Influence of interfacial interactions on the properties of polybenzimidazole/clay nanocomposite electrolyte membrane

Abstract

Despite the wide range of studies illustrating the influence of type and quantity of nanoclay on the macro properties of polymer nanocomposites, it has always remained a biggest challenge to study the chemistry of organic/inorganic interface on the final properties of polymer nanocomposites, a study that can indeed aid us in designing tailor-made materials e.g. proton exchange membranes (PEM). Hence in this study, we aimed to explore the influence of interface chemistry of nano-clay on the properties of poly(4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI)/clay nanocomposite PEM for fuel cell. Cloisite clay modified organically with three surfactant molecules differing structurally in their cationic head group - cetyltrimethyl ammonium bromide (CTAB), cetylpyridinium bromide (CPyB) and cetylimidazolium bromide (CImiB) was utilized for making OPBI nanocomposite membranes by a simple solution blending route. 13C CP MAS solid state NMR confirmed the hydrogen bonding interactions between the OPBI chains and clay particles. Formation of intercalated clay nanostructures in the OPBI matrix was revealed by WAXD and TEM analyses and this morphology was found to influence different properties in a favorable manner. TGA and DMA studies highlighted the influence of the different surfactants on the interfacial interactions in elevating/decreasing the thermal and glass transition temperatures of the nanocomposite membranes. The use of different surfactant modified clays induced sufficient hydrophobicity in the membranes that led to higher phosphoric acid (PA) loading and controlled dimensional swelling in both water and PA. The proton conductivity data particularly underscored the different degrees of interfacial interactions each of the three differently modified clays had in creating efficient proton transport pathways giving rise to higher proton conductivity while decreasing the activation energy barrier for the proton hopping. The results also revealed the critical role played by the clay particles especially by the organic modifier in preventing the leaching away of the doped acid from the nanocomposites membranes.