Phase Of Ferroelectric Liquid Crystal Research Articles

Solid-Liquid Crystal Biphasic Ferroelectrics with Tunable Biferroelectricity.

Ferroelectricity has been separately found in numerous solid and liquid crystal materials since its first discovery in 1920. However, a single material with biferroelectricity existing in both solid and liquid crystal phases is very rare, and the regulation of biferroelectricity has never been studied. Here, solid-liquid crystal biphasic ferroelectrics, cholestanyl 4-X-benzoate (4X-CB, X = Cl, Br, and I), which exhibits biferroelectricity in both the solid and liquid crystal phases, is presented. It is noted that the ferroelectric liquid crystal phase of 4X-CB is a cholesteric one, distinct from the ordinary chiral smectic ferroelectric liquid crystal phase. Moreover, 4X-CB shows solid-solid and solid-liquid crystal phase transitions, of which the transition temperatures gradually increase from Cl to Br to I substitution. The spontaneous polarization (Ps ) of 4X-CB in both solid and liquid crystal phases can also be regulated by different halogen substitutions, where the 4Br-CB has the optimal Ps because of the larger molecular dipole moment. To the authors' knowledge, 4X-CB is the first ferroelectric with tunable biferroelectricity, which offers a feasible case for the performance optimization of solid-liquid crystal biphasic ferroelectrics.

Weak anchoring resolved substrate interface and bulk mode processes in surface stabilized ferroelectric liquid crystal

Dielectric spectroscopy is employed to investigate the behaviour of bulk mode (phason mode, or Goldstone mode (GM) in ferroelectric liquid crystals (FLCs)) and surface phenomenon (partially unwound helical mode) in surface stabilized FLC (SSFLC) sample cells. The surface effect influences the basic characteristic of GM phenomenon to a great extent, even if the surface anchoring energy of the cell is kept very weak. The effect of different measuring and bias fields on the GM and surface processes has been studied in smectic C* (SmC*) phase and also at transition temperature (Tc (SmC* to SmA*)) of SmC* and smectic A* (SmA*) phases. It has been observed that in surface stabilized geometry, there exists two molecular dynamics: one near the surface of the electrodes and another in the bulk of the cell which contributes separately in the dielectric properties. The phason mode is also observed at Tc (SmC* to SmA*) and a few degrees above Tc (SmC* to SmA*) i.e. in SmA* phase of FLC in thin cell of surface stabilized geometry. The application of weak and medium measuring electric fields reverses the varying trend of GM frequency process at the phase transition from SmC* to SmA* phase. The study is intended to explore the surface effect along with the surface and bulk interface that influence the bulk response in SSFLC state.

Dielectric and electro-optical studies of Au/SnO2 core/shell nanocrystals incorporated ferroelectric liquid crystal

ABSTRACT We study the effect of Au/SnO2 core/shell nanocrystals (NCs) on dielectric and electro-optical parameters of a ferroelectric liquid crystal (FLC) material, namely KCFLC 7S. The FLC composites are characterised through polarising optical microscope (POM), photoluminescence (PL) spectroscopy and dielectric spectroscopy. Incorporation of Au/SnO2 NCs enhances the optical contrast, optical tilt angle, PL intensity and reduces saturation voltage of a FLC mixture. This modification in electrical and optical properties is attributed to change in dynamics of FLC molecules in presence of the NCs. Dielectric measurement shows that chiral Smectic C (SmC*) phase of FLC is dominated by an applied oscillating field dependent partially unwound helical mode (p-UHM) process in FLC composite with Au/SnO2 NCs. These results would be useful to fabricate power efficient and highly luminescent NCs-FLC-based electro-optical devices.

Wavelength and bandwidth control of stop band of ferroelectric liquid crystals by varying incident angle and electric field

We investigate the optical properties of the chiral smectic C phase of ferroelectric liquid crystals (FLCs) having a self-assembling helical structure that is regarded as a one-dimensional pseudo-photonic crystal. FLCs have several stop bands corresponding to full-pitch, half-pitch, and their high order bands under oblique incidence. It is expected that the center wavelength and bandwidth of the full-pitch band can be controlled by two different parameters: incident angle and electric field. By varying the incident angle and electric field, we have demonstrated independent control of the bandwidth and center wavelength of the full-pitch band.

Fast switching beam steering based on ferroelectric liquid crystal phase shutter and polarisation grating

ABSTRACTWe report on fast-switching and high-efficient optical beam steering based on a polymerisable liquid crystal polarisation grating (PG) in combination with ferroelectric liquid crystal (FLC) phase shutter. The PG was fabricated in a convenient single-step holographic exposure process using photo-sensitive azo-dye material as alignment layer for liquid crystal (LC) director. A binary electro-optical FLC was employed for circular polarisation selection, which enables the electro-tunable steering of the combined system. The efficiency of 95.7% with 82 μs switching time is obtained for 1064 nm laser. This work provides a versatile candidate for non-mechanical beam steering devices.

The design of smectic liquid crystals with an axially chiral biphenyl core: in search of a proper ferroelectric liquid crystal phase

A tricarbosilane-terminated mesogen with an axially chiral biphenyl core forms a ferroelectric liquid crystal phase.

Monte Carlo Simulations of Spontaneous Ferroelectric Order in Discotic Liquid Crystals

The demonstration of a spontaneous macroscopic ferroelectric order in liquid phases in the absence of any long-range positional order is considered as an outstanding problem of great fundamental and technological interest. We report here off-lattice MonteCarlo simulations of a system of polar achiral disklike ellipsoids which spontaneously exhibit a novel ferroelectric nematic phase which is a liquid in three dimensions, considering attractive-repulsive pair interaction suitable for the anisotropic particles. At lower temperature, the ferroelectric nematic phase condenses to a ferroelectric hexagonal columnar fluid with an axial macroscopic polarization. A spontaneous ferroelectric order of dipolar origin is established here for the first time in columnar liquid crystals. Our study demonstrates that simple dipolar interactions are indeed sufficient to produce a class of novel ferroelectric fluids of essential interest. The present work reveals the structure-property relationship of achieving long searched ferroelectric liquid crystal phases and transitions between them, and we hope these findings will help in future development of technologically important fluid ferroelectric materials.

Wavelength and bandwidth tunable photonic stopband of ferroelectric liquid crystals

The chiral smectic C phase of ferroelectric liquid crystals (FLCs) has a self-assembling helical structure which is regarded as a one-dimensional pseudo-photonic crystal. It is well known that a stopband of a FLC can be tuned in wavelength domain by changing temperature or electric field. We here have demonstrated an FLC stopband with independently tunable wavelength and bandwidth by controlling temperature and incident angle. At highly oblique incidence, the stopband does not have polarization dependence. Furthermore, the bandwidth at highly oblique incidence is much wider than that at normal incidence. The mechanism of the tunable stopband is clarified by considering the reflection at oblique incidence.

Polymer-induced improvements in ferroelectric liquid crystal

The chevron geometry in the SmC* phase of ferroelectric liquid crystals (FLC) has been a major obstacle in the use of FLC in displays as it results in poor electro-optical performance. The present paper reports a novel method to overcome this problem, by doping a small amount of polymer in the FLC matrix. In addition to the improvement in smectic ordering, polymer doping is also found to be useful in improving the vital electro-optical properties. The various electro-optical parameters like switching time, tilt angle, contrast ratio etc. show improvement in polymer mixed guest host mixture of FLC samples. POLYM. COMPOS., 31:1776–1781, 2010. © 2010 Society of Plastics Engineers.

Theoretical approximation study on the helix structure of ferroelectric liquid crystal

The expressions of average refractive index in a helix pitch of ferroelectric liquid crystal（FLC） were derived by theoretical approximation computation of the helix structure at chiral smectic C phase of FLC. A conclusion that FLC molecules in a helix pitch as a whole can be considered as a model of nematic liquid crystal in terms of the expressions of average refractive index was obtained. When the thickness of FLC with a vertical alignment along the helical axis is integer times long as the helix pitch, FLC molecules can be considered as a group composed of nematic liquid crystal models. The comparisons of experiment results of ZLI-3654 FLC with 5CB nematic liquid crystal confirmed the above conclusion. The theory and experiment are in good agreement. Theoretical guidance and a deeper understanding can be provided by this theory for preparation of deformed helix FLC and vertical alignment deformed helix FLC devices, as well as application of ferroelectric liquid crystal.

Stable field-induced ferrielectric liquid crystal phases in devices

The field-induced transitions between ferri-, antiferro-, and ferroelectric liquid crystal phases are interesting because although there are only small thermodynamic differences between them, each of these phases has different electrical and optical properties. We report an irreversible field-induced transition from an antiferroelectric phase to the ferrielectric phase in a liquid crystal device, and compare it to a system in which the transition is reversible. The two systems differ mainly in their spontaneous polarization (120 nC cm−2 for the former and 60 nC cm−2 for the latter) while the optical tilt is comparable (29° and 25°, respectively). We explain the observed transitions based on the relative magnitudes of the discrete flexoelectric and spontaneous polarizations.

Non-linear dielectric effect in the isotropic phase of ferroelectric liquid crystals

We use a phenomenological model to find theoretically the non-linear dielectric effect (NDE) in the isotropic phase above the isotropic to chiral smectic-C ( I-SmC * ) phase transition. We find an analytical expression for the NDE in the isotropic phase above the I-SmC * phase transition. The temperature dependence of the NDE is presented in the isotropic phase of the I-SmC * phase transition.

Structural and dynamical properties of the chiral smectic C phase of ferroelectric liquid crystals showing high spontaneous polarizations

Electro-optical and dielectric investigations of a ferroelectric liquid crystal (FLC) exhibiting the chiral smectic C phase (SmC*) have been carried out as a function of temperature. These experimental studies have been applied to a new FLC having high spontaneous polarization and relaxation frequency. The Goldstone relaxation mode was studied from the dielectric response of FLC cells without DC bias voltage. The rotational viscosity corresponding to molecular motion in the SmC* phase was also determined from both electro-optical and dielectric measurements. An Arrhenius-type behaviour of the rotational viscosity was obtained and the corresponding activation energies were evaluated.

Antiferroelectric and ferroelectric switching of a liquid crystal in Langmuir-Blodgett films under strong confinement conditions

An antiferroelectric liquid-crystal (LC) material formed of banana-shaped molecules in the sandwich metal-thin Langmuir-Blodgett film-metal geometry, which is typical of solid-state technology, is investigated. Upon heating the thin-film elements, the material passes to the high-temperature (68–127°C) LC B 2 phase, which, despite severe limitation on the film thickness (∼100 nm or less), exhibits antiferroelectric switching, which was previously observed only in bulk samples (10 μm thick) of the LC studied. At film thicknesses smaller than ∼40 nm, ferroelectric switching is observed, which is caused by the stabilization of the ferroelectric LC phase by the boundary surfaces. The largest values of the switched polarization of films (∼400 nC/cm2) are comparable with the polarization of bulk samples. The coercive field increases with decreasing film thickness and reaches ∼106 V/cm for the thinnest films. This value corresponds to the intrinsic coercive field of the ferroelectric under conditions of a strong surface effect, which suppresses domain processes.

Memory effect in smectic-A phase of ferroelectric liquid crystal

Memory effect has been observed in smectic-A (Sm-A) phase of ferroelectric liquid crystals (FLCs) using dielectric spectroscopy and texture observation. The dielectric relaxation processes in FLC materials with wide range of Sm-A phase and a high electroclinic coefficient have been investigated near the phase transition of Sm-C*-Sm-A phase in thin and planarly aligned cells in the frequency range 10Hz–10MHz. Here phason mode (Goldstone mode) and soft mode contribution in dielectric permittivity could not be clearly separated, near and slightly above the transition temperature Tc. If there is a contribution of phason mode after Tc in Sm-A phase then there is a possibility of getting memory even in Sm-A phase where the molecules get clamped in one direction and stay there even when the field has been removed. A comparative analysis of dielectric permittivity as well as texture in Sm-C* phase has been done on a virgin cell and on a cell in which bias has been applied in Sm-A phase just 1–2°C above Tc for 1min. The difference in dielectric permittivity and texture in the two different conditions clearly shows the continuance of memory effect from Sm-A phase to Sm-C* phase. The effect is probably due to local azimuthal variation in a finite range of Sm-A phase just above Tc.

Molecular Reorientations in the Ferroelectric Liquid Crystal with Solid Rotatory Phase

Broadband dielectric spectra of a ferroelectric liquid crystal which exhibits a solid rotatory phase are discussed. It is the first case, when the existence of such the mobile phase of ferroelectric liquid crystal is observed.

Polymerization of polymer/ferroelectric liquid crystal composites formed with branched liquid crystalline bismethacrylates

Polymerization and phase behaviour of a branched liquid crystalline bismethacrylate in a ferroelectric liquid crystal (FLC) were characterized. Addition of the monomer increases the temperature range of the smectic A phase, and, at relatively low concentrations of monomer, the temperature range increases to more than 10 times that observed in the neat FLC. Other phases such as the smectic C* phase are no longer exhibited as the monomer interferes with the inherent tilt of the FLC molecules. After polymerization, the polymer network phase separates and the phase transition temperatures return to values close to those of the FLC. The monomer also shows a high degree of orientational order before polymerization, which is retained to a large extent after polymerization. The order in the polymer network results in considerable birefringence at temperatures well above the clearing point of the pure FLC. This behaviour is induced by the order of the polymer network and interactions of the FLC molecules with the network which prevent a fraction of the FLC molecules from exhibiting typical phase behaviour. In the formation of the branched liquid crystalline bismethacrylates/FLC composites, a steady increase is observed in the polymerization rate as the polymerization temperature increases and the order of the FLC phase decreases, a behaviour significantly different from that observed in other monomer/FLC mixtures for which the polymerization rate increases as the order of the FLC phase increases. Additionally, at appropriate polymerization temperatures around the clearing point, polymerization driven endothermic phase transitions may be observed.

Bilayer smectic order in a mixture of polyphilic liquid crystals

Polyphilic compounds were previously reported @F. G. Tournilhac, L. M. Blinov, J. Simon, and S. V. Yablonski, Nature 359, 621 ~1992!# to form a longitudinal ferroelectric liquid crystal phase in the mixture M70 ~70% compound I, 30% compound II!. The ferroelectric nature was confirmed by measurements of acoustically induced piezoelectric response and hysteretic behavior in the smectic-X8 phase. We carefully examined the structure of the smectic-X8 phase with high-resolution x-ray diffraction in an aligned sample and discovered bilayer structure, suggesting that molecules actually adopt antiferroelectric arrangement. @S1063-651X~97!06604-X#

Viscosity Measurement of Ferroelectric Liquid Crystal Using Shear Horizontal Wave

The characteristics of shear horizontal (SH) wave propagation on a piezoelectric ceramic plate having the poling axis along the horizontal direction are described. The zeroth-order symmetrical (S0) mode has a nearly nondispersive property and an interdigital transducer (IDT) for the S0 mode under the configuration of no counter electrode on the opposite surface of the plate has a high electromechanical coupling constant k2 value exceeding 20% throughout the entire range of frequency times thickness (fd) values. The viscosity of a ferroelectric liquid crystal (FLC) is measured using the S0 mode SH wave device which has two acoustic wave channels. In the smectic phase of FLC, the viscosity in the direction perpendicular to the smectic layer plane is larger than that parallel to it. The difference in the viscosity between the two directions of FLC alignment is detected by measuring the acoustical phase delay difference between two acoustic wave channels.

FAST ADAPTIVE LENS BASED ON DEFORMED HELICAL FERROELECTRIC LIQUID CRYSTAL

Abstract A ferroelectric liquid crystal phase grating and its application as an adaptive lens with a variable focal length is discussed.