摘要:

Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.

ISSN:0267-8292    

DOI:10.1080/02678299508031077

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The FE-57 Mössbauer effect (ME) was used to study two solutions 0.4 wt% of ferrocenyl-4′-methoxyaniline (FMA) in the liquid crystalline material 4-n-butoxybenzylidene-4′-n-octylaniline (40.8) and 0.4 wt% of 1,1′-dinonanoyl-ferrocence in 40.8. The ME parameters were determined for the temperature range 40-300 K in the crystalline solid phase and 40–220 K in the cold smectic B phase of 40.8 for both samples. The ‘cold smectic B’ is defined as the smectic B glass and the supercooled smectic B phase. ForT< 60 K the DNF-40.8 system exhibited a ME spectrum that was composed of two quadrupole split doublets. This four-line spectrum was observed for both the crystal phase and the smectic B glass phase of the DNF-40.8 system. The FMA-40.8 system exhibited a single quadrupole doublet. The In (recoil-free intensityf) versusTdata exhibited linear Debye behaviour for the solid phase over the entire temperature range. The cold smectic InfversusTdata also exhibited Debye behaviour for this system. The smectic B glass gave a Mössbauer-Debye temperature (θlc) about 23 K lower than the crystalline phase for the FMA-40.8 system and about 25 K for the DNF-40.8 system. The intermolecular contribution to the vibrational ellipsoid (εL) declined linearly with increasing temperature for both systems.

ISSN:0267-8292    

DOI:10.1080/02678299508031078

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two phase diagrams of a six-ring double-swallow-tailed compound are presented where the mixing components involve electron-acceptor compounds. In the mixed phase region, nematic, smectic A and C, cubic and columnar phases are induced. In this way transitions between lamellar, cubic and columnar phases can be realized by variation of the con- centration. The mesophases occurring in these systems have been characterized by X-ray investigations.

ISSN:0267-8292    

DOI:10.1080/02678299508015257

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An approximate description of the elastic behaviour of smectic C* liquid crystals is introduced in order to facilitate the experimental and numerical analysis of these materials. In this way the original number of 14 independent elastic parameters is reduced to six in the approximate description.

ISSN:0267-8292    

DOI:10.1080/02678299508031079

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Time-resolved FTIR spectroscopy with microsecond resolution was applied to the switching process of FLCs using an IR polarizer attached to a microscope whose axis was set along the aligning direction (direction of SiO flux). A tilted bookshelf geometry with a layer tilt angle of approximately 34° was obtained by alignment using obliquely evaporated SiO layers in a CaF2cell with about 1.7 μm gap. Six kinds of FLC mixtures were prepared by mixing 5 wt % of a chiral molecule with host mixtures of phenylpyrimidines. By monitoring an absorption band at 1432 cm−1, assigned to vibrational modes of the pyrimidine skeleton, it was found that the molecular long axis either rotates along the upper side or along the lower side of the cone during switching. Moreover, we investigated the relationship between the rotational direction of the directors and the stability of the dark level transmission during multiplexing. The dark level transmission during rotation along the upper side of the cone may fluctuate less than that during rotation along the lower side of the cone. We found that the bias stability of the dark level transmission is closely related to the direction of the rotation along the cone. The direction of the rotation of FLCs along the cone was therefore found to play an important role in realizing high contrast ratios during multiplexing.

ISSN:0267-8292    

DOI:10.1080/02678299508031080

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Molecular ‘zip-fasteners’ consisting of short linear and cyclic siloxane backbones and the same swallow-tailed liquid crystal unit as side group were synthesized. The closing and opening mechanism was studied by dielectric measurements. Thereby an abrupt decrease of the static dielectric constant ε0∥was observed indicating the antiparallel order of the side groups in the ‘closed’ state. The dielectric relaxation frequencies of the δ- and α-relaxation for the respective oligomers do not depend on the degree of polymerization which could be the result of the high flexibility of the main chain and the relatively low mobility of the liquid crystalline unit.

ISSN:0267-8292    

DOI:10.1080/02678299508031081

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

New unsymmetrical disubstituted biphenyls have been synthesized. Two different chains (one hydrocarbon and the other fluorinated) are linked through an acetamide bond to 4-methoxy-4′-hydroxybiphenyl. Their mesomorphic properties have been characterized by light microscopy and by differential thermal analysis showing the peculiar contribution of the hydrocarbon chain and the fluorinated tail. The enhancement of the hydrocarbon moiety from one to twelve methylene groups leads to a decrease of the clearing temperatures; furthermore the enantiotropic behaviour is changed to monotropic. The variation of the fluorinated moiety generates strong effects on the transition temperatures. These mesomorphic properties are compared to those of their monocatenar perfluorinated analogues incorporating a connector of the ester or monosubstituted amide type.

ISSN:0267-8292    

DOI:10.1080/02678299508031082

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Discotic binuclear Ni(II) and Cu(II) complexes of 1,2,4,5-tetrakis-(2′-alkyl-2′-homoalkylcarbonylvinylamino)benzene ligands were synthesized. These compounds exhibit columnar liquid crystal phases.

ISSN:0267-8292    

DOI:10.1080/02678299508031083

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The isothermal annihilation kinetics of wedge disclinations in the fully liquid crystalline state were investigated using thin film specimens of a thermotropic liquid crystalline copolyester prepared by a solution-cast method. The as-cast films which are vitrified, homogeneous and amorphous were first subjected to a temperature jump to appropriate temperatures and the coarsening of the liquid crystalline texture was observed in real-time andin situby means of optical microscopy and laser light scattering. The experimental results obtained show that, in the early stages, the domain size,D, which is inversely proportional to the square root of the density of disclinations in the sample, approximately obeys a power law ofD(t)∼t0.35over the time scale covered in the experiment. In the later stages, anchoring of some of the disclination lines onto the glass surface and/or formation of crystallites from some sequences of the copolyester having a higher melting point suppress and then pin the annihilation process. The annihilation processes for pairs of wedge disclinations betweens= + 1/2 ands= − 1/2 or betweens= + 1 ands= − 1/2 were directly observedin situby optical microscopy, and the distance between them was observed to decrease with time.

ISSN:0267-8292    

DOI:10.1080/02678299508031084

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Then-butyl,n-octyl,n-decyl andn-dodecyl homologues of each of the 4-n-pentyloxyphenyl, 4-n-hexyloxyphenyl and 4-n-heptyloxyphenyl 4-(5-n-alkyl-2-thienyl)benzoates have been prepared. All the homologues give rise to liquid crystal phases and their transition temperatures are reported. Phase identification was not straightforward, especially when distinguishing between some of the higher order smectic phases, and required a combination of thermal optical microscopy and X-ray diffraction studies, including powder photographs and detailed measurements of smectic layer spacings.

ISSN:0267-8292    

DOI:10.1080/02678299508031085

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor