Role of clays structures on the polybenzimidazole nanocomposites: Potential membranes for the use in polymer electrolyte membrane fuel cell

Abstract

Polymer nanocomposites of poly (4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) were prepared with two structurally different organoclays, namely, montmorillonite (OMMT) and kaolinite (OKao), to evaluate the effect of clay structures on the nanocomposites morphology, structure, and the properties. Solid State 13C cross-polarization magic-angle spinning nuclear magnetic resonance, small-angle X-ray diffraction, and transmission electron microscopy studies suggested the formation of exfoliated structure for OPBI/OMMT, whereas and intercalated structure was obtained for OPBI/OKao. Both the nanocomposites displayed significant enhancement in the thermal stabilities compared to the pristine OPBI, and the higher thermal stability of OPBI/OMMT than that of OPBI/OKao was attributed to the higher degree of dispersion of the nanoclay into the OPBI matrix owing to the exfoliated structure of the former. Both the nanocomposites membranes exhibited large mechanical reinforcement by the clays and the superior mechanical property was obtained in the rubbery state compared to the glassy state. The storage moduli (E') for the OPBI/OKao membranes were found to be higher than that of OPBI/OMMT and recognized to the different dispersion patterns of both the organoclays along the polymer matrixes. The dispersed clay particles in the OPBI matrix shielded the polymer chains from the attack of oxidative radicals and resulted in huge enhancement of oxidative stability of the nanocomposites membranes compared to the pure OPBI membrane. The nanocomposites membranes have significantly higher phosphoric acid (PA) loading compared to the pure OPBI membrane which resulted higher proton conductivities of the formers. A continuous "forming-breaking-forming" process of weak hydrogen bonds of OPBI and the organoclays with PA was found to be the driving force for nearly a one order increase in proton conductivities for nanocomposite membranes. The low activation energy (Ea), comparable with the Ea of free PA, obtained from the temperature dependent proton conductivities suggested a faster proton conduction process. © 2011 American Chemical Society.