Thermotropic liquid-crystal-forming molecules, structure

Fibers with very high strength and modulus can be fabricated from polymers that have a molecular structure in which the chains are packed in small cross-sectional areas with strong bonds and low elongation. However, some of these aromatic polymers have melting points that are higher than their decomposition temperatures due to the rigidity of their molecules. It is therefore impossible to process them in thermotropic liquid crystal form. 

Lyotropic Liquid Crystals, Some molecules in a sulvcni form phases with orientational antl/or positional order. In these systems, the transition from one phase to another can occur due to a change of concentration, so they arc given the name lyotropic liquid cry stals Of course temperature can also cause phase transitions in these systems, so this aspect of thermotropic liquid crystals is shared hy lyotropics. The real distinctiveness of lyotropic-liquid crystals is the fact that at least two very different species of molecules nttisl be present for these structures to form... 

Thermotropic liquid crystals and also lyotropic liquid crystals generate functional molecular assemblies. lyotropic liquid crystalline phases are exhibited by amphiphilic molecules in appropriate solvents. They form nano-segregated structures because the molecular structures consist of hydrophilic and hydrophobic components. In Chapter 6, Gin and co-workers describe how lyotropic liquid crystals may be used to form functional materials. Lyotropic liquid crystals can act as templates for inorganic materials, ion conductors, catalysts, drug delivery systems, and nanofilters. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

Compounds forming small molecule thermotropic liquid crystals usually have the following molecular structural features ... 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

Molecules that exhibit fiquid crystallinity are usually long and rodlike. An important class of liquid crystals is called thermotropic liquid crystals, which form when the sohd is heated. The two common structures of thermotropic liquid crystals are nematic and smectic. In smectic fiquid crystals, tfie long axes of... 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

The liquid crystal phases of a thermotropic material are generated by changes in temperature (see Chapter 3). However, lyotropic liquid crystal phases are formed on the dissolution of amphiphilic molecules of a material in a solvent (usually water). Just as there are many different types of structural modifications for thermotropic liquid crystals (see Chapter 3), there are several different types of lyotropic liquid crystal phase structures. Each of these different types has a different extent of molecular ordering within the solvent matrix. The concentration of the material in the solvent dictates the type of lyotropic liquid crystal phase that is exhibited. However, it is also possible to alter the type of lyotropic phase exhibited at each concentration by changing the temperature. 

A different kind of mesophase is formed by amphiphilic molecules in lyotropic liquid crystals. Amphiphilic molecules exhibit a hydrophilic (polar) head and a hydrophobic (nonpolar) tail and form micelles, columns, or lamellae consisting of many molecules. These xmits can be arranged in an anisotropic structure, if the concentration of the amphiphilic substance in a solvent is suitable. In contrast to thermotropic liquid crystals, these anisotropic solutions of amphiphilic molecules are called lyotropic liquid crystals. In this book, we consider mainly thermotropic liquid crystals an overview on chiral lyotropic liquid crystals is given in Chapter 14. 

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

Even in systems where structure results from molecular self-assembly into aggregates, it is forces between molecules that drive the self-assembly process, although these can be between molecules of different types. In one-component systems such as thermotropic liquid crystals, ordering can only result from forces between molecules of the same type. It is difficult to make a quantitative statement about the precise form of the potential for any soft material, other than observing that it will be some combination of repulsive short-ranged contributions and attractive long-range contributions. 

The simplest structure with translational order is a one-dimensional layered structure (Fig. 1.2). In thermotropic liquid crystals, there are a number of such smectic phases formed by molecules in a weakly layered arrangement (Section 5.2.2). Amphiphiles also form smectic phases, but they are usually... 

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. 

We have seen that thermotropic liquid crystal molecules that tend to form ID and 2D structures in the form of smectic and columnar liquid crystals have weakly amphiphilic anisometric (rod-, disc-, banana-, and bowl-shaped) molecules, where the rigid cores tend to avoid the flexible aliphatic chains. These... 

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. 

Substances that show a liquid crystalline phase, or mesophase, are called mesogens. Several thousands of compounds, both with low molecular mass and polymeric, are now known to form mesophases. They are mainly highly geometrically anisotropic in shape, rodlike or disclike (hence the terms calamitic and discotic liquid crystals), or they are anisotropic in solubility properties, like amphiphilic molecules and, depending on their detailed molecular structure, they can exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Transitions to these intermediate states may be induced by purely thermal processes (thermotropic liquid crystals) or by the action of solvents (lyotropic liquid crystals). Each of these two categories can be further divided according to the structure of the mesophases and/or molecules Scheme 1 shows the classification of thermotropic mesophases. 

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