Tilted smectics

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. 

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.

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 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. 

Note 2 The appearance of a spontaneous polarization,, in chiral tilted smectic... 

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). 

SmB SmC SmC SmCA SmCPA SmCPp SmCo, SmIA SmX UCST XRD Smectic B phase Smectic C phase (synclinic tilted smectic C phase) Chiral (synclinic tilted) smectic C phase Chiral anticlinic tilted (antiferroelectric switching) SmC phase Antiferroelectric switching polar smectic C phase Ferroelectric switching polar smectic C phase Chiral smectic C alpha phase Chiral antiferroelectric switching smectic I phase Smectic phase with unknown structure Upper critical solution temperature X-ray diffraction... 

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... 

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...

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. 

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