Ferroelectrics chiral smectic

Note 5 This chiral smectic C phase is also known as the ferroelectric chiral smectic C phase. 

A.D.L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe and A. Fukuda, Anti-ferroelectric chiral smectic phases responsible for the tristable switching in MHPOBC, Jpn. J. Appl. Phys. 28(7), L1265-L1268, (1989). 

The symmetry approach to ferroelectricity in liquid crystals can be realized not only for individual substances but also for multicomponent systems. For low-molar-mass ferroelectric liquid crystals, most applications use LC mixtures with two main components a nonchiral matrix providing the tilted smectic structure and a chiral dopant [7]. As for the preparation of FLCPs, mixing of a smectic C polymer with a chiral dopant also results in a ferroelectric chiral smectic system [74]. Japanese authors [75,76] have carried out systematic studies on mixing tilted smectic polymers with low-molar-mass ferroelectric liquid crystals. 

Beresnev, L., Chigrinov, V. G., Dergachev, D. I., Poshidaev, E. P., Funfschilling, J., and Schadt, M., Deformed helix ferroelectric liquid crystal display a new electrooptic mode in ferroelectric chiral. smectic C liquid crystals, Liq. Cry.st., 5, 1171-1177 (1989). 

In recent years chirality in liquid crystals has developed into a very important area of liquid crystal research. Chiral materials are reqtrired for the chiral nematic (N ) phase and the ferroelectric chiral smectic C (S ) phase for a wide variety of applicatiorrs. As... 

Recently there was also considerable activity in the investigations of the SHG in nematic and cholesteric phases induced by an external electric field. Such experiments allow the high order molecular hyperpolarizabilities to be calculated. " The SHG was also observed in a ferroelectric (chiral smectic C ) liquid crystal. ... 

Most recent studies [28,29] of the dielectric properties of the SmC phase have focussed on the ferroelectric chiral smectic C phase, because of its importance in applications. The molecular interpretation of the principal permittivities is contained in Eqs. (27), with appropriate correlation fac-... 

The viscous part of the stress tensor for the SmC and the ferroelectric chiral smectic (SmC ) phase agree with one another. The flow phenomena with a fixed director orientation discussed in the foregoing section can not be observed due to the inhomogeneous director orientation in the SmC phase. However, there is a large interest in rotational movements of the director in ferroelectric displays. 

Ferroelectric chiral smectic C phases lack inversion symmetry, and are distinguished by spontaneous helicoidal electric polarization, The first experiment to measure SHG from ferroelectric liquid crystals was carried out [89] on unaligned samples under a dc electric field. Phase-matched SHG in ferroelectric liquid crystals has been carried out by using an electric field to unwind the helix [90]. Mechanical deformations in chiral smectic C elastomers have been shown to give rise to SHG [91]. A great deal of work has been carried out recently in studying SHG in ferroelectric liquid crys-... 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

Examples of chiral smectic Chquid crystals range from 2-methylbutyl (3 -4- -decyloxybenzyhdeneaminocinnamate (12), the first ferroelectric hquid crystal discovered. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

Along with the prediction and discovery of a macroscopic dipole in the SmC phase and the invention of ferroelectric liquid crystals in the SSFLC system, the discovery of antiferroelectric liquid crystals stands as a key milestone in chiral smectic LC science. Antiferroelectric switching (see below) was first reported for unichiral 4-[(l-methylheptyloxy)carbonyl]phenyl-4/-octyloxy-4-biphenyl carboxylate [MHPOBC, (3)],16 with structure and phase sequence... 

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. 

A good deal in synthesis effort has been devoted to chiral liquid crystals, especially those w ith chiral smectic C phases. The chiral smectic C phase is ferroelectric. w hich gives it properties quite useful lor applications. Perhaps the most important properly of these phases is that a lateral dipnle can produce a spontaneous polarization... 

SmB SmC SmC SmCA SmCPA SmCPp SmCo, SmIA SmX UCST XRD Smectic B phase Smectic C phase (synclinic tilted smectic C phase) Chiral (synclinic tilted) smectic C phase Chiral anticlinic tilted (antiferroelectric switching) SmC phase Antiferroelectric switching polar smectic C phase Ferroelectric switching polar smectic C phase Chiral smectic C alpha phase Chiral antiferroelectric switching smectic I phase Smectic phase with unknown structure Upper critical solution temperature X-ray diffraction... 

Earlier work on nanoparticle-doped chiral smectic-A (SmA ) and chiral smec-tic-C (SmC ) phases including some intriguing electro-optic effects in ferroelectric SmC phases were summarized in two earlier reviews [1, 2],... 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

Chirality is also an important aspect of liquid crystals. The introduction of chiral moieties into the chiral smectic phases induces functions such as ferroelectricity and antiferroelectricity. A few of the unconventional chiral liquid crystals are described in Chapter 1. The blue phase is one of the exotic chiral liquid crystalline phases. In Chapter 3, Kikuchi introduces the basic aspects and recent progress in research of the blue phase. Recently, the materials exhibiting the blue phases have attracted attention because significant photonic and electro-optic functions are expected from the materials. 

PEIs derived from AT-(4-carboxyphenyl) trimellitimide and aliphatic spacers (11) are not thermotropic, irrespective of whether the spacer is chiral or not. If the spacers are semi-aliphatic and contain a benzene ring, then thermotropic PEIs may be formed with both a smectic and nematic phase. If chiral spacers are then used a chiral smectic A or C phase may additionally be obtained. Such phases may be ferroelectric, which is extremely rare for main-chain polymers [26]. Examples of chiral and non-chiral spacers used in the copolymers (12) are ... 

Figure 6.38. The chiral smectic C phase (a and b) and the ferroelectric liquid crystal display (c and d).

The use of chiral H-bonding components yields stable chiral phases by molecular association [35-37]. For example, a ferroelectric phase derived from a chiral smectic C phase can be induced by the self-assembly of nonferro-electric molecules [35-37]. Complex 11 (n = 8) exhibits a chiral smectic phase from 109 to 123 °C [35]. In this case, the H-bonded mesogen consists of nonmesomorphic chiral benzoic acid and a stilhazole that shows a very narrow temperature range of ordered smectic phases. The value of spontaneous polarization for 11 at 115 °C is 33.0 nC/cm. When the oxygen in the alkoxy... 

The polymeric complexes derived from 4-nitro- and cyanostilbazoles also show a smectic A phase up to about 200 °C [78a]. Polysiloxane complexes 32 also exhibit thermally stable smectic A or C mesophases [79-81]. These carboxyl-functionalized polymers and stilbazoles are miscible in a whole range of composition and show stable mesomorphic behavior [26, 79]. The introduction of the chiral stilbazole for the formation of a mesogenic complex leads to the induction of ferroelectricity [80]. Polymeric complex 33 exhibits a chiral smectic C phase, while no ferroelectricity is observed for each of single components. The value of spontaneous polarization for 33 x — 0.43, n = 5) is 21.0 nC/cm at 112 °C. The hydrogen bonding between the carboxylic acid and... 

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