Molecular design of precise network polymerized ionic liquids to control aggregation and conductivity

Polymerized ionic liquids (PILs) are promising energy storage materials due to their high ionic conductivity and great thermal stability. The structure-property relationships of linear PILs have been extensively studied, yet little is known about other polymer architectures such as networks. We synthesized linear and network PILs containing precise linker lengths between charges using step-growth polymerization. Two different linkers were incorporated, either an 11 carbon chain (C11) or a tetra(ethylene oxide) chain (EO, also 11 atoms), to vary the network polarity. Wide-angle X-ray scattering measurements showed that the ion aggregation peaks were less intense in the EO systems, suggesting that the polar backbone provides better ion solvation. Network PILs had more pronounced ion aggregation peaks than their linear counterparts, indicating a role of polymer architecture on ion clustering. In addition, the EO network showed a two orders of magnitude increase in conductivity relative to the linear analogue at Tg+10°C. We hypothesize that the cross-linked structure provides greater correlation of conduction pathways. This systematic study provides a fundamental understanding on how polymer architecture and polarity can influence the aggregation and transport of ions in PILs.