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Liquid crystals activation parameters

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). [Pg.418]

Norden et al. have examined the CD spectral changes of dyes oriented in a liquid crystal matrix, on variing the order parameter q, which is estimated by NMR observation 262). Kuball et al. presented a theoretical description of the optical activity of oriented molecules, and they found that the C D of the transition A of a given oriented molecule Ae (v) is described by ... [Pg.100]

TABLE 3. formation Activation parameters for PnP In cholesteric liquid crystals. intramolecular excimer... [Pg.542]

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. [Pg.174]

TaUe 4. Dynamic parameters of cholestane spin probes in liquid crystal side chain polymers and low molecular weight analogues Rotational activation energies and anisotropy ratios... [Pg.22]

Arguably the most important parameter for any surfactant is the CMC value. This is because below this concentration the monomer level increases as more is dissolved, and hence the surfactant chemical potential (activity) also increases. Above the CMC, the monomer concentration and surfactant chemical potential are approximately constant, so surfactant absorption at interfaces and interfacial tensions show only small changes with composition under most conditions. For liquid crystal researchers, the CMC is the concentration at which the building blocks (micelles) of soluble surfactant mesophases appear. Moreover, with partially soluble surfactants it is the lowest concentration at which a liquid crystal dispersion in water appears. Fortunately there are well-established simple rules which describe how CMC values vary with chain length for linear, monoalkyl surfactants. From these, and a library of measured CMC values (35-38), it is possible to estimate the approximate CMC for branched alkyl chain and di- (or multi-) alkyl surfactants. Thus, most materials are covered. This includes the gemini surfactants, a new fashionable group where two conventional surfactant molecules are linked by a hydrophobic spacer of variable length (38). [Pg.469]

The basic problem of measurement of thermal parameters of aU solid-state devices and VLSl-chips is the measurement of temperatures of the active components, for example p n junctions, or the integrated circuit chip surface temperature. Nonelectrical techniques, which can be used to determine the operating temperature of structures, involve the use of infrared microradiometry, liquid crystals, and other took, and require that the surface of the operating device chip is directly accessible. The electrical techniques for measuring the temperature of semiconductor chips can be performed on fuUy packaged devices, and use a temperature sensitive electrical parameter (TSEP) of the device, which can be characterized/caUbrated with respect to temperature and subsequently used as the temperature indicator. [Pg.1342]

Because of the liquid-crystal-like order, the viscosity of the block copolymer is usually high and is non-Newtonian with reference to dependance on shear rate. As the repulsive interaction energy or the Flory-Huggins interaction parameter increases, the temperature dependence of viscosity decreases. For styrene-diene polymers, the activation energy of flow in the melt state is similar to that of polystyrene. As the interaction gets smaller the distinction between melt state and disordered state disappears. [Pg.22]

In the vicinity of the transition into the isotropic phase, optically isotropic uniform textures are often observed. These so-called blue phases are cubi-cally symmetric defect structures of cholesteric liquid crystals. With decreasing temperature three blue phases occur [13, 14]. All of them are optically active but not birefringent. The observation of the optical Bragg refiections allowed the determination of the structure of these phases. They are formed by a special packing of pieces of the helix into various cubic lattices. An example is shown in Fig. 1.5(b) [15]. Parameters of the lattices are of the order of the helical pitch. Due to the optical Bragg refiection firom the cubic lattice these phases are blue colored. [Pg.10]

The activation parameters found for the smectic A and B phases are significantly lower than those characterizing the nematic phase. This strange effect is similar to a behavior sometimes observed in plastic crystals, where the activation enthalpy for the reorientation in the ODIC phase is smaller, despite the higher density, than in the liquid phase. This was explained with the higher order in the solid state... [Pg.208]

As noted in Section 2 the d- and fi-isomers of optically active molecules exhibit somewhat different order parameters in liquid crystals possessing a local screw sense (such as cholesteryl derivatives). Accordingly d- and C-isomers should separate on cholesteric substrates. For 3,3,3-trichloropropylene oxide the difference in the order parameter is of the two isomers is AS z = 0.00015 (cf Section 2) for a compensated nematic mixture of cholesteryl derivatives. On the basis of Equ. (42) one would expect a separation factor of a 1.002. Up to the present the separation of optically active isomers on cholesteric substrates has not been achieved. [Pg.75]

In the study of dielectric relaxation, temperature is an important variable, and it is observed that relaxation times decrease as the temperature increases. In Debye s model for the rotational diffusion of dipoles, the temperature dependence of the relaxation is determined by the diffusion constant or microscopic viscosity. For liquid crystals the nematic ordering potential contributes to rotational relaxation, and the temperature dependence of the order parameter influences the retardation factors. If rotational diffusion is an activated process, then it is appropriate to use an Arrhenius equation for the relaxation times ... [Pg.282]

Since the first publications on this subject in 1963, NMR in liquid crystalline systems has been a wide and active field of research in many branches of organic and physical chemistry. In fact, NMR spectroscopy has revealed a powerful means of probing molecular structure, anisotropic magnetic parameters and dynamic behaviour of solute molecules dissolved in liquid crystals. Moreover, this technique has been successfully employed to investigate properties of mesophases themselves, such as their orientational ordering, translational and rotational diffusion and their effects on nuclear relaxation, and molecular organization in different liquid crystalline phases. [Pg.1179]


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See also in sourсe #XX -- [ Pg.178 , Pg.181 , Pg.183 ]




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