F. Gouda

Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals

Abstract Different contributions to the dielectric permittivity in ferroelectric liquid crystals are discussed, with emphasis on the soft mode and the Goldstone mode and their location in the dielectric spectrum. Experimentally, the complex dielectric permittivity has been studied as a function of temperature and frequency in the range 5 Hz - 13 MHz for three different ferroelectric liquid crystal materials. The main problems encountered in dielectric measurements at low and high frequency are discussed in some detail. The soft mode dielectric behaviour has been studied as a function of temperature, frequency and bias electric field. The applicability of the Curie-Weiss law for the soft mode dielectric contribution in the A* phase was analyzed. In the C* phase the temperature dependence of the dielectric contribution of the Goldstone mode has been measured. By applying a bias electric field, we have been able to study the soft mode dielectric behaviour also deep into the C* phase. In the A* and C* phases ...

The molecular aspect of the double absorption peak in the dielectric spectrum of the antiferroelectric liquid crystal phase

Abstract The dielectric spectrum of the antiferroelectric smectic C∗ phase exhibits a low and a high frequency absorption peak (Pl, Ph) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A∗ phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C∗α (82·80° to 81·91°C) and C∗γ (80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δ∊ for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E c corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E c the PH peak vanishes and the PL peak shows up at a frequency s...

The case of thresholdless antiferroelectricity: polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.

Viscoelastic Properties of the Smectic

A method for the determination of rotational viscosity and elasticity associated with the director rotation on the C* cone is proposed. The method is based on measurements of the Goldstone mode dielectric strength and relaxation frequency, supplemented with values for spontaneous polarization, tilt angle and pitch. Furthermore, the temperature dependence of the soft mode dielectric strength and relaxation frequency has been measured in the A* and C* phases. From the measurements in the A* phase we have evaluated the temperature dependence of the soft mode rotational viscosity which was found to exhibit a critical behaviour of the form γθ≈(T-Tc)-0.25.

A^{*}

Abstract The compound shows the unusual behaviour of sign reversal in the spontaneous polarization P versus temperature. This was first reported by Goodby, Chin, Geary and Patel (1). We have now a fairly complete set of data concerning polarization, tilt angle, optical response time, helical pitch and dielectric constant. These parameters are discussed, in particular concerning their temperature behaviour, in order to elucidate the mechanism behind the polarization reversal and its physical significance.

and

Abstract The influence of a bias electric field on the temperature and frequency dependence of the soft mode part of the complex dielectric constant of the chiral smectic A* phase has been studied experimentally. It was found that only close to T S*CS*A does the bias field have any influence on the experimentally determined quantities. Here the relaxation frequency increased with increasing bias field, while the corresponding dielectric strength decreases. We also present a theoretical calculation of the influence of a bias field on the complex dielectric constant of the chiral smectic A* phase which, apart from minor details, is in accordance with the observed behaviour. Finally we discuss how the nature of the electroclinic effect can be correlated with the influence of the bias field on the smectic A* dielectric constant.

C^{*}

Abstract The synthetic route to a chiral side chain copolymer exhibiting broad smectic phases is described. The copolymer shows a smectic C∗ phase from below room temperature up to 105°C. Ferroelectric properties of this copolymer are reported as well as results showing electroclinic switching. The pyroelectric effect and the dielectric behaviour of the copolymer are also demonstrated.

Phases Studied by a New Dielectric Method

Using a variety of optical and electro-optical techniques, as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of the unbranched terminal chain. The main focus of the study is the series of smectic C subphases, i.e. SmC* α, SmC* 1/3 and SmC* ¼. During switching in the SmC* α phase, a peculiar redirection of the plane of biaxiality, distinguishing this phase from SmA* and SmC*, was observed. We present a simple explanation for this behaviour which correlates well with the clock model description of the SmC* α phase. We found a zero mesoscopic polarization for the SmC* ¼ phase and a non-zero mesoscopic polarization for SmC* 1/3, observations which are consistent with a distorted clock model. The dielectric spectroscopy investigations, performed at several different cell gaps, clearly show that the dielectric response in these materials is easily dominated by surface-induced structures if the cell gap is reduced, thus reflecting that the bulk thermodynamic phase exists in very thick cells only.

Parameter characteristics of a ferroelectric liquid crystal with polarization sign reversal

Measurements of the relevant anisotropic part of the dielectric constant for ferroelectric liquid crystals allows a calculation of the dielectric torque acting upon the director during addressing conditions in a matrix‐multiplexed device. The frequency dependence of the dielectric anisotropy is presented for different commercially available ferroelectric liquid‐crystal materials in the range 103–106 Hz. The field dependence of the switching time has been measured for one of the materials in order to determine the field at the minimum value. The ratio between this field and the field at which switching ceases has been determined and found to be in good agreement with the theoretically calculated value. The results enable us to consider the torque balance as a function of field and frequency, and thereby identify appropriate driving conditions.

Experimental and theoretical study of the influence of a bias electric field on the dielectric properties of the chiral smectic A* phase

Abstract The complex dielectric permittivity of the soft mode in the A * phase and the Goldstone mode in the C* phase has been studied as a function of temperature and frequency. Using simple evaluation models, the soft mode and the Goldstone mode rotational viscosities have been determined in the A * and C* phases respectively. The models enabled us to calculate values of the elastic constant of the Goldstone mode. A correction of the usually measured dielectric anisotropy in the C* phase is also presented.