Electro-optical Properties Of Liquid Crystals Research Articles

Electro-optical properties of nematic lyotropic chromonic liquid crystals

The electro-optical properties of liquid crystals play an important role in liquid crystal displays. Lyotropic liquid chromonic crystals are attracting the interest of researchers because of its extending application in areas of chemistry, medicine, and biology. An experimental apparatus is constructed based on nematic lyotropic chromonic liquid crystal sunset yellow filled in the twisted nematic liquid crystal cell to obtain two primary parameters of the electro-optical properties, the threshold voltage and relaxation time. The threshold voltage and relaxation time of nematic lyotropic chromonic liquid crystals decrease with the increase of temperature or the decrease of concentration. The concentration of neutral ionic dopants like sodium chloride and magnesium sulfate is positively correlated with the threshold voltage and relaxation time of nematic lyotropic chromonic liquid crystals, while the concentration of alkaline ionic dopants like sodium hydroxide exhibits a negative correlation. The changes of the electro-optical properties with respect to the temperature, solute concentration, and dopant concentration are analyzed based on the alteration of the molecular self-assembly behaviors of lyotropic chromonic liquid crystals. In addition, the viscoelastic theory of nematic thermotropic liquid crystals is proved to be also applicable on nematic lyotropic chromonic liquid crystals for the first time.

Order parameter and electro-optical properties in polymer-dispersed liquid crystals

ABSTRACT Polymer-Dispersed Liquid Crystal, PDLC, are composite materials formed by liquid crystal droplets embedded in a polymer matrix. They allow to simultaneously exploit the electro-optical properties of liquid crystals and the mechanical properties of polymers, giving rise to interesting electro-optical devices such as smart windows. In this work monomer/LC mixtures in different weight ratios were polymerised in the presence of an external magnetic field to obtain PDLCs, whose director was aligned along the field direction. The time behaviour of LC order parameter was followed by 2H-NMR and the final morphology of PDLCs was investigated by SEM. Then, the electro-optical properties of PDLCs prepared in the absence of any magnetic field were investigated and correlated to the order parameter measurements, confirming the strong correlation between large order parameters and optimal electro-optical properties.

Physico-chemical and electro-optical properties of liquid crystals doped with chemically modified nanocline minerals

The object of study is nematic and smectic (ferroelectric) liquid crystals containing chemically modified nanoclay based on montmorillonite. The aim of the work is to develop new composite materials with improved physico-chemical and electro-optical properties. During the study, chemical methods were developed for modification of the surface of nanoclay; the mesomorphic, dielectric, and electro-optical properties of nematic and ferroelectric compositions doped with a nanoclay with a modified surface are studied. On the basis of experimental data, the regularities of the influence of functional groups grafted onto the surface of nanoclay on the mesomorphic, dielectric and electro-optical properties of nematic and ferroelectric compositions are established. It has been shown experimentally that the addition of a small amount of nanoclay to a nematic and ferroelectric liquid crystal material can significantly improve the electro-optical response time and reduce the threshold and saturation voltage values. For ferroelectric liquid crystals, the addition of nanoclay leads to an increase in the tilt angle in the layer and an increase in spontaneous polarization.

Electro-Optical Behavior of Liquid Crystals Doped with Low Concentrations of Various Titanate Nanoparticles.

It is well known that the physical and electro-optical properties of liquid crystals can be controlled by doping with nanoparticles. In this study, we investigated the electro-optical properties of homogeneous aligned liquid crystal devices doped with various titanate nanoparticles, such as TiSiO₄, BaTiO₃, SrTiO₃ and mixtures of BaTiO₃ and SrTiO₃. Doping liquid crystals with titanate nanoparticles decreased the threshold voltages of the liquid crystal mixture cells. In particular, the liquid crystal cells doped with SrTiO₃ nanoparticles exhibited the lowest switching amplitude of 0.643 V. The high performance of the liquid crystal cells doped with various titanate nanoparticles indicates that the proposed liquid crystal devices have strong potential for use in materials for advanced liquid crystal displays.

Amorphous Mn3 O4 Nanocages with High-Efficiency Charge Transfer for Enhancing Electro-Optic Properties of Liquid Crystals.

Improving electro-optic properties is essential for fabricating high-quality liquid crystal displays. Herein, by doping amorphous Mn3 O4 octahedral nanocages (a-Mn3 O4 ONCs) into a nematic liquid crystal (NLC) matrix E7, outstanding electro-optic properties of the blend are successfully obtained. At a doping concentration of 0.03 wt%, the maximum decreases of threshold voltage (Vth ) and saturation voltage (Vsat ) are 34% and 31%, respectively, and the increase of contrast (Con ) is 160%. This remarkable electro-optic activity can be attributed to high-efficiency charge transfer within the a-Mn3 O4 ONCs NLC system, caused by metastable electronic states of a-Mn3 O4 ONCs. To the best of our knowledge, such remarkable decreased electro-optic activity is observed for the first time from doping amorphous semiconductors, which could provide a new pathway to develop excellent energy-saving amorphous materials and improve their potential applications in electro-optical devices.

Static and dynamic electro-optical properties of liquid crystals mediated by ferroelectric polymer films.

This paper reports the electro-optical properties of high resistivity nematic liquid crystals sandwiched between ferroelectric polymer films. Interactions between liquid crystals and the film result in a series of interesting optical and electro-optical features. For example, the visualization of ferroelectric domains by means of liquid crystals has been known for decades. However, here we demonstrate that liquid crystals can also reveal the fractal dimension of multi—domain poly(vinylidene fluoride)-based films. Unidirectionally rubbed films made of poly(vinylidene fluoride)-based (PVDF) materials align liquid crystals (LC) homogeneously, with the pretilt angle on the order of 1–2 degrees. This property was implemented in the design of hybrid cells composed of liquid crystals sandwiched between PVDF-based films. The designed PVDF|LC|PVDF cells exhibit tunable electro-optical performance originating from the presence of the PVDF-based films. More specifically, (i) the threshold voltage characterizing the transition of liquid crystals from a planar to a homeotropic state can be tuned by varying the film thickness, and (ii) total fall time (turn-off time) can be controlled by varying the frequency and amplitude of the driving voltage. This frequency dependence of the fall time is strongly pronounced at a relatively high voltage applied across the cell. In the low frequency regime, an increase in the turn-off time can be approximated as a linear function of the applied electric field. An electric-field induced polarization of the PVDF-based films is considered a major reason leading to the afore-mentioned amplitude and frequency dependence of the switching time.

Electro-optical properties of liquid crystal displays based on the transparent zinc oxide films treated by using a rubbing method

Liquid crystal (LC) alignment on inorganic films has been found to be affected by surface modification via ion-beam irradiation. In this study, ZnO films treated by rubbing with a velvet cloth were shown to be capable of aligning LC molecules in the direction of the rubbing. Uniform and homogeneous LC alignment was achieved on the rubbed ZnO films. By analysing the optical axes before and after the rubbing treatment, we confirmed an increase in the anisotropy of the ZnO films; this optical anisotropy contributed to the uniform orientation of LC molecules. Further, the electro-optical characteristics of the twisted nematic cells based on ZnO films were superior to those based on conventional polyimide layers. Our results indicate that the rubbing approach could be applied in the fabrication of high-performance LC displays.

The domain mixing effect on the electro-optical properties of liquid crystals using polyimide doped with reactive mesogen

ABSTRACTThis paper investigates the enhancement of the electro-optic (EO) properties of liquid crystal (LC) devices by the domain mixing effect of vertical-alignment polyimide (PI) doped with reactive mesogen (RM). The mixture of PI and RM on a glass substrate was exposed to ultraviolet light and was thermally annealed to produce circular microdomains of the LC polymer (LCP) in the PI background. Due to the appearance of such LCP microdomains depending on the RM doping concentration, the EO properties such as the threshold voltage and the response times were significantly improved in a vertically aligned configuration of the LCs.

Reorientation of single-wall carbon nanotubes in negative anisotropy liquid crystals by an electric field.

Single-wall carbon nanotubes (SWCNT) are anisotropic nanoparticles that can cause modifications in the electrical and electro-optical properties of liquid crystals. The control of the SWCNT concentration, distribution and reorientation in such self-organized fluids allows for the possibility of tuning the liquid crystal properties. The alignment and reorientation of CNTs are studied in a system where the liquid crystal orientation effect has been isolated. Complementary studies including Raman spectroscopy, microscopic inspection and impedance studies were carried out. The results reveal an ordered reorientation of the CNTs induced by an electric field, which does not alter the orientation of the liquid crystal molecules. Moreover, impedance spectroscopy suggests a nonnegligible anchoring force between the CNTs and the liquid crystal molecules.

Formation of local electric fields in the ferroelectric BaTiO3 particles-liquid crystal colloids

ABSTRACTInfluence of ferroelectric BaTiO3 particles on dielectric and electro-optic properties of liquid crystals (LC) 5CB and H37 has been investigated. It has been shown that the presence of particles decreases the clearing temperature, increases dielectric permittivity and losses, changes the Freedericksz transition voltage and switching times. A rise time of the BaTiO3-5CB colloid improved while a decay time worsened in comparison with the pure 5CB at all applied voltages. The inverse trends were observed for the H37 matrix.Experimental results are explained by appearance of local electric fields near the polarized ferroelectric particles at application of external electric field.

Induced Magnetic Anisotropy in Liquid Crystals Doped with Resonant Semiconductor Nanoparticles

Currently, there are many efforts to improve the electrooptical properties of liquid crystals by means of doping them with different types of nanoparticles. In addition, liquid crystals may be used as active media to dynamically control other interesting phenomena, such as light scattering resonances. In this sense, mixtures of resonant nanoparticles hosted in a liquid crystal could be a potential metamaterial with interesting properties. In this work, the artificial magnetism induced in a mixture of semiconductor nanoparticles surrounded by a liquid crystal is analyzed. Effective magnetic permeability of mixtures has been obtained using the Maxwell-Garnett effective medium theory. Furthermore, permeability variations with nanoparticles size and their concentration in the liquid crystal, as well as the magnetic anisotropy, have been studied.

Orientational and electro-optical properties of liquid crystal aligned with a directly spinnable carbon nanotube web

We investigated the orientational and electro-optical properties of a nematic liquid crystal (LC) aligned with a directly spinnable carbon nanotube (CNT) web functioning both as an electrode and as an alignment layer. The LC molecules were uniformly oriented along the drawing direction of the CNT web and the spatially averaged birefringence was comparable to a rubbed polyimide sample. The CNT web sample also showed smaller residual DC and hysteresis compared to the polyimide sample.

Simulation and optimization of liquid crystal gratings with alternate twisted nematic and planar aligned regions.

Electro-optical properties of liquid crystal (LC) gratings with alternate twisted nematic (TN) and planar aligned (PA) regions are simulated. Three typical steps are introduced: first, the LC director distributions of the two different regions are simulated. Then, the phase and amplitude of the emergent light in each region are calculated through Jones matrix. Based on this information, the voltage-dependent diffraction efficiency is achieved by Fourier transformation, finally. It gives an exact explanation for the mechanism of this kind of gratings. Experiments with optimized parameters are carried out through photopatterning. The trend of the measured voltage-dependent efficiency fits the simulation result very well. This method can be used to optimize the performance of LC gratings with alternate TN and PA regions, and exhibits great potential in the simulation of corresponding photonics and display applications.

Influence of surface anchoring conditions on the dielectric and electro-optical properties of nematic droplets dispersed in a polymer network

Dielectric and electro-optical properties of liquid crystal (LC) dispersions exhibiting two different surface anchoring conditions at the polymer-LC interface are investigated. The confining surface was tuned by changing the side chains of the polymeric matrix in order to get either planar or homeotropic anchoring conditions and, in turn, bipolar or radial LC droplets, respectively. In the low frequency region of the dielectric spectra, LC dispersions exhibit a broad absorption domain, which has been assigned to a depolarization field effect (Maxwell-Wagner-Sillars (MWS) mechanism). The strength and relaxation frequency of the MWS process are closely related to the geometry of the confined LC phase. In the case of radial droplets, the electrical conductivity dominates whereas for bipolar droplets capacitive effects are predominant. From electro-optical (EO) measurements and numerical simulations of the electric field inside droplets, it is shown that bipolar droplets exhibit enhanced EO performances compared to their radial counterparts. Based on simple topological and electrodynamic approaches, this phenomenon is related to the spatial distribution of charges inside droplets.

Abnormal switching behavior of nanoparticle composite systems

We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 kHz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells.

Director field of birefringent stripes in liquid crystal/nanoparticle dispersions

The thermal stability, alignment and electro-optic properties of liquid crystals can be fundamentally altered by dispersing small amounts of solid nanoparticles in a liquid crystal host. In the present study, the local alignment of the liquid crystal in such dispersions is studied by means of polarising optical microscopy and fluorescence confocal polarising microscopy. The results of two- and three-dimensional imaging indicate that birefringent stripes, which are induced by the presence of nanoparticles, correspond to twist disclinations that are located at the liquid crystal–substrate interface. The luminescence of dispersed semiconductor quantum dots shows that the ends of disclination threads are pinned to conglomerates of nanoparticles, which stabilise these line defects.

Grafting liquid crystalline polymers from cellulose substrates using atom transfer radical polymerization.

Immobilizing liquid crystalline polymers on cellulose generates new possibilities of accomplishing addressable/responsive bio-based substrates. In this paper we report on our first steps to combine the electro-optic properties of liquid crystals with the versatility of paper as a displaying substrate. Electric current or magnetic fields can be used to manipulate the orientation of liquid crystals and thereby change the appearance and the properties of the material. Atom transfer radical polymerization (ATRP) can be used successfully to graft polymers from solid substrates in a controlled manner. In this study it is shown that the grafting of a liquid crystalline monomer, 11-(4'-cyanophenyl-4″-phenoxy)undecyl acrylate, onto cellulose by ATRP is possible, and that thicker films can be obtained by using PMA as a spacer in between the cellulose and the liquid crystalline block. The cellulose fibers become highly hydrophobic subsequent to grafting and the liquid crystalline polymer possesses mesophases accessible for further processing.

Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts

In this work we investigate electrooptic properties of nematic liquid crystal (LC) E7 from Merck filled with two types of silica nanoparticles (NPs). The particles of these types are of similar size and shape, however, they are made by different methods. The NPs of the first type, aerosil (A), are prepared by a high-temperature hydrolysis of chlorosilanes. The second type of NPs, based on hydrolysis and controlled precipitation of tetraethylorthosilicate in alcohol, yields monodispersed NP (MNPs) of silica. Transmittance versus applied voltage curves of the suspension layers are measured and analyzed. Our results show a strong dependence of the electrooptic properties on the particle preparation method. The contrast ratio (CR) 61:1 of LC-A suspension is achieved at relatively low A content (17 wt %). Unlike it, the value of the CR of LC-MNP suspensions is low (about 2:1) even at 30 wt % of MNPs in the mixture. Both types of suspensions show electrooptic memory. The maximum of the memory efficiency of LC-A suspensions is detected at 4 wt % of A, while for LC-MNP suspensions the maximum is achieved at 15 wt % of MNPs. The difference in the results for the LC-A and LC-MNP series of composites is caused by much stronger aggregation of MNPs compared with A in the LC matrix. Moreover, reactive particle fusion and formation of silica microcrystallites substantially enhances the effect caused by MNP aggregation. These processes deteriorating electrooptic performance may be stabilized by introduction of a polymer network in LC-MNP suspensions.

Effect of the dielectric layer with photo-controllable surface relief on liquid crystal devices

We present experimental results for the dielectric layer effect on the electro-optical properties of liquid crystal (LC) devices together with numerical simulations. The photo-controllable polymer is used as a dielectric material on which the alignment agent of the LC is prepared. The surface relief structure can be induced on the dielectric polymer layer by photo-irradiation for fabricating LC microlens arrays or wide-viewing LC displays. It is found that the operation voltage of the LC device decreases with increasing the dielectric constant or with decreasing the layer thickness of the polymer. The experimental data agree well with theoretical results predicted from a simple dielectric model in the continuum theory.

Electrically tunable porous silicon active mirrors

We demonstrate tunable mirrors that consist of a porous silicon microcavity infiltrated with liquid crystal molecules. We show theoretically that by utilizing the electro-optic properties of liquid crystals and the sensitivity of the microcavity resonance position to small changes in optical thickness, the porous silicon active mirror can be switched on (high reflectance) and off (low reflectance) by simply applying a voltage. We discuss the issues of obtaining uniform infiltration of liquid crystal molecules in the constricted geometry of the porous silicon microcavity and determining the necessary change in the liquid crystal orientation to achieve a high reflectance contrast. We also present preliminary experimental results showing a greater than 40% change in the reflectance of our active mirror with the application of voltage.