摘要:

This paper discusses some non-linear problems for smectic C liquid crystals based on the continuum theory proposed by Leslie et al. New restrictions on the nine elastic constants are also derived. Attention is restricted to samples involving concentric cylindrical layers in which both the layer thickness and the tilt angle are assumed to be constant. Non-linear solutions are presented for a sample contained in a wedge with an electric field applied across the bounding plates, extending earlier work by Carlsson et al., and for a sample between two coaxial concentric circular cylinders to which an azimuthal magnetic field is applied. Freedericksz thresholds, which may lead to the experimental determination of some of the elastic constants, are deduced. In the absence of an applied field it is found that, under suitable restrictions on the elastic constants, there is a critical wedge angle (or critical radius ratio in the concentric cylinder case) above which a variable non-linear symmetric solution satisfying the zero boundary conditions is energetically more favourable than the zero solution.

ISSN:0267-8292    

DOI:10.1080/026782997208992

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer (100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.

ISSN:0267-8292    

DOI:10.1080/026782997209009

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An idea for a method of experimental determination of tilt angles and order parameters of cylindrosymmetric transition moments of liquid crystals and of dyes in host-guest systems is described. Furthermore, first experimental hints are given. With freely rotating molecules, all transition moments result in an isotropic absorbance for UV/VIS-spectroscopy using polarized light. The resulting absorbance becomes anisotropic if the transition moment of the molecules is polarized, the molecules are aligned in a preferential direction, and the free rotation is suppressed. In this case, the measurement of Aiso, Atilt, and Atilt using polarized light is sufficient to calculate the related tilt angle Psi, the order parameter S, and the absorbance A as a function of the polarization angle. The local electric field and its influence on the dichroic ratio of an absorption band has not been considered. The precision of this method is still limited by the photometric accuracy of the spectrometer and the mechanical precision of the goniometric adjustment of the experimental apparatus.

ISSN:0267-8292    

DOI:10.1080/026782997209018

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Diverse 4-substituted-benzyl 1-O-alpha- and beta-D-glucopyranosides, 4-substituted-benzyl 2-deoxy2-trifluoroacetamido-alpha- and beta-D-glucopyranosides and 4-substituted-benzyl 2-acetamido2-deoxy-alpha- and beta-D-glucopyranosides have been synthesized. They were prepared directly by a one-step alkylation of commercially available carbohydrate starting materials. The 4-substituted-benzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides were cleaved to yield the corresponding free amines. The influence of hydrogen bonding, terminal dipole moments, molecular shape and chain length on the thermotropic mesomorphic behaviour of these benzyl-substituted carbohydrate amphiphiles is discussed. Comparisons between the mesomorphic behaviour of the new benzyl-substituted carbohydrates and that of the related phenyl-substituted carbohydrates, as well as the non-aromatic n-alkyl D-glucosides, are made. Although the new carbohydrates do not exhibit lyotropic phase behaviour, the 4-substitutedbenzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides have been found to gel water at very low sugar concentrations.

ISSN:0267-8292    

DOI:10.1080/026782997209027

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

It is shown that trifluoroacetic acid (TFA) can form 1 1 molar complexes with alkyloxy discotic compounds, thereby enhancing their mesomorphic properties. In particular, when hexaalkyloxytriphenylenes (THEn) (n being the number of carbons per side chain) are crystallized from, or mixed with, TFA, complexes are formed which exhibit the Dho(Colho) mesophase, as do the neat THEn's, but with a broader range of thermal stability and a higher degree of order. The mesophases of these complexes are completely miscible with those of the neat compounds. Likewise, 1 1 mixtures of hexa-alkyloxytribenzocyclononene (TBCEn) and TFA yield complexes showing enhanced mesomorphic properties for the mesogenic homologues of this series and induced mesomorphism for the lower homologues, which in the neat form are not mesogenic. The mesomorphic properties of these TFA complexes with THEn (n=3-6) and with TBCEn (n=4-6) are investigated by means of optical microscopy, differential scanning calorimetry, X-ray diffraction and NMR spectroscopy. It is suggested that the enhancement of the mesomorphic properties is due to formation of oxonium ion complexes.

ISSN:0267-8292    

DOI:10.1080/026782997209036

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The synthesis, mesophase morphology and transition temperatures of a series of 1,2,4,5-tetrasubstituted benzenes are described. The compounds have (4-tridecyloxyphenyl)carbonyloxy or (4-tridecyloxyphenyl)ethynyl as the peripheral substituents in all possible combinations ranging from the tetra-ester to the tetra-alkyne. Both the substituents used have similar structural dimensions but the greater rigidity of the alkynyl unit ensures that it projects more directly from the central core than does the more flexible ester unit. All of the seven compounds are nematogenic and five show a smectic C phase; the differences observed in the transition temperatures of the compounds are discussed in terms of molecular shape and the relationship of the compounds to appropriate calamitic and discotic systems. Molecular modelling indicates that pi-pi interactions of peripheral phenyl groups can lead to molecules with calamitic shapes.

ISSN:0267-8292    

DOI:10.1080/026782997209045

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The synthesis and mesogenic properties of a new class of liquid crystalline metal-free and metalated phthalocyanines with different numbers of alkylthio side chains are presented. The effect of the substitution pattern in a series of differently octasubstituted phthalocyanines on the width and order of the thermotropic mesophase is investigated. The exchange of the alkylthio groups from chlorine atoms alters the mesophase width and the melting behaviour. Glassy phthalocyanines, without any tendency to crystallize, can be synthesized in this way. Furthermore, mixtures of differently 4,5-disubstituted benzenedicarbonitriles are tetramerized statistically to the corresponding mixtures of substituted phthalocyanines. The effect of decreasing symmetry at the molecular level on the properties of the liquid crystalline materials has been investigated. Highly oriented columnar systems can be obtained with these mixtures. For all compounds containing less than eight alkylthio groups, lyotropic mesomorphism in hexadecane solution is also found.

ISSN:0267-8292    

DOI:10.1080/026782997209054

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A lyotropic hexagonal mesophase is identified in a mixture of a low molecular mass nematic solvent and an AB block copolymer with a liquid crystalline block A and an isotropic block B by polarizing microscopy. A highly ordered striated texture evolves spontaneously that exhibits an additional striated fine structure within the stripe domains.

ISSN:0267-8292    

DOI:10.1080/026782997209063

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Photopyroelectric measurements of heat transport parallel or perpendicular to the director have been carried out for compounds of the 4,4-dialkylazoxybenzene homologous series. A theoretical expression based on molecular length-width ratios predicts a thermal conductivity anisotropy ratio which is between 1.5 and 2 times larger than experimentally observed. However, the predicted order parameter temperature dependence seems to be consistent with the experimental data.

ISSN:0267-8292    

DOI:10.1080/026782997209072

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A successful approach in molecular design through fusing aromatic/heterocyclic rings for broadening the smectic C (SmC) phase and the synthesis of the first heterocyclic liquid crystalline (LC) molecules containing 3,7-disubstituted quinoline with one of the widest SmC phases are reported. Their naphthalene analogues are compared. The nitrogen atom in quinoline system introduces attractive forces to aid the formation of the tilted SmC phase and the flexible chain length also influences the tilted SmC phase effectively.

ISSN:0267-8292    

DOI:10.1080/026782997209081

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor