Synthesis and substituent effect of polyimide electrolyte membranes containing fluorenyl groups

A series of polyimide copolymers containing o-subustituted sulfonated fluorenyl groups (SPI-6) or sulfonated trimethylphenylene groups (SPI-7) were synthesized with various hydrophobic diamine comonomer. For investigation of polymer electrolyte properties, membranes were fabricated by casting from polymer solution. SPI-7 membranes showed better water stability than SPI-6 under accelerated testing conditions (at 140°C for 24 h). Flexible aliphatic chains turned out to cause lower water stability of the polyimide electrolytes. Water uptake was somewhat higher for SPI-7 than SPI-6. Both SPI-6 and SPI-7 membranes showed high proton conductivity under humidified conditions. SPI-6 TFMBZ with the ion exchange capacity (IEC) of 1.91 meq/g showed the highest proton conductivity of 10^-1 S/cm at 90% RH and 10^-3 S/cm at 20% RH.