Orientation control of sub-10-nm lamellar structures in amphiphilic copolymer films via water annealing

Abstract

The fabrication of polymer lamellar films with sub-10 nm periodicity and perpendicularly aligned structures is essential for applications in lithography and separation membranes. In this study, annealing an amphiphilic statistical copolymer, poly(N-octadecyl acrylamide-stat-N-(2-hydroxyethyl acrylamide)), in water induces the formation of perpendicularly oriented lamellae, where the lamellar planes align perpendicular to the substrate. The statistical copolymer with a 1 : 1 comonomer composition is synthesized via free-radical polymerization, and thin films are prepared by spin coating. The nanoscale phase-separated structures formed by annealing in water are analyzed. Structural analysis using two-dimensional grazing-incidence small-angle X-ray scattering reveals that the annealed films exhibit both parallel and perpendicular lamellae with the periodicity of 4.2–5.7 nm, and the formation of perpendicular lamellae was strongly influenced by direct contact with water. Furthermore, depth-dependent XRD studies using low-energy grazing-incidence small-angle X-ray scattering show that parallel lamellae form at the topmost film surface, while perpendicular lamellae develop in the interior. Water contact angle and Fourier-transform infrared spectra measurements further confirm this structure. Exposure to water induces alignment of the hydrophobic octadecyl side chains parallel to the substrate, promoting the formation of perpendicular lamellae in the film interior. In addition, experiments with thin films sandwiched between hydrophilic or hydrophobic glass substrates demonstrate that the nature of the interface plays a crucial role in determining the lamellar alignment. These findings emphasize the importance of interface modification in controlling thin-film structures and provide insights for designing functional materials with tailored properties.