Hierarchical Structure Analysis of Graft-Type Polymer Electrolyte Membranes Consisting of Cross-Linked Polytetrafluoroethylene by Small-Angle Scattering in a Wide- Q Range

Small-angle scattering in a wide-Q range (4 x 10(-3) to 1.5 nm(-1)) of polymer electrolyte membranes consisting of poly(styrenesulfonic acid) and cross-linked polytetrafluoroethylene (cPTFE-PEM) with various grafting degrees up to 36% was observed by focusing small-angle neutron scattering (FSANS), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). The hierarchical structure of the PEM was characterized as being composed of conducting layers (graft domains) in lamellar stacks with 48-57 nm spacing on the surfaces of 480 nm diameter crystallites and ultrasmall structures with 1.7 nm correlation distance of the sulfonic acid groups in the conducting layers. The PEMs with grafting degrees less than 15% possessed only grafting domains in the amorphous layers of the lamellar stacks of cPTFE. An increase in the grafting degree up to 5% led to an increase in the lamellar spacing of 20%, while the lamellar spacing remained constant with grafting degrees above 5% because of crystalline restriction. Moreover, with grafting degrees of greater than 15%, grafting domains were phase separated from the cPTFE substrate and covered the crystallites with a diameter of 480 nm (the length of a crystallite is above the observed Q region (>1.6 mu m). The graft domains around the crystallites were connected with the adjoining domains; accordingly, the PEMs with a higher degree of grafting had conductivity higher than that of Nafion. Furthermore, in the SAXS measurement, the nano order internal structure (1.7 nm) corresponded to the distance between the sulfonic acid groups of the graft polymers in the conducting layers of the cPTFE-PEM. The nano order correlation distances of the sulfa groups, which cannot be observed in Nafion, should result in methanol and water crossover levels lower than those in Nafion.