SSFLCs ferroelectric liquid crystals

Along with the prediction and discovery of a macroscopic dipole in the SmC phase and the invention of ferroelectric liquid crystals in the SSFLC system, the discovery of antiferroelectric liquid crystals stands as a key milestone in chiral smectic LC science. Antiferroelectric switching (see below) was first reported for unichiral 4-[(l-methylheptyloxy)carbonyl]phenyl-4/-octyloxy-4-biphenyl carboxylate [MHPOBC, (3)],16 with structure and phase sequence... 

Surface Stabilized Ferroelectric Liquid Crystals (SSFLC)116 Here all three vectors of spontaneous polarization (Fs) are initially aligned by surface effects in thin cells (ca 2 pm). The switchability is due to 180° rotation of the Fs vectors on a cone. 

The backbone affects the dynamic behavior of the ferroelectric liquid crystalline polymer. Sandwiching the two kinds of ferroelectric liquid crystals between two ITO-coated glass plates of 1.5 microns gap respectively, one constructs a SSFLC (surface stabilized ferroelectric liquid crystal) cell. The switch time between two optical states r is determined by... 

Fig. 5.2 Helical structure of SmC phase (a) and surface stabilized ferroelectric liquid crystals (SSFLC) (b and c)...

We can answer the last question if consider a constraction of the so-called surface stabilised ferroelectric liquid crystal cell or simply SSFLC ceU [9]. Such SSFLC cell is only few micrometers thin and, due to anchoring of the director at the surfaces, the intrinsic helical stmcture of the SmC is unwound by boundaries but a high value of the spontaneous polarisation is conserved. The cell is con-stracted in a way to realise two stable states of the smectic C liquid crystal using its interaction with the surfaces of electrodes, see Fig. 13.6a. First of all, in the SSFLC cell, the so-called bookshelf geometry is assumed the smectic layers are vertical (like books) with their normal h parallel the z-axis. Then the director is free to rotate along the conical surface about the h axis as shown in Fig. 13.6b (Goldstone mode). It is important that, to have a bistability, the director should be properly... 

The switching of the director in the surface stabilised ferroelectric liquid crystal cells (SSFLC) [8] has briefly been discussed in Section 13.1.2. Due to its importance for ferroelectric liquid crystal displays we shall discuss this effect in more detail. The geometry of a planar cell of thickness d is shown in Fig. 13.1.2. Now, the helical structure is considered to be unwound. We are interested in the field and time behaviour of the director or c-director given by angle cp(r), and this process is considered to be independent of z and y- coordinates. The smectic C equilibrium tilt angle 9 is assumed constant. 

This problem is overcome by Clark and Lagcrwall in their invention of the surface-stabilized ferroelectric liquid crystal (SSFLC) device [16], shown in Figure 4.9. The liquid crystal is sandwiched between two parallel substrates with the cell gap, h, thinner than the helical pitch, P, of the liquid crystal. The inner surface of the substrates is coated with alignment layers which promote parallel (to the substrate) anchoring of the liquid crystal on the surface of the substrate. The smectic layers arc perpendicular to the substrate of the cell, while the helical axis is parallel to the substrate. Now the helical twist is suppressed and unwound by the anchoring. 

The rotation around the (small) cone is another reason for the fast switching speed. For Ps = 10 nC/cm = 1 V/pm, y = 0.1 poise and = 22.5, r 10 ps The fast switching speed is a merit of ferroelectric liquid crystal devices. Regarding the bistability, on the one hand, it is good because it enables multiplexed displays of the ferroelectric liquid crystal on passive matrices on the other hand, the bistability is a problem because it makes it difficult to produce gray scales. Another issue with SSFLC is that it is more challenging to achieve uniform orientation in SSFLC than in nematic liquid crystals. 

As discussed in Chapter 4, surface stabilized ferroelectric liquid crystals (SSFLCs) have two stable states at zero field. The two states have different planar orientational angles as shown in... 

There are more possibilities for modulating light propagation using ferroelectric liquid crystals, particularly ferroelectric LC polymers, besides the ferroelectric switching in an SSFLC cell as shown in Fig. 32b. 

In the sections on smectic liquid crystals, first the alignment and molecular orientation of surface stabilized ferroelectric liquid crystals (SSFLCs) are treated in detail. Next, the alignment technologies needed for the occurrence of bistability are detailed. Furthermore, liquid crystalline devices made of AFLC materials and the applications of FLC and AFLC materials to active matrix devices are discussed. 

At first in this chapter, ferroelectric liquid crystals (FLCs) and their most interesting application as surface stabilized ferroelectric liquid crystals (SSFLCs) are briefly explained. 

Surface-Stabilized Ferroelectric Liquid Crystals (SSFLCs)... 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

Figure 5.29 (a) Bookshelf geometry in a surface-stabilized ferroelectric liquid crystal (SSFLC) display showing two states of polarization. Here the surface acts to unwind the helix, (b) Chevron geometry in an FLC. This disturbs the switchable polarization in the bookshelf geometry... 

This surface bistability is at the basis of chiral smectic C surface stabilized ferroelectric liquid crystal (SSFLC) devices [92]. As their name indicates, these devices are made of thin cells in which the walls, imposing the orientation of the molecules at the surfaces, unwind the spontaneous smectic C helix and stabilize two uniform configurations of the director in the cell. Switching between these two states can be done by applying an electric field. 

Figure 4. Layering in a surface stabilized ferroelectric liquid crystal (SSFLC) cell showing the degenerate cone for the tilt of the molecules.

In 1980 a much more brilliant approach was described by dark and Lagerwall in which the ferroelectric chancier of tte Sc phase is optimally exploited [2. They proposed the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) structure, which has the unique combined properties of fast response speed (tens of microseconds) and bistabilily. The originally suggested SSFLC structure is based on the following ideas ... 

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. ... 

In helically modulated SmC liquid crystals, the bulk polarization is vanishingly small. The helicity can be unwound by an external field applied parallel to the smectic layers. It can also be unwound by surface effects if the samples are sufficiently thin (thickness pitch), leading to the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) with a nonvanishing macroscopic polarization. 

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