Three series of polybenzimidazole (PBI) copolymers (3,5‐pyridine‐r‐2OH‐PBI, 3,5‐pyridine‐r‐para‐PBI, and 3,5‐pyridine‐r‐meta‐PBI) were polymerized and cast into membranes by the polyphosphoric acid (PPA) process. Monomer pairs with high and low solubility characteristics were used to define phase stability‐processing windows for preparing membranes with high temperature membrane gel stability. Creep compliance of these membranes (measured in compression at 180 °C) generally decreased with increasing polymer content. Membrane proton conductivities decreased linearly with increasing membrane polymer content. Fuel cell performances of some high‐solids 3,5‐pyridine‐based copolymer membranes (up to 0.66 V at 0.2 A cm–2 following break‐in) were comparable to para‐PBI (0.68 V at 0.2 A cm–2) despite lower phosphoric acid (PA) loadings in the high solids membranes. Long‐term steady‐state fuel cell studies showed 3,5‐pyridine‐r‐para‐PBI copolymers maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm–2 for over 2,300 h. Phosphoric acid that was continuously collected from the long‐term study demonstrated that acid loss is not a significant mode of degradation for these membranes. The PBI copolymer membranes' reduced high‐temperature creep and long‐term operational stability suggests that they are excellent candidates for use in extended lifetime electrochemical applications.

A new type of fluorine‐containing polybenzimidazole, namely poly(2,2′‐(2,2′‐bis(trifluoromethyl)‐4,4′‐biphenylene)‐5,5′‐bibenzimidazole) (BTBP‐PBI), was developed as a candidate for proton‐conducting membranes in fuel cells. Polymerization conditions were experimentally investigated to achieve high molecular weight polymers with an inherent viscosity (IV) up to 1.60 dl g–1. The introduction of the highly twisted 2,2′‐disubstituted biphenyl moiety into the polymer backbone suppressed the polymer chain packing efficiency and improved polymer solubility in certain polar organic solvents. The polymer also exhibited excellent thermal and oxidative stability. Phosphoric acid (PA)‐doped BTBP‐PBI membranes were prepared by the conventional acid imbibing procedure and their corresponding properties such as mechanical properties and proton conductivity were carefully studied. The maximum membrane proton conductivity was approximately 0.02 S cm–1 at 180 °C with a PA doping level of 7.08 PA/RU. The fuel cell performance of BTBP‐PBI membranes was also evaluated in membrane electrode assemblies (MEA) in single cells at elevated temperatures. The testing results showed reliable performance at 180 °C and confirmed the material as a candidate for high‐temperature polymer electrolyte membrane fuel cell (PEMFC) applications.

We report on the results of a computational investigation of the effect of fiber aspect ratio (ar) on the stiffness of composite rods reinforced with rigid spheroidal inclusions. The reinforcing spheroids are randomly placed within the containing rod and are also perfectly aligned with the tensile axis. Attention is focused in the interesting region of low (ar), where the stiffness of the composite rod is known to be most sensitive on (ar). Use of low aspect ratio fibers makes the results of this analysis suitable for a class of processed materials, such as whisker-reinforced metal-matrix composites and extruded or molded short-fiber-reinforced polymers. We consider steady-state three-dimensional deformations of composite rods containing up to 50 individual, randomly placed aligned spheroids. The equations of elasticity for the entire multi-fiber assembly are solved using the Boundary Element Method (BEM), implemented on a four-processor server and the force needed to impose a certain tensile deformation on the composite is computed. From this, an effective tensile modulus is obtained. Statistical averages of the computed effective moduli are compared to the predictions of the Mori-Tanaka model for the stiffness of short fiber composites. We find a good agreement at low values of (ar). Additionally, we investigate the effect on stiffness of random perturbations in fiber length around a mean value.