Self-organization in Polymer Gel Systems Incorporated with Active Elements

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The poly (N-isopropyl acrylamide)(PNIPA) gel coupled with Belousov-Zhabotinsky (BZ) reaction was prepared by covalently bonding the catalyst ruthenium (Ru(bpy)_3) to the polymer chain. The chemical oscillation in the BZ reaction is found to induce periodical and autonomous swelling-shrinking volume changes of the gel. In this new type of gel, conversely a volume change of the PNIPA gel affects the propagation of the chemical wave. Our attention was focused on effects of mechanical changes on the chemical wave by utilizing the thermally driven volume phase transition of the PNIPA gel. Both the velocity and the frequency of the chemical wave increased with increasing the temperature, and abruptly decreased at the volume transition temperature of the gel. The diffusion of HBr0_2, which is essential for the wave propagation, was hindered with increasing the temperature. The diffusion of HBrO_2 through the gel network in the low temperature region was explained in the same way as a simple diffusion of inactive molecules through a restricted environment. At the high temperature, in contrast, the change of the wave velocity cannot be explained by the simple diffusion theory. This is probably because non-homogeneities in reactant concentrations and the increase in the amount of bulky catalyst hinder the diffusion and the rate of reaction.Depending on the ratio of concentration of the reducing agent to that of the oxidizing agent, reduction waves appears in contrast to usual oxidizing waves. The most striking behaviors observed are self-replication and preservation of reduction pulse waves. In the former dynamics, after the propagating reduction pulse wave vanishes, it splits into two counterpropagating pulses. In the latter dynamics, counterpropagating reduction pulse waves annihilate each other upon head-on collision and then reappear.