Dual temperature-sensitive hydroxypropyl cellulose/sodium carboxymethyl cellulose (HPC/CMC) composite hydrogel for smart window, information encryption and temperature monitoring

Thermochromic hydrogels have attracted considerable interest for their potential use in smart windows and information encryption. Nonetheless, it remains a challenge to achieve flexible and adjustable multilevel phase transitions through simple strategies. In this study, a thermoresponsive composite hydrogel with tunable upper critical solution temperature (UCST) and lower critical solution temperature (LCST) was developed by introducing sodium carboxymethyl cellulose (CMC) and hydroxypropyl cellulose (HPC) into a cross-linked polyacrylamide (PAM) network via a one-pot synthesis approach. CMC not only changed the phase transition temperature, but also greatly improved the mechanical properties, anti-freezing and dimensional stability of the hydrogel. The hydrogel smart window exhibited excellent sunlight modulation capabilities and effective infrared shielding effects (T lum  ≈ 90.24 %,ΔT solar,25–2°C  = 66.88 %,ΔT solar,25–40°C  = 80.81 %), which could simultaneously meet the needs of indoor lighting and energy saving during the daytime, as well as the need for privacy at night. Furthermore, multiple encryption of QR code information and temperature monitoring were realized by combining the hydrogels with different phase transition temperatures. This strategy provides an innovative route to fabricate thermochromic cellulose hydrogels and expands their application in the field of smart materials.