Bidirectional Thermoresponsive Window via a Facilely Tailorable Double-Layer Hydrogel

Smart windows can dynamically adjust their transmittance in response to environmental conditions. Thus, they provide optimal indoor temperatures, reduce reliance on air conditioning and other equipment, thereby minimizing energy consumption. However, traditional smart window materials encounter difficulties in simultaneously achieving energy-saving and privacy functions, as the precise control of phase transition temperature remains challenging. This limitation prevents smart windows from being tailored to meet the specific requirements of various application scenarios. In this study, we developed a bidirectional optically responsive double-layer hydrogel (DHG) for smart windows by integrating two independent functional layers: a molecularly functionalized cellulose-based hydrogel acting as the lower critical solution temperature (LCST) layer and an acrylamide (AM)/acrylic acid (AA) copolymer hydrogel serving as the upper critical solution temperature (UCST) layer. The LCST (23.1-34.2 °C) and UCST (1.0-16.2 °C) were independently and precisely tuned by modifying the molar substitution degree of 2-hydroxy-3-butoxypropyl hydroxyethyl cellulose and adjusting the AM/AA molar ratio, respectively. This structural and functional independence significantly enhances the processing and customization of the smart windows. DHG smart window exhibits excellent visible light transmission (T_lum = 82.14%) and remarkable solar modulation (ΔT_sol, LCST = 76.60%; ΔT_sol, UCST = 75.28%). During daylight hours, it permits transmission of both ultraviolet and visible light when temperatures are within the comfort range, satisfying indoor illumination requirements. When temperatures exceed this range, the window blocks incident solar radiation, significantly enhancing thermal insulation. Under low nighttime temperatures, light transmission is blocked, providing nighttime privacy protection. Compared to traditional windows, DHG smart windows provide superior thermal insulation. Moreover, their precisely controllable phase transition temperature makes them ideal for intelligent temperature monitoring applications.