Self-adhesive thermotropic smart films for adaptive solar control under various climate conditions

Energy conservation has become a crucial concern for reducing carbon dioxide emissions; thus, smart windows that selectively control incident solar radiation are attracting considerable attention. While extensive research has worked towards rendering the adaptive modulation of optical transparency, the wide application of traditional glass-type smart windows is limited by their high-cost installation that requires the removal of existing glazing. In this work, we report the preparation of a smart film that can be easily attached to any surface shape. A multilayer smart film comprising a transparent polyethylene terephthalate substrate, a thermotropic poly(N-isopropylacrylamide) (PNIPAm) hydrogel, an acrylic adhesive, and a release film was prepared based on robust interface chemistry to ensure its stability. Field tests for optical switching between transparent and opaque states were demonstrated in response to the outdoor temperature. Furthermore, the developed smart film could be cut into specific shapes, namely letters, and attached to glass windows, indicating its potential to display various forms of information. The switching temperature could be finely tuned for various climatic conditions by copolymerizing the PNIPAm hydrogel with hydrophilic or hydrophobic comonomers. In addition, the attached film can be easily removed on demand without leaving any residue.