Nonexponential nuclear spin-lattice relaxation in the isotropic phase of thermotropic liquid crystals

In the isotropic phase of nematic and smectic liquid crystals T₁ of CH3 and chain protons is larger than that of ring and ring-neighboured protons being caused by fast CH₃ reorientation and internal motions in chains, respectively.     

Diffusion in thermotropic liquid crystals

Diffusion coefficients in mesophases of thermotropic liquid crystals have been measured to a considerable extent only during the last twelve years. Some theories on diffusion in mesophases have been developed too. Measurements in nematic, smectic A, smectic B, and smectic C mesophases have been carried out by mass transport techniques, detected by radio-tracers or optically, by NMR spin-echo techniques, and by quasi-elastic neutron scattering. The diffusion is anisotropic in most of the cases. In nematics self-diffusion parallel to the molecular director (D₆) is somewhat faster than perpendicular (D_⊥). Both diffusion coefficients show about the same activation energy. Impurity diffusion (small molecules dissolved in mesophases) demonstrates the same behaviour with less anisotropy. In smectics A and C the diffusion coefficients D₆ and D_⊥ can be nearly equal. The anisotropy is now shown in their respective activation energies with E₆>E_⊥. This effect is more pronounced in impurity than in self-diffusion. In addition impurity diffusion shows a strong anisotropy of the diffusion coefficients (D_⊥?D₆). In smectics B the activation energies E₆ and E_⊥ of self-diffusion seem again to be equal and the diffusion coefficients show a small anisotropy (D_⊥≥D₆).     

The influence of external fields on the orientational relaxation of nematic liquid crystals

The relaxation of the director field n, the velocity field v, and the shear and normal components of the stress tensor is theoretically investigated by numerically solving the system of nonlinear hydrodynamic equations. These equations describe the director reorientation with allowance made for the field of velocities induced by the rotation of the director field. The relaxation time and the influence of the velocity field on the relaxation processes are analyzed for a number of hydrodynamic regimes, which arise in a liquid-crystal cell under the effect of external electric and magnetic fields.     

A model for the orientational order in liquid crystals

Summary A model for the orientational-order parameter in liquid crystals from the microscopic viewpoint and in conformity with its current understanding well established in the field of phase transitions is proposed here. This model has a close resemblance with the original ideas of Maier and Saupe. However, it is independent of the nature of molecules, rigid or flexible, and enables us to describe a phase with a single-order parameter tensor as expected. Moreover, an important advantage of this model in contrast to the prevailing ones is its, ability to separate the contributions of long-range order from local information inherent in any microscopic measurement such as by NMR and EPR techniques with some reasonable assumptions. All these are demonstrated here with the observed quadrupole splittings for different deuterium sites of long-chain molecules forming the uniaxial nematic phase. The role of molecular conformations on the observed splittings is also explicitly demonstrated by recovering quadrupole coupling constants and bond orientations at different molecular sites. These results are in reasonable agreement with available data. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

A linear and non-linear electron spin resonance study of orientational relaxation in semi-crystalline polymeric liquid crystals

Summary Linear and non-linear Electron Spin Resonance spectroscopies have been designed to study the decay of the orientation correlations on different time scales. The reorientation of radicals (?spin probes?) dissolved in host phases (in the present case a polymeric liquid crystal (PLC)) may be investigated in the time window 10⁻¹¹–10⁻⁵ s. Evidence of the non-exponential regression of fluctuations is given. The temperature dependence of the correlation time of the spin probe orientation is discussed in the case of semi-crystalline PLCs and contrasted with the case of amorphous PLCs. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Short range orientational order in nematic liquid crystals

The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.     

Static dielectric pretransitional effects in thermotropic liquid crystals

The paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.     

Laser-induced nonlinear orientational effects in azobenzene nematic liquid crystals

Nonlinear optical orientational phenomena induced by laser in BAAB 1005-type nematic liquid crystals are studied experimentally. It is shown that these phenomena in the field of a single-mode laser field are displayed stronger, if the liquid crystal material is pre-irradiated with certain-frequency visible radiation. These phenomena are caused by the trans-cis transition in molecules of azobenzene liquid crystal BAAB 1005 at absorption of certain frequency radiation. Qualitative explanation of the experimental results is based on the simultaneous action of several mechanisms including thermomechanical.     

热致型胆甾酯液晶的相变研究

使用偏光显微镜观察胆甾烯壬酸酯液晶的相变，发现在不同的温度变化条件下液晶相变过程是不同的．本文讨论了其变化规律．     

A study on polymorphism of hydrogen-bonded thermotropic liquid crystals

A novel mesogenic homologous series comprising of eight hydrogen-bonded liquid crystalline complexes are isolated and analyzed by forming a hydrogen bond between p-n alkyloxy benzoic acids (where n represent alkyloxy carbon number which varies from 5 to 12) and mesaconic acid, respectively. Eight synthesized complexes are subjected to Fourier transform infra-red spectroscopy, polarizing optical microscopy and differential scanning calorimetry studies to meet the basic characterization. The variation of optical tilt angle with respect to temperature in various conventional and smectic X phases are investigated and analyzed. A phase diagram is constructed to elucidate the mesogenic behavior of novel liquid crystalline series. The Cox ratio that reveals the order of various mesophases and the stability factor, to invoke the thermal stability of mesophases, is studied under the results of DSC thermogram.     

Mössbauer spectroscopy of ferrocene-containing thermotropic liquid crystals

⁵⁷Fe Mössbauer spectroscopy has been used to study some liquid crystalline disubstituted derivatives of ferrocene (FLCn). The characteristic Debye temperatures for these compounds were found to be in the range of 25–31 K. The order parameters, and intramolecular- and lattice-contributions to the nuclear vibrational anisotropy have been obtained from the analysis of the angular dependence of the Mössbauer parameters.     

Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to ) and thick (7 to cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to ) of ( , where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far ( C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.Received: 22 February 2004, Published online: 24 May 2004PACS: 61.30.-v Liquid crystals - 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitionsSurajit Dhara: Present address: Department of Physics, Birla Institute of Technology and Science, Pilani 333 031, India.     

Molecular relaxation in homeotropically aligned ferroelectric liquid crystals

Uniform and defect-free homeotropic (HMT) alignment of ferroelectric liquid crystals (FLC) cells have been successfully achieved by a non-contact technique, namely, magnetic field-assisted alignment; otherwise such alignment is difficult by conventional methods. Frequency-temperature dependent dielectric studies, confirm the presence of stable molecular relaxation around the short axis of FLC molecule, throughout SmC*-SmA* phases with minimum low-frequency fluctuations, known as Goldstone modes. The proposed non-contact alignment method, combined with FLC-based HMT configuration, would be much promising for integrated-optoelectronic devices, such as micro-mirror displays.     

The relaxation of twist in nematic liquid crystals

A critical twist angle, above which twist can relax homogeneously, has been observed in the liquid crystals MBBA and 5CB. The results suggest that (2K₂₂ ? K₃₃) increases through zero on heating and at T_c becomes equal to K₁₁.