Influence of molecular weight dispersity of poly{2-(perfluorooctyl)ethyl acrylate} brushes on their molecular aggregation states and wetting behavior

The relationships between the molecular aggregation states and water repellency of the perfluoroalkyl (R _f) groups of poly{2- (perfluorooctyl)ethyl acrylate} (poly(FA-C ₈)) 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-C ₈) 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-C ₈) brush films with narrow MWD had peaks corresponding to the periodic lengths of bilayer lamellae at q _xy = 2-6 nm ^-1. This indicated that the orientation of R _f 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 q _xy = 12.5 nm ^-1, indicating that the R _f groups were oriented perpendicular to the surface of the silicon substrate. In the case of a poly(FA-C ₈) brush thin film with a narrow MWD, the receding contact angle (θ _R) and sliding contact angle (θ _S) versus water were ca. 80° and 35°, respectively, whereas θ _R and θ _S were 105° and 15°, respectively, at the surface of poly(FA-C ₈) with broad MWD. These results indicated that the water repellency of poly(FA-C ₈) brush surface largely depends on the molecular aggregation state and orientation of the R _f groups at the outermost surface and the MWD of the grafting polymer.