Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases

Abstract

© 2017 American Physical Society. This is an author produced version of a paper published in Physical Review E. Uploaded in accordance with the publisher's self-archiving policy.eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. A detailed investigation of the thermal and dielectric properties of a series of binary mixtures exhibiting the nematic (N) and twist-bend nematic (NTB) liquid crystal phases is presented. The mixtures consist of an achiral, dimeric liquid crystal CB7CB, which forms the nematic and twist-bend nematic phases, and a calamitic liquid crystal 5CB, which shows the nematic phase. As the concentration of the calamitic liquid crystal is increased, the transition temperatures decrease linearly, and the width of the nematic phase increases. The enthalpies of phase transitions obtained from DSC measurements show that on increasing the concentration of 5CB in the binary mixtures, the enthalpy associated with the N to NTB phase transitions reduces considerably compared to a clear first order N-NTB transition in pure CB7CB. The real and imaginary parts of the dielectric permittivity are measured as a function of frequency from 100 Hz to 2 MHz in the nematic and twist-bend nematic phases in planar and homeotropic devices. A significant decrease in the average dielectric permittivity as a function of temperature for mixtures forming the NTB phase is observed. Measurements of the imaginary part of the dielectric permittivity show a relaxation peak in the measured frequency window for all of the mixtures exhibiting the NTB phase. The activation energy associated with this relaxation process is calculated and is shown to remain constant irrespective of the composition of the mixtures.2

Variable pitch hydrodynamic electro-optic gratings utilising bent liquid crystal dimers

“…63,[66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] In addition to this, it has been shown that when the dimer is mixed with standard calamitics the resultant mixture can also show N TB phases, as well as exhibit distinctive changes in elastic constants. [96][97][98][99] The transitions for the pure CB7CB were measured as Iso 116 1C N 102 1C N TB , in agreement with literature values. 63,67,90 CB7CB has been shown to have a net positive De, due to the dominant effect of the longitudinal dipole moment of the ''hairpin'' conformer over the transverse dipole of the ''banana'' conformer ( Fig.…”

“…They exhibit values of k 11 /k 33 that are significantly higher than unity, 54,57,60,69,72,76,79,92 where standard calamitics typically exhibit k 11 /k 33 between 0.6 and 0.8. [93][94][95] It has also been shown that doping a standard calamitic liquid crystal with a bent-core or dimeric material can imbue the mixture with some similarly unusual elastic properties 56,92,[96][97][98][99] while still showing a nematic phase. For example, Parthasarathi et al 97 showed that binary mixtures of the bent dimer CB7CB with calamitic material 7OCB at certain temperatures exhibits k 11 /k 33 4 5.…”

Physics of liquid crystals of bent-shaped molecules

“…and its longer homologues (M. Cestari et al 2011;Nerea Sebastián et al 2016;Babakhanova et al 2017;Nerea Sebastián et al 2014;Robles-Hernández et al 2015;Yun et al 2015;Parthasarathi et al 2016;Robles-Hernández et al 2016;N. Sebastián et al 2017;Trbojevic, Read, and Nagaraj 2017).…”

Liquid Crystal Devices for Beam Steering Applications

“…Another phase of interest in recent years is the Twist Bend Nematic (N TB ) [155,156,158,[202][203][204][205]. As the N TB phase forms relatively periodic optical structures, there has been interest in using them for the application of diffractive beam steering devices [206][207][208].…”