Studies of Thermotropics

The application of solid state NMR spectroscopy to provide insights on the 

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

When rapid rotations exist, Sxy, Syz, and Sxz vanish, and Sxx — Syy is usually small, thereby giving 

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be 

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The study of thermotropic, as well as of lyotropic LC polymers is directly linked to a series of practical tasks, regarding the construction of polymeric materials with set properties. For instance, making use of anisotropy of the LC state in processing (particularly in moulding) of polymeric materials discloses impressive prospects for the production of so called high modulus fibers and films 18 25). 

As indicated above, much interest exists in dynamic behavior of thin aligned layers of nematic liquid crystals. It is not surprising to find, therefore, that measurement of the anisotropy of transport properties has been the objective of many studies of thermotropic systems. The literature on anisotropic thermal conductivity in nematic liquid crystals has been reviewed recently by Rajan and Picot (12). Among the studies of anisotropic diffusion are those of Yun and Fredrickson (13), Bline... 

Interest in thermotropic liquid crystals has focussed mainly on macroscopic properties studies relating these properties to the microscopic molecular order are new. Lyotropic liquid crystals, e.g. lipid-water systems, however, are better known from a microscopic point of view. We detail the descriptions of chain flexibility that were obtained from recent DMR experiments on deuterated soap molecules. Models were developed, and most chain deformations appear to result from intramolecular isomeric rotations that are compatible with intermodular steric hindrance. The characteristic times of chain motions can be estimated from earlier proton resonance experiments. There is a possibility of collective motions in the bilayer. The biological relevance of these findings is considered briefly. Recent similar DMR studies of thermotropic liquid crystals also suggest some molecular flexibility. 

Chang and co-workers [56,57] also reported studies of thermotropic MCLC polymers containing the para-PDA chromophore. Their motivation was also to develop two-dimensionally reinforced polymer materials. Aromatic copoly-... 

The cubic phase is a real curiosity in the study of thermotropic hquid crystals, although much has been done to unravel issues concerning its formation. Interested readers are directed to other sources for a detailed discussion (32) however, the phsise does crop up rather a lot in systems we have studied, so a brief introduction is in order. 

There are very few structural studies of thermotropic side-chain polymers under shear flow. Recent work [54] describes the neutron scattering by the liquid crystalline polymethacrylate labelled PMAOC4H9 (Sect. 4.1) in the SmA phase. Typical liquid crystal behavior was observed in which the layers orient... 

Blackwell, J., Dutierrez, G., Chivers, R. (1985). X-ray Studies of Thermotropic Copolyesters in Proceedings of the 2nd Symposium Div. Pofym. Chem. Polym. Liq. Ciyst. Washington E)C, New York, London, 167-181. 

Wang Jiu-fen, Zhang Na., Li Cheng-Jie. (2005). Synthesis and Study of Thermotropic Liquid-Crystalline Copolyester. PABA.ABPA.TPA. Polym. Mater. Sci. Technol, 21(1), 129-132. 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

Ward, Y. and Young, R.J. (2001) Deformation studies of thermotropic aromatic copolyesters using NIR Raman spectroscopy. Polymer, 42, 7857. 

S. Sprunt, J. D. Litster, Light Scattering Studies of Thermotropic Liquid Crystals, in Proceedings of a NATO Advanced Study Institute, Erice, Italy, (Ed. S. Martellucci, A. N. Chester), Plenum Press, New York, 1992. 

Finally, it is necessary to indicate the broad possibilities of obtaining new materials for the creation of passive elements in optical devices and media for recording and storing information. Materialsl whose use is based on unusual optical, electrical, and magnetic properties of LC polymers cannot be developed if the properties themselves are not known. In evaluating the prospects of this direction of research, it is possible to build on the advances made in the study of thermotropic LC polymers with mesogenic groups in the side chain [82]. 

Despite the variety and peculiar chemical beauty of the structures shown in Fig. 6.3, only the simplest ones have been studied, of the type illustrated in Fig. 6.2. Molecular chemical design is still ahead of the development of physical studies of thermotropic LC polymers. 

A set of heptakis-(6-S-alkyl-6-thio)-p-cyclodextrins have been produced in connection with studies of thermotropic liquid crystalsand from monotosyl-P-cycIodextrin a phosphinyl rhodium complex with the structural feature 32 has been produced. 

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