Morphology and Morphological Changes Within Nafion Membranes Induced by Mechanical Orientation

The structure of Nafion, a perfluorosulfonated ionomer, has been the subject of many studies. Morphology evaluations have used the methods of small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS), and small-angle neutron scattering (SANS). Three principal features in the small-angle scattering pattern have been observed on the H+-form Nafion. A peak at approximately Q approx. 0.2 A-1 is attributed to the clustering of the acid groups (ionomer peak), that at Q -0.03 A-1 to crystalline regions, and an upturn in intensity at the smallest Q to large-scale heterogeneities. Some of the previous works have included samples that have experienced moderate strains by mechanical elongation. In those works, the effect on the ionomer peak has been studied. The WAXS studies led to a model of the packing of the polymer molecules into ionic aggregates, while the SAXS and SANS studies resulted in models of the packing of the ionic clusters. In the present work, SANS measurements on elongated samples of Nafion have been used to obtain data that suggests a model of the structure of the material. The samples studied were elongated at two temperatures, 25 deg. and 155 deg. C. Although many of the scattering features of these two samples are similar, the samples stretched at 155 deg. C contain new information concerning the nature of the crystalline region.