Control of phase transition temperature of thermoresponsive poly(ionic liquid) gels and application to a water purification system using these gels with polydopamine

Abstract

Demand for purified water is increasing as the population grows. Water purification systems combining thermoresponsive materials and adsorbents that can be used in areas without electricity or other energy infrastructure have begun to be reported. The thermoresponsive hydrogel used in this system must be able to adsorb and desorb a large amount of water, react drastically to temperature, and have a phase transition temperature that can be easily changed depending on the environment in which it is used. In this study, thermoresponsive ionic liquid-derived polyelectrolyte gels (poly(IL) gels) were prepared as lower critical solution temperature (LCST)-type thermoresponsive materials. As a result, poly(IL) gel, which exhibits drastic swelling/shrinking and reversible adsorption/desorption of a large amount of water (44 times the amount of water to its own weight), was successfully fabricated. The phase transition temperature can be controlled between 20 °C and 35 °C by copolymerizing ILs with different structures. Then, polydopamine (PDA), which can serve as a photothermal conversion material as well as an adsorbent for contaminants, was polymerized on the surface of the poly(IL) gel. In addition to its role as an adsorbent, PDA can also serve as a photothermal conversion material. First, a poly(IL)-based material with PDA is swollen in contaminated water. Subsequent irradiation with sunlight induces an LCST-type phase transition behavior by converting light energy into heat. Finally, the contaminants remain adsorbed, and only the water is discharged from the material to obtain purified water. Using this material, we were able to show that organic dyes are removed from aqueous solutions with high efficiency using sunlight.