Molecular Aggregation State of Surface-grafted Poly{2-(perfluorooctyl)ethyl acrylate} Thin Film Analyzed by Grazing Incidence X-ray Diffraction

Abstract

Surface-initiated atom transfer radical polymerization of 2-(perfluorooctyl)ethyl acrylate on a flat silicon wafer was carried out to give poly{2-(perfluorooctyl)ethyl acrylate} (PFA-C8) brush thin film with three different thicknesses of 4, 11, and 43 nm, respectively. The water contact angle of the PFA-C8 brush surface was 120°. The molecular aggregation state of the perfluoroalkyl (Rf) group of PFA-C8 brush was analyzed by X-ray reflectivity (XR), wide-angle X-ray diffraction, and grazing incidence X-ray diffraction (GIXD) measurements. XR analysis revealed that Rf groups at the air/brush interface formed a densely packed structure, while a relatively low-density region was generated at the brush/substrate interface. The peaks of in-plane GIXD for brush films with thicknesses of 11 and 43 nm were observed at qxy=12.5 nm−1, which indicated that Rf groups at the outermost surface oriented perpendicular to the surface of silicon substrate. In an out-of-plane diffraction profile of the 43 nm-thick PFA-C8 brush film, peaks corresponding to a periodic length of the bilayer lamellae were observed. Therefore, Rf groups of the thicker brush film were crystallized and formed ordered bilayer lamellar structure at the outermost surface. In contrast, no diffraction pattern was observed from the PFA-C8 at a thickness of 4 nm by WAXD and GIXD. These results indicate that an amorphous layer was formed at the interface of the brush/substrate. The Rf groups at the anchoring region of the brush could not form a sufficiently ordered structure due to immobilization of brush chain ends on the substrate. It was suggested that the Rf groups in a PFA-C8 brush thin film at the outermost surface aggregated in a different manner from those in the anchoring region.