New insight into photoalignment of liquid crystals on coumarin-containing polymer films

Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the parts of the reacted and the unreacted coumarin moieties. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and the unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction.