Surface induced liquid-crystal alignment studied by optical second-harmonic generation

The orientational order induced in a liquid crystal (LC) monolayer of adsorbed 4-n-octyl-4′-cyanobiphenyl molecules on rubbed substrates is studied by measuring the second-harmonic response. By systematically varying the rubbing conditions and measuring the induced LC bulk and monolayer alignment, we can conclude that the homogeneity of the LC bulk alignment and the surface in-plane order parameter of the LC monolayer are closely related. The polar and azimuthal ordering of the LC monolayer, however, are found to be decoupled. By rubbing polyimide films twice and studying other rubbed surfaces, we show that two mechanisms are responsible for the LC alignment, depending on the kind of substrate that is used. For rubbed bare glass substrates long-range elastic interactions, which do not affect the monolayer alignment, are dominant. In the case of rubbed polyimide films molecular short-range interactions are responsible for the alignment of both the LC bulk and the LC monolayer.