Double Reentrance of Surface Arrangements in Freestanding Liquid Crystal Films

The molecular arrangement of tilted surface layers on freestanding films above the bulk smectic-$A$–smectic-$C^{*}$ transition has been studied by null transmission ellipsometry. In the presence of an applied in-plane electric field, a synclinic-anticlinic-synclinic-anticlinic double reentrance has been observed. Furthermore, we have characterized a novel transition from the synclinic arrangement to a structure in which the interior layers tilt in the same orientation and the outermost surface layers tilt oppositely with the interior layers.

Figure 1

$\Delta$ as a function of temperature upon cooling from a four-layer film with $E=32\mathrm{V}/\mathrm{c}\mathrm{m}$. Four upward arrows indicate where the four transitions occur. Four downward arrows indicate the temperatures at which data under rotations of $E$ in Figs. 2a, 2b, 2c, 2d were obtained. On top of the figure is the chemical structure of 8422[2F3]. (a)–(e) The tilt structures and directions of the net polarization determined from simulations at different temperature windows. (f),(g) Another two possible structures below $T_1$, which are ruled out (see text for details).

Figure 2

$\Delta$ versus $\alpha$ at different temperatures from a four-layer film with $E=32\mathrm{V}/\mathrm{c}\mathrm{m}$. Symbols are data. Solid lines are fits using the structures depicted in Figs. 1a, 1b, 1c, 1d, 1e.

Figure 3

Phase diagram for four- and six-layer films. Open and solid circles are results from ramping $E$ up and down, respectively. For four-layer (six-layer) films, $E$ was ramped between 11 and $45\mathrm{V}/\mathrm{c}\mathrm{m}$ (0 and $41\mathrm{V}/\mathrm{c}\mathrm{m}$) with 40 (70) steps. At each step, data were taken for 1 min. Solid lines connecting symbols (above which molecules adopt a synclinic structure) are guides to the eye. The four solid vertical lines in (a) and (b) are to separate the three temperature windows. Beside the lines, the temperatures are labeled as $T_1$ and $T_4$ for four-layer films and $T_5$ and $T_6$ for six-layer films, respectively. The two heavier solid curves in (a) are extrapolations of the boundary separating synclinic and anticlinic states. See text for the meaning of the three dashed horizontal paths.

Figure 4

Temperature dependence of $\Delta$ obtained upon cooling under two opposite directions of $E$. Data in (a) [(b)] are from a six-layer [four-layer] film with $E=18\mathrm{V}/\mathrm{c}\mathrm{m}$ [$E=45\mathrm{V}/\mathrm{c}\mathrm{m}$]. (c)–(f) How the transition occurs for six-layer films. The direction of the net polarization is depicted beside the tilt structure. In (a), $T_t$ is the temperature where ${\Delta }_{90}$-${\Delta }_{270}$ starts to decrease and at $T_p$ the net polarization becomes zero.