Nematic Liquid Crystal Materials Research Articles

Diphenyltriacetylenes: novel nematic liquid crystal materials and analysis of their nematic phase-transition and birefringence behaviours

Improvements in birefringence properties of LC materials are actively being pursued because of the increase in their applications in optical devices. 1,6-Diphenyl-1,3,5-hexatriyne, i.e., diphenyltriacetylenes (DPTAs; number of acetylene units, n = 3) are expected to have wide nematic mesophases and exhibit extremely high birefringence properties because of the high aspect ratio of their conjugated structures. Therefore, we synthesised DPTA derivatives as novel, highly birefringent nematic liquid crystal materials. The derivatives exhibit stable enantiotropic nematic phases with longer alkoxy carbon chain lengths (i.e., with dodecyloxy groups, m = 12). High birefringence values of Δn = 0.46 at 550 nm and 140 °C and a high extraordinary refractive index (ne) of over 2.0 were exhibited for triyne DPTA–OC6. Furthermore, we investigated the effect of the acetylene bond on the nematic transition behaviours and refractive index parameters of the conjugated oligoyne or polyyne compounds with two terminal aromatic rings. An increasing number of acetylene units (n = 1–3), i.e., extended conjugation, led to wider nematic phases and higher birefringence; the increment in Δn per acetylene unit was estimated to be approximately 0.14.

Investigation of photoinduced change of dielectric and electrical properties of indium (III) phthalocyanine and fullerene doped nematic liquid crystal

In this work, we have studied photoinduced dielectric and electrical properties of E63 nematic liquid crystal (E63-LC) materials doped with indium (III) metallo phthalocyanine (In-MPc) and fullerene (C60) in dark and under UV illumination. Doping concentration of In-MPc in E63-LC was chosen to be 5wt%, whereas a trace amount of C60 was used in investigated samples. Real and imaginary parts of dielectric permittivity were investigated in the frequency range of 100Hz–10MHz by using dielectric spectroscopy technique (DST). It was observed that doping agents and UV illumination enhanced the real part of dielectric permittivity at low frequencies, which was thought to originate from photoinduced charge transfer between In-MPc and C60 caused by extra dipole strength in LC medium. In addition to that, critical frequency (fc) and relaxation time (τ) were obtained and analyzed for all investigated samples. Photoelectrical characteristics of hybrid LC structures were carried out by current–voltage measurements. A major increase in electrical conductivity was observed in In-MPc and C60 doped LC structures under UV irradiation.

A Compact and Temperature-Sensitive Directional Coupler Based on Photonic Crystal Fiber Filled With Liquid Crystal 6CHBT

A directional coupler structure formed by a nematic liquid crystal (NLC)-filled photonic crystal fiber (PCF) represents a promising configuration in sensing applications. Because of large refractive index difference between the NLC and silica material, the mode coupling between the NLC waveguide and the silica core is more complicated than the situation of coupling between two fundamental modes of the waveguides. Therefore, it is necessary to perform a theoretical investigation of the mode properties associated with the experimental studies of the coupling characteristics. In this paper, we present a thorough analysis, both theoretically and experimentally, of the directional coupler structure, including the mode properties, coupling characteristics, and thermal sensing properties. The temperature response of the device is experimentally measured, showing a polynomial curve in nematic phase and a linear curve in isotropic phase. The nonlinearity of the temperature response of the device in nematic phase and the linearity in isotropic phase are attributed to the temperature dependence of the refractive index of the NLC. Specifically, the sensitivity is -3.86 nm/°C in isotropic phase of the 6CHBT with good linearity and shows good agreement with simulation results.

A note on regularity criterion for 3D compressible nematic liquid crystal flows

In this article, we prove a regularity criterion for the local strong solutions to a simplified hydrodynamic flow modeling the compressible, nematic liquid crystal materials.

Blow Up Criterion for Compressible Nematic Liquid Crystal Flows in Dimension Three

In this paper, we consider the short time strong solution to a simplified hydrodynamic flow modeling the compressible, nematic liquid crystal materials in dimension three. We establish a criterion for possible breakdown of such solutions at finite time in terms of the temporal integral of both the maximum norm of the deformation tensor of velocity gradient and the square of maximum norm of gradient of liquid crystal director field.

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.

Analysis of dichroic dye-doped polymer-dispersed liquid crystal materials for display devices

Dichroic polymer-dispersed liquid crystals (DPDLCs) based on nematic liquid crystal materials with azo dye were investigated in detail for the application of display devices. Polarizing optical microscopy, differential scanning calorimeter and electro-optic experiments all have shown that the DPDLC containing low concentration of dyes modifies the basic properties of these materials like optical transmission, threshold voltage, contrast ratio and absorbance factor. A minimum amount of dye needs to be added to the liquid crystal with the polymer matrix for its effective phase separation and to minimize the transmittance in the OFF state and therefore gives rise to an overall improvement in contrast ratio of the devices. Molecular orientation and dynamics in droplet sizes are readily controlled in these DPDLC materials. These findings imply that the value of the threshold electric field E th is approximately 8 V/μm for pure polymer-dispersed liquid crystal (without dye) where the threshold electric field E th values are approximately 4.0 V/μm, 2.0 V/μm, 1.7 V/μm 1.0 V/μm, for 0.0625%, 0.125%, 0.25% and 0.5% with azo dye in DPDLCs, respectively. From the results we can also infer that the maximum contrast is approximately 2.55 times the minimum contrast observed in the experiment for DPDLCs. The results show that the DPDLC with proscribed dye concentration will be possibly suitable and promising functional electronic materials for green technology flexible liquid crystal display.

Investigations of twist elastic constant K22 of new nematic liquid crystal materials using threshold IPS method

AbstractThe liquid crystal technology is one of the most popular at the present time. To the most important parameters of liquid crystals belong optical anisotropy, viscosity, and twist elastic constants. One of the most difficult and problematic to measure parameters is a twist elastic constant K22. For new liquid crystalline materials, we have to characterize their physical properties, also a twist elastic constant K22.As a final result of this work, a twist elastic constant of new nematic liquid crystal materials was investigated. In this paper, a transmission-voltage characteristic of the in-plane switching cell was observed to find Freedericksz threshold, and then twist elastic constants could have been calculated. The method was checked on well-characterized materials E7 and 6-CHBT and its accuracy was better than 20%. In addition, the temperature dependence of K22 was determined for all materials.

Controlling light fields by means of liquid-crystal diffraction elements

Methods of correcting light fields by means of optoelectronic diffraction elements based on an oriented layer of nematic liquid crystal and photopolymeric material, making it possible to form anisotropic phase diffraction structures, are experimentally investigated.

Effect of molecular weight of biscinnamated poly(ethylene glycol) oligomers on the photocycloaddition reaction

Poly(ethylene glycol) (PEG) is of great importance in the biomedical applications, owing to their hydrophilicity, nontoxicity, and absence of antigenicity and immunogenicity. Recently, much interest is focused on the utilization of light as a source of stimulus for the applications for biomedical field. In line with this photoreactive PEG has been utilized for photografting onto surface and fabrication of 3D structured hydrogels. In order to allow photoreaction for PEG, mostly conjugation of the photoreactive moiety at the hydroxyl groups in PEG is used. Cinnamoyl group is well known photoreactive group which undergoes [2+2] photocycloaddition reaction when UV is irradiated and photoalignment layer could be obtained by the irradiation of linearly polarized UV (LPUV) onto the coated films of polymer with cinnamoyl groups. Interestingly, it is reported that cinnamoyl group is spontaneously metabolized in the body and thus has been proven to be nontoxic. Additionally, nematic liquid crystal (LC) materials are emerging as promising candidates for a wide range of applications from optoelectronic devices to biosensing. Although there is lack of report on the biscinnamated PEG as a 2D in-plane orientation layer, it can be expected that biscinnamated PEG could provide biocompatible photoaligned substrate for biomedical applications. From the early studies from our group, molecular weight of PEG whether it is attached as a side chain or itself affect on the performance of application field such as ionic conduction behavior, interaction with DNA, and gelatinization of starch. Considering photoalignment layer, we also reported beneficial phenomenon of the utilization of oligomeric cinnamate rather than polymeric cinnamates in some aspects. In this note, we prepared photoalignable oligomers based on the different molecular weight of PEG oligomers and investigated the photoreactivity and the LC aligning property of the prepared oligomers triggered by the linealy polarized UV irradiation.

CdS-Nanoparticle-Doped Liquid Crystal Displays Showing Low Threshold Voltage

Low operation voltage is one of the most crucial factors for developing mobile liquid crystal display devices. Nanoparticle doping is a convenient method of attaining low threshold voltage. A type of widely studied semiconductor material, CdS nanoparticles, was doped into 4'-n-pentyl-4-cyanobiphenyl (5CB) nematic liquid crystal material. First of all, we have proved that CdS nanoparticle doping can significantly reduce the threshold voltage of 5CB twisted-nematic cells by up to 25%. The response time and frequency modulation characteristics of nanoparticle-doped liquid crystal cells were also studied. In addition, the phase transition temperature, order parameter, and dielectric constants were measured to analyze the cause of the threshold voltage reduction. All the physical properties of the doped materials have the same variation tendency with the threshold voltage as they are correlated with the nanoparticle size and doping concentration. Finally, a doped liquid crystal display device model was proposed and computer simulation was carried out on the basis of this model, yielding electrooptical results that can agree well with experimental data.

Fast Responding Robust Nematic Liquid Crystalline Gels Formed by a Monodisperse Dipeptide: Electro-Optic and Rheological Studies

Realization of mechanically robust electrically fast responding liquid crystal devices with low operating voltage is one of the current research interests. Here we report a gel system comprising a commercially available nematic liquid crystal material and a new monodisperse dipeptide liquid crystalline organogelator that results in these properties. The gels exhibit nearly 2 orders of magnitude faster switching response than the pure nematic liquid crystal while having 3 orders of magnitude higher zero shear rate viscosity, and with the attractive feature that the switching threshold voltage is hardly altered. Electro-optic and rheological studies of this system are described here.

Tunable optical properties of silicon-on-insulator photonic crystal slab structures

A photonic crystal slab structure with one-dimensional periodicity, obtained by preferential etching of a silicon-on-insulator wafer, is numerically investigated in 3-D. The etched grooves are considered to be filled either with an isotropic or with a Nematic Liquid Crystal (NLC) material. The molecular director is calculated using a consistent model of NLC reorientation under an external voltage. Different structures together with a broad range of voltage values are numerically simulated by means of a three-dimensional finite-difference timedomain method. The shifting of the photonic bandgap induced by the applied voltage, as well as its sensitivity in terms of refractometric applications, are discussed for a range of different structure geometries. [DOI: 10.2971/jeos.2009.09017]

Simulation study on terahertz vibrational absorption in liquid crystal compounds

The terahertz (THz) spectrum absorptions of nematic liquid crystal (LC) material, i.e. N-(p-methoxybenzylidene)-p-butylaniline (MBBA), and its relevant compounds are simulated in this paper by using the density functional theory (DFT) method. A strong absorptive frequency is located at 3.65 THz for the MBBA, which is in agreement with experimental data found in the literature. The result suggests that the DFT method is effective for dealing with the anisotropic nematic LC compounds.

Improvement of Phase-Shifting Properties in Coplanar Waveguide-Type Liquid Crystal Millimeter-Wave Phase Shifter by Introducing a Resonant Phenomenon

Nematic liquid crystal (LC) material is mounted on a coplanar waveguide (CPW) substrate, which is well known as an excellent transmission line for superhigh-frequency electromagnetic waves, to achieve an electronically tunable phase shifter. In our previous work, an upper electrode introduced for an LC drive significantly contributed to reducing the driving voltage with large phase-shifting properties. However, it also tended to increase transmission loss. Thus, it was clarified that it is not necessarily a desirable improvement if we evaluate the device performance by balancing the phase shift and loss. In this work, we propose a triangular pattern for the upper electrode to utilize a resonant phenomenon and to improve the phase-shifting properties. A periodic dip appears in the transmission spectra and some fluctuations appear in the phase properties near the dip frequency. It is found that the phase shift becomes very large at frequencies slightly lower than the dip frequency, although the frequency band in which the large phase shifts occur is limited.

The coupling and superposition behavior of higher-order diffraction image based on photorefractive nematic liquid crystal materials

We report some photorefractive higher-order diffraction phenomena of two kinds of photorefractive nematic liquid crystal materials. In the experiment, we observed the superposition of higher-order diffraction images, the coupling of higher-order diffraction lights, and the coupling between higher-order diffraction lights and incident beams. The observed behavior suggests the complexity of the coupling process. The properties of higher-order diffraction images are discussed theoretically, which are in good agreement with experimental results.

Capacitive Transduction for Liquid Crystal-Based Sensors, Part I: Ordered System

In this paper, the theory for tracking the average molecular orientation of a liquid crystal (LC) material via capacitive sensing of anisotropic media is presented. The candidate LC sensor structure utilizes interdigital electrodes. Two capacitive measurements in orthogonal direction can track the director axis of nematic LC material in a homogenous ordered LC film. The sensitivity for the sensor at different alignments is studied. The candidate sensors have been fabricated and experimentally verified. Both experimental and calculated values for capacitances of selected interdigital fingers sensor structures are presented.

Twisted wedges for the measurement of long pitch lengths in chiral nematic liquid crystals

A new method is described for the measurement of long pitch lengths in chiral nematic liquid crystal materials using wedges with twisted alignments on the two surfaces. *The research was performed in part at the Department of Chemistry, The University of York, Heslington, York YO10 5DD, UK

62.4: Liquid‐Crystal Etalon Device for High‐Efficiency Field‐Sequential Display Enkh‐Amgalan Dorjgotov

AbstractWe propose a pixel design using liquid crystal etalon device for high efficiency mobile field sequential color display. The new device is nearly loss‐less for monochromatic primary light sources. High efficiency transmittance is achieved through the use of a polarization insensitive, high transmittance etalon device which consists of dielectric mirrors on either side of a cavity filled with twisted nematic liquid crystal material. The device is simulated using numerical modeling program, and preliminary design results are presented.

The durability of a liquid crystal modulator for use with a high power laser

The durability of a liquid crystal (LC) modulator for use with a high power laser is measured at a wavelength of 808 nm. It is shown that the LC modulator can withstand a laser power density of 133 W cm−2. For a laser power density greater than 133 W cm−2 the phase transition is caused by the absorption of the material. In order to improve the durability, the absorption of glass, polyimide (PI), indium tin oxide and LC were measured. Results indicated that the phase transition of the nematic LC is mainly caused by the absorption of glass and PI. These results can be used for a LC modulator with nematic LC material, and it is important to use an LC modulator in optical systems with high power lasers.