S. Wróbel

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Colro, Colrd, and Colhd that were identified by X-ray diffraction. In the higher temperature Colrd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Colro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the columns axis. Triangular wave switching studies at low frequency reveal processes inside the Colro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Colro phase there is an orientationally disordered crystalline Crx phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Crx phase.

Enhanced conductivity and dielectric absorption in discotic liquid crystalline columnar phases of a vanadyl complex

A liquid crystalline vanadyl complex has been studied by DSC, polarizing optical microscopy, the reversal current technique, X-ray diffraction and frequency domain dielectric spectroscopy. The compound exhibits three columnar phases: rectangular ordered (Colro), rectangular disordered (Colrd), and hexagonal disordered (Colhd), all of which show a dielectric relaxation process at low frequencies. In the Colro low temperature phase this process seems to be connected with a slow relaxation of polarized polymeric chains inside the columns (mHz frequency range). However, in the Colhd high temperature disordered phase this relaxation is faster (Hz range). It is interesting that the liquid crystalline phases studied show enhanced conductivity which changes by four orders of magnitude from 10−9 S m−1 in the orientationally disordered crystal (an ODIC phase) to 10−5 S m−1 in the Colhd high temperature phase. Such a value of the conductivity is typical for semiconducting materials.

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...

Dielectric spectroscopy and electrooptic studies of new MHPOBC analogues

Two new MHPOBC analogues, exhibiting ferroelectric and antiferroelectric phases, have been studied by means of frequency domain dielectric spectroscopy, time domain spectroscopy and electrooptic me ...

Dielectric, electro optic and X-ray studies of a room temperature ferroelectric mixture

Abstract Dielectric spectra for the SmA* and SmC* phases of a room temperature ferroelectric liquid crystalline mixture FELIX-008 have been studied in the frequency range from a few Hz to 13 MHz. It has been found that in the vicinity of the SmA*-SmC* transition there is a pronounced soft mode dielectric spectrum, the dielectric parameters of which obey a Curie-Weiss law given by the mean-field theory. In the SmC* phase a distinguished Debye-type dielectric spectrum of the Goldstone mode has been detected, and upon increasing the bias field to 35 V it is suppressed and a new non-Debye-type dielectric spectrum-originating most probably from the so-called domain mode-shows up. Spontaneous polarization and tilt angle measurements show that the transition SmA*-SmC* is of the second order type. The dielectric and electro optic studies make it possible to compute the rotational viscosity, which is of great importance for practical applications. It turns out that the energy barrier for rotational viscosity (19.6...

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.