Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory

Phase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ∂Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.     

Landau-de Gennes theory of the core structure of a screw dislocation in smectic A liquid crystals

We present details of calculations of the core structure of a screw dislocation in a smectic A liquid crystal, using the phenomenological Landau-de Gennes free energy functional. The order parameter frustration created by topological constraints far from the dislocation core is resolved in one of three qualitatively different ways. The three types of dislocation core solution are the DT (double twist), CL (classical), and BP (broken polar symmetry) solutions, respectively. The stability requirements for these structures are discussed, as a function of temperature, smectic elastic properties, and coupling between smectic and nematic order. The effect of possible inhomogeneity between left- and right-handed conformers is also examined.     

An extension of the Landau-Ginzburg-de Gennes theory for liquid crystals

Using angular momentum representation a method is proposed that allows the systematic construction of a generalized Landau-de Gennes elastic free energy of liquid crystals, in powers of a symmetric and traceless tensor order parameter, polarization field, of external fields and all respective derivatives. By this method all linearly independent elastic invariants and surface terms are constructed for nematics and cholesterics up to fourth order terms. In particular it is shown that up to fourth order in the tensor order parameter there are nineteen bulk elastic constants and four surface terms in the free energy of a general, biaxial nematic. In addition, the stability of this expansion is studied in detail. Some special cases of the elastic free energy of liquid crystals, already discussed in the literature, are reexamined and discrepancies with our results are emphasized. Finally, a thermo-dynamically correct way of establishing contact between the generalized de Gennes elastic free energy and other theories, like those of Oseen-Frank or Meyer, is proposed by applying fluctuation theory. Thus, the degeneracy of splay and bend elastic constants is removed even when these are calculated from the standard de Gennes free energy. Restrictions on higher order elastic constants are also obtained by comparing mean field relations and stability conditions with available experimental data.     

Some novel cholesteric liquid crystals

Abstract

We investigate the possibility of inferring the absence of an ordered phase using Monte Carlo simulations. The example we have chosen is that of a one dimensional Lebwohl-Lasher model, where an analytic solution is available. We argue that Monte Carlo can be of help even in this delicate sector notwithstanding the complications created by periodic boundary conditions.     

Some novel cholesteric liquid crystals

A novel series of cholesteric liquid crystals (CNTⁿ). made by quaternization of the new mesogenic unit cholesteryl isonicotinate (CN) with n-alkyl salts, C_nH_2n + ₁ (T) (n = 2 to 8, T = 4-MeC₆H₄SO₃) has been prepared. Differential scanning calorime-try and polarizing optical microscopy studies reveal that all of these salts exhibit an enantiotropic cholesteric phase but for the cholesteryl isonicotinate unit the cholesteric phase is monotropic. For all members of the series the cholesteric-isotropic transitions are not reversible as these salts begin to decompose before their cholesteric-isotropic transition point is reached.     

Arch-texture in cholesteric liquid crystals

We have observed an arch-texture in cholesteric liquid crystals sandwiched between two glass plates making a small wedge angle. The anchoring is homeotropic on one plate and planar on the opposite one. This texture is locally periodic and composed of parallel stripes whose average direction rotates by 180° each time the sample thickness increases by p/2, where p is the equilibrium pitch of the cholesteric phase. This texture is due to a periodic modulation of the elastic boundary layer which forms near the plate treated for homeotropic anchoring.     

Phase diagrams of cholesteric films in electric fields

Planar films of cholesteric liquid crystals exhibit several types of instabilities in electric fields. A periodic equilibrium structure can appear when the field is parallel to the helix axis. Conditions for the occurrence of the modulated phase have been derived by an analytical theory, which describes long wavelength distortions in the vicinity of Lifshitz points.     

Synthesis of diaza-crown-ether cholesteric liquid crystals

Five diaza-crown-ethers were synthesized and four of them shown to have smectic liquid crystalline properties. Their properties were determined by DSC and polarized microscopy. This type of smectogenic diazacrown ether has not been reported so far. A novel lyotropic crown ether liquid crystal was obtained from the thermotropic crown ether liquid crystal 7.     

Chromaticity of polymer-dispersed cholesteric liquid crystals

The colour producing properties of the polymer-dispersed cholesteric liquid crystal colour display have been measured. We show how the central wavelength of the reflection spectrum depends on the sample voltage, chiral concentration, temperature, and viewing angle. From the reflection spectra we calculate the chromaticity coordinates for two situations and display them on chromaticity diagrams. The results indicate that these new systems may be suitable for the use in multicolour displays.     

Phase diagrams and morphology of polymer-dispersed liquid crystals: An analysis

Thermal and morphological studies of polymer-dispersed liquid crystals (PDLCs) for various compositions of liquid crystalline material 4-undecyloxybenzoic acid (UDBA) in two polymer matrices, polyvinylidenefluoride-co-hexafluoropropylene P(VdF–HFP) or polyethylene oxide (PEO), have been carried out using differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Phase diagrams for different series of PDLCs have been analysed using the Flory–Huggins theory of isotropic mixing and the Maier–Saupe–McMillan theory, to include the anisotropic contributions. Mesogenic transitions of UDBA are observed to be greatly influenced when dispersed in these polymers. The morphologies and miscibility studies of these PDLCs suggest that UDBA is highly miscible in PEO, only partially miscible in poly(methyl methacrylate) (PMMA), but almost immiscible in P(VdF–HFP).     

具有宽波反射特性的胆甾相液晶固体薄膜材料的研究

采用两步聚合法得到了具有宽波反射特性的固体胆甾相液晶薄膜,首先通过聚合物稳定胆甾相液晶制备了螺旋结构的高分子网络,然后灌入具有不同螺距的可聚合的胆甾相小分子液晶单体混合物,经紫外光照射聚合后获得了能够反射可见光区(450～780 nm)覆盖红蓝绿三色光的胆甾相液晶固体薄膜.扫描电镜测试结果表明,胆甾相液晶固体薄膜的断面...     

Spectral investigation of absorbing cholesteric liquid crystals

The investigation is made of the electron absorption spectra of pure and impure crystals of cholesteryl benzoate at 4, 2K and their Bragg reflection spectra as a function of impurity concentration (0,1 to 10%) and mesomorphic range temperature. The discrete structure of the electron absorption spectra is revealed and explained. The regularities of the mesophase order degree variation with temperature are repoted. The anomalous transmission of light by the cholesteryl benzoate mesophase (the Borrman effect analog) is measured as a function of temperature and impurity concentration.     

Surface anchoring energy of cholesteric liquid crystals

ABSTRACT

In this paper, we propose a suitable surface energy expression for cholesteric liquid crystals. We show that there exists a symmetry allowed term for chiral nematics that doesn’t appear in the traditional Rapini-Papoular surface energy form. We discuss some consequences of this new surface anchoring term.     

Optical properties of frustrated cholesteric liquid crystals

In a previous article, we proposed a model to explain the unwinding transition in an electric field of a frustrated cholesteric liquid crystal sandwiched between two glass plates imposing a homeotropic anchoring. We found that three distinct solutions exist in materials of negative dielectric anisotropy: first, the homeotropic nematic at small thickness and small voltage, second, a translationally invariant configuration (TIC) at large voltage and, third, the cholesteric fingers. In this article, we study some optical properties of these solutions. We show first that the TIC rotates the polarization of light. Its 'apparent' rotatory power is calculated exactly and is compared with the experimental data when the TIC-nematic phase transition is second order. The agreement between theory and experiment is excellent. We show in particular that there exist discrete values of the voltage for which the TIC has a pure rotatory power. We then calculated the optical contrast of the fingers when they are observed between crossed polarizers. The agreement with experiment is still satisfactory, in spite of the approximate form of the director field chosen to describe the topology of the finger.     

Flat optics with cholesteric and blue phase liquid crystals

ABSTRACT

The cholesteric (Ch) phase is an ancient liquid crystal (LC) phase, with its roots dating back to the days of Friedrich Reinitzer. It is most well known for its ability to selectively reflect circularly polarised light with the same rotation handedness as the helical structure, and have found applications in polarising and display devices. Most studies to date utilising the reflective properties of ChLCs, however, have treated ChLCs as simple dielectric mirrors in which light follows the law of reflection. We have recently shown that through controlling the phase of the Ch helix, it is possible to control the phase of reflected light. Because the phase can be controlled over 0–2π, the reflected wavefront from a planar device can be designed to possess non-specular properties, such as deflection and lensing. In this paper, we aim to provide a comprehensive understanding of the phenomenon by presenting theoretical and experimental results on the dependence of the reflected light phase on the helix phase, and the effect of chiral handedness on the phenomenon. Also, we show that wavefront manipulation based on the same concept can be achieved in Ch blue phases (BPs), which are chiral LCs with three-dimensional periodicity.     

Mechanically activated cholesteric polymer dispersed liquid crystals

In this paper we deal with a cellulose derivative cholesteric dispersed liquid crystal (CCDLC) with mechanically tuneable optical properties. The composite is formed with a matrix of acetoxypropylcellulose with embedded micrometric and submicrometric droplets of a cholesteric mixture. Polarizing optical microscopy and atomic force microscopy (AFM) measurements performed on the system are reported and it is shown that the wavelength of the reflected light can be changed by a temperature variation and it is also changeable through a mechanical deformation. The pitch of the deformed droplets can be measured from AFM photographs and compared with the wavelength reflected by the CCDLC composite material.     

Mechanically activated cholesteric polymer dispersed liquid crystals

In this paper we deal with a cellulose derivative cholesteric dispersed liquid crystal (CCDLC) with mechanically tuneable optical properties. The composite is formed with a matrix of acetoxypropylcellulose with embedded micrometric and submicrometric droplets of a cholesteric mixture. Polarizing optical microscopy and atomic force microscopy (AFM) measurements performed on the system are reported and it is shown that the wavelength of the reflected light can be changed by a temperature variation and it is also changeable through a mechanical deformation. The pitch of the deformed droplets can be measured from AFM photographs and compared with the wavelength reflected by the CCDLC composite material.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Anomalous light scattering in photonic cholesteric liquid crystals

ABSTRACT

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.