The proton conductivity of radiation-grafted ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid (ETFE-g-PSSA) and Nafion 117 membranes between 25 and -37 degrees C is reported. The freezing of water in the membranes, which strongly depends on the internal acid concentration, results in a 4-fold decrease in proton conductivity. The activation energies before and after the freezing of the membranes are approximately 0.15 and 0.4 eV, consistent with proton transport through liquid water and strongly bound water, respectively. Differential scanning calorimetry data show that up to 14 H(2)O molecules per H(+)/SO(3)(-) group remain unfrozen at subzero temperatures and are believed to be responsible for the low temperature conductivity that is observed. These results indicate that proton conductivity in membranes may be achieved via strongly bound and highly polarized water.

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Novel polymers with controlled microstructures were prepared and studied to further advance the understanding of structure-property relationships of proton conducting membranes. PAN-g-macPSSA membranes, which contained poly(styrenesulfonic acid) (PSSA) grafts of defined graft length, are compared with PVDF-g-PSSA membranes, prepared by radiation-grafting, and Nafion 1 117. The intrinsic properties of PAN-g-macPSSA membranes are insensitive to the macromonomer graft length but are highly dependent on the ion exchange capacities (IEC). Increasing the IEC increases the content of free water absorbed by the membrane. Self-diffusion coefficients of water in water-swollen PAN-g-macPSSA were found to be similar to that of N117, despite PAN-g-macPSSA's higher water content. Of the polymers studied, PAN-g-macPSSA exhibited the lowest methanol permeability, which is explained on the basis of it containing a more tortuous ionic network. Methanol permeability decreased with decreasing volume of free water.

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