Do the short helices exist in the nematic TB phase?

Abstract

Do the short helices exist in the nematic TB phase?Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical texture related to the formation of arrays of focal conic defects. Some of the focal conics show submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, suggesting surface freezing.

The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry

“…To confirm this expectation we have derived R for the twist-bend nematic phase using the Hess-Frenkel-Allen theory. The starting point for deriving the diffusion anisotropy, h R, in the twist-bend nematic phase from the Hess-Frenkel-Allen theory [26] is Equations (7) and (8). These require the principal components of the diffusion tensor in the director frame together with its scalar component:…”

“…Despite experimental [6] and theoretical [7] support for the model of bent molecules arranged in short pitch helices proposed by Dozov, [5] the structure of the phase is still open to debate [8] and other theories have been suggested. [9,10] In particular, one of these excludes the presence of the helix in favour of a polar nematic phase with molecules distributed in domains with mirror symmetry.…”

On the structure of the Nx phase of symmetric dimers: inferences from NMR

“…The presence of stripes and Bouligand arches with periodicities in the 8-10nm regime in freeze fracture TEM 12 was interpreted as formation of chiral structures on surfaces, however recent evidence based on AFM studies might suggest these features could be artefacts due to the onset of surface crystallisation 15 . Lastly, in a series of papers 8,9,16 focused on an extensive NMR characterization of CB-C7-CB it is argued that the interpretation of the NMR measurements is consistent with a helicoidal conical ("heliconical") nematic phase wherein the spatial distribution of the nematic director is 0 0 0 ( ) (sin cos ,sin sin , cos ) Z kZ kZ…”

The design and investigation of the self-assembly of dimers with two nematic phases

“…This view is supported too by freeze fracture TEM studies, [7][8][9] though AFM results point towards the possibility of surface crystallisation. 6 Solid state NMR studies of samples in the bulk, used initially to support the simple N tb model 10 cannot, according to more recent results, 11 be employed for this without invoking very unusual molecular features, such as very fast migration along helix axes. The optical defect textures of the N x phase are distinctly different to that of a typical nematic and are more reminiscent of those of smectic phases.…”