Abstract
Twelve symmetrical dimeric materials consisting of a nonamethylene (C9) spacer and either phenyl 4‐(4′‐alkylphenyl)benzoate, phenyl 4‐(4′‐alkylcyclohexyl)benzoate or phenyl 4‐(4′‐alkylbicyclohexyl)carboxylate mesogenic units were prepared and their mesogenic behaviour characterised by POM, DSC and XRD. All of the materials exhibited nematic phases with clearing points in excess of 200 °C. Four compounds were found to exhibit the twist‐bend nematic phase, with one material exhibiting a transition from the NTB phase into an anticlinic smectic ‘X’ phase. Across all three series of compounds the length of terminal chain is seen to dictate, to some degree, the type of mesophase formed: shorter terminal chains favour nematic and NTB mesophases, whereas longer terminal aliphatic chains were found to promote smectic phases.
Highlights
IntroductionIn recent years there has been a resurgence of interest in liquid-crystal dimers and bimesogens, driven by interest in wide temperature range blue phases,[1,2] flexoelectric behaviour[3,4,5] and the ability of some dimeric materials to exhibit a lower-temperature mesophase lacking lamellar organisation thereby being described as nematic.[6,7,8,9,10,11,12,13,14,15,16,17,18] This lower temperature ‘nematic’ phase is denoted NX or NTB,[6,7,8,9,10,11,12,13,14,15,16,17,18] in which ‘X’ and ‘TB’ refer to unknown and twist-bend, respectively
The mesomorphic properties of compounds 1–12 were studied by a combination of polarised optical microscopy (POM) and differential scanning calorimetry (DSC)
Phase identifications were made based on textures observed by POM, assisted where appropriate with small angle X-ray scattering data (SAXS).The transition temperatures (8C) and enthalpies of transition are given for each series of compounds in Tables 1 to 3, respectively
Summary
In recent years there has been a resurgence of interest in liquid-crystal dimers and bimesogens, driven by interest in wide temperature range blue phases,[1,2] flexoelectric behaviour[3,4,5] and the ability of some dimeric materials to exhibit a lower-temperature mesophase lacking lamellar organisation thereby being described as nematic.[6,7,8,9,10,11,12,13,14,15,16,17,18] This lower temperature ‘nematic’ phase is denoted NX or NTB,[6,7,8,9,10,11,12,13,14,15,16,17,18] in which ‘X’ and ‘TB’ refer to unknown and twist-bend, respectively. The local structure of the NX/NTB phase is still hotly debated, with the heliconical ‘twist-bend’ model proposed independently by Meyer[19] and Dozov[20] supported by 2H NMR studies, measurement of the electroclinic effect, freeze-fracture transmission electron microscopy (FFTEM) and carbon K-edge SAXS.[21,22,23,24] All three of these methods suggest a local helical structure of extremely tight pitch, in the region of 8 nm for the well-studied material CB7CB {4’,4’’’-(heptane-1,7-diyl)bis([1,1’-biphenyl]-4carbonitrile)}.[21,22,23,24,25] Polarised Raman spectroscopy has been used to measure order parameters in the nematic and NTB phases.[26] This view of the local structure has been disputed, with a growing body of experimental evidence in-
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