Unique Structural Characteristics of Graft-Type Proton-Exchange Membranes Using SANS Partial Scattering Function Analysis

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 (R g) 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 R g 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