Facile synthesis of macroscopic bilayer gels by one-shot radical polymerization in phase-separated binary solvent systems

A macroscopic bilayer gel material has potential for great research interest because of its unique anisotropic behavior; however, studies using a facile synthetic method for preparing a bilayer gel are limited since the gel layers need firm adhesion. In this study, we developed a simple and versatile one-shot synthetic method for preparing bilayer gels using radical polymerization in a phase-separated binary solvent system. Photopolymerization of t-butyl acrylate and acrylamide in a toluene/water phase-separated solution in the presence of an amphiphilic crosslinker produced a highly reproducible bilayer gel. The obtained bilayer gels exhibited selective swelling behavior and unique mechanical properties derived from the properties of each polymer layer. In addition, the mechanical properties of the bilayer gels were significantly affected by the volume of the mixed solvent layer or the emulsified layer formed at the interface. Importantly, this synthetic method has broad applicability to various solvents and monomers. For example, we successfully synthesized a bilayer hydrogel containing a thermoresponsive polymer layer using appropriate reaction solvents; these reaction solvents were substituted for water using a simple solvent substitution process. The resulting bilayer hydrogel exhibited unique functions, such as thermoresponsive anisotropic deformation and water pumping behavior.