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

Abstract

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 (T_g) 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 T_g (−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).