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 glycol) (PPG) with an average molecular weight of 4000 and poly(ethylene glycol) mono-methyl-ether with an average molecular weight of 350 (PEG) containing lithium salts. The impedance studies were made over a range of frequencies, temperatures and pressures and were used to obtain values of electrical conductivity. The DSC, conductivity and NMR data are used to characterize the PPG-based material which is a glass-forming liquid. The PEG-based material exhibits the expected tendency to crystallize. The first new result is that the effect of pressure on thr electrical conductivity (and hence activation volume) for the PEG-based electrolyte above the melting point is about half that for the PPG-based electrolyte. The same trend has been reported previously for the pressure variation of the glass transition temperature 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 relaxation in PPG. These results provide further evidence that ionic conductivity 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.