Characterization of the electroclinic effect of smectic LCs in terms of the transitional dielectric constant in the SA phase

Abstract

The electric-field dependence of the dielectric constant of some materials which show a strong electroclinic effect in the S_A phase, has been studied by measurement of the transitional dielectric constant. We suggest that the observed variation of the dielectric constant may originate from the electric field induced dielectric biaxiality; this is inherent in a material that exhibits the electroclinic effect.     

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

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.     

Degeneracy of rotational viscosities at the chiral smectic C-smectic A transition in DOBAMBC

From measurements of the electroclinic effect and the dielectric strength of the soft mode close to the chiral smectic C-smectic A transition we show that the rotational viscosities associated with the Goldstone and soft modes are degenerate at T_c, in agreement with the predictions of the theoretical model.     

Broadband dielectric spectroscopy on ferroelectric liquid-crystalline side group polymers

Broadband dielectric spectroscopy (10^-1 to 10⁷Hz, 100 to 450 K) has been applied, for the first time, to investigate the molecular dynamics of recently synthesized fast switching ferroelectric side group polymers. The softmode could be studied in detail in the S_c* phase and in the S₁* phase. At the transition a pronounced increase of the relaxational strength was found whereas its relaxational frequency remains constant The softmode is the molecular basis for the electroclinic effect which has high application potential for ferroelectric liquid crystal polymers.     

Study of the nematic electroclinic effect in mixtures

The temperature dependence of the nematic electroclinic effect has been studied in binary mixtures prepared from two chiral compounds with nematic-smectic A-smectic C and nematic-smectic C phase sequences. The data obtained show a substantial difference in the magnitude of the effect, not only with the type of phase sequence, but also with the temperature range of the subsequent smectic A phase. These results suggest that short-range smectic fluctuations can play an important role in the nematic electroclinic effect at least when a smectic C phase is close to the nematic. In addition, the dynamic behaviour of the electroclinic effect has been investigated in the compound with the nematic-smectic A transition. As in previous work, an anomaly in the electroclinic response time has been found around the nematic-smectic A transition. This fact is analysed qualitatively assuming two different mechanisms contributing to the electroclinic effect.     

Electroclinic behaviour of polymer doped low molar mass ferroelectric liquid crystals

Recently there has been much interest in the electroclinic effect due to its potential application in high speed optical modulators and grey scale displays. In present materials, however, the electroclinic effect is very dependent on temperature and falls off rapidly with increasing temperature. This limits the useful device operating range. In this paper the electroclinic behaviour of new, side chain polymer doped, low molar mass ferroelectic liquid crystal mixtures is reported. By measuring the rate of change of the electroclinic coefficient with temperature it is shown that the temperature range over which the electroclinic effect exists increases with polymer concentration. Data are also presented on electroclinic response times.     

Ferroelectric, pyroelectric and electro-optic properties of a chiral side chain copolymer with high polarization and a broad C* phase

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.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

Electroclinic behaviour of polymer doped low molar mass ferroelectric liquid crystals

Abstract

Recently there has been much interest in the electroclinic effect due to its potential application in high speed optical modulators and grey scale displays. In present materials, however, the electroclinic effect is very dependent on temperature and falls off rapidly with increasing temperature. This limits the useful device operating range. In this paper the electroclinic behaviour of new, side chain polymer doped, low molar mass ferroelectic liquid crystal mixtures is reported. By measuring the rate of change of the electroclinic coefficient with temperature it is shown that the temperature range over which the electroclinic effect exists increases with polymer concentration. Data are also presented on electroclinic response times.     

Studies of the higher order smectic phase of the large electroclinic effect material W317

Abstract

We present studies of the large electroclinic effect material W317. We found via X-ray scattering, calorimetry and optical observation that quenching from the smectic A* phase results in several higher order phases including an orthogonal (hexatic) smectic with short range in-layer translational order and no interlayer order. We characterize the electroclinic response in the quenched phase, and determine its magnitude and response time as a function of electric field amplitude and temperature.     

Experimental study of a SmA*-SmC*A phase transition

We have performed a detailed experimental study of the electroclinic behaviour in the SmA* phase above a SmA*-SmC*_A phase transition. The tilt angle and polarization were measured as a function of the applied a.c. electric field, and the dielectric constant was obtained under different values of the bias field (0≤E _d.c.≤ 3 V μm^-1). In the region of linear regime, the behaviour observed for this SmA*-SmC*_A phase transition is very similar to the one previously described for SmA*-SmC* phase transitions. The experimental results obtained under high bias field are in good agreement with the predictions of the simple theoretical model considered.     

A study of the electroclinic effect in the smectic A* phase of mixtures with strongly chiral alkoxybiphenyl-phenyl carboxylate dopants

A detailed investigation has been reported of the electroclinic behaviour in the smectic A* phase of eleven mixtures made up of a commercial achiral smectic C host and strongly chiral alkoxybiphenyl-phenyl carboxylate dopants. A new technique was used to measure the induced tilt angle as a function of electric field and temperature. For all the mixtures, the electroclinic response followed a Curie-Weiss type temperature dependence for measurements performed well away from the smectic A* to smectic C* phase transition temperature. The strength of the electroclinic response was evaluated for each mixture by determining the temperature independent ratio k/a (i.e. the electroclinic coupling susceptibility, k divided by the susceptibility coefficient controlling the induced tilt, a). Analysis of the results showed that k/a of the mixtures was dependent on the type and position of the electronegative or polar substituents that affected the net dipole moment of the chiral dopants. In fact, the value of k/a was largest when fluorine was replaced by hydrogen in the lateral position and at the chiral centre. Furthermore, relatively short alkyl chains (e.g. C₆H₁₃) at the chiral centre were preferred to longer ones (e.g. C₁₀H₂₁) for a larger electroclinic response.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

A simple electro-optic technique for studying the electroclinic effect in the smectic A* phase

The critical behaviour of the electroclinic response in the chiral smectic A* phase in the vicinity of the second-order smectic A* to smectic C* phase-transition temperature has been investigated using a new electro-optic technique. The temperature variation of the electroclinic coefficient, the relaxation frequency and the coefficient of the quartic term in the tilt angle in the Landau free energy expansion have been studied. The electroclinic coefficient diverges with decreasing temperature as the smectic A* to smectic C* phase-transition temperature is approached with a critical exponent, as predicted in the mean field Landau theory. The measured quartic coefficient varies strongly with temperature, contrary to the usual assumptions of the mean field Landau theory.     

Chiral periodic mesoporous organosilica in a smectic-A liquid crystal: source of the electrooptic response

Chiral periodic mesoporous organosilica (PMO) materials have been shown to deracemise a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded. In particular, application of an electric field E in the liquid crystal’s smectic-A phase results in a rotation of the liquid-crystal director by an angle proportional to E, which is detected optically – this is the so-called ‘electroclinic’ effect. Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid-crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids. Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level.     

Kinetics of the solvolysis of the trans-dichlorobis(1,2-diaminoethane)-cobalt(III) Ion in water + urea mixtures

Following the kinetic investigation of the solvolysis of a range of cobalt(III) complexes in mixtures of water + cosolvent wherethe cosolvent enhances the solvent structure and decreases the dielectric constant, kinetic data are now reported for such a solvolysis in water + urea where urea acts as a structure breaker and enhances the dielectric constant. A plot of log (rate constant) against reciprocal of the dielectric constant shows that differential effects of changes in solvent structure occur between the initial and the transition states and, as in theinvestigations using structure-enhancing cosolvents, the principal effect of change in solvent structure on the cobalt(III) cation occurs on the penta-coordinated ion in the transition state.     

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.     

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.     

Studies of the alignment properties of antiferroelectric liquid crystals by X-ray diffraction

It is observed optically that in a parallel rubbed antiferroelectric liquid crystal device, the texture consists of domains with two distinct optic axes, which make equal and opposite angles with the rubbing direction. It is proposed that this is caused by a large electroclinic effect at the surfaces during layer formation in the SmA* phase. This hypothesis is verified by finding the layer structure in single, parallel and skew rubbed devices by using X-ray diffraction.