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

Abstract

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 LiClO4-PPG and NaCF3SO3-PPG, and ionic liquids [bmim][PF6], [bpy][BF4], [bmpro][(CF3SO2)2N], [bpy] [(CF3SO2)2N] and [bmim][(CF3SO2)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.

Network Relaxation and Cooperativity in Ion Conducting Polymers PEO-Li: An Analysis Based on the BSCNF Model

“…As a comment, we want to point out that the dependence shown in Fig. 3 seems to differ from the behavior found in ionic liquids [15]. There, a linear relation between ln 𝐷 and ln(𝑁 B −1 ) was found, where 𝐷 is the diffusion coefficient.…”

“…The BSCNF model provides a framework to discuss the growth of the cooperativity based on the following expression [13,15],…”

“…[12]. From the figure we recognize that strong systems such as SiO2 are well characterized by small value of B and large value of C. On the other hand, fragile systems such as ZBLA (a fluoride compound) are characterized by an opposite trend of the parameters, i.e., large value of B and small value of C. The BSCNF model applied to polymers and ionic liquids[13][14][15] indicated that these materials are placed in an area of the B-C mapping characterized by large values of B and small values of C compared to other type of materials as illustrated by the parabolic region shown in Fig.1. That is, the mapping indicates that in polymer and ionic liquids, the bond strength between the constituent elements (monomers in the case of polymers) is small and its fluctuation is large.…”

“…Characterization of different materials in the B-C mapping[12]. The typical values of B and C for polymers and ionic liquids[13][14][15] are found in the marked region. The dotted curves show the behavior described by the BSCNF model.…”