Abstract

 

Uniform alignment of liquid crystals on treated substrate surfaces is very important for both LC fundamental research and application. The alignment layer plays a very important role in the performance of the liquid crystal display (LCD). Nowdays, high pretilt angle and high stable alignment layers are required in the advanced LCDs, such as super twisted nematic LCD (STN-LCD) and surface-stabilized ferroelectric LCD (SSF-LCD). We have explored using ladder-like polysilsesquioxanes (LPS) as a novel material for high performance alignment layer because of its following advantages: excellent transparency, chemical and themal stability, favorable mechanical properties and easy-modification.

 

The commercially used alignment material is polyimide (PI), but PI has some drawbacks, especially the high curing temperature can do damage to thin film transistors (TFTs) and color filters. A new alignment film based on LPS was fabricated by rubbing method. The curing temperature (150-200 °C) of LPS-based film is much lower than that of the traditionally used PI (250-300 °C). The pre-tilt angles of a nematic liquid crystal on the surface, 4-pentyl-4'-cyanobiphenyl (5CB), can be adjusted, depending on the rubbing conditions and the side chains. The annealing experiments at 100 °C and 180 °C demonstrated that the stability of these orientation films is quite well. In particular, a combined method of rubbing and photo-modification based on the photo-sensitive LPS was developed to achieve high pretilt angle of ~10°. The generation of high pretilt angle can be ascribed to the photo-isomerization of azobenzene groups lied on the surface and it can be fixed by cycloaddition of adjacent cinnamate groups. In addition, this convenient light modification technique can be used to produce patterned (multi-domain) alignment layers for improving view angle in LCD.

 

Recently, an advanced linearly polarized photopolymerization (LPP) method for preparing alignment layers was developed to overcome the drawbacks of mechnical rubbing, such as generating dusts and static charge. Cinnamate-grafted LPS was used to prepare photo-alignment layers by LPP method. The results show that the obtained alignment layer has good aligning ability and excellent thermal stability, so heralding their potential application in LCDs.

 

The 'command layer' based on photo-isomerizaton of azobenzene provides a developing prospect for a novel photo-driving mode LCD. The azobenzene-grafted LPS was used to improve the stability of the command layer. Moreover, high image quality and response sensitivity of the photo-driven cell were achieved by two ways: pre-rubbing of the command surface and applying an assisting critical in-plane mode electric field. The results show that the rise-time (4 seconds) under a weak UV intensity of 0.5 mW/cm² is shorter than that in previous report (several tens of seconds) under a UV intensity of 3-5 mW/cm². The improved photo-driven LC cell gives promise of potential application on photo-addressing or recording.

 

摘      要

 

无论对于液晶的理论研究还是在液晶显示中的应用，液晶在基板表面的均一排列都是很重要的。液晶的定向排列取决于液晶分子与基板表面之间的界面相互作用，即“锚泊作用”。预倾角——指液晶指向矢与基板表面的夹角，是液晶显示中的关键因素。目前，在高性能的液晶显示，如超扭曲向列相液晶显示、表面稳定的铁电液晶显示中，需要有高预倾角的液晶定向膜。

 

梯形聚硅氧烷作为一种新的类玻璃状成膜材料，具有优异的透明性，化学和热稳定性，良好的机械性能，并且易于进行结构修饰等优点。本论文工作的主题是研究以梯形聚倍半硅氧烷为基材制备液晶定向膜。

 

聚酰亚胺是目前工业上常用的液晶定向材料，但它的高的固化温度对液晶显示器中的彩色滤光片及薄膜晶体管等易造成损坏。我们采用摩擦定向工艺制备了以梯形聚硅氧烷为基材的液晶定向膜。这种液晶定向膜的固化温度为 150－200℃，比工业上通常所用的聚酰亚胺的固化温度(250℃～300℃) 要低得多，依摩擦条件和侧基结构不同，向列相液晶(5CB) 在所得定向膜表面的预倾角可在 2－8° 之间变化。在100℃和180℃的退火实验表明，这种定向膜的稳定性很好。特别重要的是，基于接枝偶氮和肉桂酸酯侧基的梯形聚硅氧烷，我们研究了一种摩擦和紫外光照射相结合获得高预倾角定向膜的新方法。经过摩擦处理后，定向膜的初始预倾角为1-3°，通过紫外光照射，预倾角增加到10° 左右。所得的高预倾角设想是由于偶氮侧基的光异构化导致，并由邻近的肉桂酸酯侧基原位光交联将其固定化，从而获得了稳定的高预倾角。AFM 的结果证实了上述设想。值得一提的是，这种方便的光修饰方法可以用来产生有花样的(多畴)液晶定向膜以提高液晶显示器的视角。

 

最近，一种先进的液晶定向膜制备方法——线性偏振光定向方法(LPP)——被用来克服摩擦方法带来的缺点，例如产生灰尘和静电等。我们由肉桂酸酯接枝的梯形聚硅氧烷制备了稳定的液晶定向膜。用偏光显微镜、锥光干涉观察以及电光响应测试等方法对液晶光定向膜的定向行为进行了表征，特别是通过严格的退火测验检测了定向膜的热稳定性。退火实验表明，即使液晶盒在100℃(远高于液晶清亮点)退火数小时，当温度降到室温时，液晶盒内的液晶仍能保持很好的排列状态。证明基于梯形聚硅氧烷的液晶光定向膜不仅有良好的液晶定向能力，并且有优异的热稳定性，预示了将来在液晶显示方面的应用。

 

基于偶氮苯基元光异构化的“指令层”预示了新的光驱动模式液晶显示的开发前景。我们由接枝偶氮基元的梯形聚硅氧烷制备了光活性的定向层——“指令层”——并由此组装了紫外光驱动液晶盒。该指令层具有好的稳定性和重复使用寿命。从实用器件的要求：均一的画面和足够的响应灵敏度(阈值光强和响应速度)，从两个方面进行了光驱液晶盒的改进：一是对定向层作预摩擦处理，另一方面对液晶盒加一辅助沿面电场(该电场强度控制在电光响应阈值以下，目的是使液晶分子“预活化”)，结果使光驱动液晶盒的灵敏性得到了显著提高，降低了驱动所需的紫外光强，并且加快了响应速度。结果显示在弱的(0.5 mW/cm²)紫外光照射下，励起时间(4 s)远小于以前报道的在较强(3-5 mW/cm²)紫外光下的数值(数十秒)。

 

以上几个方面的工作，已在 SCI 收录杂志上发表(包括已接受) 5 篇，并已申请专利两项。通过以上研究加深了对表面锚泊作用和预倾角形成及变化机理的认识，在实用上提供了一种高稳定性的和预倾角易调节的液晶定向膜材料，同时为新的光驱动模式的液晶显示开拓了一条可实用化的途径。