Lyotropic liquid crystals molecular structure

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. 

Membrane templating is capable of producing a range of structures in various sizes. However, the degree of control is not at the molecular scale. An alternative templating method that has molecular-level control over the pore size and the ability to produce ordered arrays of pores is lyotropic liquid crystal templates [53]. Lyotropic liquid crystals are surfactant phases produced at high percentages of surfactant to solvent. 

Charvolin, J. and Tardieu, A. Lyotropic liquid crystals Structures and Molecular Motions, in L. Liebert, ed. Liquid Crystals . Solid State Physics, Supplement 14, Academic Press, New York, NY 1978, p. 209... 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

This volume covers the structural relations between thermotropic and lyotropic liquid crystals (Chapters 1 and 2) and compares them with the micellar systems (Chapter 3). The interfacial aspects and the accompanying stability problems are covered in Chapters 5 and 6. The molecular dynamics in liquid crystals, the importance of water structure and of counter-ion binding for their stability are three essential factors for long range order systems, which are treated in Chapters 7, 8, and 9. The final chapter by E. J. Ambrose illustrates the change of order in a biological system under malignant conditions. 

A characteristic feature of molecules that form lyotropic liquid crystals is their surface activity. Because of the amphiphilic nature of the molecules, they orient upon contact with solvent molecules, giving rise to polar and nonpolar regions that are separated by the polar end groups. All structures known fit one of those made possible by the various curvatures of the interface between two liquid regions, with molecular size taken into consideration. It is therefore not surprising that the earlier treatment of the structure of lyotropic liquid crysals was unsuccessful since the molecules were regarded as stiff rods. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Fig. 4 Molecular structure of lyotropic liquid crystals. (A) lamellar (B) hexagonal (C) inverse hexagonal (D) cubic type I (E) inverse cubic type IV (F) cubic type II. (A, B, and D Adapted from Ref C Adapted from Ref E Adapted from Ref F Adapted from Ref. l)...

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. 

A few non-amphiphilic molecules are able to show liquid crystallinity in solution at a certain range of concentration, such as PBLG, DNA, the tobacco mosaic virus, etc. They are of great molecular mass, very rigid, rod-like and have a very long anisotropic shape. They are typical macromolecules and are lyotropic liquid crystals. This class of liquid crystals does not aggregate to form sphere, column or laminar structures. These lyotropic systems depend on the properties of the solvent. They are one of major interest of this book and will be discussed in detail later. 

Lyotropic liquid crystals are made up of two or more components. Generally, one of the components is an amphiphile (containing a polar head group attached to one or more long hydrocarbon chains) and another is water. A familiar example of such a system is soap (sodium dodecyl sulphate) in water. As the water content is increased several mesophases are obtained. The types of molecular packing in these mesophases are represented schematically in figs. 1.2.1 and 1.2.2, but several modifications of these structures exist. ... 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The solution properties of these materials are unusual. They form optically anisotropic solutions in both amide and acid solvent systems over quite wide ranges of concentration and polymer molecular weight. In other words they are among the few known examples of synthetic polymers which can form lyotropic liquid crystals. (That is to say liquid crystals formed by the action of a solvent.) The usual example quoted in this context is poly(y-benzyl-L-glutamate) which forms cholesteric mesomorphic solutions in certain organic solvents. The helical structure adopted by the polypeptide in these solvents behaves as a rigid rod and it is... 

Charvolin, J., Tardieu, A. Lyotropic liquid crystals structures and molecular motions. In Liebert, L., Ehrenreich, H., Seitz, F., Turnbull, D. (eds.) Liquid Crystals. Solid State Physics, pp. 209-258. Academic, New York (1978)... 

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