Negative Dielectric Anisotropy Liquid Crystal Research Articles

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.

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).

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.

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.

Tuning of Polarization-Dependent Whispering Gallery Modes in Grapefruit Microstructured Optical Fibers Infiltrated With Negative Dielectric Anisotropy Liquid Crystals

An electrically tunable whispering gallery mode microresonator based on microstructured optical fibers (MOFs) infiltrated with negative dielectric anisotropy liquid crystals (LCs) is proposed and experimentally demonstrated. Experimental results indicate that the second radial order mode of the MOF microresonator has stronger electric field response than the first radial order mode and the resonance dip for TE polarization mode is more sensitive to the applied electric field intensity in comparison with the TM polarization mode resonance dip. The Freedericksz transition threshold of the proposed MOF microresonator is experimentally found to be about $\text{2.0}\;{\text{V}}\mu {{\text{m}}^{ - 1}}$ . The electrically tunable microresonator integrated with negative dielectric anisotropy LCs is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.

P‐150: Flexoelectric Effect in Fine‐patterned In‐plane Switching (IPS) Mode

Low‐frequency driving of liquid crystal displays can greatly reduce the power consumption, but the flexoelectric effect badly occurs as an optical fluctuation in space and time, so‐called image‐flickering. In a fringe‐field switching (FFS) mode, both dynamic and static image flickering occur whereas, for an in‐plane switching (IPS) mode, dynamic image‐flicker takes place with no static flicker. However, the overall transmittance in IPS mode is relatively poor as compared to that of FFS mode. Here, we demonstrate that fine‐patterned electrodes and negative dielectric anisotropy liquid crystal can greatly enhance the transmittance as high as that of FFS mode while keeping no static image flicker. The remained dynamic flicker can be also significantly reduced by increasing the magnitude of dielectric anisotropy of liquid crystals. We report the detailed behavior of voltage‐ and time‐dependent transmittances.

49.1: Invited Paper: n‐FFS vs. p‐FFS: Who wins?

Both negative and positive dielectric anisotropy liquid crystals have been used in fringe field switching (FFS) display devices. We compare the electro‐optic performances of single‐domain p‐FFS with two‐domain n‐FFS from device and material viewpoints. Quantum‐dot backlight enhanced FFS shows vivid colors with negligible color shift, higher transmittance, and better sunlight readability.

Fast falling time of fringe-field-switching negative dielectric anisotropy liquid crystal achieved by inserting vertical walls.

We report a simulation result in which we obtained a faster falling time of the fringe-field-switching negative dielectric anisotropy liquid crystal (n-FFS) by segregating the subpixel area with vertical walls. By inserting walls near both edges of the interdigitated electrodes, the transmittance (TR) between the walls was increased by a large twist angle of the liquid crystal. The falling time of the n-FFS with 12 walls was 10.5 ms, about 50% less than the n-FFS without a wall (21.3 ms). The maximum TR (TR(max)) of the proposed n-FFS was 26.1%, which was slightly smaller than the TR(max) of the n-FFS without walls (27.8%) but still larger than the FFS with a positive dielectric anisotropy liquid crystal (p-FFS).

24.1: A Method for Analyzing the Eye Strain in Fringe‐Field‐Switching LCD under Low‐Frequency Driving

AbstractWe have presented the brightness changes in positive and negative dielectric anisotropy liquid crystal (P‐LC and N‐LC) with different alignment materials for no eye‐strain at low frequencies. We propose a method for analyzing the brightness changes and find that flexoelectric effect (FEE) is a dominant factor to affect the eye‐strain in P‐LC, and the other crucial part is brightness holding ratio effect (BHRE). This work offers an easy approach to analyze the eye‐strain quantitatively, which indicates a way to solving the problem.

High Performance Negative Dielectric Anisotropy Liquid Crystals for Display Applications

We review recent progress in the development of high birefringence (Δn ≥ 0.12) negative dielectric anisotropy (Δε < 0) liquid crystals (LCs) for direct-view and projection displays. For mobile displays, our UCF-N2 (low viscosity, negative Δε, high Δn) based homogeneous alignment fringe-field switching (called n-FFS) mode exhibits superior performance to p-FFS in transmittance, single gamma curve, cell gap insensitivity, and negligible flexoelectric effect. For projection displays using a vertical alignment liquid-crystal-on-silicon (VA LCOS), our high birefringence UCF-N3 mixture enables a submillisecond gray-to-gray response time, which is essential for color sequential displays without noticeable color breakup. Our low viscosity UCF-N2 also enables multi-domain VA displays to use a thinner cell gap for achieving faster response time.

Fringe-Field Switching with a Negative Dielectric Anisotropy Liquid Crystal

We report a high performance negative dielectric anisotropy (Δe) liquid crystal for fringing field switching (n-FFS) display. We compare the electro-optic characteristics of FFS cells using positive and negative Δe LCs. With comparable driving voltage and response time, the n-FFS cell has advantages in higher transmittance, single gamma curve, less cell gap sensitivity and slightly wider viewing angle. LC director deformation distribution is analyzed to explain these performance differences.

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

AbstractThe development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Fabrication of pseudo-pi vertical alignment mode liquid crystal devices with ultra-violet polymerisation and investigations of their electro-optical characteristics

In this study, we applied the non-contact photo-polymerisation method in fabricating pseudo-polyimide (PI) liquid crystal (LC) devices with vertical alignment copolymer films, in the hope of solving the problems associated with traditional rubbing alignment processes. We mixed different photo-polymerisation-type acrylic monomers and negative dielectric anisotropy LC at proper weight percentages, and induced phase separation behaviour between the photo-polymerisation-type acrylic monomers and the LC molecules in the LC mixture solution through ultra-violet light irradiation, thereby forming copolymer films with vertical alignment effects. We further conducted preliminary characterisation of the alignment mechanism, photo-polymerisation, etc., of the LC molecules and investigated their interactions with alkyl acrylic monomers with different carbon chain lengths (the analogue of alkyl side chains in traditionally used vertical alignment-type PI) and electro-optical properties such as the transmittance, contrast ratio, and response time.

P‐122: Enhanced Dynamic Response Time of Alq3‐Doped Vertical Alignment Liquid Crystal Cell

AbstractWe mixed Alq3 in negative dielectric anisotropy liquid crystal to improve the electro‐optical characteristics of vertical alignment cell. The fabricated vertical alignment cell shows lower operating voltage and faster dynamic response time than pure vertical alignment cell without decrease of the transmittance. We analyzed the physical characteristics of Alq3‐doped liquid crystal in terms of elastic constant and rotational viscosity. Consequently, the fast dynamic response time is caused due to the increase of the elastic constant in the Alq3‐doped liquid crystal cell.

Continuously tunable all-in-fiber devices based on thermal and electrical control of negative dielectric anisotropy liquid crystal photonic bandgap fibers

We infiltrate photonic crystal fibers with a negative dielectric anisotropy liquid crystal. A 396 nm bandgap shift is obtained in the temperature range of 22-80 degrees C, and a 67 nm shift of long-wavelength bandgap edge is achieved by applying a voltage of 200 Vrms. The polarization sensitivity and corresponding activation loss are measured using polarized light and a full broadband polarization control setup. The electrically induced phase shift on the Poincaré sphere and corresponding birefringence change are also measured. According to the results, tunable wave plates working in the wavelength range of 1520-1580 nm and a potential for realizing a polarimeter working at the 1310 nm region are experimentally demonstrated.

Liquid crystal optical phase plate with a variable in-plane gradient

We propose a nematic liquid crystal (LC) optical phase plate, with a large continuous in-plane gradient that is variable, and its application to a beam steering device with high efficiency. The device is a vertically aligned, continuous phase, optical phased array (V-COPA) that uses a negative dielectric anisotropy LC material. High steering efficiency of over 95% is demonstrated by modeling the LC director field and its effect on transmitted light. The period of the V-COPA grating can be varied by adjusting an applied voltage profile, which allows for continuous angular control of the diffraction angle.

Electro-thermal tuning in a negative dielectric cholesteric liquid crystal material

The thermal and electrical tunability of a cholesteric liquid crystal containing a negative dielectric anisotropy liquid crystal in a planar alignment was studied. The physical, optical, and electro-optical characteristics of mixtures containing different ratios of chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806 were examined. A smectic A phase was seen at room temperature for S811 loadings >20wt%. Below 20%, a room temperature cholesteric phase was observed. Upon heating mixtures with composition S811 >20%, the selective reflection notch of the cholesteric phase appeared and blueshifted with temperature. Thermal tuning from 2300to500nm was observed over the temperature range of 23–55°C. Polarized optical microscopy, differential scanning calorimetry, and x-ray studies were utilized to confirm the temperature-dependent phase behavior. Tuning of ∼50nm by the application of a direct current electric field was also observed with no onset of electrohydrodynamic instabilities for voltages up to ∼300V. Bandwidth broadening but not tuning was obtained with the application of alternating current fields. Electrical tuning is likely due to pitch contraction brought about through the annealing of defects.

Electrically switchable, polarization-independent diffraction grating based on negative dielectric anisotropy liquid crystal

An atomic force microscope is used to scribe polymer-coated substrates to create an electrically controlled liquid crystal-based polarization grating. The grating is nondiffracting in the voltage-off state and diffracting in the voltage-on state. Based upon an optical phase difference of approximately π between adjacent pixels, the grating’s efficiency is independent of optical polarization and can be prepared for diffraction in either one or two dimensions.

New Aspect of the Voltage/Confinement Ratio Phase Diagram for a Confined Homeotropic Cholesteric

We have investigated the behavior of a cholesteric mixture, consisting of the negative dielectric anisotropy liquid crystal MBBA and a chiral dopant S811(1%b.w.). In the phase diagram a.c. applied voltage versus confinement ratio, in addition to the transition previously observed, before reaching the domain of the transition TIC(Translationnary Invariant Cholesteric)–modulated TIC, we have observed a new stability domain for the topological bubble. These bubbles are now surrounded by the TIC domains and necessarily associated to an hyperbolic − 1 defect of the in plane director component.We have also obtained experimentally topological defect chains, formed by topological dipoles similar to the ones formed in 2 compound materials: liquid crystal/solid spheres or liquid crystal/liquid droplets.

First-Order Fréedericksz Transition Above the Nematic–Smectic-APhase Transition

The Fréedericksz transition is investigated for thin homeotropic cells of a negative dielectric anisotropy liquid crystal above its nematic-smectic-A phase transition temperature in a spatially uniform electric field. For very thin cells, a crossover from second- to first-order Fréedericksz behavior is observed; no additional fields are necessary. The results are consistent with the theory of Shi [Liq. Cryst. 29, 121 (2002)]].