Influence of Molecular Weight Dispersity of Poly{2-(perfluorooctyl)ethyl acrylate} Brushes on Their Molecular Aggregation States and Wetting Behavior

Abstract

The relationships between the molecular aggregation states and water repellency of the perfluoroalkyl (Rf) groups of poly{2-(perfluorooctyl)ethyl acrylate} (poly(FA-C8)) brush thin films with broad and narrow molecular weight dispersities (MWDs) were analyzed by grazing incidence wide-angle X-ray diffraction (GI-WAXD) and water contact angle measurements. MWD-controlled-poly(FA-C8) brush thin films were prepared by surface-initiated atom transfer radical polymerization on a flat silicon substrate in the absence and presence of an ionic liquid. In-plane diffraction profiles of poly(FA-C8) brush films with narrow MWD had peaks corresponding to the periodic lengths of bilayer lamellae at qxy = 2–6 nm–1. This indicated that the orientation of Rf groups were parallel to the surface of the silicon substrate. In contrast, the peak of the in-plane GI-WAXD for brush films with broad MWD brush was confirmed at qxy = 12.5 nm–1, indicating that the Rf groups were oriented perpendicular to the surface of the silicon substrate. In the case of a poly(FA-C8) brush thin film with a narrow MWD, the receding contact angle (θR) and sliding contact angle (θS) versus water were ca. 80° and 35°, respectively, whereas θR and θS were 105° and 15°, respectively, at the surface of poly(FA-C8) with broad MWD. These results indicated that the water repellency of poly(FA-C8) brush surface largely depends on the molecular aggregation state and orientation of the Rf groups at the outermost surface and the MWD of the grafting polymer.