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We investigate the origin of secondary disclinations that were recentlydescribed as a new evidence of a biaxial nematic phase in an oxadiazolebent-core thermotropic liquid crystal C7. With an assortment of opticaltechniques such as polarizing optical microscopy, LC PolScope, and fluorescenceconfocal polarizing microscopy, we demonstrate that the secondary disclinationsrepresent non-singular domain walls formed in an uniaxial nematic during thesurface anchoring transition, in which surface orientation of the directorchanges from tangential (parallel to the bounding plates) to tilted. Eachdomain wall separates two regions with the director tilted in oppositeazimuthal directions. At the centre of the wall, the director remains parallelto the bonding plates. The domain walls can be easily removed by applying amodest electric field. The anchoring transition is explained by the balance of(a) the intrinsic perpendicular surface anchoring produced by the polyimidealigning layer and (b) tangential alignment caused by ionic impurities formingelectric double layers. The model is supported by the fact that the temperatureof the tangential-tilted anchoring transition decreases as the cell thicknessincreases and as the concentration of ionic species (added salt) increases. Wealso demonstrate that the surface alignment is strongly affected by thermaldegradation of the samples. The study shows that C7 exhibits only a uniaxialnematic phase and demonstrate yet another mechanism (formation of secondarydisclinations) by which a uniaxial nematic can mimic a biaxial nematicbehaviour.

Domain Walls and Anchoring Transitions Mimicking Nematic Biaxiality in the Oxadiazole Bent-Core Liquid Crystal C7