Exposure of positive-type polyimide (PI) to linearly polarized deep UV (DUV) light (257nm) induced homogeneous alignment of nematic liquid crystals. The alignment state changed markedly at a particular energy of the UV light. Measurement of the UV absorption spectrum and retardation change generated in PI films by UV irradiation showed that anisotropic alignment was caused by a new mechanism, photodepolymerization of polyimide main chains parallel to the electric field of the DUV light.
We induced nematic monodomain homogeneous alignment by exposing positive-type PI to linearly polarized DUV light. The alignment direction was perpendicular to the electric field of the DUV, and its condition suddenly changed at 7J/cm2. We explained this phenomenon as resulting from a new mechanism, anisotropic photodepolymerization of PI.