Composite gel particles consisting of 2-phenylethyl methacrylate (PhEMA)-vinyl toluene (VT) copolymer (P(PhEMA-VT)) and poly(methyl methacrylate) (PMMA) were prepared by seeded polymerization of MMA in the presence of P(PhEMA-VT)/1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Bmim][TFSA], IL) gel particles, in which P(PhEMA-VT) exhibited lower critical solution temperature (LCST)-type phase transition behavior in IL. The particles obtained at 70 °C (above the LCST) adopted a phase-separated snowman-like shape and clearly exhibited volume–phase transition behavior in which the P(PhEMA-VT) phase reduced in size and the PMMA phase increased in size above the LCST. The P(PhEMA-VT) phase exhibited LCST-type volume–phase transition behavior and separation of IL occurred. In contrast, the PMMA phase was swollen with IL separated from the P(PhEMA-VT) phase, in which the PMMA phase could act as a preserver of IL. Interestingly, the composite gel particles exhibited similar volume–phase transition behavior in both air and vacuum without leakage of IL from the gel particles, which should be free-standing thermosensitive gel particles.
