Polarity Controlled Lithium Salt Partition in Diblock Copolymers and Polymer Blends*

The role of polymer polarity in determining conductivity of polymer electrolytes is increasing in awareness. The inherent tradeoff between polymer polarities and segmental dynamics limits ionic conductivity for single-polymer electrolytes. We hypothesize that blending two polymers, one with fast dynamics (low viscosity) and the other with high polarities (strong ionic solvation), may be a viable strategy to mitigate the aforementioned limit. It is found that upon blending, lithium salt is partitioned in two phases with a prefence to the high-polarity polymer phase. This salt partition imposes a negative derivation from the average of ionic conductivity of these two polymers. To further understand the phase separation and polymer-polymer interactions with lithium salt addition, we investigated the self-assembly of a diblock copolymer consisting of high-polarity and low-polarity blocks. By simulating SAXS patterns of this copolymer with different loadings of lithium salts, we found that the polymer-polymer interactions decreased monotonically as the salt loading increased. We also quantifiy the salt partition between two blocks and our thermodynamic analysis revealed that this partition process is enthalpy controlled. Our results advance the fundamental understanding on polymer blend electrolytes and reveal that ionic conduction can be improved by promoting simultaneous miscibility and polarity contrast between the polymer hosts.