Mojca Čepič

Influence of Competing Interlayer Interactions on the Structure of the Sm C*α Phase

Abstract In a discrete phenomenological description of tilted smectic phases, the order parameter, which describes the magnitude and the direction of the tilt of the molecules, is defined for each smectic layer. The free energy expansion in terms of order parameter takes into account interactions between nearest and next nearest smectic layers, and allows for competing interactions. By minimizing the free energy of the system the structure of the frustrated Sm C*α phase is obtained.

A phenomenological model of antiferroelectric liquid crystals

Abstract We present the analysis of static properties of a simple phenomenological model of antiferroelectric liquid crystals, capable of explaining some different sequences of phase transitions found experimentally. Stability analysis shows the existence of three simply modulated phases. Results are summed up in phase diagrams with dependence on the parameters of the model. The existence of doubly modulated incommensurate phases is predicted in parts of the phase diagrams.

Flexoelectricity and piezoelectricity: the reason for the rich variety of phases in antiferroelectric smectic liquid crystals.

The free energy of antiferroelectric smectic liquid crystals which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric, and flexoelectric interactions to the nearest layers, and dipolar electrostatic interactions to the nearest and to the next-nearest layers, induce indirect tilt interactions with chiral and achiral properties, which extend to the third- and to the fourth-nearest layers. Although the strength of microscopic interactions changes monotonically with decreasing temperature, the effective interlayer interactions change nonmonotonically and give rise to a nonmonotonic change of the modulation period through various phases. Increased chirality changes the phase sequence.

Landau-Khalatnikov dynamics of helicoidal antiferroelectric liquid crystals

Abstract A phenomenological model of helicoidal antiferroelectric liquid crystals is analysed and the relaxational eigenfrequencies of ferroelectric and antiferroelectric modes are evaluated in the smectic A phase, in the ferroelectric smectic C* phase and in the antiferroelectric phase. In the intermediate ferrielectric phase the eigenmodes are mixtures of ferroelectric and antiferroelectric fluctuations.

Reentrant Ferroelectricity in Liquid Crystals

The ferroelectric (Sm C*)-antiferroelectric (Sm C*A)-reentrant ferroelectric (re Sm C*) phase temperature sequence was observed for systems with competing synclinic-anticlinic interactions. The basic properties of this system are as follows: (i) the Sm C* phase is metastable in the temperature range of the Sm C*A; (ii) the helix handedness inverts at both Sm C*-Sm C*A and Sm C*A-re-Sm C* phase transitions; (iii) the threshold electric field that is necessary to induce synclinic ordering in the Sm C*A phase decreases near both Sm C*A-Sm C* and Sm C*A-re-Sm C* phase boundaries. All these properties are properly described by a simple Landau model that accounts for nearest neighboring layer steric interactions and quadrupolar ordering only.

Modulated general tilt structures in bent-core liquid crystals

Structurally similar bent-core compounds were studied by the X-ray diffraction method. Modulated bilayer structures were observed in lamellar and columnar B1Rev-type phases. Their formation is explained by a general tilt SmCG structure. In the case of the B1Rev-type phases the bilayer modulations were found to be incommensurate with the block structure of the columnar phase.

Low frequency dielectric dispersion in the Sm Cγ* phase of an antiferroelectric liquid crystal

Abstract A comparision between the measured dielectric spectra of a compound exhibiting ferroelectric Sm C*, antiferroelectric Sm CA* and the intermediate ferrielectric Sm Cγ* phase with a calculated spectra emerging from a phenomenological Landau type theory is presented.

Electrostatic Interlayer Interactions in Tilted Polar Smectic Liquid Crystals

Abstract In chiral tilted smectic phases transverse molecular dipoles within a smectic layer order polarly as well as quadrupolarly. Electrostatic dipolar and quadrupolar interactions vanish between two smectic layers without positiorial correlations of molecules and exist only when the positions of molecules in the two layers are correlated. Therefore the interlayer electrostatic intcractions are of short range and are expected to act only between nearest and next nearest layers. Dipolar interactions prefer antiferroelectric ordering of the tilt vectors in the two layers, while the quadrupolar interactions favour to have the tilt vectors in the two layers in two perpendicular directions.

Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals

We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters.

Anchoring transition of bent-rod liquid crystal dimers on different surfaces

Non-symmetrical dimers have been synthesised, in which bent- and rod-shaped mesogenic molecules are linked by a flexible polymethylene spacer –(CH2) n –. It has been shown that compounds in which n = 5, 6 or 7 exhibit one (nematic), two (nematic and smectic), and no mesogenic phases, respectively. These exhibit a variety of anchoring transition behaviours depending on the mesogenic molecules (n = 5 or 6) and surfaces (polyimide surfaces for homeotropic and planar alignment, or a perfluoropolymer surface). Discontinuous and continuous planar-to-homeotropic orientational changes take place with decreasing temperature. These differences may originate from different dielectric anisotropy, and steric interactions caused by the directional relationship of bent and rod mesogens, depending on the length of the spacer. A possible explanation is suggested on the basis of surface–molecule interactions.