Hydration of Nafion (R) studied by AFM and X-ray scattering

Nafion(R) is a commercially available perfluorosulphonate cation exchange m embrane commonly used as a perm-selective separator in chlor-alkali electro lysers and as the electrolyte in solid polymer fuel cells. This usage arise s because of its high mechanical, thermal and chemical stability coupled wi th its high conductivity and ionic selectivity, which depend strongly on th e water content. The membrane was therefore studied in different states of hydration with two complementary techniques: atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) combined with a maximum entropy (Ma xEnt) reconstruction. Tapping mode phase imaging was successfully used to i dentify the hydrophobic and hydrophilic regions of Nafion. The images suppo rt the MaxEnt interpretation of a cluster model of ionic aggregation, with spacings between individual clusters ranging from 3 to 5 nm, aggregating to form cluster agglomerates with sizes from 5 to 30 nm. Both techniques indi cate that the number density of ionic clusters changes as a function of wat er content, and this explains why the bulk volumetric swelling in water is observed to be significantly less than the swelling inferred from scatterin g measurements. (C) 2000 Kluwer Academic Publishers.