Mesogenic Systems Research Articles

Lateral substitution in nematic systems

Lateral substitution of mesogenic systems is shown to produce a great diversity of new liquid crystalline structures. The effect of such structural changes on the physico-chemical and electro-optical properties of laterally substituted nematic systems is discussed and compared with that of their unsubstituted analogues.

The effect of the ferrocene unit on mesomorphic properties

Abstract The synthesis of a number of 1,1′-disubstituted ferrocene-containing compounds is described; the structures of the compounds containing the ferrocene unit are related to those of mesogenic systems containing a 1,4-disubstituted benzene ring or a trans-1,4-cyclohexyl ring. Direct comparisons are made between the aryl-, cyclohexyl- and ferrocene-containing compounds in order to assess the effect on transition temperatures of replacing a benzene or cyclohexyl ring with a ferrocene unit. In comparison with conventional ring systems the ferrocene unit causes substantial reductions in clearing temperatures.

Binary Mesogenic Systems Comprised of Ester Mesogens and Non-Mesogens

Abstract With a view to evaluate the effect of non-mesogen or mesogen on the binary systems comprised of a nematogon or a polymesomorph, the present study was undertaken where the compounds have general structure (I): The binary systems are selected with variation in R and R′ (I). The binary phase diagrams have very interesting behaviour. One most common feature is rounded minimum of melting point curve suggesting large depression and resulting into ambient temperature mesophase. A non-mesogen as one of the components decreased the mesomorphic tendency of the system and, in some cases, rendered the enantiotropic phase to monotropic phase with one exception. The binary systems comprised of mesogen and polymesogen exhibited interesting behaviour. The nematic phase remained enantiotropic for most of the compositions but the nematic-smectic curve went on falling up to about 50% concentration and then the phase was eliminated from the system. Whereas in the second binary system, the monotropic- smectic phase i...

Light-scattering studies of molecular association in mesogenic systems

Results are reported of depolarized light scattering measurements on solutions of S-(+)-4-(2′-methylbutyl)-phenyl-4′-octylbiphenyl-4-carboxylate (CE8) and (±)-4-(2′-methylbutyl)phenyl-4′-octylbiphenyl-4-carboxylate (CE8R) in p-xylene and solutions of biphenyl in carbon tetrachloride. Values of the quadrupole correlation factor g2 are derived and the results are interpreted in terms of progressive molecular association. No differences were detected between g2 values for CE8 and CE8R. The scattering results are used to determine absolute values for the polarizability anisotropies of CE8 and CE8R.

Molecular Structural Effects on Mesophase Behavior of Monomers and Polymers : Azo vs Olefinic

Abstract Two monomers containing two phenylene rings, two linking units, andone phenyl group have been synthesized. The linking units were ester and azo functionalities for one of the monomers, and ester and olefinic for the other. Phase transitions, range of mesophase stabilities, textures, enthalpy, and/or entropy changes of the two mesogens are compared for the monomeric and polymeric forms. Thermal transitions observed by differential scanning calorimetry were fonfirmed by optical microscopy. The results reveal that while there are markedly different structural effects in the monomers, such effects are considerably diminished in the polymers. However, the polymers had substantially different textures as confirmed by a solutions-cast miscibility study. The data illustrate that mesogenic systems containing three phenyl rings and two linking units as opposed to two phenyl rings and one linking unit to allow one to better assess molecular structural effects on mesophase behavior.

A new model for dipole-dipole association in mesogenic systems

It is proposed that apolar association between mesogens should lead to both parallel and anti-parallel ordering of molecular dipoles in mesogenic solutions and mesophases. New measurements of Kirkwood correlation factors for solutions of four polar mesogens are interpreted in terms of a new model for dipole-association. Results suggest that there are substantial differences between the association behaviour of different mesogens, dependent on the molecular shape and polarizability.

Phase studies of binary mesogenic systems

Although several theories exist for rod-like nematogens, our understanding of thermotropic mesomorphism in semiflexible chain polymers is less well developed. Flory observed that the nematic behaviour of Kuhn chain polymers is governed by the axial ratio of the Kuhn link and is nearly independent of the contour length of the chain. Following this lead, we attempted to fit binary phase diagrams of two polymer + diluent systems using the Warner–Flory treatment of rod-like nematogens. The only modification involved replacement of the axial ratio of the rod by that of the Kuhn link. This theory assumes the nematic phase is stabilized by orientation-dependent interactions and that the axial ratio of the rod is independent of temperature. Poor agreement with the experimental phase diagrams suggests some other factor is important. We then took into account the temperature dependence of the unperturbed dimensions through appropriate modification of Flory's treatment of Kuhn chain polymers. This predicts a thermotropic nematic–isotropic transition, even in the absence of orientation-dependent interactions, at the temperature which reduces the axial ratio of the Kuhn link to the critical value. Persistence lengths measured for two polymers at different temperatures, when extrapolated to their isotropization temperatures, produce axial ratios very near the critical value. This treatment predicts that all semiflexible polymers that behave as Kuhn chains should be in corresponding states at their isotropization temperatures. The application of this concept is illustrated by examining the thermal behaviour of a number of substituted poly(p-phenylene terephthalates).

On the Observation of Liquid-Liquid Immiscibility in Binary Mixtures of Liquid Crystals

Abstract Liquid-liquid immiscibility or phase separation that is often observed in non-mesogenic systems is suggested to account for the phase behavior exhibited in several binary mixtures of mesogens. A brief qualitative thermodynamic description of the phase separation in mesogenic systems is presented.

Novel Cubane Mesogens

Abstract As part of a research programme in which we are examining the effects of introducing certain novel ring types into the core structures of suitable mesogenic systems, we now report results obtained using the 1,4-disubstituted cubane structure. From data on two types of ester, large decreases in the N-I transition temperatures have been observed compared with those for analogous esters incorporating more commonly used ring types.