Reversible photo-controllable surface wettability of epoxy-polyurethane smart coating

Abstract

Light-responsive materials, in which a rapid and reversible response can be achieved, are of particular interest for several technological applications. The transformation of external stimuli into mechanical energy is very promising in the field of molecular motors and energy convertors. Azobenzene chromophores undergo photon-driven reversible cis-trans isomerization. In these stimuli-responsive materials, reversible molecular motion can be responsible for switching surface properties, such as surface wettability. In this work, the authors synthesized a fluorinated epoxy - isocyanate thermosetting coating (EPUF) modified with an azobenzene derivative (AZO). The increase in AZO content on the surface by migration during synthesis, as a consequence fluorine presence, leads to changes in the surface hydrophobicity, when UV light is used as a stimulus. First, polyurethane precursors (PUs) based on isophorone diisocyanate (IPDI), a fluorinated diol, and AZO were synthesized. Size-Exclusion Chromatography (SEC), Fourier Transform Infrared Spectroscopy (FTIR), and Ultraviolet-Visible Spectroscopy (UV) techniques were used to characterize the precursors. The polyurethane – epoxy reaction obtained allowed to prepare AZO modified epoxy networks. The resulting materials were characterized using different techniques. Fluorinated and pristine networks were compared. The changes in contact angle, roughness, and mechanical behavior by UV irradiation were analyzed. The reversible photo-induced switching of the surface properties of the fluorine-AZO modified coatings renders them potentially useful for multiple technological applications, in which cycling surface properties are of great use.