Isotropic Phase Of Liquid Crystals Research Articles

Liquid crystal dynamics in the isotropic phase

The dynamics in the isotropic phase of the liquid crystal 1-isothiocyanato-(4-propylcyclohexyl) benzene (3-CHBT) are investigated from very short time (∼1 ps) to very long time (>100 ns) as function of temperature. The data decay exponentially only on the longest time scale (>10 ns). The temperature dependence of the long time scale exponential decays is described well by the Landau–de Gennes theory of the randomization of pseudo-nematic domains that exist in the isotropic phase of liquid crystals near the isotropic to nematic phase transition. Over the full range of times, the data are fit with a model function that contains a short time power law. The power law exponent is temperature independent over a wide range of temperatures. Integration of the function gives the empirical polarizability–polarizability (orientational) correlation function. A preliminary theoretical treatment of collective motions yields a correlation function that indicates that the data can decay as a power law at short times. The power law component of the decay reflects intradomain dynamics.

Cotton-Mouton constants and pretransitional phenomena in the isotropic phase of liquid crystals

The Cotton-Mouton constants for three homologous series of liquid crystals were measured in the isotropic phase. Some of these measurements were performed with high temperature resolution. The results are compared with theories on pretransitional phenomena. The Landau-de Gennes theory proves to be inadequate to describe the data.

On the frequency dependence of ultrasonically induced birefringence in isotropic phase of liquid crystal: 5CB (p-n-pentyl p′-cyanobiphenyl)

Theoretical expressions for the ultrasonically induced birefringence of liquids are obtained in the frame work of de Gennes’ phenomenological theory. The intensity and frequency dependence of ultrasonically induced birefringence in the isotropic phase of p-n-pentyl p′-cyanobiphenyl (5CB) was measured in order to examine the usefulness of birefringence measurements for investigating dynamical properties liquids. The observed birefringence was proportional to the square root of ultrasonic intensity. The birefringence divided by the square root of ultrasonic intensity increases with increasing frequency and appears to saturate when the ultrasonic frequency approaches the relaxation frequency of molecular reorientation. The observed values of birefringence were reproduced satisfactorily by the expression derived in this paper.

MOLECULAR BIAXIALITY AND THE LOWEST SUPERCOO-LING TEMPERATURE FOR NEMATIC LIQUID CRYSTALS IN ISOTROPIC PHASE

In the molecular field approximation, we study the effect of molecular biaxiality on the lowest supercooling temperature of nematic liquid crystals in isotropic phase. Theoretical results show that by taking into account the molecular biaxiality, the ratio of the lowest supercooling temperature in isotropic phase T* to the nematic-isotropic phase transition temperature Tc, approaches the experimental value. When the molecular biaxiality parameter δ reaches 0.3, the ratio T*/Tc reaches 0.9817, very close to the experimental value of 0.99.

Depolarized light scattering in the isotropic phase of liquid crystals

The polarized and depolarized Rayleigh light scattering spectra for the isotropic phase of p-methoxybenzylidene-n-butylaniline (MBBA) and 4-cyano-4-penthylbiphenyl (5CB) were measured in order to investigate the coupling between translational and orientational modes of molecular motion. The Rayleigh dip was observed in depolarized light scattering spectra of isotropic phase of both liquid crystals in the wide temperature ranges. The Rayleigh dip spectrum was attributed to the coupling between orientational and translational mode of molecular motions in anisotropic molecules. Data analysis based on the Alms’ theory (J. Chem. Phys. 67(12), 5779 (1977)) leads to the evaluation of the coupling parameter R. The temperature dependence of the coupling parameter of both liquid crystals near the phase transition point was discussed.

Solvation dynamics of ionic dyes in the isotropic phase of liquid crystals

We present a solvation dynamics study of Rhodamine 700 in the isotropic phase of octylcyanobiphenyl approaching the nematic-isotropic phase transition. From our experiments we detect two kinds of processes. A fast process, with decay time constants of tens of ps, shows no clear temperature dependence and is attributed to local reorientational motion of the solvent in the proximity of the chromophore. A slow process, with time constants in the hundreds of ps, shows a temperature activated behavior. This process does not seem to be influenced by the molecular order displayed by nematogenic substances and resembles the solvation dynamics behavior observed in simple polar liquids and predicted by dielectric theories.

Pretransitional nematic ordering in liquid crystals with dispersed polymer networks

The birefringence induced by partially ordered polymer networks dispersed in an isotropic phase of liquid crystals is studied. Polymer networks were formed by the polymerization of 1--4 % mixtures of prepolymer in the nematic phase of liquid crystals. The partial nematic (paranematic) ordering is analyzed in terms of the Landau--de Gennes approach using a simple model of an array of thin fibrils, which can, on a large scale, form bundles of polymer-rich material. The comparison of the theory and birefringence data clearly shows that the main building bloc of the network is a few nanometers thick fibril. The average thickness of fibrils varies slightly with the polymer concentration. The information obtained on large scale (micrometer) structures is not conclusive and must be complemented by other methods. The network on the average retains 30--50 % of the order parameter of the environment where it was formed.

Theory of universal fast orientational dynamics in the isotropic phase of liquid crystals

A theoretical treatment is presented that demonstrates universal dynamical behavior in the isotropic phase of liquid crystals on ultrafast time scales and short distance scales. The theoretical development generates a temperature independent power law for the short time scale decay of the molecular orientational correlation function. This provides a theoretical rationale for the postulate of universal behavior based on recent experimental observations on two liquid crystal systems. A temperature independent power law decay with the identical exponent, 0.63, was observed for the two systems. First, an alternative theoretical approach reproduces the Landau de Gennes results for the long distance scale, slow time scale orientational dynamics in the isotropic phase. This approach is also capable of examining the short distance scale and short time scale dynamics, and yields a temperature independent power law decay with exponent 0.5. Then critical correlations of fluctuations and local symmetry considerations are included. The Ising model of critical systems is employed. This detailed analysis yields the experimentally observed exponent, 0.63, without recourse to adjustable parameters. Modern theories of dynamic critical phenomena like dynamic scaling theory, the kinetic Ising model and the stochastic model of Karder–Parisi–Zhang are considered as alternative approaches. While these theories can generate some of the features found in experiment, it is not possible to reproduce the observed experimental results without internal inconsistencies or unwarranted adjustable parameters.

Nonlinear optical propagation and self-limiting effect in liquid-crystalline fibers

The nonlinear optical properties of the isotropic phase of liquid crystals induced by nanosecond laser pulses are analyzed in the context of nonlinear multi-mode propagation in a liquid-crystal-cored fiber. The negative thermal density nonlinearity of the core gives rise to an intensity-dependent loss in the core-guided transmission and optical action. Experiments conducted with such liquid-crystal-cored fibers show that the optical limiting threshold for nanosecond laser pulses can be as low as 0.09 J/cm2, which is one of the lowest among known nonlinear optical materials and structures, including bulk liquid-crystal films.

Nematic and smectic surface order in the isotropic phase of low molar mass and polymeric liquid crystals

Pretransitional surface ordering phenomena, as they occur in the isotropic phase of liquid crystals upon approaching the clearing point, were studied by variable angle ellipsometry and X-ray reflectivity measurements. Ellipsometric data obtained for two phenylcyclohexanes indicate a logarithmic divergence of the thickness of a nematic boundary layer with homeotropic orientation, which corresponds to a complete wetting behaviour. Data can be represented by a model based on a Landau-de Gennes approach. An investigation of a LC-side group polyacrylate showed a boundary layer with planar orientation. The thickness, which increases on approaching the clearing point, is clearly larger than in the case of the low molar mass samples. In X-ray reflectivity studies on a film of a LC-side group polymer boundary layers with smectic order were detected. They possess a remarkable stability and exist also at larger distances from the clearing point. A stepwise procedure in the data analysis enables a detailed, complete characterization.

Isotropic-liquid crystal phase equilibrium in semiflexible polymer solutions: effects of molecular weight and ionic strength in polyelectrolyte solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsotropic-liquid crystal phase equilibrium in semiflexible polymer solutions: effects of molecular weight and ionic strength in polyelectrolyte solutionsShinichiro Inatomi, Yuji Jinbo, Takahiro Sato, and Akio TeramotoCite this: Macromolecules 1992, 25, 19, 5013–5019Publication Date (Print):September 1, 1992Publication History Published online1 May 2002Published inissue 1 September 1992https://doi.org/10.1021/ma00045a030RIGHTS & PERMISSIONSArticle Views264Altmetric-Citations34LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (894 KB) Get e-Alerts Get e-Alerts

Nanosecond time scale dynamics of pseudo-nematic domains in the isotropic phase of liquid crystals

The slow reorientational dynamics of isotropic methoxybenzylidene-butylaniline (MBBA) were measured, using a transient grating optical Kerr effect experiment, over a wide temperature range (49.4°–119.7°C). The range of validity of the Landau—de Gennes theory for hydrodynamic relaxation in ordered fluids is determined. The results are compared to previous experiments on pentylcyanobiphenyl (5CB). Both liquid crystals conform to the Landau—de Gennes theory until the pseudo-nematic domain correlation length falls below three molecular lengths.

Surface-induced orientational order in the isotropic phase of a liquid-crystal material.

Orientational order in the isotropic phase of liquid crystals confined to 0.2-\ensuremath{\mu}m-diam cylindrical channels of Anopore membranes is measured by deuterium nuclear magnetic resonance. At temperatures deep in the isotropic phase, the measurements reveal the order of the first molecular layer at the cavity wall, while at temperatures near the isotropic-nematic transition the measurements are primarily sensitive to order penetrating into the cavity in a manner described by the Landau--de Gennes theory. The degree of order ${\mathit{S}}_{0}$ at the cavity wall is measured and the surface coupling constant determined for various treatments of the cavity wall. Wide variations of ${\mathit{S}}_{0}$ are found for different surface treatments. Treatments that yield both perpendicular and parallel molecular anchoring are explored. The angular dependence of the surface-induced nematic order is investigated following the ${\mathit{P}}_{2}$(cos\ensuremath{\theta}) expression independent of molecular anchoring at the surface.

Electrooptical Properties of Polymer Films Containing Nematic Liquid Crystal Microdroplets

Polymer films containing nematic liquid crystal microdroplets are useful for light modulation devices. Light scattering in these films is controlled by applied electric fields. These films were produced by cooling solutions of polymer in the isotropic phase of liquid crystal to temperatures below the clearing point (the nematic-phase-to-isotropic-phase transition temperature). In this process, sizes of microdroplets were regulated by controlling the rate of cooling. The relationship between the microdroplet size and electrooptical properties was investigated experimentally and theoretically.

Isotropic-Liquid Crystal Phase Equilibrium in Semiflexible Polymer Solutions: Application of Scaled Particle Theory

Abstract The scaled particle theory of Cotter for straight spherocylinders was extended to semiflexible polymer solutions following Khokhlov and Semenov's procedure to calculate the isotropic-liquid crystal phase boundary concentrations and osmotic pressure of the solution, with the diameter d and persistence length q of the polymer as parameters. The calculated phase boundary concentrations and osmotic pressure were compared favorably with experimental data for aqueous schizophyllan (a triple helical polysaccharide) and poly(hexyl isocyanate) in toluene and dichloromethane when use was made of the experimental q values and the d values rather close to those obtained from the partial specific volume of the polymer. Thus the thermodynamic behavior of the systems investigated was explained essentially in terms of the hard-core potential.

Electric field N.M.R. of pretransitional effects in liquid crystals A solute study

Abstract Pretransitional effects in the isotropic phase of liquid crystals are manifest in the N.M.R. spectra of solutes recorded in the presence of an applied electric field. The temperature dependence of the quadrupolar splittings of p-xylene-d 10 in the isotropic phase of the nematogen 4-n-pentyl-4'-cyanobiphenyl is investigated, and proves to be as expected from the Landau-de Gennes theory for pretransitional effects. The influence of temperature in the biphasic region of the phase diagram is very striking: the orientational ordering of the solute is temperature independent. The applied electric field not only determines the ordering of the solute, but also has a clear effect on the transition temperature. Electric field N.M.R. of solutes to study pretransitional effects is an alternative to the investigation of the nematogen itself.

Pretransitional behaviour in liquid crystals. The roles of nuclear magnetic resonance spectroscopy and molecular field theory

The liquid crystal–isotropic phase transition is weak, and so the angular correlations, characteristic of the ordered phase, grow within the isotropic phase as the transition to the liquid crystal is approached. This pretransitional behaviour has been well studied through the orientational order induced by a magnetic field, via the resulting optical birefringence. This is observed to diverge in the vicinity of the phase transition and such behaviour may be understood, at the molecular level, by theories based on the molecular field approximation. However, both the experimental and the theoretical analyses are oversimplified because they assume that the constituent molecules of a liquid crystal are rigid and cylindrically symmetric, whereas in reality the molecules are of low symmetry and flexible. To characterise the field-induced orientational order of such complicated molecules in detail it is necessary to use a technique with a far greater information content than the single number obtained from the birefringence. Deuterium n.m.r. spectroscopy has proved to be particularly valuable for investigating the details of the orientational order of liquid-crystalline phases themselves, and is now being applied, with equal success, to the study of pretransitional behaviour in the preceding isotropic phase. Here such new investigations are surveyed for pure liquid crystals, both nematic and smectic, and for liquid-crystal mixtures. The results of these unique experiments may be rationalised rather well by extending the molecular field theory to include molecular flexibility and multicomponent mixtures.

Light-Scattering Study of Thermotropic Liquid Crystals and Micellar Solutions

Abstract The accurate dynamic and static light-scattering study of pretransitional phenomena in the isotropic phase of liquid crystals and in the micellar phase of amphiphilic solutions was carried out. It was shown that the existence of the smectic ordering affects the susceptibility behaviour in the isotropic phase. In the vicinity of the micellar solution critical point there is a single characteristic length as well as in ordinary binary mixtures in spite of the large polydispersity of the amphiphilic molecules.

Nonlinear optical Properties of Liquid Crystals In The Isotropic Phase

Abstract Picosecond nonlinear absorption and nonlinear refraction were studied for several classes of isotropic phase liquid crystals, including Schiff base and ester compounds. Materials studied exhibit a large two-photon absorption coefficient (j3) at 532 nm.Values of j3 were found to be -0.6 cm/GW in several of the compounds studied. Nonlinear refraction was also observed and the nonlinear refractive index, n2 was measured for each material at 1.06 pm using an external self-focusing mangement. n2 ranged from 6 × 10-13 esu to 2 × 10-l2 esu. The combination of nonlinear absorption and nonlinear refraction in these materials result in optical limiting for input energies as low as 0.15 microjoules for 30 psec pules at 532 nm.

Degenerate four-wave mixing in the isotropic phase of liquid crystals: The influence of molecular structure

We review the ways in which the molecular characteristics of nematogenic fluids influence the efficiency of degenerate four-wave mixing and illustrate them in experiments on several materials. The nematic pretransitional effect makes the nonlinear susceptibility very large, but also slows the response times. This limits the maximum effective nonlinearity for a given laser pulse length. Although all nematogens show a universal pretransitional behavior, the rate at which the susceptibility and response time diverge, on approaching the transition point, is very dependent on the chemical structure of the molecule involved and may also be influenced by adding solvents or forming mixtures with other nematogenic molecules. We identify the influences on our results of molecular shape and polarizability, the viscosity of the fluid, and of specific intermolecular interactions, such as those leading to local smectic ordering.