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Electrohydrodynamic Effect on Phase Separation Morphology in Polymer Blend Films Tsutomu Kikuchi, Masato Kudo, Chengjun Jing, Takao Tsukada,* and Mitsunori Hozawa Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received May 20, 2003. In Final Form: December 1, 2003 We have investigated the effect of electrohydrodynamic (EHD) convection on the domain structure in a polystyrene (PS)/polyvinyl acetate (PVA) blend film to demonstrate the feasibility of using the EHD effect as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation. Here, polymers-toluene solutions were spread on a glass substrate with patterned electrodes to apply a dc electric field, and well-defined structures of EHD convection were formed in the polymer solutions. As a result, regular patterns were formed in the PS/PVA polymer blend film in which PVA-rich domains were confined within each unit of patterned electrodes, i.e., between positive and negative electrodes, at an appropriate electric voltage. In addition, it was demonstrated that such novel morphology is not due to the wetting/dewetting effect of polymer components to the Pt electrodes deposited on the glass substrate, by experiments with a SiO2-covered substrate.
1. Introduction When binary polymer mixtures dissolved in a common solvent are cast on a substrate and the solvent is evaporated, phase separation of two polymer components can be observed in the cast film. Such polymer blend films obtained by solvent evaporation are widely used in industries such as microelectronics, paints and coatings, and biotechnologies involving porous membranes, and the ability to achieve control of the phase separation morphology and consequently the mechanical, chemical, and electrical properties of the films is practically significant. Since the surface-area-to-volume ratio of polymer blend films, particularly thin films, is relatively large, the presence of both air/polymer and substrate/polymer interfaces plays an important role in determining the morphology of polymer blend thin films. Therefore, extensive studies have been carried out in which the relationship between the wetting/dewetting behavior of each polymer component to the air/polymer and substrate/ polymer interfaces and the surface morphology of the film was investigated.1-8 In addition, the morphology of polymer blend films has been modulated by the active control of substrate surface characteristics, e.g., using a patterned substrate.9-11 Most of these studies involved * Corresponding author. Phone and fax: +81-22-217-5651. E-mail:
[email protected]. (1) Geoghegan, M.; Jones, R. A. L.; Payne, R. S.; Sakellariou, P.; Clough, A. S.; Penfold, J. Polymer 1994, 35, 2019-2027. (2) Dalnoki-Veress, K.; Forrest, J. A.; Stevens, J. R.; Dutcher, J. R. J. Polym. Sci. 1996, 34, 3017-3024. (3) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232-3239. (4) Kumacheva, E.; Li, L.; Winnik, M. A.; Shinozaki, D. M.; Cheng, P. C. Langmuir 1997, 13, 2483-2489. (5) Walheim, S.; Boltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995-5003. (6) Saraf, R. F.; Ostrander, S.; Feenstra, R. M. Langmuir 1998, 14, 483-489. (7) Bergues, B.; Lekki, J.; Budkowski, A.; Cyganik, P.; Lekka, M.; Bernasik, A.; Rysz, J.; Postawa, Z. Vacuum 2001, 63, 297-305. (8) Ton-That, C.; Shard, A. G.; Bradley, R. H. Polymer 2002, 43, 4973-4977. (9) Krausch, G.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J. Appl. Phys. Lett. 1994, 64, 2655-2657.
homogeneous, i.e., spatially invariant, quenches by solvent evaporation because relatively thin polymer blend films were used. During the preparation of thicker polymer blend films by solvent evaporation, however, the quenches are nonuniform, and consequently concentration and/or temperature gradients might be formed within the film because the solvent loss occurs at the air/polymer solution interface and the interface is cooled because of latent heat of evaporation. Under these circumstances often encountered in industrial polymer processing, the concentrations of the solvent at the air/solution interface and in the layer adjacent to it remain lower than that in the bulk for considerable lengths of time. As a result, phase separation begins near the surface with little or no phase separation in the bulk, and thus, the phase separation in the bulk is delayed. Such phase separations might induce a nonuniform morphology, e.g., broad size distributions of phase-separated droplets, across the film.4,12 The degree of nonuniformity through the film thickness also strongly depends on the evaporation rate from the air/solution interface. If convection exists during phase separation of thicker polymer blend films, it might affect the solvent evaporation and concentration distributions in the film and consequently improve the film morphology. In addition, if wellcontrolled patterns of convection with a high degree of order and symmetry can be eventually trapped in the solid state by solvent evaporation, it could lead to new approaches for producing polymer blend films with periodic modulations in properties. Mitov and Kumacheva13 recently demonstrated that the ordered hexagonal patterns due to Marangoni-Benard convection are formed on the surface of polymer blend solutions for particular ranges of solvent evaporation rate and thickness of liquid layers, (10) Nisato, G.; Ermi, B. D.; Douglas, J. F.; Karim, A. Macromolecules 1999, 32, 2356-2364. (11) Cyganik, P.; Bergues, A.; Budkowski, A.; Bergues, B.; Kowalski, K.; Rysz, J.; Lekki, J.; Lekka, M.; Postawa, Z. Vacuum 2001, 63, 307313. (12) Hopkinson, I.; Myatt, M. Macromolecules 2002, 35, 5153-5160. (13) Mitov, Z.; Kumacheva, E. Phys. Rev. Lett. 1998, 81, 3427-3430.
10.1021/la0302100 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/17/2004
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Figure 1. Schematic diagram of experimental apparatus.
and that, consequently, periodic two-phase structures are conserved in polymer films after evaporation. Yamamura et al.14 also revealed that, depending on the evaporation rate, the Marangoni-Benard convection is developed and cellular structures of droplet arrays or honeycomblike patterns are formed in the polymer blend films. When a dielectric liquid between plane-parallel electrodes is subjected to a dc electric field beyond a certain critical value, Coulomb force-driven convection, namely, electrohydrodynamic (EHD) convection, occurs in the liquid layer because of the interplay between space charges present in the dielectric liquid and the electric field.15,16 Moreover, liquid motion is always present when electrodes are not plane-parallel; i.e., the liquid is subjected to a nonuniform electric field.17 Since the polymer blend solutions consisting of binary polymer mixtures dissolved in a solvent are considered to be dielectric liquids, it is expected that EHD convection occurs in the polymer solutions subjected to a dc electric field and might be utilized as a means of mixing and morphology control in the polymer blend film, similarly to Benard convection. Because the structure of EHD convection can be varied depending on the geometry of electrodes, control would be easier in comparison with Benard convection. In this work, the effect of EHD convection on the domain structure in the polystyrene (PS)/polyvinyl acetate (PVA) blend film was investigated to demonstrate the feasibility of using the EHD effect as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation. Here, polymer-toluene solutions were spread on a substrate with patterned electrodes to apply a dc electric field, and well-defined structures of EHD convection were formed in the polymer solutions. Recently, an electric field has been recognized as a new option for the modulation of the morphology in polymer blend films.18,19 Xi and Krause19 have studied the deformation of phase-separated droplets in an electric field (14) Yamamura, M.; Nishio, T.; Kajiwara, T.; Adachi, K. Drying Technol. 2001, 19, 1397-1410. (15) Pontiga, F.; Castellanos, A.; Malraison, B. Phys. Fluids 1995, 7, 1348-1356. (16) Chicon, R.; Castellaos, A.; Martin, E. J. Fluid Mech. 1997, 344, 43-66. (17) Higuera, F. J. Phys. Fluids 2000, 12, 2732-2742. (18) Venugopal, G.; Krause, S. Macromolecules 1992, 25, 4626-4634. (19) Xi, K.; Krause, S. Macromolecules 1998, 31, 3974-3984.
formed by parallel electrodes and the development of the final morphology in polymer blend films after solvent evaporation and have revealed that the conductivity ratio between the dispersed and continuous phases is extremely important in structure formation in the polymer blend film. They have focused on the deformation of the dispersed phase caused by electrical stresses at the interface but not by EHD convection in the polymer blend solutions, although they reported that a swirling motion of the bulk fluid was observed at the early stage of solvent evaporation. 2. Experimental Section To investigate the effect of EHD convection on phase separation in polymer blend films by solvent evaporation, the experimental apparatus shown in Figure 1 was constructed. A glass substrate of 55 mm diameter and 2 mm thickness, on which Pt-on-Ti electrodes with a periodic structure and approximately 0.1 µm thickness shown in the figure (inset) were formed using photolithography and sputtering techniques, was set in a Teflon duct. The periodic-patterned electrodes were used here to form well-structured, spatially and temporally stable EHD convection by constraining it within each unit of electrodes. The electrodes were connected to a dc constant voltage supply. In the experiment, first, a given amount of polymer blend solution was poured into a groove 30 × 30 × 1 mm3 on a glass substrate with patterned electrodes by a syringe, passing N2 gas saturated with toluene through a Teflon duct. Here, the polymer blend solutions were placed on top of the electrodes. Then the solvent in the polymer blend film was evaporated by flowing N2 gas with a given concentration of solvent over it, and simultaneously a dc electric voltage was applied to the electrodes. In this work, only pure N2 gas was flowed at a rate of 1 L/min to evaporate the solvent. The EHD convection and the phase separation in the polymer blend film during solvent evaporation were observed using a phasecontrast microscope. Once most of the solvent evaporated and the morphology of polymer films was frozen, the electric field was turned off. The polymers used in this work were polystyrene (PS) (Mn ) 167 000) and polyvinyl acetate (PVA) (Mn ) 119 628), and toluene was used as a solvent. The polymers were purified by reprecipitation prior to film preparation. Ruthenium tetraoxide (RuO4) (5 wt % in an aqueous solution) was used to ascertain which phase was the PVA-rich one; the PVA-rich phase became darker when the polymer blend film obtained after solvent evaporation was stained with RuO4.19 In this work, the polymer content was 4 wt %, and the polymer mixtures were prepared in a ratio of 1:1.
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Figure 2. Time evolution of EHD convection and phase separation in PS/PVA/toluene film during solvent evaporation.
3. Results and Discussion Figure 2 shows the time evolution of EHD convection and phase separation in a PS/PVA/toluene film during solvent evaporation, where the applied voltage was 300 V. The average final thickness of the film after solvent evaporation was 21 µm. At the initial stage, i.e., 20 min after evaporation starts (see Figure 2a), we observed up and down flow, EHD convection, above the electrodes, in which the liquid flows from the positive to negative electrodes along the substrate surface, ascends above the negative one, moves along the film surface, and then descends toward the positive one. Then, as shown in Figure 2b, the flow patterns changed to a pair of swirling motions that were parallel to the electrode surfaces and confined within each unit of electrodes. At this stage, definite phaseseparated droplets appeared, and their size increased because of coalescence, particularly at the stagnant center region of the swirls. With time, the viscosity of the mixtures increased significantly because of solvent evaporation, and the EHD convection ceased. Finally, regular patterns were formed in the polymer blend film in which domains were confined within each unit of electrodes, i.e., between positive and negative electrodes, as shown in Figure 2c. Here, the domains colored brown by RuO4 are the PVArich phase, while the blue region is the PS-rich phase. The white lines enclosing the PVA-rich phases are attributed to a phase-contrast microscope itself and insignificant. Figure 3 shows the effect of applied voltage on the surface morphology of polymer blend films, where the PVArich domains are shown by the white patches in the overview photographs, although they are actually darkcolored by RuO4 as shown in Figure 2c. For a relatively small voltage, i.e., 100 or 200 V, the PVA-rich domains are distributed randomly on the negative and positive electrodes and between them, while for the larger voltage of 400 V, the domains between the electrodes coalesce, and consequently the striped-patterned morphology is formed between the electrodes. As a result, at a certain applied voltage, i.e., 300 V, regular patterns were formed in the polymer blend film in which domains were confined within each unit of electrodes, i.e., between positive and negative electrodes.
Figure 3. Effect of applied voltage on the surface morphology of polymer blend film.
Figure 4 shows the time evolution of EHD convection and phase separation in a PS/PVA/toluene film during solvent evaporation, where the applied voltage was 200 V. The average final thickness after solvent evaporation was 14 µm, which is smaller than those in Figures 2 and 3. Parts a and b of Figure 4 show the observations through a phase-contrast microscope, and part c is the overview photographs of the polymer blend film stained with RuO4 after solvent evaporation in which the PVA-rich domains are shown by the white patches. The swirling motion parallel to the electrode surfaces appeared even at the initial stage, as shown in Figure 4a. The evaporation rate was larger than that in Figure 2 because of the thinner film, and thus, the large separated droplet formed by coalescence appeared between the electrodes at 14 min (see Figure 4b). In addition, Figure 4c reveals that the
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Figure 4. Time evolution of EHD convection and phase separation in a PS/PVA/toluene film during solvent evaporation when the film thickness is relatively thin.
Figure 5. Surface morphology of polymer blend film prepared by solvent evaporation when the substrate with periodicpatterned electrodes covered by a SiO2 film was used.
regular-patterned morphology is formed in each unit of electrodes similarly to Figure 3c, although the area of each domain is smaller than that in Figure 3. The issue remains that the origin of regular patterns in Figures 3 and 4c may be due to the surface effect of Pt electrodes themselves deposited on the glass substrate, i.e., the wetting/dewetting of polymers against the surfaces of Pt electrodes and the glass substrate. Therefore, another experiment for the preparation of polymer blend films in the presence of EHD convection was carried out in which the substrate with periodic-patterned electrodes shown in Figure 1 was covered by a thin film of SiO2. As a result, a morphology similar to those in Figures 3 and 4c, i.e., a spatially periodic structure, was observed in the polymer blend films, as shown in Figure 5. Strictly speaking, however, the intensity of the EHD convection and the geometry of the phase-separated domains are different from those in Figures 3 and 4c because the electric field in the film varies because of the presence of SiO2; i.e., the voltage drops through it. During the preparation of relatively thick polymer blend films by solvent evaporation, first, the solvent concentration near the film surface decreases because of evaporation from the surface, and the phase separation begins from there. However, the compositions of separated phases probably never reach equilibrium because of the low
diffusivities of the polymers in concentrated solutions. Therefore, separated phases do not grow, but a large number of small droplets with nonequilibrium compositions may be frozen near the surface of the polymer film. While it is inferred that in the bulk of film the decrease in the solvent concentration is retarded because of the large resistance to mass transfer at the surface layer with less solvent, the separated phases grow and relatively large domains are formed. Such a nonuniform phase separation through the film thickness was recently observed by Hopkinson and Myatt12 using a laser scanning confocal microscope. In contrast, since EHD convection, particularly the flow vertical to the electrode surfaces, mixes the polymer solution in the film, the concentration distributions, their time variation, and consequently the phase separation could be different from those without convection. In addition, the distribution of shear stresses, which depends on the structure of flow in the film, could vary the size distribution of the phase-separated droplets. For example, the shear flow at the boundary of two swirls induces the breakage of the droplets, while the coalescence of the droplets occurs at the center of a swirl, a stagnant region, as shown in Figure 4b. Such a flow structure affects the final morphology of the polymer blend film, although it seems that the locations of PVA-rich domains in the film and their deformed shapes at the final stage, as shown in Figure 4c, are also due to the statically electrical stresses. Here, the stresses are caused by the differences of dielectric constant and conductivity between the dispersed phase (PVA-rich) and the matrix phase (PSrich) and act at the interface of both phases.19 In this work, the morphology of the polymer blend film could not be controlled completely, but these results suggest that the EHD effect could be used as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation if the optimal geometry of the electrodes and electric voltage were determined so that the desired structure of the EHD convection is formed in the polymer blend solutions.
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4. Conclusions We have investigated the effect of EHD convection on the domain structure in a polystyrene (PS)/polyvinyl acetate (PVA) blend film to demonstrate the feasibility of using the EHD effect as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation. Here, polymers-toluene solutions were spread on a glass substrate with patterned electrodes to apply a dc electric field. As a result, well-defined structures of EHD convection was observed in the polymer film, and finally regular patterns were formed in the PS/PVA polymer blend film in which PVA-rich domains were confined within each unit of patterned electrodes, i.e.,
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between positive and negative electrodes, at an appropriate electric voltage. In addition, it was revealed that such novel morphology is not due to the wetting/dewetting effect of polymer components to the Pt electrodes deposited on the glass substrate, by experiments with a SiO2-covered substrate. Acknowledgment. This work was partially funded by a Grant-in-Aid for Scientific Research (Grant No. 12875142) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. LA0302100
