Liquid Crystals

(also the liquid-crystalline state or mesomorphic state), a state of matter in which it has the properties of a liquid (fluidity) and some properties of solid crystals (anisotropy of properties). Liquid crystals are formed by materials whose molecules have the shape of rods or elongated platelets. A distinction is made between thermotropic and lyotropic liquid crystals.

Thermotropic liquid crystals exist in the mesomorphic state within a definite temperature interval, below which the material is a solid crystal and above which it is an ordinary liquid. Examples include (I) para-azoxyanisole (in the temperature range 114°–135°C) and (II) ethyl azoxybenzoate (100°–120°C), as well as cholesterol propyl ether (102°–116°C).

Lyotropic liquid crystals are solutions of certain materials in certain solvents. Examples include aqueous soap solutions and solutions of synthetic polypeptides (poly-γ-benzyl-Lglutamate) in a number of organic solvents, such as dioxane and dichloroethane.

The relative arrangement of the molecules in liquid crystals is intermediate between the solid crystals, which contain a three-dimensional long-range coordination order (ordering of the location of the centers of gravity of the molecules) and an orientational long-range order (ordering of the orientation of the molecules), and amorphous liquids, in which long-range order is completely absent. A distinction is made between nematic and smectic liquid crystals, depending on the degree of molecular ordering (G. Friedel’s terminology). Nematic liquid crystals (para-azoxyanisole; solutions of synthetic polypeptides) are characterized by the orientation of the longitudinal molecular axes in a certain direction (long-range orientational order; Figure I, a). Ordering in the orientation of the lateral molecular axes and in the location of their centers of gravity is absent; this provides for free translational motion of the molecules. Therefore, the viscosity of materials in the nematic phase differs only insignificantly from their viscosity in the amorphous liquid state.

In smectic liquid crystals (ethyl azoxybenzoate and aqueous

Figure 1. Types of molecular arrangement in liquid crystals: (a) in nematic liquid crystals, the molecules are parallel, but their lengthwise displacement is random; (b) in smectic crystals, the molecules are in layers

soap solutions), the molecular ends are fixed in the planes perpendicular to the longitudinal axes of the molecules (smectic planes, Figure l,b). Long-range order in the arrangement of the lateral axes and the centers of gravity of the molecules is also absent. Fluidity is due to the mutual sliding of the smectic planes.

Liquid crystals of the cholesteric type (a variety of nematic liquid crystals) also exist. Such liquid crystals are formed by materials (for example, cholesterol propyl ether) whose molecules have the shape of elongated platelets arranged parallel to each other. Long-range coordination order is absent. The fluidity of the material is due to the translational motion and rotation of molecules in their plane.

The external distinction between nematic and smectic liquid crystals may be readily established by observing homogeneous layers of the materials under a polarization microscope. A particular texture corresponds to each type of liquid crystal: the structures most characteristic of nematic liquid crystals are fibers, whereas rodlike, conical, and stepped structures are most characteristic of smectic liquid crystals. The fibers in nematic liquid crystals are lines of interruption of the optical continuity. They are called disclinations, and the texture of the liquid crystals is determined by the nature of the arrangement of the molecules near the disclinations.

Some thermotropic liquid crystals may exist in two mesomorphic states. In this case, the structural transitions always occur according to the solid crystalline phase-smectic-nematic-amorphous-liquid scheme and are phase transitions of the first kind (with evolution of heat of the phase transition). The heat of transition of liquid crystals to the amorphous liquid is dozens of times smaller than the heat of fusion of solid organic crystals.

Liquid crystals exhibit anisotropy of elasticity, electrical conductivity, viscosity, magnetic anisotropy, optical anisotropy, and other anisotropic properties. The anisotropy of the properties of liquid crystals decreases with increasing temperature because of decreasing ordering of the molecular arrangement. Liquid crystals are oriented in a magnetic field with their axis of symmetry parallel to the lines of force of the magnetic field. In an electric field, the orientation of the axis of symmetry may be either parallel or perpendicular to the field’s lines of force.

The cholesteric liquid crystals have very high optical activity, exceeding the optical activity of organic liquids and solid crystals by two to three orders of magnitude. The cholesteric liquid crystals change color sharply upon changes of tenths of a degree in the temperature of the medium, as well as upon changes of a fraction of a percent in the composition of the medium.