Properties Of Nematic Liquid Crystals Research Articles

Influence of dye molecules on the birefringence of liquid crystal mixtures at near infrared frequencies

The optical properties of nematic liquid crystals have been extensively exploited in the production of devices working in the visible range of the spectrum. These same properties can be employed to make devices that function in the near infrared as required for telecommunications applications. However, it is generally observed that the birefringence of liquid crystal mixtures decreases with increasing wavelength, making it important to identify new materials, optimized for use in the near infrared region. One route to high birefringence is to operate close to an absorption band edge, which in the present context implies choosing highly conjugated materials which are potentially colored and, thus, not suited to traditional display applications. In this paper we explore the usefulness of dye molecules as birefringence enhancers in mixtures with conventional nematic liquid crystals. The optical properties, in particular, the absorption edge, polarizability, and birefringence, of families of known dyes are calculated at optical (589 nm) and infrared (1550 nm) wavelengths, using electronic density functional theory. We demonstrate the expected correlation between the proximity of the absorption edge and the magnitude of the birefringence, and estimate the birefringence enhancement occurring when each dye is incorporated in a guest-host system.

Investigation of the phase transition at the nematic liquid crystal–wall interface within nonextensivity

In this study, the nematic–isotropic phase transition is investigated for a sample in the shape of a slab of thickness d , using nonextensive formalism. The interaction potential is written as the sum of the direct interaction of a given nematic molecule with the substrate and of its incomplete interaction with the other nematic molecules due to the presence of the limiting surface. In this framework, we show the effects of the nonextensivity on the nematic–isotropic transition at the nematic–wall interface. The generalized model can shed light on the properties of nematic liquid crystal confined in small-scale structures.

Influence of the molecular structure of components on the induction of mesophases in nematic liquid crystal systems

The dielectric properties of nematic liquid crystals (NLC) are studied in the temperature range of mesophase existence with taking into account the peculiarities of the molecular structure of components. The dependence of the ordering parameter and activation energy on the temperature and composition of an LC system are obtained and investigated by analysis of EPR spectra based on the spin relaxation theory. The values of the local viscosity of the systems are estimated within the framework of the hydrodynamic model. The appearance of induced and reentrant mesophases is established. It is shown that in NLC systems with non-center-symmetric molecules the correlation between the activation energy and thermostability should be inverse to that in the Maier-Saupe model.

Tunable negative refraction in nematic liquid crystals

A tunable negative refraction mechanism based on the properties of nematic liquid crystals is proposed. It shows that negative group refraction occurs at the interface between air and the nematic liquid crystals, and that the refraction can be adjusted by an applied electric field and by the temperature. For a nematic liquid crystal (E7, Merck), the group refraction is negative for the voltages of 1.0–2.8V and reaches a maximum at 1.5V. The negative group refraction decreases monotonously with increasing temperature. Such mechanism demonstrates the feasibility of tunable negative refraction devices based on liquid crystals.

Substrate Induced Pre-tilt in Hybrid Liquid Crystal/Inorganic Photorefractives

We report on the photorefractive properties of nematic liquid crystals sandwiched between windows of cerium doped strontium barium niobate (Ce:SBN). Unlike simple glass substrates, the Ce:SBN induces molecular pre-tilts that generate electric dipole moments in the liquid crystal through the flexoelectric effect. This enables the liquid crystal to become sensitive to the sign of the space-charge field and leads to an increase in the optical amplification when the cell thickness is decreased.

Robustness of the periodic and chaotic orientational behavior of tumbling nematic liquid crystals

The dynamical behavior of molecular alignment strongly affects physical properties of nematic liquid crystals. A theoretical description can be made by a nonlinear relaxation equation of the order parameter and leads to the prediction that rather complex even chaotic orientational behavior occur. Here the influence of fluctuating shear rates on the orientational dynamics especially on chaotic solutions is discussed. With the help of phase portraits and time evolution diagrams, we investigated the influence of different fluctuation strengths on the flow aligned, isotropic, and periodic solutions. To explore the effect of fluctuations on the chaotic behavior, we calculated the largest Lyapunov exponent for different fluctuation strengths. We found in all cases that small fluctuations of the shear rate do not affect the basic features of the dynamics of tumbling nematics. Furthermore, we present an amended potential modeling the isotropic to nematic transition and discuss the equivalence and difference to the commonly used Landau-de Gennes potential. In contrast to the Landau-de Gennes potential, our potential has the advantage to restrict the order parameter to physically admissible values. In the case of extensional flow, we show that the amended potential leads for increasing extensional rate to a better agreement with experimental results.

Molecular theory of nematic liquid crystals viewed as effect of collective excitation in ferromagnetic systems

We develop a microscopic theory of the nematic phase with consideration of the effect of the collective excitation on properties of nematic liquid crystals. The model is based on the Heisenberg's exchange model of the ferromagnetic materials. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystals. Using the lattice model and the Holstein–Primakoff transformation, the Hamiltonian of the system can be obtained, which has the same form as that of the ferromagnetic substance. The relation between the order parameter and reduced temperature can be gotten. It is in good agreement with the experimental results in the low temperature region, the accordance is better than that of the molecular field theory and the computer simulation. In high temperature region close to the transition point, by considering the effect of the higher-order terms in the Hamiltonian, theoretical prediction is in better agreement with the experiment. That indicates the many-body effect is important to nematic liquid crystals.

Liquid Crystal Alignment on Chemical Nanopatterns: Control over Azimuthal and Polar Alignment

ABSTRACT We discuss alignment properties of nematic liquid crystals on surfaces that contain homeotropic and planar alignment areas on the same surface substrate. The basic idea is to locally pattern homeotropic agents (silane) on surfaces that usually lead to planar alignment (SiO2). The patterns used are linear gratings with periods smaller than 1 µm, which were produced via nanoimprint lithography on glass substrates. As the main parameters the period of the gratings and the coverage ratio between silane and SiO2 areas were varied. It is shown that simultaneous control of both polar and azimuthal orientation of liquid crystal is possible using the chemical nanopatterns as alignment layer. While the liquid crystal azimuthal angle orients along the direction of the silane lines, the polar orientation depends on the coverage ratio of the homeotropic/planar surface potential areas. Simulations for different configurations are used to explain experimental findings.

Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts

In this work we investigate electrooptic properties of nematic liquid crystal (LC) E7 from Merck filled with two types of silica nanoparticles (NPs). The particles of these types are of similar size and shape, however, they are made by different methods. The NPs of the first type, aerosil (A), are prepared by a high-temperature hydrolysis of chlorosilanes. The second type of NPs, based on hydrolysis and controlled precipitation of tetraethylorthosilicate in alcohol, yields monodispersed NP (MNPs) of silica. Transmittance versus applied voltage curves of the suspension layers are measured and analyzed. Our results show a strong dependence of the electrooptic properties on the particle preparation method. The contrast ratio (CR) 61:1 of LC-A suspension is achieved at relatively low A content (17 wt %). Unlike it, the value of the CR of LC-MNP suspensions is low (about 2:1) even at 30 wt % of MNPs in the mixture. Both types of suspensions show electrooptic memory. The maximum of the memory efficiency of LC-A suspensions is detected at 4 wt % of A, while for LC-MNP suspensions the maximum is achieved at 15 wt % of MNPs. The difference in the results for the LC-A and LC-MNP series of composites is caused by much stronger aggregation of MNPs compared with A in the LC matrix. Moreover, reactive particle fusion and formation of silica microcrystallites substantially enhances the effect caused by MNP aggregation. These processes deteriorating electrooptic performance may be stabilized by introduction of a polymer network in LC-MNP suspensions.

Alignment of liquid crystal on a polyimide surface exposed to an Ar ion beam

In this paper properties of nematic liquid crystals aligned on polyimide surfaces exposed to a low-energy Ar ion beam are investigated. We studied how the pretilt angle of a liquid crystal cell is affected by ion-beam conditions, such as the energy of the incident ions, the angle of incidence, and the exposure time. X-ray photoelectron spectroscopy data indicate that ion-beam exposure changes the chemical bonding states of the polyimide surface, which may be one of the main causes of anisotropic liquid crystal alignment. We also found that twisted-nematic cells fabricated by the ion-beam alignment method have the voltage holding time enough for application to thin-film-transistor liquid crystal displays.

Dielectric permittivity measurements of liquid crystal in the microwave and millimeter wave ranges

The fundamental dielectric properties of nematic liquid crystals were measured in the 3–33 GHz frequency range for the purpose of applying liquid crystals to adaptive devices in the microwave and millimeter-wave ranges. We proposed two kinds of measurement methods. Firstly, we used the cut back method to determine the NLCs' permittivity and loss tangent, which is the first time it has been used for this purpose. Secondly, the dielectric permittivity of nematic liquid crystals used for the microstrip-line devices were determined in the 3–33 GHz frequency range by using a newly developed inductive coupled ring resonator. In addition, the effective alignment coefficient ηa(ηa < 1) is proposed for the determination of the effective $\\varepsilon_{\\,\\parallel}^\\prime$ (the long axis of the liquid crystal molecules is parallel to the direction of the r.f. electric field Erf) because the value of $\\varepsilon_{\\,\\parallel}^\\prime$ must be modified to $\\eta_{\\rm a}\\varepsilon_{\\,\\parallel}^\\prime$ in the microstrip-line structure. This concept is useful for the design of the microstrip-line-type variable delay lines.

Measurements of the dielectric properties of nematic liquid crystals at 10 kHz to 40 GHz and application to a variable delay line

AbstractIn support of the application of liquid crystals to adaptive devices such as variable delay lines in the microwave and millimeter band, the complex permittivities and response times of five different nematic liquid crystals after voltage application were measured. We measured both the real part $\varepsilon'_{//}$ of the relative permittivity, when the long axis of the liquid crystal molecule is parallel to the high‐frequency electric field, and the relative permittivity $\varepsilon'_{\bot}$ when it is perpendicular to the high‐frequency electric field. The values $\tan\delta_{//}$ and $\tan\delta_{\bot}$ were measured. In the range from 10 kHz to 13 MHz, the capacitance and conductance of a liquid crystal cell sandwiched between ITO glass plates processed by rubbing were directly measured. For 100 MHz to 40 GHz, we propose the new cutback method, in which two coaxial tubes with different lengths are filled with the liquid crystal and the dielectric properties are measured on the basis of their respective group delay and transmission loss. Dielectric relaxation is verified in the low frequency range. In particular, in the range from 3 to 40 GHz, dielectric properties are shown for a continuous change of frequency. It is found that $\varepsilon'_{//}$ and $\varepsilon'_{\bot}$ are kept almost constant regardless of the frequency, taking values of approximately 3 and 2.5, respectively. $\Delta\varepsilon'$, which is the difference between the permittivities in two orientations, exhibits a large variation, from 0.4 to 0.8, depending on the material, which provides crucial data for selecting the optimal material. The response time was measured by placing the liquid crystal cell between two polarizers with the polarizing directions set parallel. The light from a He‐Ne laser (632.8 nm) is directed onto the liquid crystal cell, and the transmitted light is detected by a photodiode. Assuming that the liquid crystal is used in an adaptive device, the speed of operation is investigated. Based on the measurement results for two different nematic liquid crystals, the design of a variable delay line for 20, 30, and 40 GHz is considered, and a principle of liquid crystal material selection is presented. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(8): 49–60, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10130

P‐105: Out‐of‐Plane Alignment Properties of Nematic Liquid Crystals on Polyimide Surface Bombarded by Ar Ion Beam

AbstractPolyimide surfaces bombarded by a low energy argon ion beam align liquid crystals parallel to ion beam exposure direction. By ion beam conditions, such as the energy of the incident ions, the angle of incidence and exposure time, the unidirectional out‐of‐plane liquid crystal (LC) tilt angle is changed. These are related with the remaining π‐bond of polyimide which is not destroyed by ion beam bombardment. For application in LCDs, unit cells fabricated by ion beam alignment method are compared with one which manufactured by the rubbing method. And The TN cell fabricated by ion beam alignment method has enough retention time to apply to TFT(Thin Film Transitor) LCDs

Anchoring behavior, orientational order, and reorientation dynamics of nematic liquid crystal droplets dispersed in cross-linked polymer networks

The orientational ordering and the electro-optical properties of nematic liquid crystal (LC) droplets confined to cross-linked polymer networks are investigated as a function of the anchoring conditions at the polymer-liquid crystal interface. Normal alignment (homeotropic) or parallel alignment (planar) inside LC droplets was controlled by using acrylate polymers with appropriate side chains. Drastic changes in the reorientation dynamics of the confined nematic liquid crystal phase are observed, as well as in the orientational ordering of the phase-separated LC which was investigated by 13C-NMR (nuclear magnetic resonance) spectroscopy. The cross-link density of the polymer network also affects the orientational ordering and the electro-optical properties of the confined LC phase. Faster switching times and higher-order parameters were found for samples with LC droplets exhibiting planar anchoring.

QUANTUM CHEMICAL CALCULATIONS IN INDUSTRIAL LIQUID CRYSTAL RESEARCH

In advance to syntheses in the laboratory, it is very useful, to have quick and easy procedures available, allowing the calculation of physical properties of liquid crystal materials without any experimental basis. Using different quantum chemical methods, it is possible to calculate molecular data like electrical dipole moment and polarizability including the anisotropy of these data. By comparison of the results of ab-initio and semiempirical methods it can be shown, that the latter are timesaving and reliable enough for prediction of data for applications. In order to calculate dielectric properties of nematic liquid crystals, the necessary data of molar volumes and order parameters can be obtained by empirical methods without measurements. One major problem is the calculation of effective electrical dipole moments, taking in consideration the association in compounds with large dipole moments. The results of different approaches for this purpose are compared and discussed. Because approaches found in the literature fail in the case of large dipole moments, a new procedure taking in account attractive and repulsive energies of the dipole-dipole interaction has been developed. In order to improve the agreement of calculated optical data with experimental data, the dispersion of the polarizability is taken into account.

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.

Photo-induced fabrication of surface relief gratings in alternate self-assembled films containing azo dye and alignments of LC molecules

Photo-induced surface relief gratings of alternate layer-by-layer self-assembled films containing azobenzene dyes were fabricated. The alignment properties of nematic liquid crystal (LC) molecules on these films were investigated in a hybrid LC cell configuration. Poly(diallyldimethylammonium chloride) (PDADMAC) and Direct Red 80 (DR80, azobenzene dye) were deposited using the alternate layer-by-layer self-assembly method. The gratings of the PDADMAC/DR80 films were fabricated by exposure to interference patterns of Ar+ laser light at 488 nm. The formation of the gratings on the films was observed using atomic force microscopy. Furthermore, the alignment properties of LC molecules, 5CB, on the films were investigated by monitoring the change in birefringence of the LC cell in situ. This in-plane alignment was observed after ∼30 min exposure with the laser, simultaneous with the formation of the interference patterns.

Elastic constants from direct correlation functions in nematic liquid crystals: A computer simulation study

Density functional theories such as the Poniewierski–Stecki theory relate the elastic properties of nematic liquid crystals with their local liquid structure, i.e., with the direct correlation function (DCF) of the particles. We propose a way to determine the DCF in the nematic state from simulations without any approximations, taking into account the dependence of pair correlations on the orientation of the director explicitly. Using this scheme, we evaluate the Frank elastic constants K11, K22, and K33 in a system of soft ellipsoids. The values are in good agreement with those obtained directly from an analysis of order fluctuations. Our method thus establishes a reliable way to calculate elastic constants from pair distributions in computer simulations.

Electro-Optic Properties of Nematic Liquid Crystal Aligned by Photo-Oriented Films

In this work we investigated the electro-optic properties of nematic liquid crystals (NLC) aligned by photo-oriented azo-dye films. We found that the response times depend on the terms of orienting film preparation. We have measured the electro-optic characteristics of LC cells with twist orientation, obtained by photo-processed orienting films. We have found that the photo-oriented films prepared with the azo-dyes investigated are thermostable.

Dynamics of Librational Mode of Nematic Liquid Crystal Confined in Cylindrical Pores

Dielectric spectroscopy was applied to investigate the dynamic properties of nematic liquid crystal - 5CB confined in 2000 cylindrical pores of Anopore membranes with the molecules oriented perpendicular to the pore wall with lecithin treated surface. This allows the investigation of the librational (tumbling) mode since the probing electric field is parallel to the pore axis. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times in nematic phase needs the involvement of the temperature dependence of orientational order parameter. In the supercooled state, the temperature dependence of the librational mode did not follow the behavior observed in the nematic phase and shows a sharp increase while the temperature is decreased that is mainly determined by the variations of viscosity.