摘要:

We study the possibility of using hydrogen-bonding interactions to promote the stabilisation of phase biaxiality in nematic binary mixtures of oblate and prolate thermotropic mesogens. We extend Onsager's theory of the isotropic-nematic transition to allow for such selective associations among unlike species and we use it to calculate the phase diagram of binary mixtures consisting of hard spherocylinders and cut spheres. The results show that directional, short-ranged attractions between rods and discs strongly stabilise the biaxial nematic mixture against demixing and suggest that interactions of hydrogen-bonding type may provide an efficient mechanism for sustaining phase biaxiality in binary mixtures of real thermotropic nematogens. Preliminary Monte Carlo simulations designed to test such predictions are discussed.

ISSN:1058-725X    

DOI:10.1080/10587259808048442

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In the hopes of helping the discovery of a really biaxial nematic phase in the thermotropic liquid crystal family, we give a brief review on the experimental methods available for proving biaxiality in oriented or disoriented samples. We discuss the relative advantages and problems that arise with the biaxial tests based on the NMR techniques, X-ray and neutron scatterings, dynamic modes analysis, inspection of defects, or the determination of coupling constants as in conoscopy. Among these different methods, two, conoscopy and defects analysis, seem particularly useful since they may be really conclusive without excessive sophistications. They may therefore be recommended as tests for biaxiality. Conoscopy is most probably the safest way to prove biaxiality, but it needs perfectly oriented samples. In order to warrant that the samples are indeed correctly oriented, the conoscopic patterns should be observed with several entire and well-contrasted rings. Conversely, if the rings are incomplete or fading somewhere, the observed biaxiality is only apparent and likely to be produced by orientation gradients. The analysis of the disclination lines may also be used with benefit to prove biaxiality. One should then carefully control the width and contrast of the lines to keep them sharp and regular all along, in order not to confuse them with mirages. To prove biaxiality, at least two types of disclination lines should be observed with typical variations of the zigzag angles, contrasts and relaxation rates as functions of temperature.

ISSN:1058-725X    

DOI:10.1080/10587259808048443

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this article we describe research into the molecular design features for potential thermotropic biaxial nematogens of low-molar mass. We discuss how we can systematically change the shape of molecular structures in view of this aim of our work. The optical biaxialities of the materials prepared were tested through conoscopic and birefringence measurements. So far none of these materials synthesised and tested in this way have unequivocally exhibited the biaxial nematic phase. An overview of numerous further materials different in molecular shape and in part studied by others is also given.

ISSN:1058-725X    

DOI:10.1080/10587259808048444

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Oxford Workshop on Biaxial Nematic Liquid Crystals December 1996

ISSN:1058-725X    

DOI:10.1080/10587259808048445

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The authors regret that in their paper entitled: “Glass-Forming Terephthalic Esters with Lateral Phenylthio Groups and their Relaxation Behavior” by M. Pötteret al. [MCLC 312, 55 (1998)] are two errors on page 5.

ISSN:1058-725X    

DOI:10.1080/10587259808048446

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor