Lyotropic liquid crystals—anisotropic solutions

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. 

Materials such as 1, 2 and 3 are known as anionic surfactants because the polar head groups are anionic moieties. Cationic surfactants also exist and these, not surprisingly, also exhibit lyotropic liquid crystal phases. Compound 4 is a simple example of a cationic surfactant that consists of an amine with a long terminal chain that has been converted into the ammonium chloride salt. Accordingly, the ammonium cation constitutes the polar head group and, as usual, the long terminal alkyl chain completes the amphiphilic molecule in the capacity of hydrophobic unit. 

Compound 8 is an example with a more typical balance between the size of the polar head group and the non-polar alkyl chain. Amphiphilic molecules have also been prepared where the polar, hydrophilic head group is made up of a long perfluoroalkyl chain which is directly connected to a long hydrocarbon chain as the hydrophobic section. 

Compound 9 is a typical example of such a semi-fluorinated alkane in this case the lengths of the hydrophilic and hydrophobic sections are the same. However, the semi-fluorinated alkanes have been more widely studied for their thermotropic liquid crystal properties. 

Under certain conditions, larger structures than micelles form, and these generate lyotropic liquid crystal phases. Of course, on adding more water, the lyotropic liquid crystal phase would eventually dissolve to give a micellar solution. Surfactants dissolved in water have a Krafft point, defined as the temperature (Tj ) below which 

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Special attention has been given to lyotropic liquid crystal polymer solutions that are characterized by a wide range of unusual rheological phenomena (Peuvrel and Navard 1990,1991 Marrucci and Greco 1993). The viscosity of such solutions presents a particular variation with concentration. Isotropic solutions exhibit a monotonic increase in shear viscosity with their increasing concentration. The viscosity reaches a maximum value, which indicates the transition from isotropic to anisotropic state. Upon formation of the anisotropic phase, the viscosity starts to decrease. From then on, the viscosity begins to increase exponentially as the concentration continues to increase (Figure 5.3). 

Lyotropic liquid crystals are those which occur on the addition of a solvent to a substance, or on increasing the substance concentration in the solvent. There are examples of cellulose derivatives that are both thennotropic and lyotropic. However, cellulose and most cellulose derivatives form lyotropic mesophases. They usually have a characteristic "critical concentration" or "A point" where the molecules first begin to orient into the anisotropic phase which coexists with the isotropic phase. The anisotropic or ordered phase increases relative to the isotropic phase as the solution concentration is increased in a concentration range termed the "biphasic region." At the "B point" concentration the solution is wholly anisotropic. These A and B points are usually determined optically. 

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. 

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. 

Kevlar is a lyotropic liquid crystal which can be obtained from a sulfuric acid solution when the concentration reaches a critical value, e.g., 10%. However, this polymer like other LCPs is also anisotropic and its mechanical properties is directional, but less so in fibers than in the extruded plaques. The fiber properties in Table I are compared with other organic fibers and steel ( 15). On an equal weight basis, Kevlar has a strength several times that of steel. Perhaps, Kevlar is the first polymer at least comparable to metals. 

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... 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Lyotropic liquid crystals are formed by solutions. They undergo a spontaneous transformation from an isotropic to an anisotropic (nematic or smectic) ordered solution above certain threshold concentrations which are a function of their molecular axial ratio (length L to diameter D). 

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. 

Solutions of rod-like entities in a normally isotropic solvent often form liquid-crystal phases for sufficiently high solute concentration. These anisotropic solution mesophases are called lyotropic liquid crystals". Although the rod-like entities are usually quite large compared with typical thermotropic liquid-crystal mesogens, their axial ratios are seldom greater than 15. Deoxyribonucleic acid (DNA), certain viruses (e.g., tobacco mosaic virus (TMV)), and many synthetic poly-... 

In these cases, the anisotropic phase is usually not only formed in the pure polymer (in a polym melt) but also in a comparatively concentrated solution of such macromolecules. Liquid-crystal polymer melts are firequently called thoTOotropic polymeric liquid crystals (since the liquid-crystalline transition can most naturally be caused by a change in the temperature for these substances), and anisotropic polymer solutions are called lyotropic liquid crystals. 

Liquid crystal polymers (LCPs) were introduced over the last three decades. In the liquid state, either as a solution (lyotropic) or a melt (thermotropic), they lie between the boundaries of solid crystals and isotropic liquids. This polymeric state is also referred to as a mesomorphic structure, or a mesophase, a combined term adopted from the Greek language (mesos = intermediate morphe = form). This state does not meet all the criteria of a true solid or a true liquid, but it has characteristics similar to both a solid and a liquid. For instance, the anisotropic optical properties of LC polymeric fluids are like those of crystalline solids, but their molecules are free to move as in liquids. 

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