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 po lyimide films exposed to linearly polarized ultraviolet light (LPUVL) at th e 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 f unction of the rotation angle of the sample around the surface normal. The SH signal has no rotation angle dependence, independent of the energy densi ty of exposure. On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated subst rates exposed to LPUVL at 8 J/cm(2). From these results we found that the i n-plane anisotropy of the LC monolayer in contact with the polyimide film i s very small, if any, even though the polyimide film can induce the homogen eous alignment of hulk LC. The average tilt angle of LC molecules in the mo nolayers in contact with the polyimide films was also determined. We found that the average tilt angle of LC molecules measured from the surface norma l decreases with the increase of UV exposure.