Preparation of sulfonated copolyimides containing aliphatic linkages as proton‐exchange membranes for fuel cell applications

Abstract

AbstractA series of random sulfonated copolyimides containing aliphatic linkages (co‐SPIAs) with controlled degrees of sulfonic acid groups were synthesized via a conventional two‐step polyimidization method. 2,4‐Diaminobenzene sulfonic acid (2,4‐DABS) and 1,6‐diaminohexane (DAH) were used as sulfonated aromatic diamine and nonsulfonated aliphatic diamine compounds, respectively. Mixtures of diamine compounds were reacted with benzophenonetetracarboxylic dianhydride (BTDA) to obtain the co‐SPIAs. The molar ratios of DAH/2,4‐DABS were systematically varied to produce copolymers with controlled compositions that contained up to 70 mol % sulfonic acid moieties. The co‐SPIAs were evaluated for thermal oxidative stability, ion‐exchange capacity (IEC), water uptake, proton conductivity, solubility, and hydrolytic stability. The proton conductivity and hydrolytic stability of the co‐SPIAs were compared with the fully aromatic polyimide, sulfonated homo‐polyimide (homo‐SPI) (BTDA/2,4‐DABS). From thermogravimetric analysis, we observed desulfonation temperatures in the range 200–350°C, which suggested a high stability of the sulfonic acid groups. The co‐SPIAs with 40–70 mol % 2,4‐DABS showed higher proton conductivity than Nafion 117 in water. The proton conductivity values of the co‐SPIAs were mainly a function of IEC and water uptake. Furthermore, the hydrolytic stability of the 2,4‐DABS‐based sulfonated polyimide membranes were improved by the introduction of the nonsulfonated diamine with aliphatic linkages. The optimum concentration of 2,4‐DABS was found to be around 40 mol % from the viewpoint of proton conductivity, IEC, and hydrolytic stability. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008