摘要:

The liquid crystalline polymorphism (SA, SB, CrE) and the structures of the phases ofN-n-perfluoralkylethyl-(4-phenylbenzylidene)imines are described. It is shown that the smectic layer consists of two sublayers in to which the incompatible moieties of the compounds segregate. It is proved that the perfluoralkyl chains remain disordered until crystallization occurs, whereas with decreasing temperature, the aromatic parts are ordered in types of SBand CrE structure.

ISSN:0267-8292    

DOI:10.1080/02678299608032871

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Hydroxyl-terminated liquid crystalline bis[4-(5-hydroxypentyloxycarbonyl)phenylene] terephthalate (BHT) was prepared and compared with its non-hydroxylated analog, bis[4-(pentyloxycarbonyl)phenylene] terephthalate (BPT). BHT, with the possibility to form intermolecular hydrogen bonding (H-bonding), has higher thermal transition temperatures (TmandTi) than BPT. Infrared spectroscopy was applied to prove the existence of H-bonding for BHT. An X-ray diffraction study suggests that the smectic A (SA) phase is the only phase existing for BPT but for BHT, an interdigitated layered structure was found to be mixed with the SAphase. The formation of this interdigitated layered structure is mainly due to the intermolecular H-bonding between the terminal hydroxyl and the external ester groups in BHT.

ISSN:0267-8292    

DOI:10.1080/02678299608032872

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

New disc-shaped triarylamino-1,3,5-triazines were prepared by reaction of cyanuric chloride with two-fold alkoxy substituted anilines. Each of the triarylmelamines exhibits an enanti-otropic mesophase. X-ray investigations reveal the formation of columnar liquid crystalline structures with either a hexagonal or a rectangular lattice. The lattice parameters, as well as the intracolumnar ordering, depend on the length of the flexible lateral side chains. The new melamines form Langmuir monolayers at the air/water interface due to their amphiphilic nature. The monolayer organization is characterized by a side-on arrangement of the molecules. The polar central molecular parts lie flat on the water, whereas the lateral alkyl chains are oriented perpendicular to the water surface.

ISSN:0267-8292    

DOI:10.1080/02678299608032873

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Polycatenar saturated macrocycles 1,10-bis[3,4,5-tris(4-dodecyloxy)benzoy1]-1,10-diaza-4,7,13,16-tetrathiacyclo-octadecane (3b) and 1,10-bis[3,4-bis(4-dodecyloxy)benzoy1] -1,10-diaza-4,7,13,16-tetrathiacyclo-octadecane (3d) reacted with [Pd(MeCN)4][BF4]2led to the cationic complexes [4b][BF4]2and [4d][BF4]2, respectively. Complexes [4b][BF4]2and [4d][BF4]2showed smectic mesomorphism on heating as confirmed by small angle X-ray diffraction. A concurrent, irreversible structural change of the complex cations was observed and a tentative explanation is suggested.

ISSN:0267-8292    

DOI:10.1080/02678299608032874

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Four homologous series of dimeric tolans of the general formulaRC6H4C≏C6H4O(CH2)mOC6H4C'CC6H4R (m'6-10;R'H, OC6H13, OC10H21and OC14H29), denoted further as TOmOT ornOTOmOTOn, according to the number of aliphatic carbon atoms, have been synthesized. Dimers with terminal chains exhibited liquid crystalline behaviour. Crystal-crystal transitions were also observed. Resulting from the optical, thermal and miscibility studies, the following mesophases have been identified: nematic, smectics A and B, and the tilted smectics C and (probably) F or I. In the 6OTOmOTO6 series, a phase with a strong tendency to self-alignment occurred between the nematic and smectic A phases. This phase resembled the nematic in its viscoelastic properties, but also showed focal-conic textures. A transition between this ‘intermediate phase’ and the nematic was detected only from microscopic observations. On the contrary, the ‘intermediate phase’-smectic A transition was also detected by DSC (δH∼ 1kJ mol−1). Both tilted (Scand SF/I) and non-tilted (SAand SB) smectics were observed in the 10OTOmOTO10 series, while only tilted smectic phases were identified in the dimers with longer terminal substituents (14OTOmOTO14). Double melting behaviour was found in 14OTO8OTO14 and 14OTO10OTO14. An alternation of the transition temperatures and enthalpies with the odd-even alternation of the lengths of bridging groups was clearly observed. A correlation between total enthalpies of transition and solubilities of the dimeric tolans is stated.

ISSN:0267-8292    

DOI:10.1080/02678299608032875

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This work is devoted to a self-consistent account of concentration-uniformity distortion in nematic-cholesteric mixtures in an electric field. In so far as the dielectric anisotropy of the components may differ both in absolute value and in sign, we take into account the fact that the local value of Δϵ = ϵ∥— ϵ⊊may depend on the local concentration of the components. The occurrence of concentration heterogeneity has the following consequences: (a) the critical field slightly increases, (b) the cholesteric-nematic transition remains as a second-order phase transition, but the critical behaviour of the pitch transforms from logarithmic to inverse power. It was shown that when the applied field increases, the periodicity of the structure is divided into two regions: a quasi-nematic region (when the director is parallel with the external field) and a crest in the concentration wave. WhenEgoes toEc, the crest of the concentration wave is nearly constant and the quasi-nematic region grows. When the dielectric anisotropies of the components are equal, the concentration wave causes enrichment with chiral dopant of regions where the director is rapidly rotating. When Δϵ of the chiral component has a higher value than Δϵ of the nematic component, the extent of this effect also intensifies the effect of stratification of the system. Conversely, when ΔϵN> δϵch, both of the mechanisms compete and the structure of the crest becomes more complex.

ISSN:0267-8292    

DOI:10.1080/02678299608032876

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We study the influence of the anisotropy of the Frank elastic constants on the magnetic reorientation of the nematic phase of polymer liquid crystals. In the magnetic reorientation following a 90° director rotation with respect to an aligning magnetic field, a pattern of inversion walls develops which depends on the relative magnitude of the elastic constants and the magnetic coherence length. We show how this dependence can be experimentally studied by proton NMR. The transition from a homogeneous director reorientation to a distorted director reorientation is theoretically studied as a function of the rotation angle α. A critical angle of rotation αcshows up, and we study its dependence on the anisotropiesK3/K1andK3/K2. Depending on these ratios and on the wavelength of the distortion, critical angles 45° < αc< 90° are predicted for materials with positive anisotropy of the magnetic susceptibility χaand 0° < αc< 45° (mod π/2) for materials with χa< 0. Within the frame of a phase transition analogy, a Landau-like theory predicts the transition to be second order.

ISSN:0267-8292    

DOI:10.1080/02678299608032877

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A droplet, a few hundreds of microns, of liquid crystalline hydroxypropylcellulose (HPC) in water is imbedded into a polydimethylsiloxane (PDMS) matrix and subjected to a shear flow. The droplet is deformed into a thread of high aspect ratio. The textures associated with the break-up of the thread once the flow is stopped are observed by optical microscopy. After stopping the flow a banded texture appears. The droplets resulting from the break-up have a bipolar texture with the two opposite poles being along the former thread direction.

ISSN:0267-8292    

DOI:10.1080/02678299608032878

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A series of novel dis- and trisazo dyes containing a perfluoroalkyl, perfluoroalkylthio and perfluoroalkylsulphonyl groups have been synthesized. The absorption bands of these dyes were more bathochromic than the corresponding alkyl derivatives. The solubility of disazo dyes containing perfluoroalkyl and perfluoroalkylthio groups in hexane was larger than the alkyl derivatives. To obtain good dichroism, the azo dyes were required to have a largel/dratio (>2.50). These novel azo compounds could be used practically as dichroic dyes in guest-host liquid crystal displays.

ISSN:0267-8292    

DOI:10.1080/02678299608032879

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

To unravel the effects of the volume-excluding central core and the mesogenic pendant group on both the glass-forming ability and morphological stability of the thermally quenched glass, nine model compounds were synthesized that contain various nematogenic and cholesteryl pendant groups. The glass-forming ability of the melt and morphological stability of the thermally quenched glass were assessed using the DSC, XRD, and hot-stage POM techniques. With cyanobiphenyl as the pendant group, the following descending order in morphological stability against thermally activated recrystallization was established:trans-cyclohexane <all-exo-bicyclo [2.2.2] oct-7-ene < cubane <cis-cyclohexane < benzene. While the cyclohexane compound containing three cyanoterphenyl groups showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substituted by a cholesteryl group showed superior glass-forming ability and morphological stability. Additionally, withcis-cyclohexane as the central core the angular 6-(4-cyanophenyl)-naphthyl group, a stronger nematogen, showed a comparable glass-forming ability but a superior morphological ability in comparison to the cyanobiphenyl group. However, withall-exo-bicyclo[2.2.2]oct-7-ene as the central core, the angular 1-phenyl-2-(6-cyanonaphth-2-yl)ethyne, also a stronger nematogen, turned out to be inferior to the cyanobiphenyl group with respect to morphological stability. It appears that the glass-forming ability and morphological stability of the hybrid system are determined by the characters of both the volume-excluding core and the pendant group, a delicate structural balance between the two constituents, and the stereochemistry of the hybrid system.

ISSN:0267-8292    

DOI:10.1080/02678299608032880

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor