Pretransitional effects in a mesogenic mixture under an electric field

Nematogenic liquids are materials that are in high demand for many application purposes. Field-induced nematic order in isotropic liquids can be observed by the enhancement of electric permittivity during dielectric spectroscopy measurements under the bias electric field. For this reason, dielectric measurements were done on a nematogenic mixture of highly polar compounds. This paper presents a discussion on the influence of the bias electric field on the dielectric response. We observed a decrease of electric permittivity in the isotropic phase under the bias electric field. This effect is most likely caused by the field-induced creation of some complex molecular objects. In the paper, relaxation parameters of the molecular modes were analyzed. The field-induced orientational order was determined using a dichroic ratio in a guest-host system. The measurements show the significant order parameter in the isotropic phase under an electric field.

Figure 1

DSC thermogram of the heating and cooling cycles obtained for the mixture under study.

Figure 2

The isotropic-nematic phase transition textures of the mesogenic mixture at (a) 19.5 °C, (b) 19.0 °C, (c) 18.5 °C, (d) 18.0 °C, (e) 17.5 °C, and (f) 17.0 °C.

Figure 3

The temperature dependence of real components of electric permittivity ${\varepsilon }_{\parallel }^{\prime }$ and ${\varepsilon }_{\perp }^{\prime }$ measured at several frequencies for NMHP. Phase transition temperature $\left(T_{I\text{−}N}\right)$ between the isotropic and nematic phase is at 19 °C.

Figure 4

Real part of electric permittivity ε′ versus temperature $T$ at frequency $f$ of 1 kHz for the low- (a) and (b) high-bias electric fields. The red line marks the phase transition temperature $T_{I\text{−}N}$ in measurements without the dc field. The black line marks the field-induced phase transition temperature $T_{I\text{−}N}^{\mathrm{dc}}$ under the highest electric field, which is shown in the figure.

Figure 5

Real part of electric permittivity ε′ versus temperature $T$ at frequency $f$ of 1 kHz under different bias voltages. The red line marks the phase transition temperature $T_{I\text{−}N}$ observed in measurements without the dc field, while the black line marks the field-induced phase transition temperature $T_{I\text{−}N}^{\mathrm{dc}}$. Under the field of 30 V per 5 µm, the phase transition temperature shifted by 4.0 °C.

Figure 6

Real $\left(\varepsilon \prime \right)$ and imaginary $\left({\varepsilon }^{″}\right)$ parts of complex electric permittivity plotted against frequency (a) for the isotropic phase, (b) at the field-induced phase transition temperature, (c) at the temperature corresponding to the occurrence of the peak of electric permittivity, and (d) in the fully created nematic phase.

Figure 7

Calculated values of the relaxation frequency $f_{\mathrm{R}}$ versus temperature $T$ in the vicinity of the isotropic $\left(I\right)$ and nematic phase $\left(N\right)$. The filled symbols mark results for the measurements with the bias field (BIAS), which equals to 30 V. Field-induced phase transition temperature is 23 °C. Field-free phase transition temperature is 19 °C. Here, abbreviation $I\text{−}N$ stands for the field-induced phase transition region, while $N$ and $I$ indicate the presence of the same mode in the isotropic and the nematic phase. The red line marks the field-free phase transition, while the black line marks the field-induced phase transition temperature.

Figure 8

Dielectric strength $\delta \varepsilon$ of the high- and low-frequency relaxations upon temperature $T$ in the vicinity of the isotropic $\left(I\right)$ and nematic phase $\left(N\right)$. The filled symbols mark results for the measurements with the bias field (BIAS), which equals to 30 V. Field-induced phase transition temperature is 23 °C. Field-free phase transition temperature is 19 °C. Here, abbreviation $I\text{−}N$ stands for the field-induced phase transition region, while $N$ and $I$ indicate the presence of the same mode in the isotropic and the nematic phase. The red line marks the field-free phase transition, while the black line marks the field-induced phase transition temperature.

Figure 9

Distribution parameter α in the Cole-Cole model versus temperature $\left(T\right)$ in the vicinity of the isotropic $\left(I\right)$ and the nematic $\left(N\right)$ phases. The filled symbols mark results for the measurements with the bias field (BIAS), which equals to 30 V. Field-induced phase transition temperature is 23 °C. Field-free phase transition temperature is 19 °C. Here, the abbreviation $I\text{−}N$ stands for the field-induced phase transition region, while $N$ and $I$ indicate the presence of the same mode in the isotropic and the nematic phases. The red line marks the field-free phase transition, while the black line marks the field-induced phase transition temperature.

Figure 10

Field-free and electric field-induced order parameter $\left(S\right)$ of the highly polar nematogenic mixture upon the reduced temperature $T–T_{I\text{−}N}$. The order parameter was calculated using a dichroic ratio of the azo dye molecule in the guest-host system. Measurements were taken in the vicinity of the isotropic and nematic phase transition without (HG) and after applying the electric field (60 V per 6 μm).

Figure 11

Phase transition from the isotropic to the nematic phase in the investigated mixture in the homeotropically oriented cell (a) without the external field, and (a) with electric bias field. In green are the C1 molecules, while in blue and red are the C2 and C3 molecules, respectively. The quadrupoles are marked in gray.