Fast thermochromic hydrogel smart windows based on physical chain entanglement

The development of thermochromic hydrogels for smart windows faces challenges in balancing optical performance, durability, and environmental adaptability. Traditional chemically crosslinked hydrogels suffer from low transmittance and structural instability during phase transitions. Herein, we propose a physical chain-entangled thermochromic hydrogel (PNA) via a one-pot synthesis of polyvinyl alcohol (PVA), poly(N-isopropylacrylamide) (PNIPAM), and polyacrylic acid (PAA). PVA provides a network framework for the temperature-sensitive phase transition of PNIPAM, enabling a rapid (5 s) switch of the hydrogel from a highly transparent state ( T lum ≈ 95 %) to an opaque state ( T lum  < 20 %). By modulating PAA content, the lower critical solution temperature (LCST) is precisely tuned to 25 °C, aligning with human thermal comfort. Unlike traditional chemically cross-linked hydrogels, this physical chain-entangled hydrogel has a uniform pore structure, ensuring that the light transmittance of the hydrogel is independent of its thickness. This PNA smart window has demonstrated outstanding stability in long-term cycle tests (heating–cooling/ultraviolet aging), with a reduction in light transmittance of less than 5 %. DSC analysis and transmittance analysis show that this smart window has good frost resistance under sub-zero climate conditions (−20 °C freezing point). In the practical application simulation of PNA smart windows, it has reduced the box temperature by 10 °C compared with ordinary glass, demonstrating outstanding application potential in the field of energy-saving building materials.