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 (R) 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. (c) 2006 Wiley Periodicals, Inc.