S. Chandrasekhar

Bio: S. Chandrasekhar is an academic researcher from Raman Research Institute. The author has contributed to research in topics: Liquid crystal & Biaxial nematic. The author has an hindex of 31, co-authored 124 publications receiving 3368 citations. Previous affiliations of S. Chandrasekhar include Royal Institution & University of Mysore.

Theory of melting of molecular crystals: liquid crystalline phase.

Abstract: The theory of melting of mol. crystals developed by Pople and Karasz, which takes into account order-disorder processes in both the positions and orientations of the mols., is discussed in a slightly modified form. The theory is an extension of the 2-lattice model of Lennard-Jones and Devonshire so as to allow the mols. to take up 2 orientations on any site. It is assumed that the energy required for a mol. to diffuse to an interstitial site varies as V-4, as in the original formulation, but that the orientational barrier varies as V-3, in conformity with recent calcns. of the orientational potential energy in nematic liq. crystals. The thermodynamic properties of the disordered system are evaluated relative to those of the perfectly ordered system using the Bragg-Williams approxn. For small orientational barriers, the theory predicts 2 transitions, a solid state rotational transition followed by a melting transition. For larger orientational barriers, the 2 transitions coalesce, and there is a corresponding increase in the entropy of fusion. For even larger orientational barriers, the positional melting precedes the rotational melting, and there occurs an intermediate phase, similar to the nematic mesophase, that has orientational order but no positional order. The predicted entropies of transition from the liq. crystal to the isotropic phase for a certain range of orientational barriers are comparable to those obsd. in nematic compds. Theoretical curves are drawn for the degree of orientational order, the anomalous sp. heat, and thermal expansion as functions of temp. in the liq. cryst. range, and for the variation of the transition temp. with pressure. The curves reproduce the trends in the phys. properties of nematic liq. crystals. [on SciFinder(R)]

Theory of Melting of Molecular Crystals: The Liquid Crystalline Phase

Abstract: The theory of melting of molecular crystals developed by Pople and Karasz, which takes into account order-disorder processes in both the positions and orientations of the molecules, is discussed in a slightly modified form The theory is an extension of the two-lattice model of Lennard-Jones and Devonshire so as to allow the molecules to take up two orientations on any site It is assumed in this paper that the energy required for a molecule to diffuse to an interstitial site varies as V −4 , as in the original formulation, but that the orientational barrier varies as V −3 , in conformity with recent calculations of the orientational potential energy in nematic liquid crystals The thermodynamic properties of the disordered system are evaluated relative to those of the perfectly ordered one using the Bragg-Williams approximation For small orientational barriers, the theory predicts two transitions, a solid state rotational transition followed by a melting transition For larger orientational barriers, the two transitions coalesce and there is a corresponding increase in the entropy of fusion For even larger orientational barriers, the positional melting precedes the rotational melting and there occurs an intermediate phase, similar to the nematic mesophase, that has orientational order but no positional order The predicted entropies of transition from the liquid crystal to the isotropic phase for a certain range of orientational barriers are comparable to those observed in nematic compounds Theoretical curves are drawn for the degree of orientational order, the anomalous specific heat and thermal expansion as functions of temperature in the liquid crystalline range, and for the variation of the transition temperatures with pressure The curves reproduce the trends in the physical properties of nematic liquid crystals

Evidence of a tilted columnar structure for mesomorphic phases of benzene-hexa-n-alkanoates

Abstract: Optical observations on the mesomorphs of benzene-hexa-n-alkanoates, and their admixtures with benzene, show that the planes of these discotic molecules are not perpendicular to the columns in which they are stacked, as was supposed originally. This evidence comes principally from the occurrence in texture domains of circular bending, between crossed polarizer and analyser, of oblique extinction brushes (i.e., brushes not parallel or perpendicular to polarizer and analyser directions). The obliquity varies from specimen to specimen up to a maximum of about 35°. A tilted stacking, with a tilt angle (for the optic axes of the molecules) of about 35°, will fit all the observations. Four alternative variants of such structures, two with the same tilt axes for all molecules, two with a herring-bone alternation between two orientations of tilt axis, all of which give rectangular pseudo-hexagonal lattices, are discussed. It is shown that the departure from hexagonal axial ratio required by these tilted structures is small enough not to be incompatible with the X-ray diffraction evidence previously interpreted as indicating hexagonal symmetry. These structures allow several different twinning modes, and complex optical features in some domains of circular bending are explicable in terms of these twinnings.

Statistical theory of orientational order in nematic liquid crystals.

Abstract: 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 U i = −(u 0 + u 2 cos 2 θ i + u 4 cos 4 θ i + u 6 cos 4 θ i + ....), u 0 = w 00 + w 02 cos 2 θ + w 04 cos 4 θ + .... u 2 = w 20 + w 22 cos 2 θ + w 24 cos 4 θ + ...., etc., W mn = W nm , 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.

Disc-like mesogens

Abstract: The paper deals with a study of the thermotropic mesophases observed in systems of disc-like molecules, viz., pure benzene-hexa-n-alkanoates and their mixtures with benzene. The mesophases exhibit optical textures that are typical of a lamellar type of liquid crystal. The X-ray data for the pure compounds are consistent with a structure in which there is lamellar order with hexagonal symmetry in two dimensions and liquid-like disorder in the third. Preliminary NMR results are described, which again indicate that the degree of molecular order in the mesophase is intermediate between those in the solid and isotropic phases. Pressure-temperature phase diagrams are presented for two of the homologues, the hexanoate and octanoate compounds.

Handbook of Optical Materials

Abstract: CRYSTALLINE MATERIALS Introduction Physical Properties Optical Properties Mechanical Properties Thermal Properties Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties GLASSES Introduction Commercial Optical Glasses Specialty Optical Glasses Fused Silica Fluoride Glasses Chalcogenide Glasses Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties Special Glasses POLYMERIC MATERIALS Optical Plastics Index of Refraction Nonlinear Optical Properties Thermal Properties Engineering Data METALS Physical Properties of Selected Metals Optical Properties Mechanical Properties Thermal Properties Mirror Substrate Materials LIQUIDS Introduction Water Physical Properties of Selected Liquids Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Commercial Optical Liquids GASES Introduction Physical Properties of Selected Gases Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Atomic Resonance Filters APPENDICES Safe Handling of Optical Materials Abbreviations, Acronyms, and Mineralogical or Common Names for Optical Materials Abbreviations for Methods of Preparing Optical Materials and Thin Films Fundamental Physical Constants Units and Conversion Factors

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

Abstract: Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics

Abstract: Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorlander, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.