STRUCTURE OF ION-PAIRS IN SOLID POLYMER ELECTROLYTES
Abstract The known change in the optical spectrum of alkali picrates as a function of the interionic ion pair distance was used to compare the process of ion pairing in liquid and solids (polymers) of identical composition. The polymer components of the solid polymer electrolytes were chiefly comb polymers of poly(methacrylate) with pendant oligo-oxyethylene chains. Spectra of the picrate salts dissolved in the polymers and in the corresponding monomers were compared. The results show a consistent bathochromic shift for lithium and sodium picrate in the polymers relative to solutions in the monomer, suggesting increased formation of oligo-oxyethylene-complexed loose (separated) ion pairs in the solid polymers. At very high salt content only tight ion pairs are found due to insufficient chelating oxyethylene units. Addition of propylene carbonate causes a rapid rise in the fraction of free ions. The implications of our findings for the study of solid polymer electrolytes are briefly discussed.