Sulfonated polyimides (SPIs) are promising materials for polymer electrolyte membrane fuel cells (PEMFCs). Aromatic polyimides have excellent thermal stability, high mechanical strength, and superior chemical resistance. However, current SPIs show insufficient proton conductivity at low humidities and high temperatures, and they suffer from hydrolytic degradation. Influencing the nano-scale ordering of SPIs is expected to affect ionic ordering, proton conductivity, and the rate of hydrolytic degradation.[1]

We are developing materials to learn about the interrelationships between morphology and proton conductivity in SPIs. Targeted materials exhibit phase segregated nanostructure on the same length scale as ionic ordering. In our earlier study, hydrophobic polysiloxane segments were attached to SPIs to form segregated copolymers.[2] The presence of hydrophobic siloxane segments did not interfere with water swelling. However, ionomer morphology was found to be significantly different than parent SPIs.

Here we report a second approach to introducing nanostructure into SPIs. Inorganic silica nanoparticles were co-synthesized during polymerization and curing to form SPI-silica nanocomposites. Sol gel processing, using amine-terminated silanes, was employed to ensure that particles are uniformly dispersed and are connected to polyimide host. TEM and XPS studies confirmed that particles were successfully incorporated into the material. Water sorption studies show that silica particles encourage membrane hydration at lower humidity. On the other hand, silica particles also act as covalent crosslinks and influence solvent resistance. By preventing excessive swelling of the membrane, the inorganic nanoparticles are expected to improve hydrolytic stability and durability under aggressive conditions.[3] The silica content is limited to below 20 wt% to prevent loss of mechanical properties.[4-6] Impedance and water uptake measurements are reported as the first step in evaluating their potential as fuel cell membranes.

1. Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.; Watanabe, M. J. Am. Chem. Soc. 2006, 128, 1762-1769.

2. Zou, L. J.; Anthamatten, M. Journal of Polymer Science: Part A: Polymer Chemistry 2007, 45, 3747-3758.

3. Lee, C. H.; Hwang, S. Y.; Sohn, J. Y.; Park, H. B.; Kim, J. Y.; Lee, Y. M. Journal of Power Sources 2006, 163, (1), 339-348.

4. Chang, C. C.; Chen, W. C. Chemistry of Materials 2002, 14, (10), 4242-4248.

5. Chen, Y.; Iroh, J. O. Chemistry of Materials 1999, 11, (5), 1218-1222.

6. Yen, C. T.; Chen, W. C.; Liaw, D. J.; Lu, H. Y. Polymer 2003, 44, (23), 7079-7087.