摘要:

The reorientational behaviour of a nematic monodomain, in a static magnetic field, following the rotation of the mean director through an angle α ≈ π/2 to the magnetic field direction, is investigated by optical polarizing microscopy and proton NMR spectroscopy. Optical evidence supporting a reorientation process through a pure bend transient distortion of the director field is reported for poly (4,4′-dioxy-2,2′-dimethylazoxybenzene nonanediyl), a main-chain nematic thermotropic polymer. The time evolution of the distortion and flow patterns are deduced from a detailed analysis of the NMR lineshapes recorded during the reorientational process. The viscosity coefficients γ1, α1, α2, ηcand the ratioK3/K1of the bend to splay Frank elastic constants are determined and discussed in terms of the existing theories.

ISSN:0267-8292    

DOI:10.1080/02678299008036746

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The thermal properties of low molecular mass and polymeric liquid crystals of similar chemical structure have been investigated in order to see if a parallel structure-properties relationship exists between the two groups of compounds. Two series of models and of the corresponding polymers were prepared, bearing the same mesogenic moiety obtained by reacting 4-hydroxybenzoic acid with different methylated hydroquinones. Ether or ester linkages have been used to bridge the rigid core to the flexible polymethylene spacer. The thermal stability and the nature of the mesophases were examined by different techniques. An interpretation of the results on the basis of the axial ratio and the strength of the orientation-dependent attractions, is attempted for the model compounds. As far as polymers are concerned thermodynamic parameters follow the expected trend if compared with those of low molecular mass analogues. The dependence of the liquid crystal to isotropic transition temperature on the degree of substitution shows the same trend as for the model compounds. Geometric and electronic effects must be taken into account to understand the experimental results.

ISSN:0267-8292    

DOI:10.1080/02678299008036747

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A number of photochromic spiropyrans and spirooxazines containing mesogenic groups were synthesized. Only one of them, spiro-indoline-naphthoxazine with mesogenic substituent, 4-(4-heptylbenzoyloxy)benzoyloxy in 5-position, revealed mesomorphic properties. Its phase behaviour, photochromism and alignment in the electric field was investigated. A structure of the mesophase is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299008036748

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Optical microscopy, X-ray diffraction and NMR spectroscopy have been used to examine whether the hexagonal phases of representative chromonic and amphiphilic mesogens are miscible. The systems studied were octaoxyethylene-glycol dodecylether with either disodium cromoglycate or 5-n-hexyl-7-(5-methyl-sulphonimidoyl) xanthone-2-carboxylic acid. The results clearly demonstrate that the hexagonal phases of these two systems are not miscible, although miscibility does occur in the isotropic solution. These observations suggest that chromonic mesophases are a new breed of lyotropic liquid crystals.

ISSN:0267-8292    

DOI:10.1080/02678299008036749

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Sixteen new side chain liquid polysiloxanes, either homopolymers derived from poly(hydrogenmethyl)siloxane or copolymers derived from poly(hydrogenmethyl-dimethylsiloxane), are reported with side chains carrying fluoro-substituents. The known effects of fluoro-substitution in low molar mass liquid crystals are strongly echoed in the polymeric analogues and interesting comparisons are made between homo- and co-polymers carrying the same fluorinated side chains.

ISSN:0267-8292    

DOI:10.1080/02678299008036750

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

On the basis of the solution of the random flight problem of a particle between two perfectly reflecting potential barriers the incoherent scattering law for the model of the uniaxial, molecular stochastic reorientation within an arbitrary apex angle Ф0has been calculated. In conjunction with the published NMR results evidence is presented, that the along the long axis uniaxial rotation, can be described in terms of the model with an apex angle φ0of 57°. The model of the restricted uniaxial reorientation, which is expected to be of interest in nematic phases of biaxial molecules, has been applied to the investigation, by quasi elastic incoherent cold neutron scattering, of the rotational dynamics of the rigid benzylidene central part of a magnetically oriented MBBA sample in the nematic phase at 303 K.

ISSN:0267-8292    

DOI:10.1080/02678299008036751

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Neutron scattering from a mixture of normal hydrogeneous and perdeuteriated 4-n-pentyloxybenzylidene-4′-n-heptyl quiline (50.7) has been shown to contain a component which depends only on the structure and orientational distribution of the molecules. It has been used to determine the orientational order parametersPnfor n = 2, 4 or 6 in the smectic and nematic phases of 50.7. These values were much less than predicted by the Maier-Saupe molecular field theory, but in qualitative agreement with a model based on biaxial intermolecular interactions. Comparison with theP2values obtained by analysing the smearing into arcs of intermolecular interfence features, suggested that the correlation of orientation of neighbouring molecules is not very high so that the method tends to overestimate the order parameters. Other possible applications of the technique of neutron scattering from isotopically labelled mixtures are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299008036752

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Structural investigations on six combined liquid-crystalline polymers using the X-ray technique are described. The measurements have allowed a characterization of the liquid-crystalline phases and have led to our first ideas about the phase structures. All mesogenic groups are arranged parallel to each other, the shortest segment in the main-chain or in the side-chain determines the smectic layer spacing. A modified designation for liquid-crystalline phases of high polymers is proposed.

ISSN:0267-8292    

DOI:10.1080/02678299008036753

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The NMR spectrum of the protons in the biphenyl group of 4-n-pentyl-4′-cyanobiphenyl (5CB), was obtained by Sinton et al. (1984, Molec, Phys., 53, 333) and analysed by them to give an angle of twist, φmin, between the normals to the two phenyl rings of 30 ± 2°. Their analysis made the assumption that V(φ), the potential for rotation about the inter ring bond, is such that only the structures with the minimum energy need be considered when calculating averaged dipolar couplings. Re-analysis of their data by a method which allows for the whole of V(φ) to be sampled when averaging the dipolar couplings yields a value for φminof 38.4 ± 0.1°.

ISSN:0267-8292    

DOI:10.1080/02678299008036754

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The variation of the nuclear shielding of the129Xe isotope in natural xenon dissolved in various liquid crystals and liquid crystal mixtures has been studied over the temperature range from 300 to 360 K. The temperature dependence is linear in the isotropic phase of the liquid crystals. An abrupt change in the shielding is observed when passing through the nematic-isotropic and smectic A-nematic phase transitions as well as when the liquid crystal director rotates by 90° in the so-called critical mixture of ZLI 1167 and EBBA. This is interpreted as being mainly the consequence of the shielding anisotropy of xenon arising from the deformation of its electronic distribution. The shift changes observed for 4,4′-di-n-heptylazoxybenzene at the nematic-isotropic phase transition on the one hand and at the smectic A-nematic phase transition on the other are found to be opposite in sign, reflecting the change in the liquid crystal structure.

ISSN:0267-8292    

DOI:10.1080/02678299008036755

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor