Development of Interpenetrating Polymer Network Charge-Transfer Complex Polymer Films As Polymer Electrolyte Membranes

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Polymer electrolyte membranes (PEMs) are important components for PEFC and are required the several functions such as high proton conductivity, film durability, and so on. Current strategy of the molecular design for PEMs is mainly the formation of phase-separated structure like Nafion, and many hydrocarbon PEMs which were composed of block copolymers have been developed1,2. These block copolymer PEMs showed high proton conductivities and enough film durability and are the promising alternative PEMs to Nafion. On the other hand, PEMs with no phase separation were not applied practically. One of the main reasons is that highly proton-conductive homogeneous PEMs are easy to dissolve in water because large amount of sulfonic acids as a proton conductor are required. If, however, we can develop stable PEMs without any phase separation, all surfaces of PEMs can be used for proton conductivity. In order to develop stable PEMs without any phase separation, we have developed charge-transfer (CT) complex hybrid films which are composed of an electron-accepting sulfonated polyimide (SPI) and electron-donating additives3,4. Functionality of CT films such as mechanical strength and optical property can be expanded widely by using functional donor molecules. Therefore, we applied the CT films as stable PEMs without phase separation. In order to reinforce the CT films for the more practical PEMs, we developed new semi-interpenetrating polymer network (IPN) films by cross-linking reaction of the cross-linkable donor molecules. Various properties such as proton conductivity of the obtained films are reported. IPN films were prepared from electron-withdrawing SPI and electron-donating 2,6-dihydroxynaphthalene (2,6-DHN) with cross-linkers for 2,6-DHN. As cross-linkers, isocyanate, carboxylic acid chloride which can react with hydroxyl group of 2,6-DHN and 2,6-DHN with vinyl group were used. The IPN films were prepared by casting method. The DMSO solution of SPI and DHN was concentrated in vacuo, and the cross-linkers were added to the concentrated mixture and mixed in the glove box. DMSO was removed at 60 ˚C under the vacuum condition. Some of the obtained films showed dark brown color, indicating that CT complex between SPI and 2,6-DHN was formed in the films. Especially, adipoyl chloride (AC) as a typical carboxylic acid chloride, showed the proper film shape and mechanical stability. Therefore, IPN films consisting of AC were used for the further characterization. To confirm the CT formation in the films, visible spectroscopy of the obtained films was carried out. The maximum wavelength of the CT films with and without AC were 552 nm and 529 nm, respectively. The intact SPI film did not show any peaks in this region. This result indicated that CT film with AC would have different molecular structure from CT films without AC. To determine the molecular weight change after the cross-linking reaction, GPC measurement was carried out. The molecular weight of the IPN films were larger than that of the intact SPI, indicating that some of the sulfonic acid reacted with AC and the molecular weight after cross-linking would increase. Mechanical strength of the IPN films was also evaluated. The obtained results indicated that Young’s modulus tended to increase with the increase of AC, and stress and strain at break tends to decrease with the increase of AC. This result indicated that IPN introduced the film strength compared with the CT films without AC. The proton conductivity measurement of the obtained IPN films was also carried out. The proton conductivity of the obtained IPN film (SPI : DHN : AC = 100 : 25 : 25) was 80 mS/cm at 70˚C, 80%RH. The IPN films showed the slightly higher proton conductivity of the CT films without AC, indicating that enough sulfonic acid in SPI remained after cross-linking reaction and the carboxylic acid generated from AC would also work as a proton conductor. Reference K. Yamazaki, H. Kawakami, J. Membr. Sci., 43, 7185 (2010).B. Bae, K. Miyatake, M. Watanabe, Macromolecules, 43, 2684 (2010).R. Watari, M. Nishihara, H. Tajiri, H. Otsuka, A. Takahara, Polymer J., 45, 839 (2013).L. Christiani, S. Hilaire, K. Sasaki, M. Nishihara, J. Polym. Sci. A, Polym. Chem. 52, 2991 (2014). Figure 1