On the twist-bend nematic phase formed directly from the isotropic phase

Alya A Dawood,Martin C Grossel,Geoffrey R Luckhurst,Robert M Richardson,Bakir A Timimi,Neil J Wells,Yousif Z Yousif

doi:10.1080/02678292.2015.1114158

Alya A Dawood, Martin C Grossel + Show 5 more

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https://doi.org/10.1080/02678292.2015.1114158

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Abstract

The intriguing twist-bend nematic (NTB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence NTB–N–I; surprisingly, for the propane spacer, the NTB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable.

Highlights

In 1888 Reinitzer reported the synthesis of cholesteryl benzoate with its unusual double melting and striking colours seen on cooling the compound to just above the freezing point [1]
The texture observed on lowering the temperature to the twist-bend nematic phase is given in Fig, 2(b) and is a mixture of a polygonal together with a rope-like form
This is often observed for the NTB phase and is related to a periodic form resulting from the helicoidal structure with its short pitch [7, 20]

Summary

Introduction

In 1888 Reinitzer reported the synthesis of cholesteryl benzoate with its unusual double melting and striking colours seen on cooling the compound to just above the freezing point [1]. Studies of other analogous odd liquid crystal dimers [13] and their mixtures [14] confirm this result; it is observed that as the nematic range increases so the strength of the NTB –N transition decreases [15] Such behaviour is analogous to that of compounds exhibiting both nematic and smectic A phases [16]; this is not surprising given the formal similarity between the NTB and smectic A phases [4,17]. This analogy would suggest that the twist-bend nematic phase might be formed directly from the isotropic phase via a first-order transition

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