摘要:

3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA) was investigated as the central core unit in mesogenic molecules. Of particular interest was whether the BPDA unit could be self-constrained into atrans-conformation, and thus organize into a liquid crystalline phase. Two homologous series of symmetrical substituted model compounds, 3,3′,4,4′-biphenyltetracarboxy-N-N'-bis-(4-n-alkylphenyl) diimides and 3,3′,4,4′-biphenyltetracarboxy-N-N'-bis-(4-n-alkoxyphenyl)diimides were synthesized. For both series high temperature smectic A phases were observed. To our knowledge these are the first examples of liquid crystalline behaviour observed with BPDA as the mesogenic core.

ISSN:0267-8292    

DOI:10.1080/02678299508036621

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A series of polymethacrylates containing laterally methyl-substituted 4-alkoxy-4-nitrotolane, 4-alkoxy-4′-cyanotolane, and 4-alkoxy-4′-trifluoromethyltolane as mesogenic side groups are presented. The thermal behaviours of the prepared compounds were characterized by differential scanning calorimetry, optical polarized microscopy and X-ray diffraction. All of the obtained methacrylate monomers exhibit no mesophase, while most of the synthesized polymers reveal enantiotropic mesomorphism. The polymer containing 4-propyloxy-2′-methyl-4′-nitrotolane side groups was the only one to show no mesomorphic behaviour. The spacer length and the nature of terminal group have a profound influence on the type of mesophase formed and the thermal stability. Incorporation of a lateral methyl-substituent into the mesogenic groups reduces the thermal stabilities of the mesophases. X-ray diffraction reveals that polymers with a smectic A or smectic E phase have the inclination to form a bilayer structure. Increasing the temperature within the smectic phase range, the intermolecular spacing increases while the layer spacing decreases. On the other hand, introducing a lateral substituent into the mesogenic side groups leads to increase in both layer and intermolecular spacing.

ISSN:0267-8292    

DOI:10.1080/02678299508036622

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

About 50 different additives in one or more of three different nematic mixtures have been investigated to clarify the relationship between the chemical structure of the liquid crystal and the pretilt angle on a polyimide surface. The pretilts found for cells have been explained within our recently proposed population distribution model. For compounds with cyano-groups at one end, we find that the in-plane order is governed both by the surface-mesogen interaction and by the relative strength of the intermolecular interactions in the nematic phase. This strength is nearly linear in alkyl chain length for the compounds investigated. Changes in the strength due to variations in the core of the molecules can be calculated easily by using group contributions from the known Parachors. The in-plane order can be treated as a simple product of the contributions from the liquid crystal and from the polyimide. Different polar end groups will give different angles between the surface and the optical axis of the individual mesogens in the first monolayer. The cyano-group gives the highest angle and alkyl groups the lowest. For nitro-compounds the dimers formed are so strongly bound that they do not break up at the surface. Nitro-compounds will thus act as dialkyl compounds. For dialkyl compounds the pretilt angles are dominated by the difference between the chain lengths at the two ends of the molecule.

ISSN:0267-8292    

DOI:10.1080/02678299508036623

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

For the first time we have observed a surface memory effect in liquid crystal cells where both surfaces consist of rubbed polymers. In this case the azimuthal orientation of the optical axis in the cell is unchanged, while the polar orientation of the optical axis in the cell changes. This change is brought about by imposing changes in the in-plane order of the molecules at the surface.

ISSN:0267-8292    

DOI:10.1080/02678299508036624

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Four series of ferroelectric liquid crystals containing oligooxyethylene spacers have been synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC) and optical polarized microscopy (POM). The properties of the liquid crystalline phase were investigated as a function of spacer units, numbers of core aromatic rings, and different terminal asymmetric moieties. It was found that (i) the phase transition temperature decreased with the increasing oligooxyethylene spacer unit, (ii) the liquid crystalline phases were enhanced in three phenyl ring system than in two phenyl ring system, and (iii) ferroelectric liquid crystals containing different terminal asymmetric moieties exhibited novel mesophase phenomena. A twist grain boundary phase (TGBAphase) was observed in some compounds of this study. Furthermore, a wide temperature chiral smectic Crange including room temperature was achieved.

ISSN:0267-8292    

DOI:10.1080/02678299508036625

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The crystal and molecular structures of (R)-4′-(2,3-dihydroxypropyloxy)-4-cyanobiphenyl,1and 4′-(cis-cis-3,5-dihydroxycyclohexyloxy)-4-cyanobiphenyl,5have been determined. The packing of compound1occurs in sheets, with a dense hydrogen bonding network within the layer of one sheet and to the next sheet, whereby the cyano groups are incorporated into the hydrogen bonding network. The hydrogen bonding scheme of compound5involves a water molecule; there is some disorder with regard to the hydrogen atoms bound to the water molecules.

ISSN:0267-8292    

DOI:10.1080/02678299508036626

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The dynamical and conformational behaviour of a flexible tetrabenzocyclododecatetraene derivative exhibiting a columnar mesophase has been studied by a combination of deuteron solid state NMR spectroscopy and molecular dynamics (MD) simulations. As shown by two-dimensional (2D) exchange NMR, the mesophase is characterized by slow axial reorientations (∼10−3s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D spectra of the core methylene sites are sensitive to the molecular conformation and reorientation mechanism. Motional narrowing of one-dimensional (1D) spectra reveals additional fast librations due to the internal flexibility of the mesogenic moiety. The various reorientation pathways comprising interconversions and pseudo-rotations between different energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is ascribed to a complex axial motion involving rotational jumps combined with a pseudo-rotation between two symmetry related sofa forms. This is confirmed quantitatively by comparing the experimental 2D NMR spectra of the core methylene sites and the simulations which are based on the molecular geometries obtained by MD simulations. The lineshapes of one- and two-dimensional spectra of magnetically aligned samples specific to the orientation behaviour of the sofa conformer are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036627

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The synthesis of chiral liquid crystalline diacrylates is described. By mixing one of them with non-chiral liquid crystalline diacrylates. cholesteric phases are obtained of which the wavelength of reflection can be chosen by the composition of the mixture. After isothermal photopolymerization of the mixture a crosslinked network is obtained with thermally stable properties.

ISSN:0267-8292    

DOI:10.1080/02678299508036628

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Analytical solutions to the Leslie-Ericksen equations are discussed for flow through a circular capillary and a rectangular channel. The analysis allows for a director component out of the flow-gradient plane. The resulting orientation profiles match the commonly observed textures of mesophase pitch-based carbon fibres, specifically the radial and line-origin textures. Thus, this formulation would appear to be correct for the flow of discotic mesophase pitches. Furthermore, the velocity profile for capillary flow suggests the possibility of measuring the Leslie coefficient α4for a mesophase pitch using a capillary rheometer.

ISSN:0267-8292    

DOI:10.1080/02678299508036629

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Precise knowledge of thermal history (quenching and annealing) of the phases of the quenched liquid crystal MBBA has permitted us, from powder X-ray data, to attribute unequivocally a previously published structure to the C4metastable phase. Lattice parameters of the C3phase are also proposed.

ISSN:0267-8292    

DOI:10.1080/02678299508036630

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor