1Yokohama National University, Yokohama, Japan
Ionic liquids (ILs) are the third group of solvents, following water and organic solvents. They are easily available and possess unique properties such as nonvolatility, high thermal stability, and designability, which make it possible to use them on demand and under unusual conditions. Our study has been focused on understanding of the solvation and phase-separation of polymers in ILs and on their utilization as neoteric solvents for innovative polymeric materials and devices. Specifically, ILs exhibit unique solubility toward polymers (Fig. 1); this opens up a new field of stimuli-responsive materials. One of characteristic feature of solvation of polymers is that there exist competitive interactions between ILs and polymers, which induces unique phase separation of polymers. In the case of LCST phase-behavior of PBnMA (Fig. 1(c)), cation(imidazolium)- interaction enables the dissolution at low temperatures. However, the cation-anion interaction overwhelms the cation- interaction at high temperatures, which induces LCST phase-separation. Another characteristic feature is that the magnitude of thermodynamic parameters for the phase-separation in ILs is rather small, compared with such behavior in conventional solvents, which enables fine tuning of the phase-separation temperatures by slight changes in IL and polymer structures.
By utilizing these unique solvation and phase-separation behavior, the reversible micellization and sol-gel transition of thermo- and photo-responsive block copolymer solutions in ILs could be demonstrated. This transition is based on thermo/photo-responsive LCST and UCST phase-separation of certain polymers in ILs. Such sol-gel transition has been utilized to design photo-healable materials. Damaged part of materials is UV-irradiated to convert gel to sol, followed by irradiated by visible light in order to convert again sol to gel. By this procedure, the damaged part was demonstrated to be healed to the original state.