  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. 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.
