D. Krishnamurti

Optical Anisotropy of Nematic Compounds

Abstract The calculation of the principal polarizabilities of nematic compounds with reference to the two approaches using either the Vuks formulae or the Neugebauers relations is discussed. The polarizabilities and internal field constants have been calculated from the birefringence data in the nematic phase of seven compounds viz., (i) p-azoxyanisole, (ii) p-azoxyphene-tole, (iii) anisaldazine, (iv) p-methoxybenzylidene p-butylaniline, (v) 4-n-butyl-4 ′-ethoxy-α-chlorostilbene, (vi) 4-n-octyl-4′-etnoxy-α-chlorostilbene and (vii) mixture of the isomers of 4-methoxy-4′-butylazoxybenzene. The results support the validity of the Neugebauers relations rather than the Vuks formulae. The isotropic polahzability and the polarizability α‖ parallel to the long axis of the molecules are accounted for on the basis that, in the case of the conjugated bonds immediately on either side and in between the benzene rings, the respective bond polarizabilities have values which correspond to their double bond polarizabili...

Optical behavior of natural oils and fats of plant origin.

Abstract The paper describes and discusses the results of the observations made on the optical textures exhibited by sixteen naturally occurring oils and fats of plant origin. The textures observed here consist of spherulites, drops, batonnets and maltese crosses which are characteristic of liquid crystals. The refractive indices and the average isotropic polarizabilities of these substances are also discussed. Often it is found that the mesomorphic nature is more pronounced with oils and fats with a high component of saturated triglycerides and with high molecular weights. The paper is illustrated by numerous striking photographs.

Theory of birefringence of nematic liquid crystals

A theory of the birefringence of nematic liquid crystals is de- veloped taking into account the intermolecular potential energy arising from dipole-dipole, anisotropic dispersion, induction and repulsion inter- actions. The potential energy exhibits a dependence on molecular orientation and is expressible ae - (uo + u1 cost9 + uz cosat9 + u4 cos48 + u6 cosee + ...) where t9 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 mole- cules. The theory explains the experimentally observed result that the temperature coefficient of the extraordumy 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 temperahre variation of the birefringence. Assuming that the molecular librations in the liquid crystal can be represented by a system of harmonic oscillators, the rn librational mnpli- tude is evaluated for p-azoxyaniaole from recent measurements of the ultrasonic velocity. The increase in the rn amplitude With temperature in the nematic range is found to be in good agreement with that obtained directly from the experimental data on birefringence.

Optical behaviour of mixtures of nematic and cholesteric compounds

Abstract Studies have been carried out on the characteristic optical textures exhibited by mixtures of nematic and cholesteric compounds, namely, N-(P-propoxybenzylidene)p-pentylaniline (PBPA) and cholesteryl ethyl carbonate (CEC). Many of the textures are metastable and undergo a slow transformation at room temperature in the course of some days or weeks. The optical properties and textures observed near room temperature are characteristic of the cholesteric mesophase in the case of mixtures with concentrations of PBPA below 35% as also at the higher concentrations in the range of 90% and above. In the range of concentrations of PBPA lying between 40 and 90%, the optical textures observed near room temperature are characteristic of the smectic mesophase. Refractive index data obtained in the different cases confirm the above identifications. Wherever the cholesteric texture has a sufficiently large pitch, it is possible to observe the striped pattern characteristic of the periodic variation of the refrac...

Polarization field and molecular order in smectic liquid crystals.

Abstract In the smectic A and B mesophases, the polarization field is anisotropic and the extraordinary and ordinary refractive indices n e and n o are related to the effective polarizabilities αe and αo and the internal field constants γe and γo by the Neugebauers relations. The Neugebauer relations lead to the Born relation which is valid to a high degree of accuracy particularly for the long wavelengths. The paper presents a method for calculating the anisotropic internal field constants and the ratio of αe to αo, even if density data are not available. The birefringence data reported by Pelzl and Sackmann for six cinnamates which exhibit both smectic A and B mesophases are discussed in terms of the theory and results are presented with regard to the internal field constants and the ratio of αe to αo at different temperatures. Their variations are explained as due to mainly the rotational isomerism exhibited by the alkyl and alkoxy endgroups in the different compounds.

Mesomorphic Behaviour of Dodecyl Benzene Sulfonic Acid and Its Sodium Salt

X-ray and optical microscopy show that p-dodecylbenzenesulfonic acid [121-65-3] and mixts. of Na p-dodecylbenzenesulfonate [2211-98-5] with water and MeSO3H exhibit a smectic A phase. Interference studies are described which establish the mol. orientation in the droplets and batonnets. [on SciFinder(R)]

Birefringence of some nematic compounds.

Abstract This paper reports the refractive index and density data and discusses the orientational order parameters in the case of the following eight nematic compounds, viz., (1)p(p-ethoxyphenylazo) phenyl valerate, (2) p(p-ethoxyphenylazo)phenyl hexanoate, (3) p(p-ethoxyphenylazo)phenyl heptanoate, (4) p(p-ethoxyphenylazo)phenyl undecylenate, (5) 4,4′-bis(hexyloxy)azoxybenzene, (6) 4,4′-bis(heptyloxy)azoxybenzene, (7) anisylidene-p-aminophenyl acetate, and (8) anisylidene-p-aminophenyl butyrate.

Thermal, optical, x-ray, infrared, and N.M.R. studies on the α-phase of some saturated aliphatic esters.

Results are reported of the investigations carried out from diverse aspects on the I±-phase exhibited by Et, Pr, and Bu stearates and mixts. of Me stearate and palmitate, prior to their solidification into the cryst. I²-phase. Optical examn. revealed the formation of stepped drops and facetted drops, indicating a high degree of mol. order in the I±-phase. From the features observed in x-ray diffraction, it appears that the I±-phase corresponds to the ordered smectic mesophase in which there is hexagonal cylindrical packing of the mols. in the smectic layers. Infrared absorption spectra also show that in the I±-phase the zigzag chains of mols. are fully extended. In the I±-phase, the existence of rotational motions of the mols. is revealed both by infrared and proton magnetic resonance studies. [on SciFinder(R)]

Optical Effects and Molecular Ordering in Spherulites

Abstract The optical behaviour of spherulite textures exhibited by mixtures of cholesteryl acetate and cholesteryl ethyl carbonate is investigated. The observed optical effects arise due to a helicoidal arrangement of the molecules along the radial directions of the spherulites, the long axes of the molecules being oriented transverse to the radial directions. The helicoidal nature of the structure is strikingly demonstrated by two sensitive interference experiments wherein sinusoidal interference fringes are observed. The observed results are in agreement with the theory developed for the interference effects.

Proton Magnetic Resonance Absorption in n-Butyl Stearate

Abstract The proton magnetic resonance absorption in n-butyl stearate has been investigated for the range −196° to 24 °C. The second moment data are discussed and the amplitudes of molecular rotations at different temperatures are calculated.