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

Two nanosegregating siloxane-terminated mesogens with chiral (S,S)-2,3-epoxyoctyloxy side chains (E6, E11 )have been synthesized. These compounds form chiral SmA∗ and SmC∗ 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 (R,R)-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 SmC∗-SmA∗ 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 SmA∗-SmC∗ 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 SmA∗-SmC∗ phase transitions.