Photocontrol of liquid crystal alignment by polymethacrylates with diphenylacetylene side chains

Whereas diphenylacetylene (DPA) exhibits no photoisomerization because of t he involvement of a triple bond in the chromophore, prolonged irradiation o f the films with 313-nm light resulted in the reduction of absorption, lead ing to insolubilization as a result of photocross-linking. When the actinic light is linearly polarized, polarized UV absorbances perpendicular to the electric vector of the light decreased more preferentially than those in p arallel with that to generate dichroism. Homogeneous alignment of LCs was i nduced, when cells were fabricated with substrate plates covered with thin films of DPA polymers, followed by linearly polarized light irradiation at temperatures above the transition temperature of LC. Irradiation of thin fi lms of DPA polymers before cell assembly was more convenient because much l ess exposure doses were required for the LC photoalignment control. The pho togenerated LC alignment was highly thermally stable and was not altered ev en after heating at 100 degrees C for 1 week. Discussion was made on the me chanism of the photocontrol of LC alignment on the basis of the photochemic al behavior of the DPA polymers.