Chiral Nematic Research Articles

Imaging of anisotropic cellulose suspensions using environmental scanning electron microscopy.

The effect of concentration on anisotropic phase behavior of acid-hydrolyzed cellulose suspensions has been examined using conventional polarizing microscopy and the novel technique of environmental scanning electron microscopy (ESEM). Microcrystalline cellulose dispersed in water formed biphasic suspensions in a narrow concentration range, 4-12 wt % for a suspension pH of 4, where the upper and lower phases were isotropic and anisotropic (chiral nematic), respectively. It is known from previous work that within the biphasic regime total suspension concentration affects only the volume fractions of the two phases, not phase concentration or interfacial packing. As the total suspension concentration surpassed the upper critical limit (c), however, a single anisotropic phase of increasing concentration was observed. It was evident from polarizing microscopy that the chiral nematic pitch of the anisotropic phase decreased with increasing concentration, which has been attributed to a reduction in the electrostatic double layer thickness of the individual rods, thus increasing intermolecular interactions. Chiral nematic textures were also visible using ESEM. This technique has the advantage of studying individual rod orientation within the liquid crystalline phase as it permits the high resolution of electron microscopy to be applied to hydrated samples in their natural state. To our knowledge this is the first time such lyotropic systems have been observed using electron microscopy.

Study on Thermal-addressing Characteristics of (Polymer Network/Liquid Crystals/Chiral Dopant) Composite

Abstract A [polymer network/liquid crystals (LCs)/chiral dopant] composite with a smectic A (Sa) ↔ chiral nematic (N*) phase transition is prepared. A very drastic change from the light-scattering Sa phase to the transparent N* one is achieved upon heating the composite. The transparent state of the N* phase is ‘frozen’ in the Sa phase upon cooling. When an ac electric field is applied to the composite, the transparent Sa phase is changed into the light-scattering Sa one again.

Mass density measurements as a tool to distinguish between micelle size and shape in nematic and chiral nematic phases

The relative alteration in micelle size and micelle shape brought about by two sets of specific solutes have been investigated by mass density measurements at constant composition and temperature. The mass density of the first set of solutes, phenol, benzene, cyclohexanol and cyclohexane, was measured in the same host phase and found to change in the order: phenol > cyclohexanol > benzene > cyclohexane. The contribution of the solutes to the mass density of the host phase was interpreted to arise from the capability of the hydrophilic group and the hydrophobic part of the solutes to interact with the corresponding region of the host micelle. The second set consisted of racemic amphiphilic mixtures and their enantiomers, decyl- and dodecyl-esters of serine and alanine. It was found that the mass density increases in the serine phases but decreases in the alanine ones as the DL-racemic amphiphiles of the nematic phases were replaced by their L-isomers. These phenomena were shown to arise from a definite arrangement of the head groups in the racemic nematic and intrinsic chiral nematic phases. Furthermore, on the basis of the mass density difference between the achiral/induced chiral nematics and the racemic/intrinsic chiral nematics, a distinction between the induced chiral nematic phases and the intrinsic chiral ones is possible.

Phase Behavior and Effect of Enantiomerism on Potassium N -Dodecanoyl Alaninate/Water/Decanol Systems

The phase behavior of potassium N-dodecanoyl-DL-alaninate (DL-KDA) and potassium N-dodecanoyl-L-alaninate (L-KDA) in water and water/decanol systems have been investigated. In the DL-KDA/water binary system, normal micellar solution (Wm), hexagonal (H1), bicontinuous cubic (V1) phase with Ia3d symmetry, and hydrated solid (S) phases are formed successively with increase in surfactant concentration at 25 °C. The phase behavior of L-KDA is essentially similar to that of the DL-KDA system except that the melting temperature of solid phase is high. In the DL-KDA/water/1-decanol system, lamellar (Lα), reverse hexagonal (H2) and reverse micellar (Om) phases are formed successively at 25 °C with increase in 1-decanol concentrations. Addition of decanol to the H1 phase induced a H1-Lα transformation via intermediate phases. At lower surfactant concentrations, nematic (N) and discotic lamellar (LαD) phases are found, whereas at higher surfactant concentrations rectangular ribbon (R1) and defected lamellar (LαH) phases are observed. The L-KDA/water/1-decanol system shows that a chiral nematic or cholesteric (Ch) phase is present (instead of the N phase observed in the DL-KDA system) at low decanol concentrations. However, a similar phase behavior is observed at high decanol concentrations for both surfactant systems. Hence, the optical activity of the surfactant has a significant effect on the phase behavior only in the presence of a low amount of decanol. This phenomenon has been explained by the change in the effective cross-sectional area at the aggregate interfaces.

A Pairwise Additive Potential for the Elastic Interaction Energy of a Chiral Nematic

We have developed a pairwise additive potential model to describe the macroscopic elastic interactions of a chiral nematic liquid crystal. The potential is obtained from the expression for the elastic free energy density discretized onto a cubic lattice and mapped onto a suitable expansion in scalar invariants. The value of the pair potential is explored by means of a Monte Carlo lattice computer simulation. It allows, in a simple and efficient way, the simulation of liquid crystal devices and samples in confined geometries, taking into account the effect of the temperature of the sample and of the thermal fluctuations in the director distribution, as well as the elastic interactions.

A correlation between the absolute configuration of alkyl aryl sulfoxides and their helical twisting powers in nematic liquid crystals.

In this paper, for the first time, a systematic experimental and theoretical analysis of the cholesteric induction due to solutes whose chirality is originated only by a single stereogenic center has been carried out. The twisting power beta of a series of alkyl aryl sulfoxides has been determined in several nematic solvents. The sign of beta, which reflects the handedness of the induced helical arrangement of the solvent molecules, correlates with the configuration of the stereogenic sulfur in the nematic solvents E7, Phase 1083, and ZLI 2359: (S)-configurated dopants induce (M)-chiral nematics. (S)-Configurated cyclic sulfoxides, which are forced to adopt a different conformation with respect to the parent acyclic compounds, induce, instead, right-handed chiral nematics. The experimental data have been interpreted in the light of the surface chirality model, which allows the calculation of beta in terms of the molecular properties of the dopant, namely, the anisotropy and helicity of its molecular surface. The calculations reliably reproduce the behavior experimentally observed. The more flexible, open-chain compounds investigated induce chiral nematics of opposite handedness in MBBA and Phase 1053: temperature-dependent experiments point out the importance of the conformation in determining the effective sign of beta. The results have been discussed in terms of different conformation populations in these latter solvents with respect to E7, Phase 1083, and ZLI 2359.

Theory of bent-core liquid-crystal phases and phase transitions.

We study phases and phase transitions that can take place in the recently discovered bow-shaped or bent-core liquid-crystal molecules. We show that to completely characterize phases exhibited by such bent-core molecules a third-rank tensor T(ijk) order parameter is necessary in addition to the vector and the nematic (second-rank) tensor order parameters. We present an exhaustive list of possible liquid phases, characterizing them by their space-symmetry group and order parameters, and catalog the universality classes of the corresponding phase transitions that we expect to take place in such bent-core molecular liquid crystals. In addition to the conventional liquid-crystal phases such as the nematic phase, we predict the existence of other liquid phases, including the spontaneously chiral nematic (N(T)+2)(*) and chiral polar (V(T)+2)(*) phases, the orientationally ordered but optically isotropic tetrahedratic T phase, and a nematic N(T) phase with D(2d) symmetry that is neither uniaxial nor biaxial. Interestingly, the isotropic-tetrahedratic transition is continuous in mean-field theory, but is likely driven first order by thermal fluctuations. We conclude with a discussion of smectic analogs of these phases and their experimental signatures.

Thermal switching characteristics based on smectic-A↔chiral nematic phase transitions of (liquid crystals/chiral dopant) composites, with and without side chain-type liquid crystalline polymer

The light switching characteristics induced by a thermal smectic A (SmA) ↔ chiral nematic (N*) phase transition were studied for homeotropically aligned [smectic A liquid crystal (SmA-LC)/nematic liquid crystal (N-LC)/chiral dopant] and [side chain type smectic A liquid crystalline polymer (SmA-LCP)/N-LC/chiral dopant] composites. A drastic change from a transparent SmA phase to a light-scattering N* phase occurred in both composites upon heating. In the case of the heat-induced N* phase for the (SmA-LC/N-LC/chiral dopant) composite, the N* phase exhibited weak light scattering due to formation of a scroll texture. On the other hand, in the case of the heat-induced N* phase for the (SmA-LCP/N-LC/chiral dopant) composite, the N* phase showed strong light scattering due to formation of a focalconic texture. The existence of a SmA-LCP was responsible for a higher contrast ratio between the transparent SmA phase and the light scattering N* phase for the (SmA-LCP/ N-LC/chiral dopant) composite than for the (SA-LCN/N-LC/chiral dopant) composite.

Periodic deformations induced by a magnetic field in planar twisted nematic layers

Magnetic field-induced spatially periodic deformations of planar nematic layers twisted by an angle Φ were investigated numerically. Chiral nematics with pitches compatible with the twist angle and non-chiral nematics twisted by Φ ⩽ π/2 were considered. Two different modes of deformation, taking the form of stripes, were found: the so called Mode X, with periodicity parallel to the mid-plane director in the undisturbed structure, and Mode Y with periodicity perpendicular to the mid-plane director. The static director distributions were calculated for various magnetic field strengths, twist angles and elastic parameters. The influence of surface tilt was also investigated. Mode X appeared for sufficiently large Φ and was possible in nematics with typical elastic properties. Mode Y appeared provided that the k 22/k 11 elastic constant ratio and the twist angle Φ were sufficiently small. Both modes arose from the undistorted state when the magnetic field exceeded a threshold value. The spatial period of the patterns increased with field strength. At high field, regions with almost homogeneous deformation arose in the two halves of each stripe. Their width and, simultaneously, the spatial period diverged to infinity at some critical field. This divergence corresponds to the transition to a homogeneously deformed state. Diagrams were constructed showing the ranges of parameters favouring the periodic distortions.

Propagation characteristics of a shear horizontal wave in a liquid-crystal cell having temperature-dependent phase states

Propagation characteristics of the zeroth-order symmetric (S0) mode shear-horizontal (SH) wave in a cell structure containing a thermotropic liquid crystal layer with chiral smectic-C (SmC*), smectic-A (SmA), chiral nematic (N*) and isotropic (Iso) phase states, are examined. The SH wave propagation is numerically analyzed with consideration of the directional dependence of the viscosity and elastic constant of the SmA liquid crystal. Temperature dependencies of acoustic phase delay and propagation loss of the SH wave are observed. The critical behavior of the acoustic phase delay and the propagation loss of the S0 mode SH wave in the vicinity of the N*-SmA phase transition temperature are explained by the existence of the elastic constant related to layer compression. The layer compression elasticity is predominant in the SmA phase, and is determined from the comparison of numerical analysis with experimental results.

Side Arms Change Pitch

When a nematic liquid crystal is made up of chiral (or handed) molecules, the molecules themselves not only orient with respect to each other but can also adopt a helical pitch. Adding achiral molecules would be expected to decrease (or dilute) the twisting power of the chiral nematic and to increase the helical pitch. Thisayukta et al. examined what happened when banana-shaped molecules, with similar side arms but different central linkers, were added to a linear chiral nematic. Surprisingly, they observed an increase in the twisting power of the chiral nematic and a decrease in the optical pitch. Liquid crystals made of banana-shaped molecules (which lack an asymmetic carbon atom) have been known to form chiral smectic phases. These phases are believed to form as a result of a layering of the molecules, an inherent twisting of the side arms, or a helical twisting of the molecules about the director. In this case, the authors conclude that the chirality does not arise from how the molecules pack but instead is caused by twisting of the side arms. — MSL

Field-Induced Alignment of Ferroelectric Liquid Crystal with the Phase Sequence of N*-SmC* in an Uniform External Electric Field

We proposed a new field-induced alignment method for a ferroelectric liquid crystal (FLC) which exhibited chiral nematic (N*) to smectic C* (SmC*) phase transition. Homogeneous alignment for the FLC with the phase sequence of N*-SmC* can be obtained by applying an electric field to the cell at the temperature 2-3°C higher than the N*-SmC* phase transition temperature. Conventionally, the electric field is applied through indium-tin-oxide (ITO) electrodes, which is located inside of the cell, during alignment process. However, it is difficult to apply this alignment process to mass production of thin film transistor liquid crystal display (TFT-LCD) panels, where these electrodes are shielded by drive ICs after module process. In addition, the alignment of TFT-LCD panel would be destroyed by an external shock, which could not be restored afterwards. We examined the field-induced alignment states of FLC in test cells and TFT-LCD panels by putting those into an uniform electric field. We found that each test cell had a different characteristic electric field for alignment which was dependent on the spontaneous polarization. From the comparison between the TFT-LCD panel and test cell, these two exhibit different electric fields for alignment. These results were analyzed by the equivalent circuit modeling, which revealed that the alignment voltage to supply the same electric field on LC was higher in LCD panels than in test cells, even though the same liquid crystals were used for the two.

Molecular modelling of chiral nematics

The surface model that was originally developed for evaluating the twisting power of chiral solutes is generalised to the case of pure chiral nematics to rationalise the cholesteric pitch on the basis of the molecular structure. Its application to cyanobiphenyl nematogens having one chiral centre in the flexible chain, demonstrates that an important role is exerted by the conformational distribution, since the chirality contributions of conformers of a given compound differ both in magnitude and in handedness. By using reasonable estimates of the conformer populations and structure, we obtain predictions of cholesteric pitch in fair agreement with the experimental values. On the other hand, given the large number of conformers to be analysed and because of compensation effects between contributions with opposite handedness, the relation between the macroscopic chirality and the molecular structure becomes less evident than in the case of rigid solutes. Therefore we have developed a procedure which allows the identification of the contribution of specific molecular groups to the macroscopic chirality. Such a procedure, when applied to the examined cyanobiphenyl nematogens, clearly shows that the cholesteric pitch is controlled by the size of the substituents at the chiral centre.

Effects of pressure on the behavior of (hydroxypropyl)cellulose in aqueous solution

The effects of pressure and temperature on the phase behavior of aqueous solutions of (hydroxypropyl)cellulose (HPC) were in¶vestigated at low and high concentrations. In dilute solutions, the transition temperature (Tt) as measured by apparent light scattering increased with an increase in pressure at lower pressures, but decreased with increasing pressure above 100 MPa. At lower temperatures, the transition pressure (Pt) increased slightly with increasing temperature, but decreased with at temperatures above 10–15 °C. Both Tt and Pt showed a moderate polymer-concentration dependence and they also showed strong concentration dependence on salt; Tt and Pt decreased with increasing concentrations of KCl and K2SO4, whereas the addition of KI or KSCN increased Tt and Pt. The apparent absorbance of concentrated solutions (62.5%) of HPC, in a cholesteric (chiral nematic) mesophase, was also measured. The spectrum shifted to a longer wavelength under high pressure. These results could be explained by assuming that the helicoidal pitch was increased under elevated pressure, probably due to a decrease in the angle between the two semiflexible and twisted HPC molecules in adjacent planes under high pressure.

Textures of homologous 4-n-alkyloxybenzoic acids: spontaneous chirality and surface memory

By using polarizing microscopy analysis we have found that several achiral homologues of the 4-n-alkyloxybenzoic acids, displaying only the nematic phase, exhibit the optical properties of a chiral liquid crystal system. These acids possess a mesophase due to the formation of dimers via hydrogen bonding. The microtextural analysis was carried out in the temperature ranges of the isotropic, nematic and crystal phases. The nucleation of a chiral texture in small domains emerging on cooling in the isotropic phase was observed. These small domains are characterized by a conoscopic cross which presents an azimuth of 45° with respect to the polarizer axis, contrary to the usual nematic drops, for which the conoscopic cross is not rotated. On further cooling, these domains coalesce in the nematic phase close to the clearing point, thus building large chiral monodomains. Such coalesced droplets exhibit very thin stripe lines, as in the case of pure cholesterics with a tilted helix axis. Moreover, left- and right-handed chiral domains were observed, combined in regions partially separated by 'oily streaks', also typical of pure cholesterics. On cooling, the chiral nematic (N*) phase transformed through a pronounced texture transition into a normal nematic phase. However, the small chiral grains that formed from the isotropic phase are retained close to the surface, acting as 'memorizing centres'. With suitable boundary conditions, they can provide a macroscopic twist driven by the surface. Moreover, a twisted smectic B not present in the bulk phase diagram was found and interpreted as induced by the surface. Also in the crystal phase a strong memorization of the chiral N* texture was observed.

Effect of chiral dopant on a helical Sm1 phase of banana-shaped N-n-O-PIMB molecules

The chiral helical structures of the Sm1 phase of naphthalene-2,7-diyl bis[4-(4-alkoxyphenyliminomethyl)benzoates] (N-n-O-PIMB) occur with equal probability for right- and left-handed structures. In this report, we investigated the effect of chiral dopants on the formation of these structures. Two chiral homologues, N-4-O-PIMB2* with (S)-(+)-2-methylbutoxy tails and N-8-O-PIMB6* with (S)-(+)-6-methyloctyloxy tails, were synthesized for this purpose. We found that the chiral agent induces an imbalance between the two chiral domains such that the area of one of the two domains increases with chiral dopant content. The position of the chiral centre of the doping agent plays an important role in controlling the helical sense as well as the strength of the effect. A stronger effect was observed for N-4-O-PIMB2* in which the asymmetric carbon is closer to the central core. The helical pitch is not affected by the amount of chiral dopant as observed from the wavelength maximum in the CD curve. Further, it is interesting that N-4-O-PIMB2* induces preferentially left-handed helical structures, while N-8-O-PIMB6* forms right-handed structures. These unusual effects of the chiral dopant are discussed with a comparison with the conventional chiral phases such as the chiral nematic and SmC phases.

Coloring Chiral Excess

CHEMISTRY In nematic liquid crystals, weak interactions between the rod-like molecules cause them to align locally along one “average” direction. Adding a small amount of a chiral nematic (cholesteric) can cause the entire mixture to behave like the cholesteric liquid crystals used in displays,

Pretransitional optical activity of short-pitched chiral nematic liquid crystals.

We have developed, using the de Gennes theory of short-range orientational order in the isotropic phase, a closed-form expression for the temperature dependence of the pretransitional optical activity of chiral nematics. Detailed calculations are included and the results are expressed in a form that can be easily tested experimentally. The theoretical predictions are supported by experimental data.

Observation of a reentrant twist grain boundary phase.

We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- TGB(A). Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso- N*- TGB(A)-Sm- A-Re- TGB(A)- TGB(C*). Here N* and Sm- A stand for the chiral nematic and smectic- A phases, TGB(A) is the twist grain boundary phase with smectic- A blocks, and TGB(C*) that with smectic- C* blocks and exhibiting features of both the smectic- C* and TGB phases. The reentrance of the TGB(A) phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.

Fabrication of Optically Designed Multidomain Layer Structure using Azo-Dye-Doped Ferroelectric Liquid Crystal Having N*‒C* Phase Sequence

We proposed an optical fabrication method of designed multidomain structure in an azo-dye-doped ferroelectric liquid crystal with chiral nematic (N*) – chiral smectic C (SmC*) phase sequence (NC-FLC). This layer alignment control is based on the N*−SmC* phase transition induced by the photoisomerization of doped azo-dye. This photo-assisted layer switching can be applied to the fabrication of the designed multidomain structure in the electrooptic elements such as display, optical memory and optical grating. Anomalous layer switching upon the electric field has also been observed.