Showing papers by "S. Chandrasekhar published in 1971"
TL;DR: In this paper, the potential energy of a molecule in a nematic liquid crystal is derived as a function of its orientation, assuming a model based on permanent dipole-dipole, dispersion, induction and repulsion forces.
Abstract: Assuming a model based on permanent dipole-dipole, dispersion, induction and repulsion forces, the potential energy of a molecule in a nematic liquid crystal is derived as a function of its orientation. Analysis of the temperature variation of the degree of orientational order in p-azoxyanisole (PAA) and p-azoxyphenetole (PAP) indicates that the permanent dipole interactions are relatively unimportant. Making use of a mean field approximation, a statistical theory of long-range orientational order is developed and the thermodynamic properties of the ordered system are derived relative to those of the completely disordered one. Application of the theory to PAA and PAP shows conclusively that a certain degree of short-range orientational order is present in the liquid phase. Using just three parameters for each compound, viz. the two constants of the potential function and a numerical factor to allow for short range order, the following physical properties have been evaluated which are in quantitative agreement with the experimental data: the long-range orientational order parameter, specific heat and compressibility as functions of temperature in the liquid crystalline range, the latent heat and volume change at the nematic-isotropic transition point. The magnetic birefringence of the liquid phase affords an independent estimate of the short range order which supports the previous calculations.
60 citations
TL;DR: In this article, the refractive indices of anisaldazine, CH3OC6H4CH:NN:CHC4H4OCH3, have been measured in the crystalline, nematic and isotropic phases and the orientational order parameter in the mesophase has been evaluated by the application of the Vuks formula.
Abstract: The refractive indices of anisaldazine, CH3OC6H4CH:NN:CHC4H4OCH3, have been measured in the crystalline, nematic and isotropic phases and the orientational order parameter in the mesophase has been evaluated by the application of the Vuks formula. The curve for the order parameter versus the relative temperature (Te-T) is nearly parallel with those for p-azoxyanisole and p-azoxyphenetole and lies approximately midway between them.
58 citations
TL;DR: A theory of the rotatory power of thin films of cholesteric liquid crystals was developed in this article, and the results are in qualitative agreement with the theory both inside and outside the region of reflexion.
Abstract: A theory is developed of the circular dichroism and rotatory power of thin films of cholesteric liquid crystals. It is an extension of the treatment for infinitely thick specimens discussed in a previous paper. The therory predicts that the circular dichroism of a thin film plotted as a function of wave-length should exhibit a principal maximum accompanied by subsidiary maxima, and that the rotatory dispersin should be anomalous. Experimental circular dichroism and rotatory dispersion curves are presented for cholesteric cinnamate, cholesteryl-2-propyn-1-yl carbonate and spectratherm. The results are in qualitative agreement with the theory both inside and outside the region of reflexion. The rotatory dispersion curves are also in accord with the de Vries equation outside the region of reflexion.
48 citations
TL;DR: In this article, the modified form of the Pople-Karasz theory of melting of inolecualr crystatls was applied to study the thermodynamics of solid-solid and melting trasitions.
Abstract: The modified form of the Pople-Karasz theory of melting of inolecualr crystatls in a pervious paper is applied to study the thermodynamics of solid-solid and melting trasitions The results are in substantial agreement with the predictions of the theory in its original form
21 citations
TL;DR: In this paper, the dependence of the free energy of elastic deformation and the intensity of light scattering on the orientational order parameters is investigated theoretically, and it is shown that the elastic moduli should be greater and the scattering cross-section less in p-azoxyphenetole than in p -azoxyanisole.
Abstract: The constants of the potential energy function deduced from the statistical theory of orientational order in nematic liquid crystals are given for a few compounds and their significance in relation to the molecular structure is discussed. The dependence of the free energy of elastic deformation and the intensity of light scattering on the orientational order parameters is then investigated theoretically. Calculations show that the elastic moduli should be greater and the scattering cross-section less in p-azoxyphenetole than in p-azoxyanisole. The results are in quantitative agreement with the available experimental data. The theory is also able to account for the observed fact that though the order parameters and the elastic moduli decrease rapidly with rise of temperature, the light scattering remains practically constant throughout the nematic range.
5 citations