Photoreactive Langmuir–Blodgett–Kuhn multilayer assemblies from functionalized liquid crystalline side chain polymers. II. Structural characterization of homo‐ and copolymers containing azobenzene and phenylbenzoate moieties

Abstract

AbstractLiquid crystalline side chain polymers containing photoreactive azobenzene chromophores in the dark‐adapted trans form and phenylbenzoate groups as co‐mesogens have been prepared and investigated as monomolecular layers at the water/air interface and as multilayer assemblies deposited onto solid supports by the Langmuir–Blodgett–Kuhn (LBK) technique. Pressure area isotherms at variable temperatures together with reflection absorption spectra and Brewster angle microscopic pictures have been taken and analyzed (i) to derive information about the structural order and the packing properties of the mesogenic side groups within the monolayers of different homo‐ and co‐polymeric systems and ii) to optimize the conditions for the transfer of the monolayers.The resulting multilayer assemblies were primarily characterized by X‐ray reflectometry studies, which yield the film thickness, the electron density modulations indicative of any internal (double) layer periodicity, and two roughness parameters characterizing the substrate/film and the film/air interfaces, respectively. These investigations were complemented by surface plasmon, UV‐vis and IR spectroscopic measurements which provided further information as to the optical/structural properties of the samples.It was found that those systems containing only azobenzene side groups (or a high percentage of them) are composed of metastable double layer subunits which can be easily assembled to thicker films even capable of carrying waveguide modes. The azobenzene chromophores forming various types of aggregates have an average tilt angle of 36° relative to the film normal presumably with a broad angular distribution.If the azobenzene mesogens are diluted by phenylbenzoate co‐mesogens in the radio 1:2, only the first few dipping cycles show a good monolayer transfer so that only thin multilayer assemblies can be built up. Moreover, all these preparations though deposited in the Y‐type mode show no indication of a doublelayer structure in the transferred film, but instead exhibit the so‐called mesogen peak with a smectic periodicity corresponding to only a single monolayer. A structural model with interdigitating side groups is proposed to account for these experimental results.