Influence of the thickness and polarization charges on the electro-optical behaviour of twisted smectic C* cells

This paper reports on the influence of both polarization charges and thickness on the light transmitted by twisted smectic C* (TSC) liquid crystal (LC) cells. A uniaxial model, based on the de Vries eigenmodes usually used for twisted structures like cholesteric LCs, is used to calculate the transmission variation of the TSC cells as a function of applied voltage or thickness for various values of spontaneous polarization. The results obtained are in quantitative agreement with experimental results found in the literature and emphasize the importance of polarization charges on the behaviour of TSC cells. The TSC light transmission exhibits a profile resembling the so-called V-shaped switching.     

On the thickness dependence of ferroelectric polarization in smectic C* liquid crystal cells

The thickness dependence of ferroelectric polarization in a SmC* liquid crystal exhibiting a large value of polarization has been investigated. It is observed that the saturation value of the polarization shows no significant thickness dependence. This observation is in accordance with a recent theoretical framework developed by Galerne. It is shown that the wall thickness, where the polarization undergoes a reversal, and hence the self-screening effect, is insignificant in the material studied.     

On the thickness dependence of ferroelectric polarization in smectic C* liquid crystal cells

The thickness dependence of ferroelectric polarization in a SmC* liquid crystal exhibiting a large value of polarization has been investigated. It is observed that the saturation value of the polarization shows no significant thickness dependence. This observation is in accordance with a recent theoretical framework developed by Galerne. It is shown that the wall thickness, where the polarization undergoes a reversal, and hence the self-screening effect, is insignificant in the material studied.     

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     

Bistable electro-optical switching in the smectic I* phase of a ferroelectric liquid crystal

The ferroelectric switching behaviour of the highly ordered smectic I* phase has been investigated in the mixture which shows a S*_I phase at room temperature. The bistability was obtained in a 3.5μm thick cell. Director switching and the reorientation processes have been studied by applying symmetric square and triangular wave pulses. It has been found that an asymmetric switching occurs in the smectic I* phase for low electric fields due to the hexagonal ordering of the molecules in the layer. This asymmetric switching was confirmed by optical microscopy and four stable states have been observed for low electric field. For higher electric fields only one state is stabilized which results in symmetric switching by both methods in the smectic I* phase.     

Novel structures of a smectic C* phase with high spontaneous polarization in free-standing films

New stable structures of the liquid crystalline smectic C* phase are observed in free standing films of a material with high spontaneous polarization. A stable configuration of thick films is a stripe of state with an in-plane rotation of the director. In ultra-thin films an anisotropic state was observed. The director fields of the observed structures are modelled and compared with theoretical predictions.     

Dielectric relaxation spectroscopy and electro-optical studies of phase behaviour of a chiral smectic liquid crystal

Dielectric relaxation processes in various phases of a chiral smectic liquid crystal compound have been investigated over a wide range of frequencies from 10 to 300 kHz. Dielectric spectroscopy reveals the existence of two relaxation frequencies corresponding to the ferroelectric and ferrielectric Goldstone modes in the range of temperature where two different phases coexist. This phenomenon of coexistence is examined with respect to the cell thickness and reversal temperature.     

Chaotic transients in surface-stabilized smectic C* cells induced by magnetic field

Recent theoretical developments regarding the understanding of weakly chaotic transients in ferroelectric liquid crystals (FLCs), induced by electric field, are studied in terms of the interaction with magnetic field. Our research is related with the nonlinear dynamical system represented by a thin film of surface-stabilized FLC in smectic C* phase, and subjected by the swinging magnetic field. The computation of the Lyapunov exponents from the dynamic equation for the director field reveals that the director dynamics exhibits limit cycle, hyperchaotic attractor and strange attractor behavior in the dissipative nonlinear media. The transients between director’s phase space trajectories can be handled by the magnetic field parameters. The fundamental understanding of the director dynamics may have a valuable contribution to the applications of thin liquid crystal films.     

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.     

Synthesis of new smectic C* liquid-crystalline block copolymers

A series of smectic C^* liquid-crystalline (LC) block copolymers were successfully synthesized via the living anionic polymerization of polystyrene with optically active methacrylate monomers containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl-4′-carboxylate mesogens. These materials are the first reported smectic C^* block copolymers. Anionic polymerization in tetrahydrofuran (THF) at −70°C leads to LC block molecular weights of approximately twenty repeating units. The number-average molecular weight of the polystyrene block was varied from 7000 to 20000 to adjust the composition in the block copolymers. Differential scanning calorimetry and optical microscopy indicate that the smectic C^* phase is present in the systems over broad temperature ranges.     

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.     

Structural and electro-optical investigations of the smectic phase of chlorine-substituted banana-shaped compounds

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching     

Average band structure of smectic C* type random materials

We consider a Smectic C* type structure which exhibits alignment fluctuations which can be described by noise associated with both director angles. We take the propagation of axially incident electromagnetic waves, and from Maxwell equations, we establish the stochastic governing set of equations corresponding to optical phenomena. We note that this set of equations can be expressed as a linear vector stochastic system of differential equations with multiplicative noise. We use a procedure to calculate from the stochastic differential set of equations, the governing equations for the expected value of the electromagnetic transverse magnetic and electric fields for a certain autocorrelation function, and calculate explicitly their corresponding band structure for a particular spectral noise density. We have shown that the average resulting electromagnetic fields exhibit a biased decaying exponential dependence which impedes to propagate the waves in one sense while it permits them in the other sense. We have also found a remarkable widening of the band gap and the appearance of new local maxima for the modes without band gap.     

The effect of molecular chirality on the incidence of twisted smectic A phases

Four chiral chloro-esters were synthesized in order to examine systematically the effect that molecular chirality has on the temperature range and incidence of the twisted smectic A phase or the twist grain boundary phase as it has become known. We find that when the motion of the chiral centre is restricted by rotational hindrance, thereby increasing the chirality of the system, the twisted S_A phase range is increased. At low chirality the twisted S_A phase disappears altogether. Furthermore, it is shown that the transition temperatures of the chiral compounds are lower than their racemic analogues.     

CdSe quantum dots in chiral smectic C matrix: experimental evidence of smectic layer distortion by small and wide angle X-ray scattering and subsequent effect on electro-optical parameters

CdSe quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC) has been subjected to small and wide-angle X-ray scattering and atomic force microscopy to understand the molecular organization in chiral smectic C (SmC*) phase. SAXS indicates that the presence of QDs causes enhancement in the smectic layer separation. The smectic order parameter for neat FLC and FLC–QDs mixtures is obtained in the range of 0.6 to 0.85. Both smectic order parameter and structural tilt are found to be lesser for FLC–QDs mixtures as compared to neat FLC. The insertion of QDs in SmC* matrix causes localized smectic layer distortion in such a way that spontaneous polarization remains almost the same but the electro-optic switching of molecules becomes faster. We have outlined the superiority of FLC–QDs mixtures for electrical energy storage and their suitability in electronic devices.     

Unusual electro-optical behaviour of the nematic polyacrylate

ABSTRACT

High sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties.

The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated.     

Shear flow and magnetic field effects on smectic C,C*, CM and C*M liquid crystals

Abstract

The continuum equations of Leslie et al. [1] for smectic C, and the extension of this theory for chiral smectic C* [2], are applied to problems involving simple planar layer configurations which accommodate uniform layer thickness constraints. The chiral smectic C*_M and non-chiral smectic C_M [3] are considered as either biaxial smectic A phases or antiferroelectric smectic C phases and are therefore included as interesting degenerate cases of the smectic C* and C phases, respectively. The effects of static and time dependent magnetic fields on these materials are compared with related deformations occurring in nematics [4] and cholesterics [5,6]. Their reaction to applied shears is also investigated yielding examples of flow alignment, induced secondary flows and unwinding of the chiral helix and testing the validity of enforcing a constant layer thickness.