This investigation studied the resulting nanostructure of ionic membranes composed of sulfonated copolymers with thermoplastic and elastomeric blocks Linear poly(styrene-isoprene-styrene) (L-SIS) and branched poly(styrene-isoprene) (B-SI) were sulfonated to various levels of Ion exchange capacity (IEC) Since the sulfonation occurred in both the polystyrene (PS) blocks and the double bonds of the polyisoprene (PI) segments, the sulfonated polymer lost its elastomeric component The latter is critical for blocking the transport of chemicals through the membrane, therefore, physical blends of sulfonated and unsulfonated L-SIS and B-SI were casted and analyzed The newly developed membranes were characterized using several techniques including: elemental analysis (EA), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and small angle X-ray scattering (SAXS) These techniques were used to evaluate the thermal and physical properties of the membranes, which in turn, allowed for the comparison of resulting morphologies and selectivities In addition, counter-ion substitution (Mg(2+), Ca(2+), Ba(2+)) was used to cross-link the sulfonated polymers to further influence their selectivity Vapor permeabilities were measured at 308 K using dimethyl methyl-phosphonate (DMMP), a chemical compound similar to Sarin Gas (GP), and water, in order to evaluate the selectivity of the membranes and their potential application for chemical and biological protective clothing (CBPC). Significant differences were observed between linear and branched morphologies (C) 2010 Elsevier B V All rights reserved