A New Beginning for Liquid Crystals Today

Abstract

Following the formation of the International Liquid Crystal Society in Vancouver in 1990, a new periodical devoted to liquid crystals was established as the newsletter of the Society. Thus Liquid Crystals Today was born, and over the past four years it has published feature articles, book reviews, conference reports and various news items relating to liquid crystal scientists, events and developments. Liquid Crystals Today has been welcomed by the liquid crystal community, and its popular style has attracted much favourable comment. This success has encouraged the Society to look for ways of enhancing Liquid Crystals Today, and establishing it as an interface between liquid crystal technology and the more fundamental aspects of liquid crystal science. Through an agreement between the International Liquid Crystal Society and Taylor & Francis, publishers of the journal Liquid Crystals, from 1995 the new Liquid Crystals Today will be jointly published in an expanded format by the Society and Taylor & Francis.     

Liquid Crystals: A Bibliography for 1975

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (δ?) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ?_∥ in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.     

Liquid Crystals: A Bibliography for 1977

X-ray diffraction, Raman spectroscopy, and x-ray photoemission measurements indicate that the metallic conductivity of iodine-doped polyacetylene results from the intercalation of iodine between close-packed planes of the parent phase to form a charge transfer complex. The polyacetylene chains donate charge to produce I₃ and a second iodine species with lower charge density, most likely I₅. The structure of this charge transfer complex can be viewed as a perturbation of the structure of the parent polymer, which has been derived from x-ray diffraction analysis and lattice energy calculations.     

The Orientational Order in Cholesteric Liquid Crystals

The investigation of the orientational order in the mesophase of steroidal cholesterogens has been all but neglected by experimentalists, possibly because of the lack of an appropriate technique. However electron resonance spectroscopy can be used to determine the order parameter of a spin probe dissolved in a cholesteric mesophase. In addition the structure of the spin probe may be made to resemble that of the cholesteric solvent. Here this technique is employed to determine the orientational order for cholesteryl and cholestanyl benzoate, as a function of temperature. The results of these experiments yield order parameters which are comparable, or slightly smaller, than those typically observed for nematics. A comparison of these values with mean field theories suggests that the low order parameters of steroidal cholesterogens stems from deviation of the constituent molecules from cylindrical symmetry.     

6-n-Alkoxy-3-Pyridinecarboxaldehydes: New Intermediates to Liquid Crystals. Properties of Some Derivatives

The synthesis of new intermediates to liquid crystals, a series of 6-n-alkoxy-3-pyridinecarboxaldehydes is described. The mesomorphic properties of their precursors, the 6-n-alkoxy-3-pyridinecarboxylic acids are reported. Some derivatives of the carboxaldehydes, the 6,6′-di-n-alkoxy-3,3′-dipyridylmethylenehydrazines and the 6′,6”- di-n-n-alkoxy-3′,3”-dipyridylmethylene-l,4-phenylenediamines, are described and their mesomorphic properties reported. The 1,4-phenylenediamine derivatives show greater mesomorphic stability than the azines, most of the azines being non-mesomorphic while the 1,4-phenylenediamine derivatives show smectic polymorphism with wide mesophase ranges.     

Effect of Molecular Structure on Mesomorphism 8. l A New Series of Trifluoromethyl Liquid Crystals

A new series of trifluoromethyl liquid crystals, the 4-n-alkoxybenzylidene-4′-trifluoromethylanilines, has been synthesized. All members of the series with three carbons or more in the alkoxy chain are enantiotropic smectics. There is an odd:even effect in the smectic-isotropic transition temperatures. The smectic phase is of the B type. Members of this series strongly promote enhanced smectic A phases in many binary mesophase systems.     

Phase Transitions in Polypeptide Liquid Crystals: A Reentrant Isotropic Phase

Optical and C-13 NMR examinations of concentrated solutions of poly-γ-benzyl-L-glutamate (PBLG) in a mixed solvent system containing a denaturant acid, show that this polymer liquid crystal has both a low and high temperature isotropic phase. The latter is due to thermal disruption of long range orientational order of the elongated macromolecules. The former reentrant isotropic phase is a result of an intramolecular helix to random coil transition, which leads to a macromolecular conformation inconsistent with liquid crystallinity.     

Thermal Properties of Binary Mixtures of Liquid Crystals. A Review

Owing to their wide range of industrial utility, binary systems of liquid crystals are currently a topic of extensive studies. This paper is a comprehensive review of the literature from 1973, where the review of Hsu et al. ⁹⁵ ended, through mid-1981. For simplification, the organization used here is similar to that incorporated in the previous review.

Because of the volumes of literature on mesophase systems, this study includes only binary mesophase systems in which both components are thermotropic mesogens. Included here are only those systems for which phase diagrams have been constructed or systems which are similar to those whose phase diagrams have been constructed.

Much of the work on binary mixtures is done by industrial applications specialists and thus was not available for this review. Inevitably, there will be omissions from published work, although every attempt was made to include all of the available literature.     

The Failure of Asymmetric Synthesis in Cholesteric Liquid Crystals

A set of selected bimolecular reactions leading to asymmetric and dissymmetric products was carried out in cholesteric solvents with the aim of obtaining asymmetric syntheses.

In all cases, the products did not show significant optical rotation. The reasons for the lack of asymmetric induction are briefly discussed.     

New Nematic Phase in Lyotropic Binary Liquid Crystals Detected by Positron Annihilation Techniquest

A new nematic phase in lyotropic binary liquid crystalline systems consisting of sodium dodecyl sulfate-water was detected, for the first time, in the absence of additives by utilizing positron annihilation techniques. Lyotropic and thermotropic phase transitions were also investigated by the same methods. The positron annihilation parameters have also indicated a better definition of the concentration ranges at which each phase exists in the system studied. Crossed polarized light microscopy was applied to substantiate these results.     

Study of Liquid Crystals in Flow: I. Conventional Viscometry and Density Measurements

Using the technique of conventional viscometry (Haake-Hoeffler Falling Ball and Ostwald-Poiseuille viscometers), we have measured the temperature dependence of the apparent viscosity of three liquid crystals: methoxybenzylidene butylaniline (MBBA), hexyloxybenzylidene amino benzonitrile (HBAB) and cyanobenzylidene octyloxyaniline (CBOOA). The measurements with the falling-ball show a small discontinuity in the apparent viscosity of (HBAB) at ～ 91°C, where the flow properties of this compound have been found to change from one flow regime in which the “classical” molecular alignment with the flow can be observed (T > 91°C) to another regime in which this alignment cannot be observed (T < 91°C). However, in spite of this small discontinuity, the apparent falling-ball viscosity which we measured for (HBAB) is still very close to the Miesowicz viscosity, η_B, obtained by constraining the director n to remain parallel to the velocity vector during the measurements. Measurements performed with the Ostwald-Poiseuille viscometer show: (i) that the apparent viscosity and the viscous heat of the nematic and smectic-A mesophases depend upon the radius of the capillary; (ii) that the apparent viscosity, in both the flow-alignment and the non-flow-alignment regimes, is very close to η_B and (iii) that there is no convincing evidence of a discontinuity in the apparent viscosity of (HBAB) at 91°C.

Our measurements of the temperature dependence of the density of the mesophases show that the isobaric coefficients of thermal expansion for (HBAB) and (CBOOA) are different in the flow alignment and in the non-flow-alignment regimes. This is consistent with the hypothesis that there is a greater degree of molecular association in the non-flow-alignment regime.     

Chiral Induction in Liquid Crystals with Dopants Derived from 1-Phenyl-2-Aminoalcohols

New chiral additives derived from 1-phenyl-2-aminoalcohols have been prepared and tested. Especially ephedrine and pseudoephedrine as chiral auxiliaries with two asymmetric C atoms offer functional groups of divergent reactivity. Dopants prepared by acylation with promesogenic agents can be used in various electro-optic devices. A specific utilization concerns bistable cholesteric liquid crystal displays with stacked double layer construction in order to enhance the luminance of reflected light. Further, the solubility in nematic mixtures, the long term phase behavior, the dopant's impact on textures and phase behavior, the electro-optic response and the photostability of cholesteric mixtures have been investigated.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.