7 research outputs found
Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers
We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay (K₁₁), twist (K₂₂), and bend (K₃₃) 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 rod-like 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. K₃₃ 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
Domain Walls and Anchoring Transitions Mimicking Nematic Biaxiality in the Oxadiazole Bent-Core Liquid Crystal C7
We investigate the origin of secondary disclinations that were recently
described as a new evidence of a biaxial nematic phase in an oxadiazole
bent-core thermotropic liquid crystal C7. With 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 an uniaxial nematic 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 bonding plates. The domain walls can be easily removed by applying a
modest 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 tangential-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. The study shows that C7 exhibits only a uniaxial
nematic phase and demonstrate yet another mechanism (formation of secondary
disclinations) by which a uniaxial nematic can mimic a biaxial nematic
behaviour.Comment: 21 pages, 9 Figures, 1 Tabl