Statistical theory of orientational order in nematic liquid crystals

Assuming a model based on dispersion and repulsion interactions, it is shown that the orientational potential energy of a molecule in a nematic liquid crystal is expressible as, where Ui= −(u0 + u2 cos2 θi + u4 cos4 θi + u6 cos4 θi + ....), u0 = w00 + w02 cos2 θ + w04 cos4 θ + .... u2 = w20 + w22 cos2 θ + w24 cos4 θ + ...., etc., Wmn = Wnm, and θ is the angle which the long axis of the molecule makes with the uniaxial direction of the medium. Using a slightly simplified form of this function, a statistical theory of long range orientational order in the nematic state is developed. The thermodynamic properties of the ordered system are evaluated relative to those of the completely disordered one, and the conditions of equilibrium are discussed. The constants of the potential function are determined for p-azoxyanisole that lead to a theoretical curve for the degree of orientational order and a volume change at the nematic-isotropic transition point in good agreement with observations. However, the predicted latent heat of the nematic-isotropic transition is significantly higher than the experimental value suggesting that a certain degree of short range order persists in the liquid phase. The calculated latent heat of transition as well as the specific heat and the compressibility of the liquid crystal fit the experimental data when a correction factor is included in the theory to allow for the effect of short range order. The magnetic birefringence of the liquid phase gives an independent estimate of the short range order which confirms the previous calculations

Molecular theory of nematic liquid crystals

The thermodynamic conditions of nematic stability are discussed on the basis of the molecular statistical theory of orientational order developed in previous papers. Theoretical calculations of the order parameter in the nematic phase and the volume change at the nematic-isotropic transition point are presented for 2-4-nonadienic acid, 2-4-undecadienic acid, p-azoxyanisole, p-azoxyphenetole and anisaldazine. The significance of the results are discussed briefly

Nematic order in p-azoxyanisole and its dependence on pressure, volume and temperature

The extension of the Maier-Saupe treatment of nematic liquid crystals discussed in previous papers is able to give a quantitative description of the following properties of p-azoxyanisole: (a) the volume change at the nematic-isotropic transition point Tc, (b) the temperature variation of the long range orientational order parameter s1 at constant pressure, (c) the temperature variation of s1 at constant volume, (d) the volume variation of s1 at constant temperature, and (e) the pressure invariance of s1 at Tc

Dielectric relaxation studies on two systems exhibiting the induced smectic A phase

Recently we reported some physical studies (layer spacings using X-ray scattering, low frequency dielectric constants) on two systems exhibiting a maximum (indicating an induced smectic A) as well as a minimum in the A-N transition boundary. In this paper we report measurements of the dielectric relaxation of ε|| (up to 13 MHz) for a few compositions of the two systems. The activation energy in the A phase of the mixtures corresponding to the maximum in the A-N boundary is higher than that of the pure polar component. The results are discussed in relation to the molecular interactions giving rise to the induced smectic A phase

Experimental determination of the curvature-induced reduction in the smectic A-nematic transition poin

On the basis of an analogy between smectic A-nematic and superconductor-normal metal transitions, de Gennes has predicted that a twist or bend distortion should reduce TAN with respect to that of a curvature-free sample. We report measurements of the reduction of TAN on samples prepared in two different geometries. In the first, a magnetically induced twist distortion has been used to obtain preliminary data on 8 OCB and CBOOA samples. In the second, a wedge-shaped twisted nematic cell has been used to get quantitative data on the reduction of TAN of CBOOA as a function of twist distortion

Effect of skewed cybotactic structure on the dielectric constants and conductivities of some binary mixtures exhibiting the nematic phase

2-cyano-4-heptylphenyl-4'-pentyl-4-biphenyl carboxylate (7P(2CN)5BC), a nematogen with a cyano group making a large angle (~60°) with the long axis of the molecule, has a moderately strong negative dielectric anisotropy (Δε). A mixture of 85 mole per cent of 7P(2CN)5BC with 15% of 4-n-heptyl-4'-cyanobiphenyl (7CB) exhibits a reversal of the dielectric anisotropy Δε as the temperature is varied, becoming negative at higher temperatures. Δε of mixtures with higher concentrations of 7CB is positive throughout the nematic range. The conductivity anisotropy ΔS' of these systems has been investigated for a range of frequencies from 300 Hz to 30 kHz. In 7P(2CN)5BC, ΔS' changes sign becoming negative at higher temperatures for all frequencies investigated. The results are discussed in terms of the temperature variation of the tilt angle of the skewed cybotactic structure of 7P(2CN)5BC

Studies on binary mixtures of systems which exhibit a maximum as well as a minimum in the A-N transition boundary

We have studied the binary phase diagrams of (I) p'-nitrophenyl-p-n-octyloxybenzoate with (2-hydroxy)-p-ethoxybenzylidene-p'-butylaniline, and (II) p'-n-octyloxy-p-cyanobiphenyl with p-butoxyphenyl-(p'-pentyloxy)benzoate. Both of them show a strong maximum in the A-N boundary, indicative of an induced A phase, and further, a well defined minimum for a composition rich in the highly polar component. The interactions responsible for the induced phase are much stronger for the system II than for the system I. We have investigated the temperature variations of the layer spacings for various compositions of both the systems. In system I, there is a considerable thermal expansion of the spacing, the expansion coefficient attaining a maximum value for the composition corresponding to the minimum in the A-N boundary. We have given a physical interpretation of this observation. The dielectric anisotropies of various compositions of system I have also been investigated and their temperature dependences interpreted in terms of the physical model mentioned above. The conductivity anisotropy changes sign for all compositions exhibiting the A phase. The temperature of crossover in relation to TAN is the highest for the mixture which exhibits the minimum value of TAN, since the A-N transition is almost of a second order character for this composition

Dynamics of cholesteric structures in an electric field

Motivated by Lehmann-like rotation phenomena in cholesteric drops we study the transverse drift of two types of cholesteric fingers, which form rotating spirals in thin layers of cholesteric liquid crystal in an ac or dc electric field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge separation or flexoelectric charge generation can describe the drift of cholesteric fingers. We argue that the observed Lehmann-like phenomena can be understood on the same basis.Comment: 4 pages, 4 figures, submitted to PR

Theory of birefringence of nematic liquid crystals

A theory of the birefringence of nematic liquid crystals is developed taking into account the intermolecular potential energy arising from dipole-dipole, anisotropic dispersion, induction and repulsion interactions. The potential energy exhibits a dependence on molecular orientation and is expressible as - (u0 + u1 cosθ + u2 cos2θ + u4 cos4θ + u4 cos4θ + ...) where Theta is the angle which the long axis of the molecule makes with the uniaxial direction of the liquid crystal. The birefringence of the medium is evaluated in terms of the Boltzmann distribution of the oriented molecules. The theory explains the experimentally observed result that the temperature coefficient of the extraordinary index is large and negative whereas that of the ordinary index is small and positive. Analysis of the data on p-azoxyanisole and p-azoxyphenetole shows that dispersion and repulsion forces play a predominant role in determining the temperature variation of the birefringence. Assuming that the molecular librations in the liquid crystal can be represented by a system of harmonic oscillators, the rms librational amplitude is evaluated for p-azoxyanisole from recent measurements of the ultrasonic velocity. The increase in the rms amplitude with temperature in the nematic range is found to be in good agreement with that obtained directly from the experimental data on birefringence

Role of Apoptosis in Rabies Viral Encephalitis: A Comparative Study in Mice, Canine, and Human Brain with a Review of Literature

To evaluate the role of apoptosis in rabies encephalitis in humans and canines infected with wild-type street virus, in comparison with rodent model infected with street and laboratory passaged CVS strain, we studied postmortem brain tissue from nine humans, six canines infected with street rabies virus, and Swiss albino mice inoculated intramuscularly (IM) and intracerebrally (IC) with street and CVS strains. Encephalitis and high rabies antigen load were prominent in canine and human brains compared to rodents inoculated with street virus. Neuronal apoptosis was detectable only in sucking mice inoculated with CVS strain and minimal in street virus inoculated mice. In a time point study in suckling mice, DNA laddering was noted only terminally (7 days p.i.) following IC inoculation with CVS strain but not with street virus. In weanling and adult mice, apoptosis was restricted to inflammatory cells and absent in neurons similar to human and canine rabies-infected brains. Absence of neuronal apoptosis in wild-type rabies may facilitate intraneuronal survival and replication while apoptosis in inflammatory cells prevents elimination of the virus by abrogation of host inflammatory response