The Liquid-crystalline State - Molecular Shape and Structure and Liquid Crystals - Thermotropic Liquid Crystals - Lyotropic Liquid Crystals - Plastic Crystals - Structures of Thermotropic Liquid Crystals - Structures of Smectic Liquid Crystals
Structures of Smectic Liquid Crystals
The lamellar smectic state can be divided into four subgroups by considering the extent of the in-plane positional ordering of the constituent molecules, and the tilt orientational ordering of the long axes of the molecules relative to the layer planes. Two groups can be defined where the molecules have their long axes essentially normal to the layers. These two groups are distinguished from each other by the extent of the positional ordering of the constituent molecules. For example, smectic A and hexatic B are smectic liquid crystals in which the molecules have only short-range positional order, whereas crystal B and crystal E are smectic crystal modifcations where the molecules have long-range orientational order in three dimensions. Two other classes can be distinguished where the molecules are tilted with respect to the layer planes. In smectic C, smectic I, and smectic F, the molecules have short-range orientational ordering, whereas in crystal G, crystal H, crystal J, and crystal K the molecules have long-range three-dimensional ordering. Thus, as already noted, smectics C, I, and F are essentially smectic liquid crystals, whereas G, H, J, and K are crystal phases. These latter phases, however, have somewhat different properties than normal crystals, for example, their constituent molecules are reorienting rapidly about their long axes (1011 times s-1).
Structure of the Smectic A Phase
In the A phase the molecules are arranged in diffuse layers so that their long axis are on average perpendicular to the layer planes. The molecules are undergoing rapid reorientational motion about their long axis on a timescale of 1011 times per second, and also undergoing relaxations about their short axis but on a much longer timescale of 106 times per second. The molecules are arranged in such a way that there is no translational periodicity in the planes of the layers or between the layers. Therefore, the molecules have only short range hexagonal ordering extending over a few moleculer centres at most. Although the phase has been described as having a layered structure, in the direction perpendicular to the layers planes the molecules are arranged in a one dimensional density wave, indicating that the layers are relatively diffuse. As a consequence, the concept of a layered mesophase is somewhat misleading because the layers are so diffuse that in the macroscopic phase they are almost non-existent. Within these loosely constructed layers the molecules are arranged in such a way that they are often at slight angles to the layered planes. The long axes of the molecules can be tilted anywhere up to about 14 to 15° from the layer normal, and this makes the layer spacing, on average, slightly shorter than the molecular length. However, as this tilting occurs randomly across the bulk phase, the average direction of the long axes of the molecules is perpendicular to the layer planes. Consequently the director, n, is perpendicular to the layers, and the phase is therefore uniaxial. As the phase is uniaxial, it is also optically uniaxial, with the optic axis perpendicular to the layer planes.
Structure of the Smectic C Phase
In the smectic C phase the constituent molecules are arranged in diffuse layers where the long axes of the molecules are tilted at a temperature-dependent angle, q, with respect to the layer planes. The smectic C phase can be formed, via a first order phase transition, on cooling the isotropic liquid, the nematic, smectic A or the D phases, or via a second order phase transition fom the smectic A phase. Typically, for fist order phase transitions there is a jump in the value of the tilit angle at the transition, ie, from a value of zero to a large finite value of usually more than 20°. After the initial jump in the value of the tilt angle, which occurs over only a few degrees, the angle remains fairly temperature independent. For a second order transition to the smectic C phase, the tilt angle usually continues to rise with falling temperature over the entire temperature range of the mesophase. Hoever at lower temperatures there is a tendency for the tilt angle to saturate. The behaviour of the temperature dependence of the tilt angle takes the form
(θ)T = (θ)0(Tc - T)α
where (θ)T is the tilt angle at temperture T °C, (θ)0 is a constant, Tc is the smectic A to smectic C transition temperture, T is the temperature and a is an exponent theoretically predicted to equal to 0.5. This power law dependency of the tilt angle ensures that the value of the tilt angle will essentially saturate with falling temperature.
The molecules within the layers are locally hexagonally close-packed with respect to the director of the phase; however, this ordering is only short range, extending over distances of approximately 15 Å. Locally the molecules may also have bond orientational ordering, but the extent of this structural feature has not yet been fully examined. Over large distances, therefore, the molecules are randomly packed, and in any one domain the molecules are tilted roughly in the same direction. Thus, the tilt orientational ordering between successive layers is preserved over long distances.
The Liquid-crystalline State - Molecular Shape and Structure and Liquid Crystals - Thermotropic Liquid Crystals - Lyotropic Liquid Crystals - Plastic Crystals - Structures of Thermotropic Liquid Crystals - Structures of Smectic Liquid Crystals