Surface-Active Thermally Responsive Hydrogels by Emulsion Sedimentation for Smart Window Applications

Thermally responsive polymers are a subject of increasinginterestin research and development as a basis for a potential smart windowtechnology. Here, we present a concept of preparing thermally responsivehydrogels with a thin active surface layer exhibiting rapid and reversibleswitching of light scattering in the visible and near-infrared spectralranges. The process relies on the forced emulsion formation and sedimentationfrom the aqueous prepolymer solution by using a crosslinker that isengineered to serve as an antisolvent for the prepolymer and at thesame time exhibit a suitable solubility profile in the sedimentedhydrogel layer with respect to the supernatant aqueous phase. Whilethe method can be employed for different polymer and crosslinker systems,as an example, here, we employ this concept for preparing thermallyresponsive hydrogels based on ethoxylated trimethylolpropane tri(3-mercaptopropionate)(ETTMP) and glycerol-derived crosslinkers with a dimaleate functionality,enabling crosslinking by the thiol-Michael click reaction. The materialexhibits a luminous transmittance of over 95% and solar energy modulationof 59.91%. Moreover, we show that the pH and additives in the aqueousoperating solution of the hydrogel enable the choice of the transitiontemperature in a wide range. The unique thin layer on the surfaceof the hydrogel, scalability to large surface areas, and robust andfast response at the practically relevant temperature range give thismaterial a strong potential for smart window technology applications.