Structures of Smectic Liquid Crystals

The lamellar smectic state is readily divided into four subgroups by considering first, the extent of the in-plane positional or- 

smectics A, C, C i, B e, I, and F are essentially smectic liquid crystals, whereas B, E, G, H, J, and K are crystal phases. These latter phases, however, have somewhat different properties from normal crystals, for example, their constituent molecules are reorienting rapidly about their long axes (10 times per second) [19, 26]. 

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L. Zou, J. Wang, R Basnet, and E. K. Mann, Line tension and structure of smectic liquid-crystal multilayers at the air-water interface, Phys. Rev. 76,031602 (2007). 

Figure 7.1. Categories of smectic liquid crystals. From Smectic Liquid Crystals, Textures and Structures, Gray, G.W. and Goodby, J.W.G. 1984 (Leonard Hill, Glasgow). (Reproduced by kind permission of the authors and publishers.) Side elevation and plan representation of molecular ordering in each of the smectic modifications. Triangles or arrows are used to represent tilt direction.

Polarizing optical microscopy is often the method of first choice. It was with POM that the textures of liquid crystals were observed and the classification of liquid crystals was first made according to these observations. In this field, two books are recommended The Textures of Liquid Crystals by Demus and Richter (1978) and Smectic Liquid Crystals — Textures and Structures by Gray and Goodby (1984). While the latter provides readers with a practical and useful experimental guideline to the textures and classification of smectic liquid crystals of different polymorphic types together with as many as 124 reference photographs of typical textures, the former... 

Table 5.1.1. Structural classification of smectic liquid crystals...

By measuring the surface forces one can therefore obtain important information not only on the structure of a liquid crystal, but also on the influence of confining surfaces on orientational and positional ordering on a molecular level. This Chapter describes experimental techniques that are used for the measurements of surface forces and is focused on the results of the experiments that have been recently performed in the isotropic, nematic and smectic-A phases. 

Due to a pronounced optical anisotropy of ordered liquid crystals, reflection elhpsometry is a powerful experimental tool for the study of surface-modified liquid crystalline order. The sensitivity of the technique enables quantitative measurements of the nematic order parameter profile at the interfaces. Prom observation of the nematic wetting of the solid-liquid crystal interface, we have determined the values of the coupling energies between the surface and the nematic order. Even the pretransitional effects at solid-smectic liquid crystals can be studied with elhpsometry due to the strong coupling between the nematic and the smectic order. The time evolution of the complex structure of the liquid crystal adsorbate on a solid substi ate has also been successfully monitored using BAE. 

Molecules that exhibit fiquid crystallinity are usually long and rodlike. An important class of liquid crystals is called thermotropic liquid crystals, which form when the sohd is heated. The two common structures of thermotropic liquid crystals are nematic and smectic. In smectic fiquid crystals, tfie long axes of... 

Structure of thermotropic liquid crystals is rather well understood. There are three main structural types nematic, cholesteric, and smectic. In nematic liquid crystals molecules are aligned approximately in the same direction, but positionally molecules are disordered. An axis of preferable molecular orientation is called a director. More precisely, the director is defined as a unit vector n(r) that is parallel to the molecular orientation at the point r. If we use the long axis of the molecules as a reference and denote it as k, the microscopic scalar order parameter 5 is defined [16,17] as follow ... 

Introduction of chirality into the chemical structure of a liquid crystal molecule has no effect on the structure of orthogonal smectic phases (e.g., smectic A). Ne-... 

Watanabe, J., Hayashi, M., Morita, A., and Niiori, T, Structural characteristics of smectic liquid crystals in main-chain polyester. Mol. Cryst. Liq. Cryst., 254, 221-240 (1994). 

Free suspended films of smectic liquid crystals can show a change in the crystalline order from the quasi-two-dimensional to the three-dimensional one [11]. Very thin smectic B films (number of monolayers n < 10) are two dimensional with the hexatic in-plane order, and with (low temperatures) or without (higher temperatures) a tilt of molecules. With increasing n (n > 20) the transition to various three-dimensional crystalline structures is observed. 

The FLCs or AFLCs mentioned above are liquid crystalline materials showing a chiral smectic C phase or related phases. Although nematic liquid crystal possesses only directional order, smectic liquid crystals shows layer structures or periodic order of the liquid crystalline molecular centres. In this respect, smectic liquid crystalline materials have more in common with crystalline materials than with nematic liquid crystalline materials. As a result, the alignment of smectic liquid crystal is quite different from that of nematic liquid crystals. Smectic liquid crystals show a large variety of defects because they possesses highly ordered structiu-es. Moreover, the layer structures are irreversibly destroyed by applying stress. This phenomenon poses a major problem for display applications. Therefore, several techniques to prevent the application of force to the liquid crystalline materials of FLC or AFLC devices have been proposed. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

X-ray diffraction is one of the primary methods to determine the structure of a liquid crystal phase. Nematic phases are characterized by diffuse arcs at small angles which result from local smectic-type order ( cybotactic clusters . Section 5.7.1). Smectic phases are characterized by Bra spots which result from the layer periodicity (Fig. 5.14). In oriented smectic A phases, the Bragg reflections are normal to the layers and their position is reciprocally related to the smectic layer period. If q is the wavenumber defining the peak position, then from Bragg s law (Eq. 1.26) d = Injq. The molecules and layers in a smectic C phase can adopt different mutual orientations depending on how the phase is accessed. If it is obtained by cooling from an SmA phase, the layer orientation is retained and the molecules tilt. Because the layer orientation... 

Structure is unwound and h points in a fixed direction, as in Figure 1.12d, then p will point in one direction. Clearly, this and other director axis reorientation processes are accompanied by considerable change in the optical refractive index and other properties of the system, and they can be utilized in practical electro- and opto-optical modulation devices. A detailed discussion of smectic liquid crystals is given in Chapter 4. 

XL13654, and SCE9. A well-studied ferroelectric liquid crystal is DOBAMBC its molecular structure is shown in Figure 4.1 Ic. Because of these differences in the degree of order and molecular arrangement and the presence of a permanent dipole moment, the physical properties of smectic liquid crystals are quite different from those of the nematic phase. In this and the following sections we examine the pertinent physical theories and the optical properties of three exemplary types of smectics smectic-A, smec-tic-C, and (ferroelectric) smectic-C. ... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

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