Liquid-crystalline display technology

Liquid crystal display technology, 15 113 Liquid crystalline cellulose, 5 384-386 cellulose esters, 5 418 Liquid crystalline conducting polymers (LCCPs), 7 523-524 Liquid crystalline compounds, 15 118 central linkages found in, 15 103 Liquid crystalline materials, 15 81-120 applications of, 15 113-117 availability and safety of, 15 118 in biological systems, 15 111-113 blue phases of, 15 96 bond orientational order of, 15 85 columnar phase of, 15 96 lyotropic liquid crystals, 15 98-101 orientational distribution function and order parameter of, 15 82-85 polymer liquid crystals, 15 107-111 polymorphism in, 15 101-102 positional distribution function and order parameter of, 15 85 structure-property relations in,... 

Schadt M (1989) The history of liquid crystal displays and liquid crystalline materials technology. 

As the name implies, liquid-crystalline materials combine the properties of a crystal with those of a liquid in a very special way and are therefore of interest for display and data storage technology. Liquid-crystalline compounds usually consist of rod- or disc-shaped organic molecules which preferentially adopt a mutually parallel orientation [33]. A change of molecular orientation caused by application of an electrical potential transforms the optical properties and can be utilised for display applications. 

Since the description of liquid crystallinity for cholesteryl benzoate and cholesteryl acetate at the end of the 19th century by Reinitzer [1], an intense activity has been devoted to thermotropic liquid crystals, especially since the early 1970s, owing to the fabrication and application of liquid crystal displays in electronic technology. 

In recent years, investigators working in the field of physics and chemistry of high-molecular compounds have been paying a lot of attention to the problem of C3 eating liquid crystalline systems (V14). The great interest displayed in the study of properties of such systems can most probably be accounted for by two main factors firstly, by the advances in studies into the structure, properties and practical use of low-molecular liquid crystals in physics, technology and medicine, and, secondly, by the studies of the nature and salient features of the liquid crystalline state in polymers as a specific state of macromolecu-lar substeuices. 

In the mid 1960s, the first major application of liquid crystals in electro-optical display technology was identified [1]. Later, people realized that this finding was a milestone in the history of liquid crystalline materials and that... 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

The most important multicolour display technology in current use involves liquid crystal displays (LCDs), which overtook CRTs in popularity around the mid-2000s. Liquid crystals (LCs), commonly referred to as the fourth state of matter, are materials that are intermediate in character between the crystalline solid and liquid states.Unlike normal isotropic liquids in which the molecules essentially adopt a randomised orientation, liquid crystals show some time-averaged positional orientation of the molecules. In this sense, they resemble solid crystalline materials, although they retain most of the properties of liquids, notably the ability to flow. They are formed most commonly from molecules with rod-like geometry, which are referred to as calamitic. These molecules may orientate in various ways to form different types of LC phases [mesophases). There are three main mesophase types smectic, nematic and chiral nematic. In the smectic mesophase, as illustrated in Figure 11.1(a), the molecules are arranged in raft-like layers with their molecular axes parallel. These layers can pass over each other as the material flows. In the... 

Liquid crystals, polymers and liquid crystalline polymers are soft condensed matter systems of major technological and scientific interest. In liquid crystals the orientational order of the constituent molecules is associated with a reduced or absent translational order. This gives liquid crystalline systems a combination of fluidity (liquid-like properties) and anisotropic electro optic properties, similar to those of a crystal. Orientational order can be controlled easily by the application of external fields, leading to the spatial switching of bulk properties in response to external stimuli. This provides the basis for a wide range of technological applications, including displays, optical switches, adaptive optics for telescopes and many other electro-optical devices. 

A study of the miscibility of liquid crystals is of great importance from two points of view [31]. First, mixtures manifest a variety of phases separated by phase transition lines. Varying the composition of a mixture, we can study the interaction of different structural modes (the interaction of order parameters), investigate various pretransitional phenomena [32], etc. Mixing an unknown substance with a compound having well-defined phases we can also identify the structure of the unknown substance. On the other hand, mixtures are extremely important from the technological point of view. The best liquid crystalline materials for displays are, as a rule, multicomponent mixtures with a wide temperature range of operation. Here, the problem is to compose a thermodynamically stable eutectic mixture. 

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

The Schlieren texture is characteristic of the nematic phase, and the point defects seen in this phase may have two or four brushes surrounding the defect as shown in Figure 2.5. We can visualize the director field in this kind of sample because the dark and light areas of the texture correspond to different molecular orientations with respect to the axes defined by the polarizers. This concept is explained in Section 2.8.2. The nematic phase is currently the most technologically important liquid crystalline phase and can be found in most LCD screens and monitors in use today. Careful product optimization and development since the 1970s have resulted in commercial nematic materials with excellent properties for display applications. We discuss how this phase is used in LCDs and other applications later in this chapter. 

The liquid crystalline benzonitrile-based cyano compounds that had been used in PM-LCDs were gradually replaced by fluorinated compounds developed in the 1980s. As the driving technology for displays has changed over time, liquid crystal compounds have also evolved. In this section, a partial overview of the developmental trends of AM-LCDs and trends in the physical properties of liquid crystal compounds and other materials are presented. 

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