Novel acid–base polyimides synthesized from binaphthalene dianhydrie and triphenylamine-containing diamine as proton exchange membranes

The synthesis and characterization of novel acid–base polyimide membranes for the use in polymer electrolyte membrane fuel cell is presented in this paper. The sulfonated polyimides (SPIs) bearing basic triphenylamine groups were easily synthesized using 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (BTDA), sulfonated diamine of 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS), and nonsulfonated diamines of 4,4′-diaminotriphenylamine (DATPA). The effects of the structure of the dianhydride and diamines on the properties of SPI membranes were evaluated through the study of membrane parameters including water sorption, proton conductivity, water stability, dimensional changes, and methanol permeability. It was found that the BTDA-based sulfonated polyimides displayed much better hydrolytic stability than 1,4,5,8-naphthalenecarboxylic dianhydride (NTDA)-based polyimides. This is because the former has a decreased positive charge density in the imido rings compared to the latter ones. Furthermore, the present SPI membranes also displayed much better resistance to swelling than these without DATPA due to the strong interchain interaction through basic triphenylamine functions and sulfonic acid groups. The SPI membrane with IEC value of 2.21 mequiv./g shows the proton conductivity of 0.11 S/cm at 20 °C which is higher than Nafion 117 (0.09 S/cm at 20 °C). Moreover, the SPI membranes exhibited excellent mechanical properties and decreased methanol permeability.