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Macromol. Chem. Phys. 2003, 204, 125–130 125 Full Paper: Ordered macroporous materials recently have attracted much attention. A method that utilizes the condensation of monodisperse water droplets on a polymer solution is proposed for the preparation of honeycomb microporous films. Our results show that it is a general method that can be used for patterning a wide range of polymers. The presence of water vapor and polymer is necessary for the formation of regular holes in films. The formation of hexagonal packing instead of other kinds of packing takes place because the hexagonal packing has the lowest free energy. The formation mechanisms of regular hole pattern and imperfections in the hexagonal packing are proposed. Hexagonal hole structures in a PMMA film. Formation of Regular Hole Pattern in Polymer Films Juan Peng, Yanchun Han,* Jun Fu, Yuming Yang, Binyao Li State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China Fax: 86-431-5262126; E-mail: ychan@ciac.jl.cn Keywords: condensation; evaporation; macroporous polymers; patterning; thin films Introduction Ordered macroporous materials recently have attracted much attention because of their potential optical properties such as photonic bandgaps[1–4] and optical stop-bands.[5] Those with pore sizes in the range of micrometers are also of interest for applications in catalysis, sensors, separation, adsorbents, membranes, and scaffolds for composite material synthesis.[6–8] Usually, for the preparation of microporous materials, the ordered alignment of a template around which the material of interest assembles is required. These templating methods include using block copolymers,[9] colloidal crystals,[10] emulsions,[11] and polymers with rod-coil structure.[12] In all cases, structures with submicrometer dimensions are prepared, although the use of block copolymers depends too much on the selective solvent and the copolymer structure. When using colloidal crystal Macromol. Chem. Phys. 2003, 204, No. 1 templating, a fluid fills due to capillary force and solidifies in the vacant space between the colloidal crystals. Then the spheres are removed by calcinations or physical dissolution leaving behind a solid porous skeleton arranged in ordered 3D arrays. The result is a highly faithful replica of the original colloidal crystal, however, it is obvious that the dimensions of pores match those of the spheres and cannot be controlled dynamically.[13] The drawback of the emulsion method is that it requires a fractionation procedure to assure the uniformity of the droplet size. Recently, a simple non-template method by self-assembly of block copolymers and amphiphiles was used for the preparation of ordered micrometer-size honeycomb structures.[13–24] François et al.[14–17] first proposed the preparation method consisting in evaporating a layer of solution spread on a flat support under a flow of moist gas. When solvent and water droplets evaporate completely, an ordered ß WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1022-1352/2003/0101–125$17.50þ.50/0 126 J. Peng, Y. Han, J. Fu, Y. Yang, B. Li honeycomb array of holes is formed on the solid polymer surface. They found this phenomenon in carbon disulfide solutions of polystyrene-poly(p-phenylene) (PS-PPP) block copolymer and believed that the polymer with star morphology or polymeric micelles and use of carbon disulfide as solvent were determinant elements in the formation mechanism of the honeycomb structure. Srinivasarao et al.[13] further investigated this method and prepared 2D and 3D ordered macroporous layers depending on the solvent density with respect to the density of water. They also gave a vivid model for the formation of the regular structure based on thermocapillary convection. Shimomura et al.[18–23] photographed the dynamical movement of the rearrangement of water droplets during honeycomb pattern formation and used different kinds of compounds including organic-inorganic hybrid materials,[19] amphiphilic copolymers,[20–22] metalorganic,[23] and saccharide-containing polymers[23] for patterning. In this paper, we extend this method to a wide variety of polymers, including homogeneous poly(methyl methacrylate) (PMMA), linear polystyrene without any polar end group, and crown ether-containing series such as PS-crownPS, PMMA-crown-PMMA. The mechanisms of hole pattern formation have been discussed in detail. Also, the reason why the water droplets form hexagonal packing instead of other kinds of packing has been explained and a formation mechanism of imperfections in the hexagonal packing is proposed. Experimental Part The chemical structures of the compounds used are shown in Figure 1. Poly(methyl methacrylate) (1) (M w ¼ 102 600, Aldrich, 1 wt.-%), polystyrene (2) (M w ¼ 223 200, Aldrich, 1 wt.-%), PS-crown-PS (3) (M n ¼ 5 700, 0.5 wt.-%), PMMAcrown-PMMA (4) (M n ¼ 7 200, 0.5 wt.-%). For pattern formation, several microliters of dilute polymer toluene solutions were spread homogeneously on a hydrophilic glass slide over an area of ca. 1 cm2. A carrier gas (nitrogen) was bubbled through two flasks of distilled water at room temperature. The nitrogen gas saturated with water vapor was then sent from a nozzle onto the surface of the polymer film. The formed honeycomb structures were characterized by optical microscopy (XJX-2, Nanjing, China) and atomic force microscopy (SPI3800N, Seiko Instruments Inc., Japan). Results and Discussion The Formation of Honeycomb-Patterned Films After placing a drop of PMMA solution on a glass slide in a humid atmosphere (relative humidity of approximately 60%) with nitrogen flow across the surface, the solvent began to evaporate and some phenomena were observed immediately. It could be seen that the transparent polymer solution became turbid due to emulsification. After several Figure 1. Chemical structures of the polymers used. seconds, the solvent evaporated completely and left behind an opaque and white layer. Since the evaporation rate at the edge of the solution was higher than that at the central area,[25] the peripheries of the layer had a smaller thickness than the central areas. When such layers were viewed with conventional reflected light microscopy, some patches displayed iridescent colors owing to the diffraction of light at the layer with different thickness. By the use of atomic force microscopy performed in tapping mode, highly ordered, two-dimensional, micrometer-sized hole structures could be seen clearly (Figure 2A), which had a typical depth of 1.1 mm, a uniform diameter of 1.3 mm and an interval of 1.2 mm between adjacent holes. Fast Fourier transform (FFT) pattern in the inset of the image indicated a perfect hexagonal arrangement of the holes. Regular structures with similar features were also formed when using the other three polymers (polystyrene, PS-crown-PS and PMMA-crown-PMMA), with a few imperfections sometimes (Figure 2C, 2D). The size of the holes was in the range of a few micrometers and the ordered packing could cover thousands of square micrometers. Linear polystyrene also provided highly ordered structures (Figure 2B) on the contrary to the results of François et al.[14–16] and Davis.[24] François found that several polymers based on polystyrene sequences could produce regular structures: star-polystyrenes, associative polystyrenes (linear polystyrene chains with a polar terminal group able to associate in non-polar solvents such as CS2) and polystyrene block copolymers. When using usual linear polystyrene, no ordered macroporous membranes were obtained. Davis synthesized the star-polystyrene (five arm), which was used in the same way to form such regular porous structures. When adding over Formation of Regular Hole Pattern in Polymer Films 127 Figure 2. Atomic force microscopy images of the regular structures in polymer films. (A): Hexagonal hole structures in a PMMA film. FFT pattern is given in the inset of the image indicating a perfect hexagonal arrangement of the holes. (B): Hexagonal hole structure in a linear polystyrene film. (C): Hexagonal structures in a PS-crown-PS film with slight imperfections. The lower hexagon lacks a hole pattern in the central region in contrast to the upper hexagon. (D): Hexagonal structures in a PMMA-crown-PMMA film. The hexagon marks an imperfection different from that in Figure 2C. 30 wt.-% linear polystyrene to the star-polystyrene solution, the regular structure was disrupted, ultimately leading to the disappearance of the porous structure if exceeding 70 wt.-%. Srinivasarao[13] also used polystyrene with one end terminated by a carboxylic acid group and Shimomura[18] added 10 wt.-% amphiphilic polyion complex to a polystyrene solution as a stabilizer. Pure polystyrene in CHCl3 solution only resulted in a less ordered structure. The humidity of the atmosphere and the polymer solution are critical in the formation of the regular hole pattern.[23] To confirm the role of water vapor during the experiment, a droplet of toluene solution containing PMMA was placed on a glass slide and evaporated in the absence of moisture. In contrast, such ordered hexagonal structures were not observed. Therefore, we conclude that the presence of water vapor in the atmosphere is essential for the formation of an ordered array. Pure solvent was also used for comparison. When a drop of pure toluene evaporated under the same conditions, a turbid surface emerged, too. Unlike the solution containing polymer, when the solvent evaporated completely, nothing was left on the glass slide. Thus, it can be concluded that the polymer precipitates at the interface, forming a layer encapsulating the water droplets and protecting them from evaporating at the initial stage. These phenomena suggest that two conditions are determinant elements in producing this honeycomb structure: 128 J. Peng, Y. Han, J. Fu, Y. Yang, B. Li use of volatile solvent containing polymer and evaporation under moist conditions. The formation of the honeycomb morphology can be described as follows: the evaporation of toluene decreases the temperature of the solution surface, and water vapor condenses onto it. The temperature gradient between the solution surface and the substrate induces a surface tension gradient. Furthermore, the solution surface is not perfectly homogeneous and an infinitesimal small disturbance will generate some ‘‘hot’’ and ‘‘cool’’regions.[26] Because surface energy is minimized by decreasing the higher surface tension areas corresponding to the cooler region, the solution with water droplets on its surface is then pulled from the hotter area to the cooler area, driving the thermocapillary flow through Marangoni[26–28] or Rayleigh[29] convection and forming a hexagonal pattern. In the case of Marangoni convection, a temperature gradient induces a surface tension gradient driving the spreading process and the effect is expressed by the dimensionless Marangoni number established by Pearson:[30] Ma ¼ ðBDThÞ=ðrukÞ ð1Þ where B ¼
dg/dT is the variation of the interfacial tension, r, u, k, h and DT are the liquid density, dynamic viscosity, thermal diffusivity, thickness of the liquid film and the temperature variation, respectively. It is known that Marangoni instabilities can appear only when the value of the Marangoni number is larger than the critical value Mc  80.[30] Rayleigh convection results from the temperature-dependent gradients in the fluid density and is described by the Rayleigh number Ra ¼ ðgbDTh3 Þ=ðukÞ ð2Þ where b is the thermal volume expansion coefficient and g is the acceleration of gravity. Since Ra / d3 and Ma / d, Rayleigh-generated convection dominates in thick films. In our case for films thinner than ca. 1 mm, Marangoni instabilities govern the convection and drive the thermocapillary flow.[28] The evaporation process can be divided into two stages with respect to time, toluene and water droplets evaporation. At the first stage, the toluene evaporates and water droplets are immobilized on the polymer film. Then water droplets evaporate and leave behind an ordered structure at the polymer surface. Mechanisms of Formation After having shown the formation processes of ordered packing, several obvious questions arise. Why do the water droplets remain isolated from each other instead of coalescence? Why are the formed holes so uniform in size and so ordered? Why do the holes form a hexagonal packing instead of other kinds of packing? And what is the formation mechanism of imperfections in the hexagonal packing? Non-coalescence between two droplets has been studied for a long time.[29,31] Different mechanisms of noncoalescence due to electrostatic repulsion,[29] intervening lubrication film,[32] and evaporation[33] have in common the inhibition of molecular contact between the involved interfaces.[34] In this case, François[17] showed that the polymer formed at the solution/water interface inhibited coalescence between the droplets. Srinivasarao[13] assumed that such behavior was driven by a thermocapillary convection coupled with the presence of a lubricating air film between the drops. Considering the interaction between water droplets and polymer dissolved in solution, we give another explanation. Since all the polymers we used have a certain polarity more or less, there exists an attractive interaction between water droplets and polymers. It is the attractive interaction that stabilizes the droplets distributed on the solution surface thus coalescence is avoided.[35] It is known that the deformation of a liquid surface due to trapped particles gives rise to capillary forces exerted on the particles. The larger deformation is, the stronger capillary forces are between them. Furthermore, the capillary interaction turns out to be attractive and long-ranged for spheres. Such a long-range attraction appears to be strong enough to engender disorder-order phase transitions and lead to twodimensional particle aggregation and ordering.[36] When water vapor condenses onto the cold surface of the solution, it appears in the form of drops. Since water has a higher density than toluene, the droplets should sink. But actually their gravity is counter-balanced by surface tension forces. At the initial stage of the condensation process, monodisperse droplets condense onto the solution surface and form the islands that build the breath figures later.[37,38] Breath figures are the patterns formed when a cold solid or a liquid surface is in contact with moist air. The geometry of the islands can be very different and they attract each other in a much stronger way than the single water droplets because the attractive force between the islands is proportional to the sixth power of their radii.[39] Then the protective layer of polymer stabilizes the droplets by attractive interaction and inhibits their coalescence. Thus a highly uniform size of holes is obtained after the water droplets evaporated completely. Besides, capillary forces contribute to locally arrange the droplets in a well-ordered packing.[17] At equilibrium, the preferred packing arrangement of the spherical microdomains will be that which corresponds to the lowest free energy.[40] The difference in free energy for the various lattices will depend on both the amount of material overlapped and the least redistribution of material from regions of excess segmental density. The overlap fraction depends on the lattice type and the extent of overlap. For thin films containing only a single layer of spherical micelles, the hexagonal arrangement provides the lowest overlap fraction with least required redistribution of material from regions of excess segmental density.[40] Therefore, the optimum-covering lattice is hexagonal. The Formation of Regular Hole Pattern in Polymer Films 129 Figure 3. A model for the formation of regular hole pattern and existing imperfections in polymer films. (A): Water condensation occurs simultaneously with the solvent evaporation. (B): The water droplet is carried away by the evaporating solvent vapor before condensating onto the solution surface, while the other water droplets arrange in a hexagonal packing. (C): Water droplets sink into the solution. (D): Water droplets and solvent evaporate and leave the regular hole pattern with an imperfection lacking a hole in the film. (B1): Water condensation onto the solution surface forming hexagonal packing. (C1): Awater droplet is carried away by the evaporating solvent before having time to be trapped into the solution. (D1): Water droplets and solvent evaporate and leave the regular hole pattern with the imperfection of a shallower hole. close adjacent droplets are attracted to each other by the lateral capillary force that causes dense hexagonal packing. As shown in Figure 2C and Figure 2D, the hexagonal packing has slight imperfections and the formation mechanisms of the imperfections in Figure 2C and 2D are different. Figure 3 is a model for the formation of imperfections in Figure 2C and 2D, respectively. In Figure 2C, in contrast to the upper hexagonal packing, the lower hexagonal packing lacks a hole in the central region. Figure 3A to D give the explanation. It is believed that water condensation takes place simultaneously with the solvent evaporation (Figure 3A). Then the water droplet is carried away by the evaporating solvent vapor before condensation onto the solution surface, while the other water droplets arrange in a hexagonal packing due to the capillary force and the principle of the lowest free energy (Figure 3B). After that, water droplets sink into the solution (Figure 3C). Finally, water droplets and solvent evaporate and leave the regular hole pattern with an imperfection lacking a hole in the film (Figure 3D). The imperfections in Figure 2D show a little difference from that in Figure 2C. From the marked hexagon in Figure 2D, a shallower hole is formed in the central region compared to the other six holes in the hexagonal apexes. Figure 3B1 to Figure 3D1 give the explanation. When water droplets condense onto the solution surface, hexagonal packing is formed (Figure 3B1). Then a water droplet is carried away by the evaporating solvent before having time to be trapped into the solution (Figure 3C1). At last, water droplets and solvent evaporate and leave the regular hole pattern with the imperfection of a shallower hole (Figure 3D1). Conclusions We have shown that the two-dimensional, ordered structure with a uniform hole size in polymer films can be readily formed by evaporating a layer of polymer solution in a humid atmosphere. This is a facile approach to produce highly ordered porous materials. The presence of polymer and moisture has been proved to be essential for producing such ordered structures. Noncoalescence between water droplets is due to the attractive interaction between droplets and polymers stabilizing the droplets. Hexagonal packing is based on the principle of the lowest free energy. Also, the 130 J. Peng, Y. Han, J. Fu, Y. Yang, B. Li underlying mechanisms of two different kinds of imperfection formation are proposed. Acknowledgement: The authors wish to thank the National Natural Science Foundation of China for General (20274050) and Major (50290090) Program and National Science Fund for Distinguished Young Scholars of China (50125311), the Ministry of Science and Technology of China for Special Profunds for Major State Basic Research Projects (2002CCAD400), the Chinese Academy of Sciences for Distinguished Talents Program and Intellectual Innovations Project (KGCX2-205-03), and Jilin Province fund for Distinguished Young Scholars (20010101). 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