We propose to investigate the nanostructure of a new class of polymer electrolyte membranes for fuel cells. We have synthesized ionic block copolymers as an alternative to the actual benchmark perfluorinated materials as Nafion. These new membranes show very promising functional properties: high thermal stability, good mechanical properties, together with a high proton conductivity resulting from the existence of a well-marked nanophase separation between ionic domains and polymer backbone. We want to investigate by SANS their microstructure as a function of the relevant chemical parameters and membrane casting method, in order to correlate the functional properties to the morphology. This study will provide invaluable information on the key role of membrane nanostructuration on the final performances, and will allow to optimize the design of next generation high performance polymers.