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Electric-Field Oriented Polymer Blend Film for Proton Conduction Minoru Umeda*,† and Isamu Uchida‡ Department of Chemistry, Faculty of Engineering, Nagaoka UniVersity of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan ReceiVed October 24, 2005. In Final Form: March 22, 2006 A novel proton conductor has been realized by employing a ternary polymer blend system in combination with an electric-field orientation technique. A polymer film recast from a solution containing poly(acrylic acid), poly(vinyl butyral), and fluoroalkyl graft polymer under 2 kV‚cm-1 exhibited 10 times higher proton conductivity than that prepared without the external electric field. However, when the film was prepared under a field higher than 4 kV‚cm-1, proton conductivity decreased. The membranous character has been investigated by SEM and AFM observations. As a result, it has been proven that an electric-field treatment of 2 kV‚cm-1 forms the largest hydrophilic domains for proton conduction in the film. The alteration of the phase separation morphology induced by the electric field well explains the proton conductivity change.
Introduction Utilizing an external electric field for providing some functional features to polymer film is a well-known method. For instance, strong polarization of poly(vinylidene fluoride) induced by an electric field gives rise to piezoelectricity and pyroelectricity.1 The mechanism behind the strong polarization is attributed to an orientation of molecular dipoles in the poly(vinylidene fluoride) film. The same technique was applied to molecularly doped polymers, which consist of a functional molecule and a host polymer, for nonlinear optical devices.2,3 Studies on the molecularly doped polymers were performed for poly(methyl methacrylate) doped with 4-(dimethylamino)-4′-nitrostilbene or 2-methyl-4-nitroaniline, resulting in chromophore reorientation or microcrystal alignment.2,3 Moreover, the alteration of polymer blend morphology by the electric field was investigated for diblock copolymers4 and binary5 and ternary6 polymer blend films. In some cases, elongated domains oriented by the electric field are clearly observed.4,6 We previously reported that a new type of ternary polymer blend system containing poly(acrylic acid) (PAA), poly(vinyl butyral) (PVB), and vinylidenefluoride-hexafluoropropylene copolymer [p(VdF-HFP)], designed from the functional structure of Nafion,7,8 demonstrated a high proton conductivity.9 The function of PAA is similar to that of a protic group, PVB is a compatibilizer, and p(VdF-HFP) is a fluorinated hydrophobic polymer. In the blend film, a clear phase separation between hydrophilic and hydrophobic polymers is observed.9 However, for a high-conductivity appearance, more than 50 wt. % of PAA * To whom correspondence should be addressed. Tel: +81-258-47-9323. Fax: +81-258-47-9300. E-mail:
[email protected]. † Nagaoka University of Technology. ‡ Tohoku University. (1) Naegele, D.; Yoon, D. Y. Appl. Phys. Lett. 1978, 33, 132. (2) Daigo, H.; Okamoto, N.; Fujimura, H. Opt. Commun. 1988, 69, 177. (3) Lee-Yin, L.; Doraiswami, R.; Lackritz, H. S. Macromolecules 1994, 27, 5987. (4) Thurn-Albrecht, T.; DeRouchey, J.; Russell, T. P.; Jager, H. M. Macromolecules 2000, 33, 3250. (5) Shojaie, S. S.; Greenberg, A. R.; Krantz, W. B. J. Membr. Sci. 1993, 79, 115. (6) Serpico, J. M.; Wenk, G. E.; Krause, S. Macromolecules 1991, 24, 6879. (7) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. Polym. Phys. Ed. 1981, 19, 1687. (8) Ogumi, Z.; Kuroe, T.; Takahara, Z. J. Electrochem. Soc. 1985, 132, 2601. (9) Umeda, M.; Yamada, A.; Affoune, A. M.; Uchida, I. Electrochim. Acta 2004, 50, 611.
Figure 1. Schematic illustration of apparatus for polymer film recasting under external electric field.
is required, which causes low stability of the film. From these results, structural control of the phase separation seems to be indispensable to balance the two functions of the polymer blend film. In the present paper, we report the structural control of a ternary polymer blend film recast under an external electric field. The relationship between field strength and proton conductivity has been investigated. Surface and cross-sectional morphologies revealed the mechanism of the electric field dependence of the conductivities. Experimental Procedure In preparing a polymer blend solution, poly(acrylic acid) (PAA; Wako Pure Chemical Inc.), poly(vinyl butyral) (PVB; Sekisui Chemical Company, S-Lec), and a fluoroalkyl graft polymer (FGP; Central Glass Company, Cefral Soft; CAS no. 89823-13-2) were used as obtained without purification. The above functional polymers were separately dissolved in dimethylformamide (DMF; Wako Pure Chemical, analytical grade) to obtain 10 wt % solutions. Then, the aliquots of solutions were mixed to prepare the desired blend polymer in arbitrary proportions. We used the blade-coating technique to apply the blended polymer solution onto a poly(ethylene terephthalete) (PET; thickness 220 µm), resulting in a wet polymer layer approximately 400 µm thick, which was subjected to a drying procedure under an external electric field of 0-6 kV‚cm-1 by the setup shown in Figure 1. The recast sample was placed on an ITO glass, and then the counter-ITO was settled on spacers, so that the DC voltage could be applied to the
10.1021/la0528551 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
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sample using the ITO. Using the setup, the polymer layer was dried for 1 h at room temperature and under atmosphere. Subsequently, the resultant film was dried in a vacuum oven for 30 min at 140 °C. The obtained film thickness was measured by a surface profile measuring system (Sloan, Dektak III). The typical dry polymer film thickness was 40-50 µm. Nafion 117 membrane was utilized as a reference. The protonic form of the blend-polymer film was obtained by soaking it in 0.5 mol‚dm-3 H2SO4 prior to its use in conductivity measurement. The conductivity was measured in the thickness direction by an impedance method.10,11 The impedance cell consisted of the sample film fixed between two gold electrodes supported by silicon rubber and glass plates. To ensure good electrical contact, we used a micrometer under a pressure of approximately 2 atm. Prior to the measurement, excess water at the film surface was swept with tissue paper, and then the film was quickly inserted between the two gold electrodes and pressed using the micrometer. The thussandwiched sample film is considered as humidified sufficiently (100% relative humidity).10,11 The measurements were carried out at room temperature. An ac impedance spectrum was recorded from 10 Hz to 5 MHz using a Solartron 1260 Impedance/Gain-Phase Analyzer and Zplot software for windows. The membrane resistance R was determined by extrapolating the complex impedance diagram at a high frequency to the real axis. The conductivity σ was calculated from the formula σ ) l/R‚S, where l, R, and S denote the sample thickness, the membrane resistance, and the sample area, respectively. The Nafion 117 membrane was pretreated by boiling in 0.5 M H2SO4 for 1 h and then boiled in deionized water for 1 h and rinsed thoroughly in deionized water. The measured conductivity was 0.095 S‚cm-1, which is equivalent to that of proton-form Nafion.12,13 This certifies the accuracy of the proton conductivity measurement in the thickness direction. The observations of film surface and cross section were carried out by scanning electron microscopy (SEM; JEOL, JSM-5310LV) and laser scanning microscopy (Olympus, OLS1200). Cross-sectional cutting of the film was conducted using a rotary microtome (Micro Tec, CUT4060). The film surface was characterized by an atomic force microscope (AFM; Shimadzu, SPM-9500 J3). The AFM is equipped with a 55 µm scanning head and is operated in the contact mode.14 The AFM observation was conducted at room temperature and room humidity at a scan rate of 1 Hz and a 512 × 512 pixel resolution. The calibration of the piezoscanner of AFM was carried out using a gold grating sample from Shimadzu Company.
Results and Discussion In our ternary system that takes Nafion’s three-phase clustered system7,8 into consideration, the three regions could be as follows:9 • A fluorocarbon backbone region is identified in the FGP. • An ionic domain region is identified in the PAA. • An interfacial region with a relatively large amount of the compatibilizer PVB is identified. First, four solutions having different polymer compositions and containing at least 50 wt % of FGP were prepared to produce high-stability films. Subsequently, the proton conductivities of the films recast using the solutions were measured. Figure 2 shows the relationship between the proton conductivity of the film in the thickness direction and the applied electric field during the recast process for four blend systems. When we compare the conductivities of the films prepared under 0 kV‚cm-1, it is known that the higher PAA content exhibits a higher conductivity and the lower PAA content a lower (10) Affoune, A. M.; Yamada, A.; Umeda, M. J. Power Sources 2005, 148, 9. (11) Umeda, M.; Kawaguchi, S.; Yamada, A.; Uchida, I. Jpn. J. Appl. Phys. 2005, 44, L322. (12) Edmondson, C. A.; Stallworth, P. E.; Wintersgill, M. C.; Fontanella, J. J.; Dai, Y.; Greenbaum, S. G. Electrochim. Acta 1998, 43, 1295. (13) Doyle, M.; Lewittes, M. E.; Roelofs, M. G.; Perusich, S. A.; Lowrey, R. E. J. Membr. Sci. 2001, 184, 257. (14) Affoune, A. M.; Yamada, A.; Umeda, M. Langmuir 2004, 20, 6965.
Figure 2. Proton conductivity versus applied electric field for the polymer recast process as a function of polymer-blend composition.
Figure 3. Scanning electron micrographs of PAA:PVB:FGP ) 20:20:60 film prepared under electric fields of 0, 2, 4, and 6 kV‚cm-1.
conductivity. This demonstrates that the magnitude of proton conductivity is proportional to the used amount of ionic domain component. With regard to the influence of the applied electric field, the conductivity of the PAA:PVB:FGP ) 20:20:60 film prepared under 2 kV‚cm-1 demonstrates 10 times higher conductivity than that without the electric field. Whereas, proton conductivity decreases when the film is prepared under the field higher than 4 kV‚cm-1, and the conductivity of the film prepared at 6 kV‚cm-1 is the same level as that at 0 kV‚cm-1. This is the first observation that the ionic conductivity of the polymer film is dependent on the external electric field applied during the recast process. The conductivity change is presumed to be related to the film morphology transformation. For the PAA:PVB:FGP ) 30:10:60 film, the same type of electric field dependency is seen in Figure 2. On the other hand, an apparent electric field dependence is not observed for the film compositions of PAA: PVB:FGP ) 30:20:50 and 40:0:60. This points out that conductivity change does not always occur for every composition and that there exists a suitable composition for the conductivity change induced by the field. Here, we focus our attention on the PAA:PVB:FGP ) 20:20:60 film, which exhibits the most remarkable conductivity dependence on the electric field. Figure 3 shows surface SEM photographs of the PAA:PVB: FGP ) 20:20:60 film prepared under each electric field given in the figure. In the case without an electric field, an obvious phase separation is not observed; although, it is apparent that the polymers are not mixed on the molecular level. In the case where
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Figure 5. Cross-sectional photographs of PAA:PVB:FGP ) 20: 20:60 film prepared under electric fields of (a) 0 and (b) 2 kV‚cm-1.
Figure 4. AFM images of PAA:PVB:FGP ) 20:20:60 film prepared at 2 kV‚cm-1. The same spot of the film surface was measured (a) under dry condition and (b) after humidification.
2 kV‚cm-1 is applied, a honeycomblike phase separation becomes apparent. Since the composition of the recasting solution is the same, this confirms that the applied electric field induces the morphology transformation. In the case of 4 and 6 kV‚cm-1, the scale of the domain size decreases. Namely, the diameter of the dark domain changes from 50 to 100 µm at 2 kV‚cm-1 to
