Dielectric properties of mixtures of a bent-core and a calamitic liquid crystal

Dielectric spectroscopy measurements have been performed on a bent-core nematic liquid crystal and on its binary mixtures with a calamitic nematic. We have detected more dispersions in the bent-core compound than in the calamitic one, including one at an unusually low frequency of a few kilohertz. The dispersions detected in the mixtures have been identified and the spectra have been split into contributions of the constituents. In order to connect the dielectric increment with the molecular dipole moment we have applied a sophisticated conformational calculation not performed before for a large, flexible mesogen molecule with numerous polar groups.

Figure 1

Chemical structures of the bent-core ClPbis10BB and the rodlike 6OO8 molecules used in the mixtures.

Figure 2

(Color online) Phase transition temperatures of mixtures of different compositions in the binary system ClPbis10BB/6OO8 determined by dielectric measurements (symbols), and by polarizing microscopy [11] (solid lines). Phase identification is given according to [11].

Figure 3

(Color online) Frequency dependence of the parallel component of the (a) permittivity ${\epsilon }_{\parallel }^{\prime }$ and the (b) loss ${\epsilon }_{\parallel }^{″}$ in the nematic phase of ClPbis10BB at various $\Delta T=T-T_{NI}$ temperatures. (c) The corresponding Cole-Cole plot. Symbols are measured values, solid lines correspond to a fit with two relaxations. The dashed lines show the two fitted relaxations separately.

Figure 4

(Color online) (a) Frequency dependence of the loss ${\epsilon }_{\perp }^{″}$ for the perpendicular component in the nematic phase of ClPbis10BB at various $\Delta T=T-T_{NI}$ temperatures. (b) The corresponding Cole-Cole plot. Symbols are measured values, solid lines correspond to a fit with one relaxation.

Figure 5

(Color online) Temperature dependence of the static dielectric anisotropy $\left({\epsilon }_a\right)$ in the nematic phase of ClPbis10BB.

Figure 6

(Color online) (a) Frequency dependence of the loss ${\epsilon }_{iso}^{″}$ in the isotropic phase of ClPbis10BB at various $\Delta T=T-T_{NI}$ temperatures. (b) The corresponding Cole-Cole plot. Symbols are measured values, solid lines correspond to a fit with one relaxation.

Figure 7

(Color online) Temperature dependence of the relaxation frequencies in the isotropic and nematic phases of ClPbis10BB. Symbols are measured values, solid lines correspond to a fit to Eq. (2).

Figure 8

(Color online) Frequency dependence of the parallel component of the (a) permittivity ${\epsilon }_{\parallel }^{\prime }$ and of the (b) loss ${\epsilon }_{\parallel }^{″}$ in the nematic phase of binary mixtures of ClPbis10BB/6OO8 at various concentrations of the calamitic compound at the relative temperature $\Delta T=-13\text{K}$. Symbols are measured values, solid lines correspond to fits with two relaxations. The small jumps in the curves at $X=0.67$ and 0.92 concentrations are artifacts (due to change in internal sensitivity).

Figure 9

(Color online) Frequency dependence of the perpendicular component of the loss ${\epsilon }_{\perp }^{″}$ in the nematic phase of binary mixtures of ClPbis10BB/6OO8 at various concentrations of the calamitic compound at the relative temperature $\Delta T=-13\text{K}$. Symbols are measured values, solid lines correspond to a fit with one relaxation.

Figure 10

(Color online) Concentration dependence of the (a) dielectric increments $\Delta \epsilon$, the (b) symmetric distribution parameters $\alpha$ and the (c) relaxation frequencies $f_R$ in binary mixtures of ClPbis10BB/6OO8 at the relative temperatures $\Delta T=+5\text{K}$ and $-13\text{K}$, in the isotropic and in the nematic phase, respectively. The dotted lines represent linear extrapolations for the concentration dependence. The two dashed lines indicate a decomposition of $M_{\parallel 2}$ into two dispersions explained in the discussion. $M$ refers to the relaxations in mixtures.

Figure 11

(Color online) Frequency dependence of the perpendicular and the parallel components of the loss in the ${\text{SmC}}_{\text{A}}$ phase of the mixture with $X=0.5$ at 30 K below $T_{NI}$. Symbols are measured values, the solid line corresponds to a fit with one dispersion.

Figure 12

(Color online) Temperature dependence of the relaxation frequencies in the isotropic, nematic, and smectic phases of the mixture with $X=0.5$ molar fraction of 6OO8 in ClPbis10BB. Symbols are measured values, solid lines correspond to a fit to Eq. (2).

Figure 13

(Color online) Synthesized dielectric spectrum of the parallel component of the loss for binary mixtures of ClPbis10BB/6OO8 assuming three dispersions.