摘要:

A homeotropically aligned layer of smectic C liquid crystal has been studied using optical excitation of half leaky guided modes. Under the application of an AC voltage, the director configuration changes over a time-scale of the order of seconds. Fitting model results to the recorded angle dependent reflectivity data indicates firstly a change of tilt angle of the primary director, suggesting perhaps a layer tilt, and secondly a reduction of the imaginary part of the optical permittivity, implying a suppression of fluctuations. There is no evidence for a Helfrich-like deformation in the liquid crystal layer. Detailed analysis finally shows that the results are correctly interpreted as an increase in cone angle, not layer tilt, with field. Then from the fitted data, the change of tilt angle with field is obtained and hence information on the mean-field theory parameters.

ISSN:0267-8292    

DOI:10.1080/02678299508036594

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Pulsed electric fields are used to study the influence of a strong field on the nematic-isotropic phase transition for cyanobiphenyl and stilbene-type liquid crystals. Deviation of the electric field-induced optical anisotropy from the Kerr law is observed and it is shown that such electric fields can shift the transition temperature substantially. The induced birefringence and the shift of the transition temperature are measured as a function of the electric field strength. The results are explained qualitatively in the context of the Landau-De Gennes theory with two order parameters. The coefficients of the phenomenological theory are calculated using the simple density functional theory of polar nematics developed in this paper and the results for the shift of the transition temperature are compared with experiment results.

ISSN:0267-8292    

DOI:10.1080/02678299508036595

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An L-glutamate derivative having one carboxylic and two dodecylamide groups form highly-oriented aggregates in benzene, which interact with a cationic achiral dye to produce extremely strong exciton coupling around the absorption band of the dye.

ISSN:0267-8292    

DOI:10.1080/02678299508036596

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A series of 1-(4-bromophenyl) 2-(4′-n-alkoxy-2,3,5,6-tetrafluorobiphenyl-4-yl) acetylene has been prepared. Polarizing microscopic textural observation shows that they are nematic and smectic A liquid crystals.

ISSN:0267-8292    

DOI:10.1080/02678299508036597

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two homologous series of mesogenic materials which incorporate a 1,4-tetrafluorophenylene moiety and a terminal cyano- or nitro-substituent have been synthesized by using palladiumcatalysed coupling procedures. The compounds exhibit nematic and/or smectic A phases.

ISSN:0267-8292    

DOI:10.1080/02678299508036598

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε1, ε2and ε3were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε2- ε1) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036599

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Broadband dielectric spectroscopy in the frequency range from 10−4to 109Hz was employed to study the molecular dynamics of various triphenylene and benzene derivatives in their isotropic and liquid crystalline phases. For all derivatives, two relaxation processes were detected in their columnar mesophases. The β-process shows an Arrhenius-like temperature dependence with activation energies between 37 and 40 kJ mol−1. This process is assigned to a librational motion of the ester groups in the side chains. The β-process follows a Williams-Landel-Ferry temperature dependence and is correlated to the glass transition. This process is assigned to the hindered rotation of the molecular discs around the columnar axis. In the isotropic phase, the dielectric properties of the benzene derivatives are essentially different from the triphenylenes, most probably because of the difference in capability of conformational rearrangement.

ISSN:0267-8292    

DOI:10.1080/02678299508036600

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The conformation of the main-chain (backbone) of a combined main-chain/side-chain liquid crystalline polymer has been qualitatively determined by small angle neutron scattering in the oriented nematic, the smectic A and the smectic C phases. The polymer backbone presents only a weak anisotropy, of prolate shape, in the nematic and the smectic C phases. A stronger reorientation of the backbones in the direction of the applied magnetic field is measured for the SAphase. However, this anisotropy remains small compared to the stretching of a main-chain liquid crystal polymer and the smectic structure results apparently from side-chain ordering. On the other hand, hydrodynamic measurements show that the combined polymer, in solvent, is as flexible as a polystyrene chain. This result is compatible with an explanation for the weak observed anisotropy.

ISSN:0267-8292    

DOI:10.1080/02678299508036601

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Backbone anisotropy and the structure of the mesophases of a series of side-chain liquid crystal polymers have been studied in the bulk by neutron scattering. The backbone conformation is obtained by small-angle neutron scattering on mixtures of hydrogenous polymers with deuteriated backbones. The components of the radius of gyration parallel,R⊥and perpendicular, R ∥ to the magnetic field are determined as a function of temperature for both the nematic phase and the smectic phase. It is shown that the polymer backbone is deformed in both phases. When the polymer exhibits only a nematic phase, it adopts a prolate conformation, where the average backbone direction is more or less parallel to the aligned mesogenic groups. Upon transition from the smectic phase to a nematic phase, the backbone in the nematic phase assumes a slightly oblate shape. This tendency towards oblate shape is due to the smectic fluctuations which are always present in such nematic phases. The exentricity of the oblate backbone conformation in the smectic phase is always larger than in the nematic phase. This is attributed to a periodic distribution of the backbone between the mesophase layers. Then, the backbone anisotropy depends not only on the smectic structure (SA1, SAd), but also on the temperature dependence of the density of aligned mesogenic groups in the layers. On the other hand, it is shown that the isotopic mixtures are no longer ideal when polymers deuteriated in the mesogenic moieties are mixed with the corresponding hydrogenous polymers.

ISSN:0267-8292    

DOI:10.1080/02678299508036602

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Three examples of a new series of carbohydrate liquid crystals incorporating a coumarin unit are reported. A steric effect influencing their smectic A phase formation is discussed in comparison with structurally unaffected amphiphilic carbohydrate-based mesogens.

ISSN:0267-8292    

DOI:10.1080/02678299508036603

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor