sep-aration for the PS/PVME ultrathin film could be ex-plained by two factors: the negative spreading coef-ficient of PVME on the PS matrix and the remarkablyreduced conformationa
15、l entropy of a PVME chain dueto stretching.Internalphase-separatedmorphologyforimmisciblepolymer blend films has been studied by severalgroups.21-23Shiragaetal.investigatedthebulkphase-separated structure of the immiscible PS/poly(methylmethacrylate) (PS/PMMA) blend film on the basis offield-emissio
16、n scanning electron microscopic observa-tion.21Theyrevealedthatmacroscopicphase-separateddomains with a few micrometers in diameter wereformedinthebulkregionofthethickfilm. Winnikandco-workersinvestigatedthedepthprofilingofthephase-separatedstructureforthePS/PMMAblendfilmonthebasisoflaserconfocalflu
17、orescencemicroscopicobserva-tion.23They revealed that the mechanism of phaseseparationatthefilmsurfacewasfairlydifferentfromthat in a bulk phase. However, it is still questionableand interesting which macroscopic phase-separatedstructureisformedintheconstrainednarrowspaceofthe two-dimensional immisc
18、ible blend ultrathin film.Thepurposeofthisstudyistorevealthefilmthicknessdependenceofsurfacephase-separatedstructureoftheimmiscible PS/PMMA blend films.*TowhomcorrespondenceshouldbeaddressedTEL: +81-92-641-1101 (ext. 5601). FAX: +81-92-651-5606.XAbstract published in Advance ACS Abstracts, February
19、15,1996.3232 Macromolecules 1996, 29, 3232-32390024-9297/96/2229-3232$12.00/0 1996 American Chemical SocietyExperimental SectionMaterials. PolymersusedinthisstudyweremonodispersePS and PMMA. PS was synthesized by a living anionicpolymerizationmethodat293Kusingsec-butyllithiumasaninitiator. PMMAwasal
20、somadeintetrahydrofuranbyalivinganionic polymerization at 196 K using n-butyllithium-added1,1-diphenylethylene as an initiator. Table 1 shows charac-terizations of PS and PMMA. A number-average molecularweight,Mn,andamolecularweightdistribution,Mw/Mn,weredeterminedviagelpermeationchromatographywithp
21、olysty-rene standards. The radius of gyration of an unperturbedchain was calculated bywhereNisthedegreeofpolymerizationandbistheaveragestatisticalsegmentlength. ThemagnitudesofbPSandbPMMAare0.68and0.69nm,respectively.24Thesurfacefreeenergy,wasdeterminedbystaticcontactanglemeasurementonthebasis of Ow
22、ens procedure.25Film Preparation. A PS/PMMA blend solution was pre-paredbymixingeachinthetoluenesolution. ThePS/PMMAblend ratio was designated as (weight %/weight %). The PS/PMMAblendthinandultrathinfilmswerepreparedbyaspin-coating method at 293 K. Film thickness was controlled bytheconcentrationoft
23、hesolutionandthespinrates. Also,theblendthickfilmswerepreparedbyaconventionalsolventcastmethod. Thefilmthicknessofthethinortheultrathinfilmswas evaluated as follows. After a crater was formed in thepolymer film by an ion beam sputtering or a cantilever tipscratch, its average step height, correspond
24、ing to the filmthickness, was measured by atomic force microscopic (AFM)and/or scanning electron microscopic observations. The sub-stratesusedinthisstudyarethreetypes,thatis,gold,siliconwafer,andsiliconizedcoverglass,whichhavedifferentdegreesofinteractionwithPSorPMMAsegments. Thegoldsubstratedoesnot
25、fairlyinteractwithPSandPMMAsegments. Agoldsurface was coated on a silicon wafer by a sputter-coatingmethod. Itwasimpossibletomeasuretheexactwatercontactangle for the gold substrate due to its instability in air. Thecleanedsiliconwaferwasusedasthehydrophilicsurface.Thesilicon wafer was heated at 773
26、K for3hinorder to removeresidual organics on the surface and then was placed into amixed solution of concentrated H2SO4and 30% H2O2(70/30v/v) at 393 K for 1 h. The water contact angle of the cleanedSi wafer was 0. The siliconized cover glass was used as thehydrophobicsurface. Itsstaticwatercontactan
27、glewas88.7.Although it was impossible to evaluate quantitatively themagnitudes of surface free energy for all these substrates, itwasreasonablethattheorderofthemagnitudeofsurfacefreeenergy was qualititatively in the order silicon gold siliconized substrate.SurfaceCharacterization. Thesurfacemorpholo
28、gyofthePS/PMMA blend films was investigated on the basis of AFMobservation. TheAFMimageswereobtainedbySPA300withanSPI3700controller(SeikoInstrumentsIndustryCo.,Ltd.)at room temperature. The AFM cantilever used was micro-fabricated from Si3N4, and its spring constant was 0.022N m-1. AFMobservationwas
29、carriedoutinarepulsiveforcerange of ca. 1 nN. The surface chemical composition of thePS/PMMA thin film was evaluated on the basis of X-rayphotoelectron spectroscopic (XPS) measurement. The XPSspectrawereobtainedwithanESCA850(ShimadzuCo.,Ltd.)atroomtemperature. TheXPSmeasurementwasperformedunder conv
30、entional conditions with a Mg KR source at 8 kVand30mA. Theemissionangleofthephotoelectronwas90.ThemainchamberoftheXPSinstrumentwasmaintainedat10-6Pa. All C1speaks were calibrated to a binding energyof 285.0 eV for neutral carbon, in order to correct thesecharging energy shifts.Results and Discussio
31、nThick Films of 25 m Thickness. The phase-separated structure for the PS/PMMA film of 25 mthickness was investigated in order to compare withthoseofthethinandtheultrathinfilms. Figures1and2 show the AFM and the phase contrast microscopic(PCM) images of the PS/PMMA (30/70) film of 25 mthicknesscoated
32、onthegoldsubstrate,respectively.Thethick film was coated by a standard solvent-castingmethodat293K. AFMandPCMobservationsrevealedthat, although the surface was not in an apparentphase-separated state as shown in Figure 1, well-defined sea-island-like macroscopic phase-separatedstructure was formed i
33、n the bulk phase, as shown inFigure 2. Also, XPS measurement revealed that thePMMAweightfractionattheair-polymerinterfacewas8.4%,whichwasstrikinglylowerthanthatofthebulkcomposition, and in other words, PS segments wereenrichedatthefilmsurfaceduetoitslowersurfacefreeenergy compared with that of PMMA
34、even though thephase-separated structure was observed in the bulkphase. Thus,inthecaseofthePS/PMMAthickfilm,itseems reasonable to conclude that a PS rich overlayerisformedattheair-polymerinterfaceandmacroscopicTable 1. Characterizations of PS and PMMA Used inThis StudyMnMw/Mn2Rg/nm /mJ m-2Tg/KPS 90k
35、 1.05 16.6 40.2 378.2PMMA 69k 1.06 14.6 41.2 403.2Rg) (Nb2/6)1/2(1)Figure1. Atomicforcemicroscopicimageofthepolystyrene/poly(methylmethacrylate)(30/70)thickfilmof25 mthicknesscoated on the gold substrate.Figure2. PhasecontrastmicroscopicimageforthePS/PMMA(30/70) thick film of 25 m thickness coated o
36、n the goldsubstrate.Macromolecules, Vol. 29, No. 9, 1996 Surface Structure of PS/PMMA Blends 3233phase-separated structure is formed in a bulk phase.The surface structure did not quite depend on thesubstrate characteristics, for example, hydrophilic orhydrophobic properties.Thin Films of ca. 100 nm
37、Thickness. Figures 3and 4 show the AFM images of the PS/PMMA (50/50)and(30/70)thinfilmsof100nmthicknessescoatedonthegoldsubstrate,respectively. Thegoldsubstratewasused in order to prepare the surface without anypreferential adsorption of one component. In the caseof the PS/PMMA (50/50) thin film, a
38、well-defined sea-island-like phase-separated structure was observed atthefilmsurface. Theisolateddomainheightwasabout20-40nm,andthediameterofthedomainswasabout1-3 m. Also,surfacephase-separatedstructurewithcontinuous-phase-likedomainswasobservedinthecaseof the PS/PMMA (30/70) thin film. The continuo
39、us-phase-like domain height was about 20 nm. Since thedomainareafractionincreaseswithanincreaseinthePMMA bulk fraction, it seems reasonable to concludethatthedomainregioniscomposedofthePMMAphase.Figure 5 shows the AFM image of the PS/PMMA (30/70) thin film of 100 nm thickness coated on the goldsubst
40、rate after the surface etching treatment withcyclohexanefor15min,whichisagoodsolventonlyforPS segments. Also, Figure 5 shows the sectional viewalongthelineshownintheAFMimage. Itisapparentfrom Figures 4 and 5 that the difference in heightbetweenPMMAdomainsandthePSmatrixatthefilmsurface drastically in
41、creases after the surface etchingtreatment with cyclohexane. Therefore, this resultindicatesagainthatthedomainsandthematrixregionsarecomposedofthePMMAandthePSphases,respec-tively,asconcludedfromthevariationofthedomain-matrixareafraction. Also,Figures4and5exhibitthatthe PMMA continuous domains became
42、 more isolatedafter the surface etching treatment with cyclohexane.ThisindicatesthatthePMMAcontinuousdomainsarecomposed of the PMMA rich phase, in other words,slightly mixed with PS segments because the PMMAcontinuousdomainsarepartiallydissolvedwithagoodsolvent only for PS segments.Theair-polymerint
43、erfaceofthePS/PMMAthickfilmof25 mthicknesspreparedbyasolvent-castingmethodwascoveredwithaPSoverlayer,asmentionedinFigure1. Ontheotherhand,inthecaseofthePS/PMMAthinfilm of 100 nm thickness coated by a spin-coatingmethod,bothPSandPMMArichphaseswereobservedattheair-polymerinterface,asshowninFigure4.The
44、surface covered with PS segments with lower surfacefree energy is thermodynamically most stable. How-ever,inthecaseofaspin-coatingmethod,sincesolventmolecules are evaporated fairly fast from the solutionbeforeattainmentofanequilibriumandthermodynami-cally stable state, the chain conformation or chai
45、naggregationstructureisgenerallyfrozeninthethinfilmbefore the formation of the most stable surface struc-ture. Then, it seems reasonable to conclude that thediscrepancyofsurfacestructurebetweenthethickfilmandthethinonemaybeattributedtothedifferenceofthetimerequiredfortherearrangementforstablechainco
46、nformation.Figure 6 schematically shows the formation processofthesurfacephase-separatedstructureduringevapo-ration of solvent for the PS/PMMA thin film coated onFigure 3. (a) AFM image and (b) sectional view along theline in the AFM image for the PS/PMMA (50/50) thin film of100 nm thickness coated
47、on the gold substrate.Figure 4. (a) AFM image and (b) sectional view along theline in the AFM image for the PS/PMMA (30/70) thin film of100 nm thickness coated on the gold substrate.3234 Tanaka et al. Macromolecules, Vol. 29, No. 9, 1996the gold substrate. The surface area fraction of thePMMA rich p
48、hase was fairly smaller than that corre-sponding to the bulk PMMA weight fraction, and also,thePMMArichdomainsfinallyseparatedoutofthefilmsurface,asshowninFigure6c. Theformationprocessof the surface phase-separated structure can be ex-plainedasfollows. PSsegmentswithlowersurfacefreeenergytendtocover
49、theair-polymerinterfacialregionin order to minimize the interfacial free energy. How-ever, in the case of the PS/PMMA thin film, since thetime required for the surface structure is fairly shortduetoaveryfastevaporationofsolventfromthesurfacein comparison with that of the thick film, the surfacestructurecomposedofbothPSandPMMArichphasescontaining a little residual solvent is formed as shownin Figure 6b. At the stage of Figure 6b, although thelarge scale surface rearrangement cannot be attained,a local surface rearrangement of main chains can befully attained in the thin film due to the p
