Twisted nematic and supertwisted

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

The twisted nematic (TN) and supertwisted nematic (STN) liquid crystals are widely used in liquid crystal displays. The former is used in wrist watches and calculators while the latter is used in notebook computers. 

H. Seiberle and M. Schadt, LC-conductivity and cell parameters their influence on twisted nematic and supertwisted nematic liquid crystal displays. Mol. Cryst. Liq. Cryst. 239(1), 229-244, (1994). 

T. Scheffer and J. Nehring, Twisted nematic and supertwisted nematic mode LCDs, in Liquid crystals-applications and uses, Vol. 1, ed. B. Bahadur (World Scientific, New Jersy, 1990). 

Dynamical Behavior of Twisted Nematic and Supertwisted Nematic Displays 213... 

Liquid Crystal Materials for Twisted Nematic and Supertwisted Nematic Display Devices... 

See, for example, Scheffer, X, and J. Nehring. 1993. Twisted nematic and supertwisted nematic mode LCDs. Liquid Crystals Applications and Uses, B. Bahadur (ed.), Singapore World Scientific. see also Kataoh, K., Y Endo, M. Akatsuka, M. Ohgawara, and K. Sawada. 1987. Apphcation of retardation compensation A new highly multiplexable black-white hquid crystal display with two supertwisted nematic layers. Jpn. J. Appl. Phys., Part 2 26 L1784. 

The desire to improve sharjDness and viewing angle range led to the development of supertwisted nematic displays name suggests, STN displays have higher twist angles than the TN display, typically 220-270°. They are widely us laptop computers. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Certain aryl amines are endowed with unusual electronic properties. Schiff s bases such as 4-methoxybenzylidene-4-n-butylaniline were the original experimental chemicals used in the early development of electrical and electrooptical displays. Liquid crystal display (LCD) technology relies on twisted nematic or supertwisted nematic effects. Later nematogen development was based on cyanobiphenyls103. 

An advance that has extended the application of LCDs to full page computer terminals and other high information content displays without having to resort to TFTs is the supertwisted nematic device. It makes use of the fact that the electrooptic response of the nematic gets progressively steeper as the twist angle is increased, until at a certain... 

Chiral materials, not necessarily liquid crystalline, must be added to nematic mixtures for some display devices (see Chapter 13). For example, in the twisted nematic display a quarter-helix (90°) twist is caused by the perpendicular molecular alignment at the top and bottom plates and there are two possible twist directions. In order to ensure the same twist direction throughout the device, a very small quantity of a chiral material (e.g, compound 3) is added to the nematic mixture. Significantly too, a chiral material is required in the so-called supertwisted nematic (STN) displays where a twist in the nematic director of more than 90° is employed (usually between 180° and 240°). In this case, a chiral material with an appropriate helical pitch length (P) is chosen in conjunction with a particular cell spacing (d) for example, a d/P ratio of 0.75 induces a twist angle of 270°. 

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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