Jolanta Rutkowska

Some Physical Properties of Mesogenic 4-(trans-4′-n-Alkylcyclohexyl) Isothiocyanatobenzenes

Abstract Temperature dependences of density, refractive indices, and dielectric constants have been measured for mesogenic compounds of as for some of their cyano-analogues: and . The effects of substituting a phenyl ring by a cyclohexyl ring, as well as of a –CN group by an –NCS group have been analyzed. The results lead to the following conclusions: (i) the above exchange of rings leads to mesogens the long molecular axes of which do not coincide with the axes of the terminal groups, –CN or –NCS; (ii) antiparallel correlation of molecular dipoles found for cyano-mesogens does not occur in the case of isothiocyanato-mesogens.

Refractive Indices of the MHPB(H)PBC and MHPB(F)PBC Antiferroelectric Liquid Crystals

MHPB(H)PBC and MHPB(F)PBC were studied by means of refractometric measurements. The refractive indices: n “ (T), n Á (T) and n I (T) were measured using combined methods, applying both appropriately prepared Abbe refractometer and interference wedge. Values of the n “ (T), n Á (T) for the Sm A*, Sm C* and Sm C A * phases are related to the optical axis perpendicular to the proper smectic planes. The results of the optic measurements were discussed.

The Origin of High Optical Tilt in a Homologous Series of Fluorinated Antiferroelectric Liquid Crystals

The compounds from the fluorinated nF6B (n = 1,2,3,4,5,6,7) homologous series were investigated by means of optical, electrooptical, refractometrical, dielectric, densytometric, X-ray and IR spectroscopy methods. For some compounds being a subject of investigations a computer simulation of molecular properties were done. In our opinion high value of optical tilt Θo angle in comparison with a tilt angle obtained from X-ray measurements can be interpreted in terms of de Vries model of SmA* phases as well as of dimers creation process.

Technical aspects of dielectric spectroscopy measurements of liquid crystals

The dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose, the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in Sm A*, Sm C*, and Sm CA* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all possible relaxations in antiferroelectric liquid crystals (LCs).

Determination of molecular parameters of the MHPB(H)PBC and MHPB(F)PBC antiferroelectric liquid crystals

Abstract Two investigated antiferroelectric liquid crystals, MHPB(H)PBC and MHPB(F)PBC, were synthesized [1] and some their physical properties were described earlier [2]. Apart of the above, MHPB(H)PBC and MHPB(F)PBC have been studied by means of dielectric, densitometric, refractometric and X-ray measurements. Spontaneous polarization P s and tilt angle θ in ferro- (SmC*) and antiferroelectric (SmCA*) phases of MHPB(H)PBC and MHPB(F)PBC have also been investigated. Transverse α1 and longitudinal α1 molecular polarizabilities have been calculated from the ordinary n 0 and extraordinary n e refractive indices and density ρ of MHPB(H)PBC and MHPB(F)PBC, using Neugebauers, Vuks and Lorentz equations with both Haller and Subramhanyama extrapolation procedures. Effective values of the molecular dipole moment μ have been calculated from electric permittivity ε in the 1 phases of these compounds, using Maier and Meier equation.

On the Importance of the Molecular Core Interactions on the Induction of the High Optical Tilt Angle

Two smectogens being structural analogs with the opposite space orientation of the COO ester group in the molecular core exhibit extremely different tilt angles. A molecular mechanism of the induction of the high optical tilt angle is searched on the basis of extensive investigations of those compounds. Computer modeling of molecules and investigations of physical parameters of smectic phases were done. Different molecular aggregates forming smectic phases were observed. The size and shape of those aggregates are estimated on the basis of measurements and calculations. The different values of the tilt angles in studied compounds were discussed.

New High Frequency Dielectric Mode in Fluorinated Antiferroelecteric Liquid Crystals

The promising materials for applications are Antiferroelectric Liquid Crystals. Two dielectric modes were detected in AFLCs, as far [1, 2]. Low frequency mode called P L with the relaxation frequency around 100 Hz as well as high frequency mode called P H with the relaxation frequency around 100 kHz. The first (P L ) is related to in-phase motions of molecules on the cone surface (when two neighbouring layers are taken into account) while the second (P H ) is related to anti-phase motions of molecules on the cone surface. In recently synthesized fluorinated liquid crystals we detected new high frequency dielectric mode. We called it X-mode. Relaxation frequency is around 1–100 MHz and changes with the temperature.

Dielectric measurements of new antiferroelectric liquid crystals

Three new pure antiferroelectric materials, synthesized at the Military University of Technology (MUT), were investigated by dielectric means. Clear relaxation modes were detected, i.e., Soft, Goldstone, low frequency in-phase (PL) as well high frequency anti-phase (PH) modes for paraelectric Sm A*phase, ferroelectric Sm C* phase and antiferroelectric Sm CA* phase, respectively. Additionally, a high relaxation mode in Sm CA* was found. The relaxation frequencies fR of the observed modes were calculated from Cole-Cole diagrams.

Dielectric investigation of the diamagnetic anisotropy and elasticity of 4-trans-4′-n-hexyl-cyclohexyl-isothiocyanatobenzene (6CHBT)

Abstract We have measured the dielectric constants of 6CHBT. The results from studies of various alignments and thicknesses measured under different electric and magnetic fields are presented. We discuss how the dielectric properties depend on boundary conditions, sample thickness and the magnitudes of electric and magnetic fields. Experimental results and discussion in the terms of continuum theory make it possible to compute the diamagnetic anisotropy (Δχ), as well as the splay and bend elastic constants (K 11, K 33) of 6CHBT.

Composite Method for Determination of Liquid Crystal Material Parameters

Abstract A stable method for the determination of the nematic liquid crystal material parameters, such as the splay and bend elastic constants, the anisotropy of diamagnetic susceptibility, the electric permittivities and either the coupling between the nematic molecules and the substrate at the planar liquid crystal cell boundary (i.e. the director field boundary value), is presented in the work. The method consists of two stages: firstly the effective electric permittivity of planar nematics cell is measured as a function of external electric and magnetic fields, secondly the coefficient inverse problem is solved numerically using the experimental data to determine the values of material parameters. The coefficient inverse problem is posed in a variational way as a problem of nonlinear mean-square fit of the calculated nematics layer characteristics to the measured ones. The description of phenomena is based on Ericksens - Leslies theory. The unknown material parameters serve in this approach as the fit parameters, the best values of which corresponding to the best possible approximation of the experimental data by the refined mathematical model are to be found. The method was successfully used for the determination of the material parameters of PCB and can be applied to every nematics.