Cholesteric Structure Research Articles

Effect of chirality on liquid crystals in capillary tubes with parallel and perpendicular anchoring.

The director field of a cholesteric liquid crystal confined to cylindrical cavities with parallel and perpendicular surface anchoring was studied by means of optical polarizing microscopy. The chirality of the liquid crystal could be varied continuously due to a temperature-induced twist inversion of the cholesteric structure. The results indicate the occurrence of a twisted escaped radial structure for perpendicular anchoring and an eccentric double twist configuration for parallel anchoring. \textcopyright{} 1996 The American Physical Society.

Shape model for ordering properties of molecular dopants inducing chiral mesophases

An extension to chiral phases of a model derived previously to interpret orientational properties in liquid crystal solvents of molecules with arbitrary shape leads, quite naturally, to the definition of a traceless chirality tensor, useful for predicting the structures of twisted nematics induced by chiral solutes. The magnitude and handedness of the helical pitch in the induced chiral phase depend on the interplay of the chirality tensor components and the ordering of the different axes of the chiral probe in a given nematic solvent. The effect of changing the molecular structure of the solute and the temperature is discussed for a number of typical dopants, including biphenyl, binaphthyl and heptalene derivatives. Although the model has been derived in a form suitable for application to induced cholesteric structures, it contains the basic ingredients to account for the physical behaviour of chiral smectic-C* mesophases.

Numerical Resolution of the Cholesteric Unwinding Transition Problem

Some equilibrium configurations of the cholesteric unwinding transition problem are numerically computed using the Landau-de Gennes expression of the liquid crystal free energy density. The classical TIC and modulated TIC configurations are numerically obtained and the cholesteric finger structure is found to be in good agreement with the previous theoretical guesses. Moreover, we report the existence of new solutions, specifically associated with the local n → -n symmetry, and which seem to be experimentally observed.

Amorphous chiral smectic A phase of side chain copolymers

Abstract The chiral structure of the anomalous smectic phase recently observed in chiral copolymers has been studied by optical methods. The phase is amorphous, but shows a short range order, as a combination of smectic A and cholesteric structures. This phase is presumed to possess locally the TGBA structure.

Cholesteric Textures of Aqueous Hydroxypropylcellulose Solutions

Abstract Liquid crystalline textures exhibited by aqueous solutions of hydroxypropylcellulose were investigated by video-enhanced contrast polarized light microscopy. For the first time in this particular system, typical cholesteric structures, such as oily streaks and fingerprint lines, were resolved in close detail by light microscopy. Biphasic separation was observed even in samples of concentration well above the critical value generally accepted for mesophase formation at room temperature (∼40% wt). The dependence of the helical pitch on polymer concentration was also studied, by Fourier transforming images of the cholesteric textures.

Dielectric investigation of isotropic and anisotropic solutions of cellulose acetate in dioxane

AbstractThe dielectric and optical characteristics of a sample of cellulose acetate (DS = 2.45) in dioxane solutions were studied at 10–50°C of concentration 10–50 wt% to include both isotropic and anisotropic phases. The study showed that the loss maximum, εmax″, magnitude of polarization, (ε0 − ε∞), static dielectric constant, ε0, time of relation, (2πfm)−1, and refractive index, nD, steadily increase with concentration up to the critical concentration (41 wt%) and then decrease. The mean‐square dipole moment, 〈gμ2〉, decreases steadily up to the critical concentration then remains nearly constant, indicating that the isotropic solution changes to anisotropic, with smaller mean‐square dipole moment. Comparison between the results of cellulose acetate (CA) and those of hydroxypropyl cellulose (HPC) reveals that, at the critical concentration in dioxane, the cholesteric structure of HPC possesses a greater mean‐square dipole moment with higher temperature coefficient than does CA. The activation energy of the relaxation process for hydroxypropyl cellulose is higher, indicating a greater intrachain interaction compared with cellulose acetate.

Induction of Tilted Cholesteric Structures in Microconfined Liquid Crystals

Abstract 13C-NMR measurements of the chiral mesogen cholesteryl-undecylcarbonate (CHUC) adsorbed in Anopore membranes with cylindrical pores of 0.2μm diameter are presented. The pitch of CHUC in the cholesteric phase is of order of the pore diameter, and in view of the planar axial anchoring a strong influence of the walls on the director field can be expected. Simulation of the angular dependence of 13C-NMR line shape allows to determine a mean tilt angle of 60μ of the director with respect to the helical axes which are oriented along the channels. Furthermore, at T=341K a correlation time of 130μs for reorientational motions could be obtained which is connected with molecular diffusion along the pores. Optical and NMR-measurements are compatible with a tilted helical director field for the cholesteric LC material within the pores.

Preparation and characteristics of cholesteric gel from pentyl ether of hydroxypropyl cellulose

A new cellulose derivative, pentyl ether of hydroxypropyl cellulose (P-HPC), and its crosslinked gel (PG-HPC) were prepared in non-aqueous solution. The temperature dependence of the optical pitch for the thermotropic mesophases formed by P-HPC and PG-HPC was determined spectroscopically. PG-HPC displayed the cholesteric reflection colour at room temperature and its cholesteric order and structure were fixed on heating up to 90°C. The optical pitch of the redried gel of PG-HPC, after swelling in tetrahydrofuran (THF), was shifted to lower wavelength and the reflection band became narrower than that of the original, before swelling in THF.

Optical properties of cholesteric (2‐hydroxypropyl) cellulose (HPC) esters

AbstractAcetoxypropylcellulose (APC) and propionic, n‐butyric, isobutyric, valeric, isovaleric, hexanionic, and heptanionic acid esters of hydroxypropylcellulose (HPC) (respectively PPC, BPC, iBPC, VPC, iVPC, HexPc, and HepPc) were prepared and characterized by differential scanning calorimetry, chromatography, polarizing microscopy, chemical methods, and spectroscopy. All these esters form thermotropic cholesteric liquid crystalline phases. The glass (Tg) and clearing (Tc) transition temperatures were determined. The stability interval of the mesophases appears to be greater in the case of longer‐linear side chains. The mesophases of APC, PPC, iBPC, VPC, and iVPC exhibit reflection bands in the visible region, at wavelengths that depend on temperature, moisture content, size and number of substituents, and degree of polymerization ( \[ \overline {{\rm DP}} \] ). The pitch of the cholesteric helical structure increases with side chain length and with increasing temperature. At room temperature, a fingerprint‐like pattern can be observed for HepPc. No reversal in the sense of the pitch with temperature variations was observed. The pitch of BPC increases with moisture content and with decreasing values of the degree of esterification ( \[ \overline {{\rm DE}} \] ) and \[ \overline {{\rm DP}} \] . A theory for cholesteric mesophases composed of helical rod‐like species and a model of elastic bend chain have been compared to the experimentally observed changes in the pitch with temperature and with the length of the side chain substituents (for iBPC, iVPC, BPC, PPC, and APC) and \[ \overline {{\rm DE}} \] and \[ \overline {{\rm DP}} \] (for BPC). © 1994 John Wiley & Sons, Inc.

Neutron scattering experiments on magnetically aligned liquid crystalline DNA fragment solutions

Abstract Small angle neutron scattering experiments have been performed on liquid crystalline 163 basepair NaDNA fragments in aqueous solution in the concentration range 190–285 mg ml−1. To induce a macroscopic alignment, a magnetic field was applied either parallel (B ∥ configuration) or perpendicular (B ⊊ configuration) to the incoming neutron beam. The isotropic scattering pattern in the B ∥ configuration and the anisotropic scattering in the B ⊊ configuration agree with the cholesteric structure of the liquid crystalline solutions. From the anisotropic scattering in the B ⊊ configuration, information on the orientation ordering can be derived. For this purpose, the experimental data are compared to the form function of a uniform rod including a gaussian orientation distribution. The standard deviation of this distribution is approximately 20° for all concentrations investigated here.

Monte-Carlo Simulation of the Cholesteric Phase in Liquid Crystals

Abstract Starting from a simple pair interaction potential between chiral molecules, a cholesteric configuration of molecules is generated in a 15 × 15 × 15 system. The effect of temperature on the cholesteric structures is also studied. Unlike the results under the mean field approximation with the same pair potential, the present Monte-Carlo simulation shows a temperature dependence of the pitch.

Columnar and smectic ordering in a TGBA phase

Optical textures characteristic of cholesteric and columnar structures were simul- taneously detected for the first time in the same mesophase of a liquid crystal. However, X-ray diffraction measurements show that the mesophase under consideration is smectic A in nature. We show here that the compound presents, indeed, a twist grain boundary (TGB) phase within a large temperature range, and we propose an explanation of the supramolecular organization in this phase.

Cholesteric Helical Structure in Biological System

Cholesteric Helical Structure in Biological System

Piezoelectricity in cholesteric liquid crystalline structures

Using symmetry arguments we discuss the various possible helical structures that can show a longitudinal piezoelectric effect. Due to the recent experimental verification of such an effect in some elastomeric cholesteric liquid crystals, we propose that what is called cholesterics could actually represent different helical structures, which can be more complicated than the conventional (i.e. simple screw) model

Characterization and radical polymerization of (E‐CE)C/AA mesomorphic solutions

AbstractEthyl–cyanoethyl cellulose (E–CE)C/poly (acrylic acid) (PAA) composites were prepared by polymerizing acrylic acid (AA) in (E–CE) C/AA cholesteric liquid crystalline solutions. The morphology and structure and the compatibility of composites prepared in different conditions were studied by small‐angle light scattering (SALS), differential scanning calorimetry (DSC), X‐ray spectrometry, polarizing microscopy, and electron microscopy. Ho‐mogeneous composites with the cholesteric structure could be obtained by photopolymerization. During the photopolymerization, however, the morphology of the liquid crystalline domains became crylindric and the axial direction of the cylindric domain was at an angle of about 45° to the layers of the ordered molecular chains. The polymerization initiated by azobisisobutyronitrile (AIBN) resulted in the phase separation. The morphology of the liquid crystalline domains was maintained although the cholesteric structure of the composite was partly destroyed. © 1993 John Wiley & Sons, Inc.

Experimental study and modelling of a microwave cholesteric structure

A theoretical and experimental study has been carried out on a macroscopic cholesteric structure. The authors establish a physical model which allows us to calculate the reflected and transmitted electromagnetic fields on a stack of uniaxial layers submitted to a normal incidence plane wave. Experimental measurements are in good agreement with the predicted values.

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

Abstract The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.

Evidence for cholesteric morphology in films of cellulose acetate butyrate by transmission electron microscopy

Transmission electron microscopy provides direct evidence of helicoidal cholesteric morphology in films of cellulose acetate butyrate prepared by solvent casting from liquid crystalline solutions in dimethylacetamide. We believe this is the first observation reported of cholesteric structure in a mixed cellulosic ester. Cholesteric morphology was also observed in solvent cast blended films of cellulose acetate butyrate and lignin from liquid crystalline solutions. There are indications that lignin particles may be serving as nucleating surfaces or sites of direct termination for some of the observed cholesteric fingerprint pattern.

Distortion of DNA cholesteric liquid crystal quenched at low temperature : geometrical analysis and models

Freeze-fracture elecuon microscopy allows an ultrastructural analysis of deformations of a DNA cholesteric liquid crystalline sample subjected to a compressive stress applied parallel to the layers, when quickly frozen by projection onto a copper block cooled down to about 10 K. A geometrical model of these deformations is proposed. After a brief recall of the usual representation of the cholesteric structure as a succession of equidistant pseudoplanes, we show that these planes are distorted into sinusoidal surfaces whose wavelength is much smaller than the cholesteric pitch and its amplitude modulated with the average molecular orientation relative to the compressive force. The consequences of these deformations regarding double twist occurrence within the structure are analysed. When the DNA concentration in the cholesteric mesophase is low, a complex helicoidal structure is observed. A relationship between these two phenomena is considered and discussed.

The Effect of Homeotropic Surface Anchoring on the Critical Voltage for Unwinding the Cholesteric Phase

Abstract Experimental data are presented for the unwinding of the helical cholesteric structure in the presence of an applied electric field. Homeotropic surface anchoring conditions are known to reduce the magnitude of the critical unwinding voltage which depends on helical pitch and cell thickness. The experiments are unique in that the pitch is varied continuously at constant cell thickness rather than changing thickness at constant pitch. This is accomplished by employing cholesteric mixtures in which the inverse pitch changes linearly with temperature in the nematic compensation region. A model introduced by Cladis and Klemen, and extended by Goossens, is used to explain the effect of surface anchoring in the absence of an applied field. The calculations are extended to include the effect of the field, and the theory predicts a universal critical voltage curve involving the product of cell thickness and inverse pitch which is verified by experiment.