A series of polybenzimidazoles (PBIs) incorporating main chain pyridine groups were synthesized from the pyridine dicarboxylic acids (2,4‐, 2,5‐, 2,6‐ and 3,5‐) and 3,3′,4,4′‐tetraaminobiphenyl, using polyphosphoric acid (PPA) as both solvent and polycondensation reagent. A novel process, termed the PPA process, has been developed to prepare phosphoric acid (PA) doped PBI membranes by direct‐casting of the PPA polymerization solution without isolation or re‐dissolution of the polymers. The subsequent hydrolysis of PPA to PA by moisture absorbed from the atmosphere usually induced a transition from the solution state to a gel‐like state and produced PA‐doped PBI membranes with a desirable suite of physiochemical properties. The polymer structure characterization included inherent viscosity (I.V.) determination as a measurement of polymer molecular weight and thermal stability assessment via thermogravimetric analysis. Physiochemical properties of the doped membrane were studied by measurements of the PA doping level, ionic conductivity and mechanical properties. The resulting pyridine‐based polybenzimidazole membranes displayed high PA doping levels, ranging from 15 to 25 mol of PA per PBI repeat unit, which contributed to their unprecedented high proton conductivities of 0.1 to 0.2 S cm–1 at 160 °C. The mechanical property measurements showed that the pyridine‐based PBI membranes were thermally stable and maintained mechanical integrity even at high PA doping levels. Preliminary fuel cell tests demonstrated the feasibility of the novel pyridine‐based PBI (PPBI) membranes from the PPA process for operating fuel cells at temperatures in excess of 120 °C without any external humidification.

Mass-transport studies of phosphoric acid ͑PA͒-doped meta-polybenzimidazole ͑PBI͒ fuel cell membranes are described. In this study, the fundamental differences in transport properties between m-PBI/PA membranes prepared by conventional imbibing procedures and the polyphosphoric acid ͑PPA͒ process are explored. The membranes were characterized by proton conductivity and multinuclear ͑ 1 H and 31 P͒ magnetic resonance measurements. Both short-range and long-range dynamical processes were investigated by spin-lattice and spin-spin relaxation time measurements and by pulsed field gradient diffusion, respectively. Comparative data for pure PA and PPA are included. The high proton conductivity ͑0.13 S/cm at 160°C͒ of the PPA-processed membranes is correlated with rapid proton self-diffusion ͑3 ϫ 10 −6 cm 2 /s at 180°C͒. The 31 P results reveal the presence of both PA and the dimeric pyrophosphoric acid and indicate strong interaction between the phosphate groups and the m-PBI matrix, with negligible anionic transport for both kinds of membranes. The higher concentration of PA in the PPA-processed membranes and differences in membrane morphology may provide an additional proton-transport mechanism involving rapid exchange between the PA and pyrophosphoric acid species.

Planning of assembly sequences is essential to the manufacturing system design process. An algorithm is presented in this paper to generate a precedence relation qwesentatiion for mechanical assembly sequences fmn the AND/OR graph represenlatian. The equivalence of the two representation is established. The real time property for the resulting set of precedence relations is also defined and discussed. It is shown that the comm and completeness of a represenration can not guarantee the real time property.