Optical anisotropy of Langmuir-Blodgett films of long-chain acetylenic acids

The optical anisotropy of Langmuir-Blodgett films, along with its variation in the course of polymerization under UV radiation, is studied by ellipsometry at a fixed wavelength of 632.8 nm for several angles of incidence of light. Films of the Cd and Pb salts of acetylenic acids with internal and terminal triple bonds—HC≡ C(CH2)21COOH (I) and CH3(CH2)18C≡CCOOH (II)—are studied. The refractive indices and thicknesses of the films are determined on the basis of a model of a biaxial anisotropic film. It is found that the films of the Pb salt of acid II are biaxial, while the films of the Cd salt of acid II and of the Cd and Pb salts of acid I have nearly uniaxial anisotropy. The calculated thicknesses of monolayers coincide with the thicknesses determined from small-angle x-ray scattering. The anisotropy of the films is shown to vary under UV irradiation. After polymerization of the films, the anisotropy in the plane of the layer virtually vanishes, while that in the direction perpendicular to the interface is retained.