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Doping a nematic liquid crystal (NLC) with nanoparticles (NPs) of varied dimensions and properties to form LC-NP nanocomposites with improved electro-optic and dielectric properties has recently been a much-explored field of work. But still, NLC nanocomposites based on two-dimensional (2D) NPs have not been studied vividly because of the high tendency of 2D NPs to aggregate together. In the present work, we have studied the effect of incorporating hexadecanethiol functionalised graphene oxide (fGO), a 2D NP, in a conventional NLC medium under varying doping concentrations. We found that at a lower doping concentration (⁓10−4 wt.%), the NLC nanocomposite showed a substantial increase in dielectric anisotropy (Δε) and birefringence (Δµ) values as compared to pure NLC sample. A significant decrease (⁓50%) in ionic conductivity as well as reduced electro-optic response time (⁓75%) was also observed in this NLC nanocomposite. As the fGO doping concentration was increased (⁓10−2 wt.%), a reverse trend was observed for Δε, Δµ and response time values, which declined sharply with increasing doping concentration. Hence, the present work highlights two different working mechanisms at low- and high-doping concentrations of 2D NPs. [ABSTRACT FROM AUTHOR]

The calamitic nematic liquid crystal generally exhibits a discontinuous weakly first-order isotropic-nematic (I-N) phase transition. We found that, on the incorporation of CdS nanowire in the nematic host, the weakly first-order discontinuous I-N phase transition evolved as a continuous phase transition. The parallel and perpendicular components of dielectric permittivity and bulk conductivity of nanocomposites of CdS nanowires of a nematic liquid crystal (NLC) are found to follow continuous I-N phase transition in contrast to the discontinuous weakly first-order transition of the pure host nematic liquid crystal. This was further confirmed from scattered iGntensity measurements on the CdS nanocomposite samples of nematic liquid crystal in homeotropically aligned cells. Such measurements indicate a continuous phase transition for the nanocomposites of NLC having concentrations less than 0.2 wt% of CdS nanowires in NLC. The dielectric anisotropy, birefringence, threshold voltage of nanocomposites of CdS nanowire in a calamitic nematic liquid crystal with varying concentration of the nanowire have been measured. The ionic conductivity of nanocomposites of CdS nanowires of NLC increases with increasing concentration of CdS nanowires. [ABSTRACT FROM AUTHOR]

In the present work, we have prepared a liquid crystal-carbon nanotube (LC-CNT) composite by dispersing a low concentration of carboxylic group (-COOH) functionalised multi-walled CNTs (MWCNTs) in a liquid-crystal 6CHBT and studied in detail the alterations in thermal and electrical parameters. Thermodynamic studies reveal that transition processes are becoming fast due to the dispersion of MWCNTs. The change in the behaviour of transitions is credited to the reduction in ionic impurities owing to the ion trapping characteristic of CNTs. The isotropic liquid to nematic transition temperature of the composite system is increasing for the exothermic cycle as compared to that of the pure sample. The average transition enthalpies and entropies for various transitions of the composite system are increasing as compared to those of the virgin sample. Longitudinal component of the permittivity and dielectric anisotropy of the composite system is slightly reduced as compared to those of the virgin sample. Relaxation frequencies of an observed relaxation mechanism corresponding to the rotation of molecules about their short axes of the composite system are enhanced as compared to the undispersed sample. Total ionic conductivity of the composite system is boosted as compared to those of the pure sample. [ABSTRACT FROM AUTHOR]

Series of oligomeric ionic liquid crystals (OILCs) based on various anions including bromide (Br−), tetrafluoroborate (BF4−), dodecylbenzenesulphonate (A-SO3−) and (+)-10-camphorsulphonate (D-SO3−) are synthesised by use of pyridine-containing monomers, terminal bromo-substituted alkanes and anion replacement reagents. The effect of anions on the chemical structure, liquid-crystalline behaviour, and ionic conductivity is investigated. The OILCs show various mesophases including smectic, hexagonal columnar and bicontinuous cubic phase due to the change in anions. These mesophase types are determined by two incompatible segments derived from ionic mesogens and flexible alkyl chains in the microstructure. For the OILCs based on A-SO3− and D-SO3− anions, a curvature of the interfaces between the layers arises due to the increased size of one segment, resulting in columnar and cubic mesophases. The ionic conductivity is investigated in the way that a lithium ion solution passing through the oriented OILC films at room temperature. For the OILC films with various anions, ionic conductivities of the measuring system increase a little in the order A-SO3− > D-SO3− > BF4−> Br−. The values based on A-SO3− anion films show nearly twofold increase compared with those based on Br- anions, which can be attributed to the effects of both ion migration and ionic channels in the microstructures. [ABSTRACT FROM AUTHOR]

The influence of capped gold nanospheres (GNSs) on the direct current (DC) electric conductivity ( σ D C ) of a ferroelectric liquid crystal (FLC) confined in a cell with blocking electrodes was experimentally investigated. In framework of this work, several measurement techniques of σ D C , such as dielectric and conductivity spectroscopies, were utilised. However, it was demonstrated that only the capacitive technique allowed estimating values of σ D C of both pure FLC and FLC/GNSs nanodispersion confined in the cell with blocking electrodes. The experimental results confirm the assumption that the gold nanoparticles adsorb the impurities' ions in FLC which leads to the decrease in the conductivity of the FLC layer. [ABSTRACT FROM AUTHOR]

A series of novel star-shaped ionic liquid crystals (ILCs) compounds with various counterions (derived from HBF4, HPF6, CF3COOH, d-MH-SO3H, H3PO4, p-Toluenesulfonic acid) were designed and synthesised starting from precursors of pyridines and liquid crystalline monomer cholesteryl 4-bromobutanoate. Inducing various counterions by use of ionic self-assembly due to electrostatic attraction of various ion clusters was one of the most essential factors for intermolecular separation. The chemical structures, liquid crystalline properties, self-assembly behaviour and ionic conductivity of these compounds were researched during multiple experimental techniques. The star-shaped ILCs showed a smectic A (SmA) mesophase. The d-spacing of star-shaped ILCs increased slightly due to the increase volume of anion. The clearing temperatures of the pyridinium salts suggested that the effect of the stabilisation on the SmA structures was in the order H2PO4−>BF4−>TS−>D-MH-SO3−>CF3COO−>PF6−. All these star-shaped ILCs displayed ionic conductivity in mesophase. It was noted that the conductivity (σ) increased with the increase of the anion size and temperature. [ABSTRACT FROM AUTHOR]

The doping of polymer electrolytes (PEs) with liquid crystal (LC) materials has been shown to improve the performance of dye-sensitised solar cells (DSSCs). This is achieved by promoting ionic conduction and increasing optical path length through multiple-light scattering within the photovoltaic devices. In LCs, it is well known that the length of the alkyl chain plays an important role since the LC morphology and mesophase stabilisation depend strongly on the alkyl group. In this work, liquid crystal-polymer composite electrolytes (LC-PEs) are prepared using nematic LCs with different alkyl chain lengths. The morphology of the LC-PEs is investigated and correlated with their electrical properties. Subsequently, DSSCs are prepared using the LC-PEs as a direct example of its application. It is shown that increasing the alkyl chain length of the LCs reduces the efficiency of the solar devices. The longer alkyl chains are speculated to intertwine, thus trapping the mobile ions and reducing the bulk ionic conductivity. For the same reason, longer alkyl chain LCs are thought to be unable to passivate the TiO2 surface through the adsorption of cyanobiphenyl groups and hence the higher probability of back recombination reaction between the electrons in TiO2 and PE. [ABSTRACT FROM AUTHOR]

In the present work, single-walled carbon nanotubes (SWCNTs) were dispersed in a room temperature nematic liquid crystal 4-pentyl-4′-cyanobiphenyl at the concentration of 0.02 and 0.05 wt%. Differential scanning calorimetry and temperature-dependent dielectric studies suggest decrease in clearing temperature of the composite materials as compared to the pure material. Ionic conductivity increases by two orders of magnitude due to the dispersion of such a low concentration (0.05 wt%) of SWCNTs. Dielectric studies also show that the presence of the SWCNTs decreases the effective longitudinal as well as transverse components of the dielectric permittivity. For homeotropic aligned samples, a relaxation mechanism has been detected in the lower MHz region both for the pure as well as dispersed samples. Presence of SWCNTs increases the relaxation frequency corresponding to flip-flop motion of molecules around their short axes. From frequency-dependent dielectric studies, important dielectric parameters such as relaxation frequency, dielectric strength and distribution parameters have been determined. Electro-optical experiments show that the threshold voltage decreases and the steepness of the transmission voltage curve improves due to the dispersion of SWCNTs. [ABSTRACT FROM AUTHOR]

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 SmAdphases of both the nanocomposites. The optical study suggests that due to dispersion of nanoparticles, the optical band gap has decreased. [ABSTRACT FROM PUBLISHER]

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4-n-decyloxy-4'-(cholesteryloxycarbonyl-1-butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (∼200-300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material. [ABSTRACT FROM AUTHOR]

Weakly anchored hybrid aligned flexoelectric nematic layers subjected to a dc external electric field can adopt not only a hybrid distorted structure but also a planar or homeotropic one depending on thickness of the layer, d, applied voltage, U, anchoring strengths on both boundary surfaces, W 1 and W 2, as well as on dielectric anisotropy, Δε, and flexoelectric properties determined by the sum of the flexoelectric coefficients, e = e 11+e 33, of the nematic material. Recently, this problem was treated theoretically for the case of a perfectly insulating flexoelectric nematic. The formulae describing the boundaries of regions of existence of planar and homeotropic structures in the control parameters plane U,d were proposed to be used for determination of the sum of flexoelectric coefficients. In this paper, the problem is studied numerically for the more realistic case of a nematic containing ions. The regions of existence of the planar and homeotropic structures are determined for a well-purified as well as for typically contaminated nematics. The results can be described by the theoretical formulae derived for insulating nematic with sufficient accuracy only when the ion concentration does not exceed 5×1018 m-3. Methods proposed for the measurement of flexoelectric coefficients are critically analysed. [ABSTRACT FROM AUTHOR]

We have synthesized liquid crystalline polymers containing an imidazolium salt moiety and a mesogenic group by the in situ photopolymerization of a liquid crystalline vinylimidazole monomer in order to investigate the relationship between their thermal properties and ionic conductivity. A smectic phase was shown by the vinylimidazole monomer. The in situ photopolymerization of the monomer was carried out in the temperature range of the smectic phase. The polymer thus prepared displayed a highly ordered smectic phase in the temperature range between room temperature and about 200°C. The ionic conductivity of the polymer increased with increasing temperature. Anisotropic ionic conductivity behavior was observed for the polymer. The ionic conductivity of the polymer aligned homogeneously is larger than when homeotropically aligned. [ABSTRACT FROM AUTHOR]