Pretilt Angle Generation on Photoreactive Polymer Films

The mechanism of liquid crystal pretilt angle generation on photoreactive polymer films doubly exposed to linearly polarized ultraviolet (uv) light has been investigated. The first exposure for time ${\ensuremath{\tau}}_{1}$ is normal to the surface, and the film is subsequently irradiated for time ${\ensuremath{\tau}}_{2}$ with obliquely incident uv light with a plane of polarization rotated by $90\ifmmode^\circ\else\textdegree\fi{}$ with respect to that of the first uv light. The state of the polymer film after two exposures was characterized with a tensor order parameter, which then served as a boundary condition for the surface state of the nematic liquid crystal. The bulk liquid crystal behavior was investigated using Landau--de Gennes theory extended to include the possibility for weak surface interactions linear in the tensor order parameter. The pretilt angle was calculated as a function of both ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2},$ and as a function of the second uv light angle of incidence. The calculated behavior of the pretilt angle was found to be in qualitative agreement with experimental observations. Conclusions about the strength of the anchoring have also been made.