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

Abstract

A 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 non-sulfonated 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 uo 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-polyimmide (homo-SPI) (BTDA/2,4-DABS). From thermogravimetric analysis, we observed desulfonation temperatures in the range 200-350 degrees 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 funsiton of IEC and water uptake. Furthermore, the hydrolytic stability of the 2,4-DABS-based sulfonated polyimmide 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. (c) 2008 Wiley Periodicals, Inc.