摘要:

Theoretical studies are presented on discontinuous changes produced in homogeneous deformations under the competing influences of an electric fieldEapplied parallel to the sample planes and a magnetic fieldHapplied at different angles in the plane containingEandn0the initial director orientation. Orientational bistability can be affected by the tilt ofn0at the boundaries, the relative magnitudes ofHandE, the tilt angle ofHand even the order in whichEandHare applied to the sample. In particular, the bend transition can be made continuous by applyingHparallel toEin materials with positive diamagnetic susceptibility anisotropy (Xa> 0). Depending upon the tilt ofHand the relative strengths ofHandEit may be possible to change (continuously or otherwise) the nature of distortion in domains.Ab initiocalculations show that theEinduced twist transition should be continuous in samples having lateral dimensions large compared with thickness. In such a sample one can envisage the possibility of removing the discontinuity at the Freedericksz transition by tiltingn0sufficiently away from the homeotropic in a plane normal toE. A more complete theoretical analysis may be necessary to understand the discontinuous twist Freedericksz transition observed experimentally when the electrode gap is not large compared to the sample thickness. The possibility that the hysteresis width of a discontinuous Freedericksz transition might depend on domain formation above threshold cannot be ruled out. In the bend geometry non-rigid anchoring of the director at the boundaries is briefly dealt with to indicate that flexoelectricity might play a role in the initial build up of low level deformation prior to the transition whenEis static. The flexoelectric contribution may become negligible in the presence of a sufficiently strong stabilizingHwhen the anchoring is strong enough andEis time varying. On the basis of the present results and previous work it may be concluded that variation of tilt ofHmay have considerable influence on the formation of the modulated phase above the bend transition.

ISSN:0267-8292    

DOI:10.1080/02678299008047386

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The phase diagrams of four binary systems are presented in which one component is a bi-swallow tailed compound and the second is a nematogenic rod-like compound. In all of the phase diagrams studied an induced smectic A phase occurs. According to X-ray measurements the layer spacingdof the smectic A phase is considerably higher than the average molecular lengthL. Depending on the binary system and the concentrationd/Lratios between 1·17 and 2·13 were found. We propose an interpretation of the results on the basis of a simple packing model.

ISSN:0267-8292    

DOI:10.1080/02678299008047387

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A series of non-symmetrical dithiolium salts has been synthesized. Classical characterization methods (DSC and optical microscopy) show that according to the number and nature of the substituents, these dithiolium salts display crystalline or smectic A phases. TheseSAphases have been studied by X-ray diffraction and several models are proposed to describe the molecular organization in the mesophase. In some cases, it was not possible on the basis of X-ray diffraction data alone, to determine which model should apply. In these cases, however, a comparison with similar molecules shows that one of these models is more probable than the others.

ISSN:0267-8292    

DOI:10.1080/02678299008047388

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The synthesis and characterization of a new class of liquid crystals based on symmetrical bis-triazine rings connected by a flexible alkyl spacer is reported. A mechanism to account for the liquid-crystallinity in this system has been proposed. Slow kinetics for the formation of the mesophase from the isotropic phase and the crystalline phase from the mesophase make the melting transitions virtually irreversible. Potential areas of application have been identified.

ISSN:0267-8292    

DOI:10.1080/02678299008047389

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Four new series of biforked mesogens derived from 3,4-dialkoxybenzoic acids or 3,4-dialkoxybenzaldehydes have been prepared and studied. These derivatives exhibit an interesting polymorphism in which both columnar and lamellar mesophases exist in the same series and in some cases in the same compound. The short chain derivatives exhibit a nematic and/or a smectic C phase but those with long chains display the hexagonal, rectangular or oblique columnar phases. In some of them, a centred cubic phase is found.

ISSN:0267-8292    

DOI:10.1080/02678299008047390

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The contribution of flexoelectricity to the curvature elastic modulus for a symmetric lipid bilayer is calculated. The cases of permitted and forbidden charge exchange between the electrolyte on both sides of the membrane are discussed. On the basis of a simplified model of the lipid bilayer, an estimate of the difference of the moduli corresponding to these two cases is made.

ISSN:0267-8292    

DOI:10.1080/02678299008047391

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Velocity and attenuation measurements for the 5 MHz ultrasonic wave were carried out in two nematic liquid crystals with a uniform orientation created by a magnetic field. Calculations show agreement of the results with those for the viscosity coefficients measured with non-acoustic methods.

ISSN:0267-8292    

DOI:10.1080/02678299008047392

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A homologous series of poly(dialkylsilanes), with alkyl side chains from butyl to tetradecyl, has been studied using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. It was shown that these polymers undergo a first order transition from a low temperature crystalline to a high temperature columnar mesomorphic phase; the transition temperatures range from 9 to 82°C. As the length of the side chains is increased, the transition temperature initially drops drastically and then rises again. These two regimes are connected to the relative part played in the phase transition by the aliphatic side chains and the polymer backbone. The enthalpies also revealed two regimes. Starting from the butyl polymer the enthalpy first grows more and more rapidly and, beyond a chain length of about ten methylene groups, it then grows linearly. From the Y intercept of the corresponding curve it was inferred that the transition into the mesomorphic phase is followed by a stabilization of the backbone conformation. The structure of the columnar mesophase was described as a two dimensional hexagonal packing of columns consisting of straight polymer backbones surrounded by alkyl chains in a disordered conformation. From the slope of the square of the intercolumnar distance as a function of the number of methylene groups per silicon atom, and using the known value of the volume of one methylene group in the liquid state, the stacking period of the silicon atoms along the columnar axis was found to be 1·68 Å. This relatively small value indicates a coiled structure involving a significant number of gauche conformers. The Y intercept of the corresponding straight line, related to the volume of one silylene repeat unit deprived of all its methylene groups, suggests a great compacity of the atomic packing near the silicon backbone.

ISSN:0267-8292    

DOI:10.1080/02678299008047393

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We have used FTIR absorption to study the crystalline and liquid-crystalline phases of 2,3,7,8,12,13-hexa-n-tetradecanoyloxy-truxene (HATX) and 2,3,7,8,12,13-hexa(4-n-undecyloxybenzoyloxy)truxene (HBTX). Both materials show pronounced changes in the CH2stretching mode frequencies at their melting points. In addition, HATX shows a transition at about 20°C below the nominal melting point in which the alkyl tails gain significant disorder. Benzene stretching modes and the CH2deformation modes show subtle changes at the Drd→ N transition of HBTX and at the Drd→ Dhdtransition of HATX.

ISSN:0267-8292    

DOI:10.1080/02678299008047394

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The electric field induced birefringence within the blue phases and the isotropic phase of several different liquid crystals has been measured. The temperature dependences within the blue phases are found to be strong, characteristically different for each phase, and unlike what is predicted by simple theoretical arguments.

ISSN:0267-8292    

DOI:10.1080/02678299008047395

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor