Tilted Smectic Layers

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. 

Ito et al. [152] described the crystal structure of 4-[(S)-2-methylbutyl]phe-nyl 4 -hexylbiphenyl-4-carboxylate which shows a smectic A phase and a cholesteric phase. The molecules are arranged in a tilted smectic-like layer structure. Within the layers, the long molecular axes are tilted (30°). However, the compound exhibits no smectic C 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. 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

While the existence of the anticlinic minimum in conventional SmC materials has been suggested by experiments with DOBAMBC,17 by far the majority of tilted smectics only exhibit the synclinic structure of the SmC or SmC under normal conditions. When the mesogenic compound is unichiral (i.e., enantiomerically pure) or enantiomerically enriched, a macroscopic polarization with the same orientation in each layer is produced, as discussed above. In the SmCA phase of MHPOBC, however, the anticlinic structure is the global minimum structure, as shown at the bottom of Figure 8.9. In this case, given that the chirality of the molecules is fixed, the polarization must alternate... 

Figure 8.14 Illustration of first achiral antiferroelectric system bilayer tilted smectic with alternating synclinic and anticlinic layer interfaces.

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

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. 

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. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

Recently, fluorinated chiral LC with highly tilted smectic C phases and opposite tilt direction in adjacent layers (anticlinic tilt, SmCA ) became of significant interest, as Lagerwall et al. proposed an application of 90° tilted anticlinic and... 

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

Within the family of untilted smectics, there is a hierarchy of phases with order ranging from that of smectic A, in which there is no positional order within each layer, to the crystalline smectics B and E, which have long-range positional order within each layer, namely hexagonal and orthorhombic for the B and E phases, respectively. This inplane order, represented by the dots within circles or ellipses of Fig. 10-27, propagates from layer to layer, producing fully three-dimensional crystalhne order. The family of tilted smectics includes similar ordered phases J, G, K, and H. 

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. 

The polarization in a specific tilted smectic layer formed of chiral liquid crystals always has two components, as already mentioned the piezoelectric and the flexoelectric components. The piezoelectric component is to a first approximation ... 

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. 

Assuming the head-to-tail symmetry of a bent-shape molecule, the highest symmetry of a uniaxial non-tilted smectic A layer is Dooh- Then, according to molecular packing presented in Fig. 13.29a, the highest symmetry of the biaxial polar layer is 2, there is a rotation axis C2 parallel to x, and two symmetry planes xz and xy. The layer polarization is possible along the C2 axis. Had the layer consisted of the rod-like molecules tilted within the xz plane the symmetry would be C2h as in SmC. However, when the bent-core molecules are tilted in the y-direction (forward... 

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