Cholesteric texture displays

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

Fig. 5.8 A photoaddressed and multiswitchable cholesteirc LC display of 6.0 wt% chiral dopant 8 in LC host E7 in a 5 pm thick homeotropic aligned cell (1.5 in. x 1.5 in.). Top schematic cholesteric textures Middle demonstration of an image Bottom crossed polarized textures. Reproduced with permission from [76]. a Planar state, b Focal conic state, c Planar state. Copyright 2011 John Wiley Sons...

The cholesteric texture occurring with perpendicular boundary orientation, on the other hand, is not a uniform texture. Under the microscope the entire field of view is filled with right-and left-handed spirals. This cholesteric texture is known as the scroll texture and appears very much like an end-on view of a bundle of rolled-up scrolls. Capacitance measurements show that the cholesteric helical axis in this texture is still predominantly perpendicular to the plane of the layer. An exact analysis of the structure of this texture, even for the case of equal elastic constants, seems difficult to achieve. Important from the applications point of view is that the scroll texture is adopted without disclinations immediately after a display element is turned off and that this structure is essentially nonscattering. Homeotropic boundary orientation therefore solves the problem of display after-images. 

Polymer-dispersed LC (PDLC) [30], polymer-stabilized cholesteric texture (PSCT) [31], and LC gels [32] all exhibit optical scattering characteristics and have wide applications in displays and optical devices. The LC gel-based reflective LCD can also be extended to transflective... 

J. W. Doane, D.-K. Yang, and Z. Yaniv, Front-lit flat panel display from polymer stabilized cholesteric textures, Proc. Japan Display 92, 73 (1992). 

Electric fields have been used to switch a cholesteric texture between the planar, the focal conic, and the homeotropic state. The planar and focal conic states persist after the field is turned off, the choice determined by the amplitude and frequency of the applied field and the rate at which it is turned off. These textural transitions were discovered by Heilmeier and Goldmacher [32] and are used today for color display panels that exhibit gray-scale memory [33,34]. 

Slikkerveer defines a flexible display as a flat panel display made using thin, flexible substrates, where the substrate can be bent to a radius of curvature of some centimetres without loss of functionality. An example of a flexible display is given in figure 1. This display, a flexible CTLC (Cholesteric Texture Liquid Crystal) cell, is only 250 pm thick and can be bent down to a radius of two centimeter while operating. The size of the presently shown display cell is 150 125 mm. ... 

Chien and coworkers [68] devised a material scheme to manufacture red, green, and blue pixels of polymer-stabilized cholesteric textures by selectively adjusting the chiral pitch length spatially across the display substrate using... 

All other members of this series were cholesteric displaying colourful Schlieren textures. The capability to form GJ-textures was completely lost, when the molar fraction of isosorbide was increased to 20% (or higher). In other... 

A sheet of cholesteric liquid crystals is sandwiched between two glass plates separated by a gap of tens microns. The cholesteric liquid crystals on two glass plates are homogeneously aligned to form the planar texture. The cell displays a bright color. The color varies according to the view angle and temperature. This is an important characteristic of cholesteric liquid crystals—selective reflection. 

Cholesteric liquid crystal (ChLC)-driven electronic paper has been investigated mainly by Kent Display [2], Since ChLCs are chiral molecules, the particular color of light depends on the pitch denoted as P, where the pitch is the distance along the helical axis for ChLC to twist 360 and is determined by the amotmt and type of chiral additive within the liquid crystal mixture. ChLCDs are driven by switching the different textures of the ChLC electrically [1, 3], as shown in Fig. 4. 

Cholesteric polyesters were prepared from silylated derivatives of 2,3-di-(9-isopropylidene-D-threitol, DAS, or DAM with dicarboxylic acid dichlorides by polycondensation in solution [34]. Trifluoroacetic acid-water allowed an easy cleavage of the isopropylidene group without hydrolysis of the polyester. All these polyesters formed a broad cholesteric phase, and the polymers containing 5 or 10 mol per cent sugar diol displayed a blue Grandjean texture. 

The nonscattering scroll texture is quite different from the highly scattering cholesteric focal conic texture which occurs at intermediate voltages.17 in the focal conic texture the helical axis is essentially parallel to the plane of the layer. This texture never occurs in a display under the condition that all the display elements are either fully turned on (untwisted state) or completely turned off (scroll texture). This condition is generally not fulfilled in multiplexed displays. 

Figure 10.23 The reflection spectra of the cholesteric displays. P planar texture, F focal conic texture, (a) The regular polymer stabilized cholesteric display, and (b) The polymer stabilized black-white display.

The effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

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