Structural modulation of self-oscillating gels: changing the proximity of the catalyst to the polymer backbone to tailor chemomechanical oscillation

Abstract

We designed and synthesised two new polymerizable ruthenium complexes that catalyse the Belousov–Zhabotinsky (BZ) oscillating reaction and incorporated them into a copolymer to form hydrogels. The periodic oxidation and reduction of the attached ruthenium complex in the BZ reaction induces hydrating and dehydrating effects, respectively, that result in self-oscillatory volume changes of the hydrogel. We evaluated the correlation between the chemomechanical oscillation properties of the hydrogel and the proximity of the catalyst to the polymer backbone. Our results indicate that, like the change of such macroscopic parameters as temperature, reactant concentrations and pH, varying the microscopic distance between the catalyst and the polymeric chain provides a new way to tailor the chemomechanical behaviour, e.g., the initiation time, the frequency of oscillation, and the volume change of BZ hydrogels. Moreover, variation of the catalysts offers a new means to control the microstructure of the copolymer by expanding the range of monomer ratios. Modulation of molecular structure appears to be an effective way to alter the reaction–diffusion profile of species within heterogeneous chemoresponsive gels, thus contributing to the development of multifunctional, active soft materials capable of converting chemical energy into controllable mechanical forces.