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

基于聚酰亚胺的光分解反应，研究了液晶分子的光致取向机理。紫外线照射产生的不稳定聚合物碎片留在聚酰亚胺薄膜中。我们发现不稳定的聚合物碎片可以通过几分钟的超声波清洗去除。经线偏振紫外光（LPUVL）照射后，发现LC单层的面内各向异性与底层聚酰亚胺膜的各向异性成正比。比例因子约为75%。这一结果表明，LC分子与lpuvl暴露的聚酰亚胺膜接触时的排列是由聚酰亚胺与LC分子之间的短程相互作用诱导的。我们还基于主链结构中含有偶氮苯的聚酰胺酸（Azo-PAA）的光异构化反应研究了LC分子的光致取向机制。我们测定了用LPUVL辐照得到的Azo-PAA膜的面内分子序，以及热酰化得到的聚酰亚胺（Azo-PI）膜的面内分子序。发现光致各向异性在分子面内取向上很小，但热亚酰化后各向异性显著增加；即偶氮- pi膜的面内分子有序度远高于偶氮- paa膜。发现LC单层的平面内分子序参数与底层偶氮- pi膜的分子序参数几乎相等。这一结果清楚地表明，LC排列是由聚酰亚胺与LC分子之间的短程相互作用引起的。此外，我们证实了在非偏振光的斜角照射下Azo-PI的主链结构可以发生倾斜取向，这也导致了倾斜的均匀LC取向。我们成功地产生了~ 3°的预倾角。

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°.