Pressure and temperature variation of the electrical conductivity of poly(propylene glycol) containing LiCF3SO3

Abstract

Complex impedance and differential scanning calorimetry (DSC) studies have been carried out on poly(propylene glycol) with average molecular weight 1025 (PPG) and PPG containing LiCF3SO3. The impedance studies were made at frequencies from about 1 mHz to 100 MHz at pressures up to 0.3 GPa (3 kbar) over the temperature range 215–365 K. Both the impedance and DSC studies were carried out in vacuum or at atmospheric pressure over a temperature range of about 100–375 K. As a consequence, the impedance studies overlap the DSC glass transition temperature. The inadequacy of the widely used Vogel–Tammann–Fulcher or Williams–Landel–Ferry equations to describe the temperature variation of the vacuum electrical conductivity data is discussed. It is shown that the Bendler–Shlesinger formalism is a better representation of the data, particularly in the region close to the glass transition. The first pressure derivative of the electrical conductivity, and hence apparent activation volume, decreases strongly as temperature increases. In addition, the activation volumes are larger for the low molecular weight liquids than the values reported previously for related, high molecular weight rubbery electrolytes. Next, there appears to be an exponential relationship between the activation volume and the electrical conductivity. Finally, the pressure variation of the electrical conductance exhibits negative curvature which shows that the activation volume increases as pressure increases. A qualitative explanation of each of these results concerning the relationship between conductivity, temperature, and pressure is given in terms of free volume. However, whether free volume can provide a quantitative explanation remains to be determined.