A block copolymer composition AB is provided that has pendent groups --RZ extending from the A copolymer on at least 70 mol % of the A monomers, where R and Z are each an ionic species. The composition is included in a semipermeable membrane for use in a direct methanol fuel cell. Also described are articles of protective apparel comprising the composition which advantageously has barrier properties as well as superior water vapor transport properties.