S. Hiller

The domain mode in ferroelectric liquid crystals; electrooptical and dielectric investigations

Abstract The formation of ferroelectric domains in the smectic C* (SmC*) phase of ferroelectric liquid crystals with considerable high spontaneous polarization is observed using electrooptical methods. The temperature dependence of the corresponding domain modes DB (bulk) and DS (surface) is studied by dielectric spectroscopy. The behaviour of the dielectric strengths and the relaxation frequencies of these processes is investigated. The activation energies of the DB and the DS modes are given assuming the validity of the Arrhenius law.

Short-Pitch Modes Approach to the Problem of Antiferroelectricity in Liquid Crystals

Abstract The short-pitch modes (SPM) model is proposed for the description of various antiferroelectric and ferrielectric phases in the framework of phenomenological approach taking into account short-range correlations with different characteristic lengths for non-chiral and chiral terms in the free energy expansion, this small difference being dependent on the temperature. The ratio of correlation length to molecular length determines the number of SPM with wave numbers close to rational numbers of the inverse molecular length. The weak chiral interaction determines the wave number of long-pitch mode and the temperature intervals of the existence of each phase. The free energies of all the SPM are equal at the critical point that causes a mixing, i.e., a coexistence of many modes in the vicinity of this point. The alternation of ferri- and antiferroelecric phases near the critical point depends on material parameters of compounds. The existence of ferrielectric phases is determined by a neighbourhood of...

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 and pyroelectric investigations on the domain mode process in high Ps-FLC materials

Abstract Ferroelectric liquid crystals with high enough spontaneous polarization (Ps > 50nC/cm2) exibit a special periodic structure which forms after switching off a moderate electric field. One of the most important features of this new structure is the existence of inclined smectic layers with opposite signs of the inclination angle δ.In this contribution we present the results of our recent dielectric and pyroelectric investigations in the FLC-mixtures with different value of the spontaneous polarization. The temperature and BIAS dependencies of the relaxation strength and critical frequency of the domain mode process in the different mixtures were obtained. The suppression of the domain texture and the domain mode process was investigated using dielectric and pyroelectric technique.

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.

X-Ray and Optical Investigations of the Dislocation Domains in Ferroelectric Liquid Crystals

In ferroelectric liquid crystals with high enough spontaneous polarization P s≥50 nC cm-2 the formation of special periodic structure takes place after switching off the moderate value of the electric field. There are dislocation walls which separate the regions of inclined smectic layers with the opposite signs of inclination angle δ for neighbour regions (domains). We have found that 1) angle δ is proportional to the value of the spontaneous polarization, 2) the period D of the dislocaton walls is inversely proportional to P2 s, 3) during the electric field treatment the periodicity of domains and the tilt angle δ of the inclination of smectic layers can differ essentially in the bulk of FLC layer and near the electrode surfaces. Complicated character of SSFLC switching can be related with the interference of bulk and surface dislocation walls. The role of the charged dislocation walls in the phenomenon of volume bistability is also discussed.

Dielectric, Electrooptical and Magnetic Properties of Metallomesogens

Abstract The dielectric behaviour of ferroelectric liquid crystals of copper (II) Schiff bases complexes were investigated by dielectric spectroscopy. The spontaneous polarization Ps and the switching time T were determined by means of the polarization reversal method. Temperature dependent magnetic susceptibility measurements were carried out in the temperature range of 4.2 - 450 K to examine the magnetic properties of the copper (II) complexes.

Structure and kinetics of ferroelectric smectic nuclei at the chiral nematic–smectic C∗ phase transition under the influence of electric fields

Abstract In the ferroelectric liquid crystal 3-octyloxy-6-[2-fluoro-4-(2-fluoro-octyloxy)phenyl]pyridine (FFP), having the transition smectic C∗ (S∗C)-chiral nematic (N∗), the formation of the induced S∗C phase under external electric fields was investigated. In the N∗ phase, close to the transition temperature Ts∗ c∗ N∗, the induction of a strongly twisted S∗c phase is possible. In the case of AC voltage, the growth of nuclei of the S∗c phase can occur through the formation of circular smectic layers. Dependent on the frequency and amplitude of the electric field, the S∗C helix within the circles can be twisted or untwisted. This leads to different resulting colours of the circles due to the different anisotropies of the average refractive index and their wavelength dispersion. The coexistence of twisted and untwisted smectic circular nuclei with the supertwisted nematic phase looks like a flower garden (‘Darmstadter Blumengarten’).

Dielectric investigations on a liquid crystalline compound showing wide ferrielectric and antiferroelectric phases

Abstract Dielectric, pyroelectric and calorimetric measurements on the antiferroelectric compound (R) 1-Methylheptyl-4′-(4′-n decyloxybenzoyloxy)-biphenyl-4-carboxylate, denoted as 10B1M7, showing a broad ferrielectric SmCγ* phase have been performed. The investigations of dielectric permittivity and loss were done as functions of frequency, dc biasing field and temperature for the whole mesogenic range in cells with different thicknesses. The dielectric behaviour is compared with the results from DSC measurements, thermal polarizing microscopy and to literature. The determination of spontaneous polarisation by different techniques (hysteresis method, current reversal method, pyroelectric technique) has also been included. A new phase, denoted as SmCx* phase, was found below SmCγ*phase.In the higher ordered phases below the SmCA* phase a very strong activated process has been observed which is not described up to now.

Dielectric processes in ferroelectric liquid crystals, possessing chiral nematic to smectic c* phase transition

Abstract The dielectric processes in high tilt angle fetroelectric liquid crystal, possessing chiral nematic to smectic C* phase, has been studied by dielectric relaxation method in the frequency range of 10 Hz to 10 MHz in a thin, gold coated electrode cell. The studies have been reported in a homeotropic and planar aligned cell. The orientation of the molecules was done by a magnetic field. It has been observed that low frequency molecular relaxation around the short axis of the molecule is seen in chiral nematic phase and also in smectic C* phase in homeotropically aligned cell. The dielectric permittivity is dominated by the Goldstone mode in smectic C* phase in planar aligned sample. The effect of do biasing field on the Goldstone mode is studied in planar aligned cell. The temperature dependences of the dielectric relaxation processes in smectic C* and chiral nematic phase has also been reported and discussed.