Theory Of Nematic Liquid Crystals Research Articles

Electron spin resonance of symmetrical three-spin systems in nematic liquid crystals

An E.S.R. line-shape model is developed for fast tumbling three-spin systems in nematic liquid crystals. The line positions are calculated from a spin hamiltonian, which considers Zeeman, exchange, dipolar and hyperfine interactions of the three unpaired electrons. The dominant spin relaxation process, determining the line-widths, is assumed to result from the anisotropy of the zero-field splitting coupled to the molecular motion. The predictions of the theory are tested by comparison with the temperature-dependent E.S.R. spectra of trisverdazyl radicals in 4,4′-azoxydianisole. Good agreement is found between experimental and simulated spectra. A detailed analysis provides values for the solute order parameters [Pbar] 2 and [Pbar] 4. They correspond surprisingly well to predictions of the molecular-field theories of nematic liquid crystals.

Orientational order of a spin probe dissolved in nematic liquid crystals. An electron resonance investigation

The electron resonance spectra of a nitroxide spin probe, dissolved in the mesophases of several nematogens, have been measured as a function of the angle between the director and the magnetic field. The orientational order parameter, text-decoration:overlineP2, of the probe was then determined from the line separations while the novel higher order parameter, text-decoration:overlineP4, was extracted from the linewidths. The values of text-decoration:overlineP2 and text-decoration:overlineP4 measured for the spin probe are found to be in good agreement with a simple molecular field theory of nematic liquid crystals. This agreement contrasts with marked discrepancy between the simple theory and the results of Raman light scattering experiments to determine the analogous order parameters.

Ultrasonic investigation of viscosity coefficients in the nematic liquid crystal, EBBA

The attenuation of ultrasonic compressional waves in the nematic liquid crystal, N- (p-ethoxybenzylidene) –p-butylaniline (EBBA), has been measured as a function of frequency, temperature, and the angle between an aligning magnetic field and the direction of wave propagation. The results are interpreted in terms of theories of nematic liquid crystals based on hydrodynamics and micropolar continuum mechanics. It is shown that the correct expression for the attenuation anisotropy can also be derived from Leslie’s theory of anisotropic fluids. It was found that shear viscosity coefficients had the usual exponential dependence on inverse temperature with an activation energy of 10 kcal/mole whereas volume viscosity coefficients had a more complex temperature dependence. The attenuation of compressional waves in the unoriented nematic phase is compared with that of the oriented crystal.

A molecular field theory for uniaxial nematic liquid crystals formed by non-cylindrically symmetric molecules

Most molecular theories of nematic liquid crystals assume that the constituent molecules are cylindrically symmetric. However, although this may be a useful approximation the molecules of real nematogens are of lower symmetry; here we develop a theory for an ensemble of such particles based on a general expansion of the pairwise intermolecular potential together with the molecular field approximation. The dependence of the orientational properties of the uniaxial mesophase on the deviation from molecular cylindrical symmetry is calculated from the series expansion of the pseudopotential. In these calculations the number of arbitrary parameters in the orientational pseudo-potential is reduced by assuming that the anisotropic intermolecular potential originates solely from dispersion forces. The theoretical predictions for the values of the ordering matrix and the entropy change at the nematic-isotropic transition are found to be in good agreement with those observed for 4,4′-dimethoxyazoxybenzene. In addit...

An electron resonance study of the orientational order in a nematic liquid crystal

Electron resonance spectroscopy is used to determine the orientational order parameters P¯2 and P¯4 for a spin probe dissolved in a nematic mesophase. The values of these parameters, measured at three temperatures, are found to be in good agreement with those predicted by molecular field theories of nematic liquid crystals and their mixtures.

The orientation of liquid crystals by temperature gradients

It has been known for some time that the molecular orientation of liquid crystals can be affected by the presence of a temperature gradient.Stewart reported in 1936 that, whenp-azoxyanisol was placed in a vertical temperature gradient, the orientation adopted was vertical when the higher temperature was at the bottom and horizontal when the higher temperature was at the top. The continuum theory for nematic liquid crystals, proposed byLeslie, is used to explain this phenomenon. An examination of the stability of the static orientation adopted by the director in the presence of a vertical temperature gradient shows that the stable orientations are either vertical or horizontal, as found experimentally.

Interfacial effects in the magnetohydrostatic theory of nematic liquid crystals

Associates an anisotropic surface energy with a nematic liquid crystal-isotropic fluid interface. Employing continuum theory, two sets of simple experimental situations are examined, in which a solid boundary, a nematic liquid crystal-isotropic fluid interface, and an applied magnetic field act as competing influences upon the orientation of the molecules in the liquid crystal.

Molecular statistical theory of nematic liquid crystals. II. Relation between elasticity and orientational order

A relationship is derived between curvature, elasticity and orientational order in nematic liquid crystals on the basis of the molecular statistical theory developed in Part I. Calculations show that the elastic moduli of p-azoxyphenetole should be greater than those of p-azoxyanisole, in quantitative agreement with the values reported by the Orsay Group. The theoretical temperature variation of the elastic moduli is also in accord with the available data.

Alignment of Nematic Liquid Crystals

Two problems in the theory of nematic liquid crystals are formulated. One is concerned with the change of orientation due to magnetic field and the other with shear flow. The solutions of the governing differential equations explain the physical phenomena which have been observed experimentally, namely the alignment of nematic liquid crystals with the magnetic field and with shear flow. In fact, it is shown that these two phenomena are closely related to each other.

Temperature Dependence of Normal Modes in a Nematic Liquid Crystal

We have measured the temperature dependence of the intensity and spectrum of light scattered by fluctuations in the nematic phase of $p$-methoxy-benzylidene $p\ensuremath{-}n$-butylaniline. Our measurements are interpreted using two phenomenological theories of nematic liquid crystals.