Surface Orientation of Polyimide Alignment Layer Studied by Optical Second-Harmonic Generation

We have determined the orientational distribution function (ODF) of the side chains of polyimide films for liquid crystal alignment. To determine ODF, optical second-harmonic generation was measured as a function of sample rotation angle. Using nonlinear susceptibility components obtained by a fitting procedure, ODF was obtained by means of information entropy. The symmetry of the polymer side chains changes from C∞v to Cs by rubbing treatment. Asymmetry of the polar angle along the rubbing direction is induced, though the change is very small.