Molecular orientation of liquid crystal monolayers on polyimide films exposed to linearly polarized UV light

By using optical second harmonic generation (SHG), we have investigated the molecular orientation of liquid crystal (LC) monolayers in contact with polyimide films exposed to linearly polarized ultraviolet light (LPUVL) at the wavelength of 266 nm. For different exposures at 3, 8, 20, and 80 J/cm(2), the second harmonic (SH) signal from the LC monolayer was measured as a function of the rotation angle of the sample around the surface normal. The SH signal has no rotation angle dependence, independent of the energy density of exposure. On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated substrates exposed to LPUVL at 8 J/cm(2). From these results we found that the in-plane anisotropy of the LC monolayer in contact with the polyimide film is very small, if any, even though the polyimide film can induce the homogeneous alignment of hulk LC. The average tilt angle of LC molecules in the monolayers in contact with the polyimide films was also determined. We found that the average tilt angle of LC molecules measured from the surface normal decreases with the increase of UV exposure.