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Improved Vertical Alignment of Liquid Crystals with a Large Negative Dielectric Anisotropy

Abstract
Liquid 美国肯特州立大学液晶研究论文 crystals with a large negative dielectric anisotropy and large birefringence are desirable for vertical alignment applications. But they are difficult to align vertically on SiOxsurfaces. An interesting method has been found to generate high quality vertical alignment by doping the host liquid crystal with materials that have a cyan group along their long molecular axis. Using this method we propose a way to design new liquid crystal mixtures with large dielectric anisotropy and birefringence that have improved aligning ability for applications requiring vertical alignment.

1.Introduction
Vertically aligned nomadic liquid crystals with a negative dielectric anisotropy (Δε) have been widely used in many display applications mainly because of the high contrast ratio. The vertical alignment can be obtained on polyimide thin films. But in many applications like micro-displays and telecommunication devices, inorganic alignment materials such as six are preferred forgetter UV stability and less cosmetic defects due to rubbings. On another hand, liquid crystals with a large negative Δε are preferred because they require lower driving voltage and give faster response. Unfortunately, liquid crystals with a large negative Δεare very difficult to align vertically on six, sometimes even onpolyimides. In a previous letter we reported that if an appropriate amount of a positive dielectric material, such as 5CB is added tithe host material that has a large negative Δε, uniform vertical alignment can be easily achieved on both Six and polyimide (PI) surfaces. [1] Similar results have been reported by Went et al. [2]Based on this effect we suggest a means to design new liquid crystal mixtures with a large negative Δε but also the ability to form high quality vertical alignment. We also propose an explanation for this effect and provide experimental evidence to support this explanation.

2.Experimental Results
In our experiments we used two liquid crystal mixtures, LC#1and LC#2; and two do pants: 5CB, and 1-ethoxy-4-(4’-transpropylcyclohexylcarboxy)-2,3-dicyanobenzene (hereafter calledC3 whose chemical structure is shown in fig. 1 ). LC#1 has moderate negative Δε and forms vertical alignment on SiOx.LC#2, which has a large negative Δε forms non-uniform planar alignment instead

6. References
[1] Cheng Chen, James E. Anderson, and Philip J. Boss, Jon. J. pal. Phys., Vol. 44, No. 35, pp. 1126, 2005.
[2] Chien-Hui Went, Sebastian Gauze, Jun Li, Haying Wang, and hin-Tson Wu, Liq. Cyst., www.51lunwen.org/usa/ Vol. 32, No. 5, pp. 643, 2005.
[3] Xinhua Lu, K. H. Yang, T. Nakasogi, and S. J. Chewy, SID nil. Sump. Dig., Vol. 29, No. 4, pp. 446, 2000.