Properties Of Cholesteric Liquid Crystals Research Articles

Physical properties of cholesteric liquid crystals - carbon nanotube dispersions

The mesomorphic and visco-elastic properties of cholesteric liquid crystal - multiwall carbon nanotube dispersions were investigated with respect to temperature and nanotube concentration. The important rheology characteristics, such as dynamic viscosity, elastic modulus and loss modulus were studied. For the first time a 3D-model of cholesteric liquid crystal at the temperature of cholesteric phase existence was obtained by molecular dynamics simulation method. A weak effect of nanotube on the molecular orientation of the liquid crystal molecules in the N* phase was found.

Spectral Variations in Selective Reflection in Cholesteric Liquid Crystals Containing Opposite-Handed Chiral Dopants

The optical properties of cholesteric liquid crystal (CLC) containing a nematic host dispersed with a binary chiral dopant were investigated. The binary dopant was composed of a right-hand twist (RT) chiral agent and a left-hand twist (LT) chiral agent whose helical twisting power was much smaller than that of the RT one. With a fixed content of the RT additive, we found that the sample redshifts in central wavelength of the selective reflection as the concentration of the LT agent increases, presumably due to the increasing helical pitch caused by the racemic effect. Some other interesting results including the dopant-content and temperature dependence of the optical properties are addressed.

Chiroptical properties of cholesteric liquid crystals of chitosan phenylcarbamate in ionic liquids

Chitosan phenylcarbamate (CtsPC) samples were synthesized with different degrees of substitution (DS) ranging from 2.3 to 3.5, and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the concentrated lyotropic system formed a cholesteric liquid-crystalline phase and mostly exhibited vivid colorations owing to selective reflection of visible light. Ultraviolet–visible–near-infrared spectroscopy and wide-angle X-ray diffractometry were used to evaluate the maximum wavelength of selective light reflection and the distance and rotational difference between adjacent nematic thin layers in the cholesteric mesophase, respectively. The cholesteric helical pitch increased with decreasing polymer concentration, increasing temperature or increasing DS. The helical sense remained right-handed under the adopted measurement conditions, although CtsPCs are known to form a left-handed cholesteric structure in polar organic solvents, such as N,N-dimethylformamide (DMF). Accordingly, an inversion of the cholesteric sense was achieved by combination of [C2Mim][N(CN)2] and DMF as solvent; circular dichroism and optical rotatory dispersion spectroscopy verified that the sense of CtsPC cholesterics changed from left-handed to right-handed with increasing [C2Mim][N(CN)2] content in the mixed solvent. Chitosan phenylcarbamate (CtsPC) samples were synthesized and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the lyotropic system formed a cholesteric liquid-crystalline phase and exhibited vivid colorations owing to selective light reflection. The cholesteric helical pitch was controllable by polymer concentration, temperature or the degree of phenylcarbamoyl substitution. Although the helical sense of CtsPC/[C2Mim][N(CN)2] cholesterics remained right-handed under the adopted measurement conditions, an inversion of cholesteric sense was achieved by using a mixed solvent of [C2Mim][N(CN)2]/N,N-dimethylformamide.

Non‐Uniform Helix Unwinding of Cholesteric Liquid Crystals in Cells with Interdigitated Electrodes

A microspectrophotometer was used to elucidate the local optical properties of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage. The spectra collected from a spatially selective and micron-sized sampling area allow for new insights into the spectral properties of CLCs in the gaps between patterned electrodes. The microscopic electro-optic response is shown to be highly dependent on the cell thickness and the electrode periodicity. Specifically, the helix unwinding of the CLC superstructure does not always occur uniformly in the sample, as a result of field gradients through the cell thickness: for cells with relatively narrow gaps and electrodes, the redshift occurs initially only in the CLC layers closest to the substrate with the electrodes, leading to broad reflection spectra and different reflection colors depending on which side of the cell is illuminated. Theoretical estimates of the expected shift in the reflection band gap based on the critical field for a given CLC material and the spatial variation of electric field in the cell are found to be in good agreement with the complex behavior observed experimentally. In contrast, in thin cells with wider gaps, the pitch increase affects the whole CLC layer uniformly, because the electric field gradient is small.

Reflection characteristics of cholesteric liquid crystal microcapsules with different geometries

The reflection properties of cholesteric liquid crystals (CLCs) encapsulated in poly(methyl methacrylate) (PMMA) by solvent evaporation were investigated both experimentally and theoretically. The geometry of the CLC/PMMA microcapsules significantly influenced their reflection characteristics. Spherical CLC/PMMA microcapsules exhibit a color corresponding to the CLC intrinsic wavelength λ0 at the center of the microcapsule with the color changing to colorless at the fringes, whereas flattened spherical CLC/PMMA microcapsules only show vivid color at their centers. By recognizing the effect of the interactions between PMMA and CLC, an alignment model was developed and analyzed. The high correlation between the experimental and modeled color data confirms a high degree of ordering of the CLC molecules inside the microcapsules. The calculated results for a single CLC/PMMA microcapsule were in good agreement with the experimental reflection spectra of the CLC/PMMA microcapsule films. These results show how the shapes of CLC/polymer composites determine their optical properties.

Conductivity and dielectric properties of cholesteric liquid crystal doped with single wall carbon nanotube

In this work, we have studied the conductivity ( σ) and dielectric anisotropy (Δ ɛ) change in cholesteric liquid crystal (CLC) system doped with single wall carbon nanotube (SWCNT). Samples were investigated in frequency range 100 kHz to 10 MHz at various bias voltages by using dielectric spectroscopy technique. Pure CLC and SWCNT doped samples exhibit positive dielectric anisotropy as a function of the driving frequency in working range. Also critical frequency and relaxation times were determined for investigated samples. The results lead to the conclusion that the enhancement of the electrical conductivity and the change of the dielectric anisotropy in the SWCNT doped cell is mainly caused by the aligning effect of cholesteric liquid crystal system.

Laser Properties of Cholesteric Liquid Crystals with In-Plane Helix Alignment

We describe a cholesteric liquid crystal (ChLC) laser with an in-plane helix alignment where the ChLC helix lies in the in-plane direction of the cell and report an improvement in the threshold characteristics. The in-plane helix alignment of ChLCs was formed by applying an in-plane electric field in a cell with homeotropic surface treatment while cooling the sample from the isotropic phase. The energy threshold was 0.3 mJ/cm2, which was found to be less than 1/10 of conventional planarly-aligned ChLC lasers. The threshold gain was measured using variable stripe length method and was determined as 1 cm-1, which is approximately 1/100 of conventional planar devices with a cell-gap of 10 μm. The improvement of the energy threshold is attributed to small threshold gain attained in these devices.

The Effect of Weak Polymer Stabilization on the Switching Properties of Cholesteric Liquid Crystals

We examine the differences in electro-optical behavior of weakly polymer stabilized cholesteric liquid crystals (PSCLC) when formed using either a flood lit UV lamp, a flood lit UV laser, or a holographically patterned UV laser. Both samples formed with the UV laser exhibit symmetry in their dynamic electro-optical properties relative to the front and rear surface of the polymerized cell. These cells exhibit a reversible ∼100 nm blue shift tuning of the notch upon application of a field when the reflectivity is measured from either side. The UV lamp sample exhibits large differences in behavior when examined from the front or back relative to the direction of the polymerizing source. The front surface exhibits tuning of the notch while measurements from the back surface yield only the typical binary switching behavior of a CLC. The overall switching fields in flood lit UV laser and UV laser holographically patterned samples are also considerably lower. These differences are explained due to the presence of a strong polymer gradient across the cell gap in the UV lamp flood lit samples which is not present for the UV laser stabilized samples. We also present novel electro-optical results when these laser cured cells are examined in transmittance mode. Strong anchoring at the two surfaces causes a strong anisotropic local LC structure across the cell gap when a field is applied. The two local environments next to each surface act to strongly pin the CLC structure whereas within the bulk, ample reorientation of the LC molecules occurs. When probed in transmission, unusual fringe patterns which change with the strength of the applied field are observed.

Functional implications of temperature‐correlated colour polymorphism in an iridescent, scarabaeine dung beetle

Abstract 1. In many Coleoptera, iridescent colouration is generated by exoskeleton ultra‐structure, within which multilayer interference reflects only certain wavelengths. Published work indicates that the colour polymorphism shown by some iridescent beetles is under genetic control. However, the present study suggests environmental involvement in the polymorphic southern African dung beetle, Gymnopleurus humanus Macleay.2. At 24 study sites across a 1000‐km latitudinal temperature gradient, population samples of G. humanus were dominated by blue individuals in the cooler south, by cupreous individuals in the warmer north, and by locally co‐occurring blue, green and cupreous individuals in intermediate situations.3. Using digital reflectance spectrophotometry, we measured wavelength intensity values across the visible spectrum (400–800 nm) and used the 70 measured specimens to estimate maximum reflectance from a further 3912 beetles. Differences in mean reflectance values between 24 populations were strongly correlated with average annual temperatures at study sites.4. Much stronger correlations between mean reflectance values and average temperatures of the cool dry season months suggest that the cross‐climatic patterns may be related to interaction between breeding seasonality and development under different cooler temperatures.5. Published evidence suggests that inherent physical properties of cholesteric liquid crystals and their responses to different thermal conditions could, potentially, generate the different exocuticle ultra‐structure responsible for different reflected colour wavelengths. Furthermore, colour polymorphism could be advantageous across a gradient from cooler to warmer climate as a result of the different thermal properties of different colours.6. Given the correlation with temperature, it is predicted that the prevailing reflected colour balance in southern populations would shift in response to global climatic change.

Optical Properties of Cholesteric Liquid Crystals with Functional Structural Defects

The authors numerically investigate the optical transmission properties of cholesteric liquid crystals with a local modulation of pitch and refractive indices. The transmittance spectra of structures with various defect thickness, refractive indices and pitch are calculated using the 4 × 4 matrix method. Calculation results showed that single or multiple transmittance peaks arise in the selective reflection band for light incidence with the same circular handedness as the material, and their spectral positions depend strongly on the physical parameter of the defect. A pitch shortening or elongation at the defect causes the defect mode to shift in the shorter or longer wavelength region, respectively, and the tuning range of the defect mode becomes larger for a thicker defect. We also show that efficient tuning is achieved by a defect medium with a smaller birefringence, allowing the defect mode to be sweeped through the photonic band gap with a smaller modulation of pitch.

Cholesteric liquid crystal inductive asymmetric polymerisation of thiophene monomers

The thermotropic properties of cholesteric liquid crystal as a polymerisation medium for the synthesis of chiral polymers from achiral monomers were clarified. The chiral reaction medium was successfully employed to synthesise chiral polymers from thiophenes as achiral monomer. The polythiophenes thus prepared exhibit intense circular dichroism imparted by the physical chiral continuum of the reaction field. The chiral aggregate of the present polymers displays structural chirality, with optical activity derived not from the chirality of the individual main‐chain but from the chiral architecture. The chiral polymers do not exhibit a thermotropic cholesteric liquid crystal phase but nematic liquid crystal character due to the release of the chiral architecture upon heating.

Electro‐optical properties of holographically patterned polymer‐stabilized cholesteric liquid crystals

Electro‐optical properties of cholesteric liquid crystals (LCs) with holographically patterned polymer stabilization were examined. It is hypothesized that increasing the LC domain size in a single dimension, relative to a random three‐dimensional network of LC pockets separated by polymer strands, will allow enhanced electro‐optical properties of the final device. Prior to holographic patterning, polymer stabilization with large elastic memory was generated by way of high irradiation intensities and optimized material choices. High irradiation intensities are required for the holographic patterning process to maintain polymer layer formation. At optimized conditions, polymer patterning of the stabilization allowed for an approximate 20% increase in the clear state transmission of the device, and allowed for an approximate 3 V µm−1 reduction in the overall switching voltage as compared to an analogous floodlit irradiated sample. Switching times were increased at most threefold with holographic patterning, but all relaxation times were below 20 ms. The enhanced electro‐optical properties appear to stem from a single dimension domain size increase, which allows for a reduction in the LC/polymer interaction.

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H‐bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H‐bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue‐to‐orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.

Aliphatic acid esters of (2-hydroxypropyl) cellulose—Effect of side chain length on properties of cholesteric liquid crystals

A series of aliphatic acid esters of (2-hydroxypropyl) cellulose (CnPC) were synthesized via the esterification of aliphatic acid chloride and (2-hydroxypropyl) cellulose (HPC). The liquid crystalline (LC) phases and transitions were investigated using differential scanning calorimetry, wide-angle X-ray diffraction (WAXD), and polarized light microscope (PLM) techniques. This series of CnPC polymers exhibited characteristic features of cholesteric LC phases between their glass transition and isotropization temperatures. The cholesteric LC characteristics were studied utilizing an ultraviolet/visible/near infrared spectrometer in a reflection mode. It was confirmed that, with an increase in the number of methylene units in the side chains of this series of CnPC polymers, there was an increase, on the scale of nanometers, in the layer spacing values for the cholesteric LC phases measured by WAXD. This periodic layer spacing represents the thickness of the neighboring twisted layers in a helical structure. Based on a unique significant red shift of the maximum reflection peak for the LC phases in this series of CnPCs, it is evident that the pitch distance in the helical structure also increases with an increase in the length of methylene units in the side chains.

Cholesteric Polymer Guest−Host Mixture with Circularly Polarized Fluorescence: Two Ways for Phototuning of Polarization and Its Intensity

A photosensitive fluorescent cholesteric guest-host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4'-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E-Z photoisomerization and [2 + 2] photo-cycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dis-symmetry factor. At temperatures below glass transition (T(g)) the polarized light action of an Ar(+) laser (488 nm) leads to the photo-orientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dis-symmetry factor of fluorescence. UV irradiation leads to E-Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dis-symmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than T(g). Such multifunctional glass-forming guest-host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.

Effect of introducing multiple chiral defects on the optical properties of cholesteric liquid crystals

The optical characteristics of cholesteric liquid crystal (ChLC) with multiple defect layers where pitch lengths were modified have been investigated. The transmission spectrum of such multi-defect ChLCs was simulated using the transfer matrix method. A transmission band appeared in the photonic band-gap (PBG) of the ChLC. The PBG displayed a shift as the periodicity of the defect sites was decreased. Finite difference time domain simulations have been carried out to investigate the electromagnetic field distributions within the medium. Obtained results showed that coupling between the modes confined in the defect layers lead to the formation of the defect band in the PBG.

Light scattering in cholesteric liquid crystals with a large pitch

Optical properties of cholesteric liquid crystals with a pitch larger than the wavelength of light are considered. Normal waves of the medium and the Green function of the electromagnetic field are analyzed. A general algorithm based on the application of the Kirchhoff method is proposed for calculating the scattered light intensity in media with a one-dimensional periodic structure. The WKB vector method is used for calculating the spatial correlation function of thermal fluctuations of the director. It is found that the transformation of two fluctuation modes takes place is some regions. The angular and polarization dependences of the intensity of light scattered from fluctuations of the director are calculated. It is found, in particular, that the intensity of scattering is a nonmonotonic function of the size of the system.

Rheology of defect networks in cholesteric liquid crystals.

The rheological properties of cholesteric liquid crystals containing networks of defects are investigated. A network of linear defects of the "oily-streak" type is stabilized when colloidal particles are dispersed into the cholesteric liquid crystals. This network converts the rheological response of a presheared cholesteric liquid crystal from fluidlike to solidlike and leads to the formation of a "defect-mediated" solid. The frequency-dependent complex shear modulus G*(omega) is measured, for samples with and without inclusions, in both the linear and nonlinear viscoelastic regimes. The linear elastic response mediated by the defect network is discussed in terms of a model analogous to the theories of rubber elasticity. All our data for G*(omega) are fitted to a simplified theoretical form, and the values and variations of the fitting parameters, in the various regimes investigated, are discussed in terms of the properties of defect structure present in the samples. Similar rheological properties are expected to arise from particle-stabilized oily-streak defect networks in layered systems such as smectic-A and lyotropic L(alpha) phases.

Transport properties of cholesteric liquid crystals studied by molecular dynamics simulation

We have studied the transport properties of a cholesteric liquid crystal by molecular dynamics simulation. The molecules consist of six soft ellipsoids of revolution, the axes of which are perpendicular to the line connecting their centres of symmetry. The angle between the symmetry axes of two adjacent ellipsoids is 7.5°, so the molecules are twisted. At high densities they form a cholesteric phase where their twist axes are oriented around the cholesteric axis and the symmetry axes of the ellipsoids are approximately parallel to the local director. We have been particularly interested in thermomechanical coupling or the Lehmann effect, which arises when a temperature gradient parallel to the cholesteric axis induces a torque that rotates the director. The converse is also possible: rotation of the director can drive a heat current. The thermal conductivity, the twist viscosity, the cross-coupling coefficient between the temperature gradient and the torque, and the cross-coupling coefficient between the director angular velocity and the heat current have been calculated by non-equilibrium molecular dynamics simulation methods (NEMD) and by evaluation of the Green-Kubo relations from equilibrium simulations. Two ensembles have been utilized: the ordinary canonical ensemble and another ensemble where the director angular velocity is constrained to be a constant of motion. All the methods give consistent results for the twist viscosity and the thermal conductivity. The NEMD estimates of the cross-coupling coefficients agree within a relative error of 20%. This is consistent with the Onsager reciprocity relations that state that the two cross-coupling coefficients should be equal. The relative error of the Green-Kubo estimates is about 100% even though the order of magnitude is the same as that of the NEMD estimates.

Chiroptical Properties of Cholesteric Liquid Crystals Induced by Chiral Photochromic 3,3′-Dialkoxyazobenzenes

The spectroscopic features and photoisomerization behaviors of chiral photochromic 3,3′-dialkoxyazobenzenes were evaluated in an isotropic solution and a nematic solvent. The effect of photoisomerization on the helical twisting power and handedness of these chiral dopants in induced cholesteric liquid crystals were studied. Depending on the distance of the chiral center from the azo-core, elongation of the helical pitch and reversion of the handedness upon E-to-Z photoisomerization were observed. These characteristics are interesting for optical devices based on light-controlled phase switching.