Single-Step Synthesis of Sulfonated Polyoxadiazoles and their Use as Proton Conducting Membranes

Abstract

A single-step approach for the synthesis of sulfonated polyoxadiazoles from hydrazine sulfate was developed using non-sulfonated diacids in polyphosphoric acid. The post-sulfonation conditions were optimized by varying reaction time, medium and reagent concentrations in sulfuric acid, oleum and/or their mixtures. For the first time, a series of sulfonated polyoxadiazoles with ion exchange capacity (IEC) ranging from 1.26 to 2.7 meqiv. g−1 and high molecular weight (about 40,0000 g mol−1) were synthesized. The structures of the polymers were characterized by elemental analysis, 1H NMR, and FTIR. Sulfonated polyoxadiazole membranes with high thermal stability indicated by observed glass-transition temperatures (Tg) ranging from 364 to 442 ◦C in sodium salt form and from 304 to 333 ◦C in acid form and with high mechanical properties (storage modulus about 3GPa at 300 ◦C) have been prepared. The membrane stability to oxidation was investigated by soaking the film in Fenton’s reagent at 80 ◦C for 1 h. The sulfonated polyoxadiazole membranes exhibited high oxidative stability, retaining 98–100% of their weight after the test. Proton conductivity values with the order of magnitude of 10−1 to 10−2 S cm−1 at 80 ◦C and with relative humidity ranging from 100% to 20% were obtained.