Dynamics in propylene carbonate and propylene carbonate containing LiPF6

Abstract

Electrical conductivity and differential scanning calorimetry studies (DSC) have been carried out on 1 M LiPF6 in propylene carbonate (PC) from 187 to 296 K. The electrical conductivity data are analyzed in terms of Vogel–Tammann–Fulcher (VTF), Williams–Landel–Ferry (WLF), and Bendler–Shlesinger (BENSH) formalisms. In addition, literature data for viscosity and relaxation times for PC (not containing salt) are analyzed using the same formalisms. Large, systematic variations are found in both the VTF and WLF fitting parameters with temperature interval. The large variation of the parameters with temperature demonstrates the failure of both formalisms and shows that care must be taken when utilizing the VTF/WLF parameters to draw conclusions concerning the behavior of systems. One conclusion which can be reached is that the VTF results indicate a significant difference between the molecular motions in PC and PC-containing LiPF6. Another is that the WLF parameter C1 is about 11 for low temperature data, referenced to the DSC glass transition temperature, Tg, which shows that for PC and PC-based liquids the value of all three dynamical quantities at Tg is about 11 decades smaller than the value predicted for very high temperatures. Finally, all data are found to be better fit by the BENSH equation.