Abstract Thermoresponsive polymer hydrogels have great potential in the fields of smart window and information encryption due to the drastic changes in transmittance caused by the phase transition at specific temperatures. The synthesis of thermoresponsive hydrogels with a wide range and sensitive temperature response is current research goal. Here, we demonstrate a polymethacrylamide (PMAAm) hydrogel with the upper critical solution temperature (UCST) ranging from 62 to −1°C by simply polymerizing the monomer in different concentrations of acetic acid (Ac) solutions. The hydrogel exhibits sensitive temperature response with a temperature hysteresis within 2°C and can maintain stable transmittance changes after 200 cycles of testing. The phase transition mechanism is revealed by combining ultraviolet visible spectra, differential scanning calorimetry, Fourier transform infrared spectra, dynamic light scattering and scanning electron microscope measurements. This thermoresponsive hydrogel can be applied in smart window and information encryption. The smart window prepared with the hydrogel had a very high ΔTlum (The luminous transmittance modulation, 94.4), ensuring daytime illumination and privacy protection at night. Meanwhile, the smart window exhibits an excellent thermal insulation capability for energy‐saving buildings. The hydrogels with different UCST are flexibly combined to display designed patterns at different temperatures, demonstrating the ability of the hydrogels to be used for information encryption.
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