INTERFACIAL PHENOMENA IN THIN POLYMER FILMS STUDIED BY DIRECT PROFILING TECHNIQUES ŠTUDIJ POJAVOV NA MEJNIH PLOSKVAH TANKIH POLIMERNIH PLASTI Z DIREKTNIMI TEHNIKAMI PROFILIRANJA ANDRZEJ BUDKOWSKI Institute of Physics Jagellonian University Reymonta 4, 30-059 Krakow, Poland Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-12-11 Various interfacial phenomena, such as: phase coexistence, surface segregation and wetting, as well as surface directed phase separation, determine many properties of po!ymer mixtures. These phenomena are studied in blends conftned in a thin film geometry. Direct composition vs. depth profiling techniques are used, which have been developed in the last decade. Present paper describes briefly direct profiling methods and interfacial phenomena observed with their help. Discussion is illustrated by experimental results obtained for homopolymer mixtures with nuclear reaction analysis and secondary ion mass spectroscopy. Key words: polymer blends, liquid thin films, ion beam analysis, composition vs. depth profiling techniques, miscibility conditions, surface segregation, wetting, phase separation Razni pojavi na mejnih ploskvah kot so npr. soobstoj faz, površinska segregacija in omakanje na površini pa tudi s površino usmerjena ločitev faz, določajo mnoge lastnosti polimernih mešanic. Te pojave smo študirali na polimernih mešanicah (blendih) z geometrijo tankih plasti. Uporabljene so bile direktne tehnike profiliranja koncentracije z globino plasti, ki so bile razvite v zadnjem desetletju. Članek na kratko opisuje direktne metode profiliranja in pojave na mejnih ploskvah, ki smo jih opazovali s pomočjo omenjenih metod. V diskusiji so predstavljeni eksperimentalni rezultati homopolimernih mešanic z jedrsko reakcijsko analizo in sekundarno ionsko masno spektroskopijo. Ključne besede: polimerne mešanice, tekoče tanke plasti, analiza z ionskimi snopi, tehnike profiliranja koncentracije z globino, pogoji mešljivosti, površinska segregacija, omakanje, ločitev faz 1 INTRODUCTION Alloying poIymers provides an inexpensive method to produce new materials vvith desired properties, often supertor to either blend component alone1. The spatial blend structure is rarely homogeneous. Compositional inhomogeneities are related vvith internal interfaces, separating coexisting phases inside the blend, and exter-nal interfaces exposed by the blend (Figure 1). The most complex situation occurs in blends conftned in a thin film geometry. Various phase domain morphologies are encountered in thin films, and exemplified in Figure 1. Their characterization precedes an enforcement of the proper performance of many lovv- and high- tech applications. The local concentration of the surface segregation 'cuticle' of one- phase layer (Figure la) is different from the composition in the bulk, modifying e.g. the surface vvettabil-ity by paints. Complex 3- dimensional tvvo- phase structure (Figure lb), expected for the separation of polystyrene (PS)/ polybutadiene blends, results in very tough 'high impact PS'. The separation process can be directed by external surfaces leading to self- stratified films (Figure lc), vvhere individual domains have often different properties (e.g. gas permeation and mechanical characteristics in gas separation membranes). Apart from these technology oriented aspects2,3, the interfacial phenomena pose a fundamental scientific challenge to soft matter physics4. They can be catego- rized according to complexity. The internal interface specifies miscibility conditions (Section 3a). These conditions, augmented by the knovvledge of the specific segment interactions at given external interface, explain surface segregation (Section 3b). Finally, vvhen ali interfaces are treated explicit, the self- stratified films, created in the due course of surface directed phase separation (Section 3c), can be described. Nowadays the spatial structure of blend films (phase morphology and local composition) can be determined vvith a nanometer precision, comparable vvith the polmer chain dimensions. Even the 3-dimensional complex morphology of tvvo- phase systems can be resolved, e.g. vvith transmission electron microtomography5. The concentration vs. depth profiles <(>(z) are examined by modem techniques6 yielding the volume fraction of the blend component(s) as the function of depth z vvithin the thin film (Figure 1). Indirect profiling techniques, such as X-ray and neutron reflectivity, yield model dependent profiles (z) (although vvith excellent resolution 8 = 1 nm). To contrary, the straightforvvard profiles (j)(z) are obtained vvith the commonly used direct methods6. 2 DIRECT PROFILING TECHNIQUES The direct methods analyze the film composition vvith ion beams of medium (3-7 keV in dynamic Secon-dary Ion Mass Spectroscopy (SIMS)) or higher (0.7 - 7 a) vacuum/ air OC C) II z N R A ■ E ■ <' lil! ■h II 2V3He+ ~5keV ^r+\beam Figure 1: Thin polymer films eomposed of one phase (j) (a), or two coexisting phases (f> i and (b, c) are not homogeneous. External interfaces I and II cause segregation (a), internal interface i separates coexisting phases (b and c) Slika 1: Tanke polimerne plasti, sestavljene iz ene faze (j> (a) ali iz dveh soobstoječih faz in <)>2 (b. c), niso homogene. Zunanji mejni plasti I in II povzročita segregacijo (a), notranja mejna plast i ločuje soobstoječi fazi (b, c) Me V) energy (Rutherford Back Scattering (RBS), For-ward Recoil Spectrometry (FRES), Nuclear Reaction Analysis (NRA)). Bulk of polymers are made up of elements which rarely provide an effective contrast for high energy tech-niques. RBS can in principle yield composition profiles of heavy elements present in polymers, but it is excep-tionally used to trace polymers themselves6. This is because the labeling a polymer with a heavy element se-verely alters the extent of its mixing with other polymers. Therefore a deuterium labeling is commonly used instead, which allows for the profiling of 'stained' polymers in such methods as FRES and NRA. The deuterium 'staining' of one of the chemically distinct blend components introduces much smaller thermodynamic perturbation, visible only for longer chains, and easily evaluated7. Non-resonant NRA8 is the high energy method of choice providing profiling deuterium in polymer films with the highest range (even of few microns) and best resolution (5 ' 7 nm at the free surface deteriorating to 5 ' 30 nm at depth z = 600 nm). As a standard procedure thin polymer films are mounted on Si vvafers (spin čast from solution either directly on Si, or čast on mica, floated on vvater, and picked up by Si backing bearing a precast film to form a multilayer if necessary), annealed at temperatures above polymer glass transition, and measured at room (or lovver) temperature. The monoen-ergetic 3He+ beam (vvith mm diameter) impinges on and penetrates into the thin film sample, as illustrated in Fig- mam. E or H filter secondary ion detector mass spectrometer Au + sacrificial layer sample Figure 2: Schematic illustration of setups used to yield the composition (J) vs. depth z profiles (z) in thin film samples with nuclear reaction analysis (NRA) (a) and dynamic secondary ion mass spectroscopy (SIMS) (b) Slika 2: Shematska predstavitev priprav za merjenje koncentracije (|> z globino z profila ij>(z) v vzorcih tankih plasti z jedrsko reakcijsko analizo (NRA) (a) in dinamično sekundarno ionsko masno spektroskopijo (SIMS) (b) ure 2a. At different depths z the reaction: 3He + 2H ® 4He + 'H + 18.4 MeV takes plače and the magnetic or electric filter allovvs only 4He+ particles to reach the detector. The measured 4He energy is related vvith the depth z, vvhich can be calculated based on reaction kinematics and knovvn energy losses of 3He and 4He in the sample. Finally, the correction of 4He count rate vvith respect to the reaction cross section provides a relative profile (j>(z) of the polymer 'stained' by deuterium. The absolute values of the volume fractions (\>{z) are determined from the knovvn (from preparation) overall amount of the deuter-ated material in the sample9, or by profiling the studied sample covered vvith an additional reference layer built of pure deuterated material10. Typical NRA profiles are shovvn in Figures 3a and 4a. An alternative profiling method of dynamic SIMS1112 is accomplished by monitoring individual atomic and molecular secondary ions emitted vvhen the polymer sample is exposed to the primary ion (Ar+ or another) beam vvith a medium energy (Figure 2b). The primary beam is scanned over an area of mm diameter, thereby eroding the sample and forming a flat crater vvith a grovving depth. The secondary ions ejected from the central region of the crater are analyzed vvith mass spectrometer (vvith respect to their mass- to- charge ratio) and moni-tored as a function of the sputtering time. Prior to SIMS measurements, the polymer samples are covered by a polymer sacrificial layer and evaporated vvith Au. This is 0 200 600 depth z [nm] 0.2 0.4 0.6 0.8 dPS vol. fr. 0.2 0.6 dPS vol. fr. * Figure 3: ReIaxation of an initially sharp interface between pure polystyrene (PS) (N = 27.8 k) and pure deuterated polystyrene (dPS) (N = 9.2 k) leads (here after a month long annealing at varied T) to coexisting profiles ((»(z) (a). NRA profiles, tracing local dPS concentration, determine coexisting compositions, 0! and 2, used to plot experimental phase diagram (b). This is well fltted by theory (see solid line in b) to yield the specific form of segmental parameter Xsans((z> (a). NRA profili, ki spremljajo lokalno dPS koncentracijo, določajo soobstoječe sestave, $1 in 2, so bili uporabljeni za eksperimentalni fazni diagram (b). Ta se dobro ujema s teorijo (neprekinjena črta v b), tako da dobimo specifično obliko segmentnega parametra xsans(), ki je prikazan (pri T = 160°C) v (c) kot neprekinjena črta. Vrednosti Xsans(), izmerjene s SANS za polimerne mešanice v masi, so pri (c) označene z odprtimi simboli za PS (N = 15,4 k)/dPS (N = 11,5 k) in s polnimi simboli za PS (N = 8,7 k)/dPS (N = 11,5 k) ' < *_1_I—1—i—1—I—1—1— 0 100 200 depth z [nm] 0 200 400 600 depth z [nm] 0 200 400 depth z [nm] Figure 4: (a) NRA profiles (f>(z) of the surface exposed by the mixture of random olefinic copolymers [(C4H8)i.x(C2H3(C2H5))x]N : a hydrogenous h52 (x=52%, N= 1510) and a partly deuterated d66 (x=66%, N=2030), after a few hours of annealing at T= 99°C. The open and solid symboIs correspond to the surface enriched in d66 and completely wetted by the d66- rich phase 2, respectively. (b and c) SIMS profiles (z> illustrating surface directed phase separation. Its initial stages vvere recorded (see b) for the blend of dPS (N= 6.4k)/ brominated polystyrene PBrxS (N=1.7k, x=0.08) annealed for 1 day at T=180°C. Its late stages (see c) vvere observed in the blend of dPS (N=17.4k) / PS (N=27.8k) annealed for 20 days at T=190°C. (Br', 'HC7 C' and 2HC7 C" profiles are marked by crosses, open and solid symbols, respectively) Slika 4: (a) NRA profili <|>(z) za površine, ki so bile izpostavljene zmesi naključnih olefinskih kopolimerov [(C4Hg)i.x(C2H3(C2H5))x]N: vodikov h52 (x=52%, N=1510) in delno devterirani d66 (x=66%, N=2030) po nekaj urah kondicioniranja pri T=99°C. Odprti simboli ustrezajo površini, ki je obogatena z d66 in polni simboki površini, ki je popolnoma omočena z d66 - bogato fazo $2. (b in c) SIMS profili (z) predstavljajo s površino usmerjeno ločitev faz. Začetne stopnje so posnete (glej b) za polimerno mešanico dPS (N=6,4 k) / bromirani polistiren PBrxS (N=l,7 k, x=0,08), ki je bila kondicionirana en dan pri T=180°C. Kasnejše stopnje (glej c) so prikazane za polimerno mešanico dPS (N=17,4 k) / PS (N=27,8 k), ki je bila kondicionirana 20 dni pri T=190°C. (Br" je označen s križci, 'HC /C" z odprtimi simboli in 2HC"/C" s polnimi simboli) to obtain a steady sputtering state before the real sample is reached by primary ions, and to avoid charging effects, respectively. Sputtering rates are determined by NRA (FRES or ellipsometric) measurements of selected control samples. This allovvs us to evaluate the absolute depth scale for each sample. The very good depth resolu- tion of 5 = 5 nm, vvhich deteriorates only slightly vvith depth z, is the one of two main SIMS advantages. The simultaneous profiling of various species sueh as 'H, 2H, C, O, Br, Si, N, etc., labeling polymers or present in the sample, is the second virtue. Most of these species are detected hovvever only in semi- quantitative fashion. It has been concluded that the absolute local concentrations j and (j):) are expected to coexist in thermodynamic equilibrium at temperatures belovv critical point (TTC. Until recently the phase coexistence of high polymer mixtures have been evaluated vvith a dynamic method determining cloud-point loci or vvith Small Angle Neu-tron Scattering (SANS). While the first technique is problematic due to extremely lovv molecular mobility, the second one measures the segmental interaction xSans away from coexistence curve. A nevv direct approach has been developed1314 in vvhich the profiles (z) are measured (for different T) across the internal interface betvveen tvvo coexisting phases forming a bilayer morphol-ogy as in Figure lc. Such samples are obtained from bilayers composed of pure blend components as a result of an annealing process, involving the relaxation of the initial sharp internal interface and a material transport across the interface. Figure 3a presents the exemplary coexistence profiles obtained vvith this novel method for the isotopic PS blend14. The corresponding coexistence curve presented in Figure 3b shovvs Tc = 197°C, vvhich is much elevated as compared to Tc = 0.9°K characterizing the isotopic mixture of simple liquids 3He and 4He. While unfavorable segment-segment interaction is comparable in both cases, the mixing entropy is reduced only for the isotopic PS blend. The question of accordance betvveen the coex-istence conditions determined by the novel approach1314 involving thin submicron films, and evaluated by SANS for bulk samples (vvith a size of c.a. 1 mm) is addressed by Figure 3c. The composition dependence of the segmental interaction parameter %sans for the isotopic PS mixtures determined by SANS" (and marked by points) is in very good agreement vvith that (dcnoted by a solid line) based on the thin films data corresponding to Figures 3a and b. This suggests that the coexistence conditions yielded by the interface relaxation method should be valid also for polymer blends in the bulk. 3.b Surface segregation and wetting The local concentration at the external surface differs usually from the constant concentration <(>„, of one-phase in the bulk of the sample16. This is demonstrated in Figure 4a for the free surface of an olefinic blend17. The reason for this surface segregation (or enrichment) is the specific surface interaction of polymer segments, lovv-ered vvhen the surface blend concentration is changed. The amount of the segregation is a result of a trade-off betvveen this specific surface energy fs and the bulk term, expressing the free energy of mixing (three factors men-tioned in Section 3a) and opposing the segregation. For knovvn bulk concentration ())„ the segregation profile (j)(z) is generated by the bulk energy term, specified by coex-istence conditions. ,. The thickness L of the surface segregation layer can be then microscopically thin or macroscopically thick (as in Figure 4a). This corresponds to a partial and complete wetting, respectively. While a complete vvetting for polmer blends has been first observed a fevv years ago18, a partial- to complete vvetting transition has just been reported19. According to a conventional vievv point the surface of the polymer mixture is enriched in the component vvith lovver cohesive energy, regardless of the value of the composition ()>„ in the bulk of the sample. This is not necessary true. First, entropy- related forces driving the segregation have been concluded20 besides those related to cohesive energy difference. Second, an enrichment-depletion duality has been advocated by theory21 and computer simulations22, and just observed in real polmer blends23. The surface is enriched in one blend component vvhen the bulk composition is belovv a certain value ()>„ < Q, and it is depleted in this component for larger bulk concentration <)>„ > Q. 3. c Surface directed phase separation A binary mixture is thermodynamically unstable vvhen its average composition corresponds to the tvvo- phase region bounded by a spinodal curve. In such conditions spontaneous bulk- and surface- driven phase separations occur. The bulk separation, driven by thermal fluctuations, results in composition waves vvith random directions and phases. Different scenario is expected vvhen a surface segregation is observed. Then a concentration gradient, created at the surface, induces composition vvaves propagating vvith a fixed phase in a direction normal to the surface. The concentration oscillation <|>(z), characteristic for this surface directed phase separation, extends from the surface and decays inside the sample, vvhere the bulk mode of the phase separation domi-nates24. This is presented in Figure 4b for the mixture composed of deuterated and brominated PS12. Finally, at the late stages of this process, a stratified plate morphol-ogy can be obtained25 as presented in Figure 4c for the isotopic PS mixture. This layered structure can be controlled in a tunable fashion by a surface active copolymer admixed to the polymer blend25. 4 CONCLUSIONS Binary polymer blends are usually incompatible or only partly compatible. Already very vveak interactions betvveen unlike segments, as in isotopic mixtures, vvould lead to phase separation. Even miscible polymer 'alloys' are inhomogeneous due to the effects related vvith an ex-ternal blend interface. Ordered segregation and separation processes can be initiated at the external interfaces of thin films. To describe them proper!y coexistence conditions needed first to be evaluated. Nowadays thin films are studied vvith direct composition vs. depth profiling techniques, vvhich are best represented by SIMS and NRA. The advent of real 3- dimensional profiling is ex-pected, vvhich vvould enable to study directly complex tvvo- phase morphologies. ACKNOWLEDGMENTS The author thanks Dr. A. Bernasik and J. Rysz for their help in performing SIMS experiments. 5 REFERENCES 'F. S. Bates, Science, 251 (1991) 898 2 a special issue of Physics World, March (1995) 'a special issue of MRS Bulletin, January (1996) 4K. Binder, Acta Polymer„ 46 (1995) 204 5J. H. Lauer, J. C. Fung, F. S. Bates, et al., Langumir, 13 (1997) 2177 6E. J. 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