Unique Structural Characteristics of Graft-Type Proton-Exchange Membranes Using SANS Partial Scattering Function Analysis
Yue Zhao, Kimio Yoshimura, Shin‐ichi Sawada, Toshinori Motegi, Akihiro Hiroki, Aurel Rădulescu, Yasunari Maekawa
Abstract
The partial scattering function analysis was applied to determine the exact structure of radiation-grafted proton-exchange membranes, made of poly(styrenesulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA). Hydrated ETFE-g-PSSA membranes were treated as a three-component system comprising the ETFE base polymer (BP), PSSA graft polymer (GP), and absorbed water. On a large length scale, polymer grains with an approximate radius of gyration (Rg) of 150 nm and a mass fractal structure with a dimension of 2.4 were observed. These grains were formed by the aggregation of phase-separated GP domains in the BP matrix. Each individual GP domain has an average Rg of 9.5 nm and is composed of homogeneously distributed GP and water nanodomains that form a bicontinuous-like local structure with a mean separation distance of 2 nm. These structures were strongly supported by the first finding that PSSA GP and water interact attractively and repulsively in q-regions lower and higher than 2 nm–1 (i.e., ∼3 nm), respectively. The repulsion between GP and water at a molecular length level of <3 nm results in a lower hydration number and hence poorer conductivity at low relative humidity when compared to Nafion. The results of this study provided a mechanistic insight into membrane conductivity and structure correlations.