Liquid Crystal Alignment Induced by Polyimides Exposed to Linearly Polarized Light

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Through this study on photoalignment (non-robbing alignment) technique for liquid crystal (LC) devices, we have revealed that aromatic polyimides (Pis) exposed to linearly polarized UV light (366 nm) show a high alignment efficiency without significant change in the chemical structure of PIs. We extended the PI films as an alignment layer to dynamic control technique of nematic LCs (NICs). The most significant characteristic of this system is to utilize photoinduced Freedericksz transition as a trigger to control alignment of NICs. We realized the optical switching of with the photosensitive PIs containing benzophenone moiety as an alignment film. It became clear that the threshold voltage of Freedericksz transition, at which change in the LC alignment occurs, is quite different between irradiated and unitradiated conditions. It was found that photoirradiation at 366 nm resulted in the immediate change in the transmittance when bias voltage of 4.5 V was applied across the cell. Although the mechanism is not well understood at the present stage, formation of intra- or intermolecular charge transfer complexes in the surface area of PI films by photoirradiation ( i.e., change in the polarity) seems to participate in the phenomenon. In conclusion, we have shown for the first time that the photosensitive PI films enable optical control of Freedericksz transition of NLCs. This phenomenon is useful for development of polarization-controllable materials in which permanent electric polarization is controlled by change in dipolar alignment based on Freedericksz transition.