Orientational Order of Spin Probe Molecules in Liquid Crystals

We report here measurements of hyperfine splittings and g values of two nitroxide spin probes 4',4'-dimethyl-spiro-(5α-cholestane-3,2'-oxazolidin)-3'-yloxyl and 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinyloxyl methyl ester dissolved in p-azoxyanisole. Using the principal axes values of g and A tensors available, we have determined all three diagonal components of the orientational order parameter tensor S for these spin probe molecules. We find that for neither of the spin probes used here has this tensor S the cylindrical symmetry that is normally assumed to determine the orientational order parameter tensor S^(p) defined by the long molecular axis from such measurements. We show further that this conclusion about S is independent of any uncertainty in the principal axes values of the g and A tensors, and hence the relations between S and S^(p) normally used do not seem to be well justified. Probable relations are also suggested.     

Computer Simulation of Biphenyl-Phenyl Ester Liquid Crystals

Abstract

Molecular orbital (MO) calculation and molecular dynamics (MD) simulation were carried out for a set of smectic liquid crystalline molecules, 8O-O8 (4-octyloxyphenyl-4′-octyloxybi-phenyl-4-carboxylate) and 8O-8 (4-octylphenyl-4′-octyloxybiphenyl-4-carboxylate), to understand the molecular origin of different crystalline structure formation and to predict their conformational property in liquid crystalline phases. The results of MO and MD analyses indicate that the structure of 8O-O8 and 8O-8 in crystalline phase is essentially determined by intramolecular interactions and the experimentally observed structure is chosen as a consequence of intermolecular packing constraints. This paper also reports the first application of two-molecular ab-initio calculation to liquid crystalline systems.     

Gas Chromatography Analysis of Diphenyl-Diacetylene Liquid Crystals

Abstract

A gas chromatography method is used to investigate the purity of dialkyl-diphenyl-diacetylene liquid crystal components and at the same time identify the structure and concentration of the major impurities.     

Effect of Molecular Structure on Mesomorphism: 5. Comparison of Methyl and Trifluoromethyl as Lateral Substituents in the Solid and Nematic Phases.1

Two laterally-substituted nematogenic liquid crystals, methyl-p-phenylene-di-p-butoxybenzoate and trifluoromethyl-p-phenylene-di-p-butoxybenzoate, have been studied by differential scanning calorimetry, powder x-ray diffraction, and polarized light microscopy (hot stage microscopy and hot-wire stage microscopy). Comparison of thermodynamic data for the nematicisotropic transitions of these two compounds suggests a strongly repulsive electronic interaction between molecules in the nematic phase of the trifluoromethyl compound. Detailed molecular structural augments are presented to rationalize the thermodynamic data. The room temperature solid phases of the two compounds were found to have different crystal structures. However, the high temperature solid phases were found, by a novel application of hot-wire stage microscopy, to be isomorphous.     

The Use of X-Ray Diffraction in the Study of Thermotropic Liquid Crystals With Rod-Like Molecules

The paper first reviews briefly the various kinds of information that can be obtained from x-ray diffraction studies. Next, specific examples are given from the literature of determinations of each of the different kinds of data.     

Birefringence and Order Parameter of Some Alkyl and Alkoxycyanobiphenyl Liquid Crystals

The refractive indices and densities of six liquid crystals—two alkylcyanobiphenyls, three alkoxycyanobiphenyls and one mixture of two alkylcyanobiphenyls have been reported as a function of temperature. The effective polarizabilities α_eand α_o in the nematic phase calculated using the methods due to Neugebauer, Vuks and Saupe and Maier, are found to be appreciably different, though the order parameter S evaluated with those values of (α_e - α_o) are in good agreement. The preference of one method over the other two is discussed.     

Even-Odd Effect in Nematic Isotropic Transitions of Liquid Crystals Composed of Molecules with Terminal Phenyl Rings

The unusually pronounced even-odd effect in nematic isotropic transitions observed in liquid crystalline compounds with terminal phenyl ring is reproduced in a mean field model. Calculations are presented for the first five members of ω-phenylalkyl 4-(-p-cyanobenzylideneamino) cinnamates. The conformational and dispersion energies for the rigid part and the terminal ring of a molecule in the field of other molecules are considered but the aliphatic chain anisotropic interaction is neglected for simplicity. The chain conformations, however, directly influence the relative configuration of the central rigid part and the terminal ring and as a result the longitudinal polarizability of the molecules changes.     

Molecular Ordering in Liquid Crystals and the Effect of End-Chains on the Even-Odd Effect

The second and fourth orientational order parameters [Pbar]₂, [Pbar]₄ of the homologous series of 4-n-alkyl-4′-cyanobiphenyls (nCB, n = 5 ～ 8) in the nematic liquid crystalline state have been determined from the Raman depolarization ratios by the resonance Raman probe method. It was found out that not only [Pbar]₂ but also [Pbar]₄ exhibits evident even-odd effect when they are compared at the same reduced temperature. For understanding the mechanism of the even-odd effect, a theoretical calculation has been carried out on the basis of the mean field theory of Marcelja, in which the effect of the end alkyl-chain on the liquid crystalline state is taken explicitly into consideration. The observed trend in [Pbar]₂, [Pbar]₄, nematic-isotropic transition temperature were well reproduced. Detailed inspection of the results shows that the presence of anisotropic molecular field plays an important role in the appearance of the even-odd effect. The anisotropic molecular field produces redistribution of the statistical weight of various conformers in such a way that the end chains of odd-members align better along the direction of the rigid core part than those of even-members.     

Nearly Metallic [CH(13)y]x - Importance of Solitons,Crystal Order,Hopping and Band Conduction

Polyacetylene doped in the range of one to five percent has been shown to have low Pauli susceptibility yet high electrical conductivity. Earlier studies of polyacetylene doped with iodine to these nearly metallic levels show that essentially all charges go into soliton-like states. The conductivity (σ) and thermopower (S) of many samples are in quantitative agreement with charge transport via variable range hopping among soliton-like levels. Other doped samples have a lover density of states at the Fermi level with differing σ(T) and S(T). The strong sensitivity of the density of states to disorder leads to the proposal of a parallel conduction mechanism of thermal activation of charge carriers to “high mobility” extended band states. This mechanism may dominate in less disordered samples.     

Nomenclature of Liquid Crystals

Abstract

One of the signs of the maturing of a new area of knowledge is a heightened discussion and awareness of nomenclature, conventions and labels. In emerging sciences, the facts are often in dispute or open to different interpretations, and so premature attempts to categorize and standardize will only serve to confuse and delay real understanding. Liquid crystal science is now of an age when the attention of its practitioners is being drawn to considerations of nomen-clature. Indeed in 1997 IUPAC (International Union of Pure and Applied Chemistry) issued provisional recommendations concerning the definition of basic terms and symbols for liquid crystals. Members of the International Liquid Crystals Society have become involved with the preparation of a final document, which hopefully will be generally accepted and followed by scientists in the field. Some of the initial recommendations have already been reported in the new Handbook of Liquid Cryrstals, published in 1998 [1], so the process of formalising the nomenclature of liquid crystals has begun.     

Viscometry of Nematic Liquid Crystals

Viscometric experiments have played a significant part in establishing continuum theory for nematic liquid crystals. Such theory supports the hypothesis that observed non-Newtonian behaviour stems from competition between the aligning influences of flow and solid surfaces, and as a consequence predicts rather unusual scaling for the apparent viscosity. This paper first describes such scaling and its subsequent experimental confirmation emphasising the full implications of the latter. The relevance of the theory thus established, there follows a simple analysis of alignment in shear flow which leads to conditions on material coefficients necessary to ensure consistency with observations. Our discussion turns next to an account of solutions exhibiting non-Newtonian behaviour, and also mention of a recent stability analysis which attempts to discriminate between the different solutions that are possible. The final section considers oscillatory shear flow and presents an analysis of an instability which can occur in certain nematics.     

The World of Liquid Crystals

Abstract

Glenn H. Brown died at Kent, Ohio, on April 18, 1995, in the 30th anniversary year of the International Liquid Crystal Conference that he founded in 1965.     

Basal and Non-Basal Slip in Anthracene Single Crystals

The role of basal and non-basal slip in the deformation of anthracene single crystals is considered in the light of published evidence and further work to establish their relative importance is described. Compression and shear tests on suitably orientated crystals show that the critical resolved shear stress for non-basal slip is at least ten times greater than that for basal slip. It is concluded that at room temperature slip generally takes place on the (001) [010] and (001) [110] systems and that the non-basal (201) [010], (100) [010], (100) [001], (010) [100] and (010) [001] systems suggested previously are only activated to any other extent under deformation conditions which severely limit basal slip.     

Raman Scattering from Liquid Crystals

Abstract

The use of Raman scattering technique as a tool for studying local order in complex liquid crystal systems is illustrated by three examples, namely the locally anisotropic liquid L-phase, lipid membranes, and PDLC's.     

Nonlocal Continuum Theory of Liquid Crystals

A nonlocal continuum theory of liquid crystals is constructed to explain and predict the physical behavior of liquid crystals under long range intermolecular forces Balance laws consist of conservation of mass and mocroinertia, balance of momenta and energy. Constitutive equations are given for the equibilibirium and non-equilibirium parts of the stress, couple strees, free energy, entropy an nonlocal body force and couple. Thermodynamic restrinctions and material frame-indifference are studied. The theory is valid for liquid crystals having arbitrary shapes (inertia), Passage is made to the thread-like molecuels and to local theory. Applications are considered to two-dimensional problmes, steady, plane shear flows and disperison of twist waves.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.     

Structures of Complex Crystals of Alkylammonium Salts with Aromatic Molecules

Abstract

The complex crystal structures of dodecyltrimethylammonium chloride (DTAC) with catechol and hydroquinone were analysed by an X-ray diffraction method. Both complexes have isomorphous layered structures. The guest molecules locate between the interdigitated host molecules. Crystal structures are stabilized by mainly hydrogen bonds including water molecules. A cross-section balance between hydrophilic and hydrophobic parts is important for an energetically stable packing. DTAC can form the crystalline complexes with catechol and hydroquinone by both crystallizing from the solution and mixing host and guest powders in a mortar. In addition, DTAC can also make a complex with resorcinol. Powder diffraction pattern indicates that this complex has similar layered structure with complexes of DTAC / catechol and DTAC / hydroquinone. However, it is unstable in atmospheric condition.     

Field-Induced Order in Nematic Liquid Crystals

The effect of an external magnetic field on the orientational order of a nematic liquid crystal has been examined using both Landau-de Gennes and Maier-Saupe theories. In the Maier-Saupe approach a rotationally invariant form of the pseudo-potential is introduced, which in the absence of an external field leads to three degenerate isomorphic solutions for the order parameter, corresponding to alignment along three principal axes; a similar result is obtainable from the Landau-de Gennes theory. Application of a magnetic field lifts the degeneracy of these solutions, and for materials having a positive diamagnetic susceptibility anisotropy, the uniaxial solution with alignment along the field direction is always energetically favorable. For materials with a negative susceptibility anisotropy, a biaxial solution minimizes the free energy at low temperatures, but on increasing the temperature there is a transition from a biaxial phase to an uniaxial phase. The field dependence of the transition temperatures is evaluated, and for positive materials there is a critical field, corresponding to a second order transition above which the nematic and isotropic phases are indistinguishable. A contrasting behavior is predicted for negative materials, and above a certain critical field the biaxial/uniaxial transition changes from first order to second order. For weakly ordered systems it is shown that the Landau-de Gennes expression for the free energy is identical to that obtained from the Maier-Saupe theory. However, for more ordered systems, the results of the two approaches differ, and in particular the Maier-Saupe theory predicts a susceptibility divergence temperature T* which increases with applied field, in agreement with recent experiments.     

Modulated Structures in Dielectric Liquid Crystals

Review of theoretical and experimental results concerned to formation of space-modulated orientational structures in liquid crystals is given. Properties of ideal liquid crystal dielectrics are considered. Origion conditions and properties of flexoelectric and flexoelectromagnetic structures in nematics, dielectric instabilities in cholesterics and smectics, ferroelectric structures in chiral smectics are discussed.

It is well known that an electric field induces a modulation of the orientational structure of liquid crystals. There are two types of such modulated structures (MS) having different physical natures. The dissipative one is connected with the transport of electric charges and mass of the substance. The thermodynamic MS are not accompanied by the irreversible transport processes. Such MS can exist in ideal liquid crystal dielectrics. In fact there are always some ion impurities and as a result two types of MS are possible in liquid crystals. However it is possible to change the thresholds of these orientational instabilities in dependence on the concentration of impurities, anisotropy of transport parameters, boundary conditions and frequency of an external field. Thus one can exclude or induce the effects of first and second types. Below we shall consider the thermodynamic orientational transformations and conditions of their observation.     

Hall Effect in Lyotropic Liquid Crystals

It is known that Hall effect takes place in different media. The Hall effect is well studied in semiconductors. It is known quantum, quantum spin, the anomalous Hall effect, as well as the Hall effect in ionized gases. However, up to date, as evidenced from the review of scientific literature, the study of this effect in lyotropic liquid crystals (LLC) was given insufficient attention. In this paper the peculiarities of the Hall effect in the LLC were studied. It was shown that in liquid crystal systems Hall potential is approximately 100 times greater than in semiconductors. Also it was shown that after removal of the electric and magnetic fields, the Hall potential in LLC remained intact, decreasing over time, and periodically reversing its sign.