Alignment behaviour of the discotic nematic phase of 2,3,6,7,10,11-hexa (4-n-octyloxybenzoyloxy) triphenylene on polyimide and cetyltrimethylammoniumbromide coated substrates

The orientational behaviour of the nematic discotic phase of 2,3,6,7,10,11- hexa(4-n-octyloxybenzoyloxy) triphenylene ( C8OBT) on substrates coated wit h a polyimide or cetyltrimethylammonium bromide ( CTAB) was investigated by polarizing optical microscopy. The averaged order parameters and direction s of the triphenylene core and the carbonyl groups of C8OBT were evaluated by an infrared dichroic method. The discotic nematic (N-D) phase of C8OBT e xhibits a homeotropic alignment on a polyimide film, a typical nematic schl ieren texture on a glass substrate, and a tilted or planar homogeneous alig nment on a CTAB-coated substrate. The order parameter of the triphenylene c ore is higher on a polyimide film (S = 0.6) than on a CTAB-coated substrate (S = 0.2), whereas that of the carbonyl groups remains roughly constant at 0.2 to 0.3 independent of the substrate for the N-D phase.