Pyrrolidinium-Based Polyurethane Ionenes: Influence of Counterions, Chain Extenders, and PEG Blocks on Thermal Properties and Ion Conduction

We designed and synthesized pyrrolidinium-based polyurethane ionenes. These ionenes were synthesized using dihydroxyundecyl pyrrolidinium ionic liquid monomers with various anions (Br–, PF6–, or Tf2N–), along with a poly(ethylene glycol) (1000 or 4000) comonomer and one of three chain extenders: methylene diphenyl 4,4′-diisocyanate (MDI), tolylene-2,4-diisocyanate (TDI), and 4,4′-methylenebis(cyclohexyl isocyanate) (HMDI). To investigate the structure–property relationship, a comprehensive study of the thermal properties and ion conduction of the synthesized ionenes was conducted. The synthesized ionenes mostly exhibit an amorphous morphology; however, only ionenes containing PEG-4000 block show semicrystalline features. The copolymer ionenes containing PF6– anions exhibit higher glass transition temperatures (Tg) compared to those with other anions due to hydrogen bonding with the N–H groups (urethane). Also, the pyrrolidinium/PEG-4000 copolymer ionene with PF6– shows a higher degree of crystallinity. Among the synthesized ionenes, the pyrrolidinium-Tf2N polyurethane copolymer with PEG-1000 exhibits the lowest Tg (−21 °C) and the highest ionic conductivity of 1.04 mS/cm (at 70 °C). These findings suggest that pyrrolidinium-based polyurethane ionenes have potential applications in ion-conductive materials, particularly in energy storage devices such as energy storage (e.g., batteries) and conversion devices (e.g., solar cells).