Twin‐like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*–SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular‐like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin‐like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

Alignment of liquid crystals induced by a photopolymerized self‐assembled film

An alignment film derived from a photopolymerized self‐assembled film may be used to orient nematic liquid crystals after irradiating the film with linearly polarized UV (LPUV). A photosensitive cationic amphiphile was first synthesized containing two double bonds and which could be polymerized by UV. A layer‐by‐layer self‐assembled multilayer film was next prepared in an aqueous solution of the cationic amphiphile and poly(sodium 4‐styrenesulphonate); the UV‐Vis spectra showed that each layer of the LBL multilayer film was uniform. When the film was irradiated by LPUV, the photosensitive double bonds underwent [2+2] cycloaddition along the vector direction of LPUV. The polarized UV‐Vis absorption spectra also provided evidence that the film was anisotropic, i.e. the photopolymerization was along a certain direction. The anisotropic film was used as an alignment layer for nematic liquid crystals, and observations under a polarizing microscope indicated that the alignment of the liquid crystals was good, as expected, and that the orientation direction of the liquid crystals was always perpendicular to the electric vector of the irradiating LPUV.     

A novel vertical alignment technology for nematic liquid crystals based on electrostatic self‐assembled method

A novel vertical alignment film for nematic liquid crystals is reported based on electrostatic self‐assembly of alkyl sulfonic salts in aqueous solution. A series of self‐assembled films with different alkyl chain lengths were prepared and used as alignment films. It was revealed that only when the number of carbon atoms in the alkyl chain approaches 11 or larger, could the self‐assembled film induce vertical alignment of liquid crystals. We also found that the homeotropic alignment of liquid crystals was related to the surface roughness of self‐assembled films. In addition, this vertical alignment film showed good electro‐optical characteristics and excellent thermal stability.     

Reflection symmetry breaking in achiral rod-shaped smectic liquid crystals?

The SmC phase of 4'-octyloxyphenyl-4-octyloxybenzoate has been examined in light of recent reports that this phase is chiral. The results suggest that two varieties of chiral domains in LC cells of the phenylbenzoate are indeed formed, driven by interactions with surfaces. Application of sensitive probes for chirality and polarity in the absence of such interfacial influences failed to find any. Currently, there is no evidence that the subject SmC phase is chiral.     

Strain‐induced compression of smectic layers in free‐standing liquid crystalline elastomer films

The deformation of oriented smectic liquid crystal elastomer films with smectic layers parallel to the film surface was studied using optical reflectometry and small angle X‐ray diffraction. Reflectometry data show that in the chosen material, in‐plane strain causes a change in the optical thickness of the free‐standing films. Small angle X‐ray scattering was used to explore the molecular origin of this effect. The X‐ray scattering data confirm that the change in optical thickness originates from the compression of the individual smectic layers. The measured Poisson ratio in the smectic A and C* phases is close to ½, in contrast to the smectic elastomers investigated earlier by Nishikawa et. al. [Macromol. Chem. Phys. 200, 312 (1999)]. In this unique material, the molecular lattice dimensions can be reversibly controlled by macroscopic stretching of the oriented samples.     

Oscillating Poiseuille flow in photo‐aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^?6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.     

Rheological properties of chiral liquid crystals possessing a cholesteric–smectic A transition

We have measured the rheological properties of two cholesterol derivatives (cholesteryl myristate and cholesteryl nonanoate) in the vicinity of their cholesteric–smecticA transitions. The results for the two compounds differ qualitatively, and are in agreement with results based on optical observations of new defects in cholesteryl nonanoate showing that this material, traditionally considered as a typical cholesteric, in fact exhibits a TGBA phase between the cholesteric and smectic A phases.     

Overall analysis of first‐order phase transitions in some alkyloxycyanobiphenyl liquid crystals

Measurements of pressure, molar volume and specific heat as functions of temperature in the isotropic (I) phase as well as in the smectic A (SmA) and nematic (N) mesophases of some alkyloxycyanobiphenyl compounds (nOCB, n = 6–10) were carried out using differential thermal analysis under pressure, densitometry, X‐ray powder diffraction and modulated differential scanning calorimetry. Thermodynamic properties, such as latent heats and volume jumps at the different phase transitions, were determined. The coherence of this whole set of data was tested using pressure–temperature data through the slopes associated to their phase transitions, extrapolated at normal pressure in the light of the Clausius–Clapeyron equation.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.     

Study on colour characteristics depending on orientation of liquid crystal in the in‐plane switching mode

The degree of colour shift depending on viewing direction in the in‐plane switching (IPS) mode has been investigated. The IPS cell with pure twist deformation exhibits a stronger colour shift than that of the IPS cell with tilt as well as twist deformation, although the former has better luminance uniformity in the bright state than the latter. Furthermore, the IPS cell with multi‐directional LC orientation, with tilt as well as twist deformation, shows the least colour shift.     

Bent‐shaped liquid crystals containing a furan bridge in the mesogenic core

We report the synthesis and characterization of hitherto unreported bent‐shaped dimeric liquid crystals based on a furan bridge in the mesogenic core. The compound 18‐Fu2 exhibits liquid crystalline behaviour.     

Constant pressure molecular dynamics simulations of the crystal–smectic transition in systems of soft parallel spherocylinders as a model for liquid crystals

Abstract

We present the results of constant pressure molecular dynamics simulation under hydrostatic pressure for soft parallel spherocylinders. A clear first order transition from crystal to smectic phase is observed. The anisotropy of the molecular volume plays an important role in this transition. We study the anisotropy of the diffusion process in the smectic phase by the mean-square displacement in each direction. Also the structures before and after the transition are analysed by means of the pair distribution functions.     

Liquid‐crystalline glycolipids: towards understanding the roles of liquid crystals in biological and life processes

A Commentary on the paper ”Liquid‐crystalline behaviour in the n‐alkyl gluconamides and other related carbohydrates?, by B. Pfannemüller, W. Welte, E. Chin and J.W. Goodby. First published in Liquid Crystals, 1, 357‐370 (1986).     

Synthesis and ferroelectric properties of chiral liquid crystals derived from (S)‐1‐propyloxy‐2‐propanol

A new optically active chiral moiety, (S)‐1‐propyloxy‐2‐propanol, was designed and synthesized by the treatment of 1‐propanol with (S)‐propylene oxide under basic conditions. Its derivatives, the (R)‐1‐propyloxy‐2‐propyl 4‐[4‐(4‐alkoxyphenyl)phenoyloxy]benzoates, PPmPPB (m = 8–12), were prepared for the investigation of mesomorphic properties. All of the chiral materials displayed enantiotropic SmA? and SmC? phases, and the shorter alkyl chain members (m = 8–11) displayed an additional unidentified SmX? phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro‐optical response for the materials in the ferroelectric SmC? phase were measured. The electro‐optic responses in polyimide film‐coated homogeneously aligned cells exhibit thresholdless, V‐shaped switching in the ferroelectric phase.     

Phase separation in mixtures of thermotropic liquid crystals and flexible polymers

The spinodal equation and the concentration-induced anisotropic-isotropic transition equation of the mixtures of thermotropic liquid crystals and flexible polymers have been studied by using the molecular field theory The calculations of the phase diagrams of this system show that,besides the isotropic classic spinodal curve,there ex ists an anisotropic spinodal curve which has not been reported in literature.These two spinodal curves can be linked up by the concentration-induced anisotropic-isotropic transition line.In the various phase regions,demixing may take place due to different phase separation mechanisms.The phase equilibrium curve cannot always join the.spinodal curve at a critical point.These results are considered very meaningful for the understanding of the special properties of liquid crystal/polymer composites and very useful for controlling the morphology and the performance of PDLC materials     

Applying click chemistry to synthesis of chiral [1,2,3]‐triazole liquid crystals

1,3‐Dipolar cycloaddition of organic azides to triple bonds, or ‘click chemistry’ has been used in order to obtain chiral 1,4‐disubstituted [1,2,3]‐triazoles. Liquid crystalline compounds bearing such heterocycles were prepared and SmA and N* (cholesteric) phases identified. Contact experiments indicated a right‐handed helix (RH‐helix) for the cholesteric phase, and attempts to corroborate experimental and theoretical results are presented.     

Synthesis and mesomorphic behaviour of wedge‐shaped nematic liquid crystals with flexoelectric properties

The synthesis and mesomorphic behaviour of a series of wedge‐shaped liquid crystals and some reference compounds are reported. These unusual liquid crystals possess smectic C, smectic A and nematic phases. These new wedge‐shaped materials with a high degree of shape anisotropy and a large dipole moment can be used to induce an increase in the flexoelectric effects of nematic guest–host mixtures as dopants at low concentrations.