Fréedericksz transition in a thermoreversible nematic gel

A thermoreversible (physical) gel of a nematic liquid crystal in its planar configuration is investigated. The transition temperatures of the gel are thermally and rheologically determined. The temperature for the nematic-isotropic transition is higher than that for the gel-sol transition, allowing the network to grow in the oriented nematic phase. The electrical Fréedericksz transition of the gel is investigated by using both an optical and an electrical detection method. The transition can be adjusted within a large voltage range by selecting the temperature of the sample. This behavior is determined by the thermal properties of the thermoreversible gel network.

Figure 1

Gelator: $N$-(2-methyl-4-tetradecoylamino-phenyl)-cyclohexanecarboxamide.

Figure 2

SEM image of the fiberlike structure of the gel network taken from a xerogel with 0.2% gelator.

Figure 3

Polarization microscopy images showing the disappearance of the defect textures: (a) $T=20°\text{C}$, (b) $T=60°\text{C}$, and (c) $T=70°\text{C}$. The preferred direction of the director $\widehat{n}$ induced by the orientation layer is indicated by the arrow.

Figure 4

Transmitted light intensity during melting and reforming of the gel. The solid lines represent the heating processes, while the dashed lines are taken when cooling. The measurements are performed with heating and cooling rates of 1 K/min (a) and of 0.1 K/min (b).

Figure 5

Rheological properties of the nematic gel as a function of the sample temperature: storage modulus $G^{\prime }$ $(◼)$ and loss modulus $G^{″}$ $(◻)$.

Figure 6

Loss tangent of the nematic gel as a function of the frequency for several temperatures.

Figure 7

Slope $m$ obtained from the data depicted in Fig. 6 as a function of the temperature.

Figure 8

Optical detection of the Fréedericksz threshold: Transmitted light intensity with crossed polarizer optics as a function of the driving voltage for the pure liquid crystal (a), and for the liquid crystalline gel with (b) an established (gel phase) and (c) a melted (sol phase) gel network.

Figure 9

Electrical detection of the Fréedericksz threshold: Effective conductivity (a) and effective permittivity (b) as functions of the driving voltage. The solid line corresponds to the gel phase and the dashed line corresponds to the sol phase. For comparison, the dotted line shows the result for the pure liquid crystal.

Figure 10

Ohmic conductivity (a) and electrical permittivity (b) in the ground state as functions of the sample temperature: nematic gel with increasing $(▴)$ and decreasing $(▾)$ voltage; pure liquid crystal with increasing $(▵)$ and decreasing $(▿)$ voltage. The dashed lines in diagram (b) are guides to the eye.

Figure 11

Fréedericksz threshold voltage as a function of the sample temperature: nematic gel with increasing $(▴)$ and decreasing $(▾)$ voltage, and pure liquid crystal with increasing $(▵)$ and decreasing $(▿)$ voltage.