摘要:

Novel liquid crystalline 3,6-disubstituted cyclohex-2-en-1-ones and 1,4-disubstituted 2-chlorobenzenes are presented. The method of transformation of cyclohex-2-en-1-ones into chloroarenes in a one-pot procedure, together with the nature of the mesomorphic phases and some physical properties of single compounds and nematic mixtures comprising them are reported.

ISSN:0267-8292    

DOI:10.1080/02678299608032019

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

An elastic theory for Langmuir-Blodgett films is presented. The expression for the free energy density of the film is obtained in terms of the deformation tensor by means of symmetry considerations and by making use of a quasi-microscopic model. The influence of a linear term in the deformation tensor in the elastic energy density is analysed. It is shown that when the elastic constant of the linear term is larger than a critical value, the ground state of the film is periodically distorted.

ISSN:0267-8292    

DOI:10.1080/02678299608032020

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We analyse the effect of a Langmuir-Blodgett (LB) multilayer on the surface properties of a nematic liquid crystal (NLC). We show that the easy axis of the LB-NLC interface coincides with that of the LB-solid surface interface. On the contrary, the effective anchoring energy of the LB-NLC interface is lower than that associated with the LB-solid substrate interface. We show in a first approximation that the anchoring energy characterizing the NLC may be separated into three contributions: one connected with the interaction between the LB film and the solid substrate, one due to the direct LB-NLC interaction and the other one having an elastic origin. Nevertheless, to be more precise, one has to consider also the term associated with the interaction energy between the NLC and the substrate, which is screened by the LB film. The elastic contribution is of the order of the elastic constant of the LB film over the thickness of the multilayer. This quantity is estimated to be of the order of 10−2–10−1erg cm−2, as experimentally observed. Possible extensions of our model are also discussed.

ISSN:0267-8292    

DOI:10.1080/02678299608032021

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In order for a material to show a non-vanishing second order non-linear susceptibility, the material has to have a polar axis. This is hard to achieve in a liquid crystal system because of the strong quadrupolar order along the director. Electrostatic poling of polymers and polymer liquid crystals at high temperature and subsequent quenching to lower temperature only gives a small value of the polar order parameter and, in addition, results in materials which are not in thermodynamic equilibrium and therefore not stable over long times. Starting with a ferroelectric liquid crystal (which has polar order perpendicular to the director) we have succeeded in making a material with true polar order. It is not ferroelectric, although the ferroelectric properties of the starting and intermediate materials are basic for the procedure and for the final product which can be used for frequency doubling, of importance, for instance, in a wide area of optic communication applications.

ISSN:0267-8292    

DOI:10.1080/02678299608032022

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The dynamic properties of chiral smectic liquid crystals are studied theoretically by including competing interlayer interactions leading to the smectic C*αphase below the smectic A phase. In the smectic A phase only one doubly degenerate soft mode exists which becomes critical at the transition temperature for the critical wave vector corresponding to the helicoidal modulation extending over only a few smectic layers. In the tilted smectic C*αphase it splits into two modes, i.e. a phase and an amplitude mode.

ISSN:0267-8292    

DOI:10.1080/02678299608032023

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A synthetic route has been developed to a new series of homo- and co-polysiloxanes with a terminal chiral 2-(S)-butyl fragment attached to the mesogenic biphenyl core via the ketone binding group, >C˭O. The synthesis, structure and ferroelectric properties of the polymers are reported. All the polymers show in their SAphases an unusual decrease in the interlayer distance with increasing temperature.

ISSN:0267-8292    

DOI:10.1080/02678299608032024

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We have investigated the pretilt angle generated in the nematic phase of different liquid crystal materials aligned on side chain liquid crystalline polymer films. High pretilt angles (> 10 degrees) can be achieved by unidirectionally rubbing the liquid crystalline polymer layers. We have also measured the anchoring strengths of these aligning films. The results of our studies indicate that the pretilt angle and the anchoring energy are found to depend in a systematic way on the liquid crystal material and also on the chemical structure of the liquid crystalline polymer films.

ISSN:0267-8292    

DOI:10.1080/02678299608032025

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4′-carbonyloxy)-2″-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (SA), a twist grain boundary A (TGBA), and a chiral smectic C (SC*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a SAphase and a SC* phase. Moreover, a crystal E phase was observed in the short spacer chain (n= 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.

ISSN:0267-8292    

DOI:10.1080/02678299608032026

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Broadband dielectric spectroscopy (up to 109Hz) is employed to study the molecular dynamics of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) in the free bulk phase and confined in cylindrical channels of Anopore membranes having a diameter of 0.2 μm and length of about 60 μm. The bulk samples of 5CB orient almost homeotropically between the untreated metal electrodes of the measurement set-up, and two relaxation processes are observed: the slower δ-relaxation is assigned to hindered rotation (180° flips) of the molecules around their molecular short axis, and a faster second process is attributed to the tumbling of the molecules about this axis. In the confined 5CB samples, the membrane pores align the nematic director axially or radially depending upon their surface preparation. Planar (axial) alignment is always found in untreated membranes, whereas radial alignment was achieved by treatment with decanoic acid. Consequently the director field is fixed perpendicular or parallel to the electric field and we are able to study each of the two relaxation processes separately by appropriate surface treatment of the pores. The frequencies of both processes are found to be unchanged with respect to the bulk phase. We extract the frequency dependence of the dielectric anisotropy δε from the dispersion curves of ε∥ and ε⊥. Two changes of sign of δε = (ε∥–ε⊥) are detected as predicted in the literature.

ISSN:0267-8292    

DOI:10.1080/02678299608032027

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The newly synthesized palladium complex [(Azo-6)Pd(μu-C1)]2,1, [H(Azo-6)= 4,4′-hexyloxyazobenzene] has been investigated by optical observations, calorimetric measurements, and single crystal and low angle variable temperature X-ray diffraction techniques. Two different mesophases appear from 210 to 220°C (nematic) and above 220 until 235°C (smectic E) where the compound eventually decomposes. The nematic phase is not in thermodynamic equilibrium and when subjected to an annealing process transforms into a smectic phase. The molecular structure of the dinuclear cyclopalladated complex1has an intramolecular Pd-Pd distance of 3.528(1) Å. Moreover the molecules of1are arranged in pairs with an intermolecular Pd-Pd non-bonded interaction distance of 3.668(1) Å. On the basis of this structural feature and of the variable temperature low-angleXRDstudies, transitions involving dissociation of the molecular pairs (e.g. N(pair)⇌E(single), on heating; N(pair)→SE(pair), at constant temperature) are suggested.

ISSN:0267-8292    

DOI:10.1080/02678299608032028

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor