Summary: Fuel cells were designed for high temperature operations. Poly[2,2′‐(m‐phenylene)‐5,5′‐bibenzimidazole] (PBI) was synthesized in a solution of P2O5, CH3SO3H, and CF3SO3H. The PBI was dissolved in a mixture of CF3CO2H and H3PO4 and the solution was used for the preparation of Pt catalyst slurry for membrane electrode assembly. The single cell showed a current density of 280 mA · cm−2 at a cell voltage of 0.5 V with feeds of H2 and O2 at 160 °C and without external humidification. magnified image

The stress-whitened damage zone that formed ahead of a semicircular notch during slow tensile loading has been measured from optical micrographs of translucent blends of poly(vinyl chloride) (PVC) with experimental chlorinated polyethylene (CPE) resins. When the zone was small, the plane strain condition applied and from the elastic stress distribution a constant mean stress condition was found at the boundary of the crescent-shaped zone. The critical mean stress did not depend on the chlorine content or the chlorine distribution of the experimental CPE resin used in the blend. While the critical mean stress decreased as the amount of CPE in the blend was increased, the critical volume strain, calculated from the bulk modulus, was independent of composition and was thought to be the controlling parameter for stress-whitening. When the zone was larger, the shape was qualitatively described by concepts of stress redistribution in the presence of a plastic zone ahead of the notch. Macroscopic flow and necking were only detected near the maximum in the stress-displacement curve.

SYNOPSISThe brittle-to-ductile transition in continuous microlayer composites of polycarbonate (PC ) and styrene-acrylonitrile copolymer ( SAN) was investigated under the triaxial tensile stress state achieved at a semicircular notch. The availability of microlayer compositions with variations in the proportion of the components and also variations in the total number of alternating layers made it possible to examine the transition from SAN-like, relatively brittle behavior, to PC-like properties where the ductile component dominated yield and failure. Examination of the damage zone that formed a t the notch root revealed that cavitational mechanisms dominated in the brittle composites that were those with the highest proportion of SAN and fewest number of layers. Shear-yielding modes characteristic of PC dominated the damage zone of the ductile composites. Cavitational mechanisms were almost totally absent in these compositions that included those with the highest proportion of PC and the largest number of layers. A broad range of transitional behavior was observed with intermediate compositions where elements of both cavitational mechanisms and shear modes were superimposed. These compositions provided an opportunity to examine the interaction of cavitational and shear processes at the macro-and microscales. 0 1993 John Wiley & Sons, Inc.

The diffusion of a binary solvent mixture into cross-linked poly (butadiene) (PBD) was observed using fast scan FT-IR imaging spectroscopy. Spatial correlation of solvent ingress and polymer swelling was established with a temporal resolution of 20 s. Binary solvent mixtures included a good-good solvent pair of benzene-cyclohexane (50/50 wt%) and a good-poor solvent pair of toluenemethanol (50/50 wt%). A set of FT-IR images was collected over time, from which the pro les of solvent ingress and polymer sw elling were extracted. The Boltzmann transformation, which explains diffusion phenomena with respect to both time and position sim ultaneously, was used to obtain a curve, which indicates that benzene and cyclohexane diffuse similarly, but toluene and m ethanol have different diffusion behavior. The good-good solvent pair (benzenecyclohexane mixture) ingressed into polymer together, while the good-poor solvent pair (toluene-methanol mixture) showed independent ingression behavior. The good solvent (toluene) aggressively diffused into polymer while the poor solvent (m ethanol) remained in solution.