On the doping of sulfonated polybenzimidazole with strong bases

Grafting of (4-bromomethyl)benzenesulfonate onto polybenzimidazole (PBI) le ads to a proton exchange polymer of variable degree of sulfonation. At high degrees of sulfonation, the membranes display a phenomenon of dehydration and contraction, which is shown to be due to the presence of a chain cross- linking strong hydrogen bond network. Immersion of proton form benzylsulfon ate-grafted membranes in aqueous organic or inorganic base is sufficient to disrupt this network, and lead to rehydration and return to a swollen and flexible state, and base-doped N-benzylsulfonate-grafted PBI (PBI-BzSH) hav e a conductivity of 2 x 10(-2) S cm(-1) at 25 degreesC and 100% relative hu midity. Exchange of sodium form N-benzylsulfonate-grafted PBI with H+ takes place with three points of neutralisation, and in the pH range 5.5-12.4, t his polymer exists predominantly in a zwitterionic form. (C) 2001 Elsevier Science B.V. All rights reserved.