Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

The optical birefringence of rodlike nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 $\mu$m length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau–de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.

Figure 1

Optical birefringence of 7CB and 8CB measured in the bulk state, panels (a), (b), and in the silica nanochannels, panels (c) and (d), resp., as a function of temperature in comparison to fits (solid lines) based on the KKLZ model discussed in the text. The final birefringence characteristic of the paranematic phases are shaded down to the P-N “transition” temperatures, $T_{\mathrm{PN}}$. The dashed lines mark the bulk I-N and N-Sm$A$ transition temperatures. As insets in (a) and (c), the bulk isotropic ($I^b$) as well as the bulk nematic ($N^b$) phases upon homeotropic alignment, and the confined paranematic (P) and nematic (N) phases are illustrated, respectively.

Figure 2

Schematic experimental setup of a high-resolution birefringence measurement consisting of a He-Ne laser, an optical polarizer (P), a temperature controlled (TC) optical cell (OC), an optical analyzer (A), a photoelastic modulator (PEM-90), a photodiode as a detector (PD), and a “lock-in” detection and analyzing unit. The inset depicts the orientation of the silica nanochannel membrane with respect to the incident laser beam.

Figure 3

Order parameter $q_e$ in the KKLZ model as a function of reduced temperature $t$ for selected effective surface fields $\sigma$ in the absence of quenched disorder, $\kappa =0$ (solid lines) and for the $\sigma$, $\kappa$ values which yield the best fits of $\Delta n^{*}(T)$ measured for the confined LCs (dashed lines). Inset: Order parameter jump at the I-N (P-N) transition as a function of the surface field $\sigma$ ($\kappa =0$).