A series of poly(styrene-b-isoprene) block copolymers (PSI) were synthesized by sequential anionic polymerization of styrene and isoprene. Poly(styrene-b-sullonated isoprene) block copolymers (PSsI) were prepared via reacting the double bonds of the isoprene block with 1,4-dioxane/sulfur trioxide. The spherical-shaped ionic clusters within the PSsl membranes produce a comparable proton conductivity (10(-1) S/cm above 60 degrees C) but benefits from approximately 50% lower methanol permeability than that of Nafion-117. Properties, such as proton conductivity and water content values, of PSsI membranes correlate highly to the content of the sulfonated isoprene block in PSsl samples even in the cases of similar IEC. As the hydrophilic block length is increased, the water content and proton conductivity also increased. (C) 2015 Elsevier B.V. All rights reserved.