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.

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...

The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales. Locally, at the scale of a few molecules, x-ray-diffraction data demonstrate biaxial packing. However, above the mesoscopic scale, the global orientational order in all three compounds is uniaxial, as evidenced by uniform homeotropic alignment of the nematic phase which is optically tested over the entire temperature range and by the observations of topological defects induced by individual and aggregated colloidal spheres in the nematic bulk.

We investigate the origin of "secondary disclinations" that were recently described as new evidence of a biaxial nematic phase in an oxadiazole bent-core thermotropic liquid crystal C7. Using an assortment of optical techniques such as polarizing optical microscopy, LC PolScope, and fluorescence confocal polarizing microscopy, we demonstrate that the secondary disclinations represent non-singular domain walls formed in a uniaxial nematic phase during the surface anchoring transition, in which surface orientation of the director changes from tangential (parallel to the bounding plates) to tilted. Each domain wall separates two regions with the director tilted in opposite azimuthal directions. At the centre of the wall, the director remains parallel to the bounding plates. The domain walls can be easily removed by applying a moderate electric field. The anchoring transition is explained by the balance of (a) the intrinsic perpendicular surface anchoring produced by the polyimide aligning layer and (b) tangential alignment caused by ionic impurities forming electric double layers. The model is supported by the fact that the temperature of the tangentially tilted anchoring transition decreases as the cell thickness increases and as the concentration of ionic species (added salt) increases. We also demonstrate that the surface alignment is strongly affected by thermal degradation of the samples. This study shows that C7 exhibits only a uniaxial nematic phase and demonstrates yet another mechanism (formation of "secondary disclinations") by which a uniaxial nematic phase can mimic a biaxial nematic behaviour.