Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy

We report a comprehensive dielectric characterization of a liquid crystalline binary mixture composed of the symmetric mesogenic dimer CB7CB and the nonsymmetric mesogenic dimer FFO9OCB. In addition to the high-temperature nematic phase, such a binary mixture shows a twist-bend nematic phase at room temperature which readily vitrifies on slow cooling. Changes in the conformational distribution of the dimers are reflected in the dielectric permittivity and successfully analyzed by means of an appropriate theoretical model. It is shown that the dielectric spectra of the mixture reflect the different molecular dipole properties of the components, resembling in the present case the characteristic dielectric spectra of nonsymmetric dimers. Comparison of the nematic and twist-bend nematic phases reveals that molecular dynamics are similar despite the difference in the molecular environment.

Figure 1

Chemical structures of CB7CB and FFO9OCB. Schematic representation of the two main conformers for both dimers.

Figure 2

Temperature dependence of the static permittivity of the two pure compounds and the three mixtures: (□) FFO9OCB; ($●$) ${\mathrm{X}}_{\mathrm{CB}7\mathrm{CB}}=0.21$; ($△$) ${\mathrm{X}}_{\mathrm{CB}7\mathrm{CB}}=0.48$; ($◊$) ${\mathrm{X}}_{\mathrm{CB}7\mathrm{CB}}=0.82$; ($☆$) CB7CB. Discontinuous lines correspond to the phase transition $N_{\mathrm{TB}}-N$.

Figure 3

Mixture with 0.48 mole fraction of CB7CB. (a) Three-dimensional plot of the dielectric losses vs temperature and logarithm of the frequency. Frequency dependence of ${\varepsilon }^{\prime }$ (□) and ${\varepsilon }^{\prime \prime }$ (○) (b) in the $N$ phase, and (c) in the $N_{\mathrm{TB}}$ phase. In (b,c) solid lines are fits to Eq. (3).

Figure 4

Mixture with 0.48 mole fraction of CB7CB. Dielectric strength of the relaxation modes vs temperature: ($●$) isotropic phase; (Δ) $m_2$; (□) $m_{1H}$; (◊) $m_{1L}$.

Figure 5

Mixture with 0.48 mole fraction of CB7CB. Arrhenius plot of the frequency of the relaxation modes: ($●$) isotropic phase; (Δ) $m_2$; (□) $m_{1H}$; (◊) $m_{1L}$. Solid lines are fits to the VFT law, Eq. (4).