Small-Angle X-ray Scattering Study of Perfluorinated Ionomer Membranes. 2. Models for Ionic Scattering Maximum

Abstract

The origin of the “ionic scattering maximum”, attributed to the presence of ionic clusters in perfluorinated ionomer membranes, was studied by computer simulations of the observed scattering profiles and of their variations with swelling and deformation of the membranes on the basis of two models: (i) the two-phase model, in which the ionic clusters (ion-rich regions) are dispersed in the matrix of the intermediate ionic phase composed of fluorocarbon chains and nonclustered ions (single ions, small multiplets), and (ii) the core-shell model, in which the ionic cluster (i.e., ion-rich core) is surrounded by the fluorocarbon phase (i.e., ion-poor shell), the core-shell particle itself being dispersed in the matrix of the intermediate ionic phase. A clear distinction between the models turns out to be generally difficult. That is, the general aspects of the variation of the scattering profiles with swelling and deformation can be described in terms of either of the two models, and thus the distinction between the models requires quantitative investigations of the scattering profiles in terms of shifts of the peak position and height and so on. However, the swelling behavior, especially the result that the microscopic degree of swelling as determined by small-angle X-ray scattering is much larger than the macroscopic degree of swelling, clearly supports the core-shell model, the “short-range order distance” of which gives rise to the ionic scattering maximum. The deformation behavior also tends to support the core-shell model, although it is less conclusive than the swelling behavior. © 1982, American Chemical Society. All rights reserved.