Photoalignment of a nematic liquid crystal fluid and glassy-nematic oligofluorenes on coumarin-containing polymer films

The orientations of both a nematic liquid crystal fluid and a series of monodisperse glassy-nematic oligofluorenes were investigated on photoalignment films comprising a polymethacrylate backbone with 7-benzoyloxycoumarin pendants. Both classes of liquid crystalline material were found to undergo a transition from a parallel to a perpendicular orientation with reference to the polarization axis of UV-irradiation at a sufficiently high extent of dimerization. The UV-vis and FTIR spectroscopic analyses revealed photostability with irradiation up to a fluence of 10 J/cm(2), thereby excluding photodegradation as the basis for the observed crossover. A kinetic model was used to interpret the crossover behavior for irradiation at 25 degrees C, leading to the conclusion that liquid crystal molecules interact more favorably with coumarin monomers than with dimers. Through thermal annealing above T-g followed by cooling to room temperature, a glassy-nematic pentafluorene film was prepared on a photoalignment film, exhibiting an orientational order parameter comparable to that on a rubbed polyimide film. At an increasing oligofluorene chain length, however, a decreasing orientational order parameter emerged, presumably because of the increased annealing temperature that causes an orientational relaxation on the part of the dimerized coumarin moieties.