R. Douali

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from e*‖(ω) (E‖n) to e*⊥(ω) (E⊥n)allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.

Dielectric and viscous properties of 6CHBT in the isotropic and nematic phases

This paper presents the results of measurements of the principal electric permittivities ε ∥*(T, ω and ε⊥*(T, ω) and the viscosity for 4-(trans-4-n-hexylcyclohexyl)isothiocyanatobenzene (C6H13-CyHx-Bz-N=C=S, 6CHBT). In the nematic phase, the Miesowicz η2 viscosity coefficient was measured in a sample oriented due to the flow. On the basis of the temperature dependence of the static permittivities, using the Maier-Meier equations, the angle β between the dipole moment vector and the long axis of the 6CHBT molecule, the square of the molecular apparent dipole moment μ2 app 2 and the nematic order parameter S(T), were determined. From the temperature dependence of the viscosity and the relaxation time corresponding to the molecular rotation around the short axis, the strength of the nematic potential and the effective length of the 6CHBT molecule (in the isotropic phase) were estimated.

Ferroelectric properties of achiral banana-shaped and calamitic-chiral thioesters

Two thioester compounds: (S)-(+)-4-(1-methylheptyloxy)biphenyl 4′-heptylthiobenzoate (in short: MHOBS7) having rode-like, calamitic-chiral molecules, and 1,3-phenylene bis{4-[(4-octyloxybenzoyl)-sulfanyl]benzoate} (in short: 8OSOR) built of banana like achiral molecules, have been studied by complementary methods to show differences in their linear dielectric as well as ferroelectric properties. To this end the following experimental methods were used: differential scanning calorimetry, polarizing microscopy, dielectric spectroscopy and reversal current method. The first compound shows a ferroelectric SmC* phase with temperature dependent Goldstone mode displaying negative activation energy. At the N*--SmC* transition spontaneous polarization changes discontinuously showing first order character of this transition. For 8OSOR compound with bent core molecules only B1 phase shows up, which does not switch. However, it was possible to record reversal current spectra but temperature dependence of spontaneous polarization was unusual suggesting antiferroelectric order of this phase.

Phase behaviour and dynamics of exemplary MHPOBC analogue

Complementary methods were used to study one of the MHPOBC analogues: 4-(1-methylheptyloxycarbonyl)phenyl-4′nonylbiphenyl-4-carboxylate (MHPNBC). The substance shows rich phase polymorphism. In addition to the para-, ferro- and antiferroelectric phases it exhibits three sub-phases (two ferri- and alpha sub-phase) and a highly ordered room temperature phase. Typical Goldstone mode in the whole temperature range of the SmC* phase and the soft mode in the vicinity of the SmC*–SmA* transition were observed in the dielectric spectra. In the phase two relaxation processes were found: the non-cancellation mode (NCM) and the reorientation around the short molecular axis (S-process). There is only one dielectric relaxation process observed in a highly ordered phase. It is most probably the NCM, whereas the S-process seems to be frozen out.

Retardation of Molecular Relaxation in Highly Ordered Antiferroelectric Phase

ABSTRACT Dielectric and DSC methods were used to study liquid crystalline compound MHPNBC exhibiting para-, ferro-, ferri-, antiferroelectric and a highly ordered antiferroelectric like phase. Two dielectric relaxation processes were revealed in the ferroelectric phase: typical Goldstone mode in the whole temperature range and the soft mode in the pre-transition region on both sides of the Curie temperature. In the antiferroelectric phase two relaxation processes were found: anti-phase fluctuations and reorientation around the short molecular axis. Using dielectric spectroscopy it has been found that the latter process is strongly retarded at the transition to the highly ordered phase. In this phase there is only one collective dielectric relaxation process left which may be due to anti-phase fluctuations.

Ferroelectric behaviour of hexatic phases

In this article two examples of hexatic phases have been presented to show that the highly ordered phases can be either the ferro- or antiferroelectric ones. Some of them display a well-developed helicoidal structure, very high spontaneous polarization and low conductivity in contrast to liquid-like phases in higher temperature. On the other hand, some of their electrooptic parameters are not so useful for practical applications. First of all their alignment, i.e. a uniformly aligned mono-domain, is difficult in commercial cells. There are two kinds of smectic C*-hexatic transitions where the spontaneous polarisation changes either continuously or discontinuously in spite of the fact that from the thermodynamic standpoint both transitions are of the first order. The thermal, dielectric and electrooptic properties of the hexatic phases, studied by the DSC calorimetry, polarising microscopy, dielectric spectroscopy, texture observation and reversal current method have been presented for an exemplary fluorinated compound and discussed in the light of the existing theories. The compound shows an enantiotropic hexatic phase which exhibits large spontaneous polarisation.

On the phase transitions of 8CB/Sn2P2S6 liquid crystal nanocolloids.

Using differential scanning calorimetry measurements, the influence of Sn2P2S6 ferroelectric nanoparticles on the phase transition temperatures of the 8CB liquid crystal is studied. The spontaneous polarization, ionic and anchoring effects are discussed. For low concentration of dopant, the global effect leads to a decrease and an increase of the nematic-isotropic and the smectic A-nematic phase transition temperatures, respectively. For high concentrations, due to aggregates formation, the predominant anchoring effect induces a decrease of the both phase transition temperatures.Graphical abstract

Some Properties of Nematic Liquid Crystal E7/Acrylic Polymer Networks

Polymer Dispersed Liquid Crystal films were elaborated by photopolymerization using ultraviolet radiation of liquid crystal/monomer mixtures. Three acrylic difunctional propyleneglycol based monomers were used; differing only by their chain lengths in terms of their molecular weight. Infrared spectroscopy investigation made it possible to obtain the monomer conversion rates, in order to determine the effect of the presence of liquid crystal on the kinetics of polymerization and phase separation. The characterization by linear dielectric spectroscopy of the monomers and polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz.

Planar-homeotropic transition observed for B2 phase of banana-shaped thioester

1,3-Phenylene bis{4-[(4-dodecyloxybenzoyl) sulfanyl] benzoate} (12OSOR) possessing banana-like achiral molecules has been studied by complementary methods to investigate linear dielectric as well as antiferroelectric properties of its B2 phase. To this end, the following experimental methods were used: differential scanning calorimetry, polarizing microscopy, dielectric spectroscopy and reversal current method. As found, 12OSOR compound with bent-core molecules exhibits only B2 phase that happens to be antiferroelectric. Dielectric spectra have been measured using the dielectric spectrometer based on Agilent 4294A impedance analyzer. The measurements have been done using HG 5 µm cells (giving homogeneous orientation) with gold electrodes. Thermal, dielectric, and electro-optic properties of 12OSOR are discussed.

Highly ordered hexatic phases with large spontaneous polarization

In this article the physical properties of hexatic phases of three substances MHPNBC, FOOPP and FNHPP have been studied by differential scanning calorimetry, texture observation and dielectric spectroscopy. Experimental results are discussed from the point of view of existing theories. It is interesting that two of the substances studied, the FOOPP and FNHPP, exhibit enhanced spontaneous polarization in the highly ordered SmI* phase and show a jump of the spontaneous polarization in the vicinity of the SmC*–SmI* transition. In the SmI* phase of FOOPP a very high value of spontaneous polarization of the order of 530 nC cm−2 was found. Based on the results obtained the macroscopic and microscopic properties of the hexatic phases are discussed.