Using a magnetically aligned liquid crystal mixture containing a novel Se-labelled dimer and the difluoroterphenyl dimer DTC5C7, the twist-bend nematic phase (N) was studied by the resonant scattering of hard X-rays and by conventional small and wide-angle X-ray scattering (SAXS, WAXS). Resonant diffraction spots indicated a helix with a 9-12 nm pitch in the N phase and an unprecedentedly high helix orientation. This enabled deconvolution of global and local order parameters. These findings, combined with the simultaneously recorded resonant and non-resonant SAXS and WAXS data, allowed us to construct a locally layered molecular model of the N phase, where the average twisted conformation of each molecule was idealised as a helical segment, matching the local heliconical director field. The dimers were found to be less bent in the N phase than in their minimum energy conformation, and straightening further with increasing temperature. It is proposed that on further heating their low bend angle allows the transition to the normal nematic phase, where the molecules can freely move longitudinally, without the need to perform screw-like motion as in the N phase. At the low-temperature end, the increasing molecular twist becomes unsustainable, leading to a transition to a smectic phase, where no twist is required.

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.

We present structural studies of a dimeric compound composed of a central heptyl spacer linking two mesogens consisting of terphenyl units at which two adjacent fluoro groups are attached to each central ring. The terminal rings are linked to pentyl chains as terminal groups. The material exhibits a nematic-nematic transition and a low temperature modulated phase. The higher temperature nematic phase was found to exhibit an anomaly of the bend elastic constant similar to that of the dimers with N-Ntb phase sequence, and the physical properties of the low-temperature nematic phase are similar to those of the known Ntb materials. The structure of the low-temperature modulated smectic/columnar phase is described together with its ability to form freely suspended films and fibres. The relation of the modulated structure to the fibre formation and to the appearance of the labyrinthine instability in freely-suspended films is discussed.

A series of non-symmetric dimers were synthesised containing either cyanobiphenyl or difluoroterphenyl moieties on one side and a range of long, short, bent, polar or apolar mesogens on the other side of the molecules. The dielectric anisotropy of the mesogens was varied systematically. The systems were characterised by differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and detailed X-ray diffraction (XRD) studies, both in the nematic and the N x phase. The results are compared and structure-property relationships are discussed. A model for the assembly in the N x phase is developed discussing N tb structures, coaxial helices, swiss roll structures and chiral domain formation.

We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay ( 11 K ), twist ( 22 K ), and bend ( 33 K ) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rodlike mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. 33 K decreases strongly to 0.4 pN near the N -Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Keywords: dimeric nematic; elastic constants; birefringence; negative dielectric anisotropy; twist-bend nematic. N phase. Uniform bend, however, cannot be realized in space without other types of deformations, either splay or twist. Thus two different variations of the nematic with spontaneous bend have been proposed by Meyer [8] and Dozov [9]: a twist-bend nematic and a splay-bend nematic (yet to be discovered experimentally). The relative stability of the two is controlled by the ratio of the splay 11 K to twist 22 K constants. Namely, in the twist-bend phase, 11 22 /2 KK  , while in the splay-bend case, 11 22 /2 KK  [9, 10]. As for the ChOH state, Meyer [11] predicted that it can be induced by an external electric or magnetic field in a cholesteric liquid crystal only when 33 22 KK  . In calamitic nematics formed by rod-like molecules, the latter condition is not satisfied; the expected and universally observed trend follows the inequalities 33 22 11 K K K  [13]. Prior research indicates that the dimeric materials feature elastic properties very different from those of rod-like nematogens. Atkinson et al. [14] demonstrated a strongodd-even effect in the behavior of 33 K . Two dimers, one with an even number of alkyl groups in the spacer chain and another one with an odd number, were characterized by the splay Frederiks transition technique. In this approach, 11 K is determined from the threshold field of reorientation, and 33 K is obtained by extrapolating the voltage dependence of capacitance far above the threshold. 33 K of the odd homolog was about (3-4) pN, much smaller (by a factor 10 ) than the corresponding value for the even homolog; the results were in good agreement with the theoretical consideration of the odd-even effect by...