Smectic materials

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

The various orientations and conformations mesogenic groups can adopt in a layer has been studied extensively for low molar mass smectic materials, and the classification and terminology of smectic systems is entirely based on these studies. However, low molar mass smectic compound or smectic LC-side chain polymers do, of course, not allow one to elucidate the role spacers play in the layer structures of LC-main chain polymers. Therefore, poly(ester-imide)s, po-... 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

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... 

Isotactic polypropylene is known to crystallize when cooled quickly from the melt into a crystal form which was called a smectic, mesomorphic form. The X-ray diffraction pattern resembled clearly that of a smectic material. Later, this structure was also called para-crystalline to indicate the poor crystalline order. It was found that this structure is metastable below 335 K. At room temperature it has been reported to persist for over 18 months so that it should be called a CD-glass. The proposed transition mechanism to the stable crystal form involves intramolecular helix... 

The values however are much smaller than for smectic materials [157], This either implies that the aliphatic chains are not as disordered in the mesophase as in other liquid crystals or that they are not as ordered in the solid state as they usually are in their simple form in the crystal [146c], The following tables represent transitions of selected liquid crystalline phthalocyanines and porphyrins. 

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. 

A eutectic mixture of two smectic materials has a crystal to smectic transition which is lower than either of the components. 

To some extent, development of smectic materials has been slow and usually come as an off shoot of work on nematics. For example, the binary mixture shown in (38) exhibits a room temperature smectic A, a short range nematic, and is of positive dielectric anisotropy.Such properties may be used in thermally addressed displays where a transition from a scattering to a clear state forms the optical effect. This transition may also be effected by an electric field. Obviously, more work on useful smectics is required although their inherent high viscosity is a problem. 

The Austrian botanist Reinitzer, when heating cholesteryl benzoate, observed a melting point at 145.5 °C, leading to a cloudy liquid which cleared at a temperature of 179.5 °C. He had diseovered cholesteric liquid crystals. In 1922, FriedeE described a variety of different liquid crystal phases and proposed a classification scheme consisting of the three broad classes nematic, cholesteric and smectic materials. 

The word smectic comes form the Greek word anriyua, meaning soap and reflects the mechanical properties displayed by many of these materials. All smectics are layered structures having well-defined interlayer distances. Smectic materials are therefore more ordered than nematic ones and occur at a temperature below that for the nematic phase. A number of smectic phases have been identified. ... 

The optical microscopy or TOA of the smectic side-chain polymers shows the important effect that viscosity has on the behaviour of these systems, i.e. the existence of T and the fluid region, the rate of growth of texture on cooling and the immobility of the texture below r. Since smectic materials will be more viscous than the nematic phases, this suggests again that the materials should be useful for the storage of electro-optic information if this information can be written on an acceptable timescale. 

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... 

Dislocations are the most elementar defects of a SmA phase. Since they cannot end in the smectic material, they must form closed loops (at least in the thermodynamic limit of an infinite sample). The elastic energy of a dislocation line is proportional to its length [31]. It is then characterized by a line tension Yo- The total free energy per unit length is ... 

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. 

Esters provide a wide range of stable smectic materials which are suitable for physical studies. In many instances they are more useful for investigations than Schiff s bases (nOms and TBnAs) because of their better stability and purity. Simple 4-alkoxy-phenyl 4-alkoxybenzoates are an intere.sting class of materials because they provide low temperature smectic C variants (often with the addition of smectic A and nematic phases). 

Metal-containing smectic materials are of considerable interest because, as they carry metal atoms or ions, they can be effectively studied by X-ray diffraction techniques. In addition, their unique shapes and physical properties make them ideal candidates for the study of biaxiality and conductivity in smectic liquid crystals. One class of materials that stands out are the copper(II) complexes of )3-diketones. These materials can exhibit columnar as well as smectic mesophases. 

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