Orientation of liquid crystal molecules evaporated onto rubbed and photoaligned polymer surfaces

Second-harmonic generation (SHG) and polarized ultraviolet-visible (UV-VIS) absorption have been used to study the orientational distribution function of 5CB (4'-n-pentyl-4-cyanobiphenyl) liquid crystal (LC) molecules adsorbed on rubbed and photoaligned photoisomerizable polyamic acid layers. The simultaneous analysis of SHG and polarized UV-VIS absorption data using the maximum entropy method shows that the anisotropic distribution of 5CB molecules along the rubbing direction tilts up from the surface plane as the molecular density increases. More interestingly, the anisotropy is relatively low at the first stage of the evaporation and becomes gradually pronounced, indicating an important role played by the long-range intermolecular interaction among LC molecules for LC surface alignment. No orientation formation of non-liquid crystalline 4CB (4'-n-butyl-4-cyanobiphenyl) molecules on the treated surfaces supports this conclusion. It is important to emphasize that a relatively small fraction of molecules contribute to give a polar order, but the rest form dimers even at the early stage of the evaporation. This tendency is particularly marked on the photoaligned surface; no SHG activity arises irrespective of the amount of evaporated molecules.