Applications smectic materials

During the 1980s the development of ferroelectric liquid crystals continued at Bell Laboratories, and the above property-structure correlations suggested to us that, for the development of smectic C and smectic materials which would be suitable for use in applications of ferroelectric displays, it would not be wise to investigate 1-methylalkyl-substituted systems because of the... 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

The mesomorphic properties and physical properties of nematic (and smectic) materials and nltimately their suitability for applications are all fundamentally dictated by the chemical stracture of the constituent molecules. Before progressing further, several terms and their definitions need to be clarified this will be done by using the nematic phase. The term nematic phase stability refers to the upper-temperature limit (T j) to... 

The synthesis of nonchiral smectic liquid crystals is a broad topic for discussion, however, it can be divided into subsections in two different ways. For example, smectic systems can be split into metallomesogens and nonmetallomesogens, alternatively, they can be divided into materials for (1) meso-phase structure elucidation and classification [ 1 ], (2) property-structure correlations [2] and (3) host systems for ferroelectric and antiferroelectric mixtures. In the following sections template structures used for the synthesis of smectic materials will be described, followed by discussions of the syntheses of materials that have extensive histories in the elucidation of smectic phase structures, and finally of the syntheses of smectogens that are useful in applications. 

An improvement of this device has been described in which the nematic-cholesteric mixture is replaced by a smectic material. Thermal writing induces the change of the smectic from the perpendicular to a scattering texture. Unlike the nematic-cholesteric materials, selective erasure is possible with the smectic device. The thermal writing is too slow for television-rate applications because of the thermal inertia of the glass-liquid-crystal system. With a laser-beam power of 20 mW, addressing speed is approximately 10 elements/sec for the smectic device. In the projection mode, the resolution is 50 lines/mm at a contrast ratio of approximately 10 1. 

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Figure 8.13 Hypothetical smectic mesogen with hinge in center of core is illustrated. Such material could in principal switch to ferroelectric state, which we term the SmAPp, upon application of electric field in plane of layers. If this state exists in well on configurational hypersurface, then ground-state structure is antiferroelectric, denoted SmAPA.

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. 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

Patel J.S., Lee Sin-Don and Goodby J. W. Physical Properties of Smectic Liquid Crystals and Novel Electro-optic Effects. Spatial Light Modulator Technology, Materials, Devices, and Applications (Uzi Efron, Marcel Dekker, New York, 1995). 

Thus, side-chain systems can exhibit many properties in between, well-oriented and solid materials. Many applications for cholesteric, nematic, and smectic cyclic siloxanes have been proposed. Most of them use cholesterics. Cholesteric liquid crystals (n ) or tilted smectic phases reflect the incident light in a specific wavelength range and with circular polarization. The... 

Nematic materials are only one member of a large family of a variety of structurally different compounds forming liquid crystalline mesophases. Although only nematics have yet found really widespread use, mostly for display applications, some structurally highly diverse smectic phases also have unique electrooptical characteristics, for example ferroelectricity or antiferroelectricity, which can be modulated by selective fluorination [5, 51]. For 20 years intensive effort has been devoted to making practical use of these phenomena. 

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... 

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