Electrooptical smectics

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

A prerequisite for experimental determination of the anisotropic electrooptic properties (Ae, An) is the occurrence of a nematic phase with a defined order parameter S [4]. As single substances, many commercially used liquid crystalline materials have either no mesophase or a smectic phase only. As components of nematic basic mixtures on the other hand, they behave like typical liquid crystals. 

Nematic materials are only one member of a large family of a variety of structurally different compounds forming liquid crystalline mesophases. Although only nematics have yet found really widespread use, mostly for display applications, some structurally highly diverse smectic phases also have unique electrooptical characteristics, for example ferroelectricity or antiferroelectricity, which can be modulated by selective fluorination [5, 51]. For 20 years intensive effort has been devoted to making practical use of these phenomena. 

A central part of the application-oriented evaluation of liquid crystals are so-called virtual clearing temperatures, electrooptic properties, and viscosities. These data are obtained by extrapolation from a standardized nematic host mixture. 7 Af, An, and jy are determined by linear extrapolation from a 10% iv/iv solution in the commercially available Merck mixture ZLI-4792 (Tfji = 92.8°C, Af = 5.27, An = 0.0964). For the pure substances the mesophases are identified by optical microscopy and the phase transition temperatures by differential scanning calorimetry (DSC). The transition temperatures in the tables are cited in °C, numbers in parentheses denote monotropic phase transitions which occur only on cooling the sample C = crystalline, S = smectic A, Sg = smectic B, S = smectic G, S> = unidentified smectic phase, N = nematic, I = isotropic. 

The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

On the other hand, it has been shown on LMWLCs that the well-known SmC, where the molecules are tilted with respect to the layer normal, is no longer the only possibility to obtain a fluid biaxial phase [63], As a consequence, a strict determination of the chiral smectic phase structure requires not only a careful analysis of the X-ray diagrams obtained on powder as well as on aligned samples, but also a study of the electrooptic response, which allows discrimination between the ferroelectric, the antiferro-electric, and the ferrielectric behavior. 

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

In the X-ray experiments on nematic 8CB, the smectic ordering was observed at the free surface (air-nematic interface). The same phenomenon has also been observed at the solid-nematic interface by the X-ray, an electrooptical technique and molecular force measurements. The principle of the latter is shown in Fig. 10.7. For two mica cylinders submerged in nematic liquid crystal, their interaction force measured with a balance oscillates with a distance between the cylinders and the period of oscillations was found to be equal to molecular length 1. This clearly shows the periodicity in density characteristic of a smectic phase [8]. 

The coefficient y is rotational viscosity of the director similar to coefficient yi for nematics. In fact, it does not include a factor of sin cp and, in the same temperature range, can be considerably larger than the viscosity ytp for the Gold-stone mode. This may be illustrated by Fig. 13.10 the temperature dependence of viscosities y and have been measured for a chiral mixture that shows the nematic, smectic A and smectic C phases [15]. The pyroelectric and electrooptic techniques were the most appropriate, respectively, for the measurements of ya and ytp describing the viscous relaxation of the amplitude and phase of the SmC order parameter. The result of measurements clearly shows that y is much larger than y and, in fact, corresponds to nematic viscosity yj. 

By a proper treatment of the electrodes, one can obtain a texture with a uniform orientation of the smectic normal in one direction within the cell plane. Between the crossed polarizers such a cell will be black if a polarizer is installed parallel to the smectic normal. Upon application of the electric field, the antiferroelectric structure becomes distorted. At low voltages of any polarity, the electrooptic response is proportional to E the bottom part of the curves has symmetric parabolic form [35] shown in Fig. 13.24b. Above the AF-F transition, the director acquires one of the two symmetric angular positions ( 9 on the conical surface) typical of the SmC phase. At these two extreme positions the transmission is maximum. With increasing temperature from T toTi the AF-F threshold decreases due to a decrease of the potential barrier separating structures with alternating and uniform tilt. It is natural because within the SmC A phase T1 is closer to the range of the SmC phase than T2 or T3. 

Finally the three remaining Chapters 10-12 are devoted to optics and electrooptics of, respectively, nematic, cholesteric and smectic (ferroelectric and antiferro-electric) phases. In contrast to my earlier book published by WUey in 1983, only the most principal effects have been considered and the discussion of the underlying principles is much more detailed. 

The smectic A is an untilted phase in which the mass density wave is parallel to the director. The cost in free energy of buckling the layers into saddle-shaped deformations is low, with the result that it is relatively easy to construct devices that show bistability between a scattering focal conic director configuration in which the layers are buckled and a clear homeotropic configuration in which the director is perpendicular to the cell walls and the layers parallel to the walls. Transitions between these two textures have been exploited in laser-written projection displays and in both thermo-optic and electrooptic matrix displays. The various mechanisms employed are summarized in Fig. 12. 

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). 

This class of materials is of fundamental importance for the further discussion of their electrooptical properties. Traditionally, we divide these mesophases into the nematic and various smectic phases [1-5]. Nematic liquid crystals are characterized by long-range orientational order and the random disposition of the centers of gravity of individual molecules. As for an isotropic liquid, the density does not depend on coordinates... 

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