The photoalignment of a series of monodisperse glassy-nematic oligofluorenes was investigated on coumarin-containing
polymer films in the parallel regime. The orientational order parameter of a spin-cast oligofluorene film,
SOF, was found to decrease with an increasing oligofluorene length because of the increasing demand on YdSd, the
product of coumarin dimers' concentration and orientation order parameter. In addition, an increased annealing
temperature demanded by a longer oligofluorene caused an orientational relaxation of coumarin dimers to a greater
extent. Nonetheless, under favorable conditions SOF values of penta, hepta-, and nonafluorenes are comparable to
those achieved on rubbed polyimide 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 the extent of dimerization, X. The experimental data for X as a function of fluence were
used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. The
kinetic model was employed to describe the evolutions of coumarin dimer's and monomer's orientational order. 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 was successfully interpreted by
considering three factors: the relative abundance of coumarin dimers to monomers, their orientational order parameters,
and the energetics of molecular interaction.
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