Helix unwinding process in the chiral smectic C phase of MHPOBC as observed by conoscopy

Conoscopic studies have been made on the helix unwinding process in the chiral smectic C phase of MHPOBC. The unwinding process is rather unique in the sense that abrupt changes of the biaxiality and apparent tilt angle are followed by further gradual changes before the complete helix unwinding. The process is qualitatively explained by simulating the conoscopic images using the 4 x 4 matrix method with consideration of model molecular distributions, by taking account of both the ferroelectric and dielectric coupling between molecules and a field. The transmittance loss due to selective reflection was also measured under the application of a stepwise d.c. field. The shift of the dip position due to the loss, toward a longer wavelength region, was observed at low field. In contrast, the emergence of a transmittance loss at the same wavelength as that of an unperturbed state was observed. This phenomenon is explained by the ferroelectric deformation of a helix with the pitch unchanged.     

AC electric field induced helix unwinding in planar texture of a ferroelectric liquid crystal

The helix unwinding induced by an AC electric field in a thick ferroelectric S*_C sample with planar orientation has been investigated experimentally. Four patterns of instabilities have been observed below the unwinding critical field E_c. The corresponding thresholds and are found to be dependent on frequency. The formation of two instabilities (λ pattern and rhombic lattice pattern) is discussed on the basis of the present theory. The behaviour above the critical field is briefly described.     

Dielectric response of ferroelectric*C liquid crystals in a bias electric field

The dielectric response of the ferroelectric chiral smectic C phase in a bias electric field is studied theoretically within the Landau model using the constant amplitude approximation. It is argued that the response consists of two modes, one of which is related to the unwinding of the helix (the unwinding mode), whereas the other mode is associated with the distortion of the polarization distribution at constant pitch of the helix (the Goldstone mode). The relaxation frequency of the unwinding mode (fu), which is inversely proportional to the square of the sample dimension along the helical axis, is estimated to be of the order of 10^-4Hz for a sample of length l≈ 1 mm along the helical axis. Consequently, in dielectric experiments performed usually at frequencies higher than fu, only the Goldstone mode contribution is detected. In this way, we can explain the behaviour of the measured static dielectric susceptibility which goes to zero as the critical field is approached from below, although the model predicts that the total static dielectric susceptibility, being a sum of the Goldstone mode and the unwinding mode contribution, diverges at the critical field.     

Laser emission at the second-order photonic band gap in an electric-field-distorted cholesteric liquid crystal

ABSTRACT

We study the optical properties of a cholesteric liquid crystal doped with a fluorescent dye in the regime of highly distorted helix without full helix unwinding. The distortion was achieved by applying a pulsed AC electric field, perpendicular to the helix axis. If the pulse is in the millisecond range, the helix is deformed but keeps its original pitch even for electric fields higher than the theoretical critical field for helix unwinding. In this field regime, very pronounced high-order photonic band gaps are observed, in agreement with our calculations. We theoretically explore the possibility of obtaining viable laser emission at the second-order photonic band gap, and experimentally find that lasing is not only possible but has a figure of merit similar to that of the usual laser at the main-gap region. Therefore, electric-field-induced high-order photonic band gaps are potentially useful for multiline laser applications.     

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.     

Are short-pitch bistable ferroelectric liquid crystal cells surface stabilized?

Recent papers have described the short-pitch bistable ferroelectric liquid crystal effect (SBF) attributing its bistability to its stripe layer texture [1]. We have studied the bistability of this SBF effect for the short-pitch ferroelectric liquid crystal (FLC) mixture ZhKS-76 using thin planar aligned cells. Both the SBF texture (the stripe texture) and a uniform texture (uniform tilt layer structure with the extinction direction along the layer normal) were observed in different regions of a given cell when the cell was cooled down slowly from the isotropic phase to the chiral smectic C phase. Upon applying external fields, both regions are characterized by the formation of helix unwinding lines. The stripe area showed zig-zag unwinding lines and the uniform area exhibited straight unwinding lines, both running parallel to the layers. The bistability study shows a similar hysteresis curve and threshold behaviour on switching for both the SBF texture area and the uniform area, although the uniform area gcve better contrast. These facts strongly indicate that as in the long pitch FLCs, the surfaces rather than the layer stripe texture hinder the formation of the helix in the cell, and this produces dynamic bistable switching.     

Are short-pitch bistable ferroelectric liquid crystal cells surface stabilized?

Abstract

Recent papers have described the short-pitch bistable ferroelectric liquid crystal effect (SBF) attributing its bistability to its stripe layer texture [1]. We have studied the bistability of this SBF effect for the short-pitch ferroelectric liquid crystal (FLC) mixture ZhKS-76 using thin planar aligned cells. Both the SBF texture (the stripe texture) and a uniform texture (uniform tilt layer structure with the extinction direction along the layer normal) were observed in different regions of a given cell when the cell was cooled down slowly from the isotropic phase to the chiral smectic C phase. Upon applying external fields, both regions are characterized by the formation of helix unwinding lines. The stripe area showed zig-zag unwinding lines and the uniform area exhibited straight unwinding lines, both running parallel to the layers. The bistability study shows a similar hysteresis curve and threshold behaviour on switching for both the SBF texture area and the uniform area, although the uniform area gcve better contrast. These facts strongly indicate that as in the long pitch FLCs, the surfaces rather than the layer stripe texture hinder the formation of the helix in the cell, and this produces dynamic bistable switching.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Transient light scattering in helix ferroelectric liquid crystal cells

The static model to describe coherent (regular) transmittance in helix ferroelectric liquid crystal cells under the transient light scattering mode is proposed. It can be used to develop three-dimensional volumetric and two-dimensional liquid crystal displays. The dependence of coherent transmittance on cell parameters and the amplitude of the applied electric field is analysed. Conditions for increasing scattering efficiency and contrast ratio enhancement through an interference quenching effect of the coherent transmittance are indicated.     

Pyroelectric investigations of a hydrogen bonded ferroelectric liquid crystal gel by LIMM

The laser intensity modulation method (LIMM) is employed to determine spatially resolved polarization distributions in sandwich cells containing a hydrogen‐bonded ferroelectric liquid crystal (FLC) gel. At no external electric fields, contributions to the distributions at the surface of the FLC layer are dominating in all the samples with different concentration of gel former. These are attributed to non‐vanishing polarization due to surface interaction. In this case, the effect of hydrogen‐bonded network on the polarization distribution is not visible. In external electric fields, additional contribution to the resulting distribution caused by the induced polarization due to unwinding the FLC helix has been observed. Furthermore, the influence of hydrogen‐bonded network on the polarization distribution is also detected when the gel former content is increased up to 5.0 wt%. Therein the shape of the measured pyrospectra is completely different to other FLC gel samples with lower gel former concentration, where their maximum distributions still locate at the surface of FLC layer which is comparable to the initial field‐free state. These result indicate that the helical structure and orientation director of FLC are able to be stabilized effectively by the gel network even under strong external electric field. Copyright © 2004 John Wiley & Sons, Ltd.     

Electric field induced birefringence in a ferroelectric liquid crystal

We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Dielectric studies of different phases of a thin-layered ferroelectric liquid crystal with a small pitch and high spontaneous polarization

Dielectric spectroscopy of a ferroelectric liquid crystal with planar texture in the frequency range 100 Hz to 1 MHz has been carried out as a function of temperature from 25°C to 60°C at different DC fields varying from 0 to 12 kVcm^-1. From the measured dielectric strengths and relaxation frequencies, it is observed that the material has a S_c*-S_A* transition at 56·1°C and a S_A*-I transition at 58°C. The data of the S_c* phase have been assigned to the Goldstone mode. The rotational viscosity and elastic constant have been calculated from the observed data. The capacitance shows an abrupt drop at the critical DC field where unwinding of the helix occurs. The measured value of the critical field decreases with temperature and agrees to literature data from electrooptical experiments. On applying a DC field greater than 6·5 kV cm^-1 at room temperature, the liquid crystal shows the unwound S_c* and S_A* phases.     

Investigation of the helix unwinding process in thick freely suspended smectic films

Optical and electro-optical measurements have been performed on free-standing chiral smectic films sufficiently thick (10 000 layers) to preserve the natural smectic helix. The Goldstone mode appears at about 200Hz, showing that these films are a much better approximation of the 'ideal' smectic bulk state than a thick planar sample between glass plates (where the Goldstone mode is found at about 3 kHz). In these films the unwinding of the helix is studied, as a function of applied electric field, by monitoring Bragg reflections and their Fourier components. When the helix deforms, a reflection appears which at first sight might be taken for a subharmonic, but must be interpreted as the main 'full pitch mode' reflection relative to the 'half pitch' reflection from the undeformed helix. Our measurements further confirm that in anticlinic materials no helix unwinding takes place prior to the antiferroelectric-ferroelectric transition.     

Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens

A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

Chirality transfer between weakly birefringent and electric-field-induced highly birefringent B2 phases in a bent-core mesogen

Electric-field-induced transition was observed for the weakly birefringent chiral B2 phase, which is formed from the banana molecule based on the naphthalene bent core. This phase is considered to possess the twisted grain boundary (TGB)-like helical structure. When an electric field is applied, the TGB-like helix unwinds. The resulting large domain of the SmC(A)P(A) phase shows the high birefringence and simultaneously the antiferroelectric switching between SmC(A)P(A) and SmC(S)P(F) states. Through this field-induced transformation, two interesting features are obtained. First, the initially formed chiral domains are preserved even after the field-induced transformation to the unwound SmC(A)P(A) phase. This indicates the close correlation between the TGB-like helix and the layer chirality in such a way that the helical sense of the TGB-like helix is memorized as the layer chirality of the homochiral SmC(A)P(A) phase. Second, there is a critical temperature, above which the helicoidal structure is stable against the electric field. There is a competition between winding into a TGB-like structure and unwinding due to the electric field, and at higher temperatures, the helicoidal power is too strong to surpass the effect of the electric field.     

Structural dependence of phase transition and dielectric relaxation in ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s

A series of ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s, P(VDF-CTFE-TrFE), with systematically varied chemical compositions have been synthesized via a two-step approach consisting of copolymerization and dechlorination. The effect of polymer structure on polarization responses and dielectric properties has been investigated over a broad frequency and temperature range. As shown in the X-ray diffraction patterns, multiple phases coexist within the terpolymers as a result of the gauche conformation induced by the CTFE unit. The polarization hysteresis loops reveal the variation of remanent polarization and coercive electric field with the CTFE content due to the changes of crystallinity and crystalline phase. The observed broad dielectric constant peak with Vogel-Fulcher dielectric dispersion behavior suggests a transformation from a normal ferroelectric to a ferroelectric relaxor of the polymers. The relationship between the local relaxation process and relaxor ferroelectric behavior has been examined on the basis of the dielectric and mechanical loss tangents as a function of temperature.     

Magnetically tunable transparency for magnetorheological elastomer films consisting of polydimethylsiloxane and Fe3O4 nanoparticles

In this research, we demonstrate magnetically tunable transmittance for the magnetorheological elastomer films comprised of polydimethylsiloxane and Fe₃O₄ nanoparticles. The films bearing anisotropic and isotropic microtexture were prepared with and without magnetic field, respectively. The usage of toluene as diluent during the film preparation process leads to the increased tunable range of the magnetically induced transparency in comparison with our previous results. For the anisotropic film containing 1 wt% Fe₃O₄ nanoparticles, an increase of 23.01% in film transparency was observed at the wavelength of 600 nm with a magnetic field of B = ~ 80 mT applied, which is 2.67 times greater than the maximum change achieved in our previous research. The variation in transparency caused by the external magnetic field has been tentatively assigned to magnetostrictive effect. And the film microtexture might have a great influence upon the way an external magnetic field alters film transmittance.     

Electric field-induced birefringence, optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator-based novel set-up, the electric field-induced in-plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in-plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmC_A*(1/2)] compared with that for two layers [SmC_A*(0)]. On application of the electric field on the high temperature side of the SmC_A*(1/2) phase (80.1-81.5°C), a field-induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4-80.1°C) the transition takes place to SmC* via the SmC_A*(1/3) phase. The in-plane birefringence exhibits a critical power law dependence for the SmC*-SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free-standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmC_A* phase and below SmC_A*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.