Negative Dielectric Anisotropy Research Articles

P‐193: Ring‐shaped circular Patch liquid crystal display structure based on fringe field effect

The liquid crystal display (LCD) continues to hold a prominent position in the display industry. These years' key research topic is the fringe field effect, which aims to improve the LCD's performance. A new LCD structure utilizing the fringe field effect has been identified as a promising solution. This structure allows for the control of the electromagnetic field of the liquid crystal cell by regulating the voltage of the ring‐shaped and circular‐patch cell (RCC). This innovative structure is comprised of nematic liquid crystals (LCs) with negative dielectric anisotropy, a two‐electrode system consisting of a ring‐shaped ITO structure on the top layer and a circular patch structure on the bottom layer, and a SiO2 porous structure. Finite element simulation tests have verified the electric field strength, transmittance, and contrast of the cell and the 2x2 matrix structure, meeting the design objectives. This breakthrough has significant potential for application in the display field.

Comparative Study of the Optical and Dielectric Anisotropy of a Difluoroterphenyl Dimer and Trimer Forming Two Nematic Phases.

We present a comparative study of the optical and dielectric anisotropy of a laterally fluorinated liquid crystal dimer and its homologous trimer, both exhibiting two nematic phases. In the high-temperature nematic phase, both oligomers exhibit positive optical anisotropy with similar magnitude, which, however, is lower in comparison with the optical anisotropy of the monomer. In the same temperature range, the dielectric permittivity along and perpendicular to the nematic director, measured on magnetically aligned samples, reveals negative dielectric anisotropy for both oligomers, which saturates as the temperature approaches the N-N phase transition temperature. Comparison of the dielectric anisotropies of the oligomers with the corresponding anisotropy of the monomer indicates a systematic variation of its magnitude with the number of the linked mesogenic units. Results are compared with the corresponding anisotropies of the cyanobiphenyl dimers, the archetypal compounds with two nematic phases, and are discussed in terms of the dipolar structure of the mesogens and the dipolar correlations in their nematic phases.

Ferroelectric and antiferroelectric chiral multilactate liquid crystalline materials with negative dielectric anisotropy

In this work, the mesomorphic, electro-optic and dielectric properties have been discussed in the light of molecular structure-property correlations of two chiral multilactate liquid crystalline materials possessing the orthogonal paraelectric Smectic A* phase, tilted ferroelectric Smectic C* phase and the tilted antiferroelectric Smectic CA* phase, over a substantially broad temperature range. Interestingly, a reasonably rare re-entrant Smectic C* phase (SmCre*) has also been identified in one of the investigated materials. These materials differ in their linkage groups (keto or ether) and an additional chiral unit in the terminal chain. The phase transition temperatures and transition enthalpies were determined from Polarizing Optical Microscopy and Differential Scanning Calorimetry (DSC) measurements. The compounds exhibit negative dielectric anisotropy (Δε) throughout the mesomorphic range (maximum −18 and −4 in SmA* for both the compounds) and moderately high values of spontaneous polarization (∼153 nC/cm2 in SmCre* phase). The temperature dependence of the response time (τ), bulk viscosity (η) and the activation energies (Ea) throughout the mesomorphic phase have been determined from the spontaneous polarization measurements. To emphasize the structure-property correlations in more detail, dielectric spectroscopy measurement has also been performed to measure the dielectric strength, dielectric loss, frequency dependent permittivities, relaxation time and relaxation frequencies. The clear evidence of the relatively rare SmCre* phase has also been confirmed from the temperature and frequency dependence of the dielectric permittivity. These results shed important light on the emergence of these materials as a smart alternative for their application in multicomponent mixtures targeted for advanced electro-optic and photonic devices.

Formation and annihilation of electrically driven defects in nematic liquid crystals with negative dielectric anisotropy

Orientationally ordered liquid crystals (LCs) exhibit remarkable physical anisotropy and responsiveness to external fields, which give rise to distinguished physical effects and have led to the emergence of a new generation of electric-optical applications. The LCs are also renowned for their abundance of phases and topological defects, which are of significance in studying both fundamental science and practical technology. One simple approach to generating umbilic defects involves applying an electric field to a homeotropically aligned nematic LC with negative dielectric anisotropy <inline-formula><tex-math id="M8">\begin{document}$\Delta \varepsilon $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M8.png"/></alternatives></inline-formula>. However, the influence of material properties and external conditions on the dynamic process of nematic LC defects remains unclear. Here, we select seven kinds of nematic LCs with negative dielectrically anisotropy, ranging from –1.1 to –11.5, to explore the dynamics of electric-field-induced umbilics. By using a linearly increasing electric field parallel to the molecular orientation of LC, we systematically investigate the effects of material property (dielectric anisotropy) and external conditions (temperature and electric field parameters) on the formation and annihilation of umbilic defects. The experimental results show that the dynamic process of forming the umbilic defects in nematic LCs is independent of dielectric anisotropy, temperature, and electric field frequency, but follows the Kibble-Zurek mechanism, in which the density of generated umbilic defects exhibits a power-law scaling with the change of the electric field ramp rate, with a scaling exponent of approximately <inline-formula><tex-math id="M9">\begin{document}$1/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M9.png"/></alternatives></inline-formula>. Interestingly, a stronger dielectric anisotropy leads to a higher density of umbilic defects. Additionally, a change in temperature has a significant influence on the density of umbilic defects , in which higher temperature leads to greater defect density under the same external electric field conditions. Furthermore, the annihilation rate of umbilic defects is closely related to the material properties and the ramp of the applied electric field. Specifically, the annihilation rate of umbilic defects becomes faster when dielectric anisotropy is stronger or the electric field ramp is larger. This study provides valuable insights into the relationship between the formation and annihilation of defects, material properties, and external conditions in nematic LCs with dielectrically negative anisotropy, contributing to our comprehensive understanding of the dynamic process of topological defects in soft matter.

Novel properties of high-performance multi-component mixture for Vertically Aligned mode LCDs

ABSTRACT Vertically Aligned mode Liquid Crystal Displays (VALCDs) which have generated huge research interest due to their excellent contrast and wide viewing angle require materials with negative dielectric anisotropy. This work highlights the systematic development of a fifteen-component mixture from pure liquid crystalline compounds comprising laterally fluorinated bi-, tri- phenyl, tolane, bicyclohexane compounds and non-mesogenic compounds exhibiting negative dielectric anisotropy. The temperature dependence of birefringence (Δn), dielectric anisotropy (Δε), threshold voltage (Vth), bend elastic constant (K33), relaxation time (τ0) and rotational viscosity (γ1) as a function of temperature of this fifteen-component nematic mixture have been reported. This mixture exhibits a high Figure of Merit (FoM) which is desirable for reducing the device response time. Additionally, the orientational order parameter (OOP) values, activation energy (Ea), visco-elastic co-efficient (γ1/K33) and material parameters (A 0 ) have also been reported. The effect of pretilt angle in the alignment layers on the threshold voltage (Vth) and switching time (τ0) to ascertain the applicability of this material in Vertically Aligned mode Liquid Crystal Displays (VALCDs) have also been discussed.

Suppression of laser speckle contrast using negative dielectric anisotropy nematic liquid crystal doped with an ionic medium

ABSTRACT A liquid crystal (LC) with negative dielectric anisotropy called N-(4-Methoxybenzylidene)-4-Butylaniline (MBBA) doped with the ionic agent cetyltrimethylammonium bromide (CTAB) has been investigated as a speckle suppression device to reduce the laser’s speckle contrast. The electrohydrodynamic instabilities (EHDI) effect in LC is exploited to reduce speckle contrast. The EHDI effect causes chaotic turbulence in LC, resulting in dynamic scattering of the incident light beam. In order to investigate the electro-optic properties and frequency region for optimal performance, optical measurements and dielectric spectroscopy of the pristine and doped LC are performed. The dielectric characteristics of the LC demonstrate that the frequency range to produce the EHDI effect in the LC cell is broadened as the ionic agent is doped. The inclusion of ions also enhances the ion density, mobility, and conductivity in LC. The increase in ion density and mobility enhances the turbulence in the LC at a faster rate; as a result, the dynamic scattering becomes more rapid. When measured with a charge-coupled device (CCD) camera, the faster dynamic scattering provides higher suppression in the laser’s speckle contrast. This method can be used to reduce the speckle contrast in laser source-based imaging and projection displays.

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.

Novel benzoxazole-based liquid crystals with negative dielectric anisotropy and moderate optical anisotropy

ABSTRACT A series of new liquid crystal compounds with negative dielectric anisotropy was synthesised by regulating the orientation of the substituents on the benzoxazole ring, namely, 5–4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl)benzo[d]oxazole derivatives (nPP(2,3)FO). These compounds mainly display enantiotropic nematic mesophases in the ranges of 8.62–16.09°C (heating process) and 23.98–30.32°C (cooling process), moderate birefringence (Δn) ranging from 0.25–0.32 (DFT theoretical calculation) and 0.22–0.28 (experimental measurement), as well as negative dielectric anisotropy (Δɛ) ranging from −2.77 to −2.94. These characterises are ascribed to the larger perpendicular dipole moments and orientation angles of benzoxazole. This work provides new ideas for the rational design of negative dielectric anisotropy liquid crystals.

Benzoxazole-terminated liquid crystals with large birefringence and negative dielectric anisotropy

We synthesized heterocyclic liquid crystals with large birefringence and negative dielectric anisotropy, namely, 5–4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl)benzo(d)oxazole derivatives (nPFO). These compounds display enantiotropic nematic mesophases with wide mesophase range of 45.9–84.0 °C (heating process) and 61.3–119.6 °C (cooling process). Compared with reference analogues of traditional benzoxazole compound 6PFB with different orientation of benzoxazole-terminated structure, the compounds nPFO roughly give lower melting points and clearing points because of their increased dihedral angle. Meanwhile, compounds nPFO show obviously large birefringence (Δn, 0.28–0.39 and 0.39–0.48 for experimental measurement and DFT theoretical calculation, respectively) due to the enhanced π-conjugation caused by the ethynyl bridge group in molecules. Benefitting from the larger perpendicular dipole moments and orientation angles, nPFO displays negative dielectric anisotropy (Δɛ) ranging from −3.60 to −4.10. These compounds can endow a commercial negative LC mixture the larger Δn and higher values of figure of merit, suggesting they are suitable dopant to further improve the performance of some commercial LC mixtures.

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.

The influence of the helical twisting power on the bistable electro-optical performance of the liquid crystalline epoxide/mercaptan /negative-dielectric-anisotropy cholesteric liquid crystal composite films

Bistable electro-optical devices have been successfully prepared by thermal curing of the liquid crystalline epoxide, mercaptan and the cholesteric liquid crystals (N-ChLCs) with negative-dielectric-anisotropy. In order to study the influence of helical twisting power (HTP), an important parameter of the chiral dopant, on the bistable electro-optical properties of the liquid crystalline epoxide/mercaptan/N-ChLC composite films, four samples A1-A4 were prepared. Among them, four different N-ChLCs with the same Bragg reflect wavelength were prepared via blending the negative dielectric anisotropy nematic liquid crystal and the chiral dopant R811, R1011, R2011, as well as R5011, respectively. Results indicated that as the HTP increased, the transmittance in the wavelength range of 400–2500 nm became lower under the low-frequency (LF) electric field and after removing the LF electric field, which resulted from the smaller polymer microspheres. Meanwhile, samples with large HTP showed their large transmittance states under the high-frequency (HF) electric field (10 KHz, 90V) and after removing the HF electric field. Moreover, the sample A4 possessing the chiral dopant R5011 with a large HTP exhibited good bistability, and its large- and low-transmittance states can be maintained for a long time (>30 days) in the absence of electric fields.

TIC Reorientation under Electric and Magnetic Fields in Homeotropic Samples of Cholesteric LC with Negative Dielectric Anisotropy

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field B→ parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When B→ is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along B→.

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.

A bistable light shutter based on polymer stabilized cholesteric liquid crystals

In this paper, a bistable light shutter that can sustain both transparent and opaque states without applied voltage was fabricated based on the polymer stabilized cholesteric liquid crystals (PSCLCs) with negative dielectric anisotropy. By applying a high-frequency or low-frequency electric pulse, the two stable states could be switched into each other. Moreover, the role of polymer concentration and chiral pitch was also investigated. It was found that they could affect the polymer micromorphology and further influence the bistable properties and transmission spectrum. Finally, an optimized precuring formula with 3 wt% of polymerizing monomer and 5 wt% of chiral dopant was applied to prepare a light shutter, which showed good bistability and optically shielding properties. Accordingly, the light shutter was highly energy efficient.

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.

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.

Negative Dielectric Anisotropy Liquid Crystal with Improved Photo-Stability, Anti-Flicker, and Transmittance for 8K Display Applications.

Video systems such as 8K displays can provide a strong sense of presence and reality due to their extremely high resolution and wide field of view. However, high-resolution displays generally suffer from reduced transmittance, which requires the use of liquid crystals with high transmittance and high stability. In this study, negative dielectric anisotropy liquid crystal compositions with excellent photo-stability, anti-flicker capability, and high transmittance are developed, showing potential for 8K display applications. The stability of different types of negative dielectric anisotropy liquid crystal compounds is assessed under light, and the high photo-stability compounds are obtained. In addition, it is demonstrated that the flicker can be optimized from -17.6 to -47.0 by removing the compounds with a higher ion content and a larger deformation number and adding the compounds with a smaller deformation number in the negative dielectric anisotropy liquid crystal compositions. Combining with the evaluation of the factors affecting the response time, the negative dielectric anisotropy liquid crystal mixed H is designed with improved stability and flicker; thus, the response time was decreased to 9.5 ms, and the optical transmittance was 5.5% higher than that of MAT-09-1284 (for normal) and 3.1% higher than that of BY19-J01A (for 4K).

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