Thermotropic liquid crystals defined

We are all familiar with gases, liquids and crystals. However, in the nineteenth century a new state of matter was discovered called the liquid crystal state. It can be considered as the fourth state of matter (although plasmas are also candidates for this accolade). The essential features and properties of liquid crystal phases and their relation to molecular structure are discussed in this chapter. Specifically, the focus is on thermotropic liquid crystals (defined in the next section). These are exploited in liquid crystal displays (LCDs) in digital watches and other electronic equipment. Such applications are outlined later in this chapter. Surfactants and lipids form various types of liquid crystal phase but this was discussed separately in Chapter 4. Finally, this chapter focuses on low molecular weight liquid crystals, liquid crystalline polymers being touched upon in Section 2.10. 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

Like other states of matter, thermotropic mesophases are indefinitely stable at defined temperatures and pressures. Moreover, a thermotropic liquid-crystal-line material exhibits reversible phase transitions at well-defined temperatures. For example, the liquid crystal 4-n-pentyl-4 -cyanobiphenyl (5CB) melts from the solid to a nematic liquid crystal at 22.5°C and then from the nematic phase to the liquid at 35.0°C. As a consequence, the characterization and classification of thermotropic phases by microscopy also requires the use of an accurately controlled oven. 

More importantly, upon addition of a silver salt, these helical tubules were segmented into slices of tubules while maintaining helical order in these discrete nanostructures. The preservation of the shape-persistent hexameric macrocycles in this transition is responsible for the retention of supramolecular chirality. Transition from chiral tubules into discrete nanostructures with maintaining the supramolecular chirality is reported for thermotropic liquid crystals of dendritic molecules [82-84], There is, however, no study about such a transition for weU-defined nanoscale synthetic... 

Thermotropic liquid crystals are most often composed of elongated rod-like or plane disc-like organic molecules cf. Fig. 3.1, top part). However, the molecules may also take other geometries as long as they are anisotropic, e.g. a banana-like shape as found for bent-core molecules [1]. This anisotropic shape is essential, as orientational order cannot be defined for building blocks with an isotropic shape. 

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 ... 

For an isolated spin-1 system, it is convenient to define sum and difference magnetizations [Eqs. (2.84)-(2.85)] in the J-B experiment. The decay of the difference (quadrupolar order) proceeds exponentially at a rate T q, while the sum (Zeeman order) recovers exponentially towards equilibrium at a different rate. The J-B experiment allows simulataneous determination of these rates from which Ji uJo) and J2 2ujo) can be separated. Table 5.1 briefly summarizes thermotropic liquid crystals in which spectral density measurements were reported. Figure 5.4 illustrates the temperature and frequency dependences of spectral densities of motion (in s by including the interaction strength Kq factor) for 5CB-di5. The result is fairly typical for rod-like thermotropic liquid crystals. The spectral densities increase with decreasing temperature in the nematic phase of 5CB. The frequency dependence of Ji uJo) and J2(2a o) indicate that molecular reorientation is likely not in the fast motion regime. However, the observed temperature dependence of the relaxation rates is opposite to what is expected for simple liquids. This must be due to the anisotropic properties (e.g., viscosity) of liquid crystals. 

This chapter is organized as follows. The various types of liquid crystals are introduced in Section 5.2. Some important characteristics of liquid crystalline materials that result from the anisometry of liquid crystal molecules are discussed in Section 5.3. Then, in Section 5.4, the identification of liquid crystal phases is considered. Orientational order is a defining characteristic of thermotropic liquid crystals, and Section 5.5 is devoted to it. Section... 

Thermotropic liquid crystal phases are formed by rod-like or disc-like molecules, either of which can have long-range order. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This defines the director, n. With calamitic mesogens, the molecular long axes tend to lie parallel to the director, whereas the disc-like molecules in columnar phases are on average perpendicular to the director. 

The optical properties of liquid crystals determine their response to high frequency electromagnetic radiation, and encompass the properties of reflection, refraction, optical absorption, optical activity, nonlinear response (harmonic generation), optical waveguiding, and light scattering [1], Most applications of thermotropic liquid crystals rely on their optical properties and how they respond to changes of the electric field, temperature or pressure. The optical properties can be described in terms of refractive indices, and anisotropic materials have up to three independent principal refractive indices defined by a refractive index ellipsoid. 

The industrial development of thermotropic liquid crystal polymer (LCP) materials can be traced from its theoretical origins, through the identification of useful compositions, to full commercialization. The future industrial challenge will be to define and develop applications which take advantage of the unique properties of these materials. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

In order to understand the basic principles of operation of the many different kinds of LCDs being developed and/or manufactured at the present time, it is necessary to briefly describe the liquid crystalline state and then define the physical properties of direct relevance to LCDs. First, the nematic, smectic and columnar liquid crystalline states will be described briefly. However, the rest of the monograph dealing with liquid crystals will concentrate on nematic liquid crystals and their physical properties, since the vast majority of LCDs manufactured operate using mixtures of thermotropic, non-amphiphilic rodlike organic compounds in the nematic state. 

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

Lyotropic liquid crystals are formed in mixtures of amphiphiles (e.g., surfactants) and solvents, for example, detergents and water. Consequently, these phases are thermodynamically stable at defined temperatures, pressures, and concentrations. Like thermotropics, a variety of structurally distinct modifications exist, which are collectively known as lyotropic liquid crystals. 

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