Synthesis of side-chain type sulfonated polyimides and the effect of side-chain length on the properties

A series of side-chain type polyimide copolymers containing fluorenyl groups (SPI-4) were synthesized for the applications to polymer electrolyte fuel cells (PEFCs). The effect of alkyl chain length on the properties was investigated, where the carbon number of the pendant acidic groups was ranging from 1 to 12. While the obtained SPI-4 (1, 3, 4, 12) gave tough and flexible membranes by casting from the solution, SPI-4 (10) did not give a tough membrane. SPI-4 (1, 3, 4) displayed 10^-2 - 10⁰ S/cm of the proton conductivity which was comparable to that of Nafion 112 at 100% RH. However, SPI-4 (10, 10P, 12) displayed much lower conductivity. TG/DTA-MS analyses revealed two step weight losses for all SPI-4 with loss of hydrated water and acidic groups, respectively. Phosphoated SPI-4 (10P) has better thermal stability than the sulfonated analogue. The accelerated hydrolytic testing revealed that the SPI-4 with longer side chains are more hydrolytically stable than those with shorter side chains.