Dielectric Anisotropy Of Liquid Crystal Research Articles

Increasing the negative dielectric anisotropy of liquid crystals with polar substituents parallel to their long molecular axis through adding terminal 2,2-difluoroethyl group

ABSTRACT A series of 2,3-difluorophenylene-based negative liquid crystals which own different terminal groups has been synthesised in high yields through many classical named reactions such as Suzuki coupling and Williamson etherification. Thermal properties of these mesogens were tested by differential scanning calorimetry and the effects of the terminal groups on the melting points of molecules were evaluated. Dielectric anisotropies of the synthesised molecules were then measured and found that relatively long alkyl substituents would decrease the Δε value when compared with the ethyl. For the compound with a polar substitute on the long molecular axis, the introduction of 2,2-difluoroethyl would increase the negative Δε when it was put into the opposite direction of the original polar group, making this strategy a new access for the improvement of negative Δε. DFT calculations were carried out to analyse the orientation of the dipole moment in molecules and found that the introduction of 2,2-difluoroethyl had changed the orientation of the dipole moment into nearly perpendicular to the long molecular axis, which benefited the growth of the negative Δε value, revealing the mechanism of this strategy from the view of molecular level to some extent.

Field programmable topological edge array

Valley Hall topological photonic crystals, inspired by topological insulators in condensed matter physics, have provided a promising solution to control the flow of light. Recently, the dynamic manipulation property of topological photonic crystals has been widely studied. Here, we propose a novel solution for programmable valley photonic crystals, called field programmable topological edge array (FPTEA), based on the field reorientation property of nematic liquid crystals and robust valley-protected edge modes. FPTEA is composed of an array of graphene-like lattices with C3 symmetry, in which the birefringence of liquid crystal is larger than 0.5105. Due to the dielectric anisotropy of liquid crystals being sensitive to external fields such as light, heat, electric, and magnetic fields, each lattice is tunable, and the topological propagation routes and even the lattice parameters can be dynamically changed while changing the distribution of external fields. We numerically demonstrate three methods of composing an FPTEA device to design arbitrary passive optical devices by electric driving, thermal inducing, or UV writing. These results show the great application potential of liquid crystals in topological photonic crystals, and enrich the design of programmable integrated topological devices with broad working bandwidth ranging from microwave to visible light.

Installation for measuring the dielectric anisotropy of liquid crystals at low frequencies by the bridge method with constant displacement

Installation designed to measure the dielectric anisotropy in laboratory studies of liquid crystal polymer films is described. The installation operates on the principle of a balanced alternating current (AC) bridge, allowing the application of a direct external current (bias) to the liquid crystal cell. The internal resistance of the direct current (DC) source, which affects the equilibrium condition of the bridge, is compensated. The frequency of the AC current feeding the bridge and the offset voltage of the cell is regulated within a wide range, which makes it possible to study various functional dependences of the dielectric parameters of liquid crystals and their modifiers.Introduction

Introduction of 5,6-dihydro-4H-cyclopenta[b]thiophene core unit into phenyl-tolane: Expanding the mesophase range and increasing the birefringence and dielectric anisotropy of liquid crystal

ABSTRACT Six novel phenyl-tolane type liquid crystals based on 5,6-dihydro-4 H-cyclopenta[b]thiophene core were designed and synthesised by Sonogashira coupling reactions in high yields. Two reference compounds were then used to clarify the effects of this new core on the mesomorphic behaviours and physical properties of phenyl-tolane type mesogens and found that these new core-based mesogens possessed wider mesophase ranges. Both the birefringence (Δn) and dielectric anisotropy (Δε) values were increased after the employment of this new core, offering an alternative option for the design and synthesis of high-performance phenyl-tolanes. The geometry, ultraviolet absorption, polarisability (α) and dipole moment (μ) of two representative compounds and their references were calculated by density functional theory to correlate the structure–property relationship of the new mesogens. The calculated energy minimum structures of the new mesogens showed well linearity and the molecular length/width ratio was increased, which would benefit the formation of the nematic phase. The orders of the calculated polarisability (α) and dipole moment (μ) of two representative compounds and their references were in accordance with the sequences of the measured birefringence (Δn) and dielectric anisotropy (Δε) values, respectively, which might shed light on the correlation between microscopic molecular structure and macroscopic physical properties.

Signal Amplification in an Optical and Dielectric Biosensor Employing Liquid Crystal-Photopolymer Composite as the Sensing Medium.

An optical and dielectric biosensor based on a liquid crystal (LC)–photopolymer composite was established in this study for the detection and quantitation of bovine serum albumin (BSA). When the nematic LC E7 was doped with 4-wt.% NOA65, a photo-curable prepolymer, and photopolymerized by UV irradiation at 20 mW/cm2 for 300 s, the limit of detection determined by image analysis of the LC optical texture and dielectric spectroscopic measurements was 3400 and 88 pg/mL for BSA, respectively, which were lower than those detected with E7 alone (10 μg/mL BSA). The photopolymerized NOA65, but not the prepolymer prior to UV exposure, contributed to the enhanced optical signal, and UV irradiation of pristine E7 in the absence of NOA65 had no effect on the optical texture. The effective tilt angle θ, calculated from the real-part dielectric constant ε’, decreased with increasing BSA concentration, providing strong evidence for the correlation of photopolymerized NOA65 to the intensified disruption in the vertically oriented LC molecules to enhance the optical and dielectric signals of BSA. The optical and dielectric anisotropy of LCs and the photo-curable dopant facilitate novel quantitative and signal amplification approaches to potential development of LC-based biosensors.

Increasing the negative dielectric anisotropy of liquid crystals by fluorination of the terminal ethyl chain

ABSTRACT Ten new negative dielectric anisotropy liquid crystal compounds with terminal 2-fluoroethyl group were synthesised and their structures were confirmed by 1H-nuclear magnetic resonance and mass spectra. Thermal analysis shows that the four compounds bearing two rings are non-mesogenic, and among the compounds bearing three rings, two compounds exhibit only nematic phase and the others exhibit both smectic and nematic phases. Compared with the terminal ethyl analogues, the introduction of fluorine leads to a significant increase in dielectric anisotropy and a slight decrease in the clearing point. Unexpectedly, the rotational viscosity also decreased significantly. Density functional theory (DFT) calculation results reveal that the gauche conformation of 2-fluoroethoxyl benzene has the lowest total energy and it is beneficial to the increase in polar moment, which eventually leads to a large increase in dielectric anisotropy.

In-Plane Switching of Chiral Nematic Liquid Crystals with Different Dielectric Anisotropies

The electro-optic response in planar-aligned chiral nematic liquid crystals in an electric field oriented perpendicular to the helical structure axis is studied as a function of the magnitude and sign of the dielectric anisotropy of liquid crystal in order to understand the mechanism of this electro-optic effect. The main contribution to the electro-optic response is shown to be due to the interaction of the electric field with the dielectric anisotropy. The flexoelectric effect also contributes to the electro-optic response, but its magnitude is much smaller.

外加电压频率对液晶介电各向异性的影响

外加电压频率对液晶介电各向异性的影响

Flexoelectric in-plane switching (IPS) mode with ultra-high-transmittance, low-voltage, low-frequency, and a flicker-free image.

The demands for a power-saving mode for displaying static images are ubiquitous not only in portable devices but also in price tags and advertising panels. At a low-frequency driving in liquid crystal displays (LCDs) for low-power consumption, the flexoelectric effect arises even in calamitic liquid crystals and the optical appearance of this physical phenomenon is found to be unusually large, being noticed as an image-flickering. Although the inherent integrated optical transmittance of in-plane switching (IPS) mode is relatively lower than that of fringe-field switching (FFS) mode, the IPS mode shows no static image-flickering but an optical spike (the so-called optical bounce), at the transient moment between signal positive and negative frames. Here, we demonstrate an IPS mode using negative dielectric anisotropy of liquid crystals (Δε < 0) and fine-patterned electrodes (the width w of and the space l between electrodes ≤ 3 μm) with reduced operation voltage (up to 40.7% to a conventional FFS mode with Δε < 0), reduced optical bounce (up to 4.4%. to a conventional FFS mode with Δε < 0) and enhanced transmittance (up to 32.1% to a conventional IPS mode with Δε > 0). We believe the result will contribute not only to the scientific understanding of the optical appearance of flexoelectric effect but also pave the way for engineering of a superior low-power consumption LCD.

Freedericksz transition in smectic-A liquid crystals doped by ferroelectric nanoparticles

The structure, orientation and physical properties of liquid crystals is significantly affected by doping of ferroic (ferromagnetic and ferroelectric) nanoparticles into liquid crystals. Behavior of these suspensions in external fields depends on anchoring energy, volume fraction of doped nanoparticles, and sign of dielectric anisotropy of liquid crystals as well as relative orientation of director and local polarization of nanoparticles. Here, the threshold voltage for molecular reorientation (Freedericksz transition) is estimated theoretically in smectic-A liquid crystals doped by spherical ferroelectric nanoparticles, near the smectic-A to smectic-C transition temperature taking into account local electric field effects. It is shown that the threshold voltage is decreased by doping ferroelectric nanoparticles into liquid crystals.

Negative Resistance Behaviour and Molecular Reorientation Properties of Zinc Oxide Nanoparticles Based Liquid Crystals for High Image Quality Liquid Crystal Displays

The octylcyanobiphenyl (8CB) liquid crystal was doped by ZnO nanoparticles to improve the dielectric anisotropy parameters, as the image quality of liquid crystal displays (LCDs) is strongly dependent on the dielectric anisotropy of the liquid crystal. The dielectric anisotropy parameters of the liquid crystals have been determined. The magnitude of dielectric anisotropy of the 8CB liquid crystal was increased with nano-nennatic composite liquid crystal system. The current voltage chracteristics of 8CB and ZnO doped 8CB liquid crystals were determined using current voltage chracteristics. The liquid crystals exhibited a voltage-controlled differential negative resistance behaviour (VCNR). The obtained results indicate that the ZnO nanoparticles increase the dielectric anisotropy parameters of the 8CB liquid crystal

P‐117: Investigation on Movement of Ions in the Fringe Field Switching Mode Depending on Resistivity of Alignment Layer and Dielectric Anisotropic Sign of Liquid Crystal

AbstractIon movement in liquid crystal displays plays crucial role in residual dc associated with image sticking. This paper investigatesadsorption and desorption of ions at interfaces between liquid crystal and alignment layer depending on resistivity of alignment layer and sign of dielectric anisotropy of liquid crystal in thefringe field switching (FFS) liquid crystal cell. Our studies show that the resistivity of alignment layer mainly determines ion movement such that the one with lower resistivity shows faster absorption and desorption at the interface than that of the one with higher resistivity.

Design and Experimental Validation of Liquid Crystal-Based Reconfigurable Reflectarray Elements With Improved Bandwidth in F-Band

A reconfigurable reflectarray which exploits the dielectric anisotropy of liquid crystals (LC) has been designed to operate in the frequency range from 96 to 104 GHz. The unit cells are composed of three unequal length parallel dipoles placed above an LC substrate. The reflectarray has been designed using an accurate model which includes the effects of anisotropy and inhomogeneity. An effective permittivity that accounts for the ‘real effects’ of the LC has also been used to simplify the analysis and design of the unit cells. The geometrical parameters of the cells have been adjusted to simultaneously improve the bandwidth, maximize the tunable phase-range and reduce the sensitivity to the angle of incidence. The performance of the LC based unit cells has been experimentally evaluated by measuring the reflection amplitude and phase of a reflectarray consisting of 52 $\,\times\,$ 54 identical cells. The good agreement between measurements and simulations validates the analysis and design techniques and demonstrates the capabilities of the proposed reflectarray to provide beam scanning in F band.

Effect of frequency on dielectric properties of liquid crystal doped with side-chain liquid crystalline polymer

The frequency and applied bias voltage dependence of the dielectric properties and dielectric anisotropy of liquid crystal (LC) doped with side-chain liquid crystalline polymer (SLCP) mixture have been investigated using the admittance spectroscopy method ( C– V and G/ω– V) in the frequency range of 10 kHz to 10 MHz at room temperature. The liquid crystal used in this experiment is E63. The doping material used in this study is SLCP and its concentration is ensured 1 wt % in E63. Dielectric constant ( ɛ′), dielectric loss ( ɛ), dielectric loss tangent (tan δ) and real and imaginary parts of electrical modulus ( M′ and M″) of the E63/SLCP mixture was also calculated. Moreover, dielectric anisotropy (Δ ɛ) as a function of frequency was obtained. Results show that the values of the all dielectric parameters are strong functions of frequency and applied bias voltage. After a critical frequency, dielectric anisotropy has negative values according to p/n type changing.

Light efficiency of the fringe-field switching mode depending on magnitude of dielectric anisotropy of liquid crystal associated with cell gap and rubbing angle

The light efficiency of the homogeneously aligned liquid crystal (LC) mode driven by fringe electric fields depends on the magnitude of the dielectric anisotropy when using a LC with positive dielectric anisotropy. However, this dependency is not observed when using a LC with negative dielectric anisotropy. This difference is mainly associated with the molecular reorientation especially in the tilt angle of the LC between the edges and the centers of the pixel electrodes in the voltage-on state. This paper explores the dependence of the light efficiency of the device on the magnitude of the dielectric anisotropy of the LC for both types of the LCs as functions of other cell parameters such as the rubbing angle and the cell gap.

A fluid approach to microwaves

Collaborative researchers from the University of Cambridge in the UK and Technische Universitat Darmstadt (TUD) in Germany have significantly increased the dielectric anisotropy of liquid crystals in the microwave range by adding minute amounts of carbon nanotubes. This result is a significant step for the use of liquid crystals as a tunable material in a wide range of microwave devices at frequencies up to several THz.

A millimeter-wave quasi-optical grid phase shifter using liquid crystal

A new type of millimeter-wave quasi-optical variable phase shifter which exploits the dielectric anisotropy of liquid crystal (LC) materials is presented. The device is operated in transmission mode and consists of inductive and capacitive metal grids, each patterned on a dielectric substrate, which are separated by a thin layer of LC. The grids together with the LC form a parallel resonant circuit. Prototype devices with a 25-µm-thick LC layer for use at a millimeter-wave frequency of 50GHz were designed and fabricated. Experiments performed at U-band frequencies show that a phase shift of 7.8° is obtained with a low insertion loss of ∼ 0.6dB at the resonant frequency by applying a control signal of 20V. Good time response for the device is demonstrated. It is suggested that the phase shift attainable with the device can be further enhanced by using LC materials with larger dielectric anisotropy and/or thinner dielectric substrates.

Effects of dielectric relaxation on the director dynamics of uniaxial nematic liquid crystals

The dielectric anisotropy of liquid crystals causes director reorientation in an applied electric field and is thus at the heart of electro-optic applications of these materials. The components of the dielectric tensor are frequency dependent. Until recently, this frequency dependence was not accounted for in a description of director dynamics in an electric field. We theoretically derive the reorienting dielectric torque acting on the director, taking into account the entire frequency spectrum of the dielectric tensor. The model allows one to include the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor, thus generalizing a recent model [Y. Yin, Phys. Rev. Lett. 95, 087801 (2005)] limited by the single-relaxation approach. The model predicts the "dielectric memory effect" (DME)--i.e., dependence of the dielectric torque on both the "present" and "past" values of the electric field and the director. The model describes the experimentally observed director reorientation in the case when the rise time of the applied voltage is smaller than the dielectric relaxation time. In typical materials such as pentylcyanobiphenyl (5CB), in which the dielectric anisotropy is positive at low frequencies, the DME slows down the director reorientation in a sharply rising electric field, as the sharp front is perceived as a high-frequency excitation for which the dielectric anisotropy is small or even of a negative sign. In materials that are dielectrically negative, the DME speeds up the response when a sharp pulse is applied.

Liquid Crystal Tunable mm Wave Frequency Selective Surface

A frequency selective surface (FSS) which exploits the dielectric anisotropy of liquid crystals to generate an electronically tunable bandpass filter response at D Band (110-170 GHz) is presented. The device consists of two printed arrays of slot elements which are separated by a 130-mum thick layer of liquid crystals. A 3% shift in the filter passband occurs when the substrate permittivity is increased by applying a control signal of 10 V. Measured results show that the insertion loss increases from -3.7 dB to -10.4 dB at resonance (134 GHz), thus demonstrating the potential to create a FSS which can be switched between a transmitting and a reflecting structure.

Cover Picture: Synthesis and Structural Characterization of Highly Fluorinated Sulfimides and Sulfoximides as Functional Building Blocks for Materials Science (Eur. J. Org. Chem. 5/2005)

AbstractThe cover picture illustrates the conformational equilibrium of highly fluorinated sulfoximides. The conformation of this polar group critically influences the dielectric anisotropy of liquid crystals derived therefrom. The crystal structure of heterostilbene‐like fluoroaromatic sulfoximides is dominated by stacking interactions of orthogonal aromatic substructures. Details are discussed in the article by P. Kirsch et al. on p. 797 ff.