Electrochemical Synthesis of a Polypyrrole Thin Film with Supercritical

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Electrochemical Synthesis of a Polypyrrole Thin Film with Supercritical Carbon Dioxide as a Solvent Hao Yan,† Tsuyoshi Sato,‡ Daisuke Komago,‡ Aritomo Yamaguchi,‡ Kenichi Oyaizu,‡ Makoto Yuasa,‡ and Katsuto Otake*,† Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan, and Faculty of Science and Technology, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8565, Japan Received March 28, 2005. In Final Form: August 29, 2005 A conductive polypyrrole (PPy) film was successfully synthesized in a homogeneous supercritical carbon dioxide (scCO2)/acetonitrile (AN) system. The occurrence of a homogeneous supercritical state was confirmed by observations of the phase behavior of the system through a high-pressure cell with a viewing window. The concentration of a supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), significantly changed the phase behavior of the scCO2/AN system. The polymerization rate of the film in that system decreased with further addition of CO2. This result suggested that the low viscosity of scCO2 did not play an important role in improving the growth rate of the PPy film. The low polymerization rate might have been due to the electron-transfer resistance arising from the low dielectric constant of scCO2/ AN mixture. The roughness of the film prepared in the homogeneous scCO2/AN system was 1/10 that synthesized in AN itself as a solvent. The slow growth of film and the high diffusion rate of the monomer seemed to account for the smooth flat film formation.

1. Introduction Conductive polymers (CPs) used as electrode coatings, such as polypyrrole (PPy), poly(3-octylthiophene), and polyaniline, possess desirable physical, chemical, and electrochemical properties for use in many bioelectrochemical applications, such as biosensors, micro-sized reversible conductive polymer electrodes, rechargeable batteries, and actuators.1-6 Thus, the synthesis of conductive polymer films has recently been an increasingly important subject of intensive research.7-9 Though freestanding films of conductive polymer can be easily prepared by electrochemical or chemical polymerization in an organic solvent, such as acetonitrile (AN), they are usually thick and have a rough surface. Moreover, since these conventional processes may require a large amount of organic solvent, waste disposal is a major economic and environmental concern. The use of supercritical fluids (SCF) such as supercritical carbon dioxide (scCO2) in the synthesis of polymer films has provided many surprises in recent years.7,10 Because SCFs have a relatively low viscosity and high diffusivity, * To whom correspondence should be addressed: tel and fax +81-29-861-4567; e-mail [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Tokyo University of Science. (1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402-428. (2) Killian, J. G.; Coffey, B. M.; Gao, F.; Pochler, T. O.; Searson, P. C. J. Electrochem. Soc. 1996, 143, 936. (3) Beck, F.; Michaelis, R. J. Coat. Technol. 1992, 64, 59-67. (4) Vorotyntsev, M. A.; Casalta, M.; Pousson, E.; Roullier, L.; Boni, G.; Moise, C. Electrochim. Acta. 2001, 46, 4017-4033. (5) Park, Y. H.; Kim, S. J.; Lee, J. Y. Thin Solid Films 2003, 425, 233-238. (6) Iroh, J. O.; Williams, C. Synth. Met. 1999, 99, 1. (7) Kerton, F. M.; Lawlecs, G. A.; Armes, S. P. J. Mater. Chem. 1997, 7, 1965-1966. (8) Labaye, D. E.; Jerome, C.; Geskin, V. M.; Louette, P.; Lazzaroni, R.; Martinot, L.; Jerome, R. Langmuir 2002, 18, 5222-5230. (9) Cooper, A. I. J. Mater. Chem. 2000, 10, 207-234. (10) Siripurapu, S.; DeSimone, J. M.; Khan, S. A.; Spontak, R. J. Adv. Mater. 2004, 16 , 989.

they can improve the mass-transfer rate in the reaction process.11-12 From this point of view, it is expected that the addition of SCF will increase the growth rate of conductive polymer films prepared by the electrolytic polymerization, and the product will have a uniform and flat surface. Recently, Anderson et al.13 reported the first electrochemical synthesis of a conductive polymer film in supercritical CO2 containing 0.16 M pyrrole, 0.16 M tetrabutylammonium hexafluorophosphate (TBAPF6), and 0.011 mol (13 vol %) acetonitrile (AN). Their experiments were performed at 323 K and 1400 psi (about 10 MPa). This method provided a smoother film than and similar conductivity to a film synthesized in AN alone, but their paper did not provide clear proof that the synthesis was performed in the supercritical state. Several studies contain reports of the phase behavior of the CO2/AN system at temperatures ranging from 298 to 373 K over a range of CO2 molar fractions (0.0-1.0). The phase behavior measurements show that a pressure higher than 10 MPa will ensure a one-phase fluid region over the entire composition range at a temperature of 328 K for CO2/AN binary mixtures. However, the presence of a supporting salt such as TBAPF6 may have a substantial influence on the phase behavior in this system. Several authors have reported that the relative volatility of a solvent mixture containing salt increased with the mole fraction of salt added.18-20 Therefore, for the CO2/AN (11) Yan, H.; Sone M.; Sato N.; Ichihara S.; Miyata S. Surf. Coat. Technol. 2004, 182, 329-334. (12) Yan, H.; Sone, M.; Mizushima, A.; Nagai, T.; Abe, K.; Ichihara, S.; Miyata, S. Surf. Coat. Technol. 2004, 187, 86-92. (13) Anderson, P. E.; Badlani, R. N.; Mayer, J.; Mabrouk, P. A. J. Am. Chem. Soc. 2002, 124, 10284-10285. (14) Byun, H.-S.; Hasch, B. M.; McHugh, M. A. Fluid Phase Equilib. 1996, 115, 179-192. (15) Reighard, T. S.; Lee, S. T.; Olesik, S. V. Fluid Phase Equilib. 1996, 123, 215-230. (16) Ziegler, J. W.; Dorsey, J. G.; Chester, T. L.; Innis, D. P. Anal. Chem. 1995, 67, 456-461. (17) Corazza, M. L.; Filho, L. C.; Antunes, O. A. C.; Dariva, C. J. Chem. Eng. Data 2003, 48, 354-358. (18) Takamatsu, H.; Ohe, S. J. Chem. Eng. Data. 2003, 48, 277-279.

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Figure 1. Schematic diagram of experiment apparatus: (1) CO2 cylinder, (2) syringe pump, (3) sapphire window, (4) working electrode (ITO glass electrode), (5) reference electrode (Pt), (6) stirrering tip, (7) piston, (8) video-enhanced camera, (9) highpressure connect, (10) automatic polarization system, PI, pressure indicator, TI, temperature indicator, V, valve.

system containing TBAPF6, experimental confirmations of the phase behavior are indispensable. In this work, our first goal was to clarify whether the synthesis reported by Anderson et al.13 had been carried out in a supercritical state. To this end, we investigated the phase behavior of an electrolytic system consisting of scCO2, a cosolvent, a supporting electrolyte, and a monomer for electrochemical polymerization. In addition, after carrying out an electrochemical synthesis of the PPy film in the homogeneous supercritical condition, we examined the effects of several factors such as phase behavior, pressure, and mass transfer on the characteristics of the PPy film. 2. Experimental Section 2.1. Materials. Monomer pyrrole (99%) was purchased from Wako Pure Chemical Industries, Ltd. (Japan). Acetonitrile (99.0%, GC grade) and tetrabutylammonium hexafluorophosphate (TBAPF6) were purchased from Tokyo Kasei Kogyo Co. Ltd. (Japan). Carbon dioxide (99.99%) was purchased from Tomoe Shokai (Japan). 2.2. Apparatus and Procedures. A schematic diagram of the apparatus used in this work is shown in Figure 1. In brief, it consists of a special high-pressure three-electrode electrolytic cell with a viewing window, a constant-temperature heat jacket, a pressure indicator, a magnetic stirrer, a syringe pump, and a potentiostat. The variable-volume cell was made of type 316 stainless steel and had a maximum internal volume of 45 mL, which is 10 times that of the one used previously.13 A moving piston was placed inside the cell to change the pressure without changing the composition. An indium tin oxide (ITO) glass (1 × 1 cm, BAS INC., Japan), a platinum wire (1 mm in diameter), and the cell body (stainless steel) were used, respectively, as the working, reference, and counter electrodes. The ITO working electrode was cleaned by acetone before use. The compounds scCO2, AN, TBAPF6, and pyrrole were used as solvent, cosolvent, supporting electrolyte, and monomer, respectively. In our study, the preparation of the PPy film was carried out by a cyclic potential-scanning method. In a typical experiment, a solution of pyrrole monomer, AN, and TBAPF6 was put in the reaction cell. Before liquid CO2 was introduced into the reactor, the air in the reactor was removed by a vacuum pump. The system was fired to a temperature of 323 K, pressurized to a predetermined pressure with the syringe pump, and then stirred for 30 (19) Banat, F. A.; Abu, F. A.; Al-Rub, A.; Simandl, J. Chem. Eng. Technol. 1999, 22, 761-764. (20) Niehaus, D.; Philips, M.; Michael, A.; Wightman, R. M. J. Phys. Chem. 1989, 93, 6232-6236.

Yan et al. min. Once the electrochemical polymerization started, cyclic voltammetry (CV) was measured simultaneously on a HZ-3000 automatic polarization system (Hokuto Denko, Japan). In almost all CV measurements, the electrode potential range was changed from -0.6 to +1.4 V against the Pt reference electrode. The scanning rate was 100 mV/s. From the point at which carbon dioxide was introduced, the camera recorded any phase change in the cell. After a certain reaction time, the reactor was cooled to room temperature, and the ITO electrode was taken out for examination. For reference, experiments were carried out with AN as solvent for the synthesis of a PPy film at atmospheric pressure and room temperature. 2.3. Characterization. The synthesized PPy films were characterized with scanning electron microscopy (SEM) and laser microscopy (VE8500, Keyence, Japan). The thickness of the films was measured with a displacement meter, and their electrical conductivities were measured by conventional four-probe technique (K-750RS, Kyowariken, Japan). The scanning electron micrographs were taken with a S-51 electron micrographer (Hitachi Instruments Service Co. Ltd., Japan).

3. Results and Discussion 3.1. Phase Behavior of scCO2/AN System. Figure 2 shows photographs of the phase behavior in the system containing 0.16 M pyrrole, 0.16 M TBAPF6, and 13 vol % (0.11 mol) AN both before and after pressurizing. Before pressurizing (Figure 2a) we observed two separated phases in the cell, a transparent upper CO2 phase and a clear lower phase; in other words, the pyrrole/TBAPF6/AN solution was detectable. When CO2 was introduced into the cell, the volume of the lower phase increased with the dissolution of the CO2 into the solution. When the system pressure reached 10 MPa, the agitation was started to make the CO2 further dissolve into the solution. But after 30 min or more of agitation, there was no further appreciable increase in the lower phase volume. Instead, an excess CO2 gas phase could be seen in the upper part of the cell (Figure 2b). Obviously, under the same conditions (323.15 K, 10 MPa) as those reported by Anderson et al.,13 the binary state was not a supercritical state but two separated phases in our experiment. Figure 3 presents the schematic phase diagram of the CO2/AN system as given by Byun et al.14 The solid line indicates the mixture’s pressure-composition isotherm under salt-free conditions at a temperature of 328 K. Point A indicates the experimental conditions reported by Anderson et al.13 On the basis of this phase diagram, the CO2/AN system in their case was located in the one-phase region, very close to the pressure-composition isotherm. However, our result above indicated the existence of two separated phases at 10 MPa and 323 K. The reason for the difference between this phase behavior information and the results of our visual observations lies in the salt effect. Niehaus et al.20 studied the oxidation of ferrocene in scCO2 using tetrahexylammonium hexafluorosphate (THAPF6) as the supporting electrolyte. The THAPF6 reduced the ohmic distortion of the voltammograms, but its concentration was so high that liquid-liquid phase separation was observed. Consequently, the CO2/AN system with a supporting electrolyte requires a consideration of the influence of the salt on the phase behavior. Here, if the addition of a salt, such as a supporting electrolyte, causes our experimental system to shift to a high-pressure state, the curve of pressure-composition isotherm will extend from the solid line to the dashed line (as shown in Figure 3). Consequently, as we see in Figure 2b, the scCO2/AN mixture will transform into a system with two separate phases. Therefore, by controlling the concentration of the salt contained in the solution, we

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Figure 2. Photographs of phase behavior in the CO2/AN system: (a) 0.16 M TBAPF6, 0.1 MPa, 298 K; (b) 0.16 M TBAPF6, 10 MPa, 323 K; (c) 0.04 M TBAPF6, 10 MPa, 323 K.

Figure 3. Schematic phase diagram for the CO2/AN system at 328 K according to the phase diagram measured by Byun et al.14

may anticipate that the mixture’s pressure-composition isotherm will shift down to below point A (a change shown as the dotted line in Figure 3). So, we expect that a decrease in the amount of the electrolyte TBAPF6 in the system will result in a single phase, that is, a homogeneous scCO2/ AN system. In our experiments, the amount of the TBAPF6 salt added to the scCO2/AN system was reduced from 0.16 to 0.04 M, and the phase behavior in the system was observed again. At this low concentration, a completely uniform CO2/AN system was confirmed through the viewing window as shown in Figure 2c. Although the scCO2/AN mixture containing 0.16 M TBAPF6 formed two separated phases at 323 K and 10 MPa, a concentration of 0.04 M TBAPF6 produced a uniform phase. This result supports our hypothesis that controlling such factors as the additive salt concentration can significantly change the phase behavior. From these results, we can conclude that the conditions reported by Anderson et al.13 would not have led to an ideal homogeneous fluid but to a two-phase separation. It seems possible that the electrochemical synthesis in that case might not have been performed under supercritical conditions. Perhaps their reported synthesis took place in the lower phase, where the monomer solution was diluted with CO2. 3.2. Electrochemical Synthesis in Various Phase States. The electrochemical synthesis of PPy on a ITO electrode was investigated under the following conditions: (i) AN at atmospheric pressure and room temperature, (ii) the liquid phase of the two separated phases, and (iii) the homogeneous phase, both of the latter at 323 K and 10 MPa. Systems i and iii contained 0.04 M pyrrole, while system ii contained 0.16 M pyrrole. Figure 4 shows the cyclic voltammograms taken for 40 potential sweep cycles in the course of the polymerization. Although their shapes seem very similar, the anodic peak current in the homogeneous scCO2/AN was much smaller than that in the AN. This result suggests that the

polymerization is greatly affected by the CO2 component of the scCO2/AN system, and that its rate significantly decreases in the homogeneous phase. It is known that the viscosity of CO2 (30.3 µPa‚s at 328.15 K, 10 MPa)21 is approximately 1/100 that of AN (3500 µPa‚s at 300 K, 0.1 MPa).22 Thus, a large amount of CO2 can greatly reduce the viscosity of the mixed solvent and consequently cause an increase in the diffusion velocity of the monomer in the reaction system. The viscosity of the homogeneous supercritical CO2/AN system is far lower than that of AN itself and the liquid phase diluted with CO2. We therefore expected that the growth rate of the conductive polymer film formed would increase. However, the result shown in Figure 4 indicates that the polymerization rate decreased with the addition of scCO2; evidently, the low viscosity of the scCO2 did not improve the film growth rate. Generally, an electrochemical reaction rate is governed by the slowest of the mass transfer and electrontransfer processes. The low current might be due to an electron-transfer resistance arising from the low dielectric constant of the scCO2/AN mixture, itself the result of the existence of the scCO2. Compared to AN ( ) 35.9 at 298 K and 0.1 MPa),23 scCO2 produces a much lower dielectric constant ( ) 1.22 at 323.15 K and 10 MPa).24 The surface morphology of the PPy deposited on the ITO electrode was observed by SEM and laser microscopy. As shown in Figure 5, the surface morphology of the PPy formed in scCO2/AN seems to be smooth; the surface roughnesses measured by the laser micrograph were 3.94, 2.75, and 0.28 µm, respectively, for the films synthesized under conditions i, ii, and iii. This finding suggests that a uniform PPy thin film can be electrochemically synthesized in a homogeneous supercritical CO2/AN system. As summarized in Table 1, we noted that some of the physical properties of the PPy films are also greatly affected by the use of scCO2 as a solvent. The density of the film prepared in the homogeneous scCO2/AN system was higher than that prepared in the AN, a result that suggest that the film prepared in the former system may possess a highly compacted structure. Generally, the formation of a film can be divided into two processes: nuclear generation and nuclear growth process. When the speed of monomer transport to the electrode is fast enough, the first process will take place; that is, nuclear generation will occur over the whole surface of the electrode and will be followed by the film growth process, producing a uniform film. Also, the PPy film is (21) Vesovic, V.; Wakeham, W. A.; Olchowy, G. A.; Sengers, J. V.; Watson, J. T. R.; Millat, J. J. Phys. Chem. Ref. Data 1990, 19, 763-808. (22) Viswanath, D. S.; Natarajan, G. Data Book on the Viscosity of Liquids; Hemisphere Publishing Corp.: New York, 1989. (23) Riddick, J. A.; Toops, E. E., Jr. Technique of organic chemistry; vol. 7: Organic solvents: physical properties and methods of purification, 2nd ed.; Interscience Inc.: New York, 1955; Chapt. 3. (24) Moriyoshi, T.; Kita, T.; Uosaki, Y. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 589-596.

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Figure 4. Cyclic voltammograms in the course of polymerization for 40 potential sweep cycles: (a) acetonitrile only at 0.1 MPa and 298 K; (b) lower phase of the CO2/AN system with 0.16 M TBAPF6 at 10 MPa and 323 K; (c) homogeneous scCO2/AN system with 0.04 M TBAPF6 at 10 MPa and 323 K.

Figure 5. Laser micrographs (top) and SEM photographs (bottom) of the polypyrrole film on ITO electrode obtained from (a) acetonitrile only at 0.1 MPa and 298 K, (b) lower phase of the CO2/AN system with 0.16 M TBAPF6 at 10 MPa and 323 K, and (c) homogeneous scCO2/AN system with 0.04 M TBAPF6 at 10 MPa and 323 K. Table 1. Properties of the Polypyrrole Films Prepared in Various Phase Statesa medium

concn of TBAPF 6 (M)

concn of monomer (M)

P (MPa), T (K)

phase state

RMS (µm)

D (g‚cm-3)

Q/V (C cm-1)

k (×10-3 S‚cm-1)

AN ScCO2/AN ScCO2/AN

0.04 0.16 0.04

0.04 0.16 0.04

0.1, 298 10, 323 10, 323

liquid two phases homogeneous

3.94 2.75 0.28

0.11 1.84 0.68

53.8 18.9 10.3

5 140 2.5

a P and T, absolute pressure and temperature; RMS, surface roughness of the films; D, density of the films; Q/V, electrochemical capacity; k, electroconductivity.

swollen by scCO2 when it grows, and the rapid evaporation of CO2 happens when the cell is depressurized; then the surface roughness of film could be improved to a certain degree. Similar consideration was put forward by several researchers for the use of liquid CO2 or scCO2 as a solvent in polymer deposition processes to produce highly uniform film.25-28 Interestingly, despite the lower electrochemical capacity of the PPy prepared in the homogeneous scCO2/ AN system, the electroconductivity of film was found to be ca. 2.5 × 10-3 S/cm, as high as that in AN (5 × 10-3 S/cm). As a possible cause, we may look to the highly compacted structure of the film synthesized in the (25) Chernyak, Y.; Henon, F.; Harris, R. B.; Gould, R. D.; Franklin, R. K.; Edwards, J. R.; DeSimone, J. M.; Carbonell, R. G. Ind. Eng. Chem. Res. 2001, 40, 6118-6126. (26) Gallyamov, M. O.; Vinokur, R. A.; Nikitin, L. N.; Said-Galiyev, E. E.; Khokhlov, A. R.; Yaminsky, I. V.; Schaumburg, K. Langmuir 2002, 18, 6928-6934. (27) Novick, B. J.; DeSimone, J. M.; Carbonell, R. G. Langmuir 2004, 43, 515-524. (28) Wang, Y.; Liu, Z. M.; Han, B. X.; Huang, Y.; Zhang, J. L.; Sun, D. H.; Du, J. M. J. Phys. Chem. B 2005, 109, 12376-12379.

homogeneous scCO2/AN system. In addition, as shown in Table 1, the PPy film prepared in the two separated phases demonstrated a much higher conductivity and density. These characteristics were probably caused by the higher monomer concentration in those phases than that in the homogeneous scCO2/AN mixture and in the AN alone. 3.3. Effects of Pressure and Mass Transfer Process on the Surface Characteristics of the Films. As described above, the film characteristics are strongly affected by the phase behavior and the composition of the system. Our results showed that a low viscosity of the scCO2 did not improve the film growth rate but might be a significant factor in producing a uniform film. Thus we examined more closely ways in which the pressure and the transport rate of the monomer affected the characteristic of the PPy film grown in the homogeneous scCO2/ AN system. First, an electrochemical synthesis was carried out at 15 and 20 MPa by introducing more CO2 into the system. Figure 6 panels a and b, respectively, depict the SEM

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Figure 6. Laser micrographs (top) and SEM photographs (bottom) of the polypyrrole film on ITO electrode obtained from the homogeneous scCO2/AN system with 0.04 M TBAPF6 at (a) 15 MPa or (b) 20 MPa and 323 K. The pressure in the cell was changed by introducing more CO2.

Figure 7. Laser micrographs (top) and SEM photographs (bottom) of the polypyrrole film on ITO electrode obtained from the homogeneous scCO2/AN system with 0.04M TBAPF6 at (a) 15MPa or (b) 20 MPa and 323 K. The pressure in the cell was changed without changing the composition by moving the piston.

photographs and laser micrograph of the PPy film performed under these two pressures. Compared with the film shown in Figure 5c grown at 10 MPa, it is found that the roughness of the films made at higher pressures increased. Evidently, the smooth surface of the PPy film had roughened with the increase in pressure. In this case, by introducing more CO2 into the system, we had changed the pressure, the viscosity, and the CO2 molar fraction in the system at the same time. Thus the uniformity of the film was affected by both the mass transfer rate and the composition of the system. This result obtained under pressurization by introducing more CO2 indicated that the homogeneity of the film depended strongly on the pressure in the system, but the effect of the mass transfer rate remained unclear. To clarify the effect of mass transfer rate, we investigated the electrochemical film synthesis in the scCO2/AN

system, compressed from 10 to 15 and 20 MPa by the movement of a piston located within the cell, instead of introducing more CO2. By such compression, we were able to keep the cell composition constant, but the viscosity of the scCO2/AN mixture increased with each rise in pressure. Thus, we expected that the result obtained would reflect the effect of the mass transfer rate on the characteristics of the film. The SEM photographs of the films synthesized at 15 and 20 MPa are provided in Figure 7, panels a and b, respectively. When we compare these figures with Figure 5c, which depicts the film synthesized at 10 MPa, we see that the homogeneity of the electrochemical polymerized film declined with the increase in pressure. This result suggests that the transport rate of the monomer plays a important role in the formation of a uniform film, and the low system viscosity offers an advantage in obtaining it.

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4. Conclusion From the electrochemical synthesis of a polypyrrole thin film with scCO2 as a solvent investigated here, the following conclusions can be drawn: First, the concentration of the supporting electrolyte, TBAPF6, significantly changed the phase behavior of the scCO2/AN system. Second, we were able to electrochemically synthesize a smoother PPy thin film in a homogeneous scCO2/AN system. Indeed, the PPy film synthesized in the scCO2 was 1/10 as rough as those synthesized in AN. The addition of CO2 decreased the polymerization rate of the film in the scCO2/AN system; evidently, the low viscosity of the

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scCO2 did not play an important role in the growth rate of the smooth PPy film. Third, the characteristics of the PPy film obtained from that system were influenced by stereochemical factors, such as phase behavior, pressure, and the mass transfer rate of the monomer. Finally, we conclude that the transport rate of the monomer is crucial for the formation of a uniform film. Acknowledgment. We gratefully acknowledge the support of our research by the Japan Society for the Promotion of Science. LA050806I