Synthesis of novel polybenzimidazoles with pendant amino groups and the formation of their crosslinked membranes for medium temperature fuel cell applications

A series of new polybenzimidazoles (PBIs) with pendant amino groups have been synthesized via condensation polymerization of 5‐aminoisophthalic acid (APTA), isophthalic acid (iPTA), and 3,3′diaminobenzidine (DAB) in polyphosphoric acid at 190 °C for 20 h. The molar ratios between APTA and iPTA were controlled at 1:0, 2:1, 1:1, and 1:2, respectively, and the copolymerization reactions were carried out via both random and sequenced manners. The resulting polymers showed good solubility in some organic solvents such as dimethylsulfoxide (DMSO) and N,N‐dimethylacetamide (DMAc). The pendant amino groups of the PBIs were utilized to react with two kinds of crosslinkers, 1,3‐dibromopropane and ethylene glycol diglycidyl ether, to yield various crosslinked membranes. The crosslinked membranes generally showed good mechanical properties even at high‐phosphoric acid (PA) doping levels, whereas the uncrosslinked membranes highly swelled or even dissolved in PA. Fenton's test revealed that the crosslinked PBI membranes had excellent radical oxidative stability. The proton conductivities of the PA‐doped crosslinked membranes increased with an increase in temperature and high‐proton conductivity up to 0.14 S/cm at 0% relative humidity at 170 °C was achieved. The membranes with high PA‐doping levels, good mechanical properties, and high‐proton conductivities have been successfully developed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009