Tilted smectic phases

In 1995, Mandal et al. [151] described the crystal structure of N,AT-bis-(p-butoxybenzylidene)-a,a -bi-p-toluidine. They found that the two symmetrical fractions of the molecule are almost planar, but the angle between these planes is 63.5°. The molecules are arranged in tilted layers. This tilted layer-like structure is referred as a precursor to the tilted smectic phase. 

An orthogonal layered structure in the solid state of rod-like molecules is the exception rather than the rule. Therefore, there is no conclusive evidence that a tilted layer structure in the solid state melts to a tilted smectic phase. In other words, if we consider the solid state as precursor for the type of the liquid crystalline state, no real precursor for an orthogonal fluid smectic phase would exist. As demonstrated in Fig. 19, the compound B-A for example exhibiting a smectic A phase has a tilted layer structure in the solid state. 

Figure 8.5 Structure and phase sequence of DOBAMBC is shown. Terminology used to describe parts of LC mesogen is given, in addition to graphical representation (zigzag structure) of molecule as it exists in tilted smectic phase.

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. 

The enhanced chirality by doping SmC with BSMs can be explained qualitatively in the same way as in the N phase. However, the situation is more complicated in SmC because of spontaneous polarization and flexoelectric effect, and (3) must be replaced by an equation including such effects. Actually, the contribution of flexoelectric effect has been discussed by Gorecka et al. [4]. The other important effect is caused by the fact that the BSMs are in the tilted smectic phase. As mentioned above, the tilt of BSMs induces chirality as observed in the B2 phase. 

Fig. 8 Definition of layer chirality in the bent-core tilted smectic phase...

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... 

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. 

As the difference in the transition temperatures between the chiral and racemic forms of the same compound is found to be only of the order of 1 °C, it is clear that the transition is driven by intermolecular forces responsible for the tilted smectic phase and not by the dipole-dipole... 

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,... 

Finally note that the flexoelectric effect is also important in the smectic phase although the corresponding molecular theory is at a rudimentary stage. Recently a molecular model for the conventional and the so-called discrete flexoelectric effect in tilted smectic phases has been proposed... 

If the mesogens are chiral, a twisted nematic, suprarmolecular, cholesteric (twisted) phase can form [51, 52]. The achiral nonlinear mesogens can also form chiral supramolecular arrangements in tilted smectic phases. 

Schlieren textures are very common in liquid crystals and are usually easily identified and classified. These patterns are observed in homogeneously oriented nematic phases and for smectic phases where the long axes of the molecules are tilted with respect to the layer planes and the layers are arranged parallel to the surface of the glass. Thus, schlieren textures occur for the nematic phases of both calamitic and discotic materials, and for the tilted smectic phases C, I, and F. 

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. 

Year 1975 has been marked off by an outstanding publication of R. Meyer and his French co-workers [4]. As has been discussed in Section 4.9, chirality of molecules removes the mirror symmetry of any phase. The idea of Meyer was to apply this principle to the SmC phase by making it chiral. He believed that if chiral molecules formed a tilted smectic phase, its point group symmetry would reduce from to C2... 

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... 

There is one more, rather polymer-specific possibility for combining a tilted smectic matrix and a chiral dopant, namely copolymerization. Thus, a nonchiral methacrylate monomer inducing a tilted smectic phase was copolymerized with a chiral comonomer that forms a smectic A homopolymer only [77], The resulting copolymers ... 

Ferroelectricity can also be observed in other chiral tilted smectic phases of FLCPs such as smectic F, smectic I, and smectic H (Fig. 21). Those smectic phases are more ordered and therefore more viscous, so the switching processes are hindered. Nevertheless, these phases of FLCPs show such ferroelectric behavior as pyro- and piezoelectric effects [28,30,77,106]. 

FIGURE 21 Ordered tilted smectic phases (a) smectic F phase (b) smectic H phase. The smectic 1 phase has the same structure as smectic F but another tilt direction (not along the prism face but along the prism edge). 

Dielectric relaxation spectroscopy can monitor molecular and collective modes for motion of liquid crystalline molecules. In ferrolectric liquid crystals based on chiral tilted smectic phases the complex dielectric permittivity e has, in addition to molecular orientational modes, two contributions from the director fluctuations. 

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