Tilted smectic mesophase

Note 5 Tilted smectic mesophases formed by chiral compounds or containing chiral mixtures are designated by the superindex (SmC, SmF, etc.). (See, for example. Definition 3.1.5.1.3.)... 

Ferroelectric mesophase that appears through the breaking of symmetry in a tilted smectic mesophase by the introduction of molecular chirality and, hence, mesophase chirality. 

Tilted smectic mesophases formed by chiral materials are designated by the superscript. For instance, when the molecules of the SmC phase are chiral the phase is called SmC. In SmC the phase structure is basically the same except that the molecular chirality causes a minor and gradual change in the direction of the molecular tilt. However, no change in the tilt angle with respect to the layer normal occurs in SmC. This change in tilt direction from layer to layer gradually describes a helix [38],... 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

In the smectic mesophases the molecules are oriented, as in a nematic mesophase, with their principal axis roughly parallel to the director, but they are also defining layers. These layers can be perpendicular to the director, as in the smectic A mesophase (SmA), or tilted, as in the smectic C (SmC). The SmA and SmC mesophases are the less ordered and more common smectic mesophases. Other less common types of smectic mesophases are known, which differ in the degree or kind of molecular ordering within and between the layers [2]. 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

Hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with a hexagonal packing of the molecules with the director tilted, with respect to the layer normals, towards the sides of the hexagons. 

Note 1 When the numbers of layers with opposite tilt directions are not the same, the smectic mesophase has ferroelectric properties. 

Note 7 When the tilt direction alternates from layer to layer, the smectic mesophase is antiferroelectric such mesophases do not possess spontaneous polarization. They can be turned into ferroelectric structures through the application of an electric field. 

As mentioned in the introduction, chiral compounds can exhibit chiral mesophases and these are important due to the important physical properties that they may exhibit, including thermochroism, ferroelectric and electroclinic effects [15], In 1975, Meyer predicted the existence of a spontaneous polarization (Pg) in chiral, tilted smectic phases [86], and the existence of such polar order within a liquid crystal phase has important implications both scientifically and industrially [19]. The asymmetry associated with the chirality may also produce a beneficial lowering of transition temperatures. 

Bis(p-n-decylbenzoyl)methanato-copper(II), which is similar to the molecule depicted in fig. 6.1.1 (g), but with four chains instead of eight, has been reported to exhibit a smectic-like lamellar mesophase. " A tilted smectic C type of structure has been proposed, but the disposition of the molecules in each layer does not appear to have been resolved. It is worth noting that these copper complexes were the first paramagnetic mesogens to be synthesized. 

Whereas monophilic liquid crystals can show a high diversity of smectic phases (SmA-SmQ), the amphotropic liquid crystals normally exhibit only the SmA phase. Tilted smectic phases are only observed in a few cases. The first indication of possibly tilted phases was given in 1933 for thallium stearate [ 170]. A disordered SmC phase was also clearly deseribed for mesogens containing a classical calamitic core aside to their amphiphilic structure [171]. Monophilic liquid crystals can show various ordered tilted smectic phases, for example, smectic I, F, G, J, H, and K. In the case of lipids only one mesophase, the j8 phase,... 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

Watanabe efd/. [59] that the 1.3-phenylenebis[4-(4-14-tetradecyloxyphenyl) iminomethyl benzoate] (PIO-O-PIMB) molecule exhibits two types of B-phases B2 and B4.The stmctural and physical properties oftheB2 phase have been well investigated and understood [60]. It is a smectic mesophase because its n-director is tilted normal to the layer while its bent direction remains parallel to the layer, producing macroscopic polar order and chirality in addition to ferro-or antiferroelectric switching features. 

Among LC phases one distinguishes nematic phases with orientational ordering of the molecular long axes but irregular liquid-like positioning of the mass centers of the molecules (Fig. 2a) and numerous smectic phases with a layered structure. The simplest of them are smectic A, with the molecular axis normal to the layer plane, and smectic C, in which the molecular axes are tilted (Fig. 2b and c). Smectic phases A and C have short-range positional order within the layer, but layers of some other phases (e.g., smectic B) are more ordered. To date, about 20 smectic mesophases have been described, and 8 of them are tilted (smectics C, F, G, H, I,... 

The form chirahty of all of these chiral smectic mesophases takes the form of a helical stracture, but the helix manifests itself in a different way from the helix in the chiral nematic phase. In addition to being substantially the most commonly exhibited of the tilted chiral smectic phases, the chiral smectic C phase is by far the most important (least ordered and least viscous) in this category. The chiral smectic C phase is employed in the ferroelectric display device (see Chapter 13) but the helix must be unwound. 

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