Surface tensions of smectic liquid crystals

We determine surface tensions σ of smectic liquid crystals from the curvature pressure of smectic films. A new technique is introduced for the comparison of surface tensions of different smectic materials. The method is based on the relation of curvatures of smectic films drawn on communicating vessels. The measurement of the temperature dependence of σ reveals anomalies in the vicinity of phase transitions to low temperature smectic modifications. This anomalous slope dσ/dT can be related to the surface excess entropy of the material in the corresponding temperature range. The surface tension values determined for a number of mesogens fit well into the classification proposed by Mach et al.     

Polyimide aligned ferroelectric smectic C liquid crystals

Abstract

27 alkyl-aromatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectric liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained and the stability of the alignment is discussed.     

Polyimide aligned ferroelectric smectic C liquid crystals

27 alkyl-aromatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectric liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained and the stability of the alignment is discussed.     

Molecular theory of smectic C liquid crystals

We generalize an earlier density functional theory of liquid crystals by Mederos and Sullivan. The original theory took account of anisotropic hard core interactions, and for suitable intermolecular interactions predicted nematic and smectic A phases as well as isotropic liquid and vapour phases. In this generalization we also take into account quadrupolar or dipole induced dipole interactions. The modified theory now also predicts the existence of a smectic C phase.     

Electro-optic effects in smectic C liquid crystals

Electro-optic effects are observed in the smectic C states of 4-n-heptyloxybenzoic acid, 4-n-octyloxybenzoic acid. 4,4′-bis-n-heptyloxyazoxybenzene, and 4-n-decyloxy cinnamic acid under the application of an external ac electric field. The stripe domain pattern is observed for all the compounds studied above the first threshold. In the smectic C state there exists a second threshold at which the direction of the domain lines changes.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

Columnar and smectic liquid crystals based on crown ethers

Unsymmetrical benzo[15]crown-5 ethers 5 with one lateral ortho-terphenyl unit bearing alkoxy side chains of varying chain lengths (C5-C14) were prepared from 3,4-dialkoxyphenylbromides 2. Complexation with metal salts MX (M = Na, Cs) afforded the corresponding derivatives MX5. The uncomplexed crown ethers 5 h and 5 i, with dodecyloxy and tetradecyloxy side chains, respectively, exhibit liquid crystalline properties. In the series of complexed crown ethers, liquid crystal properties appeared as early as NaI5 f with C9H19 side chains. Whereas the uncomplexed 5 h,i form smectic mesophases, the complexed NaI5 g and NaI5 h exhibit textures typical of columnar mesophases. These results were supported by X-ray diffraction measurements (WAXS, SAXS), which revealed smectic (5 h,i), rectangular columnar (NaI5 g), and hexagonal columnar (NaI5 h) mesophases. As the liquid crystalline phase might retain packing features of the solid-state structure, single-crystal X-ray analyses were also performed for some of the uncomplexed and complexed crown ether derivatives. The complex NaI(3)5 a displays a sandwich-type structure, with the crown ether cores mutually antiperiplanar and maintaining an almost perfect crown conformation. In contrast, non-mesogenic uncomplexed crown ether 5 b displays a layer-type ordering in the solid phase.     

Dielectric studies under high pressure on strongly polar liquid crystals exhibiting monolayer smectic A phase

Dielectric studies have been performed at elevated pressures on two compounds of a homologous series possessing a strongly polar terminal group and three phenyl rings in their molecular structure. This is the first high pressure dielectric study on such systems that exhibit a monolayer smectic A phase. Also notable is the fact that these compounds show a dual frequency response, a promising feature for fast liquid crystal display devices. The low frequency relaxation recorded in the nematic and smectic A phases is attributed to the reorientation of the molecules about the short axis. The temperature and pressure dependence of the relaxation frequency of this mode as well as the involved activation parameters are discussed. At a given relative temperature the relaxation frequency decreases as the pressure is increased. However, the parameter that characterizes the temperature dependences of the relaxation frequency, viz., the activation enthalpy, has a very weak dependence on pressure. In contrast, the activation volume shows a linear decrease with temperature.     

A backflow effect in smectic C liquid crystals

This paper discusses the influence upon a smectic C liquid crystal cell of backflow induced by the relaxation of alignment following the removal of a strong electric or magnetic field. Our study, based upon a recently proposed continuum theory, concentrates upon the homeotropic configuration in which the smectic layers are parallel to the boundary plates, but some consideration is also given to the bookshelf geometry. Although the governing equations prove to be rather complex, some progress is possible analytically by repeating an approximation made in the corresponding problem for a nematic.     

Point-like impurity-dislocation interactions in smectic A liquid crystals

The displacement field created in the neighbourhood of a point-like impurity, its self-energy and point-like impurity-dislocation interaction are calculated for a smectic A liquid crystal in the approximation of small deformations. The binding energy of a point defect to an edge dislocation is also estimated. The use of the Peach-Kochler formula as a basis for the calculation of the dislocation interaction with other defects is discussed.     

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.     

Point-like impurity-dislocation interactions in smectic A liquid crystals

Abstract

The displacement field created in the neighbourhood of a point-like impurity, its self-energy and point-like impurity-dislocation interaction are calculated for a smectic A liquid crystal in the approximation of small deformations. The binding energy of a point defect to an edge dislocation is also estimated. The use of the Peach-Kochler formula as a basis for the calculation of the dislocation interaction with other defects is discussed.     

Polar and quadrupolar order in smectic liquid crystals

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.     

Stimulated light scattering in smectic A liquid crystals

The non-linear interaction of arbitrary polarized light with smectic layer deformations in smectic A liquids (S_A) is considered. It is shown that the combined effect of anisotropy, fluidity and a characteristic kind of deformation cause a number of specific non-linear optical phenomena. Two-wave mixing in S_A transforms into a partly degenerate four-wave mixing (FWM) when the polarization and the direction of propagation of the coupled electromagnetic (EM) waves are arbitrary. The interference of the EM waves gives rise to a dynamic grating of layer deformations without a change of mass density of S_A. In the resonant case a propagating mode of a second sound (SS) is excited. The non-linear phenomena are analysed by solving the self-consistent system of the Maxwell equations for the non-linear anisotropic inhomogeneous medium and the hydrodynamic equations of S_A in the external EM field. The explicit expressions of the EM and SS waves amplitudes are obtained. It is shown that the coupled fundamental EM waves undergo the parametric amplification and the phase cross-modulation, and their amplitudes as well as the SS wave amplitude are spatially localized. The energy transfer between the coupled EM waves is non-reciprocal. The scattering of the fundamental EM waves by the dynamic grating results in the appearance of additional harmonics with combination frequencies and wavevectors. The light induced dynamic grating also generates a longitudinal electric field due to the flexoelectric effect.     

Polar and quadrupolar order in smectic liquid crystals

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.     

Macroscopic properties of mixed chiral smectic liquid crystals

Binary mixtures of chiral and non-chiral smectogens, DOBAMBC and DBHP respectively, exhibit a chiral smectic C phase in two separated concentration regions (about 0-20 and 60-100 mol per cent of DOBAMBC) and a chiral smectic I phase in the region of 60-100 mol per cent of DOBAMBC. For the S*_cphase, the temperature and the concentration dependences of the spontaneous polarization, the tilt angle and the helical pitch were determined. The electric field switching properties of the S*_iphase have been studied. For higher concentrations, the switching is similar to that of the S*_iphase of pure DOBAMBC. When the concentration of DOBAMBC is lowered, bistable switching is observed.     

Energetics of screw dislocations in smectic A liquid crystals

We derive the self-energy of a single screw dislocation in smectic A liquid crystals allowing for bend curvature in the bulk. For the core region two models are investigated: a nematic one including bend and twist curvature and an isotropic one including surface curvature energy. The former is energetically favourable. For both models the interaction force between two parallel screw dislocations is zero within the linear theory. Taking into account non-linearities perturbatively, an interaction potential is obtained, which is proportional to the logarithm of the distance of the screw dislocations.     

Ferroelectric smectic meso-phase formed by banana-shaped achiral liquid crystals

A new banana-shaped achiral molecule, 1,3-phenylene bis[4-(3-chloro-4-n-octyloxyphenyliminomethyl)benzoate] (PBCOB) has been synthesized, and its ferroelectric properties and homeotropic alignment investigated. The presence of a lateral chloro-substituent in the Schiff 's base moiety prevents the regular stacking of molecules and results in lowering the transition temperature and the degree of crystallinity of the switchable banana phase. Their smectic mesophases, including a switchable banana phase B₇, were characterized by differential scanning calorimetry, X-ray scattering and polarizing optical microscopy. Both the left- and right-handed helical domains are spontaneously formed upon cooling from the isotropic liquid to the switchable banana phase B₇. By X-ray study, the smectic phases showed a layer spacing of 38.1 Å, compatible with the end-to-end distance of the molecule with a bent conformation. Significantly, the smectic B7 phase exhibited a periodicity of 292 Å that corresponds to a helical structure with periodicity about 7.5 times 38.1 Å. The spontaneous polarization for PBCOB is about 50 nCcm^-2 and shows a temperature dependence. The ferroelectric lyomesophase of PBCOB showed a ferroelectric electro-optical switching range extending more than 50°C, switchable at room temperature.     

Freedericksz transitions in smectic liquid crystals in annular geometries

This paper investigates the stability of simple static orientation patterns in a sample of smectic liquid crystal confined to a cylindrical annulus, when a magnetic field is applied. Four different arrangements are considered, covering cases where the layer normal is everywhere either radial or axial, and the (orthogonal) magnetic field is either radial, azimuthal or axial. A classification is given of the threshold radii for mechanical instabilities, and of the threshold magnetic fields for Freedericksz transitions for these cases, with strong anchoring at the boundaries.