Elastic properties of Nafion, polybenzimidazole and poly [2,5-benzimidazole] membranes determined by AFM tip nano-indentation

The mechanical properties of polybenzimidazole (PBI) and poly [2,5-benzimidazole] (ABPBI) membranes, possible candidates to replace Nafion as proton exchange membranes in direct methanol fuel cells (DMFC), were studied. It was observed by AFM imaging that the morphology of the ABPBI membranes strongly depends on the casting procedure, performed at high temperature from methanesulphonic acid and at low temperature from formic acid. The elastic moduli of the undoped and phosphoric acid doped membranes were determined using the AFM force spectroscopy technique and the differences observed with doped and undoped PBI and Nafion membranes, were discussed in terms of the electrostatic and swelling forces between polymer chains. The analysis of the force curves indicates differences in the mechanical behavior of doped PBI and ABPBI membranes compared to Nafion, which could have practical consequences on the stability of the membrane electrode assemblies.