Influence of Molecular Weight and End Groups on Ion Transport in Weakly and Strongly Coordinating Polymer Electrolytes

In the development of polymer electrolytes, the understanding of the complex interplay of factors that affect ion transport is of importance. In this study, the strongly coordinating and flexible poly (ethylene oxide) (PEO) is compared to the weakly coordinating and stiff poly (trimethylene carbonate) (PTMC) as opposing model systems. The effect of molecular weight (M_n) and end group chemistry on the physical properties: glass transition temperature (T_g) and viscosity (η) and ion transport properties: transference number (T₊), ion coordination strength and ionic conductivities were investigated. The cation transference number (T₊) showed the opposite dependence on M_n for PEO and PTMC, decreasing at low M_n for PTMC and increasing for PEO. This was shown to be highly dependent on the ion coordination strength of the system regardless of whether the end group was OH or if the chains were end-capped. Although the coordination is mainly of the cations in the systems, the differences in T₊ were due to differences in anion rather than cation conductivity, with a similar Li⁺ conductivity across the polymer series when accounting for the differences in segmental mobility.