摘要:

The synthesis and mesomorphic properties of several homologues of (S)-(+)-1-methylheptyl 4-[4(trans-4-n-alkoxycinnamoyloxy)benzoyloxy]benzoates (series I) and (S)-(+)-1-methylheptyl 4-[4-(trans-4-n-alkoxy-alpha-methylcinnamoyloxy)benzoyloxy]benzoates (series II) are described. The influence of the alpha -methyl group on the different mesophases and the clearing temperatures has been evaluated. Many homologues of both the series show a ferroelectric chiral smectic C (SmC*) phase while the antiferroelectric smectic C phase (SmC*A) is exhibited by compounds of series I. The mesophases have been identified by optical polarizing microscopy, differential scanning calorimetry and miscibility studies. The spontaneous polarization, the helical pitch and the tilt angle of two of the compounds have also been measured.

ISSN:0267-8292    

DOI:10.1080/026782998206740

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Mesomorphic properties of cyclohex-2-en-1-ones belonging to 14 different chemical structural types are compared and discussed. They were prepared by condensation of Mannich salts with 2-substituted acetoacetates or benzyl methyl ketones or acetylacetone in the presence of potassium hydroxide.

ISSN:0267-8292    

DOI:10.1080/026782998206759

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We present the results of molecular dynamics simulation studies of 4-(trans-4'-pentylcyclohexyl)- benzonitrile (PCH5) at different temperatures in the isotropic and nematic phases within the united atom approach. Special attention is paid to the electrostatic interactions. Different methods of determining point charges are compared and the effect of the electrostatic interactions on the molecular distribution is considered. Their decisive influence on local molecular packing and phase transition temperature is shown. We analyse various static and dynamic properties, being specially interested in the reliability of the simulation results, to check to which extent they are distorted by the confinements adopted during the simulations (and especially the size of the system). Using realistic molecular interaction models in contrast to simple model ellipsoids, we find that the number of molecules and the lengths of the simulation runs have to be increased considerably when studying macroscopic properties like the phase transition temperature and the order parameter. Much easier to access are single particle and local properties such as translational dynamics and local ordering, but also the relative dependence of the order parameters of various ranks.

ISSN:0267-8292    

DOI:10.1080/026782998206768

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A novel series of 6'-phenyl-2,2'-bipyridines (HL-n) substituted in the 4 position with promesogenic functional groups has been prepared. The materials are strongly luminescent and display nematic mesomorphism. The HL-n behave as C,N,N-tridentate, cyclometallating ligands leading to [Pd(L-n)Cl] derivatives. Metal co-ordination promotes a considerable stabilization of the crystal phase, forcing mesophases to become metastable. All [Pd(L-n)Cl] species indeed exhibit a monotropic mesophase either nematic or smectic A in character depending on the chain length of the substituents on the central pyridine unit.

ISSN:0267-8292    

DOI:10.1080/026782998206777

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The results of dielectric studies of seven members of a 5-n-alkyl-2-(4'-isothiocyanatophenyl)- 1,3-dioxane (nDBT) homologous series (n=4-10) in the isotropic and smectic A phases are presented. The complex dielectric permittivity, epsilon* (nu) = epsilon'(nu) - iepsilon'' (nu), was measured with the aid of two experimental set-ups: an impedance analyser (10kHz-13 MHz) and a dielectric time domain spectrometer (TDS, 10 MHz-4 GHz). This allowed two main relaxation processes in both the phases studied to be separated: the low frequency (l.f.) process connected with molecular reorientations around the short axes, and the high frequency (h.f.) process connected with the rotations around the long axes. The measured dielectric increments enabled us to estimate the value and direction of the dipole moment of the nDBT molecules. The l.f. relaxation process in the isotropic and smectic A phases of the nDBT compounds exhibits some peculiar features which distinguish the materials from other similar substances. The observed decrease of the relaxation times and activation enthalpy with increasing n is discussed in relation to the molecular arrangements in the smectic layers.

ISSN:0267-8292    

DOI:10.1080/026782998206786

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Experimental studies of the dielectric relaxation in the nematic and isotropic phases of 4-cyanophenyl 4-n-heptylbenzoate have been performed in the frequency range from 1 kHz to 1 GHz. In the nematic phase, measurements of the complex dielectric permittivity for two orientations of the director n, parallel and perpendicular to the measuring electric field E, have been carried out. It has been found that for E n, as well as in the isotropic phase, the dielectric spectra consist of two elementary domains, described by the Cole-Cole equation, whereas for E n three Cole-Cole-type domains can be distinguished. The possible molecular reorientational movements corresponding to these domains are discussed and comparison with the theoretical predictions is made.

ISSN:0267-8292    

DOI:10.1080/026782998206795

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The thermal properties of twelve 'single-tailed' carbohydrate-based liquid crystal materials which have a perfluoroalkyl chain (n-C4F9, n-C6F13, n-C8F17) linked to the polar sugar head group are described. All reported amphiphiles, the 6-deoxy-6-C-perfluorohexyl-L-altrose 1, the (5R)-xylopentoses 2, 3 and 4, the 5,6-dideoxy-6-C-perfluoroalkyl-D-glucofuranoses 5 and 6, the 2,3-O-perfluoroalkylidene-D-gulopyranoses 7, 8 and 9 and the 3,4-O-perfluoroalkylidene- D-altropyranoses 10 and 11 form mesophases of the smectic A type. Even a C4F9-chain (compounds 2, 7, and 10) is long enough to cause liquid crystalline behaviour.

ISSN:0267-8292    

DOI:10.1080/026782998206803

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We have studied the Goldstone-mode of three ferroelectric materials near the chiral smectic C (SmC*)-smectic A phase transition. The elastic constants corresponding to the C director in the SmC* phase have been evaluated from the dynamic light scattering measurements. In the material SCE6 we find a slow relaxing mode in addition to the normal Goldstone-mode. This slow relaxation is present only in the SmC* phase and vanishes in the smectic A phase.

ISSN:0267-8292    

DOI:10.1080/026782998206812

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A group of 4-[4-(4-n-octyloxybenzoyloxy)benzoyloxy]benzylidenemalonic acid bis(perfluoralkyl) esters and 4-{4-[2,2-bis(n-octyloxycarbonyl)ethenyl]benzoyloxy}benzoic acid 4-perfluoro- alkyloxyphenyl esters with varied lengths of the perfluorinated chains were investigated by X-ray measurements. The liquid crystalline behaviour (SmA, SmC, Col) and the structures of the phases are described. The substances show the influence of the steric interaction caused by the wedge shape of the molecules and the repulsion caused by chemically incompatible segments (fluorophobic effect). In the first group of compounds investigated the steric interaction clearly dominates the structure, so that an antiparallel packing of the molecules in SmA and SmC phases results. In the second group, the influence of the incompatibility is more and more enhanced and a low temperature phase with a columnar structure arises in which the incompatible parts are separated on a microscopic scale.

ISSN:0267-8292    

DOI:10.1080/026782998206821

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A series of semi-perfluorinated ferroelectric liquid crystals, 2-[4-(2-fluoro-octyloxy)phenyl]- 5-(omega-n-perfluoroalkylalkyloxy)pyrimidines were prepared and their physical properties evaluated. All of the fluorinated phenylpyrimidines exhibited a chiral smectic C phase enantiotropically. The results showed that high fluorination extent favours the tilted chiral smectic C phase. Also, highly fluorinated compounds exhibited a large cone tilt angle and large spontaneous polarization. However, the response became slow as the fluorination extent increased. Although the compounds showed a large spontaneous polarization in the pure state, their spontaneous polarization power as chiral dopants was so small that very little spontaneous polarization could be measured.

ISSN:0267-8292    

DOI:10.1080/026782998206830

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor