Transitions nematic liquid crystal molecular

Table 3.3 Molecular structures, transition temperatures (°C) and dielectric anisotropy (Ae measured at 20°C) for nematic liquid crystals (15-27)...

Fontana MP (1992) Molecular dynamics in nematic phases Raman and infrared spectroscopy. In Pha.se transitions in liquid crystals. NATO ASI Ser B 290 259 Force RK (1988) Anal Chem 60 1989... 

F nmr spectra have been reported for tetrafluoro-l,3-dithietane in solution with nematic liquid crystals D2h symmetry is imputed. " Theoretical calculations of the transition energies and intensities of three different tetraalkyl-1,3-dithietanes indicate a need for the inclusion of d orbitals. Theoretical calculations also have been made on a dimer of 1,3-dithietane. The UV spectrum of 2,2,4,4-tetrachloro-1,3-dithietane has been interpreted with the aid of molecular orbital calculations, interaction between the two sulfur atoms is suggested. The photoelectron spectra of tetrafluoro and tetrachloro-l,3-dithietane and 1,3-dithietane itself have been reported. The negative-ion mass spectrum of tetra-fluoro-1,3-dithietane has a base peak corresponding to fluoride ion the base peak of tetra(trifluoromethyl)-l,3-dithietane corresponds to the anion of hexafluoro-thioacetone. The oxidation potential of 1,3-dithietane does not show any unusual transannular interactions in the cation radical. ... 

With the aim of quantitatively predicting the orientational order of rigid solutes of small dimensions dissolved in the nematic liquid crystal solvent, 4-n-pentyl-4 cyanobiphenyl (5CB), an atomistic molecular dynamics (MD) computer simulation has been applied. It is found that for the cases examined the alignment mechanism is dominated by steric and van der Waals dispersive forces. A computer simulation of the deuterium NMR spectra of molecules in a thin nematic cell has been carried out and the director distribution in the cell has been studied. An experiment for the direct estimation of an element of the order matrix from H NMR spectra of strongly dipolar coupled spins that is based on the multiple quantum spin state selected detection of single quantum transitions has been proposed. The experiment also enables obtaining nearly accurate starting dipolar... 

The effect of correlated molecular motions can be observed even in the isotropic phase of nematic liquid crystals near the phase transition temperature. This phenomenon was first studied by Wong and Shen and Frost and Lalanne. They found both a pretransitional increase of the Kerr coefficient and a critical slowing-down in the relaxation process. 

Recently, particular attention has been devoted to the first-order Freedericksz transition (FT), molecular reorientation, and bistability in nematic liquid crystals (NLC s). In the last few years, Ong has shown that all of the electric and magnetic FT s and most of the optical FT s in a nonconducting homogeneous or homeotropic NLC are second-order transitions. However, applying a suitable static field, the second-order optical FT can be converted to first order in all existing NLC s. Similarly, applying a suitable optical field, the second-order electric or magnetic FT can also be converted to first order ... 

The optical-field-induced Freedericksz transition for a twist deformation by a normally incident laser beam in a planar-aligned nematic liquid crystal is studied. The Euler equation for the molecular director and the equations describing the evolution of the beam polarization in the birefringent medium are solved simultaneously in the small-perturbation limit. The stability of the undistorted state is investigated. An alternate series of stable and unstable bifurcations is found. This phenomenon has no analog in the Freedericksz transition induced by dc electric and magnetic external fields. 

E. C. Khoo, ""Optically induced molecular reorientation and third order nonlinear process in nematic liquid crystals. Phys. Rev., vol. A23, pp. 2077-2081, 1981 see also 1. C. Khoo and S. L. Zhuang, " Nonlinear light amplification in a nematic liquid crystal above the Freedericks transition, Appl. Phys. Lett., vol. 37, pp. 3-5, 1980 I. C. Khoo, ""Theory of optically induced molecular reorientation and quantitative experiments on wave-mixings and self-focusing of light, Phys. Rev., vol. A25, pp. 1636-1644, 1982 1. C. Khoo, ""Reexamination of the theory and experimental results of optically induced molecular reorientation and nonlinear diffraction in nematic liquid crystals spatial frequency and temperature dependence, Phvs. Rev., vol. A27, p. 2747, 1983. 

The majority of the existing molecular theories of nematic liquid crystals are based on simple uniaxial molecular models like sphe-rocylinders. At the same time typical mes-ogenic molecules are obviously biaxial. (For example, the biaxiality of the phenyl ring is determined by its breadth-to-thick-ness ratio which is of the order of two.) If this biaxiality is important, even a very good statistical theory may result in a poor agreement with experiment when the biaxiality is ignored. Several authors have suggested that even a small deviation from uniaxial symmetry can account for important features of the N-I transition [29, 42, 47, 48], In the uniaxial nematic phase composed of biaxial molecules the orientational distri-... 

In this chapter we consider the very simplest approach to the molecular theory of liquid crystals. We shall approach the theory phenomenologically, treating the problem of the existence of the nematic phase as an order-disorder phenomenon. Using the observed symmetry of the nematic phase we shall identify an order parameter and then attempt to find an expression for the orientational potential energy of a molecule in the nematic liquid in terms of this order parameter. Such an expression is easily found in the mean field approximation. Once this is accomplished, expressions for the orientational molecular distribution function are derived and the thermodynamic functions simply calculated. The character of the transformation from nematic liquid crystal to isotropic fluid is then revealed by the theory, and the nature of the fluctuations near the transition temperature can be explored. 

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