Synthesis and characterization of thermally stable sulfonated polybenzimidazoles

A series of sulfonated polybenzimidazoles (sPBI-IS) with controlled sulfonation degrees (SDs) were synthesized from various stoichiometric ratio mixtures of 5-sulfoisophthalic acid monosodium salt (SIPN), 4,4′-sulfonyldibenzoic acid and 3,3′-diaminobenzidine by solution copolycondensation in poly(phosphoric acid). The resulting sulfonated polymers were characterized by means of FTIR, 1H NMR and GPC, in addition to TGA and DMA. The number-average molecular weights (Mn) of the sPBI-IS are in the range of 45,500–64,000, and the polydispersity indices (Mw/Mn) vary from 1.9 to 2.4. The synthesized sPBI-IS samples present good solubilities in polar aprotic solvents and they are easy to form the transparent, flexible and tough films by solution casting. These polymer membranes show excellent thermal stabilities and dynamic mechanical properties. The thermal stability of the sodium form sPBI-IS remarkably increases with increasing SD. However, the acidic form sPBI-IS presents less thermal stability than the non-sulfonated sample (sPBI-IS0). The onset decomposition temperature (Td) and the glass-transition temperature (Tg) of the acidic form sPBI-IS70 are 439 °C and 196 °C, respectively. The sulfonated membranes show higher storage moduli and loss moduli than sPBI-IS0. The resulting sPBI-IS membranes with high hygroscopicity show potential application as the high temperature proton exchange membrane in fuel cell.