High-resolution dielectric study reveals pore-size-dependent orientational order of a discotic liquid crystal confined in tubular nanopores

We report a high-resolution dielectric study on a pyrene-based discotic liquid crystal (DLC) in the bulk state and confined in parallel tubular nanopores of monolithic silica and alumina membranes. The positive dielectric anisotropy of the DLC molecule at low frequencies (in the quasistatic case) allows us to explore the thermotropic collective orientational order. A face-on arrangement of the molecular discs on the pore walls and a corresponding radial arrangement of the molecules is found. In contrast to the bulk, the isotropic-to-columnar transition of the confined DLC is continuous, shifts with decreasing pore diameter to lower temperatures, and exhibits a pronounced hysteresis between cooling and heating. These findings corroborate conclusions from previous neutron and x-ray-scattering experiments as well as optical birefringence measurements. Our study also indicates that the relative simple dielectric technique presented here is a quite efficient method in order to study the thermotropic orientational order of DLC-based nanocomposites.

Figure 1

Illustrations of the capacitor geometry used for the dielectric measurements in the bulk state (a) and for the nanoconfined state (b). Reduced dielectric permittivity of Py4CEH vs. temperature during heating (circles) and subsequent cooling (squares) measured in the bulk state (c) and confined to mesoporous matrices of different pore radii of $p{\mathrm{Al}}_2{\mathrm{O}}_3$ (d–f) and $p{\mathrm{SiO}}_2$ (g–i). The dashed lines are ${\varepsilon }_r\left(T\right)$-dependencies extrapolated from the isotropic phase. $\mathrm{\Delta }{\varepsilon }_r$ indicate the excess permittivity.

Figure 2

Molecular ordering types of DLCs in cylindrical nanochannels. (a) Parallel axial configuration. (b) Radial, face-on configuration with isotropic core. (c) Radial multidomain face-on configuration. (d) Top-view electronmicrograph of an alumina membrane with approximately 19-nm mean pore diameter. The scale bar indicates a width of 100 nm.

Figure 3

(a) The characteristic temperatures $T_k^h$ and $T_k^c$ determined from the temperature dependencies of the dielectric permittivity (see Fig. 1) measured during heating (open circles) and cooling (solid circles), respectively. The dash-dotted line is a guide for the eye. (b) $T_k^h$ vs. the inverse channel radius $R^{-1}$ as determined by dielectric measurements (open circles) and by optical birefringence data (open squares), respectively, in comparison with a $1/R$-fit (dash-dot line) and with a function $T_k^h=T_{\text{ICD}}^o-a_1{R}^{-1}-a_2{R}^{-2}$ and the fit-parameters, $T_{\text{ICD}}^o$=365.2 K, $a_1$=52.1 $\mathrm{K}\mathrm{nm}$ and $a_2$=241.0 $\mathrm{K}{\mathrm{nm}}^2$ (solid line); see Ref. [28] and discussion in the text.