Ferroelectric smectic C liquid crystals

Polyimide aligned ferroelectric smectic C liquid crystals. Figures 1 and 3, B.O. Myrvold, Liquid Crystals, 4 (6), p. 637 (1989). Reproduced by permission of Taylor Francis. 

The ferroelectric smectic C liquid crystal display has not, at the time of writing, achieved extensive commercial use. It nevertheless stands as an important device, both because of its potential application in complex displays, which will not require an active matrix, and because of its intrinsic scientific interest. In addition, ferroelectric liquid crystal displays show faster switching rates (of the order of microseconds) than conventional nematic-based displays. 

The main energy terms for a simple model of ferroelectric smectic C liquid crystals will be introduced here. We mention the elastic energy and the energies arising from electric fields. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

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... 

Beresnev, L., Chigrinov, V. G., Dergachev, D. I., Poshidaev, E. P., Funfschilling, J., and Schadt, M., Deformed helix ferroelectric liquid crystal display a new electrooptic mode in ferroelectric chiral. smectic C liquid crystals, Liq. Cry.st., 5, 1171-1177 (1989). 

Another type of LCD uses a chiral smectic C Uquid crystal instead of a nematic liquid crystal. Chiral smectic C liquid crystals are ferroelectric, spontaneously developing an electric polarisation parallel to the smectic layers. In an tmdistorted chiral smectic C liquid crystal, the polarisation is at 90° to the normal to the layers and rotates around the normal as the director rotates aroimd a cone centred on the normal to the layers. However, if the chiral smectic C liquid crystal is placed between properly prepared pieces of glass separated by only several micrometres, it is possible to establish a texture in which the director is parallel to the glass surfaces and uniform throughout the hquid crystal. In this texture, the smectic planes are perpendicular to the glass surfaces and the... 

Recently there was also considerable activity in the investigations of the SHG in nematic and cholesteric phases induced by an external electric field. Such experiments allow the high order molecular hyperpolarizabilities to be calculated. " The SHG was also observed in a ferroelectric (chiral smectic C ) liquid crystal. ... 

Work in other display areas has of course occurred. Through the seminal work of R. B. Meyer and the research of Clark and Lagerwall [55] on surface stabilized ferroelectric liquid crystal devices based on chiral smectic C liquid crystal materials, the potential for ferroelectric devices has been fully explored in recent years. With their faster switching capability, they are attractive, and the difficulties over addressing schemes and the manufacture of ferroelectric displays will perhaps soon be overcome to give the marketplace a further liquid crystal device. 

In a chiral compound, the liquid crystalline structure itself possesses a polarity which interacts with the surface. At the surface of a smectic C liquid crystal, the ferroelectric polarization P points preferably either towards or away from the surface, depending on the material [104, 105]. Conversely, if the surface director of a chiral nematic liquid crystal is tilted, a polarization (dependent on the tilt angle) is created perpendicular to the tilt plane [106]. In principle, the presence of this polarization makes a contribution to the anchoring energy this chiral contribution is, however, too small to be measured [107]. 

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

It is possible to express ferroelectricity in a non-ferroelectric liquid crystal compound exhibiting a smectic phase with a tilt angle such as a smectic C phase by mixing with a chiral dopant. The first smectic C liquid crystal system was a phenyl pyrimidine compound. For the application in displays the following points need to be satisfied small chiral dopant amount, large spontaneous polarization, low viscosity, and optimum temperature range. 

The committee also organized the 4th International Conference on Ferroelectric Liquid Crystals in Tokyo in 1993. Many Japanese companies were very active in ferroelectric liquid crystals materials, especially a smectic C liquid crystal phase in which is growing globally against a background of the next nematic liquid crystal, and the meeting was a great success. 

Archer P, Dierking I (2009) Electro-optic properties of polymer-stabilized ferroelectric liquid crystals before, during and after photo-polymerization. J Opt A Pure Appl Opt 11(2) 024022 Archer P, Dierking I, Osipov MA (2008) Landau model for polymer-stabilized ferroelectric liquid crystals experiment and theory. Phys Rev E 78(5) 051703 Bahr CH, Heppke G (1990) Influence of electric field on a first-order smectic-A-foroelectric-smectic-C liquid-crystal phase transition a field-induced critical point. Phys Rev A 41 (8) 4335 342... 

Lahiri T, Pal Majumder T (2012) The effect of cross-linked chains of polymer network on the memory states of polymer stabilized ferroelectric molecules. Polymer 53 2121-2127 Lee K, Suh SW, Lee SD (1994a) Fast linear electro-optical switching properties of polymer-dispersed ferroelectric liquid crystals. Appl Phys Lett 64 718 Lee K, Suh SW, Lee SD, Kim CY (1994b) Ferroelectric response of polymer-dispersed chiral smectic C liquid crystal composites. J Korean Phys Soc 27(1) 86... 

Chiral smectic C liquid crystals (SmC ) have a twist axis perpendicular to the usual smectic C layers and are known to be ferroelectric. SmC liquid crystals were first shown to be ferroelectric in experiments carried out by Meyer, Liebert, Strzelecki and Keller [200] in 1975, after speculation on the possibility of such ferroelectric... 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Freely-suspended Films of Polymeric Liquid Crystals. The stabilization of freely-suspended films by using polymeric liquid crystals is obviously interesting and has been attempted previously. Unfortunately it seems to be extremely difficult to polymerize films of liquid crystalline monomers as these films were reported to always break during polymerization. It seems to be equally difficult to fabricate FS-films of polymeric liquid crystals in their smectic A and smectic C phases, most likely due to their enhanced viscosities. However, if one heats slightly into the isotropic phase it is possible to spread a film across an aperture which thins out to form a truly freely-suspended liquid crystal film after cooling into the smectic phases (57). Films of this type are homeotropic in the smectic A phase and show birefringence when cooled to the ferroelectric smectic C ... 

The ferroelectric smectics C and H are the only liquid-crystal phases with a polar structure that should make possible in them second-harmonic generation (SHG) via the quadratic nonlinearity Up to now, only nonsynchron-ous SHG was reported in a ferroelectric liquid crystal (LC). - We present here the results of an investigation of phase-synchronous SHG in the ferroelectric phase C with untwisted helicoid. The investigated substance was /7-decy-loxybenzylidene-/7-amino-2-methylbutylcinnamate (DO-MAMBC). 

Only few data are available for other liquid crystal phases (apart from nematics). For example, the azimuthal anchoring energy was recently measured in the ferroelectric smectic C phase [75]. 

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