摘要:

Detailed studies of 4,4′-di-(2-methoxyethoxy)biphenyl have shown that it displays the first recorded incidence of the crystal K phase forming directly on cooling from the isotropic phase.

ISSN:0267-8292    

DOI:10.1080/02678299508036698

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We describe the dynamics of phase separation and transition processes, in binary mesomorphic mixtures with the help of a system of two coupled partial derivative equations. We emphasize, both analytically and numerically, that, depending on the regions of the phase diagram, the dynamical behaviour may result either from a two step process (first the phase transition, then the phase separation) or from a process showing salient features of the Cahn-Hilliard spinodal decomposition (bicontinuous periodic networks in the transient stages). The dynamics of evolution of the domain patterns are illustrated with the help of numerical simulations in which homeotropic and planar anchorages are visualized.

ISSN:0267-8292    

DOI:10.1080/02678299508036699

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Using a light-beating technique and for an appropriate scattering geometry, we have measured separately the orientational diffusivitiesDsplayandDtwistfor an ‘hexa-n-alcanoyloxytruxene’ (HATX C12H25) sample which exhibits a disk-like thermotropic nematic phase (ND) between 57°C and 84°C.DsplayandDtwistare about 100 times weaker than those corresponding to rod-like thermotropic nematics. The deviation can be attributed to higher viscosities ηsplayand ηtwist.

ISSN:0267-8292    

DOI:10.1080/02678299508036700

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Three series of novel chiral twin liquid crystals consisting of a cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (III) or a 4-cyanophenoxy group (IV) and of a dihydro-cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (V) connected by an alkanoate spacer with a varying number (1–7 and 10) of methylene units were synthesized and their mesogenic properties investigated. Strong odd-even effects were observed as a function of spacer length for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phase. The compounds with an even number of methylene units have a smaller pitch than the compounds with an odd number of methylene units. Replacement of the cholesteryl group by a dihydrocholesteryl group results in an larger pitch, whereas replacement of the 4′-cyanobiphenyl-4-yloxy group by a 4-cyanophenoxy group gives a smaller pitch.

ISSN:0267-8292    

DOI:10.1080/02678299508036701

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A series of chiral triplet liquid crystalline compounds with a central biphenyl group to which two dihydrocholesteryl groups are attached via two flexible alkanoate spacers has been prepared and investigated. As a function of spacer length, these compounds show strong odd-even effects for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phases. Asymmetrical compounds with one odd and one even number of methylene groups in their spacers have properties intermediate between those of symmetrical compounds with two odd or two even spacers.

ISSN:0267-8292    

DOI:10.1080/02678299508036702

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Various members of two series of μ-halogeno-bridged tetrapalladium mesogens each with twelve dodecyloxy chains, but differing in the size of their large disc-like macroheterocyclic cores,1 a-eand2a-c, as well as one new short-chained homologue (1 c') of one of the series, as a model compound for single crystal X-ray studies, are presented. Our results on that model compound again unambiguously confirm earlier statements about the unique molecular structure of the core of these flat tetrapalladium complexes. The properties of all thermomesogenic compounds have been studied by differential scanning calorimetry, polarizing microscopy, and also by X-ray diffraction; the latter method reveals an oblique arrangement of disordered columns in most of the mesophases exhibited by the members of both series1and2.

ISSN:0267-8292    

DOI:10.1080/02678299508036703

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two new six-ring double-swallow-tailed mesogens were prepared. The phase behaviours of these compounds were studied by optical methods and by calorimetry. One of them shows the very unusual phase sequence crystalline-smectic C-cubic-isotropic-nematic-isotropic.

ISSN:0267-8292    

DOI:10.1080/02678299508036704

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The synthetic route to a chiral side chain copolymer exhibiting broad smectic phases is described. The copolymer shows a smectic C* phase from below room temperature up to 105°C. Ferroelectric properties of this copolymer are reported as well as results showing electroclinic switching. The pyroelectric effect and the dielectric behaviour of the copolymer are also demonstrated.

ISSN:0267-8292    

DOI:10.1080/02678299508036705

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The dielectric spectrum of the antiferroelectric smectic C* phase exhibits a low and a high frequency absorption peak (Pl, Ph) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A* phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C*α(82·80° to 81·91°C) and C*γ(80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δε for both processes first increases by about a factor of two, then exhibits a maximum at a threshold fieldEccorresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, atEcthe PHpeak vanishes and the PLpeak shows up at a frequency slightly lower than that corresponding to zero field. In contrast to the Δε behaviour, the relaxation frequency of the two absorptions does not show any appreciable bias field dependence forE<Ec. We attribute the PHprocess to the collective reorientation of the molecules around the cone in theoppositedirection (anti-phase in the Ø variable). PLmay be attributed to a similar collective reorientation in the same direction (in-phase) around the cone, where the coupling to the electric field is mediated by the helical superstructure, and a corresponding small shift in the local polarization directions. There is no antiferroelectric soft mode coupling to an electric field, but the anti-phase cone motion acts electro-optically in a way similar to the electroclinic effect.

ISSN:0267-8292    

DOI:10.1080/02678299508036706

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We present details of calculations of the core structure of a screw dislocation in a smectic A liquid crystal, using the phenomenological Landau-de Gennes free energy functional. The order parameter frustration created by topological constraints far from the dislocation core is resolved in one of three qualitatively different ways. The three types of dislocation core solution are the DT (double twist), CL (classical), and BP (broken polar symmetry) solutions, respectively. The stability requirements for these structures are discussed, as a function of temperature, smectic elastic properties, and coupling between smectic and nematic order. The effect of possible inhomogeneity between left- and right-handed conformers is also examined.

ISSN:0267-8292    

DOI:10.1080/02678299508036707

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor