摘要:

One phasmid and fourteen biforked mesogens with thiobenzoate end groups have been synthesized and studied. The phasmid displays a hexagonal columnar phase with a lattice parametera= 45.5 Å at 70°C. This phase is not observed for the biforked mesogens which exhibit only a nematic phase, a smectic C phase for short chains and an unknown phase for long chains. The tilt angle of the molecules in the smectic C phase is about 50° (the layer thickness is 38.2 Å for a molecular lengthl= 61 Å in the case ofIA(n= 12). The crystal structure of compoundIB(n= 12) is monoclinic and the molecules adopt a zigzag form.

ISSN:0267-8292    

DOI:10.1080/02678299308036465

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The preparation of liquid-crystalline 2,5-bis(4-alkyloxyphenyl) thiazolo [5,4-d] dithiazoles (ATTn), in which the alkyl group is CnH2n+ 1(wheren= 1 to 10, 12), has been described. Mesomorphic properties and phase transitions as determined by polarizing hot stage microscopy and differential scanning calorimetry are discussed as a function of the number of carbon atoms in the alkyloxy chain. The exhibition of tilted smectic phases (smectic C and smectic I) by these compounds is an interesting feature since there is no significant central dipole moment transverse to these molecules.

ISSN:0267-8292    

DOI:10.1080/02678299308036466

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The elastic-hydrodynamic theory for the ferroelectric smectic C* phase is reviewed and the governing equations for the dynamical behaviour of a surface stabilized ferroelectric liquid crystalline cell are written down, taking the possibility of macroscopic mass flow in the system into account. The influence of backflow effects on the dynamical cell behaviour and a control parameter, determining whether backflow effects will be of importance or not, is derived. It is shown that when backflow effects are pronounced, the response time of the switching can be considerably decreased. Also, the shape of the c-director and velocity profiles across the cell are shown to be strongly dependent on the presence of backflow.

ISSN:0267-8292    

DOI:10.1080/02678299308036467

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Three new series with a tolane core and a chiral L-lactic acid moiety have been synthesized. The theoretically predicted SA-TGBA-N* phase sequence is found in several compounds with short chains. The smectic A twist grain boundary phase (TGBA) obtained with a cholesteric texture, is characterized by different techniques. Helical pitch measurements were performed using Grandjean-Cano wedges or homeotropic drops. All pitch values are lower than those found in previously reported series exhibiting the TGBAphase. Ferroelectric smectic C (S*C) phases appear with long chains and have striated fan-shaped or pseudo-homeotropic textures. Electro-optical properties of the S*Cphase were studied with the classical SSFLC geometry. We observed high polarization, slow response time and a saturated tilt angle of 20°. These results are comparable with previous studies.

ISSN:0267-8292    

DOI:10.1080/02678299308036468

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A number of new ferroelectric side chain liquid crystal homopolymers derived from poly(hydromethylsiloxane), their copolymers with different mesogens and with poly(dimethylsiloxane) backbones have been prepared. The transition temperatures of the monomers, homo and copolymers have been determined. The homopolymers exhibit S*Cand N* phases, while copolymers show only a S*Cphase. A measured spontaneous polarization value of monomer, 9b, is larger than 500 nC cm−2.

ISSN:0267-8292    

DOI:10.1080/02678299308036469

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Studies show that branching the side chains of hexakis(acoyloxy)-benzenes (HAB) and -cyclohexanes (HAChx) does not modify the structure of the discotic mesophase formed. Shear fields appear to act on entire columns, not on individual discs, thereby causing alignment of columns in the shear direction. X-ray diffraction patterns of samples at temperatures between 30°C and 40°C above the clearing temperature provide evidence for the presence of mesophase order.

ISSN:0267-8292    

DOI:10.1080/02678299308036470

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Different nematic structures confined to a long cylindrical cavity with homeotropic surface anchoring are studied using a numerical minimization of the free energy of the uniaxial nematic liquid crystal. The stability of escaped radial structures and planar polar structures (with and without line defects) is analysed in terms of the ratio of elastic constantsK24/K11,K33/K11, anchoring strength and external magnetic field applied perpendicular to the symmetry axis of the cylinder. We draw the analogy between the stability diagram of the cylindrical structures and structures in a spherical droplet. In particular, a simple way extracting the value of the saddle-splay elastic constantK24from the stability studies is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308036471

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Five series of asymmetric diphenyldiacetylenes containing a single fluorine, two fluorines or a trifluoromethyl group at one end and an alkyl, alkyloxy or alkenyloxy group on the other end, were prepared and characterized. Most of the 4-alkylphenyl-4′-fluorophenyldiacetylenes, 4-alkyloxyphenyl-4′-fluorophenyldiacetylenes and 4-alkenyloxyphenyl-4′-fluorophenyldiacetylenes obtained exhibit an enantiotropic nematic phase and no smectic phase. However, most of the 4-alkyl-3′, 4′-difluorodiphenyldiacetylenes, 4-alkenyloxy-3′,4′-difluorodiphenyldiacetylenes and 4-alkyl-4′-trifluoromethyldiphenyldiacetylenes show no mesomorphic behaviour. The effects of alkyl, alkyloxy, alkenyloxy, fluoro, 3,4-difluoro and trifluoromethyl end groups on the mesomorphic behaviour of the diphenyldiacetylenes prepared are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308036472

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Surface tension is known to induce smectic O (SO) films at the free surface of isotropic droplets of 1-methylheptyl terephthalylidene-bis-4-aminocinnamate (MHTAC). The SOfilm has its molecules disposed in a herringbone fashion, with the layers parallel to the free surface. It constitutes a model of a 2D polar nematic liquid crystal without any contact to solid substrates. The film is oriented uniformly when applying a uniform electric field, except along disclination walls. By measuring the width of the disclination walls in the two configurations with the electric field parallel and perpendicular, we determine the anisotropy of the elastic constants in the induced Sofilms. We find the ratio of the 2D bend to splay elastic constantsKb/Ksto be ≃ 0.25 which is anomalously small when compared to the corresponding 3D ratio (K2+K3)/2K1usually found for the bulk nematic phase. This experimental result is comparable to measurements performed with 2D suspended Scfilms with, however, a different physics.

ISSN:0267-8292    

DOI:10.1080/02678299308036473

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm−2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S*1, and S*Cphases and the consequent photomechanical regulation of spontaneous polarization.

ISSN:0267-8292    

DOI:10.1080/02678299308036474

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor