Liquid Crystal Alignment Induced by Polyimides Exposed to Linearly Polarized Light

通过对液晶（LC）器件的光定向（非偏移定向）技术的研究，我们发现芳香聚酰亚胺（Pis）暴露在线偏振紫外光（366nm）下具有很高的定向效率，而Pis的化学结构没有明显变化。我们将PI薄膜作为对准层扩展到向列型lc （nic）的动态控制技术。该系统最大的特点是利用光致fredericksz跃迁作为触发器来控制NICs的对准。我们利用含有二苯甲酮片段的光敏pi作为对准膜实现了光开关。很明显，在辐照和单辐射条件下，发生LC排列变化的fredericksz跃迁的阈值电压有很大的不同。结果表明，当施加4.5 V的偏置电压时，在366nm的光照射下，透射率立即发生变化。虽然目前还不清楚其机理，但通过光辐射在PI膜表面形成的分子内或分子间电荷转移配合物（即极性的改变）似乎参与了这一现象。总之，我们首次证明了光敏PI薄膜能够光学控制nlc的fredericksz跃迁。这一现象有助于开发基于fredericksz跃迁的通过改变偶极排列来控制永久电极化的极化可控材料。

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.