Uniaxial orientation of columnar discotic liquid crystals

The present paper explores the possibility of orienting columnar discotic liquid-crystalline (LC) materials and causing a uniaxial "in-plane" orientation of the columns along the orientation direction. The discotic LCs employed were based on 2-hydroxy-3,6,7,10,11-penta-(1-butoxy)triphenylene, which was derivatized with 1-adamantenecarboxylic acid (Ada-PBT) and 10-undecenoic acid (omega-undecenoyl-PBT), respectively. As unequivocally shown by optical microscopy, uniaxial alignment of columnar Ada-PBT structures was readily achieved by spin-coating this material onto a poly(tetrafluoroethylene) orientation layer. Annealing of the films at temperatures at which the Ada-PBT was in its hexagonal plastic or hexagonal ordered mesophase slightly increased the degree of orientation. The optical absorption and photoluminescence spectra of oriented Ada-PBT films were significantly polarized, with absorption and emission dichroic ratios of 6.5 and 3, respectively. The optical anisotropy is unambiguously related to the uniaxial "in-plane" orientation of the cylindrical stacks of the LC molecules rather than an alignment of individual Ada-PBT molecules. Interestingly, this orientational behavior could not be mirrored when attempting to orient blend films of ultrahigh molecular weight polyethylene (UHMW-PE) and Ada-PBT or omega-undecenoyl-PBT by tensile deformation.