摘要:

A novel zero electric field gradient nematic liquid crystal made up from two nematic liquid crystal components is employed as a solvent for a series of molecules ranging from small molecules to mesogens themselves. Nuclear magnetic resonance is used to determine the degree of order of the solute and solvent molecules. Results are compared to those obtained for two completely different zero electric field gradient nematic mixtures. The comparison strongly indicates that for a variety of molecules largely differing in size, shape and flexibility their degree of order can be described by a single orientation mechanism. This mechanism can be adequately modelled by a simple phenomenological mean field model based on the size and shape anisotropy of the dissolved species. The use of zero electric field gradient mixtures in combination with this mean field model allows the prediction of solute order parameters at approximately the 10 per cent level.

ISSN:0267-8292    

DOI:10.1080/02678299308026307

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The liquid crystalline behaviour and nematic viscosities of a series of laterally fluorinated alkyl-oxoalkyl terphenyls are reported. The mesomorphic behaviour and nematic viscosity can be influenced by the position of the oxygen atom in the oxoalkyl chain as well as the position of lateral fluorination. Compounds have been synthesized that are suitable for use in ferroelectric smectic C (S*C) mixtures; the switching properties of a four component mixture are also reported.

ISSN:0267-8292    

DOI:10.1080/02678299308026308

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An X-ray study is performed of oriented smectic mesophases of several polyesters consisting of twop-oxybenzoyl diads interconnected by a pentamethylene segment and a polymethylene segment of variable length. The X-ray diffraction patterns are very similar for all the samples and are constituted by one or more inner reflections, lying on the meridional line, and by two broad outer haloes split into two intense portions symmetrically placed above and below the equatorial line. The former reflections are attributed to the regular piling of the smectic layers; the latter are associated with the presence of rigid mesogenic groups that are tilted with respect to the layer normal, while the average chain axis lies perpendicular to the layer planes. Accordingly, the analysed smectic mesophases can be classified as unconventional smectic C. A structural model is proposed to account for the intensity profile of the X-ray patterns.

ISSN:0267-8292    

DOI:10.1080/02678299308026309

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The frozen nematic state of a thermotropic aromatic copolyester exhibits a schlieren texture with two and four dark brushes emanating from a point observed under a polarizing optical microscope; these correspond to disclination strengthss= ± ½ ands= ± 1, respectively. The combined techniques of lamellar decoration and ruthenium tetraoxide staining have been used to reveal the director orientation around disclinations in the frozen nematic state of polymers by transmission electron microscopy. All six disclinations in the nematic mesophase of a single liquid-crystalline aromatic copolyester have been observed. Every disclination interacts and is connected with its neighbouring disclination so that disclination exist in pairs usually of opposite sign in their disclination strengths. Some pairs with the same sign disclination have also been observed.

ISSN:0267-8292    

DOI:10.1080/02678299308026310

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new class of swallow-tailed mesogen with the longitudinal dipole moment in the direction of the non-branched ‘head’ is presented. Mixtures of these compounds with ‘conventional’ swallow-tailed substances were investigated by polarization microscopy and dielectric measurements. In the mixtures, dipolar and steric interactions favour opposite arrangements of the molecules in the short range order.

ISSN:0267-8292    

DOI:10.1080/02678299308026311

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The synthesis of a number of 1,1′-disubstituted ferrocene-containing compounds is described; the structures of the compounds containing the ferrocene unit are related to those of mesogenic systems containing a 1,4-disubstituted benzene ring or atrans-1,4-cyclohexyl ring. Direct comparisons are made between the aryl-, cyclohexyl- and ferrocene-containing compounds in order to assess the effect on transition temperatures of replacing a benzene or cyclohexyl ring with a ferrocene unit. In comparison with conventional ring systems the ferrocene unit causes substantial reductions in clearing temperatures.

ISSN:0267-8292    

DOI:10.1080/02678299308026312

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (10−1Hz-109Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below 106Hz, two ferroelectric modes, Goldstone- and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between 106and 109Hz, one dielectric loss process exists, theβ-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for theβ-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free energy at the non-ferroelectric-ferroelectric phase transition. The experiment also leads to a modified understanding for the molecular origin of ferroelectricity in FLCs.

ISSN:0267-8292    

DOI:10.1080/02678299308026313

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The nucleation of bubble domains in homeotropic samples of a dielectrically positive cholesteric liquid crystal is described. These domains are found to be more stable in an electric field than the rectilinear double-twisted fingers. The electric field-induced transformation of a looped finger into a bubble domain is described in detail: it is discontinuous and irreversible, and operates only at a large enough confinement ratioC=d/p, wheredis the sample thickness andpthe quiescent cholesteric pitch. Finally, in contrast with Stieb's model [4], we propose that there are two point defects along the bubble axis and not a disclination line.

ISSN:0267-8292    

DOI:10.1080/02678299308026314

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Frequency and temperature dependent NMR relaxation measurements were performed on deuteriated benzene, pyrene and triphenylene dissolved in the nematic phase of a discotic liquid crystal. The results show a strong frequency dependence of the spectral densities. Based on the symmetries of the system and the usual model for director fluctuations this frequency dependence should be equal forJ1andJ2. From fitting the commonly used model of rotational diffusion and director fluctuations to the data we see that this is not the case for benzene and triphenylene, even though the fits themselves are satisfactory. Values for the elastic constants, effective viscosity and translational diffusion in similar discotic liquid crystals do not account quantitatively for the frequency dependence of benzene. For both pyrene and triphenylene quantitative comparison was impossible due to lack of translational diffusion data. We also find that the so-called cut-off wave-length is of the order of the dimensions of the liquid crystal molecules, just as in ordinary nematics.

ISSN:0267-8292    

DOI:10.1080/02678299308026315

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In examples of two large naphthalene- and triphenylene-centred hexaynes exhibiting only the nematic discotic (ND) type of mesophase, a new kind of phase induction was observed. These disc-shaped nematogens were doped with numerous dipolar carbocyclic compounds leading to the induction of two types of columnar mesophase (i.e. Droand Dho). The phase behaviour of these binary systems, depending on the structure of the dopant applied, was studied by polarizing microscopy.

ISSN:0267-8292    

DOI:10.1080/02678299308026316

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor