Ferroelectric mixtures

The incorporation of a chiral dopant into an achiral smectic C matrix results in the helical twisting of the structure of the mixture. In the simplest case, the wave vector of the helix is proportional to the concentration of a dopant. 

FIGURE 1.18. Inversed pitch as a function of the cholesteric compound in ne-mato-cholesteric mixtures [45, 46]. Mixtures of 4-butoxybenzyhdene-4 -butylani-line with (1) cholesteryl (Ch) chloride, (2) tiocholesterol, (3) CH acetate, (4) Ch pelargonate, (5) Ch oleate. 

At the same time the dopant induces the spontaneous polarization (Ps) in the mixture [50, 51]. The magnitude of the spontaneous polarization increases with the increasing concentration of a chiral dipolar dopant, Fig. 1.19 [51]. With doping, rather high values of Pg, of the order of 10-100 nC cm , can be obtained. 

Cholesterol itself, not being a mesogen, induces high spontaneous polarization in thermotropic smectic C matrices [52]. Moreover, it can induce ferroelectric properties even in lyotropic mesophases [53, 54]. Other biologically active molecules also induce Pg values [55]. 

Mixing two ferroelectric liquid crystals with an opposite sign of chirality and polarization we can compensate for both the twist and polarization. If the two compounds are chemically different, the two compensation ( magic ) points occur for different composition of the mixture. Fig. 1.20 [56]. With zero twist we can have a spontaneously polarized liquid crystal, that is, a nonhelical ferroelectric. The reason is that different molecular mechanisms are responsible for the twist and polarization. The pitch is determined by a chiral interaction of all types of molecules, and the polarization depends not only on chirality but also on the steric and electrical dipoles as well. The simultaneous compensation for a helix and polarization occurs only in racemic mixtures. 

---

However, many interesting effects in ferroelectric cells may be described without account of the helicity, in the approximation of a uniform SmC structure (e.g., unwound by limiting surfaces or formed by mixtures with compensated helicity). So, in this paragraph, we ignore all the space dependent terms i.e. consider a SmC structure with azimuthal angle (p 0. Going back to Fig. 13.5 this approximation may correspond to a ferroelectric mixture with qo 0. Then the free energy is ... 

When a host rrrixture has been selected, a chiral dopant is required. The chiral doparrt shottld provide the following physical properties to the ferroelectric mixture in addition to those listed above for the host rrrixture. 

The chiral dopant need not be mesogenic bnt should have a mesogenic-like stracture, preferably similar to the stracture of the host materials (to help maintain the properties of the host mixture). Consequently, some of the most interesting chiral materials in liquid ciystal mixtures are not liquid ciystalline or at least their hquid crystal phases are less important than their effect on the physical properties of the ferroelectric mixture. 

Recently, most research emphasis has been applied to chiral materials, not necessarily mesogenic, for use as dopants in ferroelectric mixtures. However, for more fundamental studies on single materials, the generation of the phase is essential. The most... 

Chiral materials for liquid crystal mixtures, especially (ferroelectric mixtures), are... 

A special strategy has been developed for composing ferroelectric mixtures. Such materials are based on chiral dipolar compounds which form the smectic C phase. The chemical synthesis of such compounds is extremely difficult. However, the problem can be solved step by step an achiral smectic C matrix with a wide temperature range may be worked out separately, and then doped by a chiral dipolar additive (the chirality and dipole moment cannot be decoupled from each other, as was discussed in Section 1.2.5). 

New ferroelectric mixtures with the helix pitch Rq < 0.3 /xm and tilt angle 9 > 30° have recently been developed for the DHF effect [22, 98-101]. The helix unwinding voltage was about 2-3 V. Later on, we will show that short-pitch FLC mixtures could be also used to obtain bistability in FLC samples [103]. 

Januszko, A. Kaszynski, P. Wand, M. D. More, K. M. Pakhomov, S. O Neih, M. Three-ring mesogens containing p-carboranes Characterization and comparison with the hydrocarbon analogs in the pure state and as additivies to a ferroelectric mixture, J. Mater. Chem. 2004,14, 1544—1553. 

Such a shift governed by the second term in the Landau expansion and strongly dependent on piezocoefficient C has been observed experimentally [14,15]. The experimental points for a multicomponent ferroelectric mixture [14] and the fit using Eq. (6) is shown in Fig. 1. 

Surface stabilized mode ferroelectric LCDs [31-35] show bistability, i. e. both the and the -6 position are equally stable. A pulse of one polarity is used to switch from 6 to -6, while a pulse of opposite polarity is used to switch from -0 to 6. The molecules can remain in either of these states (0 or -6) for a long time without any applied voltage. In the case of GH ferroelectric LCDs, the ferroelectric mixture contains an appropriate... 

---