摘要:

A range of nematogenic materials which incorporate a 2,6-disubstituted naphthyl moiety and a terminal cyano-substituent have been synthesized by using palladium-catalysed cross-coupling procedures involving arylboronic acids and alkynylzinc reagents with aryl iodides, bromides and trifluoromethanesulphonates (triflates). The compounds have very high nematic phase stability, but their melting points are also quite high. The birefringences were measured using an extrapolation technique and the values were found to be between 0·26 and 0·42.

ISSN:0267-8292    

DOI:10.1080/02678299308031946

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Samples of liquid crystalline poly(γ-benzyl-glutamate) solutions are sheared between glass surfaces with gaps,d= 10–500 μ, and shearing velocities,V= 0·05–10 000 μs−1so that the Ericksen numberE≡Vdγ1/Kis varied over a large range,E≈ 1–107. Here γ1is the rotational viscosity andK1is the Frank splay constant, with γ1/K1estimated to be approximately 1 s μ−2for our samples. We observe by polarizing microscopy a sequence of transitions with increasing Ericksen number analogous to that observed in small molecule tumbling nematics: namely rotation of the director out of the shearing plane and into the vorticity direction atVd≈ 25 μ2s−1, and formation of roll cells atVd≈ 50 μ2s−1. The roll cells become finer with increasedVdin accord with predictions of linear stability theory using the Leslie-Ericksen equations, and atVd≳ 500 μ2s−1, the cells become very irregular, producing director turbulence. The turbulence becomes finer in scale asVdincreases, reaching sub-micron, and possibly molecular scales whenVd≧ 105μ2s−1. At the highest velocities, transitions in orientation and texture are controlled by the Deborah numberDe≡λV/d, where λ is the molecular relaxation time, and uniform texture-free samples are obtained whenDe≳ 5.

ISSN:0267-8292    

DOI:10.1080/02678299308031947

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Qualitative phase diagrams were constructed using the contact method for binary mixtures of several chemically-distinct low molar mass nematogens (LMMN) with a main chain liquid crystal polymer (TPB-x) which has a mesogenic group, 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxy phenyl) butane, separated by flexible alkyl spacers of variable length,x.Several interesting effects were observed. TPB-xwas found to exhibit an odd-even variation in miscibility in the nematic state (2n+ 1 = miscible, 2n=immiscible) with 4′-pentyl-4-cyanobiphenyl (5CB), but not with 4′-pentyloxy-4-cyanobiphenyl (5OCB) in which most polymers were completely miscible. On prolonged isothermal annealing in the biphasic region in 5CB, TPB-2nexhibited spherulitic crystallization of the liquid crystal polymer. These observations are shown to be qualitatively consistent with a modification of the Flory-Huggins theory by Brochardet al.

ISSN:0267-8292    

DOI:10.1080/02678299308031948

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The geminallyC-branched 6,6-di-C-n-alkyl-d-galactopyranoses3a–dprepared in two steps from 1,2:3,4-di-O-isopropylidene-d-galacturonic acid methylester1and the even numbered C8-C12-alkylmagnesium bromides, represent a new group of ‘double tailed’ thermotropic mesogens forming columnar supramolecular structures based on hydrogen bridged (disc-like) multimers in accordance with the hypothesis of the Praefcke group. Furthermore, the thermal behaviour of the homologous 6-O-alkyl-d-galactopyranoses4a–eis discussed in connection with the revised model for the molecular arrangement in smectic A phases of ‘single tailed’ carbohydrates.

ISSN:0267-8292    

DOI:10.1080/02678299308031949

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

By means of high pressure thermal analysis we measured the pressure dependence of the clearing temperatures of nematic, 2-substituted hydroquinone-bis-(4-substituted benzoates) up to pressures of about 500 MPa. The clearing lines are strongly non-linear and can be well reproduced by a modified Simon-Glatzel equation. The gradientsdT/dpof the clearing lines decrease strongly with increasing pressure, indicating a pronounced decrease of the transition volume with increasing pressure. Compounds with long chain lateral substituents exhibit liquid crystalline behaviour because the lateral substituents tend to have an orientation parallel to the molecular long axes. We expected that the preferred orientation of the lateral substituents would be strengthened by high pressure, demonstrated by enhanced dT/dpvalues. However, despite the strong deviation of the compounds from the ideal rod-like shape, the magnitudes and the trend of the data are similar to those of classical nematics.

ISSN:0267-8292    

DOI:10.1080/02678299308031950

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence ofεand the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the NDphase. The strong temperature dependence of |k33/Δχ|, the large value ofk11/k33and the rise ink11/k33as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole correlations require that the molecules be tilted within the columns.

ISSN:0267-8292    

DOI:10.1080/02678299308031951

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The rearrangement of allylp-dimethylaminobenzenesulphonate (ASE) to form a zwitterionic product has already been recognized as an effective probe for the study of reactivity within the smectic B phase [4, 5, 19]. We have used deuterium NMR, linear dichroism and X-ray diffraction techniques to investigate the phase diagram of the ASE-OS35 reaction system. The partitioning of the reactant molecules between coexisting smectic, nematic and/or isotropic phases and the structural organization of the smectic catalytic host at different temperatures and reactant guest concentrations have been characterized. On the basis of these measurements, a model of ASE reactivity in smectic solvents has been developed. The reaction takes place provided that coexisting isotropic or nematic phases are present to act as a reservoir for the ASE reactant molecules prior to their entering the smectic phase; they then react and leave the smectic phase as a zwitterionic product. The analogy between this model of reactivity within smectic phases and the Michaelis-Menten enzyme processes is discussed. This relationship opens up the intriguing possibility of designing new experiments with which to investigate further liquid crystalline models of enzyme catalysis.

ISSN:0267-8292    

DOI:10.1080/02678299308031952

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Fluorine-19 NMR spectra have been recorded for both the phases (cholesteric and smectic C*) of the ferroelectric liquid crystal 2,3-difluoro-4-octyloxybiphenyl-4′(-4″-oxycarbonyl-(S)-1-chloro-3-methylbutyl)benzoate, using a conventional high-resolution NMR probe. The magnitude of the dipolar coupling constant,DFF0, was found to increase with decreasing temperature in the cholesteric phase with a sudden increase (of approximately 1 000 Hz) at the cholesteric-smectic C* transition, whereafter a more gradual change was observed. The order parameter, wasSFF, subsequently calculated for each temperature.

ISSN:0267-8292    

DOI:10.1080/02678299308031953

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structure and phase diagram of a binary mixture of a non-liquid-crystalline, mono-functional (meth)acrylate monomer and a liquid-crystalline compound having a cyano group-containing mesogen were examined. The monomers had mesogenic units such as biphenylene and phenyl benzoate, but did not show liquid-crystallinity. The liquid-crystalline compounds possessed a cyanobiphenyl or a cyanophenyl benzoate mesogenic unit. The liquid-crystalline 4-cyanophenyl 4′-n-octyloxybenzoate did not show a smectic A phase, while the binary mixture of the 4-butoxy-4′-(ω-methacryloyloxyhexyloxy)biphenyl monomer with 4-cyanophenyl 4′-n-octyloxybenzoate showed the induction of a smectic A phase in a nearly equimolar composition range. However, the binary mixture of the monomer, containing a phenyl benzoate group, and the liquid-crystalline compound did not induce a smectic phase.

ISSN:0267-8292    

DOI:10.1080/02678299308031954

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new method for the study of texture and director patterns in a main chain thermotropic aromatic copolyester with a flexible spacer is described, whereby the molecular chain or director orientation of the nematic mesophase becomes decorated by the formation of a banded texture during quenching, without being subjected to a shear. The pattern of the decorated banded texture may be observed directly by polarizing optical microscopy, revealing the complete texture of molecular chain orientation across the whole specimen. The molecular director orientation lies perpendicular to the long axis of the bands. Various types of disclination, including an inversion wall, in the nematic mesophase of a thermotropic aromatic copolyester have been observed. This decorating technique is particularly suited for non-crystallizable main chain liquid crystalline polymers, where the lamellar decoration technique fails.

ISSN:0267-8292    

DOI:10.1080/02678299308031955

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor