Nematic multiplexing

Figure 5.4 Twisted nematic materials for multiplex addressing.

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. 

The requirement of a very high and constant resistivity over time of nematic mixtures for TN-LCDs with active matrix addressing meant that new liquid crystals which met this and the other specifications, such as low viscosity and high positive dielectric anisotropy, were needed. The nitriles used in nematic mixtures for TN-LCDs with direct or multiplex addressing were soon found to be unsuitable for TN-LCDs with active matrix addressing. The polar nature of the cyano group leads to the solvation of ions from some of the layers on the... 

The electro-optical characteristics of multiplexed STN-LCDs exhibit a significant dependence on temperature. This has to be compensated in order to avoid variations of the optical performance of the display with temperatures. This can be achieved electronically. However, this problem can also be solved by the use of optically active, chiral dopants. The capacitive threshold voltage of a chiral nematic mixture depends on the pitch of the mixture ... 

Apolar nematic compounds usually possess a low k2,-ilk ratio e.g. 1.0 < 33/ < 1.5), partially due to the absence of molecular dimers. However, they are still essential components of nematic mixtures for STN-LCDs, since they are used to lower the viscosity and melting point of a nematic mixture of polar components as well as improve the multiplexability of the mixture due in part to the reduction in the proportion of molecular dimers of associated polar molecules. Therefore, the synthesis of the first polar alkenyl liquid crystals with high k jku ratios led to the synthesis of a series of apolar alkenyl-substituted compounds with the carbon-carbon double bond in various positions in the terminal chain. Some typical compounds (140-149) are shown in Table... 

The first TN-LCD were simple, directly addressed segment displays as still used, e. g., for wrist watches. When attempts were made to increase the information content of the displays by time-sequential addressing in rows and lines (multiplexing) the limits of the TN cell were soon met. At higher multiplex ratios [14] contrast loss occurred, because of ever shorter addressing times. The development of the super-twisted nematic (STN) cell in 1984 [15] pushed the practicable limit to higher multiplex ratios, but it did not lead to a general solution of the problem. 

MULTIPLEXING LIMITS OF TWISTED NEMATIC LIQUID CRYSTAL DISPLAYS AND IMPLICATIONS FOR THE FUTURE OF HIGH INFORMATION CONTENT LCDs... 

Fig. 2, Tradeoff between angular viewing dependence and number of multiplexed lines N. Plots are for contrast ratio versus angle of incidence 0 in principal viewing plane. Widest field of view is for low N. Polarizer alignment normal to the nematic director (n) at the adjacent LC surface emphasizes the double lobe for N=2 as discussed in the accompanying paper. ...

H. Birecki and F. J. Kahn, Effects of Cell and Material Properties on Multiplexing Levels of Twisted Nematic Liquid Crystal Displays, This volume. 

Tilt angle has also an important influence on the performance of twisted nematic liquid crystal displays (TN-LCDs) because the electrooptical threshold and angular viewing uniformity decrease as the tilt angle is increased. >2 Thus it is important to be able to measure tilt accurately. Low initial tilt angles (<10 ) are of particular interest for multiplexed TN-LCDs. 

EFFECTS OF CELL AND MATERIAL PROPERTIES ON MULTIPLEXING LEVELS OF TWISTED NEMATIC LIQUID CRYSTAL DISPLAYS... 

Multiplexed twisted nematic liquid crystal devices (TN-LCDs) are the mainstay of low power, medium information content displays. At present, the voltage threshold and the nonlinearity of the electrooptic response intrinsic to the twisted nematic structure are used to achieve multiplexing. The multiplexing level achievable using this technology is limited by the sharpness of the electrooptic response and by the variations of this response with viewing angle. ... 

LIQUID CRYSTAL FOR MULTIPLEXED TWISTED NEMATIC DISPLAYS -ITS PHILOSOPHY AND PRACTICE... 

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