M. Godlewska

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.

Phase Behavior of Didodecyldimethylammonium Bromide

Abstract Phase situation of didodecyldimethylammonium bromide (DDAB) was investigated by means of differential scanning calorimetry. X-Ray diffraction and optical microscopy methods. A complex polymorphism was detected. Apart from the crystalline phase on heating there are two liquid crystalline phases below the isotropic liquid and on cooling a third phase appears. The study shows that DDAB exhibits highly ordered layered liquid crystalline structures. The high-temperature phase which appears below the isotropic phase can be identified as SmB phase. The influence of thermal history the polymorphism of DDAB was studied. The enthalpy changes at the transitions were calculated.

Far infrared study of molecular reorientation in some alkoxyazoxybenzenes

Abstract Far infrared spectra in the frequency range 20 to 200 cm−1 for the 4,4δ-di-n-alkoxyazoxybenzene homologous series are presented for the isotropic and nematic phases and for 0.1 M solution in benzene. In addition some partially deuteriated samples were studied. The spectra consist generally of two broad bands. By comparison of the spectra for different deuteriated compounds it is possible to interpret the higher frequency band as being connected with the intramolecular torsional vibration of the end groups. The band at lower frequencies we associate with the Poley absorption of librating molecules. The confined rotator model calculations performed for this band, for all compounds studied, give the rotational correlation time which is in good agreement with the correlation times as derived from quasi-elastic neutron scattering experiments.

DSC study of the phase transitions in [Ni(D2O)6](ClO4)2 and [Ni(H2O)6](ClO4)2

DSC measurements were carried out for [Ni(H2O)6](ClO4)2 (sampleH) and [Ni(D2O)6](ClO4)2 (sampleD) in the temperature range 300–380 K. For both compounds two anomalies on the DSC curves were detected. The results for sampleH are compared to those previously obtained using adiabatic calorimetry method. For both compounds studied in this work the high-temperature transition appears at the same temperature while the low-temperature one is shifted towards higher temperatures in sampleD. Disorder connected with H2O or D2O groups is suggested in the intermediate phase between the low- and high-temperature transitions.ZusammenfassungAn [Ni(H2O)6](ClO4)2 (ProbeH) und [Ni(D2O)6](ClO4)2 (ProbeD) wurden DSC-Messungen im Temperaturintervall 300–380°C durchgeführt. Für beide Verbindungen wurden an den DSC-Kurven zwei Anomalien festgestellt. Die Ergebnisse für die ProbeH wurden mit denen verglichen, die unlängst mittels adiabatischer kalorimetrie ermittelt wurden. Für beide in vorliegender Studie untersuchten Verbindungen erscheint die Umwandlung bei höherer Temperatur bei der gleichen Temperatur, während die bei niedriger Temperatur im Falle der ProbeD in Richtung höherer Temperaturen verschoben wird. Für die Übergangsphase zwischen den Nieder- und Hochtemperaturmwandlungen wird eine Fehlordnung bezüglich der H2O oder D2O Gruppen angenommen.

Collective and molecular relaxation in ferroelectric liquid crystals

Ferroelectric liquid crystals are molecular ferroelectrics showing up in the tilted liquid crystalline systems (SmC*, SmI*, SmF*) composed of chiral molecules. In this work, we present the dielectric, electro-optic, and calorimetric studies of a single component system: 3-octyloxy-6[2-fluor-4-(2-fluoroctyloxy)phenyl]-pyridine showing interesting ferroelectric properties. The compound exhibits a first order N*- SmC* phase transition which leads to a qualitatively new behavior, for instance the relaxation frequency of the soft mode below TC seems to be temperature independent. The high frequency relaxation process, connected with the reorientation around the long axis, is practically undisturbed at the N*-SmC* transition. Yet, it was found that in the SmC* phase, the best fit was obatined with two Cole-Cole functions yielding two relaxation times to describe a biased reorientation of molecules in the SmC* phase.

Polymorphism and Molecular Dynamics in Two Selected Ferroelectric Liquid Crystals

Abstract Broad band dielectric spectroscopy, DTA calorimetry and polarizing microscopy have been used to study smectic polymorphysim and molecular dynamics of a well known ferroelectric material DOBAMBC and of fluorinated substance 3-(3-Flunoryo-nonyl)-6[4-Heptyloxyphenyl]-Pyridine (in short FNHPh-P). Both materials exhibit rich polymorphism and highly ordered phases. The SmG* phase of FNHPhP is ferroelectric with distincly longer switching times than those observed for the SmC* phase. High frequency dielectric measurements of DOBAMBC have been performed in GHz range for the first time as far as we know. The high frequency relaxation which is connected with the reorientation of molecules around their long axes has been detected in ferroelectric SmC* phases and also in highly ordered phases of both substances. The spectrum of critical frequencies is presented for both materials; for DOBAMBC the literature data were used as far as tlie radio frequency range is concerned. The spectrum is tlisscused in terms ...

Thermal behavior of selected ferroelectric liquid crystals

Thermal properties of three ferroelectric liquid crystals, namely: 3-octyloxy-6-[2-fluor-4-(fluoroctyloxy)phenyl]-pyridine (FFP), 3-(3-fluor-nonyl)-6[4-heptyloxyphenyl]-pyridine (FNHPh-P), 4-[(S,S)-2,3-epoxyhexyloxy]phenyl 4-(decyloxy)benzoate (EHPhDB) were studied using heat flux differential scanning calorimetry method. All the transitions expected in this compounds, except the SmC*-SmA* and SmC*-S3 transitions, were detected in the DSC curves. The temperatures of the phase transitions and the enthalpy changes associated with them were determined. The transition from the liquid crystalline to the crystalline state showed significant hysteresis for all three compounds studied. The following transitions: SmA*-Is, SmG*-SmC* for FNHPh-P, N*-Is for EHPhDB, N*-Is and SmC*-N* for FFP also showed a small hysteresis basing on which first-order character of all the above transitions was implied. All three substances studied in this work are characterized by a complex polymorphism in the solid state.

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.

High-pressure differential thermal analysis (DTA) of the phase behaviors of didodecyl-dimethylammoniumbromide (DDAB) and ditetradecyl-dimethylammoniumbromide (DTAB)

Dialkyldimethylammonium halides are known for their applications as commercial detergents and for their membrane-mimetic properties. They display both thermotropic and lyotropic liquid crystalline properties. In this work we establish the T(p)-phase diagrams of two representatives, namely didodecyl- and ditetradecyl-ammonium bromides. The transitions between crystal, smectic Ii and smectic I phases were recorded using a high-pressure differential thermal analysis device. For both compounds the stable range of smectic II phase is pressure-limited, resulting in a triple point crystal/smectic II/smectic I. Enthalpy changes at ambient pressure have been determined with the aid of a commercial heat flux differential scanning calorimeter. Volume changes were calculated from the slopes of the transitions lines, employing the Clausius Clapeyron equation.

Investigation of 4'-[(S)-2-propoxypropoxy]phenyl 4-(4-decyloxy) benzoate (PPPhDB) by complementary methods

Abstract Dielectric, electrooptic, DSC calorimetry studies and texture observations have been carried out for 4‘-[(S)-2 propoxypropoxy] phenyl 4-(4-decyloxy) benzoate in a wide temperature range. Dielectric spectra were taken in the frequency range from 10Hz to 10GHz. Electrooptic studies were performed on oriented SmC* phase. Texture observations and DSC calorimetry confirm the existence of a monotropic highly ordered S3 phase. On the basis of TDS measurements and electrooptic studies this phase seems to be a highly ordered non-ferroelectric phase. Low frequency dielectric spectrum of the SmC* phase is complex and consists at least of two contributions which is typical of ferroelectric liquid crystals with high spontaneous polarization.