AFLCs crystals

An unusual effect was observed in an antiferroelectric liquid crystal (AFLC) [116-125]. The switching is associated with the appearance of the third state, in addition to the two bistable up and down states known for the Clark-Lagerwall effect. The corresponding hysteresis of electrooptical switching is shown in Fig. 7.23. We can see that the third state at the zero field is stable, and can be transformed either into up or down states if not applying a rather high switching field. 

The FLCs or AFLCs mentioned above are liquid crystalline materials showing a chiral smectic C phase or related phases. Although nematic liquid crystal possesses only directional order, smectic liquid crystals shows layer structures or periodic order of the liquid crystalline molecular centres. In this respect, smectic liquid crystalline materials have more in common with crystalline materials than with nematic liquid crystalline materials. As a result, the alignment of smectic liquid crystal is quite different from that of nematic liquid crystals. Smectic liquid crystals show a large variety of defects because they possesses highly ordered structiu-es. Moreover, the layer structures are irreversibly destroyed by applying stress. This phenomenon poses a major problem for display applications. Therefore, several techniques to prevent the application of force to the liquid crystalline materials of FLC or AFLC devices have been proposed. 

In the sections on smectic liquid crystals, first the alignment and molecular orientation of surface stabilized ferroelectric liquid crystals (SSFLCs) are treated in detail. Next, the alignment technologies needed for the occurrence of bistability are detailed. Furthermore, liquid crystalline devices made of AFLC materials and the applications of FLC and AFLC materials to active matrix devices are discussed. 

New materials are discovered sometimes intentionally and sometimes accidentally. The former case can be exemplified by ferroelectric liquid crystals (FLCs), wherein the chirality was introduced into the lateral chain to reduce the symmetry, leading to a noncentrosymmetric system [1]. In the latter case, it can be referred to as antiferroelectric liquid crystals (AFLCs) which were discovered accidentally. Actually, compounds which exhibit the antiferroelectric phase had been synthesized several years before they were proven as AFLCs. The hitherto known AFLCs include three materials, as shown in Figure 9.1 (1) MHPOBC, (2) MHTAC, and (3) R) and (6 )-l-methylpentyl 4 -(4"-n-decyloxybenzoyloxy)bipheny-l-4-carboxylates. 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

Antiferroelectric liquid crystal displays (AFLCDs) have been relevant to flat panel displays since the discovery of AFLCs. Their characteristic features can be summarized as follows ... 

The phase sequence of a prototype AFLC, MHPOBC, has been known to be complex since the early stages of the investigation of antiferroelectricity in liquid crystals [1,6, 49]. The origin and the structures of the subphases have been well documented [6, 50]. However, it is not easy to identify the sub-... 

Very soon it was recognized that antiferroelectric liquid crystal (AFLC) materials exhibit not only the anticlinic antiferroelectric phase, but also three other subphases. 1 One of them, called SmC. phase, clearly has so-called "ferrielectric" characteristics, which structure is shown in Figure 8.24. 

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