Thermotropic mesogens

The molecular structure of arsphenamin is a typical representative of a thermotropic mesogen. With its symmetrical arrangement of the atoms the same holds for disodium cromoglycate, DNCG [20], which forms both thermotropic liquid crystals and lyotropic mesophases in the presence of water. Micronized DNCG powder applied to the mucosa of the nose or the bronchi absorbs water from the high relative humidity of the respiration tract and is first transformed into a lyotropic mesophase and then into a solution depending on the amount of water available. 

In contrast to these lyotropic mesogenic materials, which require the presence of a solvent in order to produce a mesophase, thermotropic mesogens are those compounds which exhibit a mesophase in the melt state at temperatures above the crystalline solid state and before the formation of an isotropic melt Mesophase-forming polymers may possess either one of the two basic structures shown in Fig. 1 the polymer may either contain the mesogenic group (the part of the polymer molecule which is responsible for liquid crystallinity) directly in the main chain, or the mesogen may be present as a pendant group in the side chain. 

A second noteworthy series is that of the dirhodium tetraalkanaotes (Figure 80 above) which have been extensively studied as thermotropic mesogens (see 5. 2. above). However, in solution in hexadecane, nematic phases are formed as evidenced by the schlieren textures observed [188],... 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

An understanding of the eorrelation between molecular structure and physical properties of thermotropic mesogens is important in order to optimize parameters such as the operating temperature range. A detailed discussion of such structure-property relationships is beyond the scope of this chapter. Further details can be found elsewhere [8 and... 

Vesicular LC phases are a new kind of LC phase with cellular structure that is formed by the spatial organization of cylindrical, spherical, or polyhedral aggregates of curved single layers, bilayers, or double layers. These phases have been considered as complicated columnar and cubic mesophases with core-shell morphology [192]. Till now only few vesicular mesophase are reported for thermotropic mesogens [193-196]. 

In contrast to the technologically extremely important thermotropic mesogens known for about 110 years and quickly growing in number, lyotropic analogues are as yet poorly studied and are much more complicated as their mesophase units do not consist of single molecules, but are made up of more or less flexible and variable aggregates of molecules in (so far mostly aqueous) solution. 

Acharya, B.R., Primak, A., Kumar, S. Biaxial nematic phase in bent-core thermotropic mesogens. Phys. Rev. Lett. 92(14), 145506 (2004)... 

Fig. 3.1 Building blocks of thermotropic and lyotropic liquid crystalline phases. The upper part of the figure shows two examples of typical thermotropic mesogens. Calamitic mesogens, such as terephthal-bis-(p-butylaniline) (TBBA) [2], can be represented by prolate ellipsoids or rigid rods, while discotic mesogens, such as benzene-hexa-n-octanoate (BH8) [4], are usually described by oblate ellipsoids or discs. The lower part of the figure shows the typical surfactant molecule sodium dodecyl sulfate (SDS), which forms lyotropic phases with water [5], Such a surfactant molecule is basically composed of a polar head group and a flexible hydrophobic tail. These amphiphilic molecules aggregate into different types of micelles, which are the actual mesogens of lyotropic liquid crystals. The shape of the micelles depends mainly on the solvent concentration...

Most thermotropic mesogens contain aromatic groups, and since the component of the diamagnetic susceptibility perpendicular to a benzene ring is greater than the inplane component, liquid crystals composed... 

The high intramolecular ordering of the thermotropic mesogenic polymer is also reflected in the strong effect of the solvent on the size of the molecules—the value of the intrinsic viscosity [ti]— in different solvents with presCTvation of the molecular weight of the sample [58]. The intrinsic viscosities [t ] (at 21 C) of mesogenic comb-shaped polymers of the polymethacrylate series in different solvents are reported below ... 

The optical characteristics of polymer 5 (Table 3.7) in comparison to the properties of polymers 15 and 18 (Table 3.5) also confirm the role of the combshaped structure of thermotropic mesogenic molecules in the formation of intramolecular axial orientational order. The values of [ ]/[ n] and aj - ot2 of polymer 5 are 2-3 times lower than the corresponding paramet for polymers 15 and 18. This difference is not determined by the simple additive contribution of the optical anisotropy of the -CjgH33 aliphatic group, which is confirmed by the values of [n]/[ti] and aj - ot2 of PCMA (polymer 5, Table 3.5). 

The set of optical, hydrodynamic, and electrooptical properties of thermotropic mesogenic macromolecules thus indicates that an intramolecular structure with very perfect (Hientational (axial and polar) order is characteristic of these polymCTs. The quantitative study of the features of the conformation of these... 

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