Polymer electrolyte membrane from polybenzimidazoles: Influence of tetraamine monomer structure

Abstract

We report successful synthesis of three types of pyridine bridged tetraamine (PyTAB) monomers namely PyTAB-COOH, PyTAB-OH and PyTAB-CF3 where additional functional groups –COOH, -OH, and –CF3, respectively have been tagged in the parent PyTAB for the synthesis of soluble polybenzimidazoles (PBI). These newly designed PyTAB monomers were polymerized with varieties of dicarboxylic acids to yield series of pyridine bridged PBIs (PyPBIs). Thermal stability, mechanical strength (storage modulus) and glass transition temperature of PyPBIs were found to be influenced by the presence of additional functionalities in the PyTAB monomer and attributed to the hydrogen bonding capability and hydrophobicity of the functional groups. Newly synthesized PyPBIs displayed greater stability in phosphoric acid (PA) when compared with the non-functionalized PyPBIs, former is stable up to 85% PA whereas later is only up to 60% PA. The increased intermolecular interactions and possibility of crosslinking between the polymer chains owing to the presence of functional groups in the functionalized PyPBIs caused the increased stability in PA. The significantly low swelling ( < 1%) in water of PyPBIs considered to be a new benchmark of water swelling of PBI based polymers. Proton conductivities of PA loaded PyPBI were considerably higher than the conventional PBIs and non-functionalized PyPBIs and also depends on the functionality, structure of DCA and solvent used to make membranes. Functionalized PyPBIs (current study) showed conductivity as high as 0.12 S/cm whereas non-functionalized PyPBIs usually displayed conductivity around 0.01 S/cm. Porous and well connected network morphology of formic acid treated PyPBI membrane were attributed to the high PA loading and conductivity of PyPBI membrane.