Facile fabrication of a magnetically assembled colloidal photonic crystal film via radical polymerization

Abstract

A facile, economical and practical technique to fabricate a magnetically assembled colloidal photonic crystal in a polyacrylamide hydrogel matrix was demonstrated by an instant radical polymerization. By taking advantage of the instantaneousness of magnetic assembly, magnetic colloids were successfully assembled into photonic crystal structures with tunable stop bands in aqueous solution. The Bragg diffraction color could be simply controlled by tuning the strength of the external magnetic field to achieve diffraction covering the entire visible spectrum. An appropriate amount of an ionic initiator, APS, was key in the radical polymerization of the photonic crystal hydrogel film. 0.3 mg mL⁻¹ of APS was chosen, not only to allow a rapid formation of the polyacrylamide hydrogel, but also to avoid coagulation of these magnetic colloids in the hydrogel. Due to the significantly instantaneous properties of magnetic assembly and radical polymerization, coagulation of these magnetic assemblies owing to their long exposure time to an external magnetic field could be prevented. Therefore, a photonic crystal hydrogel film with a uniform diffraction color could be obtained. Notably, the flexible hydrogel film displayed obvious deformation responsiveness. The diffraction light got red-shifted as the level of deformation rose. The achievement in immobilization demonstrates the system to be a major stride toward practical applications in responsive photonic materials.