Spontaneous Periodic Deformations in Nonchiral Planar-Aligned Bimesogens with a Nematic-Nematic Transition and a Negative Elastic Constant

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

“…One of the early studies of a liquid crystal dimer, with an odd spacer, showing a nematic-nematic transition was reported by Sepelj et al 1 although the nature of the low-temperature nematic phase was not identified and so was labelled N x . Similar dimers with odd spacers having a N x -N transition were reported by Panov et al 2 and the behaviour of such dimers has also been studied [3][4][5] but again without the low-temperature nematic being properly identified. Cestari et al 6 have examined an odd liquid crystal dimer and observed a nematic-nematic transition but, as the result of an extensive investigation based on numerous techniques, were able to identify the low-temperature nematic as we shall see shortly.…”

Hierarchical elasticity of bimesogenic liquid crystals with twist-bend nematic phase

“…10 In planar cells, the N tb phase also exhibited spontaneous periodic stripe patterns in micrometer-scale. 7,11 Although the latter is reminiscent of the smectic phase, X-ray scattering patterns and nuclear magnetic resonance spectroscopy indicated that the N tb phase is nematic-like rather than of the lamellar type. 2,[12][13][14] The twist-bend nematic phase was predicted by Meyer in 1973 in chiral molecules, 15 and Dozov predicted spontaneous symmetry-breaking in an achiral banana-shaped mesogen.…”

“…1(a-iv)). 1,7 In a non-surface-treated 3.8-lm-thick cell, focal-conic defect textures appeared at 99.3 C (Fig. 1(b-iii)) and fine stripes filled each focal-conic domain after further cooling ( Fig.…”

“…6 In particular, some bimesogenic liquid crystals with an odd number of methylene flexible units or a rigid bent-core mesogen showed an additional nematic phase at temperatures below the conventional nematic phase (N u ). 7,8 Recently, this phase has been identified as the twist-bend nematic phase (N tb ), 9 where the director forms periodic twisted and bent deformations with a nano-scale pitch (8-9 nm). 10 In planar cells, the N tb phase also exhibited spontaneous periodic stripe patterns in micrometer-scale.…”

“…1, polarized optical microscopy (POM) was used to investigate thin and thick planar cells as well as a non-surface-treated cell. The temperature was controlled precisely using a hot stage (TMS94 and LTS350, Linkam, USA), and in the POM observations, T Nu-Ntb was determined to be 99.6 C, at which the nematic thermal fluctuation started to disappear 7 and the opposite handedness helical domains started to appear (Figs. 1(a-ii) and 1(c-ii)).…”