On orientational order in nematic and twist-bend nematic phases: a ²H-NMR study of binary mixtures of the odd dimer,1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane, (CB9CB), and the monomer, 4-pentyl-4′-cyanobiphenyl, (5CB-d₂)

ABSTRACT

In this article, we explore those factors that influence the orientational order in both nematic and twist-bend nematic phases. The systems used for our NMR experiments are binary mixtures of a dimer and a related monomer thus enabling its behaviour to be controlled continuously, through its composition rather than chemical structure. Earlier studies have revealed that as the temperature is lowered in the nematic phase, the orientational order frequently passes through a maximum prior to the formation of the N_TB phase. A confirmative indication of this unusual behaviour is that the Haller function cannot fit this dependence over the entire nematic range; such behaviour is intriguing. In the N_TB phase, the measured order parameter is often insensitive to decreasing temperature for systems with strong N_TB–N transitions often associated with a narrow nematic range. For those mesogens with longer nematic ranges and hence weaker N_TB–N transitions, the order parameter decreases more markedly with temperature. As expected for the weak transitions, the behaviour of the order parameter is observed to be essentially continuous across the transition. Associated with the orientational order in the N_TB phase is the conical angle and an off-diagonal element of the Saupe ordering matrix.

Graphical Abstract

KEYWORDS:

• Pretransitional behaviour
• conical angle
• Haller function
• Saupe ordering matrix
• transitional entropy
• nematic range
• chiral quadrupolar splitting
• Gay-Berne

Acknowledgments

Some 46 years ago, Claudio Zannoni with his wife Nicoletta arrived in Southampton from Ravenna. At this time Claudio had taken the brave and quite unusual decision to study for his Doctorate in the Department of Chemistry at the University of Southampton. He was then armed with a small library which has continued to grow; his research was both theoretical and also guided in part by experiment. This went extremely well, indeed, it was a joy to work with him on a diverse range of projects. His thesis, not surprisingly, proved to be quite outstanding. It was examined by David Buckingham, FRS, an eminent chemical physicist from Cambridge; the Viva proved to be a Master Class for such events. Indeed, it was a real pleasure for the other examiners just to listen to the deep and wide ranging discussion between David and Claudio who were equally stimulated both by the questions and the answers.

Much has happened since then although initially Claudio did return to help teach on the undergraduate course in Liquid Crystals and Liquids made possible by his abilities as an extremely able lecturer. Such skills have undoubtedly developed over the years and it is always a real privilege to listen to one of his lectures. Related to this is the true diversity of his research as is evident by his recognition by a wide range of learned societies, the journals that they publish and the management of the science with which he is involved.

Those of us from Southampton and, no doubt, elsewhere are truly impressed, but not surprised, by his amazing achievements. We now look forward to seeing how his interests will develop further, to reading his papers and, as in the past, discussing science with him.

Disclosure statement

No potential conflict of interest was reported by the authors.