Twisted Nematic Research Articles

Experimental Study of Laser-Induced Orientational Effects in Twist and Planar Aligned Azobenzene Nematic Liquid Crystals

Laser-induced orientational effects are experimentally studied in BAAB-1005 type azobenzene NLC. Excitation of cis-form of azobenzene was induced by green laser with 0.532 μm wavelength. Nonlinearity was studied by He-Ne laser. 100 μm thick twist-aligned and planar-aligned LC-cells were used in the experiment. During the experiment occurrence of ring pattern was registered on the screen, which was formed during long time (about 10 seconds). Depending on direction of He-Ne laser polarization two types of rings were registered in case of twist-cell. This means that we have two nonlinear mechanisms. In case of planar-aligned LC cell only one type of rings was registered.

Electro-Optical Characteristics of ZrO2 Nanoparticle Doped Liquid Crystal on Ion-Beam Irradiated Polyimide Layer

It is well known that doping liquid crystals (LCs) with nanoparticles can readily change the physical and electro-optical properties of LC mixture. In this paper, we report on how the electro-optical properties and thermal stability of an LC system were enhanced by dispersing zirconia (ZrO2) nanoparticles in nematic LCs on ion-beam irradiated polyimide layers. Homogeneous LC alignment was achieved and ZrO2/LC mixture was applied in twisted-nematic (TN) mode. The addition of ZrO2 nanoparticles contributed to improvement of electro-optical properties in the TN LC cell by lowering voltage operation and decreasing response time. The TN LC cells with a ZrO2 nanoparticle concentration of 2.0 wt% showed the lowest threshold voltage of 2.0 V and the fastest response time of 15.3 ms. This enhanced electro-optical performance was likely due to van-der waals interactions and the screening effect of the ZrO2 nanoparticles in the LC medium. The thermal stability of the ZrO2/LC mixture was also improved compared to a pristine LC system.

Optically controllable bistable reflective liquid crystal display

This Letter demonstrates a photo-addressable, bistable reflective liquid crystal display that is based on a dye-doped liquid crystal (DDLC). Bistable bright and dark states can be attained using the 45 deg twisted nematic (TN) and photo-induced isotropic states (PHI) of the DDLC, respectively. Both the 45 deg TN and PHI states can exist stably for tens of hours, and each can be rapidly switched to the other by the isomerization effect using UV and green light. A bistable reflective liquid crystal display is simply fabricated, easily operated, and rapidly switched. It therefore has the potential to be used in portable information systems.

Fast-response twisted nematic liquid crystal displays with ultrashort pitch liquid crystalline materials

A new method to improve the response time of twisted nematic (TN) liquid crystal displays (LCDs) with liquid crystalline (LC) materials of pitch length shorter than twice the cell gap is proposed. It was known that by using LC materials possessing shorter pitch length, the response time could be improved. However, in previous studies, the pitch length p was limited to values larger than the twice of the cell gap d. This is because 90° TN arrangements become unstable and transform into a 270° super-twisted nematic (STN) arrangement for p < 2d. It was found that by applying voltage to 270° STN arrangement, the 90° TN arrangement could be formed and the obtained TN-LCDs could be stabilised by photopolymerisation. This is because the 270° STN arrangement contains splay deformation. The obtained TN-LCDs exhibit a fast response. By using this method, fast-response TN-LCDs could be realised maintaining moderate driving voltage.

Optical near-field excitation using liquid crystals on nanostructured photoreactive molecular thin films

Optical near-field excitations were investigated on the basis of molecular alignment control of liquid crystals (LCs) on an optically rewritable nanostructure of photoreactive molecular thin films. Twisted nematic (TN) cells of LC molecules were constructed utilizing ITO substrates with 260 nm gratings of an azobenzene molecular thin film, fabricated using standing evanescent waves. The polarization changes of light transmitted through the TN cells, which were due to the alignment changes of LC molecules locally rubbed by the azobenzene nanogratings, were observed. Furthermore, we demonstrated local plasmon excitation of Au nanowires deposited on the azobenzene nanogratings using oblique vacuum evaporation, a phenomenon that produced strong anti-optical absorption spectra. The modulation of the local plasmon resonance in metallic nanowires decorated with LC molecules was confirmed.

Fabrication of twisted nematic structure and vector grating cells by one-step exposure on photocrosslinkable polymer liquid crystals

We present a simple yet efficient method to automatically fabricate the twisted nematic structure by one-step exposure on an empty glass cell coated with photocrosslinkable polymer liquid crystal (PCLC) films. The resultant photoalignment directions of two substrates can be orthogonal to each other by controlling the difference between the exposure energy for upper and lower PCLC films and the twisted nematic (TN) structure can be automatically fabricated. The vector grating liquid crystalline cell with TN structure was also fabricated by means of a developed method, and the diffraction properties were well explained by the theoretical calculation on the basis of Jones calculus.

Temperature-Frequency Converter Using a Liquid Crystal Cell as a Sensing Element

A new temperature-frequency converter based on the variation of the dielectric permittivity of the Liquid Crystal (LC) material with temperature has been demonstrated. Unlike other temperature sensors based on liquid crystal processing optical signals for determining the temperature, this work presents a system that is able to sense temperature by using only electrical signals. The variation of the dielectric permittivity with temperature is used to modify the capacitance of a plain capacitor using a LC material as non-ideal dielectric. An electric oscillator with an output frequency depending on variable capacitance made of a twisted-nematic (TN) liquid crystal (LC) cell has been built. The output frequency is related to the temperature of LC cell through the equations associated to the oscillator circuit. The experimental results show excellent temperature sensitivity, with a variation of 0.40% of the initial frequency per degree Celsius in the temperature range from −6 °C to 110 °C.

Liquid Crystal Alignment Properties on Zirconia Doped Polyimide Layer

This paper introduces zirconia (ZrO2) use in liquid crystal (LC) alignment for uniform and effective LC orientation. We used a conventional rubbing process to induce LC alignment. As a result, uniform LC alignments were achieved in all samples with ZrO2 nanoparticles from 0.01 wt% to 0.3 wt%. Pretilt angles of LCs on ZrO2 nanoparticles doped LC alignment layers slightly increased as the percentage of ZrO2 nanoparticles rose. However, contact angles were almost the same. When twist-nematic (TN) LC cells used LC alignment layers with ZrO2 nanoparticle content of 0.1 wt%, best performance was shown. Finally, it is shown that ZrO2 nanoparticles doped LC alignment layers have more thermal budget than pristine ones in thermal stability test.

Determination of flexoelectric coefficients in nematic liquid crystals using the crystal rotation method

A crystal rotation method is employed to measure the difference between the splay and bend flexoelectric coefficients, , in liquid crystals using a twisted nematic (TN) cell with in-plane electric fields. Values of for E7 and BL087 are measured to be 7.8 1.0 pC m and 10.8 1.0 pC m, respectively. The method is robust against static changes in the internal electric field resulting from the influence of charge carriers, and this is discussed in relation to other methods.

新型多畴扭曲向列相液晶显示器

This paper proposed a wide viewing angle twisted nematic(TN) liquid crystal display(LCD) with a protrusion structure under the pixel electrode.The proposed LCD mode avoids the complex rubbing processes because of the protrusion which induces the multi-domain structure like the conventional multi-domain TN mode.As no voltage applied,the LC profile is the same as that of the normal TN cell,the LC molecules tilted four different directions to obtain the multi-domain structure while the operate voltage is applied.In this paper,the electro-optic characteristics is simulated using the professional software,and the results show that the proposed multi-domain TN LCD has the wide viewing characteristics and low color shift.

A Novel Method for Measuring the Difference in Flexoelectric Coefficients of Nematic Liquid Crystals

The difference between e1 and e3 parameters for flexoelectric polarization, as originally defined byMeyer, is measured for nematic liquid crystal materials E7 and BL087 in Twisted Nematic (TN) cells with In-Plane Switching (IPS) electric fields using the crystal rotation method, which measures transmission as a function of angle of incidence. Values of e1 − e3 for E7 and BL087 are found to be 7.2±1.0 pCm−1 and 9.4±1.0 pCm−1 respectively.

Improvement of Outdoor Readability Using Novel Switchable Transmissive and Reflective Twisted Nematic LCD

For the first time, we have fabricated a novel Switchable Transmissive and Reflective (STR) LCD based on dual gap TN (Twisted-Nematic) mode using a general a-Si TFT process. This new technology creates ultra low power LCD using reflective mode. This new device has no power consumption of backlight in ordinary room light environment. Moreover our new LCD offers superior outdoor visibility and enhanced portability to user. We will show vision as a future LCD mode for portable electronic devices.

Contrast ratio of twisted nematic liquid crystal cells and its improvement

The light leakage of twisted nematic (TN) liquid crystal displays is investigated. While the TN mode has a robust structure in regard to light scattering, due to thermal fluctuations of the liquid crystals and light leakage from imperfect rubbing defects, it maintains significant residual retardation of the liquid crystal layer in the black state. The residual retardation is induced by the azimuthally twisted liquid crystal alignment near substrate surfaces and causes major light leakage in the black state. We investigated several methods to compensate for the residual retardation, and the methods include the adjustment of the helical power of the liquid crystal material, the increase of driving voltage, the control of rubbing direction and a newly designed compensation film with a small angle twist. Though all these methods improve the contrast ratio by about 30%, each method has its own weaknesses and limitations. The development of a new compensation film would provide a good chance for future TN applications.

Super‐Fast Switching of Twisted Nematic Liquid Crystals on 2D Single Wall Carbon Nanotube Networks

AbstractWe have developed a high performance liquid crystal (LC) alignment layer of ultra‐thin single wall carbon nanotubes (SWNTs) and a conjugated block copolymer nanocomposite that is solution‐processible for conventional twisted nematic (TN) LC cells. The alignment layer is based on the non‐destructive solution dispersion of nanotubes with a poly(styrene‐b‐ paraphenylene) (PS‐b‐PPP) copolymer and subsequent spin coating, followed by conventional rubbing without a post‐annealing process. Topographically grooved nanocomposite films with two dimensionally (2D) networked SWNTs embedded in a block copolymer matrix were created using a rubbing process in which bundles of SWNTs on the composite surface were effectively removed. The LCs were well aligned with a stable pre‐tilt angle of approximately 2° on our extremely transparent nanocomposite, which gave rise to superfast switching of the TN LC molecules that was approximately 3.8 ms, or four times faster than that on a commercial polyimide layer. Furthermore, the TN LCD cells containing our SWNT nanocomposite alignment layers exhibited low power operation at an effective switching voltage amplitude of approximately 1.3 V without capacitance hysteresis.

Composite Method for Measurement of Splay, Twist and Bend Nematic Elastic Constants by Use of Single Special In-Plane-Switched Cell

A method for quick and pretty accurate measurements of splay, twist and bend elastic constants of nematic liquid crystals is proposed. The main concept relies on exploiting only electric field in a plane-parallel liquid crystal cell of special configuration with thin transparent comb-like electrodes on one cover and usual transparent plane electrode on the other. Rubbing in direction parallel to comb electrode stripes enforces homogeneous planar or homeotropic orientation of nematic molecules. Fréedericksz’ transition of pure splay, twist or bend form is initialised by applying voltage between the electrodes appropriately. The method was tested with three different comb-like electrode configurations and three nematics: 4-pentyl-4′-cyanobiphenyl, 4-trans-4′-n-hexyl-cyclohexyl-isothiocyanatobenzene and Demus’ esters.

Breathers in the one-dimensional roll structure of twisted nematics

The dynamics of nonsingular defects has been studied experimentally and theoretically in the periodic roll structures arising at electroconvection in nematic liquid crystals twisted by π/2. The presence of an axial component of the velocity of the hydrodynamic flow with the opposite direction in the neighboring rolls is characteristic of roll structures in twisted nematics. The quantitative estimates of the critical frequency of the oscillations of structural defects have been obtained on the basis of the nonlinear equation of motion for the roll displacement. It has been found that the periodic creation and annihilation of a pair of edge dislocations with the topological indices +1 and −1 occur in the process of oscillations of a defect with a nonsingular nucleus. It has been demonstrated that oscillating defects with zero topological indices correspond to the solution of the sine-Gordon equation in the form of standing breathers.

Limonene as a chiral dopant for liquid crystals: Characterization and potential applications

Abstract— The chiral molecule D(L)‐Limonene was used to induce thermotropic cholesteric‐liquid‐crystal phases. A description of the methods used to identify and characterize the chiral nematic phase and its features will be given. D‐Limonene was substituted for conventional synthesized chiral dopants in a twisted‐nematic (TN) cell and a bistable cholesteric reflective cell and showed an improvement in the response time. Further analysis showed that D(L)‐Limonene reduces the rotational viscosity of the mixture by a considerable amount. It may be considered as a “molecular lubricant.” It is surprising how many more synthesized chiral molecules have been used as the liquid‐crystal field moves forward in comparison to naturally occurring molecules. Despite the large number of synthesized molecules used as a chiral dopant, D(L)‐Limonene, can be used as a dopantfor making cholesteric phases having the advantage of being a naturally occurring substance, environmentally friendly, and with very low cost.

Carbon nanotube-induced macroscopic helical twist in an achiral nematic liquid crystal

An achiral nematic liquid crystal was doped with a small quantity of carbon nanotubes having a net chirality, and the mixture was found to exhibit an average mechanical twist over macroscopic dimensions. The nanotube-induced chiral pitch length P was determined as a function of the average nanotube concentration by measuring the radii of curvature of reverse twist disclination lines in 90° nematic twist cells. The results suggest that the nanotubes’ spatial concentration can vary significantly across the cell and that at high average concentration, the nanotubes undergo aggregation, resulting in an apparent saturation of P−1 at high concentrations. The macroscopic helical twisting power of the nanotubes has been estimated from the results.

High-brightness and wide-view transflective liquid crystal display with two in-cell imprinted optical films in an inverse-twisted-nematic geometry

An inverse-twisted-nematic (ITN) transflective (TRF) liquid crystal (LC) display, where two imprinted optical films (IOFs) with surface microstructures are embedded was developed. One of the IOFs serves as an in-cell patterned retarder with multi-optic axes, and the other behaves as a viewing-angle enhancement film. In the presence of an applied voltage, the surface microstructures on the IOFs provide the spontaneous twist of the LC from a vertically aligned state to a 90° twisted-nematic (TN) state in the transmissive part, and to a 45° TN state in the reflective part. The developed ITN TRF LC display exhibits high transmission and reflectance, fast response, and wide-viewing characteristics, along with achromaticity.

Preparation and characterization of novel polyimide with chiral side chain for twist nematic liquid crystal display

The synthesis of novel polyimide (PI) films containing chirals in side chain was investigated for utilizing in twist nematic (TN) liquid crystal display (LCD). The polyimide with amine side groups (PI4 a.m.) was prepared by two steps copolymerization of 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), 4,4′-oxydianiline (ODA), and 3,3′-diaminobezidine (4 a.m.) in N-methyl-2- pyrrolidinone (NMP). In the first step, the oligomer of 6FDA and ODA with mole ratio 4 : 3 was synthesized. In the second step, the oligomer was reacted with 4 a.m. by gradually dropping the oligomer into 4 a.m. solution with the mole ratio 4 : 5. By reacting chiral compounds, mandelic acid, menthyl chloroformate, and menthyl glyoxylate hydrate using condensation reaction with amine groups along polyimide molecules, the novel polyimides can be synthesized. Their chemical structures were confirmed by fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy (1H-NMR). The coated glass slides pairs were coated with PI4 a.m. and with each Polyimide/chiral. The liquid crystals (LCs) were inserted between two coated glass slides at above the nematic temperature under the polarized light microscope to observe the alignment of LCs. The results showed the alignment of LCs in some certain direction under Polyimide/chiral pair, regardless of the type of the chiral molecule. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011