Non-polar Nematic Liquid Crystals Research Articles

Dielectric relaxation in non-polar nematic liquid crystal devices

We investigate the dielectric relaxation in non-polar nematic planar aligned liquid crystal samples by means of both dielectric spectroscopy and a transient current method. The dielectric functions, obtained by the impedance data, are modeled by the modified Havriliak-Negami method, which allows to determine spectra in terms of permittivity, conductivity and dielectric relaxation. Further information on the involved electric charge and the ion mobility is achieved by means of current measurements. The outcomes demonstrate that the effective relaxation times originate from ionic polarization and depend on the interfaces properties, pointing out the influence of the layered structure.

Low symmetry tetrahedral nematic liquid crystal phases: Ambidextrous chirality and ambidextrous helicity

We discuss the symmetry properties as well as the dynamic behavior of various non-polar nematic liquid crystal phases with tetrahedral order. We concentrate on systems that show biaxial nematic order coexisting with octupolar (tetrahedral) order. Non-polar examples are phases with D2 and S4 symmetries, which can be characterized as biaxial nematics lacking inversion symmetry. It is this combination that allows for new features in the statics and dynamics of these phases. The D2-symmetric phase is chiral, even for achiral molecules, and shows ambidextrous chirality in all three preferred directions. The achiral S4-symmetric phase allows for ambidextrous helicity, similar to the higher-symmetric D2d-symmetric phase. Such phases are candidates for nematic phases made from banana-shaped molecules.

Difference in Steady Shear Flow Viscosity between Polar and Nonpolar Nematic Liquid Crystals in Aromatic Polyesters Derived from VECTRA

Steady shear flow viscosities was measured for nematic liquid crystals (LCs) in the aromatic copolyesters composed of 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), telephthalic acid, and biphenol whose molar contents are 0.73 − x, 0.27 − x, x, and x, respectively. Second harmonic generation (SHG) measurements showed that a SHG-active polar nematic LC formed by poly(HBA/HNA) with x = 0 is altered to a conventional nonpolar nematic LC with an increase in x more than 0.07. The shear-rate dependence of melt viscosity measured for nematic LCs of all the copolymers showed four characteristic regions; a Newtonian plateau in lowest shear-rate region 0 in addition to the well-known three regions, low-shear-rate shear-thinning region I, intermediate Newtonian plateau region II and high-shear-rate shear-thinning region III. The SHG-activity in nematic LCs reflected on the Newtonian plateau region 0. The viscosity of the SHG-active nematic LCs in this region 0 was 10 times higher than that of the non-SHG-active nematic ones. The higher viscosity of the SHG-active nematic LC was connected to the smaller domain size (or larger number of disclinations) estimated by small-angle light scattering. In the nonpolar nematic LC, many disclinations initially formed were easily annihilated because of a coalescence of two disclinations with opposite signs, whereas in the polar nematic LC, such an annihilation hardly occurred because of the polar packing symmetry.

Electro-optic behaviour of a non-polar nematic liquid crystal and its mixtures

In this work we report preliminary results on the properties of a non-polar bicyclohexane nematic liquid crystal. Moreover, its binary mixtures both with a low viscosity phenylcyclohexane and with a normal polar nematic liquid have been investigated. The elastic, viscous and electro-optical properties of these compounds are presented. The non-polar compound, and its mixture with a low percentage of the well known liquid crystal MBBA, exhibit an electrohydrodynamic behaviour in which the conducting regime is absent, while the dielectric regime spreads to low frequencies. Additionally, at higher frequencies of the applied electric field, a regime whose thresholds are linear in frequency is observed. On the other hand, the mixture formed by 50 wt % of the non-polar compound with MBBA exhibits at low frequencies the usual behaviour, followed at higher frequencies by the linear regime.

A continuum approach to electrorheology

An equilibrium thermodynamic approach is employed to derive a continuum-level expression for the electric field-induced stress in uniaxial anisotropic materials. Although this model is developed specifically to describe electrorheological and electrostrictive behavior of suspensions, it also applies to other uniaxial materials such as nonpolar nematic liquid crystals, biaxially oriented polymer films, and paper. This model introduces new electrostriction coefficients, which are material parameters that describe the strain dependence of the dielectric tensor as well as the field-induced stresses. An experimental technique for measuring the electrostriction parameters is outlined. An idealized microscopic model is presented to illustrate the relationships between microscopic parameters and the macroscopic electrostriction coefficients. The model is used to determine the stresses in common applications; predictions from the continuum approach agree with direct calculations from a microscopic approach of the normal stress and static shear modulus of electrorheological suspensions.

Anchoring transitions with polar and non-polar nematic liquid crystals on incompletely oxidized silane substrates

The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles, with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed for this symmetry breaking term.

Dielectric Study of Dipole−Dipole Interactions in Anisotropic Solutions

This paper describes an experimental investigation of dipole−dipole interactions between molecules in an orientationally ordered environment. Measurements are reported of the dielectric properties of a series of anisotropic solutions of two structurally similar dipolar solutes (1-cyano-2-fluoro-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene (CP1) and 1,2-difluoro-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene (CP3)) dissolved in a nonpolar nematic liquid crystal solvent (1-[trans-4-ethylcyclohexyl]-2-[4-ethyl-2-fluorobiphenyl]ethane (I22)). The solvent provides an orienting medium for the solute molecules, and by varying the temperature the degree of orientational order of the solutions is changed. The measurements have been made on aligned thin films of the liquid crystalline solutions for different concentrations and over a range of frequencies from 1 kHz to 10 MHz. All solutions exhibited a low-frequency relaxation associated with end-over-end reorientation of the polar solutes in the...

Fluorescence Studies on Intermolecular Interactions of Cyanobiphenyl Groups Chemisorbed on a Quartz Surface with Liquid Crystals

Fluorescence measurements were made of cyanobiphenyl (CB) residues covalently bound to a quartz plate through silylation to obtain information about molecular interactions between the fluorophore and solvent molecules as well as liquid crystal molecules at an interface. It was found that the fluorescence of surface-attached CB groups is quite different from that of corresponding solutions. A marked red shift was observed at the interface of a quartz plate and a nonpolar solvent like cyclohexane, reflecting a polar microenvironment surrounding the fluorophore residues owing to surface silanols and/or adsorbed water molecules, and a new fluorescent band possibly ascribable to an excimer of CB was also observed on the silica surface in cyclohexane. In contrast to these, the fluorescence from CB units bound on a quartz plate wetted with a nonpolar nematic liquid crystal showed that its red shift was much reduced and the emission band in the longer wavelength region was not observed. On the contrary, no alteration of the fluorescence of a pyrene photoprobe covalently attached to a silica surface was induced by contact with a nematic liquid crystal. These observations are reasonably interpreted as a result of specific molecular interactions between surface CB units and liquid crystal molecules on the interface.

Dipole-Dipole Association of Polar Molecules in a Non-Polar Liquid Crystal Solvent

Abstract Measurements are reported of the permittivity as a function of temperature and composition for strongly dipolar cyano-substituted molecules dissolved in a non-polar nematic liquid crystal solvent. Solute molecules with dipole moments perpendicular to the molecular alignment axes exhibited strong parallel association of their dipoles, even at low concentrations, and the measurements can be interpreted in terms of a ‘strong dimer’ model. Results for a solute molecule with a large dipole moment co-linear with the long molecular axis gave the major dipole correlation factor equal to 1, consistent with little or no preferred dipole-dipole correlation.

The Dependence of Volumetric Properties on the Molecular Composition of Nematic Liquid Crystals

Abstract Density studies on various terminally polar and non-polar nematic liquid crystals, measured as a function of temperature, are reported. The molar volumes were fitted to model equations by a non-linear numerical least squares fit. These equations contain non-linear terms of the form c I (T - T I c)βI for T > T I c and - C N (.T N c - T)βN for T < T N c. It is shown that the coefficients C N and cI are correlated with the extrapolated volume discontinuity ΔV c,99 at the nematic-isotropic phase transition temperature T c. Furthermore, for all liquid crystals investigated there exists a correlation between ΔV c.99 and the order parameter S zz (t r = 0.99) as determined by infrared dichroism. Our density measurements suggest that C N and C I depend on the molecular structure of the liquid crystal and are not universal for the nematic-isotropic phase transition. Assuming universal exponents βN and β1, we evaluate the average values βN = 0.388 + − 0.096 0.085 and βI = 0.135+ − 0.065 0.045.

Physical properties of nematic mixtures. I. Polar–polar and nonpolar–nonpolar systems

The thermodynamic stability, the dielectric, diamagnetic, and elastic properties of mixtures of polar as well as those of nonpolar nematic liquid crystals were investigated and compared to the properties of their components. Small deviations from the ideal behavior were observed and attributed to differences in the packing of unlike molecules which mainly depend on steric factors. Clearing points obtained by the extrapolation method are shown to appreciably deviate from the actual values.

A statistical theory of orientational ordering at the free surface of a liquid of ellipsoidal molecules

The surface tension γ of a system of ellipsoidal molecules is evaluated approximately using a generalized Fowler-Kirkwood-Buff model. The pair potential is modelled as u(r/σ) where r is the centre of mass distance and σ is an angle dependent range parameter determined by the shape anisotropy of the molecule. It is shown that if the pair correlation function g scales as g(r/σ) γ can be mapped onto the value for a system of spheres, multiplied by an angular integral which takes into account approximately both the anisotropic molecular shape as well as possible orientational order. It is shown that γ is lowered when the orientational order parameter Q ≠ 0, implying that isotropic molecular liquids may be partially ordered near the free surface. The surface favours parallel ordering of rod-like particles in a direction in the plane of the surface. For plate-like particles, ordering with the plates in the plane of the surface is favoured. In both cases the anchoring energies increase sharply with the shape anisotropy of the molecule. These results are qualitatively consistent with the surface properties of non-polar nematic liquid crystals.

Induced smectic phases

Mixtures of terminal polar with terminal non-polar nematic liquid crystals usually produce induced smectic phases (ISP). This is shown by the investigation of a great number of systems using the contact method.In general, SA phases are induced. By means of X-ray investigations SB and SE phases were also established. No solid molecular complex was found. The layer spacing for the induced smectic phases becomes minimal for mixtures about 1 : 1 at which the maximum thermal stability of the induced smectic phases was found as well. The effects observed indicate a close relation between the induction of smectic phases and the formation of the double layer structure in the smectic phases of terminal polar liquid crystals.