Abstract
ABSTRACTWe report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay (), twist (), and bend () elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behaviour 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 behaviour, described by Haller. The elastic constants also follow the standard behaviour, 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. 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.
Highlights
In the past few years, dimeric liquid crystals (LCs), formed by molecules with two rigid rod-like units connected by a flexible bridge, attracted a strong research interest because of their fascinating features [1, 2, 3, 4, 5]
The goal of this work is to determine the temperature dependencies of basic material properties, such as dielectric anisotropy, birefringence n, and elastic constants of splay K11, twist K22 and bend K33 of the N phase formed by the dimeric molecules of DTC5C9 and to compare them to the corresponding values in the N phase formed by its monomer MCT5
Behavior of DTC5C9 is dramatically different near the N-Ntb transition: the absolute value of the dielectric anisotropy starts to decrease in the pretransitional region, to the data by Borshch et al obtained for the 7:3 mixture of DTC5C9 and MCT5 [4]
Summary
In the past few years, dimeric liquid crystals (LCs), formed by molecules with two rigid rod-like units connected by a flexible bridge, attracted a strong research interest because of their fascinating features [1, 2, 3, 4, 5]. The second is a heliconical director state of a cholesteric liquid crystal, formed in the N phase doped with chiral molecules that is stabilized by an applied electric (or magnetic) field; as the external field changes, so does the heliconical pitch, usually on the micrometer length scales much larger than the molecular nanoscales [6, 7]. We denote this state as ChOH, where the subscript OH stands for “oblique helicoid”. Note that the transition might involve an appearance of other intermediate phases [12], but we do not discuss this possibility here as the focus is on the properties of the uniaxial
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