摘要:

Fluoro substitution in thermotropic liquid crystals provides a general way of modifying the properties of a parent system. Melting point, transition temperatures, mesophase types and other physical properties are affected by fluoro substitution so that frequently the behaviour of the parent compound can be manipulated and improved in a predictable manner. This review discusses the effects of fluoro substitution at different positions in a variety of core systems and briefly considers the problems arising in the synthesis of fluoro-substituted liquid crystals.

ISSN:1058-725X    

DOI:10.1080/10587259808048432

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

4-Octyloxy-2,6-di(hydroxymethyl)pyridine(2), 4(4′-octyloxyphenyl)-2,6-di(hydroxymethyl)pyridine (4), and 4(4′-octadecyloxyphenyl)-2,6-di(hydroxymethyl)pyridine (3) were synthesised. By modification of a literature method the dimethyl (or ethyl) 4-hydroxypyridine-2,6-dicarboxylate (6) was synthesised with yield greater than 90%, using thionyl chloride/DMF catalyst mixture, compared with 55–85% using methanol or ethanol/H2SO4mixture as reported in literature.

ISSN:1058-725X    

DOI:10.1080/10587259808048433

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The surface tension of 4,4′-di-n-undecanoyloxybiphenyl has been determined as a function of temperature. A distinct deviation of its temperature dependence from the normal behavior was observed below a certain inversion temperature. The surface tension of the isotropic sample decreases sharply as the falling temperature approaches the underlying bulk phase transition point. The formation of some highly ordered structures at the liquid surface is revealed, which is consistent with the suggestion that the existence of an underlying highly ordered phase favors the pretransitional surface ordering of molecules at the free surface of an isotropic phase.

ISSN:1058-725X    

DOI:10.1080/10587259808048434

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We investigate the effect of surface ordering on the bulk anchoring and anchoring transition of a nematic liquid crystal placed in the contact with a solid substrate. We model this situation by a simple lattice spin model in a semi-infinite geometry, where we impose the orientational distribution function (ODF) at the planar boundary. Mean field calculations, verified by Monte Carlo calculations, show that theanchoring transitionsin the system are fully determined by the parameters of the imposed boundary ODF.

ISSN:1058-725X    

DOI:10.1080/10587259808048435

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We propose a lattice model of elongated molecules interactingviaan angular part of the Berne-Pechukas potential. Monte Carlo simulations of this model are performed near the nematic to isotropic phase transition. We found that this transition is a stronger first order in the case of more elongated molecules. The comparison with the other simulations and the experimental data is performed.

ISSN:1058-725X    

DOI:10.1080/10587259808048436

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Polarized Fourier transform infrared (FTIR) and ultraviolet-visible (UV-VIS) spectroscopy were used to investigate the mechanism of liquid crystal alignment on polarized UV (PUV) irradiated polyimide (PI) films. PI films strongly absorb below 320 nm resulting in photochemical reaction of the PI. PUV irradiation of the PI film caused decrease of all the peak intensities in the IR, except the newly formed 1396cm−1peak, due to degradation of the PI molecules. The preferential degradation of PI molecules parallel to PUV irradiation direction results in the predominant orientation of the remaining PI molecules perpendicular to the PUV irradiation direction. But the rubbing of the PI films induced reorientation of the PI molecules parallel to the rubbing direction.

ISSN:1058-725X    

DOI:10.1080/10587259808048437

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The phase diagram ofn-C12H26+n-C15H32established by calorimetric and X-ray diffraction analyses exhibits no less than four single phase domains [T12, [O15], [Ri15] and [L], two invariants: a eutectic atT= 258.3 K and a metatectic atT= 263.3 K, and five two-phase domains. From a thermodynamic point of view, an interpretation of the phase diagram is given and an attempt is undertaken to modelise the [RI+ L] equilibrium. The extent in compositionxof this phase as well as the width in temperature of the [RI+ L] eutectic atx= 1/2, are explained in terms of degree of isomorphism εim(RI) and excess Gibbs energy functionsGE,sol(x). Those quantities as well as the melting temperature difference (ΔT) atx= 1/2, are compared and discussed with other binary systems.

ISSN:1058-725X    

DOI:10.1080/10587259808048438

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In his classic paper on the elastic behaviour of nematic liquid crystals published in 1958, Sir Charles Frank hints at the existence of a nematic phase with biaxial symmetry in addition to the conventional nematic phase with its uniaxial symmetry. Little notice appears to have been taken of this intriguing suggestion until 1970 when Freiser presented a molecular theory of nematics formed from biaxial molecules. This predicted that such materials should exhibit isotropic, uniaxial nematic and biaxial nematic phases although for a certain critical biaxiality the system should pass directly from the isotropic phase to a biaxial nematic. Despite these clear predictions it was not until the late 1980s that the claims to have prepared compounds exhibiting a thermotropic biaxial nematic phase began to appear; these originated from the laboratories of Jaques Malthtte, S. Chandrasekhar and Klaus Praefcke. However, subsequent studies of some of the materials suggested that the nematic phases claimed to be biaxial were, in fact, uniaxial. Although the formation of real biaxial nematics is in doubt, computer simulators have produced biaxial nematic phases in keeping with the theoretical predictions. In addition, about 17 years ago Alfred Saupe had discovered a biaxial nematic phase formed by a lyotropic system.

ISSN:1058-725X    

DOI:10.1080/10587259808048439

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We consider a fluid of hard boomerangs, each composed of two hard spherocylinders joined at their ends at an angle ψ. The resulting particle is nonconvex and biaxial. The occurence of nematic order in such a system has been investigated using Straley's theory, which is a simplificaton of Onsager's second-virial treatment of long hard rods, and by bifurcation analysis. The excluded volume of two hard boomerangs has been approximated by the sum of excluded volumes of pairs of constituent spherocylinders, and the angle-dependent second-virial coefficient has been replaced by a low-order interpolating function. At the so-called Landau point, ψLandau≈ 107.4°, the fluid undergoes a continuous transition from the isotropic to a biaxial nematic (B) phase. For ψ ≠ ψLandauordering isviaa first-order transition into a rod-like uniaxial nematic phase (N+) if ψ > ψLandau, or a plate-like uniaxial nematic (N−) phase if ψ < ψLandau. The B phase is separated from the N+and N−phases by two lines of continuous transitions meeting at the Landau point. This topology of the phase diagram is in agreement with previous studies of spheroplatelets and biaxial ellipsoids. We have checked the accuracy of our theory by performing numerical calculations of the angle-dependent second virial coefficient, which yields ψLnndau≈110° for very long rods, and ψLandau≈90° for short rods. In the latter case, the I-N transitions occur at unphysically high packing fractions, reflecting the inappropriateness of the second-virial approximation in this limit.

ISSN:1058-725X    

DOI:10.1080/10587259808048440

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A microscopic theory is developed for the bulk and surface elastic constants of biaxial nematic liquid crystals. Our expressions depend on generalized orientational order parameters and on the direct pair correlation function.

ISSN:1058-725X    

DOI:10.1080/10587259808048441

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor