Smectic substances

The Oseen theory embraces smectic mesophases, but is not really required for this case. The interpretation of the equilibrium structures assumed by smectic substances under a particular system of external influences may be carried out by essentially geometric arguments alone. The structures are conditioned by the existence of layers of uniform thickness, which may be freely curved, but in ways which do not require a breach of the layering in regions of greater extension than lines. These conditions automatically require the layers to be Dupin cyclides and the singular lines to be focal conics. Nothing, essentially, has been added... 

Smectic substances are crystalline-liquid substances in which elongated filament or ribbon-like molecules are arranged in such positions that both ends lie preferably in given planes. Fig. 71 will show a diagrammatic representation of this type of arrangement. Apart, however, from this grouping of the individual particles, they are free to move to a large extent they can rotate around their axes, and be displaced in such a way that the axes remain parallel and the number of the chains comprised in a bundle, as shown in Fig. 71, can vary molecules from one set can pass over to another, etc. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

The best known low molar mass substances forming cholesteric phases are esters of cholesterol, which are responsible for the name cholesteric phase. Therefore it was straightforward to attach these derivatives to a polymer in order to obtain cholesteric polymers. These experiments, however, failed because in all cases not cholesteric but isotropic, smectic or crystalline polymers resulted 22) (refer also to Shibaev and Plate, this issue). 

The fact that the structure of a solid monomer influences its polymerization substantially now seems obvious. It is not as clear whether structural phenomena can effect polymerization if the monomer is a liquid. It has long been known that ordered regions or clusters exist in liquids, and several years ago it was assumed that in some cases these regions in liquid monomers can influence the polymerization. One of the most vivid examples—namely, polymerization in the liquid-crystalline state—was accomplished by Krentzel and co-workers (I, 2, 3). The object of their study was p-methacrylylhydroxybenzoic acid, which forms conventional crystals in the pure state and does not polymerize in the solid state. However, when mixed with alkoxybenzoic acid, it forms liquid crystals of both smectic and nematic forms. Polymerization of p-meth-acryllylhydroxybenzoic acid in various forms of liquid crystals was compared with polymerization of the same substance dissolved in dioxane and dimethylformamide (DMF). 

Liquid crystals represent a transition between solid crystalline substances and isotropic liquids. On heating, mesophases are formed that have ordered structures which can be nematic, smectic or cholesteric. On further heating, the orientation is disturbed and the phases are converted into an isotropic liquid. The long structure of liquid crystals causes isomers with more drawn-out shapes to be readily dissolved in the ordered liquid crystal substrate ( mesophase ) thus yielding stronger sor-bat-sorbent interactions,... 

The rotational crystalline phase has conceptually the same meaning as that of the smectic liquid crystal [8]. Molecules of substances forming liquid crystals have, as a rule, an anisometrically elongated shape and low symmetry. The main structural feature of the liquid-crystalline state is a parallel array of molecules with very light contacts between them. 

If the ratio f lf2 I is greater than unity the torques induced by the symmetric and antisymmetric strain rates respectively will never cancel out and the antisymmetric pressure will never vanish. This means that the director continues rotating for ever. The liquid crystal is said to be flow unstable and complicated flow patterns arise. TTiey have been studied comprehensively both experimentally and theoretically [30]. Some nematic liquid crystals are flow stable whereas others are not. For example, 4-n-pentyl-4 -cyanobiphenyl (5CB) is flow stable whereas 4-n-octyl-4 -cyanobiphenyl (8CB) is flow unstable. The only difference between this two substances is the length of the hydrocarbon chain attached to the cyanobiphenyl skeleton. Nematic liquid crystals that are flow stable usually become flow unstable close to the nematic-smectic A transition. The reason for this is that there is an emergent layer structure in the fluid that is incommensurate with the strain rate field. 

Compare the natures and extents of order in the smectic liquid-crystal and isotropic liquid phases of a substance. Which has the higher entropy Which has the higher enthalpy ... 

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. 

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. 

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

Case A planar alignment, Ca <0 0. This is the most studied, classical case, since the conductivity anisotropy of usual nematics (substances without a smectic phase) is typically positive. As for , there is a wide range of materials with negative dielectric anisotropy. 

There are two principal categories of mesophases, thermotropic and lyotropic. Thermotropic liquid crystals are formed within a particular range of temperature in a molten material, with no solvent present, whereas lyotropic liquid crystals are formed by some substances when they are dissolved in a solvent. Within each of these categories there are three distinct classes of mesophases, which were first identified by Friedel in 1922. The simplest of these to describe are the nematic and smectic classes, illustrated schematically in fig. 12.16. These phases are formed by long thin rigid molecules which tend to line up parallel to each other. 

Substances that form liquid crystals are often composed of rod-shaped molecules that are somewhat rigid in the middle. In the liquid phase, these molecules are oriented randomly. In the liquid crystalline phase, by contrast, the molecules are arranged in specific patterns as illustrated in A FIGURE 11.32. Depending on the nature of the ordering, liquid crystals are classified as nematic, smectic A, smectic C, or cholesteric. 

SECTION 11.7 A liquid crystal is a substance that exhibits one or more ordered phases at a temperature above the melting point of the sohcL In a nematic liquid crystal the molecules are aligned along a common direction, but the ends of the molecules are not lined up. In a smectic hquid crystal the ends of the molecules are lined up so that the molecules form... 

It often happens that a substance possessing a smectic liquid crystalline phase just above the melting point passes into a nematic liquid crystalline phase at a higher temperature. Account for this type of behavior. 

Ilquld crystal A substance that flows like a liquid but has some order in its arrangement of molecules. Nematic crystals have long molecules all aligned in the same direction, but otherwise randomly arranged. Cholesteric and smectic liquid crystals also have... 

---