Liquid ferroelectric properties

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

None of the complexes with three and four chiral chains was liquid crystalline. In addition, the number and position of these chains drastically influenced the ferroelectric properties, particularly spontaneous polarization, as well as the... 

Recently, Wischerhoff et al. [229] reported the synthesis, the thermal behavior, and the ferroelectric properties of liquid-crystalline copolysiloxanes 49 containing chiral... 

Figure 6 The first chiral oxadiazole-based liquid crystal exhibiting ferroelectric properties.

Another interesting biological example that shows ferroelectric properties is voltage-dependent ion channels. These channels are glycoproteins located in the cell membrane, and they are found in either open or closed conformations. In the open conformation, these proteins are ion conductors. In the closed conformation, they become non-conducting, yet they still retain their ferroelectricity, even showing liquid crystalline properties [36,48]. 

Flexible spacers in the semirigid-rod molecule allow the molecule to orient with order parameter greater than 0.8 when coated and cured upon a rubbed polyimide layer. Kinetics of the photo-polymerization [62,63] and surface-induced orientation [49] as well as mechanical [64,65], optical [59,66,67], and ferroelectric properties [68-70] of the anisotropic networks were thoroughly examined. These investigations were mainly carried out by the Philip s research group and were applied to develop liquid crystalline display devices [58,69]. 

FELCD Ferroelectric liquid crystal device a device exploiting the ferroelectric property of chiral tilted smectic phases (especially smectic C ). 

One of the disadvantages of the LCD display is that the response time is slow (approximately 50 (jls) and the liquid-crystal phase is too symmetric to allow vector order. To overcome this problem, tilted smectic phases with ferroelectric properties (polarisation can be reversed by an electric field) can be used, providing that the liquid crystal is chiral. One such ferroelectric liquid crystal is... 

Shen, D. Tschierske, C. Diele, S. Wirt, 1. A novel class of non-chiral banana-shaped liquid crystals with ferroelectric properties. Chem. Commun. 1998,23, 2573-2574. 

As mentioned earlier, the helical structure of the smectic C phase should be untwisted by an electric or magnetic field, or suppressed by a surface effect, to observe ferroelectric properties of the phase. In the first publication on ferroelectric liquid crystals [5] an approach to nontwisted ferroelectric LC materials was suggested. By mixing two individual ferroelectric liquid crystals having opposite signs of P but different absolute values, one can compensate the helical twisting without zeroing the polarization. That has been done for low-molar-mass liquid crystals... 

Chiellini, E., Galli, G., and Cioni, F., Chiral liquid crystal polymers with potential ferroelectric properties synthesis and characterization, Ferroelectrics, 114, 223-228 (1991). 

Poths, H Wischerhoff, E., Zlentel, R., Schonfeld, A., Henn, G., and Kremer, F., From monomeric to polymeric ferroelectric liquid crystals. A comparative study of ferroelectric properties, Liq. Cryst., 18, 811-818 (1995). 

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]. 

Smectic Liquid Crystals Ferroelectric Properties and Electroclinic Effect... 

If a smectic-C phase is formed by chiral molecules—regardless of whether a chiral compound exhibits a smectic-C phase by itself or a smectic-C phase of nonchiral molecules is doped with a chiral additive—a helical structure appears which is in some aspects similar to the helical structure in a cholesteric liquid crystal. The helical structure of the chiral smectic-C phase had been recognized in the early 1970s [13], [14], [15], well before the ferroelectric properties of this phase were realized. 

By mixing a chiral liquid-crystal compound with its optical antipode, systems possessing arbitrary values of the enantiomeric excess can be designed. If a chiral compound shows smectic-C and smectic- I phases, the racemate, i.e., the 1 1 mixture of the two antipodes, also exhibits these phases but the ferroelectric properties of the smectic-C phase and the electroclinic effect in the smectic- phase are lost. This offers the unique possibility to study a given system with and without ferroelectridty or with a variable markedness of its ferroelectric properties. 

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