Positive Dielectric Anisotropy Research Articles

Size-dependent optical and dielectric anisotropy of CdS nanowire doped high-birefringent nematic liquid crystalline compound

Highly birefringent liquid crystals have grabbed ample attention because of their potential applications as an active medium for devices in visible, infrared and other spectral regions. Here, we have attempted to study the effect of the size and concentration of CdS nanowires on a nematic liquid crystal. Anisotropy in liquid crystals is one of those distinguishing traits that gain their superiority over other materials. We hereby have analyzed how optical and dielectric anisotropy respond to variations in the size and concentration of the dopant. The study is of significant scientific value because there is very little literature available on how dopant size can alter liquid crystalline properties and also introduces a method to tune the properties of pure liquid crystals (LCs) by varying the size and concentration of the dopant. The study explains how the size of the nanowires changes the analyzed parameters and up to what extent. The obtained results yield a significant increase in both optical and dielectric anisotropy by doping CdS nanowires making them an excellent choice for dopant selection. CdS-doped nematic LC mixtures have proved to be highly birefringent and possess a high positive dielectric anisotropy as well. Such a compound can be of great application in device fabrications such as laser beam steerers, electronic lenses, attenuators and light shutters, etc.

Influence of the molecular structure of compounds differing with terminal polar group on the induction of the smectic A phase. Part III

ABSTRACT The aim of this work is to determine the influence of the terminal polar group of nematic compounds, namely -OCF3, -NCS, -CN, -F, on the induction of the SmA phase in their mixtures with compounds having terminal non-polar groups. The former group of compounds has positive dielectric anisotropy, and the latter negative. The phase transition temperatures of binary mixtures were measured using polarising thermomicroscopy, and phase diagrams for investigated systems were constructed. The strength of the SmA phase induction increases upon the increase of the polarity of compounds. The strongest induction is observed in systems with compounds having a terminal trifluoromethoxy group. Although compounds with the cyano group have the highest polarity, their ability for the SmA phase induction is smaller due to their dimerisation.

Electric field effects on N–Sm-A–Sm-C phase transitions

A liquid crystalline system formed by rod-shaped molecules that have a small transverse dipole moment is analyzed in the presence of an external electric field normal to the smectic layers. The approach focuses on the phase transition diagram displaying the N–Sm–A-Sm-C phases, when both negative and positive dielectric anisotropies are considered. The analysis is carried out within the framework of an extended mean field theory, which shows the emergence of a re-entrant Sm-C phase in the strong field regime, accompanied by a suppression of the translational ordering of the Sm-A phase, in the case of a negative anisotropy. A different scenario is observed in the case of positive anisotropy, as the electric field tends to suppress the Sm-C phase because the molecular alignment is driven by the field along its direction. In this case, the emergence of a residual nematic phase is also observed, which replaces the isotropic phase and is characterized by a low-value orientational order parameter.

Influence of the molecular structure of compounds with terminal isothiocyanate group on the induction of the smectic A phase: Part II

ABSTRACT The influence of the structure of compounds having terminal isothiocyanate group in binary mixtures with compounds having terminal alkyl chains on the induction of the smectic A phase is shown. Compounds with a triple bond in the molecular core exhibit a weaker induction of a SmA phase than compounds without it. Molecular lengths, dipole moments and polarisabilities of investigated compounds were calculated. Dielectric spectroscopy studies were done for the chosen system to show that single compounds have the opposite signs of the dielectric anisotropy, while the mixture with maximum temperature of SmA-N phase transition in the nematic phase has a positive dielectric anisotropy. In addition, the dielectric spectroscopy results confirm that the SmA phase is indeed nucleated in the mixture. It has been concluded that there is a competition of different factors, like the shape of molecules, their polarity and polarisability affecting the induction of SmA phase. The shape of a molecule appears to be a crucial factor here since it determines the distance between molecules and influences the contribution of dispersion forces.

Recent Advances in Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLC) are molecules that can self-assemble into helicoidal superstructures exhibiting circularly polarized reflection. The facile self-assembly and resulting optical properties makes CLCs a promising technology for an array of industrial applications, including reflective displays, tunable mirror-less lasers, optical storage, tunable color filters, and smart windows. The helicoidal structure of CLC can be stabilized via in situ photopolymerization of liquid crystal monomers in a CLC mixture, resulting in polymer-stabilized CLCs (PSCLCs). PSCLCs exhibit a dynamic optical response that can be induced by external stimuli, including electric fields, heat, and light. In this review, we discuss the electro-optic response and potential mechanism of PSCLCs reported over the past decade. Multiple electro-optic responses in PSCLCs with negative or positive dielectric anisotropy have been identified, including bandwidth broadening, red and blue tuning, and switching the reflection notch when an electric field is applied. The reconfigurable optical response of PSCLCs with positive dielectric anisotropy is also discussed. That is, red tuning (or broadening) by applying a DC field and switching by applying an AC field were both observed for the first time in a PSCLC sample. Finally, we discuss the potential mechanism for the dynamic response in PSCLCs.

Temperature dependence of dynamic response of liquid crystal materials

Most properties of liquid crystal (LC) materials will change with the change of temperature, which makes the performance of liquid crystal display (LCD) and other devices change. However, at low temperature, the properties of LC will mutate with the decrease of temperature, which directly leads to the defects of LC devices. This paper makes use of the developed homemade temperature control device to realize the construction of high temperature to low temperature environment, using precision LCR meter (Agilent E4980A) and self-built optical path, combined with LC cell capacitance model and dual-cell model, the dynamic response properties of LC materials with temperature change law was explored. The results show that the decay time τ d of the LC with positive dielectric anisotropy increases from 12.04 ms to 159.7 ms, and the rotational viscosity coefficient γ 1 increases exponentially from 12.04 mPa·s to 1582.45 mPa·s. While τ d of the LC with negative dielectric anisotropy increases from 13.10 ms to 408.1 ms, γ 1 also increases exponentially from 162.73 mPa·s to 7903.69 mPa·s. The conclusions presented in this paper provide some guidance for the improvement of low temperature properties of LC.

INVESTIGATION OF DIELECTRIC PROPERTIES OF MBBA-5CB NEMATIC LIQUID CRYSTAL STRUCTURES

The dielectric anisotropy (Δε') is the response of the LC molecules to an applied electric field. In this study, the dielectric properties of 4-pentyl-4'-cyanobiphenyl (5CB) and N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystals with positive and negative dielectric anisotropy were investigated in detail. In addition, a structure with zero dielectric anisotropy was obtained by mixing MBBA and 5CB liquid crystals in certain rate (1:0.02). The behavior of the samples under 0V and 20V voltages was investigated in detail. Dielectric measurements were made with dielectric spectroscopy technique, which is a powerful method to investigate the molecular interaction mechanisms of liquid crystal materials, at a frequency of 1 kHz-10 MHz and a voltage of 0-20 V at room temperature. Some important parameters such as dielectric anisotropy (Δε'), relaxation frequency (fR), relaxation time (τ) were determined for the samples. When electric field is applied to nematic liquid crystals, it is seen that, depending on the voltage, the relaxation frequency decreases, and the relaxation time increases.

Zinc Oleate Nanorod-Induced Vertical Alignment of Nematic Liquid Crystal.

Nanocomposite zinc oxide nanorods capped with oleic acid (ZOR) with positive dielectric anisotropy liquid crystal (LC) 4'-octyl-4-biphenylcarbonitrile (8CB) filled in unaligned cells exhibit homeotropic alignment of host LC molecules. Further, systematic investigation of the textural, dielectric, and conductivity properties of nanocomposites filled in planar cells is performed with increasing concentration of nanorods. At a nanorod concentration ≤0.2 wt % in 8CB, the order parameter of nanocomposite samples is found to be increasing and ionic conductivity is found to be decreasing as compared to pure LC. Beyond 0.3 wt % concentration of nanorods in 8CB, vertical alignment (VA) of host LC is observed even in a planar aligned cell. The VA of LC molecules in ZOR nanocomposites is confirmed through attenuated total reflection-Fourier transform infrared absorption spectra studies.

Lens-shaped nematic liquid crystal droplets with negative dielectric anisotropy in electric and magnetic fields

ABSTRACT Recently, plano-convex spherical lens-shaped liquid crystal (LC) sessile droplets with positive dielectric anisotropy were studied in magnetic and/or AC electric fields. Here, we present experimental observations in magnetic, AC electric, combined fields and theoretical considerations of plano-convex lens-shaped nematic LC droplets with negative dielectric anisotropy. In purely magnetic field, our results are the same as for positive materials, i.e. an inversion wall forms normal to the field that moves towards the periphery. In contrast to previous observations on nematic LCs with positive dielectric anisotropy, in electric fields applied normal to the base plane, we do not find any wall formation, but only a radial director structure with a central defect line along the field. Above a threshold electric field, the radial symmetry becomes twisted due to elastic constant anisotropy. Applying a magnetic field perpendicular to the vertical electric field, a twisted inversion wall forms together with a vertical central defect line. When the electric field is applied parallel to the base plane of the droplet, a homeotropic central region forms along the electric field. When this field is applied together with a magnetic field applied in the same direction, the homeotropic central region becomes perpendicular to the applied field.

Influence of the molecular structure of calamitic compounds with trifluoromethoxy terminal group on the induction of the smectic a phase. Part I

ABSTRACT For the liquid crystalline mixtures of compounds with positive and negative dielectric anisotropies, the induction of the liquid crystal smectic A phase can be observed. This effect can be very favourable for definite specific purposes. The main aim of this work is to determine the influence of the molecular structure of compounds with a polar trifluoromethoxy (-OCF3) group on the induction strength of the orthogonal smectic A (SmA) phase. In order to reach this target, the phase diagrams of the mixtures were established using the polarised optical microscope observations. The presence of a triple bond, phenyl and cyclohexyl rings as well as fluorine atoms in the core and the length of terminal alkyl chains of the tested compounds were considered. The length of the rigid core has the greatest effect on the induction strength of the SmA phase, while the length of the alkyl chain (3–5 carbon atoms) has relatively minor influence. Explaining the correlation between the molecular structure and the induction strength, the dipole moments and the polarisability of the molecules should be taken into account. For shorter molecules, changes in the molecular structure have more significant influence on the temperature and concentration range of the orthogonal SmA phase.

Induction of the Orthogonal Smectic A Phase in Mixtures of Nematic Tolanes with Positive and Negative Dielectric Anisotropy

Induction of the orthogonal smectic A phase occurs in mixtures of nematic compounds with positive and negative dielectric anisotropy. The aim of this work is to determine influence of a molecular structure of compounds on the induction strength of this phase. Cyclohexyl tolanes with the terminal group OCF3, differing with a lateral substitution of a rigid core with fluorine atoms, were mixed with tolanes with two fluorine atoms in a lateral position and different lengths of a terminal alkoxy chain. Based on phase diagrams, obtained with the use of a polarising optical microscope, the influence of a structure of both types of compounds on the induction of the SmA phase is presented. Both, an increase in a number of fluorine atoms in a rigid core and a decrease in an alkoxy chain length cause a decrease in the strength of the SmA phase induction in the investigated systems.Keywords: chemistry, liquid crystals, smectic A phase induction, miscibility studies

Equilibrium configurations in a nematic pi-cell under an electric field

In this work, we study the equilibrium configurations in a nematic pi-cell under an electric field using the one-dimensional Oseen-Frank director model. The equilibrium orientational configurations that can be attained in a pi-cell are known as splay, bend, and twist. Among those, bend and twist are topologically equivalent and can be transitioned into one another as voltage varies. The transition can be continuous or abrupt depending on the material parameters. On the other hand, the splay configuration becomes asymmetric for sufficiently high voltages if the liquid crystal has a positive dielectric anisotropy. We determine those threshold voltages and characterize the order of transitions in terms of the elastic constants and the pretilt angle at the boundary.

Induction of the smectic A phase in liquid crystalline mixtures formulated using non-chiral compounds with positive and negative dielectric anisotropy

ABSTRACT The induction of the SmA phase in mixtures of non-chiral nematic compounds with the positive and the negative dielectric anisotropy is presented which is obtained by different substitutions of fluorine atoms. One component is a three-ring tolane with polar group – OCF3 in position 4 and two fluorine atoms in positions 3 and 5 in the terminal phenyl ring. The other component is two ring tolane with two or four fluorine atoms in the lateral position. Based on phase diagrams, the influence of the alkyl chain length of both types of compounds on the induction phenomenon is shown. Results of the X-ray measurements confirm that the induced phase is a monolayer SmA phase. The increase of the alkyl chain length causes the increase in the strength of the induction in mixtures, i.e. the induced SmA phase is observed for a wider range of the temperature and concentration of mixture components.

Effect of Different Polar Groups on the Mesomorphic and Physical Properties of Synthesized Schiff Base Liquid Crystalline Compounds

Five compounds of calamitic Schiff base with liquid crystalline behavior were synthesized and characterized. The terminal of compounds were substituted with different types of polar groups are - OCH3, -COCH3, - Br, and -COOC2H5, -CF3. The chemical structures of all compounds were characterized by FTIR spectroscopy while the phase transitions and optical textures of the samples were observed by polarized optical microscope. Furthermore, the transition temperatures of the compounds and their liquid crystalline temperature range were determined by using differential scanning coulometer. All compounds showed a higher tendency to exhibit an antiotropic nematic mesophases with different transition temperature range. Moreover, the dielectric and electrical conductivity were investigated. Using different polar groups in the synthesized liquid crystalline compounds is accompanied by significant changes in mesophase thermal stabilities as well as physical properties and all samples are with positive dielectric anisotropy.

Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB).

In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang etal. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.

Observation and simulation of toron polymorphism: Effects of surface anchoring, elasticity and electric field in cholesterics with smectic-A phase beneath

Toron is one of the fundamental chiral topological textures appearing in many spintronic and soft matter systems. Here we report on the observation of torons in cholesteric liquid crystals with positive dielectric anisotropy (Δε > 0). In addition, the materials show smectic-A phase beneath the cholesteric phase, enabling us to see the pretransitional effect on the formation of the torons. In detail, we investigated the relation of toron formability and the external stimuli in terms of temperature and electric field. Importantly, the divergence of the elastic constants (K22 and K33) in the vicinity of the nematic-smectic-A phase transition causes the transition from the toron to the homeotropic alignment. The results reveal that both the homeotropic anchoring strength and the frustrated confinement of the cholesteric helix are important to determine whether the toron texture is selected among other textures like fingers. The confinement ratio, i.e. the ratio of cell thickness to cholesteric pitch, for the incidence of torons was found to be in a wide range of about 0.82 ∼ 1.5. We also show that the torons can be polymer-stabilized so that they retain over a wide temperature window. Even more importantly, the orientational state of the polymer-stabilized toron can be changed by an electric field, realizing fast switching between the dark and bright states. The system may find applications in electric-driven diffraction gratings or privacy protection films with a sub-millisecond response time.

Optical, dielectric and elastic studies of high birefringent ambient temperature nematic liquid crystal mixtures having similar molecular cores

We report various physical properties such as optical, dielectric, and elastic studies of two high birefringent climate temperature nematic liquid crystal mixtures having similar molecular cores such as quarter phenyl and phenyethnyltolane cores. In both the mixtures, one compound is common, and, in another compound, the lateral group ethyl group is replaced with a methyl group. These mixtures exhibit high birefringence and high elastic constants in comparison to regular liquid crystals. Both the compounds exhibit high birefringence, positive dielectric anisotropy whereas the parallel component shows anomalous temperature dependence. Our results show that with a variation of lateral group, correlation among the cores varies and that reflects in optical and dielectric anisotropies. These studies will be useful in synthesis and exploring appropriate liquid crystals for various display and smart window applications.

Electrophoresis of metal-dielectric Janus particles with dipolar director symmetry in nematic liquid crystals.

We study the electrophoresis of metal-dielectric Janus particles with dipolar director symmetry in two nematic liquid crystals (LCs) having the same sign of conductivity anisotropy (Δσ) but opposite signs of dielectric anisotropy (Δε). The applied ac electric field is parallel and perpendicular to the director for positive and negative dielectric anisotropy LCs, respectively. We show that the Janus dipolar particles propel faster than the non-Janus dipolar particles in both LCs. The propelling speed of the Janus dipolar particles is also significantly higher compared to the quadrupolar Janus particles studied previously. We map the electroosmotic flow fields surrounding a Janus dipolar particle using microparticle image velocimetry (μ-PIV) and show that the flow on a metal hemisphere is stronger than that on a dielectric hemisphere. Altogether, Janus dipolar particles demonstrate efficient electrophoresis compared to both Janus and non-Janus quadrupolar particles. These findings may be useful for applications in active matter, microrobotic and microfluidic devices.

Comparative dielectric study of bent-core liquid crystal doped 5CB

Bent-core liquid crystal (LC) is used to dope the (4-cyano-4′-pentylbiphenyl) liquid crystal (5CB). The polarizing microscopy results show that 5CB and LC exhibit Nematic (N) and Banana-Shaped (B1) mesophases, respectively. In this paper, we use dielectric spectroscopy to determine the dielectric properties of the 1% LC doped with 5CB liquid crystal. The dielectric parameters of these materials have been determined by different applied voltage. In this respect, dielectric properties of 5CB + 1% LC have been compared with 5CB. The dielectrical anisotropy of 5CB and 5CB + 1% LC liquid crystal composite changes from the positive dielectric anisotropy to negative with the frequency of the applied voltage. The critical frequency fc values of dielectrical anisotropy for 5CB and 5CB + 1% LC composite were found to be 688 and 534 kHz, respectively. The real (ε′) and imaginary components (ε′′) of dielectric constant have been calculated by capacitance method. It has been also determined that adding LC to 5CB decreases the voltage dependent dielectric strength and increases threshold voltages.

Ferroelectric Particles in Nematic Liquid Crystals with Soft Anchoring.

A theoretical evaluation of the electric Freedericksz transition threshold and saturation field is proposed for a liquid crystals composite with ferroelectric particles. Existing models consider a strong anchoring of nematic molecules on the glass support of the cell, but in this paper a soft molecular anchoring of molecules on the glass support and also on the ferroelectric nanoparticle’s surface is assumed. Thus, a finite saturation field was obtained in agreement with real systems. Calculations are made for planar configuration of positive dielectric anisotropy liquid crystals. The results are compared with data obtained on similar systems from different publications and the differences are discussed.