Physical studies of nematic azoxybenzenes. II. Refractive indices and the internal field

Molecular orientational distribution function of a chiral de Vries smectic liquid crystal from birefringence measurements

An alternative method for determining the orientational distribution function and the order parameter from the electric field-induced birefringence measurements of a chiral liquid crystal compound in its Smectic A^* is being introduced. A chiral mesogen based on a 5-phenyl-pyrimidine benzoate core terminated by a trisiloxane group on one side and the chiral alkyloxy chain on its opposite side is designed and synthesized to exhibit the "de Vries" smectic characteristics. The compound exhibits first order Smectic A^*-Smectic C^* phase transition, evidenced by the results of differential scanning calorimetry. The material is being investigated by electro-optical experiment in its smectic phases. We present a model that incorporates the generalised Langevin-Debye model which includes the Maier-Saupe effective mean-field potential term in order to explain the change in birefringence with the electric field. A good agreement between the experimental results and the predictions from the model leads to the determination of the molecular orientational distribution function in Smectic A phase. Furthermore, the temperature dependency of the Saupe orientational order parameter ⟨P₂⟩ is obtained using the parameters of the model. Based on the experimental and theoretical results, we show that de Vries Smectic A^* phase exhibits a broad volcano-like tilt angle distribution with the two maxima occurring at finite tilt angles closer to the Smectic A^*-Smectic C^* transition temperature, and a sugarloaf-like distribution occurs in the tilt for temperatures close to the Isotropic-Smectic A^* phase transition.

Effect of nanoparticle chain formation on dielectric anisotropy of nematic composites

A general theory of the dielectric constant of nematic liquid crystal mixtures is presented including the particular case of nematics doped with polar nanoparticles. The results are used to estimate the contribution of chains of polar nanoparticles to the static dielectric anisotropy and birefringence of the nematic composite taking into account contributions from chains of different lengths. The dependence of the dielectric anisotropy on the dipolar interaction strength is considered in detail and it is shown that formation of polar chains of nanoparticles enables one to explain a significant increase of the dielectric constant of the composite as observed experimentally.

Universality of the nematic mesophase

This paper presents a detailed comparative study of published order-parameter experimental data of nematic liquid crystals. Throughout a simple linear transformation, it is shown that three distinct nematic mesophases exhibit the same and unique universal behavior for all temperatures below the nematic-isotropic phase transition. This paper generalizes a previous one [M. Simões, D. S. Simeão, S. M. Domiciano, and A. de Campos, Phys. Lett. A 372, 5346 (2008)], in which only the nematic mesophase delimited by the crystalline phase at low temperature was studied. Here we have assembled order-parameter experimental data of 61 different compounds, forming a set with data of 96 experiments, aiming to show that the nematic universal behavior is independent of what is the actual phase or mesophase bordering the nematic phase at low temperatures; our results show that all experimental data of all nematic mesophases coalesce along a common line that extends over the entire range of mesophases.

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.

Corresponding states of order parameter in nematic liquid crystals

In this work, experimental data points from which the profile of the order parameter of nematic liquid crystals can be obtained, mainly magnetic susceptibility anisotropy and dielectric anisotropy, will be used to show that when rescaled they coalesce along a common line that extends from the nematic-isotropic phase transition region, until the nematic-crystalline phase transition region. A large set of experimental data of a diversity of nematic compounds that share the presence of a nematic-isotropic and a nematic-crystalline phase transition, without another phase within, has been collected from the liquid crystals literature. Taking for each of them the temperatures of these two phase transitions as fixed points, a common temperature scale has been constructed and, in this scale, the value of each physical quantity has been uniformly fixed at the nematic-crystalline phase transition temperature. This procedure has revealed the existence of a law of corresponding states that covers the entire nematic phase. As the values assumed by these physical quantities can be used to measure the macroscopic order parameter, the use of this temperature scale suggests that it presents a common behavior that covers the entire nematic phase.

Dispersion of the ordinary refractive indices of smectic liquid crystals in free standing films

The ordinary refractive indices of some smectic-A liquid crystals have been measured by means of interference spectra of free standing films as a function of wavelength and temperature. Numerical data are presented for further use. In the Sm-C phase one of the principal indices can be measured. If the three principal indices in the Sm-C phase would be known the tilt angle could be measured by means of the interference technique with high precision. Predictions of the refractive index of liquid crystals based on quantum chemical calculations of polarizabilities are presented. Using an expression derived by de Jeu and Bordewijk [J. Chem. Phys. 68, 109 (1978)] values for the principal refractive indices have been calculated that can be compared with our experimental results.

Calculation of the birefringences of nematic liquid crystals at optical and infrared wavelengths

In this paper we critically examine computational methods for predicting the birefringences of nematic liquid crystals, with a view to screening molecules for potential use in infrared applications. Using the liquid-crystal 5CB as a test molecule, we calculate molecular electronic polarizabilities using ab initio quantum-mechanical techniques and a wide range of basis sets. We show that the polarizabilities tend to a limiting value as the quality of the basis set is improved. However, the biggest hurdle remains the determination of the refractive index from the polarizability data. We examine several methods for performing this conversion and conclude that the simplest equation, due to Vuks, is adequate for predicting the birefringence, given the uncertainties involved in other parameters. The agreement between calculation and experiment is best described as "semiquantitative." We perform similar calculations for a wide range of nematic liquid crystals at both 589 and 1550 nm, taking into account the likely impact of molecular vibrations at the longer wavelength. We demonstrate that there is a simple scale factor, for conventional nematics, between the birefringence at visible wavelengths and in the infrared. Thus knowledge of the birefringence at optical wavelengths, as widely available in the literature, is a good guide to the usefulness of conventional nematic liquid crystals as active elements for optical switching in the telecommunications industry.