Liquid crystalline state lyotropic

Some drug substances can form mesophases with or without a solvent [19-26]. In the absence of a solvent, an increase in temperature causes the transition from the solid state to the liquid crystalline state, called thermotropic mesomorphism. Lyotropic mesomorphism occurs in the presence of a solvent, usually water. A further change in temperature may cause additional transitions. Thermotropic and/or lyotropic liquid crystalline mesophases of drug substances may interact with meso-morphous vehicles as well as with liquid crystalline structures in the human organism. Table 1 presents drug substances for which thermotropic or lyotropic mesomorphism has been proved. 

Deuterium NMR Analysis of Polyly-benzyl L-glutamate) in the Lyotropic Liquid-Crystalline State ... 

The structures of liquid crystals are intermediate between the amorphous and crystalline states. They have some short-range orientational order. Some also have positional order. Thousands of organic compounds exhibit liquid crystal structures. Most have molecules that are very long and thin, but some have molecules that are flat and pancake shaped. Many compounds may exist in more than one liquid crystalline state. Transitions from one state to another may be thermotropic (caused by temperature change) or lyotropic (caused by change of solute concentration). 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

Gray has provided an excellent book on the liquid crystalline state as known in 196222. It includes many detailed definitions. Although he notes the wide occurrence of liquid crystals in biological tissue, he limited his discussion of such crystals to the introduction. Materials in the liquid crystalline state can be divided into two major groups, those that are thermotropic and lyotropic. 

The change from a crystalline into a liquid crystalline state can be brought about by changes in, for example, temperature or pressure. Furthermore, some molecules may be induced to form liquid crystals by the addition of a solvent such as water. This behavior is in reality a liquid crystalline formation in a two component system and is called solvent-induced liquid crystal formation or lyotropic mesomorphism (Small, 1986, p. 49). 

Both theoretical approaches qualitatively describe the "thermotropic" and "lyotropic" liquid crystalline state of rod-like molecules ( see also D.B. DuPre, R. Parthasarathy, this book). Combination of both theories (Flory, Ronca)(7) slightly improves the predictions compared to the experimental findings. Anisotropic dispersion interactions and/or anisometric molecular shape can thus be the basis for explaining theoretically the appearance of "lyotropic" and "thermotropic" liquid crystalline phases. 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

It stabilizes the lyotropic liquid crystalline state of biological assemblies relative to the crystalline state, due to the so-called chiral bilayer effect, which will be discussed in more detail in Section 4.2. For example, 10-nonacosanol, extruded from the lipophilic wax layer of pine needles, forms fluid lipid tubules rather than crystals. Although it is difficult to establish the enantiopurity of the natural product, the fact that synthetic pure enantiomers produce tubules while the racemate gives platelets suggests that the biologically relevant morphology is attained because of the enantiopurity of the biomolecule. °... 

Whang and Wu [3] have described the liquid crystalline state of polyimide precursors and shown that certain polyamic acids derived from pyromellitic anhydride exhibit lyotropic behaviour. Liquid crystal phases have also been observed by Wenzel et al. [4] in polyimides derived from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenyl ene diisocyanate. Dezern [5] has disclosed a synthesis for linear polyamide-imides derived from benzophenone dianhydride but the occurrence or otherwise of mesophases is not mentioned. 

The amphiphilic molecules can occur in lyotropic and thermotropic mesomorph, and the organization of lipid bilayers such as liposomes can exist in a temperature range where all the mesophases from gel stale to liquid crystal state are favored. In the gel state the phospholipid acyl chains are closely packed and the molecular movements are deteriorized, while in the liquid crystalline state, the faity acid moieties are in a more fluid state and are able to move more freely. 

The precipitation process is assumed to lead either from the lyotropic, nematic liquid crystalline state via predpitation with water under maintenance of ordered water-polymer layers to the crystal form II, or with other solvents (and also from low polymer concentrations with water) through disordered solvent-polymer structures to crystal form I In both crystal structures the ultimate polymer crystal layers are H-bonded in the aystallographic bc-plane (100), as shown in Fig. 6.2. In crystal form 1 the second molecular chain goes through the center of the unit cell (Pn or P2j/n space group, 2 chains per unit cell, monoclinic, pseudo-orthorhombic)... 

Since the discovery of Kevlar, scientists world-wide have attempted to synthesize many thermotropic LCPs that the liquid crystalline state can be formed in a melt instead of in a solution as lyotropic LCPs. Generally, two kinds of thermotropic LCPs can be prepared depending on the position of mesogens (J9.) ... 

A final point of similarity between these two polymers is the occurrence of a critical concentration for a given polymer-solvent combination. Above this point the viscosity falls with further increase in concentration as illustrated in Fig. 6 for poIy(p>benzamide) in hydrofluoric acid. This is a consequence of the transition to a lyotropic liquid crystalline state. 

The recently published reviews by Papkov on lyotropic liquid crystalline polymer systems (12 ) and by Shibaev and Plate on the liquid crystalline states in polymers (13) should be regarded as the first attempts to systematize the great body of available experimental data with a view to elaborating adequate techniques of producing liquid crystalline polymer systems. 

The three elastic constants of a liquid crystal are important physical parameters which depend on the interaction between the molecules in the liquid crystalline state. While a large number of theoretical and experimental investigations on the elastic constants are contained in the literature for thermotropic liquid crystals, very little is known about them in the case of lyotropic polymer liquid crystals such as those formed by poly-Y-benzyl-L-glutamate (PBLG) in various organic solvents. Some theoretical investigations have been carried out 3 the experimental data is limited largely to measurements of the twist elastic constant and a few recent measurements of the bend and splay constants. ... 

To obtain the liquid crystalline state in a polymer network, several strategies are conceivable. They are all based on well known principles evaluated during the last few decades for linear liquid crystalline polymers. The monomer units of the network have to consist of mesogenic moieties, which are either rigid rods or discs in the case of thermotropic polymorphism or amphiphiles in the case of lyotropic polymorphism. The mesogenic units can be attached either as side chains to the monomer units yielding side chain elastomers (Fig. la, b) or directly linked... 

It must be emphasized that neither the lyotropic nor the thermotropic polymers are ordinarily used in the liquid crystalline state. The LC state is highly convenient for processing, yielding highly ordered structures with low... 

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