Multifunctional cholesterol-based peroxide for modification of amino-terminated surfaces: Synthesis, structure and characterization of grafted layer

A multifunctional cholesterol-based peroxide modifier - monoperoxided monocholesteryl pyromellitate (PChP) with residual acid chloride groups has been synthesized. The structure of the peroxide modifier was confirmed using IR and 1H NMR spectroscopies. Grafted PChP coating was successfully fabricated onto amino-terminated glass surfaces. Thickness and refractive index of the coated layer, its morphology, wettability, and alignment of elongated PChP molecules, attached to the surface at different grafting times were characterized by means of ellipsometry, AFM, measurements of wetting contact angle and testing alignment of a nematic liquid crystal on the coatings. In a flat cell assembled of a pair of glass substrates coated with the PChP layer nematic molecules align preferentially tilted with respect to the cell normal, though in some place one finds homeotropic alignment, where nematic molecules are perpendicular to the surface. Liquid crystal textures visualize inhomogeneities in surface profile of the coated layer. Homeotropic alignment is observed in places where roughness of the layer is completely randomized in agreement with AFM data, providing visualization of evolution of the surface profile. Concentration of grafted molecules per area of the surface deduced from ellipsometry data suggests that PChP molecules are grafted to the surface rather densely already in about ten minutes.