Polybenzimidazole Co-polymers; Synthesis, Morphology and High Temperature Fuel Cell Membrane Properties
Polybenzimidazole (PBI) random co-polymers containing alicyclic and aromatic backbones were synthesized by using two different dicarboxylic acids (viz., cyclohexane dicarboxylic acid and terephthalic acid) by varying their molar ratios. The synthesized co-polymers were characterized by inherent viscosity (IV), Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The co-polymer composition was determined by 1H NMR spectroscopy. The cyclohexyl based PBI possessed proton conductivity lower (114 mS/cm) than terephthalic acid based PBI (220 mS/cm). The aromatic PBI has high tensile modulus of 11 GPa whereas modulus of cyclohexyl PBI is only 2 GPa. By suitably selecting the monomer concentration, co-polymer properties can be altered (both proton conductivity and mechanical property). Out of different co-polymers, one synthesized using 30 mol % cyclohexane dicarboxylic acid and 70 mol % terephthalic acid exhibited good elongation (8 %), modulus (10.5 GPa) and improved proton conductivity (242 mS/cm). In the doped condition, the co-polymer registered an elongation of 52 % and tensile modulus of 170 MPa. High conductivity of this composition is attributed to the presence of ordered domains (field emission scanning electron microscopy) present in the co-polymer in the doped condition. The co-polymers are thermally stable and thermal stability increased with increase in aromatic content. Thus alicyclic-aromatic copolymerization is a viable technique to prepare high temperature proton exchange membranes.