Study of Nematic Liquid Crystal Alignment using Surface Control

Abstract

The directional control of anisotropic molecules have attracted much attention to the organic electronics including liquid crystal displays (LCDs) because it is important to the fundamental research and has a potential for device applications. Particularly, LCs are very useful in several key areas of flat panel displays and optical devices. LCs are orientationally ordered as a result of steric constraints. The alignment of LC molecules which are governed by contact with solid substrates of different chemical constitution plays an important role in understanding and improving the electro-optical properties of LC devices. It has long been known that LC orientation can be directed by the anisotropy of a surface, which can be introduced by surface modifications using various contact and non-contact methods. However, these methods produce either planar or vertical alignment of the LC molecules, not tilted alignment between planar of vertical one. Various additional applications are available if we can easily control the pretilt angle (tilt angle of LC on the surface) over the whole rage (i.e., 0?a~90?a). Various methods have been studied to control the pretilt angle over the whole range. Though control of the pretilt angle from 0?a~90?a was achieved, there are many problems such as reproducibility, thermal stability, adoptability to large size devices. In this thesis, surface controlling methods are investigated to align the nematic LC molecules from the viewpoints of interesting physical phenomena as well as practical applications. We focus on the controlling method of continuous pretilt angles from 0?a to 90?a with two kinds of methods, stacking of a two kind of alignment materials and using directionally polymerized reactive mesogens (RMs) within alignment layer. First, many kind of approaches for producing the intermediate pretilt angles are introduced. The brief concept and the characteristics are presented and their merits and demerits are analyzed. Next, the controlling method for continuous pretilt angles of nematic LCs by vertically stacked planar and vertical alignment layers are discussed in the viewpoints of the scientific studies and the optical devices. The anchoring competition generated by the vertical alignment layer which screened the anchoring energy of the underlying planar alignment layer based on the van-der Waals interaction was found to depend on the thickness of the layer. The theoretical model is proposed to describe the physical mechanism based on the competition of surface anchoring energy in the vertical direction. For applications, no-bias ?k-cell which is stable at low bend state without bias voltage was experimentally demonstrated with intermediate pretilt angle, as a result, low driving voltage and fast response times as well as wide viewing characteristics could be realized. And, as another controlling method of securing continuous pretilt angles, we present the complex system of alignment layers composed of UV curable RMs, which are produced directionally polymerized RMs directly governed by the underlying LC distribution. The directional polymerization of RM over a LC phase arises from the isotropic diffusion of RM monomers in an alignment layer that contacts the LC. We also discuss a mechanism for the filament formation, in which the RM monomers on the alignment layer dissolve in the LC molecules and anisotropically aggregated by the gradient force of their monomer density. As one of an application, we suggest the multi-domain surface-controlled vertical alignment mode which has different pretilt angles in one pixel, as a result fast response time and wide viewing angle characteristics, specially at off-axis, could be achieved. In conclusion, throughout this thesis, the controlling the direction of LC molecules by the surface modification has been extensively explored from the viewpoints of scientific researches and device applications. The continuous pretilt angles of LC molecules from 0?a to 90?a are realized by controlling a surface of alignment layer. The theoretical analysis and several device concepts introduced here may be a foundation for the future scientific researches and applications.