IONIC INTERACTIONS AND TRANSPORT IN A LOW-MOLECULAR-WEIGHT MODEL POLYMER ELECTROLYTE

AC impedance, FT-Raman and pulsed field,gradient (pfg) NMR measurement s have been conducted on solutions of poly(ethylene oxide) dimethyl et her (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the v(S)(SO3) and delta(S)(CF3 ) vibrational band envelopes of the CF3SO3 anion, respectively, the re lative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation o f associated ionic species into spectroscopically ''free'' anions with increasing salt concentration in dilute solutions. The relative abund ance of associated ionic species increases with increasing temperature . Pfg-NMR measurements show that D_(F-19) and D+(Li-7) are very simila r fur all concentrations (i.e., O:Li greater than or equal to 53:1) an d temperatures (25-80 degrees C)investigated. Most notably. the diffus ivity of the oligomer solvent, D(H-1), is significantly faster than th e self-diffusion coefficients of the dissolved ions in all cases, Pred icted values for the ionic conductivity were obtained from the NMR dif fusivities, using the Nernst-Einstein relation, and compared with thos e from direct measurement. We find that the calculated values an highe r for all concentrations; the discrepancy increases with decreasing sa lt concentration and increasing temperature. A good correlation is fou nd between the concentration dependence of the ionic redissociation pa ttern, as determined from the v(S)(SO3) Raman band envelope, and an in crease in equivalent ionic conductivity with increasing salt concentra tion in dilute solutions (i.e., O:Li greater than or equal to 110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved io ns and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer-salt complexes. (C) 1998 American Institute of Physics.