Bidirectional Optical Response Hydrogel Realized by Adjusting Intermolecular Interactions

ABSTRACT Hydrogels with switchable optical transparency are highly desirable for applications in smart windows and information anti‐counterfeiting. They are usually fabricated by employing components with distinct critical transition temperatures, and display an opaque–transparent–opaque (O‐T‐O) transparency change upon heating. In this contribution, a bidirectional optical response hydrogel featuring a unique transparent–opaque–transparent (T‐O‐T) transition upon heating is successfully developed. It was fabricated via the direct polymerization of hydroxyethyl methacrylate (HEMA) in an aqueous solution containing the nonionic triblock copolymer, Pluronic F127. Notably, neither the polymerized HEMA (PHEMA) nor F127 can exhibit an optical response to temperature changes individually. The critical transition temperatures of the as‐prepared hydrogel can be readily tuned by adjusting the content of each component in this system. This novel T‐O‐T bidirectional optical response can be ascribed to the temperature‐dependent changes in the hydrophilicity of F127, which induce alterations in its conformation and aggregation states. These variations further modulate the intermolecular interactions between F127 and PHEMA, and subsequently influence the microphase separation degree of PHEMA, leading to transparency variation of the hydrogel. Owing to the tunable temperature range of white opacity, this bidirectional optical response hydrogel holds great promise as a material for on‐demand information writing and temperature‐controlled segmented decoding.