Macroporous temperature-sensitive gels with fast response: Comparison of preparation methods

Application of thermo-responsive gels as soft actuators for microfluidic devices with repeating “open–close” cycles requires high rates of water transport driven by changes in temperature. These rates are conventionally evaluated by comparison of water retention curves in T-jump tests with fixed initial and final temperatures. A shortcoming of this method is that it does not allow changes in diffusivity with temperature to be assessed quantitatively. To characterize cooperative diffusivity of water molecules, we propose a novel approach based on modeling the responses of temperature-sensitive gels in equilibrium swelling tests and T-jump experiments. Analysis of observations on poly(N-isopropylacrylamide) gels synthesized in aqueous solutions of pore-forming agents reveals that formation of a macroporous structure in a gel is a necessary, but not sufficient condition for acceleration of water transport. The effect of cosolvents used under preparation (phenol, ethanol, methanol, diethyl ether, dioxane, sucrose, and dodecyl dimethyl benzyl ammonium bromide) on water diffusivity is studied in detail. It is shown that the most pronounced enhancement of diffusivities under deswelling and reswelling is reached when the gels are synthesized in aqueous solutions of the surfactant at subzero temperatures.