Investigation of photoinduced alignment mechanism of liquid crystal molecules using polyimide alignment layers

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We have investigated the photoinduced alignment mechanism of liquid crystal (LC) molecules based on the photo-decomposition reactions of polyimide. Unstable polymer fragments produced by ultraviolet (UV) irradiation remain in the polyimide film. We found that the unstable polymer fragments can be removed by a few minutes of ultrasonic cleaning. The in-plane anisotropy of the LC monolayer was found to be proportional to that of the underlying polyimide film, which was washed after irradiation of linearly polarized UV light (LPUVL). The proportionality factor was about 75%. This result suggests that the alignment of the LC molecules in contact with the LPUVL-exposed polyimide film is induced by a short-range interaction between the polyimide and LC molecules.We have also investigated the photoinduced alignment mechanism of LC molecules based on the photo-isomerization reactions of polyamic acid (Azo-PAA) containing azobenzene in the backbone structure. We determined the in-plane molecular order of the Azo-PAA film irradiated with LPUVL, as well as that of the polyimide (Azo-PI) film obtained by thermally imidizing it. We found that the photoinduced anisotropy in the in-plane molecular orientation was small, but it increased significantly after thermal imidization ; i.e. the in-plane molecular order of the Azo-PI film was much greater than that of the Azo-PAA film. The in-plane molecular order parameter of the LC monolayer was found to be almost equal to that of the underlying Azo-PI film. This result clearly shows that the LC alignment is induced by a short-range interaction between the polyimide and LC molecules. In addition, we confirmed that inclined alignment of the Azo-PI backbone structure can be induced by oblique angle irradiation of un-polarized UV light, and that this also induces tilted homogeneous LC alignment. We succeeded in generating the pretilt angle of 〜3°.