Nematic analog displays

When nematic liquid crystals are arranged in thin layers, their ability to transmit scattered or polarized light changes by applying an electric field. This phenomenon can be utilized for alphanumeric and analog displays, image converters, and matrix-type picture screens. 

Deschenaux has shown that the 1,3-disubstituted ferrocene complexes (Figure 57) are far better at supporting mesomorphism when compared to the analogous 1,1 -disubstituted complexes [112]. The former complexes characteristically show enantiotropic Sc, Sa and nematic phases. Asymmetric 1,3-disubstituted ferrocenes have been synthesized and display a rich mesomorphism, for example, Sa, Sc and nematic phases [113]. 

According to Eq. (7.1) P is zero for the two cases of uniform director fields and pure twist. Hence both cases can serve as a zero state as far as flexoelectric excitations are concerned. It is important to note that a twist is not associated with a polarization (i.e. C2 is identically zero, cf. Fig. 7.2). An imstrained nematic has a centre of symmetry (centre of inversion). On the other hand, none of the elementary deformations - splay, twist or bend have a centre of symmetry. According to Curie s principle they could then be associated with the separation of charges analogous to the piezoeffect in solids. This is true for splay and bend but not for twist because of an additional symmetry in that case if we twist the adjacent directors in a nematic on either side of a reference point, there is always a two-fold symmetry axis along the director of the reference point. In fact, any axis perpendicular to the twist axis is such an axis. Due to this symmetry no vectorial property can exist perpendicular to the director. In other words, a twist does not lead to the separation of charges. This is the reason why twist states appear naturally in liquid crystals and are extremely common. It also means that an electric field cannot induce a twist just by itself in the bulk of a nematic. If anything it reduces the twist. A twist can only be induced in a situation where a field turns the director out of a direction that has previously been fixed by boundary conditions (which, for instance, happens in the pixels of an IPS display). 

The structures typically obtained as a consequence of shift non-uniqueness are analogous to nematic phase liquid crystals in the sense that there exists a unique chain direction, but the monomer repeat units in neighboring chains are not well correlated in longitudinal position. As a consequence, the x-ray rotation photographs about the chain axis direction (c) display either (1) sharp hkO reflections and progressively more diffuse hk reflection with increasing Z (2) diffuse reflection for all layers combined with a few sharp hkO reflections or (3) diffuse reflections on... 

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