Anti-ferroelectric liquid crystal

In the last few years the anti-ferroelectric liquid crystal Sca was discovered by Chandani et al. (1988, 1989) in MHPOBC... 

Figure 6.39. The molecular packing of unwound samples (a) ferroelectric and (b) anti-ferroelectric liquid crystals. (From Goodby et al, 1993.)...

Compared with conventional ferroelectric liquid crystals, anti-ferroelectric liquid crystals have the following advantages ... 

Boemlburg et al. (1991) first discovered the anti-ferroelectric liquid crystal phase in the chiral side chain liquid crystalline polymer, Sca phase. Several other research groups followed with more such side chain liquid crystalline polymers. Boemlburg et al. (1992) reported an anti-ferroelectric liquid crystal in the molecule... 

Anti ferroelectric Liquid Crystal Materials with Unusual Chemical Structures... 

The application of this fluoro-epoxide for the synthesis of dimeric liquid crystals which exhibited st le anti ferroelectric chiral Smetic C Riase and were good dopants for anti-ferroelectric liquid crystals was shown by Hiyama and co-workers (Figure 12) (30). 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

If a mesophase possesses ferroelectric properties in the bulk its spontaneous polarization Pg can interact with a solid surface. The Pg-vector can be oriented, for example, in the direction toward substrates on both interfaces of a liquid crystal layer, that is, anti-symmetrically. In this case, the director of the smectic C phase, which is rigidly coupled with Pg, will change its orientation in the bulk of the layer resulting in the appearance of speciflc defects [33]. 

It was recognized as early as 1981 [54] that in order to achieve the ferroelectric state, a chiral liquid crystal is necessary. Chirality puts two molecules in the ideal unit cell, and this allows new modes of fluctuations and also new forms of organisation which also give possible minima in the free energy [ structure ] function. An electric field can now take the liquid crystal into one of these alternative forms of molecular packing, some of which can have a net ferroelectric moment per unit cell and therefore an overall ferroelectric moment. Columnar pyrenes made recently by Bock and Helfrich [55] have saturation polarization of -llOnCcm" and switch from the anti- to the ferro-electric state on a time scale of = 10 s with switching fields -10 V tm . Similar behavior has been... 

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