A new class of polyelectrolytes, poly(phenylene sulfonic acids) and its copolymers as proton exchange membranes for PEMFC's.

摘要

A novel rigid rod liquid crystalline poly(biphenylene disulfonic acid), PBPDSA, was synthesized for the first time using the Ullman coupling reaction. The resulting water soluble, polymer showed a complex aggregation behavior in solution, which complicated the estimation of its molecular weight. The proton conductivity of PBPDSA was higher than that of Nafion over the whole tested range of relative humidities and temperatures. The unparalleled properties of this material were attributed to its liquid crystalline lamellar solid state structure. In order to obtain water insoluble membranes, PBPDSA was modified by grafting bulky or crosslinkable hydrophobic groups. The resulting grafted copolymers showed a solid state structure similar to that of PBPDSA, as well as an analogous anisotropy in some of its properties. The in plane proton conductivity of these materials, measured as a function of relative humidity and temperature, was higher or comparable to that of Nafion. The membranes performance at low relative humidities and high temperatures is remarkable, showing conductivity values up to 2 orders of magnitude larger than those found for Nafion. TGA and FTIR studies indicate that the polymers are stable up to 175°C. The most important discovery was that this class of materials forms almost perfect MeOH vapor barriers. A 20mu film was more than 1000 times less permeable than Nafion 117. The effect of the bulky and crosslinkable groups on the conductivity, mechanical properties and dimensional stability of the copolymer membranes was evaluated. However, an unequivocal correlation between polymer structure and its properties was complicated by the presence of structural defects generated during the grafting process. Experimental conditions allowing the control but not the elimination of such defects were found and used to prepare grafted copolymers in a controlled and reproducible manner. The initial results of an effort to produce random copolymers using new comonomers amenable to copolymerization using the Ullman reaction are discussed. An extension of this approach, with better structural control and understanding of the membranes' final solid state structure, will likely produce new membranes whose properties will greatly surpass those of the state of the art materials widely used today.