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Thin Film Phase Separation on a Nanoscopically Patterned Substrate Kenji Fukunaga, Hubert Elbs, and Georg Krausch* Lehrstuhl fu¨ r Physikalische Chemie II, Universita¨ t Bayreuth, 95440 Bayreuth, Germany Received August 24, 1999. In Final Form: December 15, 1999 We investigate phase separation in a thin film of a binary polymer blend A/C in the presence of a solid substrate covered with immobilized nanoscopic domains of the respective polymers. The patterned substrate is realized by physisorbing a microphase separated brush of an ABC triblock copolymer. We observe a marked suppression of phase separation in the blend on the pattern as compared to a laterally homogeneous substrate. The effect is studied as a function of blend film thickness. The results are compared to a simple estimate of the interfacial energies.
The effect of boundary surfaces on the phase separation behavior of liquid mixtures has recently attracted considerable interest both theoretically1 and experimentally.2-5 Quite generally, differences between the wetting properties of the two coexisting phases at the respective boundaries will influence the kinetics of phase separation as well as the resulting equilibrium domain structures in the thin film. Obviously, these effects will be most pronounced for very thin films exhibiting a large surfaceto-volume ratio. In the experimental studies of thin film phase separation, polymers have played a dominant role as model liquids owing to their low vapor pressure and the fact that the slow dynamics inherent of long chain molecules facilitates the experimental observation of the phase separation process.6 Quite generally, the phase exhibiting the lower interfacial energy with a boundary surface will tend to accumulate at this surface. In consequence, the near-surface domain structure will be altered and the overall morphology of the film will be different from the respective bulk situation. Given suitable wetting conditions at the film boundaries, highly anisotropic morphologies can occur. Due to the rather large correlation lengths in high molecular weight polymer blends, thin film domain structures can be boundary dominated for films as thick as some 1 µm.7 There have been various attempts to utilize the boundary effects during phase separation for a controlled manipulation of the resulting domain structures. Selforganized polymeric multilayers7 and laterally ordered domain structures8-10 of different periodicity and termination have been successfully created via surface-directed phase separation on substrates with suitably chosen surface energy (patterns). Common to these studies is the fact that the spatial arrangement of the domains was (1) Binder, K. Adv. Polym. Sci. 1999, 138, 1-89. (2) Jones, R. A. L.; Norton, L. J.; Kramer, E. J.; Bates, F. S.; Wiltzius, P. Phys. Rev. Lett. 1991, 66, 1326-1329. (3) Bruder, F.; Brenn, R. Phys. Rev. Lett. 1992, 69, 624-627. (4) Steiner, U.; Klein, J.; Eiser, E.; Budkowski, A.; Fetters, L. J. Science 1992, 258, 1126-1129. (5) Reich, S.; Cohen, Y. J. Polym. Sci. 1981, 19, 1255-1267. (6) Krausch, G. Mater. Sci. Eng. Rep. 1995, 14, 1-94. (7) Krausch, G.; Dai, C. A.; Kramer, E. J.; Marko, J.; Bates, F. S. Macromolecules 1993, 26, 5566-5571. (8) Krausch, G.; Kramer, E. J.; Rafailovich, M. H. J. S. Appl. Phys. Lett. 1994, 64, 2655. (9) Bo¨ltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877. (10) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602.
controlled via surface interactions, while the typical dimensions of the resulting structures were largely determined by the phase separation process. In the present contribution, we describe an attempt to control the domain size in a thin film polymer blend via surface interactions. We demonstrate that the interface between a microphase separated brush of an ABC triblock copolymer and an A/C polymer blend significantly decreases the domain size in the phase separating blend provided that the blend film thickness is small enough. The experimental results are compared to a rough estimate of the interfacial energies involved. As a model system, we study a (50/50) blend of polystyrene (PS) (Mw ) 104k) and poly(tert-butyl methacrylate) (PtBMA) (Mw ) 80k) spun cast onto different substrates from a common solvent (tetrahydrofuran, THF). As substrates, we used polished silicon wafers coated with an ultrathin layer of poly(styrene-block-2-vinylpyridineblock-tert-butyl methacrylate) (Mw,PS ) 51k; Mw,PVP ) 68k; Mw,PtBMA ) 50k).11,12 For comparison, plain silicon wafers without polymer coating were used. The wafers were first cleaned following standard procedures. The triblock copolymer thin films were deposited onto the wafers by spin casting from THF. The sample was then thoroughly washed in pure THF for several minutes. After drying, the surface of this layer was imaged with scanning force microscopy in tapping mode. Here and in the following, for (average) film thickness determination part of the polymer film was removed by applying a scratch to the specimen. Subsequently, AFM TappingMode topography images have been taken at the border of the scratch to measure the height of the polymer film with respect to the solid substrate.13 The average film thickness of the triblock copolymer layer was 5 nm. As is shown in Figure 1a, the surface of this layer exhibits a characteristic lateral structure with a periodicity of some 80 nm. This structure is expected to result from the selective physisorption of the polar poly(2vinylpyridine) middle block onto the (polar) SiOx substrate surface followed by microphase separation of the PS and PtBMA end blocks. Atomic force microscopy (AFM) experiments after exposure of this structure to solvent vapors of different selectivity strongly corroborate (11) Giebeler, E.; Stadler, R. Macromol. Chem. Phys. 1997, 198, 3815. (12) Giebeler, E. Dissertation, Universita¨t Mainz, 1997. (13) For TappingMode imaging, the free amplitude of the cantilever was set to some 10 nm and the set-point ratio amounted to typically 95%.
10.1021/la991140t CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000
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Figure 1. (a) AFM TappingMode topography image of the surface of a 5 nm thick film of a P(S-b-2VP-b-tBMA) triblock copolymer grafted onto a silicon wafer. The height scale ranges between 0 and 5 nm. The root mean square roughness of the layer amounts to some 3 nm. (b) Topography image of a 20 nm thick film of a PS/PtBMA blend spun cast onto a plain silicon wafer. The height scale ranges between 0 and 50 nm. In the inset (top left corner) a larger area scan of the same sample is shown. (c) Topography image of a 20 nm thick film of a PS/PtBMA blend spun cast onto a silicon wafer coated with a graft layer of P(S-b-2VP-b-tBMA). The height scale ranges between 0 and 30 nm.
this notion.14 As is discussed in ref 14 in more detail, the formation of a laterally microphase separated layer of an ABC triblock copolymer with strong affinity of the middle block to the boundary surface is in qualitative agreement with recent self-consistent field calculations15 predicting the melt equilibrium morphology of thin triblock copolymer films confined between two solid walls. We start with the results obtained on uncoated silicon substrates for later reference. After spin casting a 20 nm thick film of a 50/50 blend of PS and PtBMA from THF, the film surface is characterized by round-shaped protrusions with a characteristic lateral dimension of some 1 µm (Figure 1b). Long-term annealing (12 days at 160 °C) under vacuum does not significantly change the observed morphology. Comparison to earlier work on similar systems indicates the formation of PS-rich and PtBMArich domains.16,17 Typically, the material which was less soluble in the common solvent used for spin coating protrudes over the respective coexisting phase after solvent evaporation. In our example, the round domains predominantly consist of PtBMA and float in a bicontinuous matrix of PS. The domain morphology shown in Figure 1b is characteristic of thin film phase separation during solvent extraction in a binary polymer blend. As the lateral domain size is of order 1 µm, it typically results in slightly opaque films. The situation changes quite markedly, when we coat the silicon substrate by a thin layer of the triblock copolymer prior to blend deposition. Figure 1c shows an AFM micrograph of a PS/PtBMA thin film prepared under identical conditions as the one shown in Figure 1b, however spun cast onto a precoated substrate. In this case, a lateral structure is still observed on the blend surface; however, the characteristic size of the features has decreased considerably. As an obvious consequence, the resulting films appear optically transparent. Again, the domain structure does not significantly change on annealing. One is tempted to discuss the observed phenomenon in terms of a minimization of unfavorable contacts between (14) Elbs, H.; Fukunaga, K.; Sauer, G.; Stadler, R.; Magerle, R.; Krausch, G. Macromolecules 1999, 32, 1204. (15) Pickett, G. T.; Balazs, A. C. Macromol. Theory Simul. 1998, 7, 249-255. (16) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232-3239. (17) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995-5003.
the two phases A and C. Obviously, when the characteristic lateral size of the homopolymer domains is significantly larger than the lateral domains on the triblock copolymer brush, roughly half of the triblock copolymer/homopolymer interface will consist of AC or CA contacts, respectively. The same was true if a laterally homogeneous, layered domain of one of the coexisting phases was in contact with the triblock copolymer brush. One may therefore lower the interfacial energy by decreasing the size of the lateral domains such that it matches the dimensions of the microdomains formed in the triblock copolymer brush. This will of course lead to an increased interfacial area between the homopolymer domains, which grows linearly with the thickness of the homopolymer film. Qualitatively, one would therefore expect that the effect of the triblock copolymer microstructure shall be limited to rather thin films. For a rough quantitative estimate, we may assume that the microphase separation of the triblock copolymer brush leads to a chessboard pattern of square areas of A and C with a characteristic size a2 (Figure 2). We denote the homopolymer film thickness as d and calculate the area of unfavorable contacts F between A and C. For simplicity, we normalize F to a section of the film of lateral extension a2. We also neglect entropic contributions to the interfacial energy. Therefore, for homopolymer domains of lateral size a2 perfectly aligned with respect to the underlying triblock coploymer microdomains, unfavorable AC contacts are formed at the interface between homopolymer domains only yielding an area F⊥ ) 2ad. For increasing lateral dimensions of the homopolymer domains, the normalized area F⊥ between domains decreases (and eventually vanishes), while the average planar contribution to the area of unfavorable contacts F| takes a constant of F| ) 1/2a2. One can therefore estimate a maximum thickness dmax, below which the formation of nanoscopic domains should be favorable over the formation of larger domains. In the chessboard geometry, one finds dmax ) 1/4a. Different geometries will give slightly different prefactors; however, we take from this simplified model that blend thin films with a thickness comparable to the lateral size of the microdomains should be stabilized, while thicker films should become unstable and form larger domains. To test this hypothesis experimentally, we have repeated the experiment shown in Figure 1c for homopolymer films
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Figure 2. Schematic presentation of the model used to estimate the interfacial energy contributions for different lateral domain sizes in the A/C homopolymer blend.
of different thickness. The result of this procedure is shown in Figure 3. In all cases the average thickness of the homopolymer films was determined by AFM after removing part of the film by applying a scratch to the sample surface (see above). We find that with increasing film thickness, an increasing number of larger domains is
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observed in addition to the nanoscopic domains found in the ultrathin films. Eventually, the sample surface is characterized by rather large features indicating that the size of the domains formed on phase separation is considerably larger than the lateral size of the block copolymer microdomains. While a detailed analysis of the domain structure in thicker films is beyond the scope of this work, Figure 3 clearly shows that small domains can only be obtained in thin enough films. In this respect, the result displayed in Figure 3 is in qualitative agreement with the above considerations. We note that the above arguments assume that the films have reached their thermodynamic equilibrium morphology. This assumption seems questionable given the fast solvent evaporation during the spin-casting process. Even though extended annealing did not change the resulting domain morphologies, we cannot exclude that a nonequilibrium structure has been frozen in during film preparation and stabilized against equilibration by kinetic barriers. We may compare the scheme outlined above to the more common procedure of using a surfactant for blend compatibilization. In the latter case, one would typically mix the A/C homopolymer blend with an A-C block copolymer. The block copolymer would then self-assemble at the interfaces between coexisting A-rich and C-rich phases thereby lowering the respective interfacial energy. As a consequence, the average size of the domains would decrease. To compare the efficiency of the two procedures for the system studied here, we have investigated the domain size in thin films of an A/C homopolymer mixture after addition of a symmetric A-C diblock copolymer. As substrates, we used uncoated silicon wafers as in Figure 1b. With increasing block copolymer content, the average size of the domains is indeed reduced. This is shown in Figure 4. However, even at a block copolymer concentration of 24 wt %, the domains formed on the bare silicon substrate are still significantly larger than the respective domains formed from the pure A/C blend on a laterally patterned substrate (Figure 1c). The major difference between both procedures is the fact that the triblock copolymer chains are immobilized by strong physisorption of the P2VP middle block on the silicon substrate. Therefore the dimension of the A and C domains is determined by the molecular parameters of the ABC triblock copolymer and stays constant after addition of the A/C blend. As discussed above, the blend morphology will follow the underlying microdomain pattern as long
Figure 3. AFM TappingMode topography image of thin films of a PS/PtBMA blend spun cast onto a silicon wafer coated with a graft layer of P(S-b-2VP-b-tBMA): (a) 20 nm, (b) 40 nm, (c) 80 nm thickness. The height scale ranges between 0 and 50 nm.
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copolymer chains. We conclude that for thin enough films, the use of an immobilized microphase separated layer of A and C blocks can be superior in blend compatibilization as compared to the more common use of A-C block copolymers. Obviously, this difference only holds for sufficiently thin homopolymer layers, while for thicker layers the latter approach will be more effective. We note in passing that a closer inspection of the domain structure in Figure 4 reveals smaller features (some 10 nm in size) in addition to the several hundred nanometer diameter domains. Given the rather large diblock copolymer concentration, these structures may be due to diblock copolymer micelles formed within the continuous homopolymer-rich phase. We shall not follow this issue however, as it is of no relevance to the issue discussed in the present paper. In conclusion, we have demonstrated that thin films of an A/C blend can be efficiently compatibilized by use of an immobilized layer of an ABC triblock copolymer. The results indicate a most simple and fast procedure to stabilize thin films against large scale phase separation and thereby improve their technological (e.g., optical) performance. Figure 4. AFM TappingMode topography image of a thin film of a PS/PtBMA blend containing 24 wt % of P(S-b-tBMA) diblock copolymer.
as the films are thin enough. In a blend containing free diblock copolymer chains on the other hand, the block copolymer chains will accumulate at the A/C interfaces no matter how large the domains may be. The domain size is now determined by the interfacial energy, which is somewhat lowered by the presence of the block
Acknowledgment. The authors gratefully acknowledge financial support through the Deutsche Forschungsgemeinschaft (Schwerpunkt “Benetzung und Strukturbildung an Grenzfla¨chen”, Contract No. Kr 1369/9). The authors are indebted to E. Giebeler and R. Stadler for synthesis and characterization of the ABC triblock copolymer. LA991140T
