Polyethylenimine containing benzimidazole branching: a model system providing a balance of hydrogen bond network or chain mobility enhances proton conductivity.

A series of multibenzimidazole functionalized branched polyethylenimine (MPEI) molecules with varied benzimidazole substitution are designed and synthesized to study how hydrogen bonds of benzimidazole can be enhanced through the branching structure of polymer chains. The reduction of H-bonding and the increment of interatomic distance distribution initiate an increase in proton conductivity with temperature as detailed analyses by temperature dependence Fourier transform infrared spectroscopy and radial distribution function calculated from temperature dependence X-ray diffraction technique. MPEIs with a higher benzimidazole substitution form a greater number of hydrogen bonds together with the lowering of chain mobility. In combination with the proton conductivity evaluation, bPEI with 19.7% benzimidazole substitution is a preferable condition since at this condition both hydrogen bond and chain mobility are in good balance and favor the proton transfer resulting in a significant proton conductivity ∼10(-5) S cm(-1) in the case of the pure sample in pellet form and ∼10(-4) S cm(-1) in the case of the blend with PVA in the membrane form measuring at 190 °C under anhydrous condition.