Thermotropic liquid crystal phases

There are a large number of thermotropic liquid crystal phases that have been identified and classified. Their molecular structures are varied and range in complexity. However, they all exhibit common properties of anisotropy and fluid-like behavior in at least one dimension. In the following sections, we discuss the most commonly encountered liquid crystal phases, describing their molecular structures and key properties. Novel, interesting liquid crystal phases are still being discovered as new materials 

---

Liquid crystal phases can be divided into two classes. Thermotropic liquid crystal phases are fonned by pure... 

Molecular Structure in Thermotropic Liquid Crystal Phases. 58... 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

In addition to barotropic liquid crystals of flexible polymers, there are also a few reports on the observation of the liquid crystalline phase of polymers containing no mesogenic units of rigid rods. Aharoni (1988), for example, was very lucky to have found a series of the following polymers that form a thermotropic liquid crystal phase. 

Thermotropic liquid crystal phases are formed by rodlike or disclike molecules. However, in the following we consider orientational ordering of rodlike molecules for definiteness, although the same parameters can be used for discotics. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This is known as the director, which is a unit vector denoted n. 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

Table 4.2 Symmetry and structural features of the most popular thermotropic liquid crystal phases consisting of rod-like molecules (for the nomenclature we follow [28])...

Just as there are many different types of thermotropic liquid crystal phases there are also several different types of lyotropic liquid crystal phases. In general, the lyotropic liquid crystal phases of surfactant systems have been extensively investigated over the whole concentration range. This intensive research parallels the commercial importance of soap and detergent products. 

Fig. 1. Schematics of thermotropic liquid crystal phases in between the isotropic fluid and the crystal, arranged from left to right in order of increasing order and decreasing temperature.

The difference between direct ordering of molecules and the indirect ordering of molecules via supermolecular aggregates is one of the distinctions between different types of soft material. Thermotropic liquid crystal phases result from partial orientational and translational order of the molecules. In contrast, the symmetry of lamellar, micellar and bicontinuous phases is specified by the location of supermolecular aggregates. The molecules within the aggregate do not have the same orientational and translational order as the mesoscopic structure itself in fact, they can be relatively disordered . 

Thermotropic liquid crystal phases are formed by anisotropic molecules with long-range orientational order, and in many types of structure some degree of translational order. The main types of mesogens are those that are rod-like or calamitic and those that are disc-like or discotic. 

All thermotropic liquid crystal phases are characterized by long-range orientational order of the mesogens. However, the extent of translational order... 

Here we give an overview of fluctuation dominated thermotropic liquid crystal phase transitions with a few hints of emerging aspects. From this perspective, the situation may be fairly summarized by noting that while analogies to phase transition models in spin-space (e.g. XY model with two components for the order parameter or Ising model with one) or momentum space (superconductivity) are a powerful tool to predict qualitative behavior for fluctuation dominated, real space, high temperature liquid crystal phase transitions, there is a significant gap between several quantitative (and qualitative) expectations and experimental measurements. 

Liquid crystal materials can be grouped into two main classifications, thermotropics and lyotropics. A thermotropic phase is one that can form by heating or cooling a material. Just as we see a phase transition between solid and liquid as we heat and melt ice, in thermotropic liquid crystals, additional melting points can be observed in between the solid and the liquid phases. These are the thermotropic liquid crystalline phases. A lyotropic liquid crystal phase is formed by molecules dissolved in a solvent, and phases form at certain concentrations in that solvent. In this chapter, we focus on descriptions of the thermotropic liquid crystal phases. Lyotropics, although also liquid crystals, are described in detail in Chapter 3 on surfactants. 

Liquid crystals have structural order intermediate between conventional liquids and solids. Thermotropic liquid crystal phases form in pure compounds or homogeneous mixtures as the temperature is changed. Lyotropic liquid crystals form when amphiphilic molecules are dissolved in water, or another suitable solvent, and concentration is the main physical variable. Polymeric liquid crystalline order occurs in fluid polymer melts and solutions. We shall limit our discussion to thermotropic liquid crystals, for which the knowledge is most complete. 

The above developments had two major consequences. First, the identification of the various hexatic phases led to a detailed classification of the large variety of thermotropic liquid crystal phases. The essential structural details of these phases are illustrated in the Appendix8>9. Second, the diffraction patterns yield novel crossover critical phenomena. In three dimensions, this presented single experiments which yield a simultaneous measurement of many anisotropy crossover exponents, previously obtained from separate complex experiments. For very thin layers, the patterns allow detailed studies of the crossover from two to three dimensions, and provide new quantitative information on the two dimensional hexatic phase. 2 The present paper aims at a brief review of these developments. 

Thermotropic liquid crystal phase of ionic liquids... 

---