Influence of Quadrupolar Ordering Factor on Supermolecular Structure in Ferroelectric Sm C*

A ferroelectric liquid crystal confined in cylindrical nanopores: reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations

The orientational and translational order of a thermotropic ferroelectric liquid crystal (2MBOCBC) imbibed in self-organized, parallel, cylindrical pores with radii of 10, 15, or 20 nm in anodic aluminium oxide monoliths (AAO) are explored by high-resolution linear and circular optical birefringence as well as neutron diffraction texture analysis. The results are compared to experiments on the bulk system. The native oxidic pore walls do not provide a stable smectogen wall anchoring. By contrast, a polymeric wall grafting enforcing planar molecular anchoring results in a thermal-history independent formation of smectic C* helices and a reversible chevron-like layer buckling. An enhancement of the optical rotatory power by up to one order of magnitude of the confined compared to the bulk liquid crystal is traced to the pretransitional formation of helical structures at the smectic-A*-to-smectic-C* transformation. A linear electro-optical birefringence effect evidences collective fluctuations in the molecular tilt vector direction along the confined helical superstructures, i.e. the Goldstone phason excitations typical of the para-to-ferroelectric transition. Their relaxation frequencies increase with the square of the inverse pore radii as characteristic of plane-wave excitations and are two orders of magnitude larger than in the bulk, evidencing an exceptionally fast electro-optical functionality of the liquid-crystalline-AAO nanohybrids.

Sign inversion of the spontaneous polarization in a "de Vries"-type ferroelectric liquid crystal

In contrast to common ferroelectric smectic C* liquid crystals, the siloxane-terminated smectic mesogen E6 is characterized by an unusual temperature variation of the spontaneous polarization. The polarization starts to grow from nearly zero despite the first-order SmA*-SmC* transition, and increases faster than linearly over a large temperature interval while the tilt angle rapidly saturates. To study this behavior in more detail, binary mixtures of different concentrations of E6 in the achiral SmC material C8Cl, which has a similar chemical structure, were investigated. Surprisingly, all mixtures show a temperature dependent polarization sign inversion, which shifts towards the SmC*-SmA* transition with increasing E6 concentration. For the pure E6 the inversion temperature meets the SmA*-SmC* phase transition temperature. In a second binary mixture with E6 and a conventional material C9-2PhP we found out, that the dependence of the inversion temperature on the concentration of E6 changes qualitatively when the nanosegregation is partially destroyed. A molecular theory of the polarization sign inversion in smectics C* with strong polar intermolecular interactions is developed which enables one to explain the concentration dependence of the inversion temperature in both mixtures.

Collective rotations of ferroelectric liquid crystals at the air/water interface

We study the Langmuir monolayers of four different ferroelectric liquid crystals on water surface. Two of them are attached to water surface by their polar groups, and the chiral groups, at the opposite ends of the elongated molecules, remain well above the interface. The other two ferroelectrics have both groups (polar and chiral) at close proximity, and therefore the chiral group is also attached to the surface or even submerged in water. We demonstrate that only when the chiral group of the ferroelectric liquid crystal in Langmuir monolayer is not attached to the interface and stays in the air does the system exhibit the collective rotations induced by evaporation of water (described for the first time by: Tabe, Y.; Yokoyama, H. Nat. Mater. 2003, 2, 806). The isotherms of surface pressure and surface potential versus molecular area of four compounds were measured with simultaneous observations using Brewster angle microscopy. Experimental data of the compression isotherms are described with a van der Waals model with very good accuracy, and the fitted parameters were used for calculations of compressibility coefficients for different phases found in the compounds under investigations. The ability of the two compounds for rotation and the disability of the two others is discussed in a context of thermodynamic properties of the monolayers.

Mechanism of sign inversion of spontaneous polarization in ferroelectric SmC* liquid crystals

The ferroelectric liquid crystal FLC-117, which is known to exhibit at a certain temperature the sign inversion of spontaneous polarization P(s), is studied under an electric field. From the analysis of the electro-optic response and the direct texture observations, it is concluded that the inversion temperature of P(s) has the applied voltage dependence. To interpret our experimental result, we introduce a coupling term between the molecular dipole moments and the magnitude of the applied electric fields to the asymmetric rotational potential about the molecular long axis. The simulated results using this potential are in accordance with our experimental result.