摘要:

The synthesis of a homologous series of amidic derivatives of azacyclic compounds, i.e. 3,4-dialkoxybenzoyl ('two chain') N',N',N''-substituted hexahydro-1,3,5-triazines with alkoxy chains-OCnH2n+1 having n = 5, 6, 8-14 and 16 carbon atoms, and of one corresponding amine derivative, i.e. 1,3,5-tris(3,4-didecyloxybenzyl)hexahydro-1,3,5-triazine, is described. The thermal behaviour of the compounds is characterized by means of polarizing microscopy and differential scanning calorimetry. The amide with n = 6 is a monotropic liquid crystalline material, whereas the amides with n = 8-14 and 16 exhibit enantiotropic mesomorphism. The amine derivative with n = 10 is not a thermotropic liquid crystalline compound. The optical textures of the mesogenic compounds are spherulitic. X-ray diffraction measurements for the amides with n = 8, 10, 12, 13 and 16 show evidence of a hexagonal columnar structure (Colh) in the mesophase. The lattice constants of the mesophase are compared with the values from CPK models.

ISSN:0267-8292    

DOI:10.1080/026782998205949

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A homologous series of siloxane based dimesogens was synthesized in racemic and enantiomeric form. The flexible part of the core is composed of a central hexamethyltrisiloxane unit and two polymethylene chains of variable length. Increasing the number of methylene units in the core promotes smectic phase behaviour whose polymorphism was studied by polarization optical microscopy. Transitions into metastable phases were detected and investigated by differential scanning calorimetry using different heating rates. Information about the crystallization tendency was obtained from annealing experiments by changing annealing temperature and duration. In contrast to short dimesogenic homologues which predominantly showed monomesogen-like behaviour, polymer-like behaviour was observed for the longer homologues. Small angle X-ray diffraction provided data on the temperature dependence of the smectic layer spacing of mesogens and their corresponding dimesogens linked by the siloxane spacer. No significant differences between the layer spacings of the two classes of compounds were detected which suggests chain folding in the dimesogenic compounds and a similar mesogenic orientation for the mesogens and dimesogens.

ISSN:0267-8292    

DOI:10.1080/026782998205958

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The basic structural differences between three thermotropic liquid crystals TB4A, TB7A and TB10A have been interpreted by analysing their Raman spectra. Three spectral regions, 925-1025, 1275-1475 and 1525-1650 cm-1, have been chosen in this context to incorporate their structural dissimilarity. The induced planarity of the backbone, as an effect of increasing chain length, has been studied by observing the changes in the respective Raman spectra of the three compounds. A tentative assignment of all the modes observed in the region 400-1700cm-1 is made in this context. The molecular conformations of the three compounds have been predicted in their solution states, and are compared with the molecular environment that exists in their liquid crystalline states. Structural disorder at the solid-SmG transition is discussed and the changes are incorporated systematically.

ISSN:0267-8292    

DOI:10.1080/026782998205967

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The structures of the cubic mesophases of a chiral and an achiral hydrazine derivative have been investigated by means of X-ray diffraction experiments. As seen from the phase diagram of the binary system, both cubic phases are incompletely miscible which can be explained by the different space groups of the cubic lattices. In contrast to the isotropic liquid, the cubic phase of the chiral compound exhibits a considerable optical rotation indicating phase chirality.

ISSN:0267-8292    

DOI:10.1080/026782998205976

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

First results of investigations of electro-optical properties of liquid crystalline (LC) dendrimers in solution are presented. Measurements of electric birefringence (Kerr effect) and dielectric polarization of first generation carbosilane dendrimers with different ester linked terminal mesogenic groups (cholesteryl, cyanobiphenylyl and 4-methoxyphenyl benzoate) have been carried out using dilute solutions in CCl4. The results show that the dielectric polarization is proportional to the second power of the electric field in accordance with Kerr law. The Kerr constants calculated are close to those of the low molar mass analogues of the corresponding mesogenic groups. Thus the electric birefringence of the LC dendrimer solutions is mainly determined by the electro-optical properties of their terminal mesogenic groups oriented in the electric field independently of the main chain.

ISSN:0267-8292    

DOI:10.1080/026782998205985

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Chiral oxovanadium(IV), copper(II) and palladium(II) beta-diketonates show a room temperature tilted columnar mesophase (rectangular P21). The dielectric response of these compounds was measured in the frequency range 10-2-109Hz. Three different relaxation processes have been observed in the columnar phase. One of them is only present in this mesophase and we suggest that it is a Goldstone-like mode, similar to the one present in the helical SmC* phases. In our case the presence of the helix was confirmed by circular dichroism studies.

ISSN:0267-8292    

DOI:10.1080/026782998205994

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this paper we report an accurate optical method for measuring the azimuthal director angle and the azimuthal anchoring energy for a nematic liquid crystal. The method is fully automated and provides a direct measurement of the azimuthal angle. The experimental procedure exploits the dependence of the reflectivity tensor on the surface director orientation. The measurement of the azimuthal angle does not require any knowledge of the optical parameters of the nematic material or of the substrate, and provides an absolute accuracy better than 0.2 in the whole range 0-360 and a sensitivity of 0.01. If the optical parameters of both the nematic sample and the substrate are known, the surface polar angle can also be obtained from the experiment.

ISSN:0267-8292    

DOI:10.1080/026782998206001

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The genetic algorithm (GA), written to allow automatic analysis of optical reflectivity data obtained from liquid crystal cells using the half-leaky guided mode technique, has been developed to the point where liquid crystal cells can be analysed successfully giving greater detail of optical parameters and director profile than yielded by any other technique. The technique models the liquid crystal layer as a set of discrete, independent sub-layers which can map out the variation of the director through the thickness of the cell. Given sufficient high quality data, it is now possible automatically and accurately to fit the parameters of a complete liquid crystal cell. Using this highly adapted GA, half-leaky guided mode optical reflectivity data from the nematic, smectic A and smectic C* phases of SCE13 in a surface stabilized ferroelectric liquid crystal have been fitted to reveal director profiles and optical parameters of the cell in each phase.

ISSN:0267-8292    

DOI:10.1080/026782998206010

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Instead of studying the cholesteric-smectic A transition from the analogy with superconductors, we present a direct investigation of the properties of this transition. The transition is of first order and a very simple relationship between the four basic lengths (correlation length, penetration length, layer spacing of the smectic structure, and pitch of the cholesteric structure) exists at the transition temperature. We calculate the wall energy separating a cholesteric region from a smectic region in two different configurations. For one of them, we recover the definition of the type I (positive wall energy) and type II (negative wall energy) smectics A. We do not find a specific value of the Ginzburg-Landau parameter kc which gives the distinction between the two types of smectics; kc is a function of the product of the perpendicular correlation length and the wave vector of the smectic density modulation. We recover the well known value from the superconductor analogy, kc 1/tick 2, only if this product is large and the rotation of the director inside the wall is small. For the second configuration, the wall energy is always positive and there is no analogy with superconductors. Finally, we discuss briefly two different possibilities of mixed phases.

ISSN:0267-8292    

DOI:10.1080/026782998206029

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report proton NMR experiments on the liquid crystal material N-(p-methoxybenzylidene) p-n-butylaniline (MBBA) at 100MHz in the temperature range 110-350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.

ISSN:0267-8292    

DOI:10.1080/026782998206038

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor