Structure and coordination properties of facilitated olefin transport membranes consisting of crosslinked poly(vinyl alcohol) and silver hexafluoroantimonate

Abstract

The high selectivity of solid-state crosslinked poly(vinyl alcohol) (CPVA) membranes containing silver hexafluoroantimonate (AgSbF6), with respect to olefin/paraffin mixtures, was previously reported. The structure and coordination properties of CPVA/AgSbF6 complexes were investigated in this study with wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and theoretical ab initio calculations, and they were compared with those of poly(vinyl alcohol) (PVA)/AgSbF6 complexes. Contrary to expectations, the measurements of the intersegmental d-spacings and glass-transition temperatures indicated that the chain mobility in the PVA/AgSbF6 membranes was lower than that in the CPVA/AgSbF6 membranes. The different extents of transient crosslinking in the two systems were attributed mostly to their different coordination structures; silver ions in PVA/AgSbF6 were coordinated with hydroxyl oxygens located near the polymer main chains, whereas those in CPVA/AgSbF6 were coordinated with aldehyde oxygens located far from the main chains. According to WAXS spectra, AgSbF6 was completely dissolved in both PVA and CPVA, and this disrupted the crystallinity of the polymers. However, our DSC study showed that the silver ions dissolved in both polymer matrices recrystallized into silver oxide at elevated temperatures. The binding energy of Ag3d(5/2), as determined from XPS spectra, shifted to lower values with the addition of increasing amounts of the polymer matrix, indicating the increasing coordination of silver ions with polymer chains. The presence of various oxygen species with and without coordination to silver ions was confirmed from O1s XPS spectra of CPVA membranes containing silver ions. (C) 2004 Wiley Periodicals, Inc.