Polymers in Ionic Liquids and Ionic Liquids in Polymers: Critical Factors for Compatibility and Materialization

Abstract

Ionic liquids (ILs) are recognized as a third class of solvents, following water and organic solvents. ILs can dissolve gases, inorganic salts, non-ionic organic compounds, and even polymers. In this article, the critical factors governing the compatibility of "polymers in ILs" are discussed. The distinctive features of ILs arise not only from their unique physicochemical properties but also from their characteristic solvation mechanisms. Because a pure IL consists solely of strongly interacting cations and anions, solute-IL interctions compete with the Coulombic interactions between the constituent ions. As a result, polymer compatibility in ILs cannot be simply classified as soluble or insoluble. In some systems, upper critical solution temperature (UCST) and lower critical solution temperature (LCST) phase behaviors are observed. In particular, in LCST phase separation, competitive interactions play a crucial role: directional polymer-IL interactions at low temperatures, such as hydrogen bonding and cation-π interactions, are weakened relative to cation-anion Coulombic interactions, leading to a transition from soluble to insoluble with increasing temperature. Based on this understanding of polymer compatibility in ILs, the latter part of this article addresses the materialization of "ILs in polymers." In compatible systems, such materials are referred to as ion gels (ionogels). ILs impart transport properties, thermal stability with negligible vapor pressure, and chemical and electrochemical stability, while the polymer provides mechanical integrity and processability. Certain applications of ion gels are presented. Another fascinating class of IL-based materials is stimuli-responsive systems based on UCST and LCST phase transitions. By employing amphiphilic block copolymers containing UCST and/or LCST segments, molecular-level conformational changes can be transformed into changes in molecular assembly. This approach enables the precise design of temperature-and photo-responsive materials.