Nematic Liquid Crystal Mixture E7 Research Articles

Optical properties and zeta potential of polyvinyl pyrrolidone capped gold nanoparticles dispersed nematic liquid crystal mixture E7

The effect of dispersion of polyvinyl pyrrolidone (PVP) capped gold nanoparticles in nematic mixture E7 on the optical properties (absorbance and photoluminescence, fluorescence life time and Raman imaging) and zeta potential is studied. The gold nanoparticles of 0.25, 0.5 and 1.0 wt% concentrations are dispersed into E7. It has been found that the absorbance and photoluminescence intensity of dispersed systems are increased and depend on the GNPs concentration. These increase correspond to the coupling between the radiation field (electromagnetic waves of the light source) and the GNPs low-energy phonons and constructive coupling of emissions between the E7 and GNPs, respectively. The change in the fluorescence lifetime of pristine and GNPs dispersed E7 systems has confirmed the energy transfer between E7 and GNPs molecules. The study of Raman spectrum shows the variations in the intensity of the Raman peaks with different orientations. Due to dispersion of GNPs in E7 the values of zeta potential are increased. This ensures the good stability of the dispersed systems. The conductivity of dispersed system has increased drastically. In conclusion, the dispersion of GNPs in host nematic mixture E7 has influenced significantly its properties, thereby making the dispersed material more promising for the potential applications.

Design and Realization of a Compact Efficient Beam Combiner, Based on Liquid Crystal Pancharatnam–Berry Phase Gratings

We demonstrate a laser beam combiner based on four photo-patterned Pancharatnam–Berry (PB) phase gratings, which is compact and has high diffraction efficiency for incident circularly polarized light. The nematic liquid crystal mixture E7 is used as anisotropic material, and the thickness of the layer is controlled by spacers. The beam combiner can bring two parallel laser beams closer to each other while remaining parallel. This work shows the potential to realize components based on flat optical LC devices.

Photonic NO2 Gas Sensing with Binaphthyl‐Based Dopants

AbstractA series of reactive binaphthyl‐diimine‐based dopants is prepared and investigated with respect to their potential for the chiral induction of structural coloration in nematic liquid crystal mixture E7 and the selective photonic sensing of nitrogen dioxide (NO2). Studies of the helical twisting power (HTP) in 4‐cyano‐4′‐pentylbiphenyl (5CB) reveal HTP values as high as 375 µm‐1 and the tremendous impact of structural compatibility and changes of the dihedral binaphthyl angle on the efficiency of the chiral transfer. Detailed investigation of the sensing capabilities of the systems reveals an extraordinarily high selectivity for NO2 and a response to concentrations as low as 100 ppm. The systems show a direct response to the analyte gas leading to a concentration‐dependent shift of the reflectance wavelength of up to several hundred nanometers. Incorporation of copper ions remarkably improves the sensor's properties in terms of sensitivity and selectivity, enabling the tailored tweaking of the system's properties.

Influence of carbon quantum dots on electro–optical performance of nematic liquid crystal

A simple synthesis of carbon quantum dots (CQDs) from green sources has attracted increasing attention nowadays. Here we report the synthesis of water-soluble fluorescent carbon quantum dots from mandarin juice by hydrothermal method. When excited at 364 nm, CQDs give maximum fluorescence at 462 nm. As verified from Transmission Electron Microscopy, obtained CQDs were very small, with an average diameter of 2.37 ± 0.42 nm. These very small-sized nanoparticles were dispersed to nematic liquid crystal mixture E7. As is known, particle size is a significant parameter for nanoparticle dispersed liquid crystal applications. The threshold voltage of pure and CQDs doped liquid crystals was derived from dielectric spectroscopy measurements, and it was observed that the threshold voltage increased with the increasing CQDs doping concentration. The nematic to the isotropic phase transition temperature of samples was investigated with DSC thermograms and POM. The phase transition temperatures gradually decreasing with increasing CQDs doping concentration were revealed.

Electro-optical, thermal and dielectric properties of ternary mixture of E7/6CB/6BA liquid crystal mixture complex

In this study, a novel liquid crystal mixture complex was designed and synthesized from the nematic liquid crystal mixture E7, hexylcyanobiphenyl (6CB) and hexylbenzoic acid (6BA). The thermal and morphological properties were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The DSC and POM results show that the liquid crystal complexes exhibit liquid crystalline properties with smectic and nematic phase. While the E7 ratio is 80%, the mixture complex has the widest nematic range and exhibits E7 properties. When the E7 weight ratio is less than 80%, with the formation of hydrogen bonds the number of mesogenic phases increases and the nematic range decreases. The electrical properties of obtained samples were measured by impedance spectroscopy technique. The birefringence and contrast ratio of samples also investigated by the help of light transmittance experiment.

The effects of carbon nanotubes on the clearing transition of the antiferroelectric liquid crystal MHPOBC

abstractWe present a first attempt to study the effect of carbon nanotubes (CNTs) on the clearing transition, from SmA* to isotropic liquid phase, of the antiferroelectric liquid crystal 4'-Octyloxy-biphenyl-4-carboxylic acid 4-(1-methyl-heptyloxycarbonyl)-phenyl ester (MHPOBC), following two methods to introduce the CNTs. On the one hand, they are dispersed before in the nematic liquid crystal mixture E7 and then mixed with MHPOBC. While it allows decent quality of the CNT dispersion, the effects of E7 itself on the MHPOBC phase sequence is very strong. As a complementary approach to avoid this effect, we therefore also dispersed dry CNT powder directly in MHPOBC.

Supramolecular liquid-crystal gels formed by polyfluorene-based π-conjugated polymer for switchable anisotropic scattering device.

To overcome the problem of high driving voltage and low contrast ratio in the switchable scattering device of conventional liquid-crystal (LC) physical gel, a new type of supramolecular LC physical gel has been developed and fabricated through the fibrous self-assembly of the polyfluorene-based π-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in nematic LC mixture E7. It was found that the rubbed interface between the LC molecules and polyimide layer can induce the LC physical gels to demonstrate fantastic light scattering characteristic. The gels with oriented self-assembled supramolecular structures exhibiting significant anisotropic light scattering in the main-chain direction of the F8BT molecules under an extremely low driving voltage (ca. 2.7 V) are reported for the first time. In addition, the contrast ratio can be reached exceeding 1000. In contrast to conventional LC physical gels, the large reduction of driving voltages of the supramolecular gel provides great possibility for application in various electro-optical devices such as tunable polarizers, transflective displays, and polarized light modulators.

Magnetic field effects on the electric modulus properties of nematic mixture E7

The molecular dynamics of the homogeneously aligned nematic liquid crystal mixture E7 subject to a magnetic field has been studied. The dielectric spectra study has revealed a low bias magnetic field effect on the evolution of dielectric relaxation spectra occurred at lower (∼kHz) (δ-relaxation) and higher (∼MHz) (α-relaxation) frequency regions. The complex electric modulus, which converted from experimental dielectric spectra, has been analyzed with theoretical model of Debye relaxation. The obtained fitting parameters of relaxation time and strength of dielectric components are shown to vary systematically with the strength of applied magnetic field. A microscopic molecular dynamic model has been proposed to describe the two-step variation of E7 molecular under the bias magnetic field. The results provide implication for magneto-modulation of liquid crystal molecular dynamics under the bias magnetic field.

Electro-optic steering of Nematicons

We study the angular steering of spatial solitons in nematic liquid crystals, i.e. nematicons, exploiting their electro-optic response. Approaches to electro-optic steering include acting on the optic axis and walk-off, propagating the soliton through a voltage-adjusted refractive interface or getting it totally internally reflected. Using comb electrodes in a planar cell, maximum angular deflections of 70° are expected with the standard nematic liquid crystal mixture E7. Full Text: PDF

Switchable waveguiding in two liquid-crystal-filled photonic crystal fibers

Switchable waveguiding is investigated in two liquid-crystal-filled photonic crystal fibers with a solid core using the nematic liquid-crystal mixture E7 under planar and homeotropic anchoring conditions. Addressing experiments using ac voltages show polarization-dependent and -independent effects with response times down to a few ms. It is shown that the attenuation spectra of the two liquid-crystal-filled photonic crystal fibers can be changed dramatically by just varying the boundary conditions. Electromagnetic field simulations are presented, which are in good agreement with the experimental findings.

Tunable NIR filter based on a free-standing porous silicon film containing nematic liquid crystal

We present the design of a thermally tunable interference filter for the NIR range. The filter is based on a free-standing porous silicon multilayer filled with the nematic liquid crystal mixture E7. The porous silicon stack is constituted by a quarter-wave microcavity sandwiched between two distributed Bragg reflectors. Reversible thermal tuning of the microcavity resonance is obtained in the range of 13 nm. Thermal dependence of the effective refractive index of the liquid crystal in the sample is characterized using variable angle spectroscopic ellipsometry. The non-axial liquid crystal director configuration is found inside silicon mesopores. The distribution of the liquid crystal director within pores is simulated using the Frank's free energy approach. Assuming the molecular configuration of the liquid crystal to be escaped radial, we have determined the value of the surface anchoring strength in the mesoporous silicon to be less than 10−6 J m−2. By fitting the transmittance spectra, using an effective medium approximation and a transfer matrix method, the liquid crystal is found to fill 84% of the void inside the porous sample. The good agreement between the experimental data and the theoretical curves justifies the Frank's approach and the hypothesis of escape radial configuration of nematics confined in silicon mesopores.

Characterization of a nematic mixture by reversed‐phase HPLC and UV spectroscopy: an application to phase behaviour studies in liquid crystal–CO2 systems

In the present work a simple but selective reversed‐phase high performance liquid chromatographic method (HPLC) was developed for the analysis of the nematic liquid crystal mixture E7 to determine precisely the composition of the liquid crystal mixture used in PDLC film preparation. Ultraviolet absorption spectrophotometry experiments were carried out to determine the HPLC detection wavelength and to characterize the absorptivity constants of E7 constituents. The technique developed is applied in the case of equilibrium solubility studies for E7 in supercritical carbon dioxide (scCO2). The results indicate unambiguously that scCO2 can fractionate the mixture towards the E7 components. The four single component peaks of the E7 mixture were distinctively separated by this method, which enabled the determination of the solubility of E7 constituents in the scCO2.

Molecular orientational and dipolar correlation in the liquid crystal mixture E7: a molecular dynamics simulation study at a fully atomistic level

Molecular dynamics simulations are reported for the four component nematic liquid crystal mixture E7, which is used commercially. We are able to show the growth of a nematic phase directly from an isotropic liquid over a 100 ns period for an all-atom model, and study orientational and dipole order within the nematic phase. The simulations show that the cyanoterphenyl component of the mixture, 5CT, is more ordered than the three cyanobiphenyl components. The simulations show also that both parallel and anti-parallel dipole correlation take place in E7 but that the strong anti-parallel dipole correlation is localised to particular arrangements of molecules. It is possible to identify two key preferred configurations for molecular pairs in the fluid, which explain the form of the dipole correlation function, g(1)(r).

Effects of Perfluoration of Epoxy-Amine Polymer Matrices on the Electro-Optical Performance of Polymer Dispersed Liquid Crystal Films

Polymer dispersed liquid crystal (PDLC) films were obtained by combined thermally and polymerization induced phase separation processes initiated by the temperature controlled polycondensation reaction of monomers carrying either epoxy or amine groups. The epoxy diglycidyl ether of bisphenol A (DGEBA) and two different amines very used: an aliphatic amine and an aromatic fluorinated amine. The nematic liquid crystal mixture E7 was employed. The electro-optical performance of the obtained PDLC films was analyzed as a function of reaction cure in the absence and presence of the fluorinated amine. It was found that the addition of the fluorinated amine has a strong influence on the transmission vs voltage curves. A remarkable decrease of threshold- and saturation voltages as well as an increase of the transmission values in the on-state was observed.

Electro-Optical and Morphological Characterization of Electron Beam Cured Liquid Crystal-Polymer Composite Materials

Abstract Polymer dispersed liquid crystals (PDLCs) are promising new materials for electro-optic applications such as switchable windows and flexible displays. Thin films were prepared via a polymerization induced phase separation process using electron beam radiation. The PDLC material was obtained from a blend of a eutectic nematic liquid crystal mixture E7 and a polyester acrylate based polymer as a precursor of the matrix The electro-optic behavior of the obtained composite material has been examined To understand the optical response of these materials, the relationship between the electro-optic properties and the morphology of the PDLC films has been investigated. The size, size distribution, shape, volumetric number density and spatial distribution of droplets are important factors influencing the electro-optic performance. Scanning electron microscopy (SEM) was used to characterize quantitatively PDLC film morphology

Polymer‐dispersed liquid crystal materials prepared by electron‐beam‐initiated polymerization

AbstractPolymer‐dispersed liquid crystal (PDLC) films were prepared using electron beam radiation by a polymerization‐induced phase separation process. The composite material was obtained from a blend including a polyester acrylate and the nematic liquid crystal mixture E7. The electro‐optic properties of the PDLC films were investigated as a function of film thickness. The obtained transmission vs. voltage curves exhibit a good reproducibility, low threshold voltages, and a high transmission in the on state. © 1995 John Wiley & Sons, Inc.

Evaluation of electro-rheological fluids incorporating liquid-crystalline materials

Liquid-crystalline materials have been employed in electro-rheological fluid dispersions. Four commercially available dispersed phases were used in conjunction with a nematic liquid-crystal mixture E7 to form liquid-crystalline electro-rheological fluids displaying good yield stresses. Both the dielectric anisotropy and the highly organized nature of the molecules of the nematic phase exhibited by the liquid-crystalline materials have been shown to contribute to the electro-rheological activity exhibited by these novel fluids.

Temperature Dependence of the Anisotropy of Turbidity and Elastic Constants of Nematic Liquid Crystal Mixture E7

Abstract The three principal elastic constants, K 11, K 11, and K 33, of the eutectic nematic liquid crystal mixture E7 have been determined as a function of temperature by following changes in the anisotropy of turbidity of a magnetically aligned specimen. The agreement with values of K 22 obtained by a previous capacitance-voltage method is excellent, whereas the values of K 11 and K 33 derived from the absorption coefficients are systematically higher.