Development of a Model for Analyzing the Temperature Dependence of the Viscosity of Ion Conducting Polymers and Ionic Liquids

The bond strength-coordination number fluctuation (BSCNF) model of the viscosity developed by the authors considers that the viscous flow occurs by breaking and twisting the connections between the structural units that form the melt. The analytical expression of the viscosity that results from such processes is written in terms of the average bond strength, the average coordination number, and their fluctuations of the structural units. In the present study, we use the BSCNF model to investigate the temperature dependence of the viscosity of ion conducting polymers LiClO_4-PPG and NaCF_3SO_3-PPG, and ionic liquids[bmim][PF_6], [bpy] [BF_4], [bmpro][(CF_3SO_2)_2N], [bpy] [(CF_3SO_2)_2N] and [bmim][(CF_3SO_2)_2N]. For ion conducting polymers, the analysis of the α-relaxation process is also presented. A case study done for ionic liquids indicates that the cooperativity for molecular motion which is evaluated from the viscosity analysis can be correlated with the diffusion coefficients and the ionic conductivities. The results of this study indicate that the BSCNF model is an effective model that could be used to analyze and interpret the measured temperature dependence of the viscosity