Wide-angle X-ray scattering studies on the structural properties of polymer electrolytes containing silver ions

Abstract

Wide-angle X-ray scattering (WAXS) has been used to study the structural properties of polymer electrolytes containing silver ions. The WAXS spectra of pure poly(2-ethyl-2-oxazoline) (POZ) and poly(vinylpyrrolidone) (PVP) showed two broad amorphous peaks. The first of these peaks was assigned to the interchain distance, and the second peak was assigned to the distance between the pendant groups of the polymer chains. When silver salts such as AgBF4, AgCF3SO3, or AgClO4 were introduced into the polymer matrices, their silver ions coordinated with the carbonyl oxygens on the pendant groups to form polymer/silver salt complexes. In the case of the PVP/silver salt complexes, the first peak shifted to lower angle and became narrower with increasing silver concentration, indicative of an increase in the interchain distance and ordering. In contrast, the position of the second peak remained almost constant. For the POZ/silver salt complexes, however, the positions of both peaks shifted to higher scattering angle with increasing silver concentration until the mole ratio of carbonyl oxygen to silver reached about 3:1. Further increase of the concentration caused the first peak to shift to lower angle while the second peak remained almost constant, which is similar to the behavior of the PVP/Ag salt complexes. The difference in the behavior of the PVP/Ag and POZ/Ag salt complexes may arise from different strengths of the transient cross-links formed by silver cations with the pendant groups of POZ and PVP. The Bragg d spacing results calculated from the second peak indicate that the silver ions coordinate more strongly with the pendant groups in the POZ/AgClO4 complex than in the POZ/AgCF3SO3 and POZ/AgBF4 complexes. It is worth noting that the Bragg d spacings for the POZ/silver and PVP/silver salt complexes were very different at low silver concentrations but approach almost the same value at high silver concentrations (e.g., at a mole ratio of [C=O]:[Ag] = 1:1). These findings are consistent with our previous results on facilitated olefin transport and glass transition temperature behavior in silver polymer electrolyte membranes. We therefore conclude that the facilitated olefin transport and glass transition temperature behavior in polymer electrolytes are strongly related to the structural change that occurs upon coordination of silver cations with the carbonyl oxygens of the polymeric chains.