Electro-optic and dielectric studies of silica nanoparticle doped ferroelectric liquid crystal in SmC* phase

We present the results based on the electro-optic and dielectric properties of silica nanoparticle (SNP) doped ferroelectric liquid crystal (FLC) in SmC* phase. Switching time, spontaneous polarization and rotational viscosity decreases with increase in the silica concentration. An improvement in switching time after doping the silica nanoparticle is due to enhancement in anchoring energy exist between silica nanoparticle and ferroelectric liquid crystal. We noticed that the dielectric permittivity and dielectric strength decreases with increasing the concentration of silica nanoparticle in SmC* phase. Relaxation frequency increases with increasing the silica concentration and temperature in SmC* and decreases as we approaches towards transition temperature.     

Linear and non-linear dielectric properties of a short-pitch ferroelectric liquid crystal stabilized by a polymer network

Linear and non-linear dielectric measurements were carried out on a ferroelectric liquid crystal stabilized by an anisotropic polymer network. The polymerization process was achieved at room temperature. It was performed from an achiral monomer in the ferroelectric chiral smectic C phase, exhibiting a very short helical pitch and a large polarization. The linear and non-linear dielectric spectroscopy were also completed by textural morphology as well as structural and ferroelectric characterizations. All these measurements were carried out on a pure ferroelectric liquid crystal material and on composite films containing two polymer concentrations. The increase of the polymer network density leads to a decrease of the dielectric strength determined in the linear and non-linear dielectric spectroscopy. The complementarity between the linear and non-linear dielectric measurements and their confrontation with a theoretical model allowed the simultaneous determination of some physical parameters such as macroscopic polarization, rotational viscosity and twist elastic energy. We also discuss the effect of the polymer network density on the obtained physical parameters.     

Characteristic dielectric parameters of para-, ferro- and antiferro-electric phases of (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate

Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC_A*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmC_A* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmC_A* phase.     

Reentrant ferroelectricity in liquid crystals

The ferroelectric (Sm C*)-antiferroelectric (Sm C*A)-reentrant ferroelectric (re Sm C*) phase temperature sequence was observed for systems with competing synclinic-anticlinic interactions. The basic properties of this system are as follows: (i) the Sm C* phase is metastable in the temperature range of the Sm C*A; (ii) the helix handedness inverts at both Sm C*-Sm C*A and Sm C*A-re-Sm C* phase transitions; (iii) the threshold electric field that is necessary to induce synclinic ordering in the Sm C*A phase decreases near both Sm C*A-Sm C* and Sm C*A-re-Sm C* phase boundaries. All these properties are properly described by a simple Landau model that accounts for nearest neighboring layer steric interactions and quadrupolar ordering only.     

Distortion and unwinding of the helical structure in polymer-stabilized short-pitch ferroelectric liquid crystal

We report the effect of an anisotropic polymer network formed from an achiral photoreactive monomer in a short-pitch chiral SmC* phase on the distortion and the unwinding of the helical structure of the ferroelectric phase. The electro-optical behaviour and ferroelectric properties were experimentally determined for films containing various polymer concentrations. The critical field, E_u, for the transition from the distorted structure to the homogeneous state was measured as a function of polymer concentration. A linear increase of E_u versus polymer concentration was observed, showing that the helical structure of the short-pitch SmC* phase was stabilized by the polymer network. This behaviour was expected to be a consequence of the increase of the apparent elastic constants of the ferroelectric liquid crystal stabilized by the anisotropic polymer network films. The polymer network morphology was investigated using atomic-force microscopy, revealing a twisted structure of the polymer fibers. This twisted structure was transferred onto a polymer network during the polymerization process within a short-pitch SmC* phase. The increase of the apparent elasticity can then be interpreted by a strong interaction between polymer network and the liquid-crystal molecules. From our experimental data, the coupling coefficient, W_p, characterizing this interaction was evaluated for all studied polymer concentrations.     

Dielectric relaxation spectroscopy of guest-host ferroelectric liquid crystals: influence of anthraquinone dye

The dielectric spectroscopy of a short pitch and high spontaneous polarization ferroelectric liquid crystal mixture and its guest-host derivatives with different wt/wt ratio of anthraquinone blue dichroic dye has been studied over a wide frequency range of 50 Hz-1 MHz. The increase in dye concentration results in the decrease of the permittivity of the material in the SmC* phase, however, an opposite effect is observed in the SmA phase. The influence of bias voltage on the dielectric parameters has also been investigated. A new relaxation mode has been observed with a relaxation frequency of ∼300 kHz and dielectric strength of ∼5 at room temperature.     

Effect of multi-walled carbon nanotubes on dielectric and electro-optic properties of a high tilt antiferroelectric liquid crystal

By dispersing a small amount of multi-walled carbon nanotubes in a high tilt antiferroelectric liquid crystal, a nano composite is prepared. Though the phase sequence remains the same, the transition temperatures of the composite get changed, the stability of SmA* and SmC* phases increases whereas that of SmC_A* phase decreases. Pitch of the helicoidal structure shows a discontinuous change at SmC*–SmC_A* transition. Absorption strength and critical frequency of the anti-phase antiferroelectric mode are reduced in the composite. The dielectric increment of the Goldstone mode (GM) decreases and the critical frequency increases due to increased elasticity of the composite. A two-fold increase in the critical field for suppression of the GM is observed which signifies more stable helical structure in the composite. The switching time is reduced by 56% in the nano composite as a result of decrease in rotational viscosity but the high tilt remains intact. An observed lower value of conductivity indicates ion trapping in the composite.     

Dielectric measurements of new antiferroelectric liquid crystals

Three new pure antiferroelectric materials, synthesized at the Military University of Technology (MUT), were investigated by dielectric means. Clear relaxation modes were detected, i.e., Soft, Goldstone, low frequency in-phase (P_L) as well high frequency anti-phase (P_H) modes for paraelectric Sm A*phase, ferroelectric Sm C* phase and antiferroelectric Sm C_A* phase, respectively. Additionally, a high relaxation mode in Sm C_A* was found. The relaxation frequencies f_R of the observed modes were calculated from Cole-Cole diagrams.     

Dielectric spectroscopy of the SmQ* phase

Liquid crystal possessing two biphenyl moieties in the molecular core and lateral chlorine substitution far from the chiral chain has been studied by dielectric spectroscopy. On cooling from the isotropic phase, the material possesses the frustrated smectic Q* (SmQ*) and SmC_A* phases. It has been confirmed by dielectric spectroscopy that the SmQ* phase can be related to the SmC_A* anti-ferroelectric phase. However, only one relaxation process has been observed in the SmQ* phase, while in the SmC_A*, two relaxations are clearly detectable. It seems that the mode found in the SmQ* can be connected with high-frequency anti-phase mode observed in the SmC_A* phase. Its relaxation frequency is similar to P_H relaxation frequency, but is weaker. The same relaxation has been observed even a few degrees above the SmQ*–Iso phase transition. Another explanation for the mode detected in SmQ* and isotropic phases can be molecular motions around short molecular axis.     

Shifting of transition temperature of ferroelectric liquid crystals due to addition of dye-An optical and dielectric study

In order to study the shifting of phase transition temperature of ferroelectric liquid crystals due to addition of dye molecules, we have investigated two ferroelectric liquid crystal materials (Felix 16/030 and Felix 16/100) and their five mixtures with Anthraquinone dye (1%, 2%, 3%, 4% and 5% wt/wt). The phase transition scheme has been investigated and analyzed by results obtained from the optical transmittance and the dielectric permittivity study with variation of temperature in the range of 30 to 100 °C. Both the samples clearly show the shifting of phase transition temperature with dye concentration, especially the SmC*-SmA phase transition temperature. It is also clear from the study that SmC*-SmA phase transition phenomenon also becomes stronger with the addition of dye molecules. A theoretical explanation has also been given for shifting of phase transition temperature. The amount of shift in transition temperature agrees well as obtained from optical and dielectric studies.     

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T _C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for f_NRM, δε^NRM and distribution parameter α_NRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT _C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.     

Self-assembling properties of lactic acid derivative with several ester linkages in the molecular core

The effect of several polar ester linkage groups incorporated in the molecular core of a chiral lactic acid derivative on self-assembling properties has been investigated by polarizing optical microscopy, small angle X-ray diffraction, differential scanning calorimetry, optical and electro--optical studies. The compound possesses the paraelectric smectic A* (SmA*) and ferroelectric smectic C* (SmC*) phases over a broad temperature range. Mesomorphic behaviour, spontaneous polarization, birefringence, optical transmission, dielectric anisotropy and structural properties of the self-assembled chiral material have been determined. The obtained results are discussed and compared with that of other liquid crystalline materials. Experimentally determined spontaneous polarization and tilt angle values are also used to elucidate the nature of SmC* to SmA* phase transition. The effect of polar ester linkages in the molecular core has also been discussed.     

Dielectric and optical properties of polymer-liquid crystal composite

Dielectric properties of polymer-liquid crystal mixture, having constituent polymer, poly-butyl methacrylate (PBMA) and liquid crystal, cholesteryl nonanoate, are reported as a function of frequency and temperature. The measurement has been done in a temperature range of 300-375 K and frequency range of 100 Hz-10 MHz. The dielectric permittivity and dielectric loss shows significant changes with the addition of polymer molecules in liquid crystal. The significant feature of composite formation is that the pure liquid crystal and polymer do not show dielectric relaxation in the frequency range covered, while the composite shows relaxation peak at a particular frequency. The optical transmittance of pure liquid crystal and composite has also been measured and compared.     

Chain dynamics in a chiral C phase by deuteron spin relaxation study

Molecular reorientations and internal conformational transitions of an aligned chiral liquid crystal (LC) 10B1M7 are studied by means of deuterium spin-lattice relaxation in its smectic A (SmA) and smectic C* (SmC*) phase. The motional model which is applicable to uniaxial phases of many LCs is found to be adequate even when the phase is a tilted SmC* phase. The deuterium NMR spectrum in this phase cannot discern rotations of the molecular director about the pitch axis. The basic assumption is that the phase biaxiality is practically unobservable. However, the relaxation rates can be accounted for by the tilt angle between the molecular director and the layer normal in the SmC* phase. The tumbling motion appears to show a higher activation energy upon entering from the uniaxial SmA into the SmC* phase.     

Low frequency dielectric spectroscopy of two room temperature chiral liquid crystal mixtures

Dielectric spectroscopy of two room temperature chiral liquid crystal mixtures (W-96 and W-97) have been carried out in the frequency range of 100?mHz–10?MHz. Low frequency dielectric relaxation modes corresponding to collective behavior of molecules (Goldstone- and soft-modes) in the SmC* phase have been found to be masked by the ionic conductance. Two slow modes of dielectric relaxation due to the ionic conductance have been detected (below 15?Hz) in planar-aligned samples. It has been observed that the effect of ionic conductance decreases with the number of thermal annealing cycles on the materials. With large number of thermal annealing cycles it has been possible to wipe out the effects of ionic conductance and then to detect other weak modes of dielectric relaxation which are otherwise masked.     

‘Guest–host’ effect in liquid crystal mixtures

The most important goal of our research is to show the influence of the ‘guest’ (bent-core mesogen, 1,3-phenyldicarboxylatebis{4-[(4-octylbenzoyl)sulphanyl]phenyl} [IFOS8], banana-shaped liquid crystal [BLC]) on the ‘host’ (calamitic liquid crystal [CLC], (S)-(+)-1-methylheptyloxybiphenyl-(4-n-octylphenyl)thiobenzoate [MHOBS8]), on the stability and the destabilization of the antiferroelectric B2 and the ferroelectric smectic C* (SmC*) phases, and change of the temperature ranges of other phases in the binary liquid crystal mixtures. This work is focused on polymorphism of three new binary liquid crystal mixtures, exhibiting a ‘guest–host’ (guest liquid crystal–host liquid crystal [GH-LC]) effect. MHOBS8 has, among others, a ferroelectric SmC* phase, and IFOS8 assumes the B2 phase with antiferroelectric properties. The observed properties of the mixtures, such as variation of the phase transition temperatures, spontaneous polarization, tilt angle and switching time, are characteristic of a ‘guest–host’ mixture. The influence of BLC on the character of the interactions within the CLC host is discussed, with particular attention paid to electro-optical properties of the GH-LC mixtures.     

Dielectric spectroscopy of liquid crystals. Electrodes resistivity and connecting wires inductance influence on dielectric measurements

Dielectric spectroscopy is a very useful experimental method for liquid crystal investigation. Electrodes made from indium tin oxide (ITO) or gold are widely used in measuring cells. During the dielectric spectroscopy measurements performed for smectic liquid crystalline mixture it was found that detection of some important relaxation modes in paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC_A* phases for frequencies higher than 0.2–0.5 MHz is not possible. The measuring setup does not allow us to measure such relaxations due to its own dielectric response covering the dielectric response of liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to non-zero resistivity of electrode material or non-zero inductivity of connecting wires. In this paper, the new model was introduced. Its final equations show how to calculate parameters of relaxations observed in liquid crystals, from dielectric response of the empty and filled measuring cell. The experimental proof of strong influence of measuring setup properties on effective (measured) values of dielectric permittivities was shown.     

Optical second-harmonic generation in ferroelectric chiral smectic polysiloxane

The second-order nonlinear optical properties of ferroelectric liquid crystalline polymer have been studied. Angular phase-matched Second-Harmonic Generation (SHG) is observed in the chiral smectic C (SmC*) phase of siloxane copolymer with chiral mesogen in the side chain. The effective nonlinear optical coefficient d_eff under the phase-matching condition is determined. The intense SHG is observed even in the crystalline phase by cooling down from the SmC* phase under the electric field. The effective coefficient of SHG in the crystalline phase is more than ten times larger than that of the phase-matched SHG in the SmC* phase. The enhancement of SHG in the crystalline phase is observed only in a homeotropically aligned cell and maintained even in the non-biased state for at least several days. The angular dependence of the SHG in the crystalline phase is confirmed.     

Grayscale-Memory Spatial Light Modulator with Polymer-Dispersed Phase-Transition Liquid Crystal

A new storage-type spatial light modulator has been developed which uses a polymer-dispersed phase-transition liquid crystal film and a photoconductor for high-brightness projection displays. The liquid crystal film containing micrometer-sized chiral nematic liquid crystal domains subdivided by a unique honeycomb polymer network functions as a memory layer for an input image. The liquid crystal domains independently maintain the binary alignment states in the opaque (light-scattering) cholesteric phase or transparent nematic phase. Grayscale images can be displayed according to the spatial distribution of the bistable liquid crystal domains. The device exhibits rapid response of a few ms and a limiting resolution of 32 lp/mm. It has the advantages of high transmittance without polarizers and ease of fabrication due to the free-standing liquid crystal film.