Mesomorphic phase transition

Mesitylene, production from acetone, 1 164 Mesityl oxide, 14 589-590 characteristics of, 16 337 hydrogenation, 16 337-338 hydrogen peroxide treatment of, 16 338 Z-menthol from, 24 520 production of, 16 336-337 production from acetone, 1 164, 174 Mesogenic diols, 25 460 Mesogenic molecules, solids of, 15 82 Mesogens, 24 53, 54 Mesomixing, 16 683 Mesomorphic behavior, 24 53-54 Mesomorphic phase transitions, 15 102 Mesomorphism, 15 81. See also Liquid crystalline materials Mesophase pitch-based carbon fiber, 26 734-735... 

An exception to the rule that lowering the Iciiiperaiure cutises transitions to phases with increased order sometimes occurs for polar compounds w hich form the smectic l. phase (a layered structure formed by molecular dimers). Decreasing the temperature cutises u transition front nematic to smectic. 1,.,. but a further lowering of the temperature produces J transition back to the nematic phase tcalled the reentrant nematic phase). Electric or magnetic fields also may induce mesomorphic phase transitions. 

Mesomorphic phase transition and mesogenicity of the cyclotetraphos-phazenes (80) and (81) have been studied and compared with their cyclotriphos-phazene analogues. Compound (81) appears to be the first cyclotetraphos-phazene that exhibits mesogenetic behavior. A related study of the mesomorphism of cyclotriphosphazenes has shown that relatively small changes of the mesogenic side groups have a considerable effect on the mesomorphic properties. ... 

Table 1. Characteristic times of lipid mesomorphic phase transitions established by time-resolved x-ray diffraction... 

The theory of mesomorphic phase transitions, developed at Princeton, has been extensively described in the literature (I, 3) and will only be briefly summarized here. (Charvolin and colleagues independently developed similar ideas at about the same time. See reference 13 for a summary.) The basic... 

Figure 8. Spectra for unfused lithium stearate sample taken through mesomorphic phase transition, cooled to 137°C., and heated again...

Molecular theories of mesomorphic polymers may serve to give a better insight into the mechanisms involved in the formation of ordered phases in macromolecular liquids. The importance of parameters such as chain length, chain flexibility... can be explored and the results used in synthesis of new systems with well-defined characteristics. Studies of the influence of solvents on the mesomorphic phase transition and the possibility of ordering by external fields (electric, magnetic, flow) are problems of technological in-ter>est in the spinning of fibers and in the formation of new polymer blends. 

Fig. 4. Phase diagram for mesomorphic phase transition for a liquid crystal polymer A (degree of polymerization = 100, persistence length q = 1D] in solution in a liquid crystal solvent B ( ) exact ( ] Landau de Gennes...

In order to learn more about the am polymorph we crystallized PDES isothermally from the melt. A somewhat different phase diagram results then. By quenching the material from the melt to the isothermal crystallization temperature, bypassing thus the mesomorphic state, the crystallinity of the sample diminishes and the peaks move to lower temperature (266 K for the high temperature to mesomorphic phase transition and 290 K for the low to high temperature transition). This could be due to smaller crystals. The position of the peaks however, is not affected by the crystallization conditions and the subsequent thermal treatment. The ratio of the heats of fusion of the two peaks has also changed. These are indications that the phase... 

The Arrhenius plots of the diffusive permeability coefficient, P of water for the pol)nner/artificial amphiphile composite membranes reveal a distinct jump in the vicinity of the phase transition temperature of artificial amphiphiles. This striking increase of P may be caused by activation of thermal molecular motion which is closely related to the crystal-mesomorphic phase transition behavior. 

The a priori evaluation of the melting temperature of a crystal requires a separate (and accurate) evaluation of the enthalpies and entropies of the solid and of the liquid as a function of temperature, which is so far an impossible task, and could be seen as a major challenge in computational chemistry. The melting temperature and the enthalpy of melting can be readily measured by calorimetry, although the experiment is sometimes made difficult by mesomorphic phase transitions near the melting point. 

The transition temperature at which this change from a crystalline to a mesomorphic phase occurs varies with the chain length and with the unsaturation. The effect of water is to lower this transition tempera-... 

Solid-state NMR experiments showed the dynamic nature of the disordering. It has been possible to monitor the type and the changes of segmental motions within the mesomorphic phase and at the corresponding phase transitions for typical examples. The thermal... 

In addition to this thermotropic mesomorphism, a lysotropic mesomorphism is observed [98]. The phase transition temperature, Tt, for the transition from the crystalline to the liquid crystalline state decreases as a function of water content. The decrease in Ttis due to destabilization of the crystal lattice in the head group region by water molecules. This, in turn, decreases the interaction between the fatty acid chains. When the water content reaches a certain level, the phospholipids assume a thermodynamically optimal arrangements whereby the fatty acids are directed to the... 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

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