Twisted nematics

The first stable commercial liquid crystal display (LCD) device was the twisted nematic (TN) [110], still widely... 

Schadt M and Flelfrich W 1971 Voltage-dependent optical activity of a twisted nematic liquid crystal Appl. Phys. Lett. 18 127-8... 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Fig. 16. Twisted nematic LCD showiag the V dark state (right), where D is the threshold voltage of the ceU.

A slight variant of the classic twisted nematic display has been iatroduced due to its wider viewiag angle and improved switching characteristics. 

Figure 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

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... 

Figure 8.11 Illustration of Mauguin twisted nematic cell, reported in 1911. Substrates are thin mica plates, which are uniaxial with their optic axis parallel to plane of plates. Apparently, uniaxial crystal stmcture of mica produces strong azimuthal anchoring of nematic LCs of Lehmann, such that director is parallel (or perpendicular) to optic axis of mica sheets at both surfaces. Mauguin showed that method of Poincard could be used to explain optics of system if it was assumed that LC sample created layer of material with uniformly rotating optic axis in twisted cells.

A physicist would say the liquid crystal adopted a twisted nematic structure. 

Spontaneously twisted nematics, 15 92 Spontaneous polarization, in compound semiconductors, 22 152 Spontaneous polymerization ofVDC, 25 695... 

Supertwisted nematic display, 15 114 Super twisted nematic liquid crystal display (STN-LCD), 9 340 Super ultra-low emissions vehicle (SULEV), 13 855... 

Twin-wire paper formers, 18 120 Twisted nematic LCDs, 15 113 characteristics of, 15 114 Twisted pair wire cable, 7 714 Twisted yarn, 11 178... 

Twisted nematic liquid crystal sandwiched between two glass plates, with the director aligned parallel to the plates, with one of the plates turned in its own plane about an axis normal to it. 

Time required for the light intensity viewed through crossed polarizers to increase to 90% of the final value from the off-state to the on-state of an electro-optical twisted-nematic cell. 

Time required by an electro-optical twisted-nematic cell for a light-intensity change from 90% to 10 % of the maximum intensity on going from the on-state to the off-state. 

Twisted polarised light Twisted nematic displays... 

In the case of those LC materials which exhibit negative dielectric anisotropy, cells can be constracted which align vertically and twist on applying a field, exactly the converse to the twisted nematic effect from positive anisotropic LCs. Cells of this type are of interest because they can form a superior black state. 

Figure 5.3 Twisted nematic materials for direct drive displays.

Figure 5.4 Twisted nematic materials for multiplex addressing.

Colour Displays from Twisted Nematic Liquid Crystals... 

Figure 5.5 Twisted nematic materials for active matrix addressing.

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