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 near a smectic A-chiral smectic I transition

The electroclinic effect in the S_A phase of (S) (4'-decyloxy-2'-hydroxybenzylidene)-4-amino(1-ethoxycarbonylethyl) cinnamate has been studied. The induced tilt angles have been measured as a function of temperature near the S_A-S*_I transition at 87°C. The response of the material to different voltages and frequencies has been analysed by using two complementary optical techniques. At low A.C. probe fields the amplitude of the induced tilt shows a relaxational behaviour which can be described by a single relaxation time. The relaxation frequencies have been found to be linear in temperature near the transition, in a similar way to the S_A-S*_C transition.     

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.     

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.     

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 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.     

Relaxation phenomena in PPG model networks near the glass-rubber transition

The viscoelastic and dielectric properties of polypropyleneglycol (PPG)-tris-(4-isocyanatophenyl)thiophosphate (Desmodur^®) model networks have been studied. Attention was focused on the relaxation behaviour near the glass transition. Two relaxation mechanisms were observed in the glass transition range, one due to the PPG chain segments and one due to the less mobile cross-link moieties, i.e. the networks show motional heterogeneity. Mechanically only the first mechanism was observed while dielectrically both mechanisms were found. This motional heterogeneity has a profound effect on the dependence of the glass transition temperature on the composition of these networks. The effect of the stage of cure on the temperature of maximum loss and the magnitude of these two relaxation mechanisms was studied dielectrically. Finally, the effect of exchanging the cross-linker molecules with chain extender molecules with similar structure, thus lowering the cross-link density, was studied.     

Giant surface electroclinic effect in a chiral smectic A liquid crystal

We report the observation of a very large surface electroclinic effect in the smectic A* phase of a chiral liquid crystal. In planar-aligned cells of enantiomerically pure W415, the smectic A* phase grows in from the isotropic state with the layer normal rotated ψ = - 24° from the rubbing direction, a consequence of the surface electroclinic tilt θ_s of the director. The sign of θ_s depends on the molecular handedness, with θ_s ≡ 0 in the racemate, and increasing linearly with moderate enantiomeric excess before saturating as ee → 1. A uniform layer structure can be achieved using cross-rubbed alignment layers, in which case thin cells of W415 in the smectic C* phase display V-shaped (analogue) electro-optic switching.     

A study of the electroclinic effect using half-leaky guided modes with a homeotropically aligned liquid crystal

High resolution voltage dependent tilt angle studies using optical excitation of half-leaky guided modes have been conducted on a homeotropically aligned ferroelectric liquid crystal mixture (Merck SCE13) in the S_A phase. Uniform homeotropic alignment is realised, with no surface aligning layer, by the application of an in-plane DC electric field when the liquid crystal is in the S_C* phase. The applied field unwinds the pitch of the S_C* chiral helix and gives a uniformly tilted homeotropic monodomain. On warming into the S_A phase, detailed studies of the voltage induced tilt, the electroclinic effect, are then conducted at various temperatures. Because there is no influence of surface anchoring forces, the linear relationship between the induced tilt angle and the DC field is obtained even under very weak fields. Further, the relationship between induced tilt and temperature confirms the predictions of a second order Landau mean-field theory with a coupling term between the tilt angle and the DC field.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

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.     

Evidence of electroclinic effect in some TGBA mesophases

Two members of a chiral tolane series exhibiting a new TGBA phase and a S*_C ferroelectric phase have been investigated by means of electro-optical measurements. These materials belong to the optically active series of (R)-4-(1-methylheptyloxycarbonyl)-4'-(4'- alkyloxy-3'-fluorobenzoyloxy) tolanes and are the tetra- and octa-decyloxy derivatives. Due to the large pitch values in these TGBA mesophases, complete unwinding of the helix is found to be possible in thick cells. This also allows measurements of the electroclinic properties, such as the electric field induced tilt angle, response time, and the ferroelectric properties in the S*_C phase.     

Linear optical switching in a FLC/waveguide composite device

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

Fast switching ferroelectric side-chain liquid-crystalline polymer and copolymer

New ferroelectric side-chain liquid-crystalline polymers, a copolymer and a homopolymer, with siloxane backbone and a triaromatic mesogen as the side group have been synthesized. The materials exhibit a chiral smectic C phase over a large temperature range extending to room temperature. They possess high values of spontaneous polarization: 105 nC cm^-2 for the homopolymer and 180 nC cm^-2 for the copolymer. The electro-optic switching time in the chiral smectic C phase is extremely fast (150 μs). In the smectic A phase, an electroclinic effect with switching times less than 100 μs and with field induced tilt angles of 18° is observed.     

Electroclinic properties of chiral smectic a liquid crystalline polymers and block copolymers

The scope for the synthesis and investigation of chiral smectic A liquid crystalline (LC) polymers and block copolymers is discussed. LC block copolymers can combine the molecular order of liquid crystals and the supramolecular order typical of block copolymers, thus allowing to attain information on mesomorphic responses in restricted geometries. We present a general overview on several aspects of the syntheses and properties of LC block copolymers, specifically those obtained by starting from azomacroinitiators. These materials can exhibit an electroclinic effect, that is an electrically induced molecular tilt, which is characterized by a linear dependence on the applied field and a very fast response time in the paraelectric smectic A phase. The current progress in their potential application in electrooptics is outlined.     

Linear optical switching in a FLC/waveguide composite device

Abstract

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

Electroclinic effect in the chiral smectic A and cholesteric phases at the proximity of a N*–SmA–SmC* multicritical point

We studied the electro-optic and dielectric properties of three pure ferroelectric liquid crystal materials (C10, C11 and C12) of the same series exhibiting cholesteric (N*), smectic A (SmA) and chiral smectic C (SmC*) phases. From electro-optic investigations, the tilt angle and spontaneous polarisation were determined as a function of temperature. In the dielectric measurements carried out without a dc bias field, we studied the soft-mode relaxation in the SmA phase. From experimental data and using the results of a Landau model, we evaluated the soft-mode rotational viscosity and the electroclinic coefficient in the SmA phase. A soft-mode like mechanism was also observed in the N* phase for compounds with shorter chains (C10 and C11). This relaxation process is not detected for the homologue with a longer chain (C12). The observation of this mechanism is related to smectic order fluctuations within N* phase whose amplitude is increased when approaching the SmC*–SmA–N* multicritical point.