A. Mikułko

Dielectric and Electro-Optic Properties of New Ferroelectric Liquid Crystalline Mixture Doped with Carbon Nanotubes

In scope of this paper, dielectric and electro-optic properties of planar aligned ferroelectric liquid crystals/carbon nanotubes dispersions were investigated. It is demonstrated that even a small concentration of nanotubes greatly affects the performance of the liquid crystal cells. The obtained experimental results are explained by the trapping of ions through the carbon nanotubes, which results in a significant modification of the internal electric field as well as the space charges distribution inside the liquid crystal cells.

Electrooptic study of antiferroelectric mixtures for display application

The aim of this paper is to study the influence of electric field on alignment of para-, ferro- and antiferroelectric phases in the vicinity of SmA* — SmC* or SmC* — SmCA* phase transitions as to obtain mono-domain cells. Four mixtures studied (W-193B, W-193B-1, W-201, W-204D) show the SmCA* phase in a wide room temperature range. Measurements of the spontaneous polarization versus temperature by using reversal current method give an answer to the question, what kind of the transitions take place between para-, ferro- or antiferroelectric phases using the Landau mean field theory. Optimal electrooptic parameters for different compositions of the mixtures such as tilt angle, spontaneous polarization and saturation voltage have been measured to compare parameters of the mixtures studied.

Antiferroelectric Modes Behaviour in a Wide Temperature Range SmC*A Phase

ABSTRACT Two liquid crystalline partially fluorinated compounds: (S)–(+)–4-(1-metyloheptyloxycarbonylphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) biphenyl-4-carboxylate, in short MHPP(9F)HBC and (S)–(+)-4-(1-metyloheptyloxy-carbonylbiphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) benzoate, in short P(9F)HPMHBC, exhibiting paraelectric, ferroelectric and antiferroelectric phases have been studied by dielectric spectroscopy and spontaneous polarization measurements. These two compounds have the same molecular weight but their terminal chains at para positions are interchanged. Influence of molecular structure on the phase diagram and dielectric behaviour is presented. Two relaxation process revealed in the antiferroelectric phase are interpreted as collective modes: anti-phase and in-phase fluctuation of phase and amplitude. The reorientation around the short molecular axis does not contribute to the dielectric spectra.

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.

Ferroelectric Behaviour of a Chiral Thioester

In this contribution a chiral thioester showing paraelectric – ferroelectric transition has been studied by DSC, texture observation, dielectric spectroscopy and reversal current method. The substance studied shows a direct cholesteric (paraelectric) to tilted smectic C* (ferroelectric) transition which is a first order type.

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.

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.

Molecular Structure and Physical Properties of Chiral Liquid Crystalline Compounds

Influence of molecular structure on physical properties of chiral compounds is presented. It is shown that fluorinating of the achiral chain have great influence on physical properties of liquid crystalline phases. As an example, two compounds: (S)-(+)-4-(1-Metyloheptyloxy-carbonylbiphenyl) 4′-(6-perfluoropentanoyloxyhex-1-oxy) benzoate and (S)-(+)-4-(1-Metyloheptyloxycarbonylbiphenyl) 4′-(6-pentanoyloxyhex-1-oxy) benzoate have been selected. Phase transition temperatures and stability of these compounds have been studied by DSC calorimetry, texture observation, electrooptic measurements and dielectric spectroscopy. Based on these studies it was found that the SmC*–SmA* transition is a continuous one for both compounds studied.

Influence of Molecular Structure on Phase Diagram and Ferroelectric Behaviour

Discussion on the influence of molecular structure on dynamics and ferroelectric properties of two compounds with cyanic group in achiral chain is presented. Physical properties of these two compounds have been studied by dielectric spectroscopy, DSC calorimetry, texture observations and reversal current method. Based on the results of these studies the phase sequence of the compounds and the character of the ferro- paraelectric transition have been determined. Temperature dependence of the spontaneous polarization and dielectric parameters of the soft mode suggest that the SmC* - SmA* transition is of the second order type for both compounds studied.