Chiral liquid crystals, surface alignment

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

We fnst consider the 0°/360° BTN, where the alignment directions on the two ceU surfaces are parallel, as shown in Figure 10.13. Chiral dopants are added to the nematic liquid crystal to... 

Cases (c) and (d) both represent truly ferroelectric behavior, which can be found in smectic C phases of chiral molecules only if the helix is suppressed by the boundary conditions at the liquid crystal glass interfaces. Because it is nearly impossible to align columnar phases uniformly with rubbed or otherwise anisotropic polymer surfaces (the switching is generally studied in polydomain or sheared samples), and because the ferroelectric behavior remains present in thick cells, surface induction of ferroelectricity can be excluded. It must be assumed that the two-dimensional column lattice suppresses helix formation in these materials. 

For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

The chiral smectic C phase has the unique property of a dipole perpendicular to the tilt direction of the mesogens. This results from the lack of a mirror plane due to the chirality of the mesogens. However, a macroscopic polarization is not observed, as the lilt direction changes from layer to layer to form a helical superstructure. The twist can be unwound by surface alignment and electrical fields in a so-called surface-stabilized ferroelectric liquid crystal (SSFLC) cell. ... 

---