Unconventional ferroelectric behavior in nanosegregating liquid crystals with de Vries–like behavior

Two nanosegregating siloxane-terminated mesogens with chiral $\left(S,S\right)$-2,3-epoxyoctyloxy side chains (E6, E11 )have been synthesized. These compounds form chiral $\text{Sm}{A}^{\ast }$ and $\text{Sm}{C}^{\ast }$ phases and show an untypical behavior of the spontaneous polarization which increases with decreasing temperature in a convex manner while the tilt angle saturates. We compare these results with results obtained for two similar mesogens with chiral $\left(R,R\right)$-2,3-difluorooctyloxy side chains (F6, F11), which both show a typical concave curvature with decreasing temperature. A theoretical explanation is given for the unexpected temperature dependency of the spontaneous polarization. The materials also exhibited first-order $\text{Sm}{C}^{\ast }\text{-Sm}{A}^{\ast }$ phase transitions and hence, very high values of the tilt angle. All substances show increased de Vries character in the range of 50%, which is substantially higher than 20% for the nonsiloxane analogs. Furthermore, for the latter materials, second-order phase transitions are common, while the siloxane materials exhibit first-order $\text{Sm}{A}^{\ast }\text{-Sm}{C}^{\ast }$ phase transitions. These results clearly suggest that the achievement of nanosegregation is a powerful tool to induce de Vries behavior and to promote first-order $\text{Sm}{A}^{\ast }\text{-Sm}{C}^{\ast }$ phase transitions.

Figure 1

Chemical structures and phase transition temperatures in $°\text{C}$ of siloxane phenylpyrimidines E6, E11, F6, and F11.

Figure 2

Reagents and conditions: (a) $\left(E\right)\text{−}\left(S,S\right)$-2,3-epoxyoctan-1-ol, DIAD, ${\text{Ph}}_3\text{P}$, THF, $25°\text{C}$; (b) 6-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)hexanol or 11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecanol, DIAD, ${\text{Ph}}_3\text{P}$, THF, and $25°\text{C}$.

Figure 3

(Color online) Layer spacing $d$ as a function of temperature $T$ obtained from small-angle x-ray scattering (SAXS) measurements on heating, and the molecular structure of E6, E11 derived from the molecular modeling at the AM1 level.

Figure 4

(Color online) Temperature dependence of the spontaneous polarization (triangle) and tilt angle (circle) of a) F6, F11 and b) E6, E11. In these nanosegregating FLC materials the tilt angle saturates while the polarization continues to grow with decreasing temperature.

Figure 5

Temperature dependence of the ration of spontaneous polarization and tilt angle for a) F6, F11 and b) E6, E11. The solid lines are the best fits of the data obtained in the $\text{Sm}{C}^{\ast }$ phase using Eq. (12)..