M. Marzec

Collective and molecular processes in a new class of ferroelectric liquid crystals

Abstract Electro-optic and dielectric studies have been performed for a thioester (with a chiral chain incorporating the lactic acid ester moiety), which exhibits a monotropic room temperature ferroelectric Sc* phase. Moreover, there are some hints that this substance also shows the recently discovered helicoidal SA* phase. The dielectric and electro-optic measurements have been carried out using ITO coated glass plates (d = 10 μm). Planar alignment was obtained by means of an AC electric field (v = 1Hz and V p-p = 70V). The tilt angle and spontaneous polarization temperature dependences show that the SA*-Sc* transition is of second order. Soft mode behaviour in the pretransitional region also confirms this result. The monotropic Sc* phase exhibits a rather strong Goldstone mode (GM) with pronounced temperature dependence of the dielectric parameters. The molecular reorientation about the long axis has been found for both principal alignments. The homeotropic effective alignment was obtained by applying a...

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.

Goldstone mode and domain mode relaxation in ferroelectric phases of 4′-[(S,S) - 2,3 - epoxyhexyloxy] phenyl 4-(decyloxy)benzoate (EHPDB)

Abstract Dielectric and electrooptic studies of 4′-[(S,S)-2,3-epoxyhexyloxy] phenyl 4-(decyloxy) benzoate (in short EHPDB) have been done and Goldstone mode (GM) and domain mode (DM) relaxation have been observed. The dielectric measurements have been done in the temperature range between 57˚C and 80˚C covering the N*, SmA*, SmC* and a metastable S3 phase. Temperature dependences of the dielectric parameters of GM and DM show that the phase known before as S3phase is one of the ferroelectric phases (SmI* of SmF*). The electrooptic measurements have been performed on uniformly aligned samples using polarizing microscope. Temperature dependencies of the dielectric and electrooptic data of the pitch of helix and the critical field for unwinding the helix show that the SmC*-(SmA*)-N*transition is of the first order type.

Dielectric and electro-optic behaviour of two chiral compounds and their antiferroelectric mixtures

Dielectric and electro-optic properties of two chiral compounds (MHPOBC analogues) and their three antiferroelectric mixtures have been studied. Mixtures of both compounds show SmA*, SmC*α, SmC* and an induced antiferroelectric phase, which is not present for pure substances. All materials were studied using differential scanning calorimetry, frequency domain dielectric spectroscopy and the reversal currents method. Spontaneous polarisation of the mixtures is between 200 and 280 nC/cm2; in the antiferroelectric phase it depends linearly on temperature. Ferroelectric and antiferroelectric phase of mixtures display a mono-domain texture grown in a strong electric field upon slow cooling from the SmA* phase. In the SmC* a domain mode was observed under the bias field, while the antiferroelectric phase shows two characteristic dielectric relaxation processes that are weakly bias field dependent. The low frequency relaxation is not a single Debye-type process, and it also shows up without a bias field. The dielectric data will be discussed based on the Parry-Jones–Elston theory.

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.

Electro-optic and dielectric properties of new binary ferroelectric and antiferroelectric liquid crystalline mixtures

ABSTRACT Several new binary liquid crystalline mixtures have been designed and their properties were studied by complementary methods. It has been shown that even both pure components used for the mixture design possess the ferroelectric behaviour; the induced antiferroelectric smectic phase has been detected for one of the prepared mixtures. The phase diagram has been constructed and the existence of the antiferroelectric phase was confirmed by switching time and dielectric spectroscopy measurements. Some of the resulted mixtures possess very high values of the tilt angle that reaches close below 45° degrees at saturation. Values of spontaneous polarisation were found within 50–200 nC/cm2 in dependence of the mixture’s composition. Due to specific properties, the obtained mixtures might be interesting for further design of multicomponent mixtures and formulation of the advanced nanocomposite systems. Graphical Abstract