Electrical conductivity, DSC and NMR studies of PEG and PPG containing lithium salts

Complex impedance, differential scanning calorimetry (DSC) and Li-7 nuclear magnetic resonance (NMR) studies have been carried out on poly(propylene g lycol) (PPG) with an average molecular weight of 4000 and poly(ethylene gly col) mono-methyl-ether with an average molecular weight of 350 (PEG) contai ning lithium salts. The impedance studies were made over a range of frequen cies, temperatures and pressures and were used to obtain values of electric al conductivity. The DSC, conductivity and NMR data are used to characteriz e the PPG-based material which is a glass-forming liquid. The PEG-based mat erial exhibits the expected tendency to crystallize. The first new result i s that the effect of pressure on thr electrical conductivity (and hence act ivation volume) for the PEG-based electrolyte above the melting point is ab out half that for the PPG-based electrolyte. The same trend has been report ed previously for the pressure variation of the glass transition temperatur e for related host polymers. The second new result is that the curvature in the effect of pressure on the electrical conductivity is opposite for the two electrolytes. It is pointed out that the curvature for PPG:LiCF3SO3 has the same sign as the effect of pressure on the relaxation time of the cu r elaxation in PPG. These results provide further evidence that ionic conduct ivity is controlled by large-scale segmental motions of the polymer chain. Further explanation of the data is given in terms of free volume. (C) 2000 Published by Elsevier Science B.V.