Synthesis, proton conductivity and methanol permeability of a novel sulfonated polyimide from 3-(2′,4′-diaminophenoxy)propane sulfonic acid

A novel sulfonated diamine monomer, 3-(2′,4′-diaminophenoxy)propane sulfonic acid (DAPPS), was successfully synthesized and the sulfonated polyimide (SPI) was prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The resulting SPI, NTDA–DAPPS, was soluble in common organic solvents. The SPI membrane displayed proton conductivity σ values of 0.12–0.35 S/cm at temperatures ranging from 35 to 90 °C in liquid water, which were similar to or higher than those of Nafion 117 and sulfonated hydrocarbon polymers. The σ of the SPI membrane decreased significantly with decreasing relative humidity (RH) and became much lower than that of Nafion 117 at 30% RH. The SPI membrane displayed good water stability at 80 °C and was thermally stable up to 240 °C. It showed reasonable mechanical strength of a modulus of 1.3 GPa at 90 °C and 90% RH. Its methanol permeability PM was 0.57×10−6 cm2/s at 30 °C and 8.6 wt% methanol in feed, which was a fourth of that of Nafion 117. As a result, its ratio of σ/PM was 21×104 S cm−3 s, which was about 4 times larger than that of Nafion 117, suggesting potential application of the SPI membrane for direct methanol fuel cell.