Dirk Kilian

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.

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 ...

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.

High temperature antiferroelectric smectic phase composed of banana-shaped achiral

Abstract In this paper a report is given on a fluorinated compound, composed of banana-shaped molecules snowing antiferroelectric behavior in the high temperature smectic 63 phase. Dielectric studies performed in the frequency range between 10 Hz and 13 MHz revealed two broad peaks in the low frequency dielectric spectrum of B2 phase. There is only one complex relaxation process in the B1 phase that transforms into a glassy state where the molecular process responsible for the dielectric spectrum is being frozen out. The Vogel-Fulcher model has been applied to describe temperature dependence of the relaxation time in the vicinity of the glassy transition that is characterized by To = 303.95 K and Tg = 316.46 K. At the end some possible mechanisms of dielectric relaxation processes observed will be discussed. Spontaneous polarization computed by integrating of the polarization current peaks is equal to ca. 400 nC/cm2 for the B2 phase.

Influence of Electric Fields on Antiferroelectric MHPOBC Analogue with Perfluorinated Terminal Chain

Abstract Frequency domain dielectric spectroscopy (FDDS), differential scanning calorimetry and polarising microscopy were used to study phase transition and dielectric properties of a new fluorinated MHPOBC analogue, exhibiting ferroelectric, ferrielectric and antiferroelectric phases. Dielectric spectra were measured versus temperature and as function of measuring electric field and bias field on sample aligned by means of 1.2 T magnetic field. Very large dielectric absorption observed between SmA and SmCA* phase with a characteristic break is most probably due to the presence of a ferrielectric SmCsub* phase. Up to four dielectrically active modes were detected within the intermediate SmC subphase. The intermediate phase shows pronounced bias and measuring field dependencies of dielectric spectrum.

Dielectric relaxation in four cyclic liquid-crystalline oligosiloxanes with different ring size and spacer length

Using dielectric spectroscopy in the frequency range 100 Hz to 13 MHz, the molecular dynamics of cyclic liquid-crystalline oligosiloxanes have been investigated. Up to two relaxation processes were found, depending on different molecular dipole components. The compounds were aligned and as well as temperature-dependent spectra, bias-field-dependent spectra were recorded to show clearly the behaviour of each component while aligned in the electric field. The influence of the ring size on the dielectric relaxation behaviour is described as well as the influence of the spacer length.

Surface Effect on Goldstone Mode in Ferroelectric Liquid Crystals

Abstract Dielectric studies of a single component ferroelectric liquid crystal material, showing chiral nematic to smectic C* phase transition have been investigated using a thin, highly conducting electrode cells. The main emphasis was given to the Goldstone mode which comes due to phase fluctuations in tilt angle of the molecules in Sm C* phase. To see the surface effect on the behaviour of Goldstone mode, one of the electrodes of the cell was highly anchored. The dielectric permittivity, particularly due to Goldstone mode has been studied for positive and negative polarity of the bias field in Sm C* phase. The Goldstone mode behaves differently during the positive and negative polarity of the bias field. The unusual dielectric behaviour has been explained by considering the charge accumulation phenomenon. The results agree with the electro-optical studies.

Dielectric Investigations of a Racemic AFLC Host (+/−)-12F1M7

Abstract Dielectric spectra of a racemic liquid crystal (+/−)12F1M7 synthezised recently were measured as a function of temperature on planar and homeotropically aligned samples. The complex dielectric behaviour inside the apparent mesogenic phases studied is presented. Cells prepared on gold-coated glass substrates were used to obtain good characteristics in a wide frequency and temperature range. Additionally transparent indium-tin-oxide (ITO) with rubbed polyimide-coated glass substrates were also used for surface aligned samples. A pronounced difference between “ITO” and “gold” cells emerges due to surface conditions. The results are discussed with respect to earlier measurements on optically pure S(+)-12F1M7.