Biphenyl was grafted on rigid-rod aromatic poly(p-phenylenesulfonic acids) (IEC ∼ 8 mequiv g −1 ) to generate cross-linkable polyelectrolytes. Cross-linking conditions and film properties before and after cross-linking were studied. Films equilibrated between 20 and 30% relative humidity (RH) had tensile moduli of 2 to 1 GPa and broke at 5 to 9% elongation. The moduli decreased as RH increased and dropped drastically at high humidity if the films were not cross-linked. Grafted films had conductivities at 80 °C 4−5 times that of Nafion NR-212 over the whole relative humidity range even after cross-linking. The sample reported in detail here had a conductivity of 0.10 S cm −1 at 120 °C and 30% RH. As is usual for this class of materials, even after the loss of 24% of starting sulfonic acid groups by grafting and cross-linking, ionic conductivity was high at low humidity. A membrane electrode assembly (MEA) prepared with this rigid-rod poly(phenylenesulfonic acid) proton exchange membrane and tested in a fuel cell exhibited performance and properties similar to those of Nafion NR-212 films. The power density was ∼95% of that of the Nafion MEA over the operating range in spite of relatively high hydrogen crossover due to the presence of nanocracks in the membrane.

Water insoluble poly(para-phenylene disulfonic acid) and its copolymers were synthesized by direct polymerization of 1,4-dibromobenzene-2,5-disulfonic acid and 4,4 0 -dibromobiphenyl-3,3 0 -disulfonic acid lithium salts using Ullmann coupling and subsequent grafting of long-tail alkylbenzene groups onto the polymer backbones. Copolymers with ion exchange capacities of 4.3 to 7.7 meq. g À1 were obtained. Polymers and copolymers prepared under optimized polymerization conditions were characterized by NMR, TGA, DSC and viscometry. The physicochemical characteristics of the copolymers were tailored by adjusting monomer compositions and by varying the grafting reaction temperature and time. These polymers could hold eight or more strongly bound water molecules per acid group, which facilitated their high conductivity. In the dry state, the polymers were very brittle. Membranes prepared from these polymers exhibited proton conductivity as much as ten times higher than that of NafionÒ 212, at elevated temperature and low relative humidity.

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Rigid-rod poly(p-phenylene sulfonic acids) were shown to have "frozen-in" free volume that enabled them to retain water at low relative humidity, generating very high conductivity; the best was 0.1 S/cm at 15% RH and 75C. Efforts to generate dimensional stability by grafting alkyl benzenes on the polymers worked well for poly(biphenylene disulfonic acid). Poly(phenylene disulfonic acid) grafting has not yet produced water stable PEMs.