Threshold twisted nematic cell

Fig. 6. Schematic operation of a twisted nematic cell for (a) no voltage and (b) a voltage several times the threshold (14) applied across the nematic layer. The cell components are as follows (1) and (2), polarizer and analyser respectively with their polarization directions orthogonal as indicated by the dark arrows (3) glass substrates (4), conducting transparent electrodes and (5), alignment layers with the rubbing directions orthogonal between the upper and lower substrates.

The threshold voltage for deformation of the twisted nematic cell is... 

Figure 4.6 Working principle of a twisted nematic (TN) cell in the normally white" configuration (left), and the change of transmission with increasing applied voltage (right). In the cell configuration sketched above the threshold voltage (V,, ) for the electrooptical response corresponds to approximately V90 for 90% of maximum transmission.

At the present time, as shown by the pocket calculator, materials are available with adequate electro-optic performance where the duty cycle is 1 in 2, 3 or 4. Where temperature compensation is used, 1 in 7 duty cycle is possible O with mixtures of cyanobiphenyls and benzoate esters. For a duty cycle of 1 in 10, temperature compensation is a must and the electro-optic performance is determined principally by the threshold sharpness. By way of example. Fig. 36 shows a viewing cone plot for ZLl 1253 (dotted line curves) compared with a sharper threshold material (1.65) (solid line curves) in an 8y, low tilt cell. By single frequency addressing, a 1 in 10 duty cycle is approaching the maximum for an acceptable performance in a twisted nematic display. 

In an extensive work on polysiloxane polymer liquid crystals, Finkelmann et al. examined their performance in a twisted nematic electro-optical cell. In parallel with the polymer measurements, analogous studies were carried out on low molar mass nematic compounds of equivalent dielectric anisotropy (Ae). Both the low molar mass nematic materials and the polymer side-group moieties were mixtures of methoxy- (or alkyloxy-) and chlorine-terminated benzoate esters. Cells were prepared using rubbed polyimides and in the polymer case the samples were annealed for several hours to obtain defect-free aligned textures. Measurement of the threshold voltages, for both the low molar mass compounds and the polymer, showed ... 

The twist transition can neither be evoked straightforwardly with electric fields nor detected easily with capacitive measurements, although this is possible, in principle, with lateral electrodes (e.g. Pashkovsky et al. [20]), but the director deformation is then complex and two-dimensional. Instead of using a pure twist cell, one can measure K22 from a 90° twisted nematic (TN) planar cell [21-23]. The threshold voltage is given by [8,21]... 

Figure 37. Schematic diagram of a twisted nematic electrooptic cell for (a) zero voltage and (b) a voltage above threshold, V,h(TN). Note that some chiral nematic mesogens remain anchored in a planar arrangement on the alignment surface, which then provides the coupling for the field-off decay back to the twisted structure. The weak chiral nature prevents back flow.

Dielectric anisotropy is an additive molar property. Thus, a small amount of PEBAB [Ae 10] (about 10-15 mol %) dissolved in MBBA [Ac —0.2] will provide a material suitable for twisted nematic devices. The threshold will, of course, be higher for this mixture than for a pure positive one such as 4-pentyl-4 -cyanobiphenyl, where the dielectric anisotropy is much higher. There are other influences on the threshold voltage for liquid crystal cells, principally the materials elastic constants and, in the case of dynamic scattering, material viscosity. The response times also are dependent upon elastic constants, viscosity, and dielectric anisotropy. These factors are discussed at length in a review by Goodman." ... 

We first discuss the classical Freedericksz transitions and critical thresholds for a nematic. The understanding of these phenomena is crucial to the basic traditional idea of switching liquid crystal cells by fields having magnitudes above the critical threshold. The commercial exploitation of these results in liquid crystal display devices, especially the twisted nematic display to be discussed in Section 3.7 below, has greatly increased the general interest in theoretical and experimental aspects of Freedericksz transitions, and vice-versa. 

Figure 4.9 Elastic deformations of calamitic, rod-like liquid crystals in the nematic phase. The corresponding elastic elasticity constants are K, (splay), /<2 (twist), and Kj (bend). K, has the largest influence on the threshold voltage, of TN cells [23f].

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