Poly(ionic liquid) Star Polymers and Block Copolymers

Abstract

Poly(ionic liquid) block copolymers (PILBCPs) have been investigated as promising materials in lithium-ion batteries due to their unique combination of properties, including high solid-state ionic conductivity, high electrochemical stability, and nanostructured morphology. However, to date, most of these studies are exclusive to linear block copolymers. Star polymers are of interest due to documented property enhancements compared to their linear analogs. In this work, novel star poly(ionic liquid)s (PILs) and star PILBCPs were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization with a core-first approach. A systematic study of the effect of initiator concentration and number of chain transfer agents (CTAs) per RAFT agent on the polymerization kinetics was conducted to fundamentally understand the synthetic parameters that would yield well-defined star PILs. Increasing the initiator concentration resulted in faster monomer conversion at the cost of increased star-star coupling, while decreasing the number of CTAs per RAFT agent resulted in a decrease in initial rate retardation and less linear polymer formation. Using these results, well-defined 3-, 4-, and 6-arm star poly(VBMIm-TFSI) PILs were synthesized. The impact of star polymer architecture on polymer properties was explored by measuring the thermal, mechanical, and ion transport properties of each star PIL compared to those of an analogous linear PIL. The star PILs simultaneously exhibited an up to two-fold higher ionic conductivity and 150% higher elastic modulus compared to an analogous linear PIL. Additionally, the Williams-Landel-Ferry (WLF) equation was utilized to relate the improved ion transport in the star PILs to an increase in polymer fractional free volume. Lastly, three targeted compositions of 4-arm star PILBCPs were synthesized. The thermal, morphological, and ion transport properties were measured and analyzed in comparison to linear analogs to investigate impact of star polymer architecture on PILBCP properties. The increased polymer segmental motion in the ion conductive domains were a major contributing factor to the improved ion transport in star PILBCPs compared to their linear analogs at similar block compositions. Overall, this work demonstrates the successful synthesis of novel star PILs and PILBCPs and highlights their potential as promising materials for lithium-ion batteries.