摘要:

The dynamics of liquid crystal systems, consisting of either small molecules or polymer chains with main-chain or side-chain mesogenic groups, can be investigated using the same spectroscopic techniques which are applied to isotropic systems. This article briefly reviews1H,13C and2H N.M.R. and E.S.R. spectroscopy as well as dielectric relaxation. The particular features related to the oriented nature of the liquid crystal phases are developed. Typical examples of studies performed on small-molecule liquid crystals using these various spectroscopic techniques are described.

ISSN:0267-8292    

DOI:10.1080/02678298808086346

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Pretransitional effects in the isotropic phase of liquid crystals are manifest in the N.M.R. spectra of solutes recorded in the presence of an applied electric field. The temperature dependence of the quadrupolar splittings ofp-xylene-d10in the isotropic phase of the nematogen 4-n-pentyl-4'-cyanobiphenyl is investigated, and proves to be as expected from the Landau-de Gennes theory for pretransitional effects. The influence of temperature in the biphasic region of the phase diagram is very striking: the orientational ordering of the solute is temperature independent. The applied electric field not only determines the ordering of the solute, but also has a clear effect on the transition temperature. Electric field N.M.R. of solutes to study pretransitional effects is an alternative to the investigation of the nematogen itself.

ISSN:0267-8292    

DOI:10.1080/02678298808086347

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Ultraschalluntersuchungen an 4-Hexyloxyphenol-4'-Methoxybenzoat bestätigen für diese Verbindung eine kritische Absorption in der nematischen Mesophase. In der untersuchten chemischen Verbindung ist die kritische Absorption von Ultraschallwellen mit der Phasenumwandlung, nematische Mesophase—isotrope Flüssigkeit verbunden. Die charakteristischen Größen für den Prozess der kritischen akustischen Relaxation, Frequenz und Relaxationszeit, wurden errech-net. Sie wurde mit der Frequenz und Relaxationszeit im Fall der dielektrischen Relaxation verglichen. Die Beschreibung der Frequenz und der akustischen Relaxationszeit unter Anwendung von kritischen Exponenten wird beschrieben.

ISSN:0267-8292    

DOI:10.1080/02678298808086348

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Small-angle X-ray scattering curves, CD. spectra and textures of the liquid-crystalline phase formed from poly(I) · poly(C) in water-salt solutions containing poly(ethyleneglycol) have been obtained at different temperatures. It was found that heating the poly(I) · poly(C) liquid-crystalline phase is accompanied by two types of transitions, the first being cholesteric → compensated structure → cholesteric and the second being cholesteric → isotropic state transition. The latter transition takes place at a temperature which corresponds to that of separation of chains of a double-stranded poly(I) · poly(C) molecule. The temperature dependence of the abnormally high optical activity of poly(I) · poly(C) liquid crystals allows us to estimate the order parameter of poly(I) · poly(C) molecules in both the liquid-crystalline phase and the liquid-crystalline microphases.

ISSN:0267-8292    

DOI:10.1080/02678298808086349

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A molecular theoretical derivation of the Landau free energy of chiral smectic C liquid crystals is presented on the basis of a molecular theoretical approach. The excess free energy concerned with the ferroelectric helicoidal structure is derived within a mean field approximation. Several material constants such as the Curie temperature, the elastic constant, the helical pitch, the piezo- and flexoelectric constants and the dielectric susceptibility are expressed in terms of some effective intermolecular interaction parameters between molecules. A molecular theoretical consideration of the chiral smectic C-smectic A transitions is given in comparison with a few previous works. By comparing with some available experimental data forDOBAMBC, effective interaction parameters are numerically estimated.

ISSN:0267-8292    

DOI:10.1080/02678298808086350

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Binary mixtures are composed of compounds belonging to the 4'cyanobiphenyl-4-yl 4'-n-alkylbiphenyl-4 carboxylate homologous series and halogenophenyl 4-(trans-4-n-decylcyclohexyl)benzoate (10XPCHB) have been studied. Compounds CBnAB are smectic A1forn≤ 7 and smectic Adforn≥ 8; compounds 10XPCHB are smectic A, only. It was found that the SAdphase of compounds CBnAB exhibits ideal miscibility with the SA1phase of compounds 10XTCHB, and that the SA1phases of both compounds show abnormal behaviour. The SA1phase of compounds CBnAB is destabilized and the range of the SA1phase is enhanced on the side of 10XPCHB. The reasons for this behaviour are discussed.

ISSN:0267-8292    

DOI:10.1080/02678298808086351

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

We consider the nematic-isotropic transition temperature for a rigid oligomer formed by linking identical mesogenic units together. In particular, we seek to relate the transition temperature for the oligomer to that of the constituent units, within the framework of the Maier-Saupe molecular field theory and its various parameterizations.

ISSN:0267-8292    

DOI:10.1080/02678298808086352

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The handedness of lyotropic cholesterics formed by decylammonium chloride, ammonium chloride, water and chiral dopants was determined by circular and linear dichroism and, independently, by circular intensity differential scattering. From the study of the twisting power of a few dopants and from the magnetic orientation of cholesterics with long and short pitches, it is concluded that distortion of the micelles is likely to be determinant for the formation of the cholesteric phase.

ISSN:0267-8292    

DOI:10.1080/02678298808086353

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The crystal structure of this compound, a substance which exhibits smectic meso-morphism, is triclinic,P1space group, with lattice constantsa= 11.280(5) Å,b= 13.598(1) Å,c= 25.650(2) Å, α = 97.85(1) Å, β = 96.76(3)° and γ = 100.37(3)°. Molecules with two different conformations are present in the unit cell, and the heptyl and hexyl side-chains are extended nearly parallel to each other. The molecules are packed together in layers with their major axes inclined at 60° to the layer plane. The observed structure explains why the liquid crystal mesophases of this and related compounds are not discotic.

ISSN:0267-8292    

DOI:10.1080/02678298808086354

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

This paper reports the results of the measurement of refractive indices and densities ofp-ethoxyphenyl trans-4-butyl cyclohexane carboxylate at different temperatures. The molecular polarizabilities have been calculated from refractive indices using Vuks' formula and orientational order parameters are determined from the polarizability values. The order parameter values have been compared with those obtained from Maier-Saupe theory. The compound shows anomalous behaviour regarding variation of density, ordinary refractive index and order parameter values with temperature. This is consistent with our previous X-ray study of this compound. The possible reason for this peculiar behaviour has been discussed.

ISSN:0267-8292    

DOI:10.1080/02678298808086355

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor