Cholesteric Pitch Research Articles

Periodic anchoring condition for alignment of a short pitch cholesteric liquid crystal in uniform lying helix texture

Due to flexoelectricity, a cholesteric liquid crystal unidirectionally aligned in a sandwich cell, with its helix lying parallel to the confining cell substrates, exhibit a flexoelectro-optic response when subjected to an electric field applied perpendicular to the helix axis. The clarity of the flexoelectro-optic response is dependent on the quality of the uniformly lying helix texture of the cholesteric which often is difficult to achieve. Herein, a method for alignment of a short pitch cholesteric in such a texture by means of a periodic surface anchoring condition with periodicity matching that of the cholesteric liquid crystal is described.

Optical Monitoring of Gases with Cholesteric Liquid Crystals

A new approach to optical monitors for gases is introduced using cholesteric liquid crystals doped with reactive chiral compounds. The approach is based on cholesteric pitch length changes caused by a change in helical twisting power (HTP) of the chiral dopants upon reaction with the analyte. The concept is demonstrated for monitoring carbon dioxide via reversible carbamate formation and for oxygen using the irreversible oxidation of a chiral dithiol to a disulfide. Monitoring of CO(2) was achieved by doping a commercial cholesteric liquid crystalline mixture (E7) with 1.6% mol of the 1:1 complex of an optically pure diamine with a TADDOL derivative. Upon exposure to carbon dioxide, the reflection band of a thin film of the mixture shifted from 637 to 495 nm as a consequence of dissociation of the complex after carbamate formation of the diamine. An O(2) monitor was obtained by doping E7 with a chiral binaphthyl dithiol derivative and a nonresponsive codopant. The reflection band of the oxygen monitor film changed from 542 to 600 nm, due to the conformational change accompanying oxidation of the dithiol to disulfide. These monitoring mechanisms hold promise for application in smart packaging, where carbon dioxide and oxygen are of special interest because of their roles in food preservation.

Addition effects of imidazolium salts on mesophase structure and optical properties of concentrated hydroxypropyl cellulose aqueous solutions

Addition effects of N-alkyl-substituted methylimidazolium salts ([CnMim][X]) on the mesophase structure and lower critical solution temperature (LCST)-type phase-separation behavior of concentrated hydroxypropyl cellulose (HPC) aqueous solutions were investigated mainly by spectrophotometry. In the cholesteric mesophase formation, at concentrations of more than 50 wt% HPC, helical pitch (P) was confirmed to shift upward according to the chaotropic strength of X–, for example, in a manner satisfying the order of Cl−<Br−<NO3−<I−. Organocations generally elevated P relative to the nonionic reference, the effectiveness being pronounced in the order of [C2Mim]+<[C4Mim]+<[C6Mim]+. With regard to LCST behavior, imidazolium additives raised the cloud point (Tc) in the isotropic solutions of ⩽40 wt% HPC, whereas the Tc value of mesomorphic solutions was prone to be lowered by the addition. Discussion of these observations took into consideration the differences in the N-alkyl structure and amphiphilic nature between cationic imidazolium varieties, as well as the difference in the chaotropic strength between the counter anions. It was also exemplified preliminarily that this kind of salt-containing lyotropic system of HPC exercised an electro-optical function when coupled with an electric circuit. Addition effects of a series of N-alkyl-substituted methylimidazolium salts ([CnMim][X]) on the mesophase structure and LCST-type phase-separation behavior, and the ensuing optical properties of concentrated hydroxypropyl cellulose (HPC) aqueous solutions, were investigated by spectrophotometry and complementary X-ray diffractometry. Salted HPC solutions exhibited distinct shifts in both the cholesteric pitch and the cloud point, relative to the nonionic reference. Discussion of the observations took into consideration the differences in the N-alkyl structure and amphiphilic nature between the imidazolium varieties, as well as the difference in a socalled chaotropic strength between the anions used.

Colloids in liquid crystals: a lattice Boltzmann study

We propose a hybrid lattice Boltzmann algorithm to simulate the hydrodynamics of colloidal particles inside a liquid crystalline host. To validate our algorithm, we study the static and the microrheology of a colloid in a nematic, with tangential anchoring of the director field at the particle surface, and we confirm theories and experiments showing that the drag force in a nematic is markedly anisotropic. We then apply our method to consider the case of a colloid inside a cholesteric, and with normal anchoring at the surface. We show that by tuning the ratio between particle size and cholesteric pitch it is possible to control the defect configuration around the particle, and to stabilise novel figure-of-eight or highly twisted loops close to the colloid surface.

Electrospun microfibres with temperature sensitive iridescence from encapsulated cholesteric liquid crystal

We apply coaxial electrospinning to produce core-sheath polymer composite fibres with encapsulated short-pitch cholesteric liquid crystal, giving the fibres iridescent colours due to selective reflection within a narrow band of the visible wavelength spectrum. By modifying the feed rate of the liquid crystal during spinning we can tune the fibre diameter from the sub-micron range to about 7 μm, other ranges being accessible via further modifications of the spinning parameters. We demonstrate that the thinnest fibres display quantised colours, determined primarily by the core diameter, whereas the thicker fibres allow a quasi-continuous change in colour if the cholesteric helix pitch changes. Because of the strong response function of liquid crystals, phases as well as structures changing in response to small changes in the environment, the resulting non-woven fibre mats have potential for smart textiles, in particular in sensing applications.

Small amplitude oscillatory shear permeation flow of cholesteric liquid crystal polymers

We investigate the small amplitude oscillatory shear permeation flow (SAOSPF) of cholesteric liquid crystal polymers (CLCPs) using a mesoscopic model obtained from the Doi kinetic theory for flows of CLCPs. We model the system by Stokes hydrodynamic equations coupled with orientational dynamics and study the frequency-locked solutions of the system by employing a coarse- grained approach. The leading order solutions exhibit boundary layers of thickness of the order of the cholesteric pitch in low frequency regimes but in the order of 1 √ ! in high frequency regimes, where ! is the plate driven frequency. The response of CLCPs has been calculated. Viscoelastic behavior is only observed at intermediate frequencies and the material is essentially viscous at low and high frequency regimes. At both low and high frequencies, the real component of complex viscosity controls its magnitude which is of the order of the reciprocal of the cholesteric pitch at low frequencies while the order of 1 at high frequencies.

Chiral amplification in a cyanobiphenyl nematic liquid crystal doped with helicene-like derivatives

The addition of a chiral non-racemic dopant to a nematic liquid crystal (LC) has the effect of transferring the molecular chirality to the phase organization and a chiral nematic phase is formed. This molecular chirality amplification in the LC provides a unique possibility for investigating the relationship between molecular structure, intermolecular interactions, and mesoscale organization. It is known that axially chiral or helical-shaped molecules with reduced conformational disorder are good candidates for high helical twisting power derivatives. In particular, biaryl derivatives are known to be efficient chiral inducers in biaryl nematic mesophases. In this paper, we focus on a new series of helicene-like molecules of known absolute configuration. We have integrated cholesteric pitch measurements with geometry optimization by DFT calculations and analysis of the twisting ability by the Surface Chirality model to shed light on the structural features responsible for the analogies and differences exhibited by these derivatives. The investigation of these dopants with well-defined geometry, by virtue of the low conformational freedom, and the substituents variously distributed around the core, allows us to extend our knowledge of the molecular origin of the chirality amplification in liquid crystals and to confirm the simple relationship “molecular P-helicity” → “cholesteric P-handedness” for helical-shaped helicene-like derivatives.

Generalized van der Waals theory for the twist elastic modulus and helical pitch of cholesterics

We present a generalized van der Waals theory for a lyotropic cholesteric system of chiral spherocylinders based on the classical Onsager theory for hard anisometric bodies. The rods consist of a hard spherocylindrical backbone surrounded with a square-well potential to account for attractive (or soft repulsive) interactions. Long-ranged chiral interactions are described by means of a simple pseudoscalar potential which is appropriate for weak chiral forces of a predominant electrostatic origin. Based on the formalism proposed by Straley [Phys. Rev. A 14, 1835 (1976)], we derive explicit algebraic expressions for the twist elastic modulus and the cholesteric pitch for rods as a function of density and temperature. The pitch varies nonmonotonically with density, with a sharp decrease at low packing fractions and a marked increase at higher packing fractions. A similar trend is found for the temperature dependence. The unwinding of the helical pitch at high densities (or low temperatures) originates from a strong enhancement of the local nematic order and the corresponding increase in the twist elastic resistance associated with near-parallel local rod configurations. This contrasts with the commonly held view that the increase in pitch with decreasing temperature as often observed in cholesterics is due to layer formation resulting from presmectic fluctuations. The increase in pitch with increasing temperature is consistent with an entropic unwinding as the chiral interaction becomes less significant than the thermal energy. The variation of the pitch with density, temperature, and contour length is in qualitative agreement with recent experimental results on colloidal fd rods.

A model liquid crystalline system based on rodlike viruses with variable chirality and persistence length

We compare the phase behavior of a mutant filamentous virus, fd Y21M, to that of a conventional fd wild-type (wt). We find significantly different macroscopic phase behavior despite the only microscopic difference between the two viruses being in a single amino acid of the major coat protein pVIII. Compared to fd wt, the location of the isotropic–cholesteric phase transition for fd Y21M shifts to lower densities. This is attributable to a significant difference in the flexibility of the two viruses. The persistence length of fd wt is 2.8 ± 0.7 µm, whereas the persistence length of fd Y21M is 9.9 ± 1.6 µm. The large persistence length of fd Y21M makes it an essentially rigid rod, thus allowing for the first time a quantitative test of the Onsager theory for the isotropic–nematic phase transition. Even more striking, is the difference in the chiral phase behavior of the two viruses. Both viruses form cholesteric phases, with the fd wt forming a left-handed cholesteric helix, and the fd Y21M forming a right-handed one. At a given density, the magnitude of the cholesteric pitch between the two systems is different by fivefold. Using mixtures of the two viruses, we create a liquid crystalline system with a tunable control over its macroscopic chirality.

Molecular Structure and Liquid-Crystalline Characteristics of Chitosan Phenylcarbamate

Chitosan phenylcarbamate (CtsPC) samples were synthesized to have different degrees of substitution (DS) ranging from approximately 2.7 to approximately 3.7, and the lyotropic liquid crystallinity was mainly characterized by spectrophotometry. The products of DS>2.8 formed a cholesteric type of mesophase in concentrated solutions of >44 wt % with polar aprotic solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO), and some of the solutions imparted vivid colorations because of selective visible light reflection. The cholesteric helical pitch increased with increasing temperature and with decreasing polymer concentration, but the helical sense remained left-handed under the adopted measurement conditions. It was also found that the helical pitch increased in proportion to the increment of DS, whereas an average degree of phenylcarbamoyl polyaddition in the side chains (DPs) rather affected the pitch distribution measuring an orderliness in the cholesteric structure. With a small increase in DPs (e.g., from 1.01 to 1.04), the cholesteric orderliness decreased and the temperature sensitivity of the pitch turned sluggish. Wide-angle X-ray diffractometry was also used for evaluating the distance and azimuth difference between adjacent nematic thin layers in the cholesteric mesophase.

DNA Cholesteric Phases: The Role of DNA Molecular Chirality and DNA−DNA Electrostatic Interactions

DNA molecules form dense liquid-crystalline twisted phases both in vivo and in vitro. How the microscopic DNA chirality is transferred into intermolecular twist in these mesophases and what is the role of chiral DNA-DNA electrostatic interactions is still not completely clear. In this paper, we first give an extended overview of experimental observations on DNA cholesteric phases and discuss the factors affecting their stability. Then, we consider the effects of steric and electrostatic interactions of grooved helical molecules on the sign of cholesteric twist. We present some theoretical results on the strength of DNA-DNA chiral electrostatic interactions, on DNA-DNA azimuthal correlations in cholesteric phases, on the value of DNA cholesteric pitch, and on the regions of existence of DNA chiral phases stabilized by electrostatic interactions. We suggest for instance that 146 bp long DNA fragments with stronger affinities for the nucleosome formation can form less chiral cholesteric phases, with a larger left-handed cholesteric pitch. Also, the value of left-handed pitch formed in assemblies of homologous DNA fragments is predicted to be smaller than that of randomly sequenced DNAs. We expect also the cholesteric assemblies of several-kbp-long DNAs to require higher external osmotic pressures for their stability than twisted phases of short nucleosomal DNA fragments at the same DNA lattice density.

Simple method for accurate measurements of the cholesteric pitch using a “stripe–wedge” Grandjean–Cano cell

The classical method for measuring the inherent periodicity, or “pitch”, in the order parameter in a cholesteric liquid crystal implies using a wedge cell with well‐defined boundary conditions. But the error can be large due to difficulties in controlling the wedge angle, α. A modified method is presented that significantly reduces the measurement error. A relief of indium tin oxide stripes on one of the plates shifts the disclination lines a distance proportional to both relief amplitude and to the wedge angle. With this “stripe–wedge Grandjean–Cano cell” the pitch can be accurately determined without a priori knowing α.

Synthesis and characterization of hydroxyethyl chitosan grafted by carboxyl ending DOVOB dendrimer: A novel liquid crystalline polymer

A novel chitosan derivative, hydroxyethyl chitosan grafted 3,4,5-tris[ p-( n-dodecyloxy)- m-methoxybenzyloxy]benzoate (HECS-g-DOVOB) was prepared via grafting dendrimer 3,4,5-tris[ p-( n-dodecyloxy)- m-methoxybenzyloxy]benzoic acid (DOVOB acid) onto hydroxyethyl chitosan (HECS). The structure of HECS-g-DOVOB was investigated by means of FTIR, 1H NMR, UV and circular dichroism (CD). The degree of substitution was measured to be about 0.39 per glucose unit. The DOVOB acid was able to self-assemble into a thermotropic hexagonal columnar liquid crystalline phase showing fan-like texture, the aqueous HECS solution formed lyotropic cholesteric liquid crystal showing fingerprint texture. Whereas, HECS-g-DOVOB formed lyotropic cholesteric liquid crystal in concentrated DMSO solution with a planar texture, which was different to that of both DOVOB and HECS. The cholesteric pitch of HECS-g-DOVOB is much smaller than that of HECS. The fluorescence spectrum of HECS-g-DOVOB shows excimer fluorescence which is very similar to the dendrimer is a good indication for light-emitting polymeric material.

Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell

Spatial tuning of lasing wavelength in a dye-doped cholesteric liquid crystal wedge cell is demonstrated. The wedge cell possesses a series of dislocation lines along the wedge direction. In the lasing operation, we find that the lasing wavelength is continuously tuned in the region between two dislocation lines, while the lasing wavelength jumps when crossing the dislocation lines. The observed one-dimensional spatial continuous tuning is attributed to the presence of a gradient in the cholesteric helical pitch, while the laser wavelength jumping originates from the pitch jump owing to a change in the number of half-turns of the cholesteric helix.

Structure of DNA cholesteric spherulitic droplet dispersions

The aggregation of short (294-base-pair) linear double-stranded DNA molecules intocholesteric spherulitic droplets in a brine solution of polyethylene glycol has been studiedusing polarized light microscopy. The DNA concentration within the droplet is found to beconstant and therefore the cholesteric pitch is independent of the total DNA concentration.The size of the droplets grows as a power law of the total concentration of DNA. Theexponent of the power law relation is calculated using a progressive nucleation and growthmodel. The exponent is found to be equal to two-thirds and is in good agreementwith the experimental data. By analysing fields of spherulitic droplets at lowmagnification using either hydrophobic or hydrophilic slides and cover-slips, wedemonstrate that the positions of the droplets are correlated in solution. The longrange correlation between the droplets indicates some kinetic arrest of the phaseseparating solution that may have been induced by the finite size of the system.

The diverse world of liquid crystals

Orientationally ordered soft matter is exceptionally responsive to a variety of excitations. That’s the basis for its great range of applications.

Influence of the magnetic and laser fields on the director structures of a ferrocholesteric liquid crystal in homeotropic cells

We study the influence of the magnetic and laser fields on the director structures of a ferrocholesteric liquid crystal in homeotropic cells. Using the analytical method based on the Euler–Lagrange equations, we find a correlation between the fields intensities and the confinement ratio r = d / p , ( d is the cell thickness and p is the cholesteric pitch) at the limit of the transition from the homeotropic alignment to the translationally invariant configuration (TIC) with uniform in plane twist. We discuss this correlation as a function of the sign of the magnetic and dielectric anisotropies. If both anisotropies are positive and the magnetic field and the laser beam are perpendicular to the cell walls, the magnetic field preserves the homeotropic alignment while the laser beam pushes the system towards the TIC. The control parameters of the transition are the laser beam intensity and the confinement ratio. If the magnetic anisotropy is negative and the dielectric one is positive both fields concur in driving the system towards the TIC. The spinodal surface separating the metastable homeotropic configuration from the instable TIC is an ellipsoid whose halfaxes are smaller that in the case of a pure liquid crystal by a factor depending on the material constants and the cell thickness. We find also the total twist angle across the sample witch can be varied between a lower and an upper limit by changing the light intensity or the magnetic field strength. Our results can be useful in designing magneto-optical devices.

Photo-optical properties of photopolymerizable cholesteric compositions

Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the long-wavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated.

Cholesteric liquid crystals with a helical pitch gradient: Spatial distribution of the concentration of chiral groups by Raman mapping in relation with the optical response and the microstructure

Cholesteric liquid crystals (CLC) selectively reflect light when the helical pitch is of the order of the wavelength of the incident beam propagating along the helix axis. The wavelength bandwidth, related to the optical anisotropy, is typically limited to a few tens of nanometers in the visible part of the spectrum, which is insufficient for applications such as white-or-black polarizer-free reflective displays and smart windows for the control of the solar light and heat. A way to make cholesteric films reflecting in a broad wavelength band consists in associating various cholesteric pitches in the same film. In this work, it is shown how a study by confocal micro Raman spectrometry mapping makes it possible to have access to information accounting for the local organization of CLCs in the case of graded pitch materials. These investigations will be correlated to the optical response and the transverse microstructure of the CLC material as investigated by transmission electron microscopy. An accurate analysis of the vibrational behavior evolution of the C==O can be correlated to the evolution of the populations of the chiral and achiral groups in the case of the interdiffusion of two CLC substances with various stoechiometries. Besides an easy measurement of the Raman spectrum gives the opportunity to quantify the relative ratio of the mesogenic species and thus to go up by a simple way to the pitch of the helical structure.

Rotational Reorganization of Doped Cholesteric Liquid Crystalline Films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.