摘要:

The polymorphism of three members of the homologous series hexa(p-alkoxyphenoxymethyl) benzene (benzene pizzanes) with 5, 6 and 7 carbons in the alkoxy chains and several of their deuteriated isotopomers have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and deuterium NMR spectroscopy. These homologues exhibit several solid phases and a high temperature M phase, which is isomorphic in the three homologues and whose nature is discussed. In the solid phases, the benzene cores of the molecules remain rigid, but the side chains are mobile, as reflected by rapid pi-flips of the benzene rings in the side chains. It is found that there are two types of such benzene rings, differing in the rates of flips. In the M phase the molecules undergo fast overall reorientation and the side chains are even more disordered than in the solid phases. However the X-ray measurements do not provide a clear cut determination as to whether this phase is crystalline or mesomorphic. Mixing of the benzene pizzanes with p-xylene yields lyomesophases, which appear to belong to the Dho class.

ISSN:0267-8292    

DOI:10.1080/026782997209603

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Existing calculations of the Leslie coefficients for rigid rods endowed with the Maier-Saupe potential are here extended to investigate the consequences of relaxing the constraint of a complete molecular rigidity. In particular, the recently proposed model of the Slightly Bending Rod (SBR) is used to calculate the ratio alpha3/alpha2, the sign of which controls whether the nematic is of the tumbling or flow-aligning type. The results show that SBRs are more prone to tumbling than fully rigid rods. The flow-aligning to tumbling transition, which takes place by increasing the order parameter (e.g. by decreasing the temperature) is shifted, for SBRs, to a smaller value of the order parameter.

ISSN:0267-8292    

DOI:10.1080/026782997209612

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Three optically active oxovanadium(IV)-containing metallomesogens have been prepared and their mesomorphic properties investigated. It is found that enantiotropic chiral phases are stabilized when the molecular system promotes the anti-ferromagnetic coupling of individual molecules into associated pairs. The molecular optical rotation of the non-mesomorphic ligands is greatly enhanced on complexation to an oxovanadium(IV) metal centre to give the mesogenic complex. The mesophase alignment can be retained by cooling to room temperature and this may be utilized in the fabrication of a low cost, multi-session, ‘write-read-erase’ optical storage device with low power consumption.

ISSN:0267-8292    

DOI:10.1080/026782997209621

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The neat phases of a series of quaternary ammonium halides based on trioctadecylamine have been investigated by optical microscopy, differential scanning calorimetry, and H NMR spectroscopy. Phase behaviour depends primarily on the structure of the fourth substituent on nitrogen and the thermal history of the samples. For instance, the iodide salts with methyl, ethyl, or propyl as the fourth substituent form enantiotropic nematic phases, while those with a proton or a longer alkyl substituent exhibit complex phase behaviour that does not include a liquid crystalline phase. Possible reasons are advanced for the unprecedented formation of nematic phases.

ISSN:0267-8292    

DOI:10.1080/026782997209630

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The electro-optical Kerr effect was studied for mixtures of low molar mass liquid crystals within the isotropic phase in the neighbourhood of the nematic phase. The mixtures are characterized by components having similar or antagonistic dipole configurations, as well as different molecular geometries (calamitic and discotic). The investigations reveal that simple mixing rules can be applied for the description of the Kerr response of the mixtures in spite of the fact that the Kerr response displays a set of unusual features, among them a temperature induced sign inversion and synergistic effects.

ISSN:0267-8292    

DOI:10.1080/026782997209649

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A study has been undertaken of the structure-activity relationship of eight new chiral compounds having either a pyrazole (series P) or an isoxazole (series I) ring as a central bridge in the mesogenic core. The presence of dimers in the pyrazole compounds accounts for their lower P s values in comparison with the isoxazole analogues. The corresponding four beta-diketone precursors have also been studied and these, as expected given their bent molecular shape, show much worse mesomorphic and ferroelectric behaviour. In order to complete the study, the molecular dipoles of the three types of derivative have been determined using AM1 calculations. Two types of chiral tail have been incorporated into the compounds: alkoxy and alkanoyloxy. The latter tail gives rise to the best mesomorphic and ferroelectric properties. A study of the tail conformations by MM2 calculations provides an explanation of these results. The highest P s value (137 nC cm) has been obtained for the isoxazole derivative with the (2S)-2-butyloxypropanoyloxy chiral tail (compound I4). The potential of all twelve compounds as chiral dopants for FLC mixtures has been evaluated by a study of 10 mol% binary mixtures in a standard host system.

ISSN:0267-8292    

DOI:10.1080/026782997209658

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Some novel disaccharide-derived amphiphiles, in which the hydrophobic long chain alkyl group and an alpha -D-glucose moiety are separated by a hydrophilic five carbon spacer or a hydrophobic furan residue, were investigated as to their mesomorphic properties using DSC, thermomicroscopy and various X-ray methods. All compounds exhibit a smectic A phase, characterized by aggregation to a bilayered S structure. The layer thickness d, measured by A X-ray, correlates with the presence of polar d and non-polar moieties, adjoining molecules interacting via hydrogen bonding. Based on the known structural concepts for sugar-derived liquid crystals, a model is suggested to account for the observed S mesomorphic behaviour of two N -alkanoyl derivatives of (glucosylox dymethylfuryl) methylamine, i.e. species in which sugar and long chain fatty acid are separated by a quasi-aromatic furan residue, is unusual, as different X-ray methods invariably give the same odd indicative of a smectic layer thickness of three to four molecules. layer arrangement. The A d -value indicative of a smetic layer thickness of three to four molecules.

ISSN:0267-8292    

DOI:10.1080/026782997209667

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Phase diagrams of binary mixtures of the non-mesogenic N -(p -methoxy- o -hydroxybenzylidbe ene)- p -aminopyridine with a series of p - n -alkoxybenzoic acids ranging from methoxy to hexadecyloxy were established using differential scanning calorimetry and polarising optical microscopy. The key results obtained are: (1) the formation of 1 1 hydrogen bonded complexes between the pyridine derivative and the alkoxybenzoic acids, (2) the stability of the alkoxybenzoic acid mesophases over a wide range of compositions (up to slightly over 50 mol% of the pyridine derivative), (3) the absence of additional mesophases corresponding specifically to the 1 1 complexes, and (4) the complete miscibility of the acids with the complexes in the mesomorphic state. With alkoxy chains from methoxy to heptyloxy, mixtures produce only nematic phases; they produce both nematic and smectic phases with chains from octyloxy to dodecyloxy, and only smectic phases with chains from tetradecyloxy to hexadecyloxy. The formation of hydrogen bonded complexes was investigated at various temperatures using FTIR spectroscopy. Molecular ordering was studied by X-ray diffraction as a function of temperature and composition both for the crystalline and the mesomorphic states.

ISSN:0267-8292    

DOI:10.1080/026782997209676

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Measurements of the proton cross-relaxation rate in the polymer dispersed 4-pentyl4-cyanobiphenyl (5CB) droplets were performed by the selective magnetization inversion NMR technique. This method makes possible direct determination of the magnitude of the fast cross-relaxation rate in theMHz regime where the standard spin-lattice relaxation measurements only indicate its presence. In the isotropic phase, the cross-relaxation rate is found to be more than ten times larger in the droplets of pure 5CB than that reported earlier for the liquid crystal mixture E7 in the same polymer. The difference is discussed in terms of dynamic processes in the surface layer.

ISSN:0267-8292    

DOI:10.1080/026782997209685

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Broadband dielectric spectroscopy (102-105Hz) is used to study the opto-dielectric effect in statistical polysiloxane copolymers with side groups containing chiral mesogenic units and others containing azobenzene moieties. Below 64 C, the unexposed copolymer is ferroelectric. UV-exposure induces a photoisomerization of the azobenzene to the cis -configuration, and the phase transition, ferroelectric/non-ferroelectric (SC*/SA), is shifted to lower temperatures. In the temperature interval between the phase transition temperatures ferroelectric/nonferroelectric of the exposed and unexposed samples, an opto-dielectric effect is observed. The efficiency of this opto-dielectric switching depends on the packing of the chromophore within the smectic layers.

ISSN:0267-8292    

DOI:10.1080/026782997209694

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor