Twist-bend nematics, liquid crystal dimers, structure–property relations

ABSTRACT

One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.

Graphical Abstract

KEYWORDS:

• Twist-bend nematics
• liquid crystal dimers
• local-structure correlations
• N_TB–I transition
• molecular curvature
• inhomogeneity and anisotropy
• orientational order

Acknowledgements

We are grateful to a number of colleagues who have helped us with this project. They are Mr Herbert Zimmermann of the Max-Plank-Institut für Medizinische Forschung, Heidelberg for the sample of CB7CB-d₄; Professor Corrie Imrie of the University of Aberdeen for allowing us to measure optical textures of certain dimers; Professor Ghazwan Faisal of the University of Duhok for assistance with the calculations of the chemical structures and dimensions of the dimers and Mr Kevin Adlem of Merck Chemicals for allowing us to use their DSC equipment.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

The supplemental data for this article can be accessed here.

Additional information

Funding

The Ganesha X-ray scattering apparatus used for this research was purchased under an EPSRC Grant ‘Atoms to Applications’ Grant ref. EP/K035746/1. We also wish to thank the University of Zakho for the award of a split-site PhD Grant, number 1112/9, to Dr Alya Dawood.