A spiropyran-based polymer with a stimulus response to water temperature and water content

Abstract

Temperature-responsive spiropyran-functionalized polymers usually require a thermo-sensitive polymer. However, their temperature response range is limited by the lower critical solution temperature (LCST) of the thermo-sensitive polymer, which does not exceed 37 °C. In this work, a hydrophilic polymer (PHEA-SP) sheet was prepared by photo-initiated copolymerization of hydroxyethyl acrylate (HEA) and a spiropyran crosslinking agent (SP). In water, swelling and hydrogen bonding can increase the ring-opening isomerization probability of spiropyran at the PHEA-SP crosslinking point, thus amplifying the discoloration of spiropyran induced by temperature change. PHEA-SP is very responsive to water temperature in the range of 25–55 °C, due to the amplification of spiropyranoid discoloration described above. This method avoids the dependence of the temperature responsive spiropyran-functionalized polymer on a thermo-sensitive polymer and additional UV light, while increasing the upper limit of the water temperature response to 55 °C. In addition, PHEA-SP also shows responsivity to water content in ethanol solution from 0.3% to 100%.