Ionic interactions and transport properties in methyl terminated poly(propylene glycol)(4000) complexed with LiCF3SO3

Alternating current (ac) impedance, restricted diffusion, and vibrational spectroscopic (Raman and IR) measurements have been conducted on complexes of methyl capped poly(propylene glycol) of molecular weight 4000 and LiCF3SO3 salt. The relative concentrations of anions in different chemical environments have been calculated from an analysis of the symmetric anion SO3 stretch (Raman) over a wide concentration range. Comparisons are made to previous studies on hydroxyl capped PPG systems, and we find that polar polymer end groups play an important role in the solvation of the salt. The relative fraction of anions interacting directly with lithium cations is considerably higher over the entire concentration range in the present study, and we infer the existence of negatively charged ionic aggregates in the solutions. We also note that the fraction of spectroscopically ''free'' anions increases with increasing salt concentration in the ether oxygen to alkali metal cation ratio (O:M) range 502:1 to 12:1, contrary to a decrease reported for the analogue hydroxyl terminated electrolytes. The ionic conductivity has a more pronounced concentration dependency in the methyl capped system; notably, the molar conductivity (Lambda) increases dramatically with increasing salt concentration passing through a relatively sharp maximum at O:M = 20:1 at room temperature. Restricted diffusion data are also reported which show that the salt diffusion coefficient increases by approximately 60% between O:M 40:1 and 20:1. The results are discussed in terms of ionic interactions and ion transport in nonideal polymer ion conductors. We comment on the characteristic concentration dependency of Lambda and observations of low and even negative cationic transference numbers in similar complexes.