Class Of Liquid Crystals Research Articles

ChemInform Abstract: A New Class of Liquid Crystals Containing an Oxirane Ring.

AbstractIt is shown that the trans stilbene epoxides (II) prepared from their stilbene precursors (I) (no yields given) represent a new class of nematic and smectic liquid crystals.

Alkyl chain flexibility in discotic columnar mesophases

A steric model with a single adjustable parameter is employed to compute configurationally averaged quadrupolar splittings of deuterium labeled alkyloxy chains in a homologous series of hexaalkyloxytriphenylenes (THEn). A comparison with new experimental data [Goldfarb et al., J. Chem. Phys. (in press)] permits inferences about the alkyl chain flexibility in discotic mesogens. The parameter, a control of the constraints on the chain mobility, is first optimized by fitting deuterium NMR data in a homolgous series of nematogens with labeled alkyloxy chains. The same optimum value of the parameter is subsequently used to model the quadrupolar splittings in the columnar phase of THEn discotigens and succeeds in qualitatively accounting for the experimental observations. This result indicates that alkyl chain mobility is quite comparable in these two classes of liquid crystals. The distinctly different features of the quadrupolar splitting patterns observed for nematogens and columnar discotigens are shown to originate from the different orientation of the molecular order tensor relative to the mean direction of chain propagation in the two types of mesogens.

Anisotropic Dispersion Interactions In Liquid Crystals

Abstract The polarizability and shape anisotropics of a new class of aliphatic liquid crystals, 4,4′-di-n-alkyl-bibicyclo[2.2.2]octanes (n, n-BBCO) have been calculated as a function of the alkyl chain length and its degree of flexibility to inquire about the relative importance of attractive and repulsive interactions in stabilizing the liquid crystalline phase. The calculated polarizability anisotropics are found to be much smaller than that of conventional mesogens. In the context of the Maier-Saupe theory, this indicates that anisotropic dispersion interactions make only a minor contribution to the stability of this class of liquid crystals. The cylindrical symmetry exhibited by the n, n-BBCO homologues and their rigid molecular structure suggest that they might be ideally suited for critical testing of existing theories of the nematic-isotropic phase transition.

Some proposals for the application of liquid crystals to photoelastic instrumentation

Liquid crystal displays (L.C.D.'s) are well known for their application to electronic watches, calculators, games and digital thermometers, the technology involved being derived from electro‐optic effects observed in liquid crystals since about 1968. This paper describes proposals for a totally different application of one particular class of liquid crystal, as an optical element in a photoelastic transmission or reflective polariscope.The proposals are currently under development* and considerable work remains before this particular application of liquid crystals is perfected. Early results are promising and if successful they could well produce a revolution in the design of optical equipment with particular attraction to automatic systems incorporating microcomputer control.

New Liquid Crystal Materials; Physical Properties and Performance in Displays for Automobile, High Information Density and Guest-host Applications

Abstract Four novel liquid crystal classes are presented, namely bi-, tri-, and tetracyclic ethanes, pyridazines, and tetracyclic chloro-diesters. Each class exhibits distinct physical characteristics, such as low viscosities η − 12 cP, low elastic ratios k33/k11 ∼ 1, large mesomorphic ranges −40°C. 250°C, large negative dielectric anisotropies δ e − 9 and/or low dielectric relaxation frequencies f e ∼ 1 kHz. It is shown that the dielectric Debye relaxation time τ characterizing the hindered rotation of polar nematic molecules around their short axes is much more affected by molecular structural factors than by their rotational viscosity γ1. The material and electro-optical properties of mixtures for twisted nematic and guest-host LCDs comprising the new compounds are presented. The mixtures cover a wide range of applications such as high contrast, fast and multiplexable automotive TN-LCDs; positive and negative contrast guest-host LCDs; high-information density matrix, and dual-frequency addressable LCDs.

Viscous Properties and Dynamic Electrooptical Performance in Twisted Nematic Displays of Several Liquid Crystal Classes

Abstract The temperature dependence of the rotational viscosities of several physically markedly different binary mixtures belonging to ten different liquid crystal classes is measured. An attempt is made to correlate the results with class-specific structural properties such as hydrogenated rings, molecular polarity and structure of flexible side-chains. It is show that the measured small-scale dynamic properties of twist cells compare well with approximative theoretical calculations and experimentially determined class-specific material parameters. For the switching times corresponding to large-scale twist deformations heuristic approximations lead to satisfactory results.

Effect of Liquid Crystal Classes on the Improvement of Dynamic Scattering by Electron Donor-Acceptor Dopants

Abstract Effect of chemically different nematic liquid crystal (LC) classes are described with the improvement of their dynamic scattering (DS) cut-off frequency fc by electron donor and acceptor dopants. The fc improvement was found to be typically classified into the following categories, according to the LC classes:

Dielectric Heating and Relaxations in Nematic Liquid Crystals

Abstract Novel experiments on dielectric heating were made to induce changes of temperature ΔT in nematic liquid crystal layers. Measurements of the dependence of ΔT on cell voltage, driving frequency, electrode spacings, external cell temperature and liquid crystal material parameters are correlated with molecular relatxation proceses occurring in the frequency range 0-10 MHz. Temperature increases ΔT up to 40°C were achieved at low driving voltages in nematics belonging to two different liquid crystal classes which exhibit large positive static dielectric anisotropies. It could be shown that homeotropic wal alignment leads to a markedly different dependence of ΔT(w) in th vicinity of the cross-over frequency ωc, of the lowest dielectric relaxation process, compared with unoriented or parallel aligned layers. The material requirements and limits for a possible application of dielectric heating in liquid crystal displays are discussed.

Measurements of the Bend and Splay Elastic Constants of a Polymer Liquid Crystal by the Frederiks' Distortion

Abstract The three elastic constants of a liquid crystal, Kii (i=1,2,3) are fundamental measureable constants, which determine the way the material is held together and its response to an external stimulus. While extensive measurements have been carried out to determine the elastic constants of thermotropic liquid crystals,1 very little is known about them in an important class of liquid crystals formed by macromolecules.

Liquid Crystalline 4′‐Pentylphenyl 4‐Alkyloxydithiobenzoates

AbstractThe 4′‐pentylphenyl‐4‐alkyloxydithiobenzoates (1a–e), prepared for the first time, form enantiotropic nomatic phases. Thus, the –CS.S– linking unit qualifies as a central group in the construction of new classes of liquid crystals. The compounds investigated are red in colour and exhibit only a slight tendency to form smectic phases.

Substituted phenylcyclohexanes and cyclohexylcyclohexanes two new classes of liquid crystals as anisotropic solvents in optical absorption spectroscopy

The suitability of nematic phenylcyclohexanes and cyclohexylcyclohexanes as solvents for linear dichroism measurements has been investigated. Especially the cyclohexylcyclohexanes orient solute molecules very well and they are broadly applicable, because their absorption edge lies below 200 nm.

Studies on Electrolytic Substitution Reactions. XII. Synthesis of 4-Alkoxy-4'-cyanobiphenyls -- a Class of Liquid Crystals -- via Anodic Cyanation of 4,4'-Dialkoxybiphenyls in Emulsion Systems.

Studies on Electrolytic Substitution Reactions. XII. Synthesis of 4-Alkoxy-4'-cyanobiphenyls -- a Class of Liquid Crystals -- via Anodic Cyanation of 4,4'-Dialkoxybiphenyls in Emulsion Systems.

Nematic-Like Flow Behavior in a Non-Steroid Cholesteric Mesophase

Abstract Steady flow viscosity data were obtained on a compound which exhibits a cholesteric liquid crystalline mesophase but does not contain the steroid moiety. The compound is (+)-2-methyl-butyl p-[N-(p-methoxybenzylidene)amino] cinnamate. This cholesteric liquid crystal rheology was found to the similar to that of the nematic class of liquid crystals. The material's liquid crystalline nature lowers its viscosity. A viscosity maximum in seen at the cholesteric-isotropic phase transition. This effect differs from that previously reported for cholesteric liquid crystals.

Liquid Crystals in Molten Salt Systems

BECAUSE of the strong long range forces acting between the molecules, molten salts constitute a highly distinctive class of liquids. Up to now, however, the variety of stable ionic melts studied has been largely confined to the simplest ions, such as inert gas type halides, nitrates, and network forming oxides and fluorides. Through the establishment of procedures for achieving chemical stability in molten salts with organic anions1,2, the variety of these ionic melts available for investigation and use is potentially greatly increased. Distinctive microstructures and unusual properties might be expected for at least some of these liquids. We illustrate these possibilities here with new observations on liquid crystals, stable over a wide interval of temperatures, formed by melts of sodium isovalerate and related salts. This group of substances differs in several ways from well known classes of liquid crystals, which include chiefly rod-shaped molecules, with long rigid parts3.