摘要:

The phase behaviour of three alkyl cyanoterphenyls has been investigated with the particular objective of elucidating their unusual phase behaviour and determining the structure of the new, and so far uncharacterized, phase (E′) appearing between the smectic Adand the crystal E′ phases. In the course of this work complications arose from the appearance of yet another phase which appears only on cooling but for which no characteristic X-ray diffraction pattern could be obtained. The E phase appears to have the orthorhombic symmetry characteristic of the E phase but has a complex temperature dependent packing in thecdirection. This is driven by the competition between a density wave and an antiferroelectric pair ordering which results in a locked-in structure with a periodicity of three times that of the underlying density wave.

ISSN:0267-8292    

DOI:10.1080/02678299008047326

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Aqueous solutions of decylammonium chloride and decylammonium chloride/ammonium chloride between 20 and 50 per cent of DACl have been studied by Raman scattering. The spectral behaviour was followed as a function of temperature and phase transitions were observed for all samples. We have focused our study on the intensity variations of both the symmetric and the antisymmetric stretching vibration modes of CH2groups situated in the region 2800–3000cm−1. The study of the high frequency part of the spectrum showed the characteristics of order disorder transitions already studied for other materials. We have observed that the existence of a nematic phase in the ternary systems is caused by the addition of the salt.

ISSN:0267-8292    

DOI:10.1080/02678299008047327

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The behaviour of α,ω-diacid and α-monoacid guest molecules (with carbon atoms ranging between 6 to 22) in the lamellar and hexagonal mesophases of dipotassium thapsate (KOOC(CH2)14MCOOK)-water mixtures have been investigated, using deuterium NMR spectroscopy. Addition of short diacids to either the lamellar or hexagonal phases results in phase separation and essentially no incorporation into the mesophase structure. The deuterium spectra of the guest molecules exhibit characteristic patterns which are qualitatively interpreted in terms of their ordering properties. In no case were separate signals observed due to folded diacid guest molecules which have their carboxyl groups on the same side of the interface.

ISSN:0267-8292    

DOI:10.1080/02678299008047328

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The hole lattice model of rectangular parallelepipeds is presented to describe the structural and excess thermodynamic properties of nematic-non-mesogenic mixtures. The molecular attractions are taken into account within the quasi-chemical approximation. A procedure for evaluating model parameters from data on the thermodynamic characteristics of pure components and the activity coefficients of the non-mesogen at infinite dilution at the nematic-isotropic transition temperature of the mesomorphic component is proposed. The mixing functions (enthalpy and volume), activity coefficient of the non-mesogen and the order parameters of the components are calculated at a molecular level for systems composed of 4-methoxybenzylidene-4′-propylaniline and a non-mesogen (tetrachlormethane, benzene andn-heptane). The calculated results are in quite good agreement with experiment in the temperature rang from 319·2 to 335·4 K.

ISSN:0267-8292    

DOI:10.1080/02678299008047329

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The orientational ordering of a series of 4-alkenyl-substituted bicyclohexyl liquid crystals was studied by natural abundance13C NMR spectroscopy. A combination of the methods of variable angle spinning (VAS) and separated local field spectroscopy (SLF) was used. Rapid sample spinning about an axis forming an angle of about 65° with respect to the magnetic field forces the nematic director to align parallel to the spinnin axis, leading to narrow peaks in the13C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled13C signals in the ω2dimension and first-order C-H splitting patterns in the ω1dimension, from which the C-H dipolar coupling constants for individual pairs of nuclei can be obtained. Then, the order parameters for different molecular segments can be calculated. The method was applied to five different 4-alkenyl-substituted cyanobicyclohexanes. For the compounds studied, the dependence of the ring order parameters on the alkenyl chains shows the same trend as the melting and clearing temperatures and the elastic constants.

ISSN:0267-8292    

DOI:10.1080/02678299008047330

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Biforked or polycatenar mesogens derived from 3-(3,4-dialkoxyphenyl)-propanoic acid and with ester bridges between all of the aromatic rings are described. These derivatives exhibit both lamellar, columnar and cubic mesophases. One example of reversed phase order is found (when the sample is cooled from the isotropic liquid as compared to when it is heated from the crystal) which is demonstrated by DSC measurements and microscopy. The relationships between molecular structure and polymorphism are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299008047331

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A nuclear spin lattice relaxation study of the liquid crystal butoxybenzylidene hexylaniline (40.6) has been carried out as a function of Larmor frequency (4 to 30 MHz) and temperature covering all its mesophases (N, SAand SB), with a view to obtain information on different dynamic processes at the molecular level. These studies are also supplemented by a nuclear dipolar relaxation study as well as anisotropy of these relaxation rates with respect to the static magnetic field. The analysis of this data in nematic and smectic A phases based on a composite model, including the contributions from self diffusion (SD), order director fluctuations (ODF), and molecular reorientations (R), indicate that the nuclear spin relaxation is effectively mediated by diffusion and reorientation processes at frequencies above 10 MHz while at lower frequencies ODF also seem to contribute appreciably. The relative contribution from these mechanisms at different temperatures covering nematic and SAphases are evaluated, and relevant dynamic parameters are calculated. The onset of the SBphase is marked by sudden increase inT1and subsequent frequency independence of this data at lower temperatures. The dipolar relaxation data show a dramatic decrease at this transition temperature. These observations are attributed to a sudden slowing down of diffusion in the SBphase.

ISSN:0267-8292    

DOI:10.1080/02678299008047332

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Low molecular mass compounds and analogous side group polymers with a thermotropic phase behaviour crystalline-smectic liquid-crystalline-isotropic have been synthesized for a comparative structural study. As characteristic features of the compounds a biphenyl group has been chosen for the mesogenic core and alkoxy parts of various lengths as terminal and spacer groups. The phase behaviour has been studied with differential calorimetric (DSC) measurements and polarization microscopic observations. The low molecular mass compounds form crystalline phases at room temperature and exhibit predominantly mosaic textures in the polarization microscope at elevated temperatures indicating high order of the packing of the molecules. The arrangements of the molecules in the crystalline and liquid-crystalline phase can best be described as layered structures according to X-ray diffraction measurements. A structural analysis of a solution grown single crystal provides valuable information on the conformation and packing of the compounds investigated in this study.

ISSN:0267-8292    

DOI:10.1080/02678299008047333

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A short review of the scope and limitations of liquid-crystalline behaviour in carbohydrate derivatives is presented. In order to investigate the influence of structure variations on the thermal behaviour of monosaccharide amphiphiles, six homologous series of D-glucose and 2-deoxy-D-glucose derivatives with an aminolinkedn-alkyl chain were prepared. The observed thermal behaviour could be readily explained with the aid of a refinement of the qualitative model that was presented earlier [1]. All compounds were found to be mesogenic, and the observed mesophase was smectic Ad(i.e. a partially overlapping bilayer structure) in all cases.

ISSN:0267-8292    

DOI:10.1080/02678299008047334

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We show the existence of point defects connected by strings in columnar phases. The most interesting consequence is the increase, by several orders of magnitudes, of the bend elastic modulus as compared to values for nematics.

ISSN:0267-8292    

DOI:10.1080/02678299008047335

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor