Liquid, definition thermotropic

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. 

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

Thermotropic liquid crystal phases are formed by rodlike or disclike molecules. However, in the following we consider orientational ordering of rodlike molecules for definiteness, although the same parameters can be used for discotics. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This is known as the director, which is a unit vector denoted n. 

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. 

A cholesteric phase represents a special case of a nematic phase. The planes of adjacent molecules are also parallel, but the longitudinal axis of adjacent planes are turned about a definite angle. The result is a screw structure of the system. Cholesteric thermotropic liquid crystalline phthalocyanines are also known. ( + )-2,3,9,10,16,17,23,24-octakis[4-(dodecyloxy)-2-oxapentyl]phthalo-cyanine (preparation see Schemes 13 and 14) shows a texture typical for cholesteric phases [155], At 160°C, this compound gives a fluid isotropic phase. Upon cooling an anisotropic phase appears at 153 °C. The texture similar to platelets (blue phase) changes at 66 °C to typical cholesteric fan-shape structure and remains fan-shaped down to room temperature. Except for the discotic... 

Thus, all types of liquid crystals are thermodynamically stable over a definite temperature range. In depending on the conditions of LC formation, liquid crystals are divided into two groups. An LC phase formed by heating a solid or cooling an isotropic liquid is usually called thermotropic liquid crystals. ... 

The monofunctional P-U complex (supermolecule 19, Table 3) has been shown to form disklike dimers that can self-assemble into columns displaying thermotropic behavior. Each disk has a thickness/diameter ratio of 0.1 [155], The formation of columnar mesophases by similar discotic supermolecules is described in the previous section when cases in which liquid crystallinity and growth are hierarchically related, coupled, or totally uncoupled were considered. Lack of data on the equilibrium constants, or DP, prevents a definite assessment of the assembling mechanism of discotics based on supermolecule 19. 

Relaxation measurements provide another way to study dynamical processes over a large dynamic range in both thermotropic and lyotropic liquid crystals (see Sec. 2.6 of Chap. Ill of Vol. 2A). The two basic relaxation times of a spin system are the spin-lattice or longitudinal relaxation time 7] and the spin-spin or transverse relaxation time T2. A detailed description, however, requires a more precise definition of the relaxation times. For spin 7=1, for instance, two types of spin-lattice relaxation must be distinguished, related to the relaxation of Zeeman and quadrupolar order with rates 7j"2 and Jfg. The relaxation rates depend on spectral density functions which describe the spectrum of fluctuating fields due to molecular motions. A detailed discussion of spin relaxation is beyond the scope of this... 

The liquid crystalline phase of dinuclear copper stearate has been known since 1938, and was reinvestigated in 1964 [38] but it was not until 1984 [39] that this mesophase was definitely characterized by Giroud-Godquin et al. as columnar discotic by X-ray diffraction. This was the first example of a thermotropic hexagonal discotic meso-... 

Liquid crystallinity can appear for more than one reason. Materials in which liquid crystalline properties are induced by the presence of a solvent are called lyotropic. If liquid crystallinity appears in definite temperature intervals, we have thermotropic liquid crystals. Hsiao, Shaw and Samulski" found that liquid crystalline properties can be also brought about by elevation of pressure I have called such LCs barot-ropic. Their existence is not surprising, since pressure and temperature changes produce similar (although not identical) effects in terms of affecting free volume. 

The existence of the liquid-crystalline state of matter is not only a function of temperature. A large number of organic materials show liquid crystallinity in properly chosen solvents. Systems which exist in the liquid-crystalline state in a definite range of temperature are called thermotropics, while the second group is known as lyotropics. Both groups show a rich polymorphism. While mesophases of rod-like thermotropic liquid crystals can essentially be subdivided into two... 

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