Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

Abstract

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific propertiesCertain nematic liquid crystals, e.g., those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N * tb) can be observed below the conventional chiral nematic (N * ) phase, while under electric field the N * phase is transformed into a specific state known as "heliconical cholesteric". In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChD) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analyzed, and selective reflection in the visible range could be observed in the N * phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N * tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.

Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB)

“…The presence of this compensation point is surprising and was not reported before to the best of my knowledge. This behavior contrasts with that previously observed in similar mixtures doped with larger concentrations of chiral molecules in which the pitch seems diverge at T NB [12]. It turns out that this unexpected property is very interesting to test the present theory on the heliconical instability induced by confinement.…”

“…Another interesting point would be to change the concentration of R811 to see if the compensation point disappears. This should be the case if I refer to a recent work by Kasian et al [12] in which there is no mention of a compensation point in similar mixtures doped with a larger concentration of R811 (more than 4 % in their paper) .…”

Heliconical-fluctuation-induced compensation point in the diluted cholesteric phase of mixtures containing the flexible dimer CB7CB

“…This interpretation was indeed proposed by the authors of Ref. [18] to explain the large increase of the pitch measured in their mixtures at large concentration of chiral dopants. This interpretation is certainly partly correct, but fails to explain the twist reversal at low concentration of chiral molecules.…”

“…Another example was recently found by one of us (PO) in a mixture of a flexible dimer CB7CB (see below) and a rodlike molecule (8CB) doped with a very small amount of the chiral molecule R811 [17]. In this mixture, a surprising compensation point appeared in the vicinity of the twist-bend nematic phase (N TB ), contrasting with previous studies only mentioning a divergence of the pitch at the transition [18]. We recall that in the N TB phase, the bent-core molecules spontaneously arrange into a doubly degenerate heliconical structure at a nanometer scale (for a review, see Ref.…”

Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

“…Similar behavior of the Ch pitch is observed near the transition to the smectic A [5] and N*TB phase [24,30]. The trend could be again explained by the proximity to pseudo-layered NTB and N*TB phases, but Oswald and Dequidt [24] proposed an additional mechanism, rooted in the possibility that N*TB nuclei impose an opposite sense of chirality onto the surrounding Ch background [24].…”