Preparation and characterization of polybenzimidazole membranes prepared by gelation in phosphoric acid

Phosphoric acid doped poly (2, 2'-(m-phenylene)-5, 5'-bibenzimidazole) (PBI) membranes were prepared by dissolving PBI powders in 85% phosphoric acid at 190-200 degrees C and then promoting gelation of the PBI by cooling the solutions to -18 degrees C. The extent of acid doping of the PBI membranes was controlled by immersing the membrane in aqueous phosphoric acid solutions of different concentrations (acid de-doping). The process of the acid de-doping was faster than acid doping of membrane cast from N,N-dimethylacetamide (DMAc). The de-doping process caused shrinkage of the PBI membrane and thus an increase in the membrane strength due to the packing of PBI chains according to the X-ray diffraction analysis. The tensile stress and proton conductivity of the obtained PBI membranes with different acid doping levels were measured. For a PBI (eta(IV): 0.58 dL.g(-1)) membrane with an acid doping level of 7.0 (molar number of doped acid per mole repeat unit of PM), the stress at break and proton conductivity at 120 degrees C without humidification were 2.6 MPa and 5.1 x 10(-2)S.cm(-1), respectively. These results were comparable to those of the membranes cast from PBI solutions in DMAc. Copyright (C) 2009 John Wiley & Sons, Ltd.