Macromolecules in ionic liquids: Progress, challenges, and opportunities

Ionic liquids (ILs) are ambient temperature molten salts and have attracted considerable attention because of their unique properties such as nonflammability, negligible volatility, high ionic conductivity, and thermal stability. Here, we show that the combinations of macromolecules with ILs as solvents and additives could offer new progress, challenges, and opportunities in polymer materials science. Compatible binary systems, where ILs are solidified (gelled) by polymers, can be used as ion-conducting polymer electrolytes (ion gels). The ion transport in ion gels is found to be decoupled from the segmental motion of the polymers, leading to relatively high ionic conductivities even at their glass transition temperatures. The flexibility of the molecular design of ILs allows us to prepare many new ILs having electrochemical functionalities such as proton, electron, and lithium ion transport. The new ILs and their ion gels appear to be promising candidates as new materials for electrochemical applications. In contrast, certain combinations of macromolecules in ILs exhibit lower or upper critical solution temperature type phase separation by changing the temperature, where the phase separation temperatures are easily altered by the change in the polymer and IL structures. Moreover, cross-linked polymer gels in the ILs show reversible volume phase transition. The volume phase transition of polymer gels in ILs could usher in a new era of environmentally stable and smart gels since the swelling/shrinking behavior can be observed reversibly in an open atmosphere without the need to consider solvent evaporation, which is a serious drawback of conventional smart gel materials.