S. Wrobel

Alpha sub-phase in a new ferroelectric fluorinated compound

Dielectric and DSC methods were used to study a new fluorinated liquid crystalline compound exhibiting ferroelectric and paraelectric phases as well as an intermediate alpha sub-phase. Two dielectric relaxation processes were revealed in the SmC* phase: a typical Goldstone mode over the whole temperature range and a soft mode in the pre-transition region on both sides of the SmC*–SmA* transition. From the temperature dependencies of the dielectric increments and critical frequencies for the dielectric relaxation processes observed in all the liquid crystalline phases, as well as from texture observations, it was shown that there is a SmC*α sub-phase between the ferroelectric SmC* and paraelectric SmA* phases.

Antiferroelectric liquid crystals studied by different methods

Abstract Liquid crystalline substances, exhibiting ferroelectric and antiferroelectric phases, have been studied by means of frequency domain dielectric spectroscopy as well as electrooptic and DSC calorimetry methods. Strong Goldstone mode in the ferroelectric SmC* phase and soft mode in the paraelectric SmA* phase have been revealed. In the antiferroelectric phase two dielectric relaxation modes, showing temperature independent dielectric increments and relaxation frequencies, have been observed. It has been found that the reversal current spectrum exhibits one peak in the ferroelectric SmC* phase whereas the antiferroelectric phase shows two separated peaks. Spontaneous polarization has been evaluated as a function of temperature.

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. au2009uniformly 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.

Ferroelectric and antiferroelectric phases studied by dielectric spectroscopy

Two MHPOBC analogues, exhibiting ferroelectric and antiferroelectric phases, have been studied by means of frequency domain dielectric spectroscopy, time domain spectroscopy and electrooptic methods. Four relaxation processes at low frequencies and one broad spectrum at high frequencies have been revealed in the antiferroelectric phases of both substances studied by dielectric spectroscopy.

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 530u2009nCu2009cm−2 was found. Based on the results obtained the macroscopic and microscopic properties of the hexatic phases are discussed.

Anomalous behaviour of antiferroelectric SmCα* phase

New liquid crystalline substance has been studied by means of frequency domain dielectric spectroscopy (FDDS) and electro-optic methods. Typical soft mode is visible in paraelectric SmA* phase. Temperature dependencies of dielectric increment and critical frequencies in SmCsub* are more complicated what suggests existing of SmC(alpha )* phase. There is no evidence of the Goldstone mode in the monotropic SmC(alpha )* phase what supports the idea of its antiferroelectric nature. It is confirmed by measurements of spontaneous polarization by the reversal current method. Two peaks appear on the opposite sides of the zero field. The temperature dependence of spontaneous polarization as well as FDDS measurements show that transition between the SmC(alpha )* and paraelectric SmA* phases seems to be a first order transition.

Recent results on some columnar paramagnetic metallomesogens

The liquid crystalline and selected physical properties of some vanadyl and manganese(III)-porphyrin-TCNE complexes are discussed. These materials form linear chains from a magnetic point of view and columnar discotic LC phases. Some of the manganese complexes exhibit a bulk magnetic phase below ∼20 K. The discussed materials are studied by means of DSC, X-ray powder diffraction, dielectric spectroscopy, reversal current characterization and magnetic susceptibility measurements. A broader view on some physical properties of columnar paramagnetic metallomesogens is given.

Antiferroelectric liquid crystals studied by DSC, electro-optic, and dielectric methods

Thermal properties of four liquid crystalline substances exhibiting antiferroelectric SmCA* and ferroelectric SmC* phases were studied using differential scanning calorimetry, texture observation, electrooptic measurements and dielectric spectroscopy. The measurements were performed both on heating and cooling of the samples. All four substances studied in this work are characterized by a complex polymorphism. The temperatures of phase transitions and enthalpy changes associated with them were determined. The transition from the liquid crystalline to the crystalline state showed significant hysteresis for all four substances studied. Textures observations and electrooptic measurements were performed using ITO cells having thickness from 6 to 10 micrometers . The measurements of spontaneous polarization were performed by means of reversal current method. Spontaneous polarization was measured for a few frequencies of the triangular voltage applied. Temperature dependencies of spontaneous polarization have been studied as a function of the side chain structure.

Induced Goldstone mode in a highly ordered ferroelectric phase

Dielectric, electrooptic and pyroelectric properties of unknown SmX* phase of (S)-(1-propyl-oxycarbonyl) ethyl 4(dodecyloxythiobenzyloxy) benzoate are studied. Low frequency dielectric studies performed with a 10micrometers EHC cell show that this SmX* phase exhibits an induced Goldstone mode dielectric spectrum. The spectrum is a single Debye-Type process with the relaxation time of the order of 10-5 s. The intensity of the spectrum increases non- linearly with the bias field strength. Studies of pyroelectric coefficient in the vicinity SmA*-SmX* transition show, that the SmX* phase is not a typical ferroelectric or antiferroelectric phase, and that the character of the transition depends on the field strength applied.

Electro-optic properties of new antiferroelectric mixtures

Spontaneous polarization and threshold voltages have been measured for four multicomponent mixtures based on new MHPOBC analogues. Electrooptic measurements were performed using ITO cells having thickness 7 and 10 micrometers . Spontaneous polarization has been measured by reversal current method. It was found that the spectrum of polarization currents depends strongly on the frequency of voltage applied as well as on temperature. Temperature dependencies of spontaneous polarization and threshold voltage have been analyzed using the mean-field theory of the second order transition.