摘要:

Theory predicts that a compound whose structure possesses both rod-like and disc-like characteristics should exhibit a biaxial nematic phase. With this in mind we have synthesized and characterized two new series of non-symmetric dimers containing rod-like and disc-like mesogenic units linked by a flexible spacer. The two series differ in that in one the perimeter of the disc-like unit is decorated with alkyl chains, whereas for the other it is not. The liquid crystalline properties of the two series were investigated both as pure systems and in their equimolar binary mixtures with an electron acceptor. The nematic phase formed by the equimolar binary mixtures is monotropic. However, a small entropy change at the nematic-isotropic transition could be determined. The nature and structure of the monotropic nematic phase is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036611

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The game of molecular engineering of liquid crystalline systems via a combination of molecular architecture and functionalization of mesogens has opened interesting perspectives in the field of liquid crystals. Here we report on the synthesis and characterization of star-like heptameric triphenylenes. The discotic subunits in these molecules have been arranged in a hexagonal fashion around a central discotic core, thus leading to a new type of molecular arrangement for discotic liquid crystals. These oligomers do not crystallize and combine a well-defined structure with a discotic polymer-like processing.

ISSN:0267-8292    

DOI:10.1080/02678299508036612

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Discotic liquid crystals represent a promising class of new materials, for example, with respect to their photoconductivity properties. To tailor the processability and mesophase behaviour of such materials, specifically functionalized cores are required as precursor molecules for discotic oligomers, polymers and networks. The paper presents a simple synthetic strategy leading to unsymmetrically functionalized triphenylene and dibenzopyrene derivatives. Furthermore new symmetrical discotic octa-alkoxy-substituted dibenzopyrenes have been synthesized applying this route.

ISSN:0267-8292    

DOI:10.1080/02678299508036613

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

As new chiral dopants for ferroelectric liquid crystals (FLCs), some optically active α-trifluoromethylbenzyl derivatives were synthesized, utilizing optically active 3-(4-methoxy-phenyl)-4,4,4-trifluorobutanoic acid. The magnitudes of the spontaneous polarizations (Ps) and the response times depended on the core structures and the type of linkage between the optically active and the core blocks. FLC mixtures containing the chiral dopants having an ether bond between the optically active block and the core blocks showed largePsvalues compared with those having an ester linkage, while the response times were inversely proportional to the values ofPs.

ISSN:0267-8292    

DOI:10.1080/02678299508036614

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Cholesteric lyotropic mesophases are prepared by addition of 2,3:4,6-di-O-isopropylidene-α-L-sorbofuranose or diacetone-sorbose, DAS, to nematic matrices based on several amphiphiles. The orientation of the DAS solute is investigated by deuterium NMR of the labelled DAS-d6derivative. This compound was obtained by exchanging one acetone ligand for perdeuteriated acetone. The resulting deuteriated methyl groups are observed as coincident or distinguishable doublets. Quadrupolar splitting values are used for the calculation of parameters of the solute's Saupe ordering matrix, defined in a molecule fixed coordinate system. Decylsulphate lyomesophases containing both caesium and sodium counter-ions are also investigated. For these mesophases, caesium-133 NMR spectra of the counter-ion and deuterium NMR of the residual HDO are also obtained. The present work points out that the DAS ordering in lyomesophases is dependent on the charge and nature of the amphiphile headgroup. The exchange of Na+for Cs+counter ions in decylsulphate mesophases affects the anisotropic properties of the liquid crystalline medium, changing the ordering of the DAS solute.

ISSN:0267-8292    

DOI:10.1080/02678299508036615

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Thirty-eight non-substituted and laterally fluoro-substituted 4-nitro-, 4-isothiocyanato-, 4-fluoro-, 4-trifluoromethyl- and 4-trifluoromethoxyphenyl 4-(5-alkyl-1,3,2-dioxaborin-2-yl) benzoates have been synthesized and their phase transition temperatures and enthalpies of transition measured. The influences of both terminal and lateral substituents in both benzene rings (in the acidic and phenolic moieties) on the phase transition temperatures in general and on the stability of the smectic A phase in particular are compared. The clearing points of the compounds (a), the thermal stability of the smectic A phase (b) and the tendency to form a smectic A phase (c) decrease in the following way:

ISSN:0267-8292    

DOI:10.1080/02678299508036616

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A previously presented model is used to simulate the dynamics and microstructure of spatially invariant uniaxial discotic nematic liquid crystals in isothermal, incompressible, irrotational, extensional (shear-free or elongational) flows. Numerical and analytical solutions of the directornand alignmentSare presented for given uniaxial extensional, equi-biaxial extensional and planar extensional start-up flows. The unit sphere description of the director is used to discuss and analyse the sensitivity of the director trajectories and the alignment relaxation to the initial conditions (no,So), to the alignment Deborah number (De), and to the type of flow. The numerical results are used to characterize the relaxation of the tensor order parameterQand to compute the steady flow birefringence. The various flows are classified according to their orienting strength and alignment strength, and according to whether they generate geodesic (shortest path) director orbits. Equi-biaxial extensional and planar extensional flows are found to be strongly orienting and strongly aligning flows, while uniaxial extensional flow is a weakly orienting and weakly aligning flow. The number of strain units required to achieve steady state are shown to depend on whether the flow is geodesic (uniaxial extensional and equi-biaxial extensional flows) or not (planar extensional flow).

ISSN:0267-8292    

DOI:10.1080/02678299508036617

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The behaviour of a large number of paramagnetic metallo-mesogenic molecules with Cu and VO in different mesophases in a magnetic field was investigated by EPR techniques and magnetic susceptibility measurements. The investigation of the angular dependence of the EPR spectra enabled conclusions to be reached concerning the molecular orientation in the external magnetic field. Temperature dependence magnetic susceptibility measurements were carried out in order to obtain information about the overall susceptibility anisotropy. The good agreement between experimental results and calculated data based on the known increment scheme is obvious. It is shown that the direction of orientation of the molecules in a magnetic field is predetermined by the sum of the anisotropy of the phenyl ring and the chelate core in the molecular structure.

ISSN:0267-8292    

DOI:10.1080/02678299508036618

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Liquid crystal induced circular dichroism (LCICD) measurements were made to investigate the pretransitional phenomena in the SAphase just above the SA-S*Cand SA-S*CAphase transitions of both the first and the second order. The pretransitional LCICD in SAwas observed in the second order phase transition to S*cand S*cA, suggesting the existence of a dynamic helical structure in SA. Such behaviour disappears when the transition is of the first order. It seems that the handedness of the dynamic helix in SAis the same as that in S*ceven when the lower temperature phase is S*cA. This is explained as a result of a dominant contribution of ferroelectric soft mode.

ISSN:0267-8292    

DOI:10.1080/02678299508036619

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Magnetic Freedericksz transition measurements were performed on samples of chiral and racemic mixtures in the cholesteric/nematic phase. We found that the quantity Ф, which is defined as the threshold fieldHthtimes the cell thickness, is larger for the chiral than for the racemic mixtures. This result is inconsistent with chiral contributions to the anchoring strength potential. Instead, the data support a model by Lubensky wherein the transition temperature to the smectic A phase is suppressed in a chiral liquid crystal.

ISSN:0267-8292    

DOI:10.1080/02678299508036620

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor