Effects of source correction on positron annihilation lifetime spectroscopic analysis of graft-type polymer electrolyte membranes

Abstract

Introduction: Recently, the free-volume hole features of poly(styrene sulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) have been studied using positron annihilation lifetime spectroscopy (PALS) to determine the relationship between gas crossover through a PEM and a fuel cell. As one such series, this work investigates the source correction in PAL spectroscopic analysis for ETFE-PEM. Method: ETFE-PEM was prepared by radiation-induced graft polymerization and subsequent sulfonation. The free-volume hole characteristics of ETFE-PEM with a grafting degree (GD) of 106% were determined using PALS with and without source correction. Results: After source correction, the original-ETFE and ETFE-PEM strains exhibited increases in r3 (smaller radius of free-volume holes in the lamellar amorphous regions, the PSSA grafts, and the interface zones inside the lamellae) and r4 (larger radius of free-volume holes in the mobile amorphous layers and the PSSA grafts outside of the lamellae). In addition, the full width at half maximum (FWHM) of r3 is much greater than that of r4 due to the rearrangement in the mobile amorphous region and the polystyrene layers outside the lamellar structure. Conclusion: Source correction causes a significant change in the distribution curves of r3 for the original-ETFE and ETFE-PEM. Thus, source correction of positron annihilation lifetime spectroscopic analyses is an important issue for determining the o-Ps lifetime of polymers, which is near the lifetime of positrons in source materials.