Recognized Ionic Structures in Large Dimension of Graft-type Polymer Electrolyte Membranes Using Pair Distribution Function Expanded for Small Angle X-Ray Scattering

This study reports the pair distribution function (PDF) analysis of combined wide- and small-angle X-ray scattering (WAXS/SAXS) profiles of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) within a wide grafting degree (GD) of 0%–117%. The PDF analysis of WAXS profiles (from Cu-Kα₁ radiation) provides a measure in size of the crystallite domains (5.1–8.7 nm). The extension of the PDF analysis for only SAXS profiles shows the distances of crystallite layers of 25.1–32 nm. In particular, SAXS-PDF analysis is effective in showing the existence of newly generated graft domains with distances ≈60–64 nm, which can not be determined previously by the conventional SAXS analysis. The high similarity in local and higher-order structures observed for polystyrene grafted ETFE films and ETFE-PEMs suggests that the hierarchical structures including the spatial arrangement of large amorphous contents in the membranes can be determined at the graft polymerization step. Note that the presence of newly generated PSSA graft domains at large dimension can explain well the comparable or higher proton conductivity of ETFE-PEMs as compared with commercial Nafion membrane.