摘要:

Previous studies using dideuterium as a solute have demonstrated the importance for orientation of the interaction between the solute molecular quadrupole moment and the average electric field gradient present in liquid crystals. With the aim of learning about additional orientational mechanisms, we have studied the temperature dependence of the2H-NMR spectra of the liquid crystal 5CB-d19as the solute in three liquid crystal mixtures: 55 wt % 1132/EBBA, 56·5 wt % 1132/EBBA and 70 wt % 5CB/EBBA. In these mixtures, the contribution from the environment to the average electric field gradient at the2H nucleus of dideuterium is zero. The spectra of 5CB-d19in the mixtures 55 wt % 1132/EBBA and 56·5 wt % 1132/EBBA are equivalent, but are different from those in 70 wt % 5CB/EBBA. The spectra of 5CB in 55 wt % 1132/EBBA and 70 wt % 5CB/EBBA are analysed using two different models for the short range potential, and param eters of the models are used to compare the potentials in the different mixtures. For a given spectral splitting of the chain C1deuteron, the reduced short range potential is the same in all three mixtures studied. The spectral differences observed are a consequence of different nematic-isotropic phase transition temperatures combined with the effect oftrans/gauche-isomerization in the hydrocarbon chain.

ISSN:0267-8292    

DOI:10.1080/02678299408036733

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We describe and analyse in this paper the behaviour of solutions of side-chain liquid crystalline polymers (LCP) in molecular compounds in their smectic A phases. We give evidence for a straight correlation between phase separation and the difference in smectic layer spacings of the components of the binary system. In addition, making use of materials containing fluorinated segments, we show that fluorocarbon moieties greatly enhance the incompatibility of the mixture.

ISSN:0267-8292    

DOI:10.1080/02678299408036734

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We report the NMR study of the liquid crystalline phases of aqueous solutions of the dye benzopurpurin (BP-4B). Upon changing dye concentration, the system exhibits a phase transition between two ordered phases at about 3·5 wt %. The structure of these phases was not determined but the evidence suggests that they consist of columns or helically twisted columns of stacked dye molecules which are randomly oriented, similar to nematic phases.

ISSN:0267-8292    

DOI:10.1080/02678299408036735

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The effect of adding salts NH4OH and CsOH to aqueous solutions of NH4PFO and CsPFO, respectively, was studied for concentrations in the range 0 to 2 wt %. In both systems, the isotropic to nematic and nematic to lamellar phase transition temperatures were elevated. There was an insignificant effect on the width of the nematic range in the NH4PFO system, while the nematic phase was widened by as much as ∼3°C in the case of CsPFO. High-resolution X-ray scattering measurements were performed to determine the micellar size and its dependence on temperature for 52 wt % NH4PFO solution in water.

ISSN:0267-8292    

DOI:10.1080/02678299408036736

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Following studies concerning mesomorphic heteroaromatic salts (pyrylium, thiopyrylium, dithiolium), we present here new thermotropic liquid crystal materials based on 1,5diazapentadienium (vinamidinium) salts and on the pseudocyclic vinamidine system (1,5-diazapentadiene). The mesomorphic properties of these systems have been studied by microscopic observations, DSC and X-ray diffraction.

ISSN:0267-8292    

DOI:10.1080/02678299408036737

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The selective synthesis and the thermal behaviour of somecis, cis-(3,5-dihydroxycyclohexyl) 3,4-(alkenyloxy, alkyloxy)benzoates (monofunctionalized ‘two chain’ diols) are described. Thus, several 3-(alkyloxy)-4-(undecenyloxy)benzoic acids and 4-(decyloxy)-3-(undecenyloxy)benzoic acid have been obtained. The monofunctionalized ‘two chain’ diols form a hexagonal columnar mesophase through hydrogen bonding. Subsequently, low molecular weight liquid crystalline siloxanes, model compounds for polymers, i.e. two twins and one cyclic product, were synthesized via a hydrosilylation reaction. With respect to the ‘two chain’ diols, the observed hexagonal columnar mesophase was stabilized and the intercolumnar distance was extended by the siloxane moieties. Remarkably, the thermal behaviour of the cyclosiloxane differs from that of the twins. A cubic mesophase, which can be observed very rarely in thermotropic mesogens, was formed at temperatures below the hexagonal columnar phase.

ISSN:0267-8292    

DOI:10.1080/02678299408036738

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Anchoring interaction of MBBA and MBBA + 5CB nematic layers with monomolecular films of CTAB on ITO glass supports is studied by videomicroscopy in the presence of an electric field. Planar-planar or homeotropichomeotropic electrode substrates are used to make the nematic cells. The substrate symmetry is broken by coating only one electrode with self-assembled CTAB film. In a DC electric field we observe some new effects; (i) polarity-dependent breaking of anchoring and switching to two oblique states in dielectrically stable planar cells; (ii) a polarity-dependent flow-induced metastable anchoring transition in homeotropic cells to a planar or tilted alignment after the field is switched off. These results are discussed in terms of a surface transition assisted by electric transport of biphilic CTAB ions and by a surface memorization of the flow-induced planar alignment.

ISSN:0267-8292    

DOI:10.1080/02678299408036739

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299408036740

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A new series of chiral homologous thiobenzoates with non-chiral end chains ranging from heptyloxy to octadecyloxy has been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy on pure compounds and mixtures, DSC, and electro-optical and pitch measurements. They display a very rich polymesomorphism including SA, S*Cα, S*C, S*CFI, S*CAphases with reference to the compound MHPOBC through miscibility tests. Peculiarly noticeable are: (i) two S*CFIphases sandwiched between S*Cand S*CAphases in some compounds; (ii) full miscibility between the S*CAphase and the S*Ophase of the racemic reference compound MHTAC which leads us to question whether the S*CAor S*Osymbol should be used to designate the antiferroelectric smectic C* phase; (iii) odd-even cancellation of the S*CAphase in relation to both the chiral chain and the alkyloxy chain; (iv) a reentrant sequence, C SCSCASCSAI, in a pure compound assessed by a phase diagram. The electro-optical measurements were carried out with the classical SSFLC geometry. The field threshold is determined for each titled mesophase. One can note: (i) a low threshold whatever the phase; (ii) a first plateau at 0·4 Vμm−1and a saturation at 1 Vμm−1in the S*CFIphase; (iii) strong pretransitional variations of the pitch at the S*CA-S*CFIand S*CFI-S*Ctransitions.

ISSN:0267-8292    

DOI:10.1080/02678299408036741

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In the literature it has been reported that the rigid cyanobiphenyl core of 4-n-alkyl-4′-cyanobiphenyl molecules deposited onto polyimide-coated substrates makes an average angle of ≈ 78° with respect to the surface normal, independent of the rubbing conditions applied and independent of the polyimide materials studied. A simple anchoring model based on a combination of dipole-dipole, van der Waals and steric interactions explains this angle.

ISSN:0267-8292    

DOI:10.1080/02678299408036742

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor