Dynamics in propylene carbonate and propylene carbonate containing LiPF6

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-Tam mann-Fulcher (VTF), Williams-Landel-Ferry (WLF), and Bendler-Shlesinger (BE NSH) 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 para meters with temperature interval. The large variation of the parameters wit h temperature demonstrates the failure of both formalisms and shows that ca re 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 molecula r motions in PC and PC-containing LiPF6. Another is that the WLF parameter C-1 is about 11 for low temperature data, referenced to the DSC glass trans ition temperature, T-g, which shows that for PC and PC- based liquids the v alue of all three dynamical quantities at T-g is about 11 decades smaller t han the value predicted for very high temperatures. Finally, all data are f ound to be better fit by the BENSH equation. (C) 1999 American Institute of Physics. [S0021-9606(99)51411-6].