Study on application of UV-processed molecular alignment in liquid crystal devices

Lias, Jais (2011) Study on application of UV-processed molecular alignment in liquid crystal devices. PhD thesis, Nagaoka University of Technology.

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Abstract

Photoalignment technique in liquid crystal devices (LCDs) fabrication processes is classified into a non-contact process which provides an anisotropy in liquid crystal (LC) alignment layer by exposing ultra violet (UV) light. The greatest benefit for using this technique is to avoid electrostatic charges and impurities on the substrate. Moreover, photoalignment can realize the structures which has the required liquid crystal director azimuth with the selected area of the cell, thus viewing angle characteristics can be improved by the pixel dividing. In this study, the photoalignment technique was applied to the fabrication of a bistable type LCD and polymer-stabilized blue phase (PSBP) LCD. Most liquid crystal devices are monostable, which possesses only one possible state in absence of field. They requires continuous voltage application and frequent image refreshment, which is the cause of the energy consumption and limits the multiplexability. In other word, monostable devices have no intrinsic pixel memory and they need an active matrix or other external storage elements to obtain high multiplexing levels. Bistable type LCD have two (or more) stable states. Once an image displayed, the director state in each pixels is memorized for a long time, ranging from seconds up to years, until when the new image is overwritten. This intrinsic memory capability is a peculiar advantage of the bistable type LCD, provides a potential to reduce the power consumption, especially for the specific application which is unnecessary frequent update. In this study, to fabricate a bistable LCD (BLCD) by using unpolarized UV light irradiation, single-step laser patterning to photoalignment layer was proposed. Bistability can be achieved by two equilibrium configurations of LC director profile which is induced by a periodically patterned alignment layer on a substrate. The patterns were formed by stripes of alternating random planar and homeotropic anchoring in the order of 0.5 um. In this work, two possible configurations of bistable LCD that can be obtained by combining a micropatterned surface formed with alternating random-planar- and homeotropic-alignment with planar- or homeotropic-alignment surfaces were proposed. The alignment properties of the two proposed BLCD models such as twisted angle, pretilt angle and its microscopic switching behavior and memory effect were investigated and determined. It was assumed that the formation of the two bulk orientational states will depend on the degree of depolarization of the laser light, the stripe periodicity and the effective anchoring strength of the patterned surface. Recently, blue phase (BP) liquid crystal (LC) has attracted the attention of many researchers because of its exotic structures and properties. BP with a structure of self-assembled three-dimensional cubic structure formed by double-twisted arranged LC cylinders is usually observed in a cooling process from isotropic to chiral nematic (N*) phase LC. Such a molecular arrangement exhibits an optical isotropy, selective reflection to circular polarized light, a reflective band switched by external fields, and microsecond response. These characteristics show some potential applications of BP such as large-screen flat panel displays and tuneable photonic band gap devices. The very narrow stable temperature range of the BP, however, typically a few Kelvin, is a serious problem for device applications. Although, much effort has been made to expand the narrow temperature range of the BP, Kikuchi et al. firstly reported that the temperature range of the BP can be expanded over 60 'C by adding a small quantity of precursor polymers within the BP materials, referred to as a polymer-stabilized blue phase (PSBP). To realize a fast response LCD, PSBP LCD was successfully fabricated by applying the UV photoalignment technique to expand the temperature range of blue phase (BP). As a coupling coefficient, the polar anchoring strength between the BP LC and the diacrylate type polymer was evaluated by means of the oblique incidence transmission spectroscopic ellipsometry using two methods which are SOITE method and Hung's method. In the two methods, different type of LC materials and chiral dopant were employed. As a result, the polar anchoring strength measured with SOITE and Hung's methods were determined in the order of 10-4 J/m2. The polar anchoring strength also compared with the other materials which was used for polymer dispersed LCD. We found that the host BPLC materials and the diacrylate-type polymer in a PSBP LC having vertical (homeotropic) alignment. It is assumed that the strong polar anchoring strength between the BPLC and the diacrylate-type polymer allows the PSBP LC to maintain and stabilize its structure. Thus, the BP temperature range can be broadened by adopting a diacrylate-type polymer into host BPLC materials. The finding that the polar anchoring strength is relatively high offers an important insight with which to understand and optimize the polymer stabilization of BP.

Item Type:Thesis (PhD)
Subjects:T Technology > T Technology (General)
ID Code:2704
Deposited By:Normajihan Abd. Rahman
Deposited On:28 Aug 2012 16:46
Last Modified:28 Aug 2012 16:46

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