Small-angle X-ray scattering analysis of ionic domain features in graft-type polymer electrolyte membranes

Abstract

Introduction: Although poly(styrene sulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) are potential polymer electrolyte membranes for fuel cells, there are only a few reports on the effect of synthesis steps and grafting degree (GD) on the features of ionic domains. These ionic features are related to the conductance and thus directly affect the fuel cell performance. Accordingly, this work reports SAXS analysis to determine the features of ionic domains, including domain sizes and interdomain distances, in ETFE-PEMs according to GDs.

Methods: ETFE-PEMs were prepared via the irradiation of polystyrene onto the original ETFE matrix (grafted-ETFE) and subsequent sulfonation. The structural features of the ionic domains were investigated by the Ornstein–Zernike (OZ) and Teubner–Strey (TS) models based on the fitting of small-angle X-ray scattering (SAXS) profiles.

Results: According to the OZ model, the polystyrene (PS) and PSSA grafts can be ordered up to 0.8 and 1.1 nm, respectively. Moreover, the TS model suggested that the interdomain distances of the PS and PSSA grafts were approximately 0.7 and 1.1 nm, respectively.

Conclusion: The above SAXS results suggest that the grafted-ETFE films have the capacity for self-organization of graft domains. Moreover, phase separation occurred strongly at the sulfonation step, leading to the self-organization of ionic domains at larger dimensions compared to those of the corresponding graft layers.