Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4′,4″-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4',4'-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

Dielectric studies of a ferroelectric liquid crystal

Measurements of the complex dielectric permittivity in the frequency range 30 Hz-13MHz have been made for the chiral smectic C and smectic A phases of the mixture ZLI-3654. In the S*_C phase a large contribution to the electric permittivity coming from the Goldstone mode was found. For the pretransitional region S*_C-S_A, only the soft mode has been detected. It is found that the molecular relaxation originating from the reorientation around the short axis is well separated from the soft mode even in the pretransitional region. The influence of different relaxation modes on the dielectric anisotropy is presented.     

Dielectric, viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M-smectic C

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

Measurement of spontaneous polarization in ferroelectric smectic liquid crystals

This paper presents measurements of spontaneous polarization made on two ester-bridged materials (SCE1 and CE8). The two materials are chosen to be representative of long pitch (SCE1) and short pitch (CE8) materials. The measurements were made by two different methods, Diamant bridge and field reversal; these methods are compared and the results shown to be in good agreement. The variation in results between different samples of these materials is discussed and the results compared (where possible) with the work of other authors. Special attention is paid to the variation which occurs between the published spontaneous polarization measurements of different authors. The reasons for this variation are examined and variation in sample alignment and the determination of the transition temperature by different authors are shown to be major causes.     

The influence of spontaneous polarization on symmetry recovery addressing schemes for ferroelectric liquid crystal devices

Symmetric addressing is investigated in both low and high spontaneous polarization ferroelectric liquid crystal devices. For devices containing low polarization materials the approach is shown to prevent image ghosting. High polarization materials however show a locking effect at a particular grey level. This extends over a range of pulse amplitudes and frequencies, and is explained as an ionic effect. It may limit the exploitation of symmetric schemes to low polarization materials.     

Use of a gelator in a ferroelectric liquid crystal: pitch compensation and nanofibres

A new azobenzene-containing gelator for liquid crystals, AG2, was synthesized and used to prepare a ferroelectric liquid crystal (FLC) gel. The FLC gel shows interesting features. On cooling from the isotropic phase into the N * phase, the dissolved AG2 acts as a chiral dopant and has a compensation effect on the helical pitch of the N * phase. With 0.5 wt% of AG2 in the FLC host, a homogeneous alignment of the FLC molecules is formed in the N * phase, ensuring the bulk alignment in the SmC * phase, even on quenching the mixture from the isotropic phase. This alignment under fast cooling contrasts sharply with the slow cooling rate required for the alignment of a pure FLC. After formation of the bulk alignment, the aggregation of AG2 occurs in the lower temperature SmA or SmC * phases, and the gelator molecules self-assemble into nanometer-sized fibres (about 100nm diameter) that are aligned and located between the smectic layers. As the gelator is microphase-separated from the FLC in the SmC * phase, it exerts little disruption on the electro-optic properties of the FLC cell.     

Spontaneous polarization,tilt angle and dielectric measurements on a ferroelectric liquid crystal

A new ferroelectric liquid crystalline compound 4-(p-butyloxy-2-chloro-tyrosinate)-2-hydroxy-benzylidene-4'-nonylaniline, abbreviated as BCTHBN, was characterized by various techniques. Phase sequence and transition temperature data were obtained by thermal microscopy and DSC. The values of spontaneous polarization, tilt angle, response times and viscosity, along with the wide thermal range of the SmC* phase, make this compound a candidate for applicational studies. Dielectric studies included variation of capacitance with temperature and calculation of activation energies in the soft and Goldstone modes.     

The high frequency dielectric response of a ferroelectric liquid crystal

Abstract

The temperature and frequency dependences of the complex dielectric susceptibility of a ferroelectric liquid crystal near the smectic C*-smectic A phase transition have been calculated using the classical and generalized Landau models. It is shown that although the dielectric response of the S*_C phase consists generally of four modes (soft, Goldstone, and two high frequency polarization modes) only three bands appear in the dielectric loss spectrum of ferroelectric liquid crystals at the S_A–S*_C phase transition. These results are in agreement with recent experimental data.     

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     

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.     

Microscopic origin of spontaneous polarization in ferroelectric SC* liquid crystals

The origin of spontaneous polarization in the ferroelectric smectic C* phase is investigated within a mean-field microscopic model which describes the coupling between the tilt of molecules from the normal to the smectic layers and the rotation of a molecule around its long axis. The mean-field potential is studied which takes into account a chiral polar and a non-chiral quadrupolar biasing of the rotation of molecules around the molecular long axes. Each molecule is characterized by three transverse molecular axes: the chiral axis which turns parallel to the macroscopic C₂ axis at small tilts, the polar axis in the direction of the transverse dipole moment and the quadrupolar axis which tends to be parallel to the C₂ axis at very large tilts. A numerical analysis of the model shows that there are four different types of spontaneous polarization dependent on the temperature, including the sign-reversal type. The influence of three microscopic parameters, i.e. two angles between the three characteristic axes and the relative strength of the chiral versus the non-chiral biasing, on the type of spontaneous polarization is investigated. The relationship between the microscopic and the equivalent Landau model is established and discussed.     

Effect of molecular rotational hindrance on the spontaneous polarization in the S*C phase of a ferroelectric liquid crystal

The spontaneous polarization, P_s, and the tilt angle, θ, have been measured simultaneously in the S*_C phase of binary mixtures of two ferroelectric liquid crystals as a function of temperature and concentration. A molecular rotational model was used in order to calculate the temperature dependence of the P_s/θ ratio. A two parameter fit of the experimental data was carried out which showed good agreement. The molecular rotational potentials have been determined from the fit parameters and are presented as a function of concentration. It is shown that the chiral part of the potential depends very weakly on the composition in contrast to the non-chiral part which shows a strong negative non-linear deviation with a minimum corresponding to the concentration at which the smectic C* phase width is maximal.     

A chiral induced ferroelectric liquid crystal phase transition with a vanishingly small enthalpy

Abstract

Information on molecular interactions that give rise to the stabilization of various ferroelectric, liquid-crystalline mesophases is important to the realization of their potential utility in a wide variety of optical devices. Understanding the roles that optical activity and optical purity play in the formation and properties of the various smectic phases is therefore of particular interest. In order to study this relationship three new liquid-crystalline materials were prepared, one as a racemic mixture, the other two as the optically active analogues. The optically active isomers appear to exhibit a different mesophase morphology from the racemate. The chiral compounds apparently possess two extra ferroelectric mesophases in comparison to the racemic mixture. The transitions to and from these phases have extremely small enthalpies. An attempt is made to explain the results for the chiral compounds in terms of differing dipolar couplings in the chiral ferroelectric phases. In the racemic mixture these interactions are compromised or scrambled by a loss of asymmetry thus destabilizing these extra phases.     

A chiral induced ferroelectric liquid crystal phase transition with a vanishingly small enthalpy

Information on molecular interactions that give rise to the stabilization of various ferroelectric, liquid-crystalline mesophases is important to the realization of their potential utility in a wide variety of optical devices. Understanding the roles that optical activity and optical purity play in the formation and properties of the various smectic phases is therefore of particular interest. In order to study this relationship three new liquid-crystalline materials were prepared, one as a racemic mixture, the other two as the optically active analogues. The optically active isomers appear to exhibit a different mesophase morphology from the racemate. The chiral compounds apparently possess two extra ferroelectric mesophases in comparison to the racemic mixture. The transitions to and from these phases have extremely small enthalpies. An attempt is made to explain the results for the chiral compounds in terms of differing dipolar couplings in the chiral ferroelectric phases. In the racemic mixture these interactions are compromised or scrambled by a loss of asymmetry thus destabilizing these extra phases.     

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH₂- unit between the chiral carbon and the dipole moment.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Abstract

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.