摘要:

A thorough numerical analysis of a recently proposed cubic bond orientational model for Blue Phase III (Longa, L., and Trebin, H.-R., 1993,Phys. Rev. Lett.,71, 2757) is presented. In addition to the standard quadrupolar tensor fieldQαβ(r), describing the cubic space groups of BPI and BPII, a spatially constant fourth-rank hexadecupolar tensorB4αβγδof cubic point group symmetry is used to describe a cubic bond orientational order. While in BPI and BPII both order parameters are present, in BPIII only the hexadecupolar tensor is non-zero. Hence, BPIII is viewed as a phase of long-range cubic order. Within this model distinct phase diagrams are computed up to four stars ofk-vectors, the elements of a star being related by the point group symmetry operations. In particular, it is possible to account for some of the details found experimentally, such as the dominance of BPI over BPII for high chiralities. If, however, the artificial body centred cubic structure O5is being made metastable, then BPI also vanishes from the phase diagram.

ISSN:0267-8292    

DOI:10.1080/02678299608032830

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Compounds with two mesogenic groups connected by a hexamethylene chain and terminal butyl groups were synthesized by the Wittig reaction from the newly described bis-Wittig salts 1,6-bis[4-(triphenylphosphoniomethyl)benzyl]hexane dichloride (1) or 1,6-bis[5-(triphenylphosphoniomethyl)-2-thienyl]hexane dibromide (2) and various aromatic aldehydes. All derivatives containing two or three vinylene units and three or four aromatic rings in each mesogenic group have liquid crystalline phases. These aromatic rings are 1,4-disubstituted benzene and/or 2,5-disubstituted thiophene rings. The transition temperatures of these compounds increase with increasing numbers of benzene rings in each core. All-E-1,6-bis{4-[4-(4-butylstyryl)styryl]phenyl}hexane (3) (which is one of the few examples of conventional rod-like liquid crystalline materials without any hetero-atoms) has the highest transition temperatures compared to all the other substances. The formation of smectic phases of a high order is dependent on the number of thiophene rings in each core. Therefore compounds with two thiophene rings and one benzene system in each core (8, 14, 15and16) normally have only nematic phases. The only exception is all-E-1,6-bis{4-[5-(5-butyl-2-thienylvinyl)-2-thienylvinyl]phenyl}hexane (7) which shows a smectic phase both on heating and cooling the sample. Most of the compounds with two benzene rings and one thiophene ring form smectic phases. It is obvious that the formation of a smectic phase is more probable if the mesogenic group is connected to the hexamethylene spacer by a benzene than by a thiophene system (see for example4and7). For compounds3and6, X-ray studies have been made. The compounds with four aromatic rings in each core show only nematic phases. They decompose either in the mesophase or the isotropic phase.

ISSN:0267-8292    

DOI:10.1080/02678299608032831

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In this paper we use the second virial approximation to study the nematic-isotropic (N-I) transition for a Gay-Berne liquid for a broad range of parameters. For Gay-Berne hard Gaussian overlap (HGO) fluid particles we found that a N-I transition exists as a function of density for all length to breadth ratiosk> 4, in reasonable agreement with other theoretical studies and results of Monte Carlo simulations. For the full Gay-Berne potential (GB) the location of the N-I transition as a function of density for different temperatures was studied for several values of shape and energy anisotropy parameterskandk′. It was shown that the transition temperature increases or the transition density decreases with increasingkand/or decreasingk′. Wherever the molecular dynamics (MD) or Monte Carlo (MC) data were available, comparison was made. For each system, coexistence density and pressure were calculated, and, wherever possible, also compared with MC or MD data to show qualitative agreement. The reported study is considered to be a first step in the calculation of N-I transition for rigid liquid crystal (LC) molecules.

ISSN:0267-8292    

DOI:10.1080/02678299608032832

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Results are presented of studies on the low frequency (f' 250 kHz) non-linear dielectric effect on approaching the nematic and smectic A phases in 4′-cyano-4-n-alkylbiphenyls (nCB, for n = 6, 7, 8, 10, 11, 12). In all cases, the mean-field behaviour was valid up to the clearing temperature. In the case of the isotropic-nematic transition (6CB, 7CB, 8CB), a quantitative agreement with the relation based on the Landau-de Gennes model was also ascertained. The specific feature of the research method applied was that the characteristic time introduced by the measurement field (T′ ≊ 1/f) was always greater than the relaxation time τ, i.e. the condition: (T′/τ)>1 was satisfied up toTc.

ISSN:0267-8292    

DOI:10.1080/02678299608032833

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A range of chiral biphenylcyclohexanes of high enantiomeric excess has been prepared by asymmetric synthesis. These materials were designed as chiral dopants for ferroelectric mixtures based on fluoro-substituted host materials. Accordingly, fluoro substituents were strategically incorporated into the aromatic core of the chiral biphenylcyclohexanes in order to determine their effect on the spontaneous polarization of the ferroelectric mixtures. Chiral hydroboration was used to generate the chiral cyclohexane units which were attached to the core by using palladium-catalysed cross-coupling reactions with arylboronic acids; the synthetic methods used are discussed. The spontaneous polarization of the chiral materials was evaluated in H1 host mixture and the results are reported and discussed in comparison with the non-fluoro-substituted analogues.

ISSN:0267-8292    

DOI:10.1080/02678299608032834

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Mesogenic butadiene-tricarbonyliron complexes6and 7 have been obtained in optically active forms and they exhibit columnar, cholesteric, smectic A and smectic C properties. Both smectic mesophases can potentially be used in switchable bistable electro-optic devices and all the mesogenic enantiomers can act as dopants by inducing ferroelectric properties when dissolved in a non-chiral smectic A or C phase.

ISSN:0267-8292    

DOI:10.1080/02678299608032835

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0267-8292    

DOI:10.1080/02678299608032819

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor