Isotropic Liquid Crystal Research Articles

Phase Equilibria in Systems of Water, Naphthenic Acids, and Phenols

A partial phase diagram has been determined for the system based on 5‐phenylvalerate, 4‐pentyphenol, and water at 25 °C. The system showed a very rich phase behavior in which many different isotropic solutions and liquid crystals were found. Both normal and reverse self‐assembly structures of the micellar and hexagonal types were noted. In the middle of the phase diagram, a lamellar liquid crystalline phase with a large swelling capacity was observed. When the aromatic alcohol was replaced by a long‐chain alcohol the reverse hexagonal structure disappeared. The effect of temperature and salinity on the phase behavior was also studied. Raising the temperature increased the micellar regions, while the lamellar phase was slightly reduced and the reverse hexagonal phase disappeared. Addition of salt gave the lamellar phase a smaller region of existence and the large extension towards the water apex disappeared. Introduction of an acid to the system resulted in a remarkable change of the phase behavior: both the normal micellar and lamellar regions were significantly reduced, while the reverse micellar region was significantly increased.

The dynamic influence of photoisomerization on optical reorientation in absorbing isotropic liquid crystals

The induced transient dichroism and the transient optical Kerr effect (OKE) response of azo-doped isotropic liquid crystals were studied in picosecond time scale. Photoisomerization occurs when the azo-dye molecules are resonantly excited. As a result, a significant enhancement of OKE signal was found in the dye doped liquid crystals with respect to the pure liquid crystals. The enhancement was attributed to the extra torque exerted on liquid crystals after anisotropic orientational distribution of trans and cis isomers were generated. A modified mean field theory model was applied to treat the interactions of trans and cis isomers with liquid crystals. A small-pump enhancement factor of 3.8 was obtained, which is comparable with that of the well-known anthraquinone dye doped LC system.

The isotropic and nematic liquid crystal phase of colloidal rods

The density-dependent distribution function of the orientation of the long axis of rod-like colloidal particles in the liquid crystalline nematic phase is numerically calculated as the solution of a nonlinear integral equation. From this solution, which is obtained by an iterative method that is explained in detail, the free energy, the pressure and the chemical potential can be computed, as well as the nematic order parameter. Moreover, the first-order phase transition from the disordered isotropic fluid phase to the nematic phase is analysed numerically. This study, presented at the level of advanced undergraduates or starting graduate students, not only illustrates the existence of liquid crystalline ordering for nonspherical particles, but also shows explicitly how thermodynamics, structure and symmetry are related.

Terahertz spectroscopy in smectic phases of a bent-core molecule

The dynamics of a bent-core molecule in two smectic phases is studied using terahertz (THz) spectroscopy, which enables us to observe some rapid molecular motions and segmental motions reflecting intermolecular interactions. The isotropic liquid, liquid crystal and crystal phases of the bent-core liquid crystal P-8-PIMB are clearly distinguished by THz spectra. Unexpectedly, the transmittance in the B3 phase, which has been assigned to either a highly ordered smectic or crystal phase from results of x-ray diffraction analysis, is almost the same as that in the switchable liquid-crystalline B2 phase. This suggests that the molecular dynamics in the B3 phase is liquid-crystal-like on this time scale.

A system of Kerr effect spectroscopy for light absorbing liquids

We developed experimental system of the optical Kerr effect spectroscopy for the light absorbing liquids. To avoid the problems due to the light absorption, such as the thermal lens effect, our new system used a gated rectangular light pulse with the alternative polarization modulation as the pump light. The experimental results of the transient behavior of the signal showed that our system could clearly distinguish the optical Kerr effect from the thermal lens effect. Careful monitoring of the signal enables us to suppress the temperature increase within the experimental latitude. We examined the previous results that the optical anisotropy of the isotropic liquid crystals induced by the external field is enhanced dramatically compared with the pure liquid crystals by doping a small amount of the dye from the viewpoint of relaxation dynamics. The information on the dynamic molecular orientation relaxation obtained by our spectroscopy would help us understand a microscopic interaction among liquid crystals and dye molecules, especially in the vicinity of the transition temperature. The system would be useful widely for the measurement of the nonlinear optical properties in the materials, which show strong light absorption or scattering.

Well-ordered mesoporous silica prepared by cationic fluorinated surfactant templating

We describe the synthesis and characterization of ordered mesoporous silica using the template 1H,1H,2H,2H-perfluorooctylpyridinium chloride. This surfactant forms several lyotropic mesophases at room temperature, including hexagonal close packed cylinders, an isotropic liquid crystal, and a lamellar phase. Using this surfactant, mesoporous silica is synthesized by room temperature precipitation and surfactant extraction. Both the surfactant and product material are thoroughly characterized. From nitrogen adsorption, the product has a specific surface area of 982 m 2/g and a pore diameter of 2.6 nm. X-ray diffraction and transmission electron microscopy confirm two-dimensional close-packed hexagonal long-range ordering. This is the first example of cationic fluorinated surfactant templating of an ordered porous ceramic material.

Effect of external electrical field on phase behavior and morphology development of polymer dispersed liquid crystal

Phase diagrams for mixtures of liquid crystal (LC)/monomer with and without an external electrical field applied have been established using polarized light microscope (PLM).The (isotropic + nematic) coexistent phase region and (isotropic + isotropic) phase boundary of LC/monomer mixtures were observed to shift upward to higher temperatures when the external electrical field exists. It was found that the electrical field applied during the cross-linking polymerization has a significant influence on the phase diagrams for the LC/polymer mixtures by rendering the coexistent phase regions shift upward to higher temperatures. The influence of the external electrical field on the processes of the isotropic–isotropic phase separation and liquid crystal ordering in PDLC formation has also been investigated. The results revealed that both the processes could be highly accelerated by the electrical field.

Linear splay elasticity in surface-induced films of tilted smectic liquid crystals.

The prefrozen films that may be observed at the surface of isotropic liquid crystal droplets, close to the isotropic-smectic phase transition, or surface-induced films, are essentially asymmetric. If moreover, the molecules are tilted inside the smectic layers, as in the smectic-C (SmC) or smectic-C(A) (SmC(A)) phases, the c director that we may define as the order parameter of the film, is a real vector. Thus, the surface-induced films of MHTAC exhibit vectorial or polar properties, though the molecules are not chiral. The film free energy therefore contains a surface-elasticity term, K(1)c(2) c, that is a linear function of the splay distortion, and that may be negative enough to promote a mechanical instability. A spontaneous c distortion, or c modulation, then invades the whole film and produces an array of parallel stripes, with a typical four-fringe periodicity when observed between crossed polarisers. Here, we present optical measurements of the distortion for different film thicknesses, and we propose a linear analysis of the data to test our model. Due to the limitations inherent to the Fourier expansion that we use, the calculations are valid only between two limits: a large q limit where splay domains collapse into disclination lines, and a small q borderline below which the distortion evolves towards a system of independent solitons. We find that the sign of the spontaneous splay elastic constant K1 alternates as a function of the number of layers, N, a property that is reminiscent of the alternate structure of the SmC(A) phase. We argue that the two-dimension elastic constant, K1, originates from the interactions between the molecules in contact to the isotropic phase, and we deduce K1 approximately -(-1)(N)x10(-11) N and the elastic anisotropy, with a ratio of the splay over bend elastic constants K(s)/K(b) approximately 4.5. Similar properties could be observed also in other types of ultrathin films, e.g., in free-standing, ferroelectric (SmC*) or antiferroelectric (SmC(*)(A)), films, in Langmuir films, and even in particular biological films. In some cases, a second, electric instability may occur and superimpose onto the elastic one.

Rotational dynamics of nondipolar and dipolar solutes in an isotropic liquid crystal: Comparison with an isotropic liquid

Rotational dynamics of a nondipolar solute, 2,5-dimethyl-1,4-dioxo-3, 6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and a dipolar solute, coumarin 6 (C6) has been studied in the isotropic phase of a liquid crystal, 4′-methoxybenzylidene-4-n-butylaniline (MBBA) to understand the influence of pseudonematic domains on the dynamics of dopant molecules. The reorientation times of both DMDPP and C6 follow the Stokes–Einstein–Debye hydrodynamic model instead of the Landau–de Gennes model, which is used to describe the rotational relaxation of neat isotropic liquid crystals. However, comparison of the data for both DMDPP and C6 in MBBA to that in an isotropic solvent, 1-decanol reveals that the probes are rotating considerably slower in the liquid crystal. These results indicate that ordered local structures of the liquid crystal exert more friction on the rotating solute molecule compared to an isotropic liquid.

Local structure in nematic and isotropic liquid crystals

By computer simulations of systems of ellipsoids, we study the influence of the isotropic/nematic phase transition on the direct correlation functions (DCF) in anisotropic fluids. The DCF is determined from the pair distribution function by solving the full Ornstein–Zernike equation, without any approximations. Using a suitable molecular-fixed reference frame, we can distinguish between two qualitatively different contributions to the DCF: One which preserves rotational invariance, and one which breaks it and vanishes in the isotropic phase. We find that the symmetry preserving contribution is barely affected by the phase transition. However, symmetry breaking contributions emerge in the nematic phase and may become quite substantial. Thus the DCF in a nematic fluid is not rotationally invariant. In the isotropic fluid, the DCF is in good agreement with the prediction of the Percus–Yevick theory.

Dynamics in supercooled liquids and in the isotropic phase of liquid crystals: A comparison

A comparison is made of the dynamics observed over wide ranges of time and temperature between five supercooled liquids and four isotropic phase liquid crystals that have been previously studied separately. Optical-heterodyne-detected optical Kerr effect (OHD–OKE) measurements were employed to obtain the orientational relaxation dynamics over time scales from sub-ps to tens of ns. For the supercooled liquids, the temperatures range from above the melting point down to ∼Tc, the mode coupling theory critical temperature. For the liquid crystals, the temperatures range from well above the isotropic-to-nematic phase transition temperature TNI down to ∼TNI. For time scales longer than those dominated by intramolecular vibrational dynamics (≳1 ps), the fundamental details of the dynamics are identical. All nine liquids exhibit decays of the OHD–OKE signal that begin (>1 ps) with a temperature-independent power law t−z, where z is somewhat less than or equal to 1. The power law decay is followed in both the supercooled liquids and liquid crystals by a crossover region, modeled as a second power law. The longest time scale decay for all nine liquids is exponential. In supercooled liquids, the exponential decay is the α relaxation (complete structural relaxation). In liquid crystals, the exponential decay is the Landau–de Gennes decay (relaxation of pseudonematic domains). As Tc (supercooled liquids) and TNI (liquid crystals) are approached from above, the time range over which the “intermediate” power law can be observed increases, until near Tc and TNI, the power law can be observed from >1 ps to many ns. The data for all nine liquids are described accurately by the same functional form and exhibit a scaling relation in common. The nature of the dynamics in the liquid crystals is understood in terms of pseudonematic domains that have a correlation length ξ, which increases as TNI is approached. It is conjectured that the similarities between the liquid crystal data and supercooled liquid data are produced by the same underlying physical features: that is, like liquid crystals, supercooled liquid dynamics is a result of structural domains even at relatively high temperature.

Excitation of convective motions in isotropic and anisotropic liquids by light

The possibility of excitation of convective motions of the Rayleigh — Benard and Marangoni type in isotropic liquids and nematic liquid crystals upon absorption of light with a spatially periodic intensity distribution is demonstrated theoretically and experimentally. It is shown that gravitational and thermocapillary surface hydrodynamic waves are observed in the case of a running interference pattern. The possibility of control and the stability of convective motions are investigated. Benard cells become unstable when the light intensity is high. These instabilities are of thermal origin because the Prandtl number for the medium under investigation is considerably larger than unity. The competition between the gravitational and thermocapillary mechanisms of photohydrodynamic reorientation of the nematic liquid crystal director is also studied. The effect of convective motions on the thermodynamic phase transition is observed and explained.

Reaction Center Models in Liquid Crystals: Identification of Paramagnetic Intermediates

Photoinduced charge separation and recombination to the triplet state in a carotene (C) porphyrin (P) fullerene (C 60 ) triad have been followed by time-resolved electron paramagnetic resonance (EPR). The electron transfer process has been studied at different temperatures in both a glassy isotropic matrix (2-methyltetrahydrofuran) and uniaxial liquid crystal (E-7). In both media, the triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge separated state (C ” + -P-C 60 ” m ), and charge recombination to the triplet state, localized in the carotene moiety. The carotenoid triplet state is initially polarized according to the mechanism of recombination of a radical pair with singlet precursor. Both the photoinduced spin-correlated radical pair and the carotene triplet are observed in E-7 starting from the glass to the liquid crystal phase. The exchange interaction between the electrons in the radical pair (J=1.2 Gauss) has been evaluated by simulation of the EPR spectrum in the isotropic glass and comparison with the corresponding spectrum in the oriented medium.

Thermally induced nonlinear optical effects in an isotropic liquid crystal at 10.6 microm.

We investigate thermally induced nonlinear optical effects on a CO2 laser beam that passes through an isotropic liquid crystal film. We evaluate the thermal nonlinear coefficient of the refractive index at a wavelength of 10.6 microm by means of a thermographic technique combined with an optical method based upon the measurement of the self-defocusing effect caused by the heating of the liquid crystal induced by the laser beam. The novelty of our work is the application of thermography in order to measure the temperature field on the liquid crystal film caused by the partial absorption of the laser radiation by the liquid crystal. These measurements provide a precise evaluation of the thermal diffusion lengthof the liquid crystal under investigation. Moreover, thermography results in a straightforward experimental technique that can be used to investigate the thermal properties of a wide class of other fluids.

Isotropic-nematic phase equilibrium and phase separation of kappa-carrageenan in aqueous salt solution: experimental and theoretical approaches.

The behavior of chiral-nematic and isotropic phases of helical kappa-carrageenan in aqueous solution of sodium iodide was compared with that of the anisotropic biphasic phase that contains both these phases. On the basis of birefringence, rheology, chemical analysis, average molecular weight, and polydispersity index measurements, we derived a number of characteristic differences as well as similarities between these phases, over a range of polysaccharide concentrations obtained by the dilution of each phase. For example, we assessed the critical concentration of an isotropic-anisotropic transition (C(i)), the temperature of the anisotropic-isotropic phase shift during thermal heating-cooling cycles, and the viscosity changes due to the phase shift and due to the diminishing of the helical conformation. We also demonstrated how the different phases and their dilutions behave under the effect of shear and frequency of oscillation and how the viscoelastic properties vary in each phase and discussed the isotropic and anisotropic liquid crystal controlling behavior mechanisms. From a theoretical point of view, we propose to combine the wormlike chain model for semiflexible polyelectrolytes interacting via both hard-core and electrostatic repulsion to assess the concentration of isotropic-nematic transition, to assess the coexistence concentration range, and to determine the effects of charge by applying the effective diameter and a twisting effect.

Effective index of refraction, optical rotation, and circular dichroism in isotropic chiral liquid crystals.

This paper concerns optical properties of the isotropic phase above the isotropic-cholesteric transition and of the blue phase BP III. We introduce an effective index, which describes spatial dispersion effects such as optical rotation, circular dichroism, and the modification of the average index due to the fluctuations. We derive the wavelength dependence of these spatial dispersion effects quite generally without relying on an expansion in powers of the chirality and without assuming that the pitch of the cholesteric P is much shorter than the wavelength of the light lambda, an approximation that has been made in previous studies of this problem. The theoretical predictions are supported by comparing them with experimental spectra of the optical activity in the BP III phase.

Lattice Boltzmann Simulation of 3D Nematic Liquid Crystal near Phase Transition**The project supported by National Natural Science Foundation of China under Grant number 19974007, and the Research Grant of National Education Ministry of China

Phase transition between nematic and isotropic liquid crystal is a very weak first order phase transition. We avoid to use the normal Landau–de Gennes's free energy that reduces a strong first order transition, and set up a data base of free energy calculated by means of Tao–Sheng–Lin's extended molecular field theory that can explain the experiments of the equilibrium properties of nematic liquid crystal very well. Then we use the free energy method of lattice Boltzmann developed by Oxford group to study the phase decomposition, pattern formation in the flow of the liquid crystal near transition temperature.

Infrared Electroabsorption Spectroscopic Study of Association Structures of 5CB in the Solution, Isotropic Liquid and Nematic Liquid Crystal States

Abstract The association structures of 4-n-pentyl-4’-cyanobiphenyl (5CB) in the solution, isotropic liquid, and nematic liquid crystal states have been studied by infrared electroabsorption spectroscopy. In a 0.5 mol dm-3 carbon tetrachloride solution, the determined value of dipole moment of 5CB (4.9D) agrees excellently with a literature value (4.85D) for monomer, indicating that the 5CB molecule exist as a monomer. Monomeric response of the 5CB molecule to the external field has also been found for the isotropic liquid state. For the nematic phase, marked deviation from the monomeric response has been found, showing the domain formation of the 5CB molecules. The size dependence of the domain response to the applied electric field has also been elucidated.

Generalized Young-Laplace Equation for Nematic Liquid Crystal Interfaces and its Application to Free-Surface Defects

This paper uses the classical theories of liquid crystal physics to formulate the Young-Laplace equation of capillary hydrostatics for interfaces between viscous isotropic (I) fluids and nematic liquid crystals (NLC's). The equation provides an expression of pressure jumps across interfaces and determines the geometry of the interface. In addition, the force balance equation at a capillary point or line singularity containing a nematic surface defect is also derived from first principles. It is shown that the areal pressure balance and the line or point force balance equations provide a complete set to determine the geometry of an interface containing a defect. A new expression for the Peach-Koehler force for surface defects is developed. The contribution of the surface Peach-Koehler force to areal pressure balances is also identified. The equations are applied to solve for the interfacial geometry of a NLC whose free-surface contains a point defect. Consistency of the new approach developed in this paper with previously presented results is demonstrated.

Analysis of Displacement Fields near Dislocation Cores in Ordered Polymers

We have significantly improved a method to characterize the displacement fields near edge dislocations in ordered polymers. Our extended analysis now makes it possible to predict and explain the variation in tilt of different lattice planes in the vicinity of dislocations in isotropic solids, anisotropic crystals, and liquid crystals in terms of their elasticity constants. Direct images of the dislocation cores were obtained in three different polymer systems using bright field transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). A b[010] = 63 nm edge dislocation was imaged in the ABC triblock copolymer polystyrene-block-poly(ethylene-co-butylene)-block-poly(methyl methacrylate) (SEBM). The isotropic displacement fields according to theories by Taylor and Burgers were compared to the SEBM data. Fitting the theoretical displacement fields to the displacements measured from the image, an estimate of the elastic constant anisotropy was obtained. For this material the ratio o...