摘要:

Several series of phasmids and biforked mesogens or more generally of polycatenar mesogens, i.e. rod-like core mesogens ending in six, five, four or three aliphatic chains (respectively hexa-, penta-, tetra- and tricatenars) are reviewed. According to the number and positions of the aliphatic chains, lamello-columnar, columnar, cubic, smectic and nematic phases appear.

ISSN:0267-8292    

DOI:10.1080/02678299308026292

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The nematic director alignment in the liquid crystal cell was reversibly regulated by the geometrical isomerization of α-hydrazono- β-keto esters chemically linked on the substrate surface. Two methods for the surface modification were employed successfully. Method I was based on direct modification of substrates with α-hydrazono-β-keto ester units having a silylating group. Cells fabricated with these substrates showed altering reversible liquid crystal alignment through a combination of photoirradiation and heating. In method II, ω-acryloyloxyalkyl α-hydrazono-β-keto ester units were introduced through Michael addition to substrates modified with 3-aminopropyltriethoxysilane. The liquid crystal alignment in this case altered reversibly according to the reversible photoisomerization of the α-hydrazono-β-keto units on the substrates. The results strongly support the observation that a change in molecular shape, due to geometrical isomerization, is responsible for regulating the nematic director alignment.

ISSN:0267-8292    

DOI:10.1080/02678299308026293

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A series ofN, N-di-n-alkyl-N,N-dimethylammonium bromides was synthesized and characterized. Its thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. A novel smectic mesophase was thus identified, in which the lateral packing of the molecules within the layers is ordered and tetragonal in symmetry.

ISSN:0267-8292    

DOI:10.1080/02678299308026294

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The mesogenic behaviour of a series of thirty-one 1-alkyl-4-(or 2-)alkylpyridinium salts and of a homologous series of four 1-methyl-4-n-alkoxycarbonylpyridinium iodides is described. The occurrence and stability range of the thermotropic phases depend dramatically on the structure of the surfactants. Mesophases are only observed if the 1-alkyl group is methyl. Furthermore, an unusually strong dependence of the mesogenic behaviour on the number of carbon atoms in an unbranched alkyl chain is found. Alkyl chain branching, especially close to the headgroup, in 1-methy1-4-(C12-alky1)Pyridinium iodides lowers the clearing point and, to a lesser extent, the melting point. Apparently, the mesophase is destabilized due to the lack of space to accommodate a protruding side chain. The mesophases of the unbranched as well as the branched compounds are identical to the smectic A phase found for carbohydrate mesogens with one alkyl chain (smectic Ad). The mesophase of the peg-shaped compound 1-methyl-4-(17-tritriacontyl)-pyridinium iodide could not be assigned with certainty. Based on molecular shape, textural characteristics and the observed miscibility with the columnar hexagonal phase of a non-ionic carbohydrate amphiphile, the compound most likely exhibits a Dhdphase. However, the single reflection, corresponding to ad-spacing of just over one molecular length, suggests a smectic phase. It is assumed that all compounds described belong to the general class of amphiphilic mesogens. 1-Methyl-4-n-alkoxycarbonylpyridinium iodides are visibly thermochromic, whereas the corresponding 4-alkyl derivatives are not. This probably stems from a shift of the chargetransfer absorption band to higher wavelengths, induced by the presence of the ester group in conjugation with the pyridinium ring. Lyotropic liquid-crystalline phases of some surfactants were also examined. There is no apparent correlation between the occurrence of thermotropic and lyotropic phases in individual compounds. The types of lyotropic phases observed are dependent on the overall shape of the anhydrous molecule, but, surprisingly, it was not always possible to correlate the mesophase behaviour at maximum hydration with the type of aggregate formed in dilute solution.

ISSN:0267-8292    

DOI:10.1080/02678299308026295

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The modes of both the director configuration and the electrical field in dielectric domain stripes are found for a pretilted supertwisted nematic layer. Good agreement between the calculated data and the reported experimental data is obtained. The domain voltage range as well as the threshold voltage can be calculated.

ISSN:0267-8292    

DOI:10.1080/02678299308026296

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Small amplitude oscillatory shear has been used to study thermotropic liquid-crystalline polymers that have mesogenic groups pendant to flexible backbones. The polymers studied form nematic and smectic glasses, enabling viscoelastic response to be studied over a wide range of frequencies using time-temperature superposition. In contrast to main chain liquid-crystalline polymers, the nematic side chain polymers exhibit linear viscoelastic response over a wide range of strain amplitudes that is independent of thermal and shear histories. Viscoelastic response is very sensitive to smectic-nematic and smectic-isotropic transitions, but insensitive to the nematic-isotropic transition, as time-temperature superposition applies across this transition. We compare viscoelastic data with diffusion data by calculating the time τ that it takes a polymer to diffuse a distance equal to its coil sizeR(τ=R2/D). At frequencies lower than 1/τ side chain polymers in their nematic show the terminal response characteristic of viscoelastic liquids. In their smectic, they are still strongly viscoelastic at frequencies lower than 1/τ and approach the terminal response of a viscoelastic solid at the lowest frequencies. Implications of such behaviour are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308026297

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

First examples of amphiphilic alkyl pentakis(phenylethynyl)benzene ethers containing functional groups at the terminal position of their alkoxy chains were synthesized by etherification of pentabromophenol with 11-bromoundecan-1-ol and 11-bromoundecanoic acid ethyl ester, respectively, and subsequent palladium catalyzed, fivefold coupling reactions with the substituted phenylacetylene. Two undecanoic acid derivatives with different pentakis(phenylethynyl)phenoxy substituents in the ω-position were prepared by ester cleavage from the corresponding ethyl esters. The thermal behaviours of five penta-alkynes in their pure states, as well as of their charge transfer complexes formed from mixtures with 2,4,7-tri-nitrofluorenone, were examined.

ISSN:0267-8292    

DOI:10.1080/02678299308026298

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The cyclopalladated complex [Pd(L3)(μ-I)]2([Pd(L3)(μ -I)]2= bis-{5-(1-nonyl)-2{[4′(1-nonyloxy)phenyl-2′-ato]}pyrimidine-N′,C2′]-μ-iodo}dipaladium (II); C→105·1°C (C1)→128·1°C (SA)→202·5°C (I)) has been investigated by X-ray diffraction and the electron density profile along the director, in the smectic phase, determined. [Pd(L3(μ -I)]2is a mesogenic species with a lateral-lateral fused molecular shape whose main features are: (i) the solid C1phase when observed with a polarizing microscope gives a polygonal texture like a smectic A phase, and (ii) to describe properly the electron density profile along the director a model which takes into account a high electron density area (i.e. the four aromatic rings and the Pd2I2fragment) extending perpendicular to the director is required.

ISSN:0267-8292    

DOI:10.1080/02678299308026299

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The conformations adopted by the alkyloxy chains in the α,ω-bis(4-phenyloxy)alkanes (PONOP) with 2, 3 and 4 methylene groups in the alkyl chain have been investigated for an isotropic solution in acetone for the compound withN= 2, and for samples dissolved in three nematic solvents for all three compounds. The energy difference,Ecctg, betweengaucheandtransconformers produced by rotation about the C-C bond in PO2OP dissolved in acetone was derived from the scalar coupling constants obtained by analysing the proton spectrum from molecules containing a13C atom at one Cαposition. This gaveEcctgin the range-0·58 to 0·49 kJ mol−1. The deuterium NMR spectra of fully deuteriated samples of the PONOP dissolved in the nematic solvents were used to obtain local order parameters from the quadrupolar splittings. These were compared with values calculated by the mean field method of Emsley, Luckhurst and Stockley (ELS) to give values ofEcctgin the liquid-crystalline phases. The values obtained forEcctgchange from being negative for PO2OP, through zero for PO3OP to 2·5 kJ mol−1for PO4OP. The deuterium NMR data obtained previously for the mesogenic molecules α,ω-bis(4-cyanobiphenyloxy)alkanes in their nematic phases have been re-interpreted and it is shown thatEcctgin these compounds is also strongly dependent on chain length.

ISSN:0267-8292    

DOI:10.1080/02678299308026300

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Novel liquid crystals containing a siloxy chain as an end tail group instead of an alkyl chain were synthesized. The substitution effects were studied for ferroelectric liquid crystal materials. It was found that the temperature range for the chiral smectic C phase was reduced and shifted to lower temperature in comparison with the analogous alkyl chain derivatives. The crystallinity of the siloxy chain derivatives decreased and cholesteric phases were not observed. The influence of siloxy chains on ferroelectric liquid crystal properties, especially spontaneous polarizations and tilt angles, also greatly depended upon the mesogenic group structure. The X-ray diffraction results showed that the end tail group occupied a larger thickness in the chiral smectic C layer for the siloxy chain derivative than that for the alkyl chain derivative.

ISSN:0267-8292    

DOI:10.1080/02678299308026301

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor