Mesomorphic, definition

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Note 4 The pendant groups in these polymers have structures compatible with liquid-crystal formation, that is, they are mesogenic but not intrinsically mesomorphic. See the examples given in Definitions 2.10 2.11.2.1. 

Q = 1.3gcm , lu = 5.2 A and M = 265 (for CA), one obtains = 26 according to Eq. (13), and, from Eq. (11), Vp = 0.284 at incipience of a nematic (or cholesteric) phase. The experimental values in Table 3 are generally somewhat greater than this result, but the differences are not beyond the uncertainty in the characteristic chain dimension, here represented by o/n. Inasmuch as this ratio depends on the solvent, as is well known, dependence of the threshold Vp on the solvent is to be expected. Available data do not permit a definitive correlation, however, between Vp at incipience of the mesomorphic phase and the influence of the solvent on chain flexibility. 

As discussed earlier, structural anisotropy is paramount in importance when considering the intermolecular interactions necessary to promote mesomorphism. The structural anisotropy is related to the molecular shape and we shall now endeavour to investigate its effect upon the mesomorphic properties of several complexes. In several instances we have discussed N, Sa and Sc phases arising from calamitic molecules, and in later sections we shall discuss disc-like molecules exhibiting columnar mesophases. Unfortunately such rigid definitions of molecular... 

Smectic and cholesteric phases have been somewhat less extensively investigated until recently, probably because of the lack of gross structure in the NMR signals from these phases. Dunell and his co-workers (1, 9, 10, 15, 29) have published extensively on the temperature dependence of the NMR signal from various alkali soaps. Their work, however, has been principally concerned with the definition of the crystal-mesomorphic transitions by NMR. Recently Lawson and Flautt (16, 17) have studied the NMR signals of the mesomorphic states of several surfactant and surfactant-water phases under high resolution conditions. 

There are many metal complexes, metallomesogens, which deomonstrate liquid crystal or mesomorphic properties, and the organization of metal complexes in fluid phases is definitely supramolecular behaviour. These compounds were well reviewed in 1991 [20a], 1992 [20b] and 1993 [20c] the strength of this research in Europe and Japan is evident. 

Liquid crystals is one of the names by which certain mesomorphic phases or mesophases are known, and as the name implies refers to materials which show a high degree of structural order, yet are liquid. The topic has been reviewed recently. " Those most used in NMR work have been the thermotropic variety which have a definite meso-phase range between the melting point and a second transition producing the isotropic liquid. Both pure substances (e.g. p-azoxyanisole) and mixtures (e.g. butyl-/7-(p-ethoxybenzoyl)phenyl carbonate and p-(/7-ethoxyphenylazo)phenyl heptanoate) which provide a mesophase at room temperature, have been used. 

The films of common amorphous p state are optically isotropic. In c birefringence observed in the unload cates that a definite orientational chain arrangement (in the absence of tial lattice) similar to the nematic liquids. This property is a direct high rigidity and regular structure molecules O favoring the formation o gions with mesomorphic structure. 

The mesomorphic properties and physical properties of nematic (and smectic) materials and nltimately their suitability for applications are all fundamentally dictated by the chemical stracture of the constituent molecules. Before progressing further, several terms and their definitions need to be clarified this will be done by using the nematic phase. The term nematic phase stability refers to the upper-temperature limit (T j) to... 

TLCs are optically active mixtures of organic molecules. The correct name for TLCs used for temperature measurements is chiral nematic or cholesteric liquid crystals. TLCs are characterized hy well-analyzed reflections of visible light (color play) within a definite bandwidth of temperature. A certain temperature leads to reflections of an explicit spectrum of wavelengths, with a local maximum and a narrow bandwidth. Below the start temperature of the color play, called red-start temperature, the TLCs are transparent, when applied in thin layers the bulk looks milky and white. In this state the molecules, which are elongated like a cigar, have a typical size of about 2-5 nm [ 1 ], are well ordered and close to each other like in a solid crystal (see Fig. 2a). When the temperature increases the arrangement passes through the mesomorphic state, which characterizes the TLCs. The crystals are stiU optically inactive or transparent, hut the molecules are separated in layers. 

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