摘要:

A relatively rare phenomenon, the induction of thermotropic mesophases from combinations of homologues of non-mesogenic compounds, is examined in some detail. Thus, monotropic nematic phases have been obtained from binary mixtures of 4-alkyl-N-(4-cyanophenyl)piperidines (I). Except for the hexyl homologue, the compoundsIwith butyl to nonyl as alkyl groups are non-mesomorphic as neat materials. From phase diagrams, the nematic clearing temperatures of the binary mixtures are found to vary linearly with composition and to increase as the molecular lengths of the components are increased. By introducing a methyl group at the 1-position of the alkyl chain, the melting points of the neat components are depressed, and the nematic clearing temperatures of the binary mixtures are lowered. Factors associated with the induction of this mesomorphism and with the ‘kinetic’ and thermodynamic stabilities of the monotropic nematic phases are discussed. Among these is the orientation between the rings of the core groups inIand in the more common mesogens, thetrans-4-alkyl-1-(4-cyanophenyl) cyclohexanes,II.

ISSN:0267-8292    

DOI:10.1080/02678299408037332

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The polymorphism of sodium ricinoleate has been examined using differential scanning calorimetry, optical microscopy and low angle X-ray diffraction. There are three low temperature phases, separated by small transition enthalpies, where the molecules are packed in layers with little molecular disorder. At high temperatures (>160°C), two semi-molten mesophases are formed. Both have distinct optical textures under polarizing microscopy, that of the highest temperature phase being the typical fan pattern of a hexagonal phase. X-ray diffraction shows that this phase is comprised of hexagonally packed oblate spheroids arranged in layers according to an ABAB type pattern. It has not been reported previously for surfactants. The head groups form the cores of the spheroids with molten alkyl chains filling the remaining space. In the lower temperature mesophase, the head groups pack into rods joined in fours to give layers of a tetragonal array. The layers stack to give a body centred tetragonal structure. Again, the head groups form the rod cores, while alkyl chains fill the remaining space.

ISSN:0267-8292    

DOI:10.1080/02678299408037333

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Results are presented of temperature and pressure studies of the non-linear dielectric effect (NDE) in the isotropic phase of MBBA (N-4-methyoxybenzylidene-4′-n-butylaniline) and EBBA (N-4-ethoxybenzylidene-4′-n-butylaniline). In both nematogens the pretransitional increase of the NDE is described by the classical relation obtained on the basis of the Landau-de Gennes model. The value of the ratio of amplitudes describing the pretransitional effect of MBBA and EBBA shows a dependence on the measurement frequency of the NDE (a few MHz). In nematogens with a large positive anisotropy of the electric permittivity, change in the measurement frequency even leads to a change in sign of the NDE on approaching the clearing temperature.

ISSN:0267-8292    

DOI:10.1080/02678299408037334

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Starting from the Landau-de Gennes free energy expression, the author has numerically analysed the director pattern in a nematic droplet of polymer dispersed liquid crystals. The nematic director has been understood as the eigenvector, which corresponds to the largest eigenvalue of the tensor order parameter. To investigate the droplet structure influence, all equations have been treated on the curvilinear coordinate system which is generated along the droplet boundary. In the case of spherical and spheroidal droplets with normal strong anchoring, the director exhibits an axial configuration and a disclination ring. The ring radius and the capactiance of the system change without hysteresis with the applied voltage.

ISSN:0267-8292    

DOI:10.1080/02678299408037335

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The isotropic-liquid crystalline transition of concentrated DNA solutions is investigated using freeze-fracture electron microscopy in order to understand the first steps of the DNA condensation process. Between the isotropic liquid and the cholesteric mesophase, we report the existence of double twist DNA bundles and describe their long range ordering into 3D networks. This organization corresponds to the formation of ‘blue phases’ already observed in thermotropic liquid crystals, but never reported in lyotropic systems. In addition, the size of the DNA molecule, about ten times that of most thermotropic materials, allows here the molecular resolution imaging of blue phase structures. Since such structures recall chromatin organization of some Procaryotes and lower Eucaryotes, we suspect that they may be widespread and of potential interest in the regulation of chromatin functions.

ISSN:0267-8292    

DOI:10.1080/02678299408037336

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

DSC and X-ray diffraction studies on a series of 1,3-phenylene bis[4-(4-alkoxyphenyliminomethyl)benzoates] are presented. The only mesophase exhibited by the methoxy to hexadecyloxy homologues is of the smectic C type. The reversal of the iminomethyl linkage reduces drastically the incidence of a mesophase; that is, only the first five homologous members exhibit a smectic C phase in the series of 1,3-phenylene bis[4-(4-alkoxybenzylideneamino)benzoates]. Almost all the members in the second series become nematogenic by the chloro substitution at the 4-position of the 1,3-phenylene moiety. In addition, a smectic C phase is observable for the ethoxy to pentyloxy and also the tetradecyloxy and hexadecyloxy members. The second chloro-substituent introduced to the 6-position of the same central ring eliminates completely the smectic C phase and enhances the nematic thermal stability.

ISSN:0267-8292    

DOI:10.1080/02678299408037337

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In smectic phases molecules can reorientate around their long and short axes. The motion of a molecule around its long axis is non-cooperative and influenced by both stochastic and deterministic forces exerted by neighbouring molecules. Solving the Smoluchovsky equation for such a reorientation process, two-time correlation functions are calculated which are related to frequency-dependent susceptibilities. The results are used for investigating how deterministic forces in higher ordered smectic phases have an effect on microwave spectra. It can be concluded that the quadrupolar ordering produces a splitting of the relaxation frequency. This splitting, which should occur at the transition from the smectic A to the smectic C phase, is only weakly influenced by an additional dipolar (ferroelectric) ordering.

ISSN:0267-8292    

DOI:10.1080/02678299408037338

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

X-ray studies of a homologous series of rod-shaped liquid crystal molecules with one tail perfluorinated and the other protonated, reveal large decreases in the smectic A layer spacing with increasing temperature. These materials form unique dimer phases in which the smectic layer spacing is dependent on the length of the perfluorinated tail and independent of the length of the protonated tail. The chain statistics of the perfluorinated tail significantly influence the thermal expansion coefficient since the length of the fluorinated tail defines the smectic layer spacing. Thermal expansion coefficients for the layer spacing observed here are negative and nearly an order of magnitude greater than for typical protonated rod-shaped thermotropic liquid crystals in the SA phase.

ISSN:0267-8292    

DOI:10.1080/02678299408037339

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Two novel series of pyrazine-containing esters, together with an analogous phenyl system have been prepared, and their liquid crystalline phase behaviour compared in order to establish how the replacement of a phenyl ring by a pyrazine ring affects the liquid crystal behaviour of these esters. Both of the heterocyclic-containing ester series show significant suppression of the smectic phases, and some suppression of the nematic phase, when compared with the non-heterocyclic series.

ISSN:0267-8292    

DOI:10.1080/02678299408037340

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Novel series of liquid crystalline compounds having an enyne unit, 1-(4-alkylphenyl)-4-(trans-4-alkylcyclohexyl)-but-3-en-1-yne and 1-(4-alkoxyphenyl)-4-(trans-4-alkylcyclohexyl)-but-3-en-1-yne are presented. The synthetic methods, mesomorphic phases, and some physical properties of these series are reported.

ISSN:0267-8292    

DOI:10.1080/02678299408037341

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor