Investigation of an oleophobic-hydrophilic polymer brush with switchable wettability for easy-to-clean coatings

Abstract

The combination of different wetting states, such as hydrophilic and oleophobic, in an adaptive polymer layer provides new opportunities in the field of easy-to-clean and anti-fog coatings. In this work, the synthesis of new end-functionalized well-defined block copolymers consisting of a hydrophilic poly(acrylic acid) (PAA) block and an oleophobic poly(1,1,1,3,3,3-hexafluoroisopropyl methacrylate) (PHFIPMA) block of different block length was investigated. This was achieved by parallel synthesis of the single blocks separately using atom transfer radical polymerization (ATRP), and subsequent block coupling by copper(I)-catalyzed click chemistry. A hydroxy end-group functionalization allowed these polymers to be assembled into dense polymer brushes via a grafting-to approach. The architecture of theses brushes and the orientation of the chains within the films were studied using angle-resolved X-ray photoelectron spectroscopy (ARXPS). To study the influence of the lengths and ratio of the two blocks on the switching of the wettability, contact angle measurements were performed demonstrating the correlation between structure and functionality. It was found that an increasing length of the hydrophilic block, while keeping the length of the oleophobic block constant, leads to an increase of the switching amplitude. Molecular Dynamic Simulations, which gave deeper insights into changes of the brush architecture during switching, enabled in combination with chemical analysis at the nanometer scale (ARXPS) conclusions about the influence of molecular rearrangements inside the brush on macroscopic changes at the outer surface of the brush due to external stimuli. Finally, oil spray-off experiments demonstrated the potential of these new adaptive polymer brushes as easy-to-clean coatings.