Journal of Mechanical Science and Technology 25 (1) (2011) 33~36 www.springerlink.com/content/1738-494x
DOI 10.1007/s12206-010-1010-3
Process optimization of preparing honeycomb-patterned polystyrene films by breath figure method† Yanqiong Zheng1,2, Yuki Kubowaki1,2, Makoto Kashiwagi1,2 and Koji Miyazaki1,2,* 1
Life BEANS Center Kyushu744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Mechanical and Control Engineering, Kyushu Institute of Technology1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan
2
(Manuscript Received October 5, 2010; Revised November 19, 2010; Accepted November 19, 2010) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Abstract Honeycomb-patterned polystyrene film is prepared by breath figure method. Pore size with a few hundred nano-meter to several micro-meter diameters is generated by controlling various parameters, such as the polymer concentration, humidity, casting area and solvent. The pore size becomes larger with increasing humidity and temperature difference between air and substrate. Porous film can be formed by drying under high humidity. The density of solvents affect the pore shape and pore depth. Video observation indicates that in the formation process of the porous pattern water droplet condenses gradually on the surface, along with the migration of the droplet formation boundary line from the edge to the center, when the boundary line reaches the center, hexagonally packed porous template is completed. Keywords: Breath figure method; Porous film; Condensation ; Density of solvent ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Introduction Polymer films with micro-meter pore size are attractive materials with potential applications, such as in photonic crystals, biosensors, templates, catalysis, size- and shape-selective separation media, and even in paints, photonic papers, and cosmetics due to their properties (e.g., structural colors). A variety of methodologies has been developed to create microor nano-scale ordered porous films, including direct writing of polymer patterns [1], soft-lithographic methods [2], the use of photo- or electrochemically polymerizable precursors [3], and electric-field-induced patterning of block copolymers. In particular, the template approach and “breath figure” method are two of the most widely used techniques for fabricating ordered porous polymer films. “breath figure” method is first discovered by Bernard Francois group [4], after that with this approach many factors were studied, such as polymer material’s type, substrate’s hydrophilic property, different pore arrangement etc.. The mechanism of breath figure method is based on the self-assembling of water droplet which comes from air, because in this process highly volatile solvent is necessary, the evaporation of solvent is a decalescence process that induces substrate temperature to decrease, the water droplet in air will automatically condense on the substrate surface. After water †
This paper was presented at the MECT-10, Seoul, Korea, March 2010. This paper was recommended for publication in revised form by Guest Editors Seung Jin Song, S. Mochizuki * Corresponding author. Tel.: +81 93 884 3137, Fax.: +81 93 884 3137 E-mail address:
[email protected];
[email protected] © KSME & Springer 2011
droplet evaporates, porous structure with hexagonal array or square array is formed. Due to the strong surface tension so the most stabilized packing mode is hexagonal pattern [4]. The breath figure procedure is simple, quick and inexpensive, thus, it has been used for preparing many types of porous polymer films, such as polystyrene-polyfluorene block copolymers [5], polystyrene (PS) [6], poly (styrene-butyl acrylate- acrylic acid) [7, 8], poly (methyl methacrylate) (PMMA) [9], cellulose acetate butyrate (CAB) [10, 11]. In this paper, the influence of solution concentration, humidity, and solvent besides mechanism of pore formation is investigated. We also report a simplified process of using a drop casting method from a dip coating method [12]. The contribution of the paper is to present the real-time observation image for the uniform pore formation process, and analyze the underlying principle of solvent effect and humidity on pore size from chemistry and physicochemical point of view.
2. Experimental section Polystyrene (Mn=208,000 g mol-1) is used in all the experiments. All the solvents including chloroform, THF, toluene, benzene and dichloromethane are of analytical grade, and were purchased from Wako Chemical Company (Japan). Polystyrene is dissolved in chloroform by magnetic stirring to prepare different concentrations. The preparation procedure of porous film is carried out in the equipment as Fig. 1, in this chamber there are temperature sensor, temperature controller,
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Fig. 1. Equipment image of preparation for porous polymer film.
and moisture meter. Highly moist air is poured into this chamber to control the humidity. The 40 μl polystyrene solution is dropped on a clean glass slide (20×20×1mm) under different humidity and temperature. Then the glass slide is put on a cool substrate, the temperature of the cool substrate is well controlled. After solvent evaporates porous polystyrene film is fabricated. The porous structure is observed by Optical Microscope (E600FN, Nikon, Japan), Atomic Force Microscope (CP-Ⅱ,VEECO, USA), and Scanning Electronic Microscope (JSM-7000FSK, Japan Electronics).
3. Results and discussions 3.1 Influence of solution concentration, humidity, and casting area The porous films were prepared using different concentrations of polystyrene solution in chloroform (0.5, 0.75, 1.0, 2.0, 3.0 wt%), the pores are scattered at 0.5 wt%, because the solvent evaporates qickly and the water droplets aggregate easily. When concentration is over 1.0 wt%, the viscosity of solution is higher that the film becomes thicker, however the pore depth is lower because water droplets float on the surface, aspect ratio of pores becomes smaller. In order to get uniform pore and relatively high aspect ratio, in the experimental case, 1.0wt % is probably suitable. At the same temperature difference 5.0 oC between air and substrate temperature, the effect of humidity was studied from 67.0% to 89.7%, the pore distribution of samples made at the five humidities is shown in Fig. 2(a). Within this range, the average pore size of the film increased with humidity, from 4.4 to 11.0 μm (Fig. 2(b)). When the porous film was air-dried under 85% humidity, a ~3.0 μm pore size formed due to water condensation (Fig. 2(c)). About the mechanism of humidity’s effect on pore size, it maybe be explained from evaporation rate and thermodynamic point of view. One factor is that, the water vapor pressure becomes bigger at higher humidity (Eq. (1)), which blocks the increase of solvent’s vapor partial pressure, so the chloroform evaporates more slowly. Longer drying time induces more water droplet to coalesce into bigger pore. Another influence factor can be explained from the point of view of thermodynamic (Eq. 2) and Young- Laplace Eq. (Eq. 3). H=
18 P0 29 Ptotal − P0
(1)
Fig. 2. (a) Pore size distribution at different levels of humidity; (b) Effect of humidity on pore size (inset: Atomic Force Microscope (AFM) image of the porous film obtained under 78.7% humidity; (c) AFM image of porous film prepared at air-dried under 85% humidity.
Pc = P0e − (2γ / r )(1/ P0 ) − P '− Pc =
2γ r
2γ r
(2) (3)
where H is humidity, P0 is the partial pressure and Ptotal is the total pressure, the partial pressure is the vapor pressure in the closed system, r is the radius of curvature, γis the surface tension, P’ is the gas pressure within the curvature of a surface, and Pc is the liquid pressure outside the curvature of a surface. Due to the presence of capillary force, it is enhanced in highhumidity ambient systems, according to the relation of humidity to vapor pressure (Eq. 1), humidity increase induces higher vapor pressure. In a closed system with controlled humidity, Eq. (2) is the thermodynamic equation. Pc presents positive correlation with P0. According to Young-Laplace Eq. (3), Pc is increased that the radius of curvature r is increased [13]. Therefore the pore coming from water droplet becomes larger at high humidity. The relationship between pore size, pore number and humidity are given in Fig. 3, curve a indicates pore size increases with humidity, curve b shows that the pore number on 4,800 μm2 made at different humidity. Small pore size, big pore number and big covering area are optimal, because they correspond to small numerous pores. At the temperature difference 5.0 oC and room temperature 14 oC, 70.9% humidity results in the best porous structure. When the temperature difference between air and substrate is bigger, the pore size is bigger, because the water droplets become motionless on the surface
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Fig. 3. Relationship between pore size (curve a) and pore number (curve b) with humidity.
(a) ρsolvent > ρwater (b)ρsolvent < ρwater
Fig. 5. AFM images of Polystyrene porous film with chloroform (a), benzene (b), THF (c) as solvent and SEM cross section image with chloroform as solvent (d).
(c) ρwater-soluble < ρwater Fig. 4. Schematic diagram of pore shape and pore depth made by different solvents.
and coalesce. Casting area also affects the pore size, because larger casting areas result in thinner films, which dry faster (less time for the water droplets to aggregate). 3.2 Effect of solvent Many solvents are used to dissolve PS, such as benzene, toluene, chloroform, THF, and dichloromethane. The boiling point (BP) of benzene is 80 oC; thus, film-drying time is longer, which results in larger pore size. The BP of Toluene is 111 oC, and under normal conditions, it requires reduction of the air pressure in order to obtain the porous film. Dichloromethane has a BP of 40 oC; therefore, its evaporation is quick under normal pressure and temperature, which results in scarce pores. Chloroform and THF have been found to be better than the previous solvents. However, THF is water-soluble, and its stock solution always needs to be freshly prepared. As the water condenses on the surface of polymer solution film, water droplets on the surface (as is the case when solvent density is larger than that of water) cause shallow pores. However, when the density of solvent is smaller than that of water, the water droplets sink into the polymer solution, and induce small pore sizes and larger pore depths. The schematic for these phenomena is given in Fig. 4. AFM images of the polystyrene porous film with chloroform, benzene and THF as solvents are shown in Fig. 5, The pore shapes in chloroform and THF are similar ; here, the water droplets sink directly and cause round pore shapes. The pores with benzene are conical. Pore depths correlate with solvent density, as in the case of benzene and THF, whose densities are lower than that of water. Benzene and THF result in pore depths of 90 and 60 nm, respectively. These are larger than the pore depth with chloroform as solvent. From the SEM cross-section image, the pore shape with chloroform as solvent is elliptical (Fig. 5). Thus,
Fig. 6. Video sequence of the formation process (from a to d) of the hexagonal arrays on glass substrate.
m
Fig. 7. Video sequence of the solvent evaporation and coalescence of water droplets.
the density of solvent, as compared with that of water, affects the pore shape and pore depth. 3.3 Mechanism of pore formation The literature describes the mechanism of the water figure method as occurring through, the condensation of water droplets after solvent evaporation. That is, solvent evaporation and water molecule coverage are not synchronous. However, after micrographic observation, solvent evaporation and water molecule coverage on the whole surface are found to take place simultaneously. The results of video observation are shown in Fig. 6. The formation process of the porous pattern
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occurs as the droplet formation boundary line moves from the edge of the solution to the center (as the arrow direction in Fig. 6a), and the line range shrinks from (a) to (b). In figure (c) the ellipse shows the bubble formation after the boundary line moves to center. After the bubble in Fig. 6c breaks, due to the capillary force the uniform pore pattern is formed, as is seen in Fig. 6d. In the whole process along with the migration of this line water droplet condenses gradually on the surface of the polymer solution. When it moves to the center, the fabrication of porous template is finished. The coalescence process of water droplets on the polymer surface is observed by microscope (Fig. 7). Initially, small water droplets condense on the surface, because of the surface tension within the polymer solution. Neighboring water molecules are pulled toward the polymer film center. The water droplets easily merge into a big droplet, and the pore size becomes larger. The final porous film is formed through the procedures mentioned above.
4. Conclusions In the process of preparing polystyrene porous film by breath figure method, the pore size increases with humidity. Large temperature differences between air and substrate induce bigger pore size. For a given amount of polymer solution bigger casting area results in smaller pore size. The density of solvent affects the pore shape and pore depth. Chloroform as solvent is optimal for long-term use and preparation of the porous film under normal pressure and room temperature. In the process, the pore formation moves along the droplet formation boundary line from the edge to the center. When the boundary line reaches the center the porous structure is completed. The final pore size is obtained after the water droplets coalesce.
Acknowledgment This work is supported by NEDO (Bio Electromechanical Autonomous Nano Systems (BEANS) project), Japan.
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Yanqiong Zheng received the Ph.D. degree from Huazhong University of Science and Technology in 2009, China, now as a postdoc Researcher of Bio Electromechanical Autonomous Nano Systems (BEANS) Institute and Kyushu Institute of Technology, Japan. Currently she is concerned with research of thermoelectric material fabrication at nano-scale.. Koji Miyazaki received the Ph.D. degree from the Tokyo Institute of Technology, Tokyo, Japan, in 1999. He was a Lecturer in the Department of Mechanical and Control Engineering, Kyushu Institute of Technology from 1999 to 2000. Then as a Visiting Scholar at the Universtiy of California, LosAngeles (UCLA) from 2000 to 2001, and at Massachusetts Institute of Technology, Cambridge, from 2001 to 2002. Currently he is an Associate Professor in the Department of Mechanical and Control Engineering, Kyushu Institute of Technology and a Senior Researcher of BEANS laboratory, Japan. His research interests focus on heat transfer at the nano-scale area.
