X-RAY STUDY OF SUBSTRATE-INDUCED ALIGNMENT OF A SMECTIC-A LIQUID-CRYSTAL

We have performed a structural study of the liquid crystal (LC) octylc yanobiphenyl (8CB), deposited on gratings and flat surfaces, using hig h resolution X-ray scattering as a function of film thickness. 8CB is a room temperature smectic A2', with a layer spacing of 31.6 angstrom. Glass was used as substrate and treated with either one of the organi c surfactants MAP or DMOAP. Surface tension forces cause the liquid cr ystal molecules to align perpendicularly with respect to the plane of the substrate at the air interface. Competing with the LC-air interfac e, which is a strong aligner, a grating at the LC-substrate interface produces distortions in the smectic layering with an excess of elastic energy, which favours alignment parallel to the substrate and the gro oves. Our purpose was to detect the onset and evolution of parallel al ignment as a function of film thickness. The studies used 9 keV (1.403 angstrom) X-rays focused to a spot size of 2 mm2 at the sample positi on. In-plane scans, which detect the smectic layers perpendicular to t he plane of the substrate, were done at angles phi = 0-degrees and 90- degrees with respect to the gratings to ascertain the molecular orient ation, at a nominal X-ray incidence angle of alpha = 0-degrees. In ord er to observe regions of varying smectic layer orientation within the film, we performed a series of scans where the out-of-plane tilt angle chi changed from 0-degrees, corresponding to scattering in the plane of the film, to 90-degrees, which corresponds to scattering normal to the surface of the film. The results from these scans were fitted to a multilayer model where the orientation of the smectic layers varies a s a function of film depth. The analysis confirmed our earlier observa tions that surface tension at the air interface plays a dominant role in the alignment of the LC molecules.