Preparation of highly porous heat-resistant polybenzoxazole network films and their electrical conductivities

Highly porous rigid polybenzoxazole (PBO) network films were prepared using a precursor-mediated fabrication method. The obtained PBO network films possessed high porosities of ~40%, as calculated from their apparent densities. In addition, the 5%-weight-loss temperatures of the films were ≥570°C under nitrogen atmosphere, demonstrating an excellent thermal stability. The electrical conductivities of the obtained PBO network films and phosphoric-acid-doped PBO network films were also evaluated. In addition, PBO network films containing pyridine rings were prepared and subjected to phosphoric acid doping. The resultant films were found to exhibit the highest conductivities of the films considered in this study owing to proton conduction both between phosphate groups and between the pyridine rings. The highest conductivity was found for a film prepared from a phosphoric-acid-doped PBO network containing pyridine rings, that is, 2.09 × 10⁻¹ S/cm at 150°C, which was higher than that of Nafion™. Therefore, these films can be used at higher temperatures than that of Nafion™.