Water Absorption and Conductivity in Poly(Ethylene Oxide)-Lithium Tetrafluoroborate Films.

Water-vapor absorption and conductivity in spin-cast polyethylene oxide- lithium tetrafluoroborate films were investigated near 21 deg C by in situ near-infrared spectroscopy and ac conductance measurements. The film conductivity was less than or .0002 ohhmscm until the water content exceeded that corresponding to LiBF4.3H2O. It increased with additional water, reaching .003 ohmscm at the composition equivalent to LiBF4.20H2O. Most of the water above the trihydrate level was unstably absorbed, however, and began to escape from the film within a few minutes. This syneresis was attributed to spontaneous restructuring or crystallization of the polymer. The conductivity data were interpreted on the basis of a two-phase model in which one portion of the salt remained in a nonconductive solid region, while the other portion dissolved in a more fluid conductive polymer-water region.