Dielectric relaxation study of a H shaped liquid crystal dimer

The present article reports dielectric relaxation study of an unusual shaped liquid crystal dimer (H shape) in the temperature range 40–85°C. The study of this liquid crystal dimer is important due to the presence of azo central linkage, which is photosensitive. The dielectric relaxation study indicates coupling between the electric field and the liquid crystal molecule of H shaped dimer. Various dielectric properties for this liquid crystal dimer are quite different from the conventional liquid crystals. The dielectric relaxation observed in the smectic phase is due to the reorientation of molecule along the director in the presence of an applied electric field. The temperature dependence of the dielectric parameters, such as relaxation frequency, relaxation strength and distribution parameter have been evaluated for this liquid crystal dimer for both planar as well as homeotropically aligned cell and then compared. The Cole–Cole equation has been used to evaluate the aforementioned parameters.     

Proton spin relaxation in a smectic liquid crystal

A proton spin relaxation study in the liquid crystal ethyl-[(methoxybenzylidene)-amino] cinnamate is presented. A “phase change” is observed at ≈ 103°C within the smectic A phase. Some liquid-like mobility exists below this temperature.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

Comparative study of temperature dependent orientational relaxation in a model thermotropic liquid crystal and in a model supercooled liquid

Recent optical Kerr effect experiments have revealed a power law decay of the measured signal with a temperature independent exponent at short-to-intermediate times for a number of liquid crystals in the isotropic phase near the isotropic-nematic transition and supercooled molecular liquids above the mode coupling theory critical temperature. In this work, the authors investigate the temperature dependence of short-to-intermediate time orientational relaxation in a model thermotropic liquid crystal across the isotropic-nematic transition and in a binary mixture across the supercooled liquid regime in molecular dynamics simulations. The measure of the experimentally observable optical Kerr effect signal is found to follow a power law decay at short-to-intermediate times for both systems in agreement with recent experiments. In addition, the temperature dependence of the power law exponent is found to be rather weak. As the model liquid crystalline system settles into the nematic phase upon cooling, the decay of the single-particle second-rank orientational time correlation function exhibits a pattern that is similar to what has been observed for supercooled liquids.     

Deuteron relaxation study of D2O motions in a lyotropic liquid crystal

The water molecules adsorbed at the soap—water interface in the mesophases of potassium laurate—D₂O system reorient rapidly while diffusing along the interface; our results also suggest that the dynamical behaviour of the aqueous and paraffinic media are strongly correlated.     

Ionic relaxation in nematic liquid crystal cells

We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

Abstract

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Thermodynamic study of a liquid crystal as a liquid phase in gas-liquid chromatography. II. A cholesteric liquid crystal.

Gas-liquid chromatography is utilized for the determination of thermodynamic solution parameters for various organic solutes at infinite dilution in the meso- and isotropic phases of cholesteryl palmitate. The thermodynamic data and trends in values of the activity coefficients for the solutes are discussed in relation to their structure and to the orientations of the liquid crystal.     

Nuclear spin relaxation in biaxial liquid crystal phases

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J_mLm'L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the 'biaxial' spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Nuclear spin relaxation in biaxial liquid crystal phases

Abstract

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J _mLm′L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the ‘biaxial’ spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Proton NMR relaxation study of molecular dynamics of chromonic liquid crystal Edicol Sunset Yellow

Proton nuclear magnetic resonance (¹H NMR) relaxometry, over about five decades in Larmor frequency, and pulsed field gradient NMR were used to study the molecular dynamics in the chromonic nematic and isotropic phases of stacked molecules of the binary mixture composed by Edicol Sunset Yellow (ESY) and deuterated water. Our results evidence that in both phases collective motions are responsible for the spin-lattice relaxation dispersion in the Larmor frequency range below 1 MHz. In the nematic phase, the collective motion are attributed to columnar undulations within the stacked molecules, while, in the isotropic phase, the results are explained by local order fluctuations due to the formation of the stacks. The high frequency dispersion was explained by individual molecular motions like rotations around and perpendicular to the stack axis, and also self-diffusion.     

Rotational dynamics of a discotic liquid crystal studied by deuteron spin relaxation

We report on measurements of deuteron quadrupolar splitting and spin-lattice relaxation times T_1Q and T_1Z in the columnar phase of a ring-deuteriated hexaoctyloxyrufigallol at 46 MHz as a function of temperature. To describe small-step rotations of these molecules within each column in the columnar phase, a space-fixed frame is used to diagonalize the molecular diffusion tensor. The principal diffusion constants in this so called 'anisotropic viscosity' model D_alpha and D_beta are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities at seven temperatures in a minimization procedure was carried out. We found that D_alpha > D_beta, which is consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column.     

Dielectric relaxation of liquid crystal molecules in anisotropic confinements

Abstract

The dielectric relaxation of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (K15) in the presence of an anisotropic network has been studied. Anisotropic networks containing K15 molecules were prepared by in situ polymerisation of liquid-crystalline diacrylate molecules in a mixture containing K15. By changing the network concentration, the effect of the network molecules on the behaviour of the K15 molecules, which were not chemically attached to the network, was investigated. With increasing network concentration it was found that the mean relaxation times of K15 molecules shifted to lower temperatures and that their distribution became broader. The activation energy associated with the relaxation, however, remained almost constant before showing some increase at high network concentrations.     

X-ray study of substrate-induced alignment of a smectic A liquid crystal

Abstract

We have performed a structural study of the liquid crystal (LC) octylcyanobiphenyl (8CB), deposited on gratings and flat surfaces, using high resolution X-ray scattering as a function of film thickness. 8CB is a room temperature smectic A₂, with a layer spacing of 31·6 Å. Glass was used as substrate and treated with either one of the organic surfactants MAP or DMOAP. Surface tension forces cause the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate at the air interface. Competing with the LC-air interface, which is a strong aligner, a grating at the LC-substrate interface produces distortions in the smectic layering with an excess of elastic energy, which favours alignment parallel to the substrate and the grooves. Our purpose was to detect the onset and evolution of parallel alignment as a function of film thickness. The studies used 9 keV (1·403 Å) X-rays focused to a spot size of 2 mm² at the sample position. In-plane scans, which detect the smectic layers perpendicular to the plane of the substrate, were done at angles φ = 0° and 90° with respect to the gratings to ascertain the molecular orientation, at a nominal X-ray incidence angle of α = 0°. In order to observe regions of varying smectic layer orientation within the film, we performed a series of scans where the out-of-plane tilt angle χ changed from 0°, corresponding to scattering in the plane of the film, to 90°, which corresponds to scattering normal to the surface of the film. The results from these scans were fitted to a multilayer model where the orientation of the smectic layers varies as a function of film depth. The analysis confirmed our earlier observations that surface tension at the air interface plays a dominant role in the alignment of the LC molecules.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Dielectric relaxation of liquid crystal molecules in anisotropic confinements

The dielectric relaxation of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (K15) in the presence of an anisotropic network has been studied. Anisotropic networks containing K15 molecules were prepared by in situ polymerisation of liquid-crystalline diacrylate molecules in a mixture containing K15. By changing the network concentration, the effect of the network molecules on the behaviour of the K15 molecules, which were not chemically attached to the network, was investigated. With increasing network concentration it was found that the mean relaxation times of K15 molecules shifted to lower temperatures and that their distribution became broader. The activation energy associated with the relaxation, however, remained almost constant before showing some increase at high network concentrations.