Ravindra Dhar

Enhancement of the display parameters of 4′-pentyl-4-cyanobiphenyl due to the dispersion of functionalised gold nano particles

We report electro-optical and dielectric studies of the gold nano particle (GNP) dispersed room-temperature liquid crystalline material 4′-pentyl-4-cyanobiphenyl (5CB) at a concentration of 0.1 wt%. Due to the dispersion of GNPs, the nematic to isotropic transition temperature is significantly increased. The threshold voltage required to switch molecules from the planar orientation (bright state) to the homeotropic (dark state) orientation is decreased. The dielectric parameters, namely, the longitudinal component of the dielectric permittivity and hence the dielectric anisotropy, are highly affected. The relaxation frequency corresponding to flip-flop motion of the 5CB molecules about their short axes has increased due to the presence of GNPs.

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.

Electrical and electro-optical parameters of 4ʹ-octyl-4-cyanobiphenyl nematic liquid crystal dispersed with gold and silver nanoparticles

Thermodynamical, dielectric, optical and electro-optical characterisation of pure 8CB and its composites with gold and silver nanoparticles have been studied. Thermodynamical studies suggest a decrease in clearing temperature of the nanocomposite systems as compared to the pure system. Dielectric parameters of pure nematic liquid crystal and nanocomposites in the homeotropic and planar aligned samples have been measured in the frequency range of 1–35 MHz. Ionic conductivity increases significantly in nematic and smectic Ad (SmAd) phases, whereas dielectric anisotropy is almost unchanged for both the nanocomposites. Threshold voltage for Freederick transition, switching voltage and splay elastic constant have decreased in the case of nanocomposite systems. Relaxation frequency and activation energy of an observed relaxation mode corresponding to molecular rotation about the short axis increase in the SmAd phases of both the nanocomposites. The optical study suggests that due to dispersion of nanoparticles, the optical band gap has decreased.

Enhanced electro-optical properties of a nematic liquid crystals in presence of BaTiO3 nanoparticles

Composites of nematic liquid crystals (NLCs) and ferroelectric barium titanate (BaTiO3) nanoparticles (NPs) have been prepared. The alignments of NPs in the host medium have been demonstrated. Effect of NPs doping on various display parameters of NLCs, namely, threshold voltage, dielectric anisotropy and splay elastic constant has been studied using electro-optical and dielectric studies. The nematic ordering of host supports alignment of NPs parallel to the director which consequently improves electro-optical parameters in the composite system. The dielectric and electro-optic properties of LC–NPs composites have been discussed in frame of conventional theories of NLCs.

Effect of dispersed colloidal gold nanoparticles on the electrical properties of a columnar discotic liquid crystal

A dispersion of colloidal gold nanoparticles (GNPs) in a triphenylene-based discotic liquid crystal (DLC), namely 2,3,6,7,10,11-hexabutyloxytriphenylene (HAT4), possessing a hexatic plastic columnar phase (Colhp) phase, has been studied by differential scanning calorimetry, polarized light microscopy, UV-Vis absorption spectroscopy, X-ray diffraction and dielectric spectroscopy. GNPs have been dispersed at three different concentrations in pure HAT4 viz. 0.2, 0.6 and 1.2 wt%. It has been observed that with the increase of GNPs concentration, the Colhp–isotropic liquid transition temperature decreases substantially but the crystal–Colhp transition temperature does not changes significantly. Electrical conductivity increases by at least five orders of magnitude for the highest concentration of GNPs (1.2 wt%) as compared to the pure material i.e. HAT4. Observed results suggest that although extremely low concentrations of GNPs are not very useful, a moderate concentration is highly useful for increasing the conductivity of the Colhp phase and also to implant surface plasmon resonance. These results may be exploited to enhance the efficiency of electro-optical devices by using HAT4–GNPs composites.

Effect of γ-irradiation on liquid crystalline properties of cholesteryl pelargonate (nonanoate)

Abstract Effect of Co60 γ-irradiation at a dose 500 kGy on dielectric and thermodynamic properties of cholesteryl pelargonate (nonanoate) (ChP) has been studied by differential scanning calorimeter (DSC) and electric impedance analyzer. From UV spectra of irradiated ChP a charge transfer complex peak has been observed at 290 nm besides the peak of pure ChP at 240 nm making it a bicomponent mixture. Irradiated sample shows depression in crystal-cholesteric (KCh) transition by 4.0°C and cholesteric-isotropic (ChI) transition by 9.8°C while cholesteric-smectic A (ChSmA) by 15.4°C. Enthalpies (ΔH) of the KCh, CSmA transitions are found to decrease on irradiation. The dielectric constants ϵ⊥′ and ϵ∥′ and losses(ϵ⊥″ and ϵ∥″ in planar and homeotropic alignmen the sample have been measured in the frequency range 100 Hz to 10 MHz at different temperatures between 30 to 100°C. The dielectric losses (ϵ⊥″ and ϵ∥″) of irradiated sample are observed to increase significantly (150%) whereas the dielectric anisotropy Δϵ (ϵ⊥K′ − ϵ∥′) decreases.

Thermal and morphological studies of liquid crystalline materials dispersed in a polymer matrix

Polymer dispersed liquid crystal (PDLC) films have been prepared by a combination of solvent-induced phase separation and thermal-induced phase separation techniques, for three mesogenic materials: 4-(undecyloxy) benzoic acid (UDBA), 4-(decyloxy) benzoic acid (DBA) and 4-(dodecyloxy) benzoic acid (DDBA) in poly(methyl methacrylate) (PMMA) as polymer matrix and tetrahydrofuran (THF) as common solvent. Differential scanning calorimetry and polarising optical microscopy are used for thermal and morphological investigations of prepared PDLCs, and findings are explained with the help of a suitable phase diagram. It is found that isotropic liquid–nematic transition temperature (TIN ) for these mesogens in the PMMA matrix decreases by about 2.8–5.6°C as compared with the pure mesogens. Methods based on the solubility parameter and the fraction of liquid crystal (LC) in LC-rich domains inside the PMMA matrix at different concentrations have been adopted to achieve an optimised condition for uniform dispersion of the film over a larger area. Remarkable changes in the droplet morphology of the mesogens confined in PMMA matrix as compared with their bulk counterparts have also been observed. Bipolar droplets and Maltese-type crosses are observed by POM for 75 and 30 wt% UDBA in the PMMA matrix.

Dielectric anisotropy and AC conductivity of bicomponent mixtures of liquid crystals cholesteryl pelargonate and nonyloxybenzoic acid

Abstract Dielectric constant (e′) and electrical conductivity ([sgrave]) of the binary mixtures of cholesteryl pelargonate (ChP) and nonyloxybenzoic acid (NOBA) of different concentrations have been measured in the frequency range of 100 Hz to 10 MHz from room temperature to 150°C both in planar and homeotropic alignments. These mixtures do not show the dielectric dispersion upto 10 MHz. Dielectric constants parallel and perpendicular to helical axes (e′∥ and e′⊥) and the dielectric anisotropy (Δe′ = e′∥ - e′⊥) have been determined in different mesophases. Dielectric anisotropy (Δe′) of the mixtures changes sign from negative to a positive value for NOBA concentrations above 75 mole %. From the measured frequency dependent conductivity [sgrave](f), d.c. conductivity [sgrave](dc) has been obtained using the universal power law equation. Activation energy (W a) of the d.c. conduction process has been determined in different mesophases.

Effect of alumina nanoparticles on dielectric permittivity, electrical conductivity, director relaxation frequency, threshold and switching voltages of a nematic liquid crystalline material

In the present work, alumina (Al2O3) nanoparticles (Al-NPs) were dispersed in a room temperature nematic liquid crystal 4-pentyl-4ʹ-cyanobiphenyl (5CB) at the concentrations of 0.2 and 0.6 wt%. Differential scanning calorimetry studies suggest decrease in clearing temperature of the composite materials as compared to the pure material. Dielectric studies show that the presence of the Al-NPs decreases the effective longitudinal component () of the dielectric permittivity while has no effect on the transverse component (). Ionic conductivity decreases for low concentration of the nanoparticles. In the case of homeotropic aligned samples, a dielectric relaxation mode has been detected in the MHz frequency region. Presence of the Al-NPs decreases the relaxation frequency corresponding to flip-flop motion of molecules about their short axes. From the frequency-dependent dielectric measurements, important dielectric parameters have been determined. Electro-optical experiments show that the threshold voltage decreases and the steepness of the transmission–voltage curve improves due to dispersion of Al-NPs.

Change in Dielectric and Electro-Optical Properties of a Nematic Material (6CHBT) Due to the Dispersion of BaTiO3 Nanoparticles

Thermodynamical, dielectric and electro-optical studies of a ferroelectric nanopowder (0.5 weight % Barium Titanate, BaTiO3) dispersed in 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) have been carried out. Change in the dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode, threshold and switching voltages of electro-optic response (transmission-voltage characteristic) have been determined for the dispersed and pure samples in their nematic mesophases. These studies suggest that isotropic to nematic transition temperature for BaTiO3 dispersed 6CHBT is lowered. Changes in other properties are also discussed.