Perfluorosulfonic acid (PFSA) ionomer membranes are ion-conducting polymers with high water sorption capacity and thermo-mechanical stability The morphology of PFSA changes during sorption affecting the mechanical and physical properties of the membrane. In this work, we investigate the structure-property relationship in swollen PFSA membranes using three proposed nanostructural descriptions and determine Young's modulus of the membrane at a wide range of temperatures (-20-85 degrees C) and water volume fractions (0-0.5) for these configurations Comparing the mechanics-based models with experimental data we propose that ion-rich water domains in PFSA membrane are spherical at low water content, spherical with connecting channels at intermediate water content and cylindrical at high water contents. Furthermore, our findings indicate that the scaling behavior for Young's modulus of PFSA ionomers is similar to that of aerogels This suggests, from a mechanics perspective, that aerogels and ionomers may have a similar interconnected porous nanostructure where some solid regions do not contribute to the mechanical load-bearing capacity. (C) 2010 Elsevier Ltd. All rights reserved.