Ferroelectric properties of smectic C liquid crystals with induced helical structure

Ferroelectricity has been observed in liquid crystalline mixtures of chiral smectic C type with a helical structure induced by chiral guest molecules (pitch p ≈ 100 μm). Samples of thickness d?p form monodomains with finite zero-field electric polarization because the spontaneous polarization of the smectic layers is not compensated by the helical twist. The temperature dependence of the spontaneous polarization has been studied near the phase transition smectic C→ smectic A.     

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.     

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.     

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.     

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.     

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.     

A smectic C*-smectic A tricritical point in a mixture of two compounds exhibiting high spontaneous polarization

X-ray diffraction, D.S.C. and miscibility studies have been performed on inary mixtures whose constituent compounds exhibit large values of spontaneous polarization. These studies have led to the observation of a tricritical point for the smectic C*-smectic A transition. The origin of this tricritical behaviour is discussed in relation to the range of the smectic A phase and the large spontaneous polarization.     

A smectic C*-smectic A tricritical point in a mixture of two compounds exhibiting high spontaneous polarization

X-ray diffraction, D.S.C. and miscibility studies have been performed on inary mixtures whose constituent compounds exhibit large values of spontaneous polarization. These studies have led to the observation of a tricritical point for the smectic C*-smectic A transition. The origin of this tricritical behaviour is discussed in relation to the range of the smectic A phase and the large spontaneous polarization.     

Preliminary communication Dispersion of the optical axes in smectic C* liquid crystals

Optical axes dispersion denotes the dependence of the orientation of the indicatrix and the optical axes on the wavelength of light. Theory predicts optical axes dispersion in optically biaxial phases with low crystallographic symmetry, like the C 2h-symmetry of SmC or the C2 symmetry of SmC* phases. The magnitude of this axes dispersion was measured electrooptically for two liquid crystal materials exhibiting SmC* phases using several wavelengths of light. Far below the phase transition temperature, the wavelength dispersion of the optical tilt is about 1-2 (5-10% of the total tilt) over the range of visible wavelengths.     

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical     

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.     

Optical observations of layer ordering in helial smectic A* liquid crystals

We report the first optical microscopic observation of characteristic textures and the associated layer ordering in a recently discovered, helical smectic A* phase on a multiple domain substrate. The experimental results are quite unusual but consistent with a mean-field model of the twisted grain boundary phase.     

Backflow effects in the Goldstone mode dielectric response of chiral smectic C* liquid crystals

Employing the elastic-hydrodynamic theory of the ferroelectric smectic C* phase, it is easily seen that in most dynamic processes in the system, a coupling between director rotations and macroscopic mass flow exists. Taking this possibility of backflow into account, the equations governing the Goldstone mode dielectric susceptibility are rederived, and it is shown that the corresponding rotational viscosity, measured in a conventional dielectric experiment, is renormalized due to backflow effects. An expression for this renormalization is given and the crucial parameter, determining whether backflow effects are of importance or not in the dielectric experiment, is derived.