摘要:

A molecular simulation study of a cyclic siloxane macromolecule based on a pentamethylcyclosiloxane core and biphenylyl 4-allyloxybenzoate mesogenic units is reported. Molecular dynamics and semi-empirical calculations were used to provide insight into the conformation and the dielectric properties of the material. Out of three proposed conformations of the molecules, a cylindrical conformation was found to be the most probable. The intermolecular interactions were found to be optimized for the case where the mesogenic groups were planar and parallel to each other. The calculated mesogen length and inter-mesogen distances were consistent with available X-ray data. Electrostatic interactions were found to make a very significant contribution to the total energy. For the cylindrical model, the major component of the dipole was calculated to be along the long axis of the molecules. This is consistent with the alignment of the molecules parallel to a low frequency applied electric field as found experimentally.

ISSN:0267-8292    

DOI:10.1080/02678299408027863

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense.

ISSN:0267-8292    

DOI:10.1080/02678299408027864

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The temperature dependence of the nematic electroclinic effect has been studied in binary mixtures prepared from two chiral compounds with nematic-smectic A-smectic C and nematic-smectic C phase sequences. The data obtained show a substantial difference in the magnitude of the effect, not only with the type of phase sequence, but also with the temperature range of the subsequent smectic A phase. These results suggest that short-range smectic fluctuations can play an important role in the nematic electroclinic effect at least when a smectic C phase is close to the nematic. In addition, the dynamic behaviour of the electroclinic effect has been investigated in the compound with the nematic-smectic A transition. As in previous work, an anomaly in the electroclinic response time has been found around the nematic-smectic A transition. This fact is analysed qualitatively assuming two different mechanisms contributing to the electroclinic effect.

ISSN:0267-8292    

DOI:10.1080/02678299408027865

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The synthesis and characterization of a new class of liquid crystalline alternating copolymers based on s-triazine rings, connected by flexible alkyldiamine spacers and other selected bridging groups is reported. The propensity of the system to demonstrate liquid crystallinity as a function of the bridging group is investigated. Unexpectedly, the homopolymer of the triazine ring connected by a flexible alkyldiamine spacer is also a liquid crystalline polymer. A new polymerization technique is reported in which the alkyldiamine serves the dual role of monomer and proton acceptor. These polymers form liquid crystalline glasses which are transparent and have good mechanical properties. This suggests that they could be interesting materials to be further studied in the context of non-linear optics.

ISSN:0267-8292    

DOI:10.1080/02678299408027866

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The behaviour of a chiral nematic mixture with low frequency dielectric relaxtion was studied between two electrodes, with homeotropic anchoring, in the presence of an AC electric fieldE. The phase diagram in the parameter plane (d/p, E) was determined for this mixture which could be dielectrically negative or positive in different frequency ranges. The determination was carried out by changing the sample thicknessdand both the amplitude and frequency of the electric field at a constant cholesteric pitchp.

ISSN:0267-8292    

DOI:10.1080/02678299408027867

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δεi, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique.

ISSN:0267-8292    

DOI:10.1080/02678299408027868

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The molecular and crystal structure ofN-(4-n-butyloxybenzylidene)-4′-ethylaniline (4O.2) and the homologueN-(4-n-heptyloxybenzylidene)-4′-hexylaniline (7O.6) have been solved (at room temperature) by direct methods. The crystals of both compounds belong to the triclinic system with space group P1with two molecules per unit cell. 4O.2:a= 5·531(2),b= 7·592(3),c= 19·746(7) Å, α = 86·66(2),β= 88·15(2),γ= 90·29(2)° 7O.6:a= 5·420(2),b= 8·307(3),c= 28·057(7) Å, α = 91·69(2),β= 89·76(2),γ= 108·02(2)°. The structures were refined by full-matrix least-squares calculations toR= 0·036 for 2297 observed reflections for 4O.2 and toR= 0·037 for 2150 reflections for 7O.6. The conformations in the asymmetric units of the two compounds differ considerably: The planes of the two phenyl rings of 4O.2, forming the mesogenic core of the molecule, are twisted at 61·2° to each other and the butoxy group contains agaucheconformation. In contrast the heptoxy chain of 7O.6 forms an alltrans-conformation which lies almost in one plane with the two coplanar phenyl rings. The hexyl group also exists in an extended form, in a plane which is rotated against the plane of the mesogenic unit. The packing in the crystalline state of the two homologues exhibits a layered structure alongc*; in 4O.2 these layers are separated, but in 7O.6 they are interdigitated. The compensation of the dipole moments of the C‒O‒C and C˭N‒C bonds occurs similarly in both structures: molecular orientations parallel to thea, c-plane in which the long axes of the molecules points in the same direction are packed in antiparallel fashion alongb*.

ISSN:0267-8292    

DOI:10.1080/02678299408027869

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Some molecules of unusual shape and apparently non-linear geometry, with mesogens tied directly to a central unit (for example, tetrahedral pentaerythritol), have been investigated for liquid crystalline behaviour. It was found that these three-, four- and six-armed ‘star’ molecules generated liquid crystalline mesophases, which were characterized by DSC, optical microscopy and X-ray diffraction. The temperature ranges of the fluid mesophases for the thermotropic compounds were above 158°C for the three-armed molecules and above 219°C for the four-and six-armed ‘stars’. All the liquid crystalline compounds exhibited a smectic phase, which appears to be smectic A, with the molecules in their fully extended conformations within the layers. Some of the compounds also had a smectic phase of higher order or a nematic phase.

ISSN:0267-8292    

DOI:10.1080/02678299408027870

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We have considered a classical system, consisting of cylindrically symmetric particles, associated with a three-dimensional, simple cubic lattice, and interacting via nearest neighbour pair potentials of the form

ISSN:0267-8292    

DOI:10.1080/02678299408027871

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The alignment of nematic liquid crystals on micro-groove patterns was studied. It has been found that the order of the alignments is determined by the edge shape, spacing, and line pitch of the micro-groove patterns. On coarse patterns, with pitches greater than 2 μm, striped patterns of the liquid crystal alignment were observed using polarized light which manifested different orientations of the liquid crystals on the top, edge, and bottom of the grooves, respectively. On fine patterns, with pitches less than 2 μm, a uniform device-quality alignment has been realized, with which twisted nematic cells were constructed in combination with the rubbed alignment layer on the opposite substrate. Their viewing angle characteristics and tilt orientations of the director were also investigated.

ISSN:0267-8292    

DOI:10.1080/02678299408027872

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor