Properties Of Liquid Crystal Cells Research Articles

Electro-optic properties of liquid crystal cells with nanowall electrodes

Electro-optic properties of liquid crystal cells with nanowall electrodes

Influence of mechanical vibration actions on the memory effect of PDLC films doped with SiO<sub>2</sub> nanoparticles

The effect of mechanical vibration in the frequency range from 200 Hz to 2500 Hz on the memory effect in 5CB polymer dispersed liquid crystals doped with SiO2 nanoparticles has been studied. As a result of capacitance-voltage measurements, the dependences of the dielectric properties of liquid crystal cells with 0.1 wt% SiO2 on the bias voltage. It has been established that the memory effect detected by dielectric hysteresis in polymer dispersed liquid crystals with silicon dioxide nanoparticles monotonically decreases with increasing oscillation frequency and completely disappears at a frequency of 2000 Hz due to the destruction of the network of silica nanoparticles and, respectively, the appearance of an isotropic orientation liquid crystals molecules.

Effect of WS2 nanoparticles on the refractive properties of liquid crystal compositions

The spectral dependences of the refractive index of liquid crystal compositions sensitized with different concentrations of tungsten disulfide (WS2) nanotubes are considered in the present study in the visible spectral range. A correlation between the refractive index and the dynamic properties of liquid crystal cells is established. The characteristics of laser-induced breakdown under the influence of a pulsed laser with a wavelength of 1.54 µm are also considered. The concentration of the WS2 nanoparticles doping the liquid crystal mesophase, at which the phase transition to a quasi-smectic state occurs, was determined. This finding significantly enhances the application range of liquid crystal cells in optoelectronic systems.

Diffraction properties of liquid crystal cell with beat structure formed by photoalignment substrates.

The diffraction properties of liquid crystal (LC) cells fabricated by photoalignment substrates with different periodic optical anisotropic distributions are demonstrated. The twist angle distribution structure inside the LC cells formed by the grating period difference between photoalignment substrates is called "beat structure." In addition to diffraction corresponding to the grating periods of photoalignment substrates, beat structure itself also functions as a diffraction grating. The diffraction properties can be controlled by varying the retardation. The fabrication method and diffraction properties of the LC cells with beat structure are explained by both experiment and theoretical analysis.

Nanoparticle-induced vertical alignment liquid crystal cell with highly conductive PEDOT:PSS films as transparent electrodes

ABSTRACTIn this work, high transmittance vertical alignment (VA) liquid crystal (LC) cells were accomplished by using highly conductive PEDOT:PSS films and polyhedral oligomeric silsesquioxane (POSS) nanoparticles. PEDOT:PSS films could be used as the transparent electrode and treated with rubbing process. Therefore, high contrast ratio VA cell without disclination lines could be achieved. Moreover, the electro-optical properties of LC cells fabricated by the rubbed PEDOT:PSS films and POSS were comparable to those constructed by indium tin oxide (ITO) and polyimide. Our work shows the potential of POSS-induced VA on rubbed PEDOT:PSS films for ITO-free and flexible LC displays.

Highly Conductive PEDOT:PSS Film by Post-Treatment with Dimethyl Sulfoxide for ITO-Free Liquid Crystal Display

In this study, a simple method to enhance the conductivity of poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) thin films from 0.7 to 1185 S/cm by post-treatment with dimethyl sulfoxide (DMSO) is demonstrated. After the rubbing technique was applied, the DMSO-treated PEDOT:PSS film was used as both transparent electrode and alignment layer to fabricate ITO-free liquid crystal (LC) cells. The electro-optical properties of LC cells fabricated by rubbed PEDOT:PSS were compared to those constructed by ITO and polyimide. This work indicates that highly conductive PEDOT:PSS film is a promising material for ITO-free LC devices.

Liquid crystal alignment behavior on transparent cellulose films

The liquid crystal (LC) alignment properties of LC cells fabricated with plant-based cellulose films were investigated.

Amplification of the Capacitance Containing Nematic Liquid Crystal Embedded with Metal Nanoparticles

Herein, we report the dielectric properties of liquid crystal cells embedded with the nanoparticles of Pd, where each of which is covered with a diffusion cloud. It is shown that an amplification of the capacitors with these media occurs with the gain,Ac=12.5,when the concentration of nanoparticles is 0.3 wt% and in the frequency region below the dielectric relaxation frequency, 158.5 Hz. This phenomenon is explained by an equivalent circuit model together with a compatible explanation of the dielectric strength and the relaxation time. It is claimed that the occurrence of the capacitance amplification may be attributed to a special nature of the oscillating extra charges, which appear in the region between the host medium and inclusion, and produces an effective negative dielectric constant of the special nanoparticles. This explanation was made by formulating an independent auxiliary equivalent circuit equation that enables to determine the numerical condition of the production of the negativity in the dielectric constant of inclusions (nanoparticles), and, thus, we succeeded in getting the numerical value of this dielectric constant and that of the gain of the capacitance amplification.

Liquid crystal alignment properties of n-alkylsulphonylmethyl-substituted polyoxyethylenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of n-alkylsulphonylmethyl-substituted polyoxyethylenes (#S-PEO, # = 4, 6, 7, 8 and 10), where # is the number of carbon atoms in the n-alkyl side groups having different n-alkyl chain length, were investigated as a function of the rubbing density. The LC cells made from unrubbed #S-PEO (# ≥8) films having more than eight carbon atoms in the n-alkyl side groups showed homeotropic LC alignment. The homeotropic LC alignment behaviour correlated well with the surface energy values of the unrubbed #S-PEO films; homeotropic LC alignment was observed when the surface energy values of the unrubbed #S-PEO films were smaller than about 21.62 mJ m−2. The LC cells made from rubbed #S-PEO (# ≥7) films having more than seven carbon atoms with a rubbing density of 150 showed homeotropic LC alignment. It was also found that the tilt angle of the LCs on the rubbed #S-PEO films was affected not only by the n-alkyl chain length of the polymers, but also by the rubbing density, regardless of the surface energy value of the #S-PEO film.

Liquid crystal alignment properties of 2-naphthoxymethyl-substituted polystyrenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of 2-naphthoxymethyl-substituted polystyrenes with different contents of naphthoxymethyl side groups were investigated. The polymer films exhibited good optical transparency in the visible light region (400–700 nm). The LC cells made from the unrubbed films of polymers having more than 57 mol% of 2-naphthoxymethyl containing monomeric units showed homeotropic LC alignment with a high pretilt angle of about 90o. Good electro-optical characteristics, such as the threshold voltage, response time, voltage holding ratio and residual DC voltage were observed for the LC cells fabricated with the polymer having 100 mol% of 2-naphthoxymethyl containing monomeric units as an LC alignment layer.

Liquid crystal alignment properties of polystyrene derivatives containing coumarin side groups

The liquid crystal (LC) alignment properties of LC cells fabricated with films of polystyrene derivatives containing coumarin side groups (P7COU#, where # is the molar content of coumarin side groups in %) were investigated. Good optical transparency of the P7COU# films was observed in the visible light region. LC cells fabricated with rubbed P7COU# films with more than 51 mol. % of coumarin‐containing monomeric units exhibited LC alignment perpendicular with respect to the rubbing direction with a high azimuthal anchoring energy (1×10−5–7×10−5 J m−2). LC cells made from rubbed P7COU# films with more than 65 mol. % of coumarin‐containing monomeric units exhibited a shift of the LC alignment direction by approximately 10° compared with those made from a polystyrene film. The electro‐optical performance of the LC cells fabricated with the rubbed P7COU82 film was found to be as good as that of the LC cells fabricated with a rubbed polyimide film.

Molecular orientation and photorefractive effects in single-wall carbon nanotubes doped nitrile–ester mixture liquid crystals

We report the electro-optical properties of liquid crystal cells containing pure nitrile–ester mixture liquid crystal (NEMLC) and its doped form with 0.05% (wt/wt) single-wall carbon nanotubes (SWCNT). Diffraction efficiencies of 441-nm pump and 632-nm probe beams were measured in two-wave mixing experiment. Efficiencies and rise time were found to increase in SWCNT doped nematic liquid crystals. A maximum diffraction efficiency of ∼15% was found for cell doped with SWCNT, while cells without SWCNT had a maximum efficiency of 7%. We also have observed that photoconductivity in SWCNT doped liquid crystals is highly dependent on the light intensity. This is attributed to the doping, which enhances the efficient photo-charge generation, combined with enlarged conductivity.

Synthesis of New Vulcan-Shape Aromatic Polyimides for IPS-Mode and Their Alignment Properties

In plane switching (IPS)-mode liquid crystal display (LCD) is one of the most useful technologies for a wide range of viewing angles. For IPS-mode application, we synthesized novel diamine containing a methyl group as a bridge and homogeneous alignment materials. A 1 ∼ 2 low pretilt angle manifested by changing dianhydrides (BTDA and BPDA) and bridged diamines (ODA and PACM). We measured their pretilt angles using the crystal rotation method with positive liquid crystal (LC). The electro-optical properties of LC cells using these polyimides as alignment films were also investigated. Results show all polyimides manifest a low pretilt angle of below 2 degrees.

Influence of Longitudinal Electric Field on Time Characteristics of Freedericksz Transition

Electrooptic properties of liquid crystal cells with semiconductor substrate are studied. A DC electric field has been applied along the semiconductor substrate. It is shown, that presence of such field results both in reduction of Freedericksz threshold and in essential reduction of characteristic reorientation and relaxation times. For more understanding of the mechanism of current flow through the liquid crystal – semiconductor interface, the Volt-Ampere (V-I) and spectral characteristics of photocurrent for such system have been investigated.

The geometrical optics approach for multidimensional liquid crystal cells

We present an approach based on the geometrical optics approximation (GOA) for analysis of liquid crystal cells whose director varies in more than one spatial dimension (multidimensional liquid crystal cells). The GOA is applied to calculate light transmittance and far field diffraction patterns for two- and three-dimensional nematic liquid crystal films. Important features of the GOA, such as a method of eliminating non-convergence problems that can occur during the iterative numerical solution of the equations for the amplitudes of the electromagnetic field, are described. We compare the results obtained from the GOA with those produced by the quasi-one-dimensional Jones calculus and the beam propagation method, where the latter is applicable. It was found that the refraction (or ray bending) effects, produced by the GOA, are more important than effects of diffraction and light scattering, which means that the GOA (unlike the Jones calculus) is accurate for the considered type of liquid crystal cells, whose director varies on the micron scale. The GOA is about as fast as the Jones method and is applicable for calculating optical properties of liquid crystal cells with any number of dimensions of director variations.

Chemical structural effects of polyimides on the alignment and electro-optical properties of liquid crystal cells

The nature of the nematic liquid crystal (LC) alignment induced by the rubbed polyimide (PI) alignment layers (ALs) and the electro-optical (EO) properties of the LC cells are expected to depend on the chemical and physical features of the PI. By employing five pyromellitic dianhydride (PMDA)-type PIs having different functionalities, we have studied the effects of the PI’s structure and chemistry on the alignment characteristics and the cell’s EO properties. Increasing the flexibility of the PI increases the pretilt angle and tends to improve the alignment stability. On the other hand, the rigid, fluorinated PI displays poor stability for LCs and induces a less stable/uniform LC alignment and, subsequently, a small pretilt angle. It also transpired that fluorination of the PI deteriorated the voltage-transmittance characteristics and the voltage holding ratio; increasing the flexibility of the PI structure improves these EO properties. The finding that the qualitative trends for the PI’s functionalities are similar for both the alignment and EO properties suggests that the EO properties are closely related to the alignment characteristics, which are determined by short-range interactions between LC and PI molecules.

Electrooptical and Response/Relaxation Properties of Liquid Crystal Cells in In-plane Switching Mode with Polyvinylcinnamate Photoinduced Alignment Layer

Using polyvinylcinnamate (PVCi) photoinduced alignment layers and rubbed polyvinylalcohol (PVA) alignment layers, we produced PVCi-PVA and PVA-PVA in-plane switching (IPS) cells. Transmission properties of the two kinds of IPS-mode cells show that it is possible to decrease the driving voltage for the IPS-mode cell with a weak anchoring alignment surface. The response times of the PVCi-PVA cells are almost the same as those of the PVA-PVA cells, however, the measurement results of the relaxation times for the PVCi-PVA cells indicate that they depend on the time of the voltage applied, and the relaxation times increase with the increase of the applying time. Furthermore, we investigate the decline effect of the twist angle of a PVCi-PVA cell with time after a voltage has been applied for a very long time and then removed.

P‐68: Influence of Polyamide Structures on Electrical Properties of Active‐Matrix Liquid‐Crystal Cells

AbstractThe electrical properties of liquid crystal cells were estimated using various kinds of polyimide(PI) films. It is clarified that the PI structures and its layer structures affects electrical properties of LC cells. Based on the result, we developed a new alignment layers that exhibit a high voltage holding ratio and a low residual DC voltage at the same time.

Molecular Orientations and Optical Transmission Properties of Liquid Crystal Cells with Slit-Patterned Electrodes

The molecular orientations in slit-patterned electrode liquid crystal (LC) cells with homogeneous initial alignment parallel to the slit pattern are investigated in detail using a 3-dimensional polarization observation method. In addition to conventional observations of interference fringes, we study the molecular orientations from cross sections of the LC cells using photoelasticity experimental techniques and a UV-curable LC material. A model of LC molecular orientations referred to as the electrically induced hybrid twisted nematic (EHTN) molecular orientation, is proposed. Moreover, optical transmission properties of such LC cells with different structural parameters are measured as a function of the applied voltage. The possibility of application of these kinds of LC cells to LC display devices is discussed.

Properties of liquid crystal cells of pyrolyzed polyimide Langmuir-Blodgett films

Dependence of the conductivity on the pyrolyzing temperature of pyrolyzed polyimide Langmuir-Blodgett (PPI LB) films was examined and compared with the results of absorption spectra of the films. The PPI LB films pyrolyzed at 1000 °C showed good electro-optic properties as electrodes and aligning films for twisted nematic liquid crystal cells.