摘要:

A part of homotopy theory is applied to classify planar structures in chiral nematic liquid crystals confined to cylindrical cavities. The resulting classification is exact in the approximation of undeformed chiral nematic surfaces. Within this approach the relative stability of possible planar structures with surface and bulk disclination lines is discussed. The number and the shape of these disclinations, which in some cases form spiral structures, are predicted. Further approximate analytical expressions for non-singular director fields close to disclination lines with integral strength are introduced. Our predictions, which are also in agreement with some previously suggested pictures of such director fields, are used to improve stability considerations of the confined planar chiral nematic structures in tubes and droplets.

ISSN:0267-8292    

DOI:10.1080/02678299308027709

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Measurements of complex electric permittivity of room temperature ferroelectric liquid crystal mixtures have been made on aligned samples with the electric measuring field being parallel to the layer planes. The spontaneous polarization, the tilt angles and pitch have been measured in these mixtures. By theoretical fitting of the experimental points of electric permittivity for the Cole–Cole modification of the Debye equation dielectric parameters, the dielectric strength, relaxation frequency, and distribution parameter for the Goldstone mode have been computed. The dielectrically observed Goldstone mode in our mixtures is shown to have both DC bias field and AC field dependences.

ISSN:0267-8292    

DOI:10.1080/02678299308027710

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Time-resolved FT-IR spectroscopy was performed to monitor the behaviour of ferroelectric liquid crystals on a microsecond time scale. A bookshelf geometry was obtained by alignment with SiO obliquely evaporated layers in a CaF2cell with 2 μm gap. Six kinds of FLC mixtures were examined, all of which were three-component systems consisting of chiral molecules of 5 wt% and two kinds of phenyl pyrimidine molecules of 47·5 wt% each. All IR spectra were dominated by vibrational modes from the phenyl pyrimidines. Typical vibrational modes from core and alkyl chain parts are selected in order to compare the movement of the alkyl chain as a function of the chiral components. A novel parameter α = (intensity of largest peak at 2925 cm−1/intensity of smallest peak at 2925 cm−1)/(intensity of largest peak at 1432 cm−1/intensity of smallest peak at 1432 cm−1) for evaluating the alkyl chain movement is introduced. It was found that the length of the rigid or flexible parts of the chiral molecules and the parameter α are correlated. Moreover, relations between the cone angle and α and between the rise time and α were found.

ISSN:0267-8292    

DOI:10.1080/02678299308027711

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for exampled22= 5 pm V−1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*cand in the glass state, are reported. Moreover their waveguiding properties are presented.

ISSN:0267-8292    

DOI:10.1080/02678299308027712

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm−2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.

ISSN:0267-8292    

DOI:10.1080/02678299308027713

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this work, a set of Landau–Ginzburg equations to investigate the dynamic properties of ferro- and antiferro-electric smectic phases is formulated on the basis of the elastic continuum theory of compressible smectics. In the present framework, the polarization electric field is consistently taken into account through the Poisson equation as seen in our previous work. As a practical application, a few numerical results are presented for the surface-stabilized geometry with inclined and chevron layer structures. An asymmetric bistable switching is found to be achieved in the chevron layer structure under an alternating external field. In an inclined layer structure, however, a symmetric switching is found to be possible. In addition, it is first presented from a theoretical standpoint that the compressible smectic layer structure may be drastically deformed in the chevron and inclined layer structures with a sufficiently large external field.

ISSN:0267-8292    

DOI:10.1080/02678299308027714

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The electric-field dependence of the dielectric constant of some materials which show a strong electroclinic effect in the SAphase, has been studied by measurement of the transitional dielectric constant. We suggest that the observed variation of the dielectric constant may originate from the electric field induced dielectric biaxiality; this is inherent in a material that exhibits the electroclinic effect.

ISSN:0267-8292    

DOI:10.1080/02678299308027715

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture ofS-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308027716

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Spatial dispersion effects in chiral liquid crystals are reviewed. New spatial dispersion phenomena are observed in the vicinity of the Bragg reflection wavelength–an anomaly of the refractive index and an optical anisotropy of the cubic blue phases. These effects can be explained by taking into account the spatial dispersion correction of the dielectric tensor of the medium of higher orders in the ratio of the light wave vector to the structural wave vector of chiral phases.

ISSN:0267-8292    

DOI:10.1080/02678299308027717

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Nematics MBBA, EBBA, PBBA, BBBA, 5CB, 8OCB and PCH5 doped with chiral 2-(4′-phenylbenzylidene)-p-menthan-3-one are studied. A numerical solution in terms of the Landau-de Gennes theory is obtained to describe the experimental temperature dependence ofP/P0, whereP0corresponds toPin zero magnetic field. The angle of tilt θ from the plane perpendicular to the helical axis is evaluated and the domain wall energy in a constant magnetic field analysed. To take into account the helical structure deformation, the helicoidal order parameterGis defined, so thatG= 0 in the N phase andG= 1 withdϕ/dz= const. and θ = 0 in the N* phase. The relaxation of the helical structure to the equilibrium state (N* → N) is shown to be determined by the domain wall energy with the magnetic fieldH≠ 0.

ISSN:0267-8292    

DOI:10.1080/02678299308027718

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor