Temporal Control of Self-Oscillation for Microgels by Cross-Linking Network Structure

Self-oscillating microgel, which undergoes an autonomic and periodical swelling/deswelling oscillation, has been reported. The cross-linked microgels were synthesized by the copolymerization of N-isopropylacrylamide (NIPAm) with ruthenium tris(2,2′-bipyridine) [Ru(bpy)3] as a catalyst for the Belousov−Zhabotinsky (BZ) reaction by surfactant-free aqueous precipitation polymerization. The self-oscillation of the microgels was detected by changes in optical transmittance. The microgels showed not only the swelling/deswelling oscillation, which is synchronized with the redox oscillation of the Ru(bpy)3 complex immobilized in the microgels, but also flocculating/dispersing oscillation around the phase transition temperature of the microgels, with a remarkable change in optical transmittance. These microgel oscillations were further studied by comparing the optical transmittance changes of the microgels and the redox potential change of Ru(bpy)3 immobilized in the microgels. In order to analyze the microgel self-oscillation in detail, three characteristics (induction period, oscillation period, and waveform) of the microgel oscillation were compared with those of the bulk solution of the BZ reaction. Because of the effect of Ru(bpy)3 immobilization in the microgels on the BZ reaction, induction time for the microgels became longer than that for the bulk solution; however, the oscillation periods became shorter. The latter can be well explained by the Field−Körös−Noyes (FKN) mechanism.