摘要:

We discuss the different transitions from hexagonal to lower symmetry phases which result from a molecular tilt inside the columns. Several orientational structures are explained and others are predicted. Our model also applies to other physical phenomena, in particular the elliptical deformation of columnar aggregates in lyotropic liquid crystals

ISSN:0267-8292    

DOI:10.1080/02678299008033808

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The long time dynamical response of a nematic liquid exhibiting banded textures (inversion walls) during the twist Freedericksz transition is presented. A dynamical model of approach to equilibrium through defect interaction and the resulting dissolution of the banded textures is presented. A linear stability analysis shows that splay-bend inversion wall defects are unstable to two dimensional infinitesimal perturbations. A model of inversion wall segment collapse with production of a disclination line pair is given. The energy-momentum tensor gives the force of interaction between inversion walls and disclination lines. A perturbation analysis gives the evolution of the director field in closed form. Entropy production gives the velocity of each line. The growth law governing the wall dissolution is given.

ISSN:0267-8292    

DOI:10.1080/02678299008033809

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Mixtures of water and the dipotassium thapsate form hexagonal and lamellar lyomesophases with structures similar to those found for monosoaps. X-ray diffraction from a high intensity synchrotron radiation source exhibits the first four reflections of the hexagonal phase and the first two from the lamellar phase yielding lattice parameters of about 28 Å and 22 Å, respectively. Within the stability range of the mesophases these parameters do not vary significantly with the disoap concentration and the temperature. An intermediate phase, found previously at about 120°C between the hexagonal and the lamellar phase was shown to be an isotropic liquid rather than a cubic mesophase. Combined X-ray diffraction and N. M. R. experiments on magnetically aligned samples showed that the lamellar and hexagonal phases have respectively negative and positive anisotropic magnetic susceptibilities. Samples which are allowed to equilibrate in the isotropic-mesophase boundary region for several hours, in the absence of a magnetic field, undergo surface-induced alignment. The lamellar phase aligns homeotropically while the hexagonal phase aligns homogeneously, but with the directors perpendicular to the capillary axis and with the hexagonal lattice vectors inclined at a constant orientation to the surface. Deuterium N. M. R. measurements on perdeuteriated disoap samples indicate that the amphiphile molecules in both mesophases are highly disordered, partly due to isomerization equilibria and partly due to orientational disorder. Based on the X-ray and N. M. R. results molecular models for both the lamellar and hexagonal phases are proposed. These models resemble very closely the corresponding mesophases of the monosoaps.

ISSN:0267-8292    

DOI:10.1080/02678299008033810

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A nearly symmetric alkoxyphenylbenzoate monomer is found to exhibit a transition from perpendicular to tilted alignment at a lecithin-treated glass substrate several degrees below the nematic-isotropic phase transition. By means of the Freedericksz transition the coefficientBof the anchoring energy has been obtained. It is found thatBrapidly decreases, and the tilt susceptibility increases on approaching this transition.

ISSN:0267-8292    

DOI:10.1080/02678299008033811

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Dielectric data of a polymer siloxane derivative have been analysed. The deviation from the Debye behaviour has been interpreted using the many body, cooperative model. Comparison with the results for liquid crystals composed of elongated molecules has been made from the point of view of short and long range correlations.

ISSN:0267-8292    

DOI:10.1080/02678299008033812

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The nematic phases of cyano derivatives of different chemical classes, in particular pyridine derivatives, and their mixtures with weakly polar matrices, 4,4′-butylmethoxyazobenzene and 4-transbutyl cyclohexanecarboxylic acid, have been investigated by X-ray diffraction over the entire concentration range. Values have been obtained for the fluctuation layer structure period,d, and the longitudinal correlation length, ζ, for two incommensurate density waves corresponding to monomers and dimers of the cyano derivatives. Specific features of the concentration behaviour ofd1andd2depending on the structure of the cyano derivative and the weakly polar matrix have been analysed. A linear correlation between the elastic constant ratioK3/K1andd22for cyano derivatives has been found, as well as an anticorrelation betweenK3/K1, the viscoelastic ratio,ns/K1, and a correlation length, ζ1for the mixtures. In the latter case a similar concentration behaviour ofnB/K3and ζ2andd2has been established.

ISSN:0267-8292    

DOI:10.1080/02678299008033813

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The liquid-crystalline properties and structural characteristics of copoly(dimethyl-methylhydrogeno)siloxanes substituted with different proportions of long mesogenic cyano groups are investigated. Comparison is made with the phase behaviour of the chemically related low molecular weight molecules. Some of the unusual features of the polymorphism of these polar compounds are preserved. The compatibility of these copolymers with their low molar mass parents is tested through the study of binary phase diagrams.

ISSN:0267-8292    

DOI:10.1080/02678299008033814

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We study the influence of pressure on the stability, ordering and thermo-dynamic properties of the discotic nematic-isotropic transition. In the present calculation we include both the second and fourth Legendre polynomial order parameters in the evaluation of the angle-dependent term in the expression of the excluded volume and the perturbation term.

ISSN:0267-8292    

DOI:10.1080/02678299008033815

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

As part of a programme investigating mesogenic diacetylenes a symmetrically disubstituted diacetylene has been synthesized and polymerized. Thermal, thermo-optical and IR studies have been carried out to investigate the different crystalline forms of the diacetylene.

ISSN:0267-8292    

DOI:10.1080/02678299008033816

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In order to further understand the structure of the blue phase III (BPIII) phase of chiral liquid crystals, we have observed the effect of electric fields on the reflectivity of BPIII phases in which the liquid crystal hasnegativedielectric anisotropy. The reflectivity increases dramatically with increasing field, in contrast to the behaviour for materials ofpositivedielectric anisotropy. We compare the data with cubic domain and double-twist models for BPIII structure and we find that the data best agree with the double-twist model and possibly with an O5cubic domain model.

ISSN:0267-8292    

DOI:10.1080/02678299008033817

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor