Electrical orientation behaviour of a nematic liquid crystal with negative dielectric anisotropy above the microelectrodes

Factors for the electrical orientation on interdigitated microelectrodes deposited on a substrate, a microdielectrometric sensor (or a sensor), were discussed experimentally using a model nematic liquid crystal with negative dielectric anisotropy. The electrical orientation behaviour of the nematic liquid crystal was investigated using microdielectrometry and polarised optical microscopy. The experimental results were discussed in relation to electric-field line, surface topology, anchoring of the liquid crystalline molecules to the microelectrodes, and transversal rotation of a rode-like molecule of the liquid crystal with negative dielectric anisotropy.     

Nematic compounds and mixtures with high negative dielectric anisotropy

Syntheses, mesogenic and dielectric properties of three-ring alkylcyclohexyl, alkoxy and alkylphenyl benzoates substituted with four fluorine atoms or with two fluorine atoms and simultaneously a cyano group or two cyano groups are described. Their properties are compared with analogous three-ring phenyl tolanes. Conformation analysis and the calculation of dipole moments with the quantum chemistry method are performed to explain the experimental results. Several four fluorine-substituted two-ring tolanes and biphenyls are also prepared, which are used as a solvent decreasing viscosity, melting point and clearing point of the formulated mixtures. Examples of mixtures prepared from the esters and tolanes with high dielectric anisotropy are given. Curiosity is observed that 4?-(4-ethoxyphenyl)-4-pentyloxy-2,2?,3,3?-tetrafluorotolane is not the mesogenic compound, while analogous ester 4?-ethoxy-2,3-difluorobiphenyl-4-yl 2,3-difluoro-4-pentyloxybenzoate exhibits a broad temperature range of the nematic phase.     

Fast switching of vertically aligned negative liquid crystals by optically hidden relaxation

We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.     

A single-cell-gap transflective liquid crystal display with a vertically aligned cell

A single-cell-gap transflective liquid crystal display with a vertically aligned cell using square ring electrode is demonstrated. The top substrate has a top planar common electrode, a square ring pixel electrode is coated on the bottom substrate, while a bumpy reflector is coated under the bottom substrate. In this device, the planar common electrode and square ring pixel electrode generate a strong longitudinal electric field in the transmissive region (T region) and a weak fringe field in the reflective region (R region). As result, the T and R regions accumulate the same optical phase retardation. The simulation results show that the display exhibits reasonably low operating voltage, high transmittance and well-matched voltage-dependent transmittance and reflectance curves. Besides, fabrication process of the transflective liquid crystal display is very simple.     

Electroconvection characterization of guest-host nematic liquid crystals for dynamic scattering mode applications

ABSTRACT

The threshold voltage for electro-hydrodynamic convection (EHC) in nematic liquid crystals in the low-frequency convective regime is examined. The addition of dopant species is known to strongly effect the frequency response of this regime. In this work, we investigate the effect of adding optically anisotropic species on the low-frequency threshold, and, demonstrate that certain dichroic dyes have a marked effect. We have measured electrical transport properties of suitably doped liquid crystal mixtures in order to make a comparison with theoretical predictions based upon linear stability considerations. Our results show satisfactory agreement with these predictions. Our findings indicate future directions for new applications of dynamic scattering mode technologies with more accessible operating parameters previously known.     

Fluoro-substitution in hydrogen bonding liquid crystal benzoic acid: dielectric,electro-optic and optical proprieties and inducing polar nematic phase

ABSTRACT

Dielectric and electro-optic properties have been studied in order to understand the effect of the fluoro-substitution on hydrogen bonded liquid crystal proprieties. The fluoro-substitution alters the characteristics of pure LC structure and changes its physical parameters. In the present paper, variation in physical parameters like dielectric permittivity, response time, elastic constant, rotational viscosity etc. has been explained based on many factors. In addition, the fluorine substitution has lead to the appearance of the polar nematic phase.     

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.     

Analysis of electro-optical and dielectric parameters of TiO2 nanoparticles dispersed nematic liquid crystal

The present investigation is focused on to find out the role of TiO₂ nanoparticles (NPs) on altering the dielectric and electro-optical parameters of nematic liquid crystal (NLC). In addition to this, we also optimized the concentration of dopant (0.25 wt%) for a saturation value of permittivity and dielectric anisotropy in the doped system. Dielectric spectroscopy has been performed with the variation of frequency and temperature to investigate the various dielectric parameters, which demonstrate that the investigated NLC is of positive dielectric anisotropy; the observed result shows a decrement in the value of relative permittivity and dielectric anisotropy; however, the permittivity value increases for higher concentration of dopant but remains less than that of pure NLC. Electro-optical measurements have also been performed to compute the optical response of pure and dispersed NLC. It is found that optical response decreases for the NP-doped systems. This optimized concentration of NPs in NLC matrix can have various credential applications in the field of active matrix display and holography.     

Electrically induced anchoring transition in nematics with small or zero dielectric anisotropy

The orientational transitions induced by electrically controlled ionic modification of surface anchoring in liquid crystal cells based on the nematics with small or zero dielectric anisotropy Δε are considered. The type of director reorientation is shown to be independent of the sign of dielectric anisotropy and can be the same for the nematics with both negative and positive Δε. Besides, the orientational transition and corresponding switchable optical states do not depend on the Δε value and can be effectively realised even for the nematics with zero dielectric anisotropy.     

Free energy in vertically aligned liquid crystal mode

Vertical alignment (VA) is a widely applied operation mode for liquid crystal displays. To achieve optimum brightness, the electrode of VA is often patterned with fish-bone fine slits to generate fringe field, so the negative liquid crystal aligns along the fine slits when the electric field is applied. VA is usually simply modelled by the bend geometry along the cell gap. However, defects, domain boundaries and periodical splay induced by the fine slits also exist in real pixels and disturb the liquid crystal alignment. Polymer-stabilised VA test cells with various fine slit pitches which lead to various strength of fringe field were fabricated to observe the deformation of liquid crystal. Then the models of liquid crystal deformation nearby the defects and in the fine slit area were proposed to calculate the electromagnetic (f_EM) and elastic free energy (f_elastic). The results show that the key factor to regulate f_EM and f_elastic is the pitch of the fine slits, and the optimum liquid crystal alignment is obtained when f_EM and f_elastic are equal. The models are useful for further investigation on the dynamics of liquid crystal alignment and applications in industrial products.     

Investigation of alignment direction in wide view film and rubbing angle of twisted nematic liquid crystal display mode

Wide view (WV) film is an important material in the polariser. It is a hybrid-aligned, discotic liquid crystal (DLC) on an alignment layer on the tri-acetyl cellulose film, which is used as compensation film in twisted nematic (TN) mode liquid crystal display (LCD). The relation between the alignment direction of the DLC in the WV film and the rubbing direction of the polyimide on the glass substrate in the TN mode LCD was investigated. The results indicated that the contrast ratio (CR) of the TN mode LCD can be increased by adjusting this angle . When an 88°?rubbing angle was used in this work, the CR of the TN mode LCD could reach 1000:1 and the horizontal and vertical viewing angles were 170 and 160°, respectively.     

Fast fringe-field switching of vertically aligned liquid crystals between high-haze translucent and haze-free transparent states

We report control of the haze value in a liquid crystal (LC) cell driven by a fringe electric field. When a fringe field is applied to a vertically aligned (VA) cell, a large spatial phase difference with a short grating period is induced in the LC layer. The average grating period of a VA cell driven by a fringe field is a quarter of the pitch of the interdigitated electrodes, which is half of the grating period of a VA cell driven by an in-plane field. Moreover, a sharper spatial phase profile is built around the edges of the interdigitated electrodes, which led to a high haze of 84.3% in the translucent state. The device was haze-free in the transparent state owing to the use of an LC layer without a polymer structure. To increase the haze value of the LC device while retaining a short response time, we developed an LC cell with crossed interdigitated electrodes where a large spatial phase difference is induced with little dependence on the azimuth angle. By applying a fringe electric field to a 20 μm thick LC cell using crossed interdigitated electrodes, we demonstrated a very high haze of 95.4% and a response time of less than 5 ms.     

Synthesis,mesogenic and dielectric properties of fluorosubstituted isothiocyanatoterphenyls

The synthesis, mesomorphic and physical properties of 14 homologous series of laterally fluorinated 4?-alkyl-4-isothiocyanatoterphenyls were described. Influence of the number of fluorine atoms and their position in the terphenyl core on the phase transition temperatures, nematic range, dielectric and optic anisotropy as well as bulk viscosity were discussed. The compounds with the most optimal properties for formulation of nematic mixtures were selected, and mixture examples with low viscosity and medium or high birefringence were presented.     

Dielectric properties of selected laterally fluoro-substituted 4,4′′-dialkyl,dialkoxy and alkyl-alkoxy [1:1′;4′:1′′]terphenyls

Measurements of the complex permittivity, ε*?=?ε′ – iε″, within the frequency range 200 Hz to 10 MHz for 15 laterally fluoro-substituted terphenyls have been conducted. In most cases the substances exhibited the nematic phase over a broad temperature range. All substances were characterised by negative dielectric anisotropy, and are potentially useful for vertical alignment mode systems. The static permittivity tensor components have been analysed in relation to the dipole structure of the molecules. Dielectric relaxation processes observed in the liquid crystalline (LC) and solid rotator (R) phases (obtained by slow cooling of the samples) are characterised by calculation of the relaxation times and activation barriers. The rotation motions around the short axes are typical for LC phases, whereas rotations about the long axes, accompanied in some cases by internal motions, are present in the R phase.     

Molecular vibration mode assignment of nematic liquid crystal 5CB on Terahertz spectra

Using DFT/B3LYP/6-311++G** method, the molecular structure and absorption spectra in terahertz (THz) range of liquid crystal 5CB are investigated. In a frequency range 0–15 THz, an assignment of the vibrational modes corresponding to absorption frequencies is performed using potential energy distribution for the first time. It is found that the cyano group radical (–CN) do actively take part in the strongest THz absorption of 1.743, 3.942, 5.169 and 14.769 THz in different vibration modes. The results suggest that the strong polar group should be avoided in designing liquid crystal molecule and mixtures in order to reduce the absorption intensity in THz range.     

Elastic constants,viscosity and dielectric properties of bent-core nematic liquid crystals doped with single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) are dispersed in (4’-fluoro phenyl azo) phenyl-4-yl 3-[N-(4’-n-hecyloxy 2-hydroxybenzylidene)amino]-2-methylbenzoate (6–2M-F) a bent-core nematic (BCN) liquid crystalline medium composed of bent-shaped molecules with short core, reduced bend angle possessing polar fluoro substituent in longitudinal direction and methyl group in bent direction. Such molecules are at the borderline of typical bent-core and rod-like molecules resembling hockey stick shape with intermediate properties. The elastic anisotropy is negative for 6–2M-F (bend elastic constant K₃₃ < splay elastic constant K₁₁); similar to other BCNs reported earlier with smectic-like clusters; but turns to high positive (K₃₃ > K₁₁) value by insertion of SWCNT (concentration ≥0.05 wt.%) in 6–2 M-F. The ratio of K₃₃/K₁₁ becomes comparable to the calamitic liquid crystals (LCs) in doped system. Dielectric anisotropy increases in the nanocomposite implying enhanced nematic ordering due to π–π electron interaction between CNTs and the LC molecules. Threshold voltage at first increases and then decreases with increasing CNT concentration owing to the respective variations in splay viscosity of the system. The present study demonstrated the interaction of SWCNTs with BCN molecules and reveals significant modifications in viscoelastic, dielectric and ionic properties of the host.     

A simple transflective liquid crystal display with composite dielectric layer

ABSTRACT

A simple transflective liquid crystal display with a vertically aligned cell using a composite dielectric layer is demonstrated. The top substrate has a top planar common electrode, two transparent dielectric layers with different dielectric constants are coated on the bottom planar pixel electrode to generate linearly varying electric potential from the transmissive region (T region) to the reflective region (R region), while two bumpy reflectors are coated under the bottom substrate. In this device, with the composite dielectric layer, the common and pixel electrodes generate a strong electric potential in the T region and a relatively weak electric potential in the R region. Consequently, the T and R regions accumulate the same electro-optical characteristics. The simulation results show that the display exhibits reasonably low operating voltage, high optical efficiency and well-matched voltage-dependent transmittance (VT) and reflectance (VR) curves. Besides, the driving mode and the fabrication process of the transflective liquid crystal display are fairly simple and it is suitable for mobile applications.     

Reverse-mode polymer dispersed liquid crystal films prepared by patterned polymer walls

This article proposes a methodology to prepare polymer dispersed liquid crystal (PDLC) films working in the reverse-mode operation, where the ion-doped nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε) were locked by polymer walls. On-state and off-state of films were controlled by an electric field. In the absence of an electric field, it appears to be transparent. In the field, the homogeneous alignment NLCs form dynamic scattering, giving rise to opaque. The effect of the cylindrical holes with different diameters of photo masks and liquid crystal Δε on the electro-optical properties and transmittance wavelength range of 400–3000 nm light of samples were investigated. It was found that it exhibited very good electro-optical characteristics, high contrast ratio and excellent infrared energy-efficient of films used as switchable windows.     

Effects of carbon nanotubes on electro-optic characteristics in vertically aligned liquid crystal display

Size- and aggregation-controlled dispersion of thin multiwalled carbon nanotube (t-MWCNT) in negative dielectric anisotropic liquid crystal (LC) material exhibits remarkable improvement in electro-optic response time in vertically aligned LC cells. The physical properties such as birefringence, dielectric anisotropy and clearing temperature of nanotube dispersed LC material appear to be almost invariant to that of pristine LC. Nevertheless, the response time shows noticeable improvement, especially in decaying time associated with transition from maximum to minimum transmission, hence important for faster switching LC devices. The effect is attributed to that vertically aligned t-MWCNTs along the field direction play role of vertical alignment layer between LCs, consequently resulting in increased bend elastic constant of LCs.