Thermoresponsive Physically Cross-Linked Hydrogels with Bidirectional Optical Response for Smart Windows Application

Hydrogel-based smart windows have the potential to reduce the energy consumption associated with air conditioning and lighting systems. Most existing hydrogels exhibit a unidirectional response to a specific temperature, limiting their applicability. In this work, a bidirectional temperature-responsive hydrogel was developed by incorporating hydroxypropyl cellulose (HPC) into a physically cross-linked copolymer matrix of N-(2-hydroxyethyl) acrylamide (HEAA) and acrylamide (AM), with cetyltrimethylammonium bromide (CTAB) used to stabilize lauryl methacrylate (LMA) micelles in a deep eutectic solvent (DES)/H₂O binary solvent system. At higher temperatures, the hydrogel becomes opaque through phase separation driven by the lower critical solution temperature (LCST) of HPC. At lower temperatures, another optical transition seems to be governed by the growth of micelles formed from LMA. The temperature operating window can be tuned by changing the composition, keeping a rapid optical switching response of less than 30 s. Furthermore, due to the dynamic reversibility of hydrophobic associations and hydrogen bonding, the hydrogel exhibits excellent mechanical strength and self-healing capability at room temperature. The presence of the DES also contributes to its antifreezing performance, allowing the hydrogel to retain flexibility even at -20 °C. With its integrated functionalities, this material represents a highly promising candidate for smart window applications in real-world environments.