Interpretation of the small-angle X-ray scattering from swollen and oriented perfluorinated ionomer membranes

Small-angle X-ray scattering (SAXS) and bulk volumetric measurements were p erformed on the perfluorinated ionomer membrane "Nafion". The swelling expe riments were conducted using controlled environments of different relative humidities. The membranes were oriented using uniaxial draw, both before an d after conversion of the precursor material, as well as both sequential an d simultaneous biaxial draw. Using a novel model-independent maximum entrop y method, it is shown that the most statistically probable scattering model for Nafion is an ion clustered morphology with a hierarchical scale of str ucture. The smallest scale is comprised of arrays of roughly spherical ioni c clusters. The clusters are agglomerated into higher-order structures, who se shape is determined by the spatial coherence of the clusters, as inferre d from the SAXS measurements. Anomalies in the magnitudes of the microscopi c and macroscopic swellings are shown to be caused by the rearrangement of ionic material, producing changes in the number density of clusters.