Tilt SmC phase

Electroclinic effect118,119 Close to the phase transition from the tilted SmC phase to the orthogonal SmA phase the tilt angle becomes soft (soft mode). Consequently, one can realize faster switching times than in SSFLC and DHF cells. 

Let us first consider a fictitious material, which has a tilted SmC phase, whose molecules are chiral but not polar. Let us also keep in mind that such a material does not exist in reality. There exist materials, which at a certain, well-defined temperature, exhibit a sign reversal of their polarization. At exactly the temperature of the polarization reversal the polarization does not exist due to several reasons the average of the coexistent chiral conformations is not polar or the hindrance of the rotation is such that the polarization coincides with one of the two axes x and z) or both. But this is not the case for a wider temperature range. The molecule itself always has some polarization because of its structural asymmetry. 

O-Cyano Chains. The fo-cyano chains associate the flexibility of the paraffins with the polarity of the cyano group. They are used in order to obtain orthogonal lamellar mesophases such as an SmA phase instead of tilted SmC phases. In fact, the biforked mesogens 42a displayed an SmA phase... 

Schematics of the arrangement of the more realistic SmA and SmC phases. It can be seen that the tilted SmC phase has a closer packing (hexagonal) than in the orthogonal smectic A (rectangular). These fllustralions, of course, assume short-range in-plane packing, which is still compatible with the macroscopic fluid nature.

This transition is usually second order [18,19 and 20]. The SmC phase differs from the SmA phase by a tilt of the director with respect to the layers. Thus, an appropriate order parameter contains the polar (0) and azimuthal ((]i) angles of the director ... 

Figure 8.6 Three-dimensional slice of C2 symmetrical SmC phase, showing tilt cone, polar axis (congruent with twofold symmetry axis), smectic layer planes, tilt plane, and polar plane.

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Apparently this switching mode is disfavored since, in fact, the chirality of the layers does not change upon switching to the ferroelectric state rather the layer interface clinicity changes. This occurs when the molecules in alternate layers simply precess about the tilt cone in a manner exactly analogous to antiferroelectric to ferroelectric switching in the chiral SmC phase. As shown in Figure 8.25, the ferroelectric state obtained from the ShiCsPa antiferroelectric phase is a ShiCaPf structure, an achiral macroscopic racemate with anticlinic layer interfaces. 

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. 

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... 

Fig. 30 Selected examples of chiral rod-like mesogens with one fluorinated chain (77° C) one enantiomer is shown as example (SmCA = antiferroelectric SmC phase SmC = ferroelectric SmC phase SmCpi = ferrielectric SmC phase SmCa = helical SmC phase SmI = chiral tilted low temperature phase) [197-199]...

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. 

Finally, the difference of chirality enhancement in the N and SmC phases should be mentioned. As shown in Sect. 2.1, enhancement rate in SmC is about one order of magnitude larger than that in N. In the SmC chirality enhancement is attributed to two effects (1) the interaction between bent-core and chiral host molecules and (2) the coupling between ee, tilt, and spontaneous polarization. The latter effect is absent in the N phase and is an additional effect in SmC. Moreover, the chiral discrimination parameter AU is expected to be larger in SmC than in N because of a confined geometry, i.e., smectic layer. 

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