Polymers in ionic liquids and ionic liquids in polymers: critical factors for compatibility and materialization

Abstract

Ionic liquids (ILs) are often described as a third class of solvents, following water and organic solvents. They can dissolve a wide range of substances, including gases, inorganic salts, organic molecules, and even polymers. This perspective discusses the key factors that control the compatibility of “polymers in ILs”. The unique behavior of ILs arises not only from their physicochemical properties but also from their distinctive solvation mechanisms. Because ILs consist entirely of strongly interacting cations and anions, interactions between a solute and the IL must compete with the coulombic attraction between the ions themselves. As a result, polymer compatibility in ILs cannot be simply described as soluble or insoluble. Instead, more complex phase behaviors, such as upper critical solution temperature (UCST) and lower critical solution temperature (LCST), are often observed. In LCST-type systems, directional interactions between the polymer and the IL, such as hydrogen bonding and cation–π interactions, stabilize mixing at low temperatures. At higher temperatures, these interactions weaken relative to ion–ion interactions, leading to phase separation. Based on this understanding, the inverse concept, “ILs in polymers”, is then considered. When ILs and polymers are compatible, they form ion gels (ionogels). In these materials, ILs provide ion and gas transport, thermal stability, and chemical and electrochemical robustness, while the polymer matrix ensures mechanical strength and processability. Representative applications of ion gels are also highlighted. Finally, stimuli-responsive IL-based materials that exploit UCST- and LCST-type phase transitions are discussed. By using amphiphilic block copolymers with thermoresponsive segments, molecular-level conformational changes can be translated into controlled changes in molecular assembly. This strategy enables the rational design of temperature- and photo-responsive materials.