Supertwisted nematic device

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... 

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. 

Thus studies of the significance of the position of the location of the double bond in a terminal alkyl chain led to the alkenylhi-cyclohexane systems [75], which today provide excellent materials for supertwisted nematic devices, and the work on ferroelectric materials led to the discovery of the antiferroelectric phase. 

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°. 

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

SEE Supertwist birefringence effect device effect in a 180° to 270° twisted nematic structure that uses a combination of interference and polarization guiding to achieve an optical effect with sharp threshold behavior, making it particularly suitable for multiplexing. 

The current (third) period, which may be called a colonization, involves wide electrooptical investigations of novel effects in ferroelectric liquid crystals [9, 10] and a study of exotic materials like polymeric and lyotropic mesophases, blue phases in cholesterics, well-ordered smectics, and so on. For conventional (nematic and cholesteric) phases the accent was shifted to the optimization of the material properties for electrooptical devices, though novel phenomena like the supertwist effect [11] and a gamma of linear electrooptical effects [12-14] have also been discovered. 

Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc. Industrial processing of these nematic cells, as well as the transparent conductive coating of the cell windows for electro-optical device applications, is understandably more elaborate. 

---