Travelling fingers in two-frequency cholesteric liquid crystals: fragments,loops and spirals

We review the different dynamical patterns that cholesteric fingers of the first type (CF-1) and of the second type (CF-2) form in an a.c. electric field near the coexistence line with the homeotropic nematic phase. Videomicroscopy and computer image analysis were used for investigation of the patterns in polarized light. We show that CF-1s can form stable rectilinear fragments that crawl at constant velocity along their axes, whereas CF-2s form only unstable curved fragments that drift perpendicularly to their axes. Observations of CF-2 staple-shaped fragments which continuously lengthen in their centres are also reported. Finally, we describe in detail the experimental conditions in which CF-2 loops and spirals grow, collapse and destabilize.     

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.     

Bilayer structures in cholesteric, cyclic-siloxane liquid crystals

X-ray diffraction has been used to investigate a series of commercially available liquid crystals based on a cyclic penta(methylsiloxane) with combinations of cholesteryl-4'-allyloxybenzoate and biphenyl-4'-allyloxybenzoate mesogens pendant on the siloxane ring. A lamellar, (smectic-like) structure is indicated in the mesophase and quenched glassy solid state. Extended molecular dimensions of the mesogens generally account for the observed d spacings. The mesogens may interdigitate within lamellae but the packing and extent of interdigitation is dependent on the ratio of cholesteryl to biphenyl mesogens. There is probably also a structural contribution from the nano-aggregated (nano-phase separated), immiscible, siloxane-rich layer delineating the lamellae interfaces as this mesophase shows unusually good definition; diffraction patterns exhibit high order reflections (up to 6th order). The lamellae have a macroscopic helicoidal twist about an axis lying in the plane of the lamellae, a cholesteric supramolecular structure, which derives from the chiral, steroid mesogenic component. Surprisingly, this mesophase composed of low molar mass cyclic siloxanes may be drawn into fibres tens of metres in length, to give a morphology having the lamellae normal to the fibre axis.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Lehmann rotatory power: a new concept in cholesteric liquid crystals

By doping a nematic phase with a chiral molecule one obtains a cholesteric phase. Each chiral molecule is characterised by its helical twisting power (HTP) which is defined as HTP?=?q/(2πC) where q is the equilibrium pitch of the cholesteric phase and C the concentration (in wt%) of chiral molecules. In a similar way, we define the Lehmann rotatory power (LRP) as LRP?=?v/(2πC) where v is the thermomechanical Lehmann coefficient. By making compensated mixtures, we measured the HTP and the LRP of five chiral molecules (R811, S2011, CC, CB15 and CE4) dissolved in an eutectic mixture 8CB/8OCB. We found that, although these quantities were different, their ratio R?=?LRP/HTP changed little from one molecule to another. This result shows that the Lehmann effect is closely, but not completely, related to the twist of the phase.     

Vitrifying star-shaped liquid crystals: synthesis and application in cholesteric polymer networks

In this paper, the formation of glass-forming reactive mesogens, that do not crystallize upon cooling, but vitrify and form supercooled LC phases, is described. These molecules exhibit broad range LC phases and enable us to carry out photopolymerization in a broad range of temperatures. From such reactive mesogens densely crosslinked networks in which the liquid crystalline order is permanently fixed are formed by photopolymerization. For this purpose eight novel low molecular mass LC materials with photopolymerizable acrylate groups have been synthesized and the detailed experimental procedures are given. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. Comparing the twin molecules which we have described before with the novel three- and four-armed stars, we found that the supercooled LC phase in the three-armed stars has a stability superior to that in both twin molecules and four-armed stars. In the three-armed star triple-4 with a suitable substituent pattern, the supercooled nematic phase is stable at room temperature for at least nine months. Photo-DSC experiments show that the final conversion after 10 min of UV-irradiation for the threearmed star molecule triple-4 is as high as for the smaller molecules twin-4 and mono-4 over the whole temperature range. Doped with a suitable chiral molecule the novel nematics formed cholesteric phases which were used to make cholesteric polymer networks by photopolymerization.     

Vitrifying star-shaped liquid crystals: synthesis and application in cholesteric polymer networks

In this paper, the formation of glass-forming reactive mesogens, that do not crystallize upon cooling, but vitrify and form supercooled LC phases, is described. These molecules exhibit broad range LC phases and enable us to carry out photopolymerization in a broad range of temperatures. From such reactive mesogens densely crosslinked networks in which the liquid crystalline order is permanently fixed are formed by photopolymerization. For this purpose eight novel low molecular mass LC materials with photopolymerizable acrylate groups have been synthesized and the detailed experimental procedures are given. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. Comparing the twin molecules which we have described before with the novel three- and four-armed stars, we found that the supercooled LC phase in the three-armed stars has a stability superior to that in both twin molecules and four-armed stars. In the three-armed star triple-4 with a suitable substituent pattern, the supercooled nematic phase is stable at room temperature for at least nine months. Photo-DSC experiments show that the final conversion after 10 min of UV-irradiation for the threearmed star molecule triple-4 is as high as for the smaller molecules twin-4 and mono-4 over the whole temperature range. Doped with a suitable chiral molecule the novel nematics formed cholesteric phases which were used to make cholesteric polymer networks by photopolymerization.     

Study of phase transformations in various cholesteric liquid crystals

Journal of Structural Chemistry -     

Flow activation free energy in cholesteric lyotropic liquid crystals

The flow activation free energy ( ΔG *) is the minimum energy necessary to induce a flow in a system that is at rest. It can be calculated from the system viscosity using the Andrade--Eyring theory. In the present work the flow activation free energy of cholesteric lyotropic liquid crystals was studied as a function of D-(+)-mannose concentration added to a nematic mesophase. The results obtained showed that all the systems can be characterized as Newtonian, but a smooth time-dependent effect can be observed mainly in systems with a lower chiral inductor concentration. It has also been observed that an increase of cholestericity leads to a decrease of ΔH * values until a limit is reached at 1 mol % of inductor. From a positive ΔS * variation it was possible to verify the existence of a locally less organized transient state during the process of micellar diffusion. When the cholesterization process was considered, the increase of the inductor concentration leads to a decrease of ΔS * and consequently to a system, as a whole, more orderly, possibly as a result of the restriction of movement caused by the presence of chiral interactions.     

Shear-stress measurement in aerodynamic testing using cholesteric liquid crystals

The change in the selective reflection spectra of cholesteric liquid crystals with shear is described, and the results indicate that, although it appears to be a promising technique for flow visualization in aerodynamic testing, the method is only suitable for approximate measurement of shear. An improved method for visualising flow is proposed that utilizes a shear-induced texture change from the uncoloured focal-conic state to the selectively reflecting Grandjean state. The sensitivity of the transition to shear is investigated and the effects of adding silica particles to modify the viscosity is examined.