A Comparison Between Highly Crystalline and Low Crystalline Poly(phenylene sulfide) as Polymer Electrolyte Membranes for Fuel Cells

Sulfonation-induced changes in the crystalline structures of highly crystalline (HC) and low crystalline (LC) poly(phenylene sulfide) (PPS) electrolyte membranes and relative humidity-induced changes in the morphology of those sulfonated PPS (SPPS) membranes were characterized by wide-angle X-ray scattering, small-angle X-ray scattering, and atomic force microscopy. The correlations between the hydrated morphology and the development of proton channels in both kinds of membranes are discussed in the paper. For HC-PPS membranes, crystallinity decreased steeply up to an ion-exchange capacity (IEC) of 1.6 meq/g during sulfonation, but in the case of LC-PPS membranes, crystallinity decreased up to IEC = 0.9 meq/g, but not as steeply as for HC membranes. In the varied hydration conditions of the membranes, water molecules were not predominately located within the ionic aggregation but, rather, were gradually dispersed and rearranged through the whole membrane with increasing hydration level. For both kinds of membranes, HC-SPPS and LC-SPPS, Porod plots showed a "positive deviation" that revealed that the polymer/water interface under varied hydration conditions was not smooth, but diffused, and well-developed proton channels did not form in the membranes. LC-SPPS membranes showed about a 10% rougher (from alpha value) polymer/water interface than HC-SPPS membranes.