X-ray study of liquid crystal alignment films and discotic liquid crystal strands

We present x-ray diffuse scattering measurements from freely-suspended strands of discotic liquid crystals, and x-ray reflectivity measurements of polyimide films on glass. The diffuse scattering measurements are the first quantitative measurements of the diffuse scattering from a discotic-hexagonal phase. We report a toroid of diffuse scattering in the basal plane, consisting of constant-$\vert$Q$\vert$ arcs surrounding the Bragg $\u3c$ 100 $\u3e$ points. The toroid has an anisotropic cross-section in the HOL plane, and exhibits a surprising sinusoidal variation in intensity as the $\phi$-angle is varied. We compare our results to the diffuse scattering expected from phonons in the discotic-hexagonal phase. We also report x-ray reflectivity measurements of mechanically rubbed polyimide films and dye-doped polyimide films that have been exposed to linearly-polarized laser light. We find anisotropic off-specular diffuse scattering in the mechanically rubbed films, which we attribute to grooves created by the rubbing process. Our x-ray reflectivity analysis suggests that the optically aligned dye-doped polyimide film has laterally distributed regions which are slightly thicker and much less electron-dense than the surrounding polyimide. An anisotropic roughness has been measured suggesting that these regions have some alignment. We suggest that the molecular-scale anisotropy in these regions gives clues to the mechanism of nematic liquid crystal alignment on the dye-doped optically aligned polyimides