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).     

Ordering in nematic liquid crystals from NMR cross-polarization studies

The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining proton-proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented.     

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.     

Diffusion studies in confined nematic liquid crystals by MAS PFG NMR

Pulsed field gradient (PFG) NMR and magic-angle spinning (MAS) NMR have been combined in order to measure the diffusion coefficients of liquid crystals in confined geometry. Combination of MAS NMR with PFG NMR has a higher spectroscopic resolution in comparison with conventional PFG NMR and improves the application of NMR diffusometry to liquid crystals. It is found that the confinement of the liquid crystal 5CB in porous glasses with mean pore diameters of 30 and 200 nm does not notably change its diffusion behavior in comparison with the bulk state.     

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.     

Universal power law in the orientational relaxation in thermotropic liquid crystals

We observe a surprisingly general power law decay at short to intermediate times in orientational relaxation in a variety of model systems (both calamitic and discotic, and also lattice) for thermotropic liquid crystals. As all these systems transit across the isotropic-nematic phase boundary, two power law relaxation regimes, separated by a plateau, emerge, giving rise to a steplike feature (well known in glassy liquids) in the single-particle second-rank orientational time correlation function. In contrast to its probable dynamical origin in supercooled liquids, we show that the power law here can originate from the thermodynamic fluctuations of the orientational order parameter, driven by the rapid growth in the second-rank orientational correlation length.     

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．     

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.     

The twist-disclination line in planar oriented samples of liquid crystals

In a planar oriented layer of a nematic liquid crystal regions of differing chirality, separated by a twist-disclination line, are observed. At least four modifications of this disclination are observed in a parallel layer (zero twist) and three in a twisted layer. The nature of these modified twist-disclinations is described.     

NMR in rapidly rotating samples of nematic liquid crystals

By rapidly rotating a bulk nematic sample in a magnetic field it is possible to obtain a stable configuration where the directors all lie in the plane of rotation. A random distribution of orientations in the plane of rotation gives rise to a unique NMR “powder pattern”. The use of this technique in identifying spin—lattice relaxation mechanisms as well as its possible use in observing biaxial molecular order is given.     

Surface transitions in nematic liquid crystals oriented with Langmuir-Blodgett films

In this paper two types of surface transitions in nematic liquid crystals are theoretically analyzed with a simple model in which the anchoring energy is the result of both steric and electric interactions. The physical model is proper to explain both the transition temperature experimentally observed in homeotropically aligned nematic liquid crystals, and the change in the mean molecular direction due to the variation of the number of surfactant films, piled on the boundary surfaces with the Langmuir-Blodgett technique.Partially supported by the TEMPUS program     

Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T ₂ ^?1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.     

Extended x-ray absorption fine structure studies of oriented single crystals

Linearly polarized synchrotron radiation is used to study the orientation dependence of the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of single crystal zinc. The spectrum some 5–1200 eV above the K photoabsorption threshold is found to be markedly anisotropic, as would be expected from the established formalism generalized to crystals of lower than cubic symmetry. Zinc thus provides a prototype for molecular crystals of low symmetry where one wishes to study particular bond directions.     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

Proton NMR signals of intra- and interpair dipolar order in liquid crystals

We demonstrate the existence of magnetic intrapair and interpair dipolar order in aligned liquid crystalline samples. The relaxation times of the corresponding dipolar signals are different and depend strongly on the system.