Electrical conductivity and dielectric constant measurements of liquid crystal–gold nanoparticle composites

The behaviour of the anisotropic electrical conductivity of liquid crystal–gold nanoparticle (LC‐GNP) composites consisting of a commercially available room temperature nematic compound doped with alkylthiol‐capped GNPs has been investigated. The nematic–isotropic transition of the composite decreases nearly linearly with increasing X, the concentration of GNP (in weight %) at a rate of about 1°C /weight %. The inclusion of GNPs increases the electrical conductivity of the system with the value increasing by more than two orders of magnitude for X = 5%. However, the anisotropy in conductivity, defined as the ratio of the conductivity along (σ_∥) and orthogonal (σ_⊥) to the director shows a much smaller but definite decrease as X increases.     

Structure peculiarities and optical properties of nanocomposite: 5CB liquid crystal–CTAB-modified montmorillonite clay

The effects of the modification of natural layered montmorillonite (MMT) clay by cetyltrimethylammonium bromide (CTAB) cations on the structure and optical properties of the composite material based on this mineral (4.5%mass) and a nematic liquid crystal (LC), 4-pentyl-4'-cyanobiphenyl (5CB), have been investigated. As shown by small-angle X-ray diffraction and infrared (IR) spectroscopy experiments, this modification results in a significant expansion of the interplane spaces in the MMT nanoparticles and a considerable growth of their surface affinity to the 5CB molecules, which allows the LC molecules to penetrate into the MMT galleries and additionally expand these galleries. According to IR studies, this heterosystem possesses van der Waals interactions between its components on the phase separation boundary and, as a result, orientation alignment of the molecules in the near-surface layers occurs. These interactions specify the electro-optical properties of the composite. When an electric field is applied to a system, the light transmittance of the material increases due to the induced orientation of the LC dimers. This LC ordering remains even after the voltage is shut off, i.e. the system shows an electro-optical memory effect.     

Mechanism of the emergence of the photo-EMF upon silicon liquid crystal–single crystal contact

The effect light has on a silicon liquid crystal–single crystal contact at different temperatures of the surface doping of silicon, and when BaTiO₃ nanoparticles are added to the composition of a liquid crystal, is studied. The mechanism of the emergence of the photo-EMF in the liquid crystal–silicon structure is explained.

     

Molecular dynamics and alignment studies of silica-filled 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal

A comprehensive study of the dielectric properties of 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal filled with silica particles (particle size 30–80 nm, concentration 2, 3, 5, 10 and 15 wt%). Dielectric spectroscopy in the frequency range 100 to 10 7 Hz was applied to investigate the influence of the filler on the dynamic behaviour of the liquid crystal molecules in both the nematic and isotropic phases. In this frequency range one relaxation process is observed (at f>10⁶ Hz). The dynamical behaviour of the 5CB liquid crystal is described by the Cole-Cole relaxation function. The temperature dependence of the relaxation time obeys the empirical Arrhenius equation. The activation energies are approximately 75 kJ mol¹ for the pure 5CB sample in the nematic phase and 50 kJ mol¹ for the 5 wt% silica-filled 5CB sample. These values are compared with the corresponding literature values. The reversible electro-mechanical response of these samples under the influence of an applied a.c. electric field is investigated.     

Alignment and ordering mechanisms at a liquid crystal–solid interface

Abstract

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal–solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0·2 μm in diameter are chemically modified using an aliphatic acid (C _n H_2n+1 COOH) as a surface coupling agent and filled with the liquid crystal compound 4′-pentyl-4-cyanobiphenyl deuteriated in the α position of the hydrocarbon chain (5CB-αd ₂). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n ≥7 while chain lengths n ≤6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n = 15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.     

Synthesis and properties of organic–inorganic hybrid liquid crystal gels

Organic–inorganic hybrid liquid crystal (LC) gels have been synthesised by the thiol-ene reaction of a multifunctional cyclic siloxane, 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (TVMCTS) and alkane dithiols, 1,6-hexanedithiol (HDT) or 1,9-decanedithiol (DDT), in LC matrices, 4-cyano-4?-pentylbiphenyl (5CB) or 4′-n-octyl-4-cyano-biphenyl (8CB). The LC gels were prepared in an isotropic phase at 70°C or mesophases at 25°C using radical initiators. The phase transition temperatures from a mesophase to an isotropic phase of the resulting gels were lower than those of the original LCs. The gels containing 8CB (8CB gels) prepared at 25°C showed two phase transitions: smectic-to-nematic and nematic-to-isotropic transitions. By contrast, the 8CB gels synthesised in the isotropic phase showed only one phase transition from smectic phase directly to isotropic phase. Reaction conversions in the LC gels prepared at 70°C were higher than that in the gels prepared at 25°C. Scanning microscopic light scattering analysis of the LC gels cleared homogeneous small size mesh with a small amount of large defect. Polarisation micrographs of the LC gels showed framed optical textures derived from the LC molecules at room temperature. The LC gels containing more than 90 wt% of LC showed electro-optic response.     

Nafion–ZrO2 nanoparticle–ionic liquid nanobiocomposite for the direct electrochemistry of myoglobin

Journal of Solid State Electrochemistry - A nanobiocomposite was perpared by mixing zirconia nanoparticle with ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate and Nafion together, which...     

Field dependence of the optical activity of a chiral liquid crystal near the isotropic–smectic A transition

Abstract

Chiral liquid crystals exhibit molecular optical activity in the isotropic phase. We have studied the evolution of the optical activity as a function of an applied electric field on a 76·2 μm film of the chiral liquid crystal W7, which exhibits an isotropic–smectic A transition at approximately 40°C. We measured the optical activity by recording the rotation of the plane of polarization of an incident linearly polarized ray of light, provided by a He–Ne laser. The applied biasing electric field is parallel to the direction of the incident beam. We find that at 41·0°C, the plane of polarization shifts from ?1·1° for an applied voltage of 30 V to a maximum of ?4·0° at 70 V. The absolute value of the signal decreases beyond this voltage. These shifts are in the direction of the smectic A phase and are in general larger than those observed as a function of temperature. Close to the isotropic–smectic A phase transition, molecules inside the liquid coalesce to form dynamic coherent groups, which have smectic nature. These groups are randomly oriented with respect to each other in the absence of an electric field. The application of an electric field causes the molecules within these groups to align along the direction of the field and to contribute coherently to the optical activity of the system. The way the molecules align with the field depends on the relative values of the polarizability α, which contributes to the alignment of the long axis of the molecule, and the dipole moment p, which contributes to the alignment of the short axis of the molecule. Our preliminary results and calculations suggest that for small fields, the electric field couples with the dipole moment p, whereas for fields in excess of 70 V, the field couples with the polarizability of the long axis of the molecule, causing a rotational reorientation of the molecules in the isotropic phase. The value of the field at which this reorientation occurs may be controlled by temperature.     

Photo-induced control of bubble domains in chiral–nematic liquid crystal by a violet laser beam

The stable bubble domains generated by mixing 10% of chiral molecules into an azobenzene liquid crystal (LC)-doped nematic host can be optically controlled by a violet laser beam (415 nm). The photon-induced reversible trans–cis photo-isomerisation of azobenzene changes the helical twisting power (HTP) of LC mixtures in which the HTP of cis-azobenzene LC is lower than trans-azobenzene LC. Under the irradiation of an optical field (>20 mW cm^???2), the helical pitch distance, which is inverted proportional to the HTP, increases and the bubble domains disappear. Immediate obstruction of laser light irradiation initiates cholesteric nucleation, merging of domains and the subsequent generation of stably dispersed bubble domains.     

Dynamics of the acousto‐optic effect in a nematic liquid crystal

In a nematic liquid crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double‐exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale qualitatively consistent with the dynamics of the Fréedericksz transition, but the slow mode shows novel behaviour associated with the acousto‐optic effect.     

Electrosynthesis of polyphenylpyrrole coated silver particles at a liquid–liquid interface

This communication reports the production of polyphenylpyrrole coated silver nanoparticles at the liquid/liquid interface by an EC-type mechanism. In the electrochemical step of the reaction N-phenylpyrrole facilitates the transfer of the silver ion from an aqueous to an organic phase. This step is followed by a slow homogeneous electron transfer reaction from the N-phenylpyrrole to the silver ion followed by polymerization and metal cluster growth.     

Voltammetric determination of perchlorate ion at a liquid–liquid microscopic interface

A method is proposed for the electrochemical determination of perchlorate ion by voltammetry at the interface between two immiscible phases (water–o-nitrophenyl octyl ether). A demountable original-design amperometric ion-selective electrode based on a laser-microperforated polymeric membrane was fabricated for voltammetric measurements. The conditions of analytical signal recording in the determination of ClO₄ ^? were determined. The effect of interfering ions was assessed and amperometric selectivity coefficients were calculated. The accuracy of the procedure was verified by the added–found method. The developed electrode was applied to the determination of perchlorate in natural and drinking waters.     

Investigation of acid–base and surface characteristics of a thermotropic semifluorinated salicylaldimine liquid crystal by inverse gas chromatography

The surface energy of a semifluorinated salicylaldimine liquid crystal has been characterised by inverse gas chromatography over the temperature range 303 to 323 K using n-alkanes, tetrahydrofurane, dichloromethane, chloroform, acetone and ethyl acetate molecular probes. The dispersive component of the surface free energy of the adsorbent surface studied was calculated according to the approaches of Fowkes and Dorris–Gray in the infinite dilution region. The specific free energy, enthalpy and entropy of adsorption of polar probes on the liquid crystal were determined. The values of the specific enthalpy of adsorption were correlated with both the donor and the acceptor numbers of the probes to quantify the acidic and the basic parameters of the liquid crystal surface. The surface of the semifluorinated salicylaldimine liquid crystal was found to show a basic nature, which determined the nature of its interaction with the polar probes.     

Dielectric properties of magnetic nanoparticles’ suspension in a ferroelectric liquid crystal

Colloids of elongated γ-Fe₂O₃ magnetic nanoparticles (NPs) in a ferroelectric liquid crystal (FLC) were studied. Decreasing the dielectric strength of the Goldstone mode and changing the value of Cole–Cole parameter were found in the suspensions. It was also shown that the effect of introducing magnetic particles into a FLC consists of increasing the electric field strength magnitude required for unwinding its helical structure. Effect of magnetic field on dielectric properties of the FLC colloid was also studied. Dielectric constant measured under static magnetic field is different for the FLC host and FLC doped with the NPs.     

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmC_A* and ferrielectric SmC_γ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmC_A* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmC_γ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.     

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.     

Synthesis and mesomorphic properties of trifluorobenzoate liquid crystal

Four series of trifluorobenzoate liquid crystals have been synthesized. Their phase transition temperatures have been also measured by texture observation in a polarizing microscope and confirmed by DSC. The influence of the lateral fluoro-substitution and triple bond has been also discussed.