Smectic A and C Phases

Smectic A and C phases are characterized by a translational order in one dimension and a liquid-like positional order in two others. In the smectic A phase the molecules are oriented on average in the direction perpendicular to the layers, whereas in the smectic C phase the director is tilted with respect to the layer normal. A simple model of the smectic A phase has been proposed by McMillan [8] and Kobayashi [9] by extending the Maier-Saupe approach for the case of one-dimensional density modulation. The corresponding mean field, single particle potential can be expanded in a Fourier series retaining only the leading term ... 

On passing from the least ordered smectic A phase down to the more ordered crystal H and K phases the layer planes tend to sharpen up. In the smectic A and C phases the layers are therefore very diffuse, and can be thought of as one-dimensional density waves relative to the director. Thus, locally these two phases are very similar in structure to the nematic phase. 

Liquid crystalline polymers containing ferrocene in their side chains have been reported.235-240 Deschenaux used free-radical synthesis to prepare thermotropic liquid crystalline polymethacrylates containing ferrocene (Scheme 2.46).235 Polarized light microscopy showed that monomer 171 and its corresponding polymer 172 exhibited enantiotropic smectic A and C phases. 

As the temperature is further cooled, the molecules begin to segregate into planes giving rise to a smectic A or smectic C phase. In addition to the orientational order that the nematic phase shows, the smectic A and C phases exhibit a one-dimensional translational order, and can therefore form layered structures. There is a liquid-like motion of the rods in each layer and no correlation of the molecular positions from one layer to the next. In each layer, the mass centers of the molecules are randomly distributed as... 

In the smectic A phase, molecules tend to be perpendicular to the smectic layers. The layer thickness d is roughly the same as the molecular length l. The thickness of the layers in the case of liquid crystalline polymers is about the order of the monomer s length. But in the smectic C phase, the molecules in the layers are parallel and tilted in arrangement with respect to the normal of the layers by a tilt angle 0. The layer thickness of the smectic C phase is d = l cos 6. The ordering of the smectic A and C phases are both higher than the nematic phase so that they appear at a lower temperature than the nematics do. The smectic A phase appears first as the temperature decreases if a compound shows both the smectic A and C phases. 

FIGURES One-dimensional translational order in smectic A and C phases in smectic A the thickness of the layer is close to monomer length /, whereas in smectic C it is / cos 0 with 6 being the tilt angle to normal. 

The orderings in smectic A and C phases are higher than in the nematic phase. They appear at lower temperatures as the nematics. In systems that show smeetic A and C, phase A appears first as the temperature decreases. 

Smectic A and C phases are the most common smectic phases. They exist in several texture variants. Similar to the nematic phase, smectic A phase can exhibit the homogenous texture. The corresponding texture of the smectic C is the schlieren texture, In this texture, the layers are parallel to the sample surface. The schlieren indicates strong distortions of the director field [1]. 

Liquid crystals manifest a number of transitions between different phases uprm variation of temperature, pressure or a craitent of various compounds in a mixture. All the transitions are divided into two groups, namely, first and second order transitions both accompanied by interesting pre-transitional phenomena and usually described by the Landau (phenomenological) theory or molecular-statistical approach. In this chapter we are going to consider the most important phase transitions between isotropic, nematic, smectic A and C phases. The phase transitions in ferroelectric liquid crystals are discussed in Chapter 13. 

Smectics exhibit positional disorder in two dimensions but have a center-of-mass correlation along the third dimension. Each layer can be likened to a two-dimensional fluid. There are various smectic phases. The smectic A and C phases are represented in Fig. Ic and d. The direction of molecular alignment is perpendicular to the layer planes in the smectic-A and is at a tilt to the layer normal in the smectic C phase. Numerous other smectic phases have been identified and are distinguished by various in-plane order and correlations between the layers. 

Just as with the smectic A and C phases, examples of each of these more ordered phases with a director both perpendicular to the layers and at an angle to the layer normal have been observed. For the tilted phases, the director can point to one of the nearest neighbour molecules or toward the midpoint of the line joining two nearest neighbours. 

In the smectic A phase, the director is perpendicular to the planes, while in the smectic C phase, the director is tilted at an angle less than 90° to the planes. In the smectic A and C phases, the molecules diffuse randomly and as a result, no positional order exists within the planes (positional order exists only in one dimension). However, other smectic liquid crystal phases exist in which the molcules have some degree of order within each plane that results in three-dimensional positional order (or quasi-three-dimensional order). In this case, molecules diffusing through the plane spend more time at certain locations than at other locations. 

The mesophases differ from each other regarding the positional order of the molecules (Fig. l). In the nematic phase there is no long range positional order at all just as in isotropic liquids. Nematics are normally uniaxial, however biaxial nematics were discovered very recently. In the smectic phases the centre of masses of the molecules are concentrated in layers forming a one-dimensional density wave. In the smectic A and C phases there is no long-range positional order within the layers. The smectic A phase is uniaxial, the director (n) is parallel with the layer normal, 1. In the C phase the director is tilted with respect to the layer normal. This phase is biaxial although the deviation from uniaxiality is usually small. There are further smectic phases in which the molecules form two-dimensional lattices within the layers (ordered smectic phases). The difference between ordered... 

Arkas et al. [281] have prepared cyano-propylalkyldimethylammonium bromides that show smectic A phases. There are also materials that show smectic A and C phases [282]. The smectic A phases of substituted pyrimidinium salts are completely miscible with non-polar pyrimidine derivatives [283]. 

Polymerization of organoiron monomers has also resulted in the production of liquid crystalline polymers containing ferrocene units in their sidechains. " Scheme 6 illustrates Deschenaux s free-radical synthesis of ferrocene functionalized thermotropic liquid crystalline polymethacrylates Monomer 31 and its corresponding polymer 32 exhibited enantiotropic smectic A and C phases. 

Fig. 1.9 Smectic A and C phases. These have highly mobile, disordered layers, which can be regarded as two-dimensional liquids...

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