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 (lEC) 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, H-1 NMR, and FTIR. Sulfonated polyoxadiazole membranes with high thermal stability indicated by observed glass-transition temperatures (T-g) ranging from 364 to 442 degrees C in sodium salt form and from 304 to 333 degrees C in acid form and with high mechanical properties (storage modulus about 3 GPa at 300 degrees C) have been prepared. The membrane stability to oxidation was investigated by soaking the film in Fenton's reagent at 80 degrees C for I 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 degrees C and with relative humidity ranging from 100% to 20% were obtained. (c) 2007 Elsevier B.V. All rights reserved.