Small-angle X-ray and neutron-scattering techniques have been used to probe the structure of swollen Nafion membranes in the range 10-10000 Å. From analyzing the scattering data as a function of the polymer volume fraction and using a contrast variation method for the neutron experiments, we suggest a new structural model of Nafion in the hydrated state. It is based on the aggregation of the ionomer chains into elongated polymeric bundles with a diameter on the order of 40 Å and a length larger than 1000 Å, surrounded by the electrolyte solution.

The structure of hydrated Nafion membranes neutralized by N(CH 3 ) 4 + ions has been investigated by the small angle neutron scattering technique. First, the spectra show that the counterions are condensed at the interface between hydrophilic and hydrophobic phases. More importantly, by applying a contrast variation method, the normalized scattered intensity of membranes swollen in several D 2 O/H 2 O mixtures was analyzed. The N(CH 3 ) 4 + counterions were used as a probe to identify the nature of the scattering entity that is polymeric aggregates surrounded by ionic groups and water molecules, contrary to the generally accepted model. This new insight into the Nafion structure will allow one to reconsider the swelling process and the degree of mesoscopic orientation in such perfluorinated systems.