Magdalena Włodarska

Dielectric spectroscopy of polyazomethine with vinylene moieties in the main chain

We have explored the dielectric properties of polyazomethine with vinylene moieties in the main chain (PAZ-PV). The dialdehyde 2,5-bis(hexyloxy)-1,4-bis[(2,5-bis(hexyloxy)-4-formyl-phenylenevinylene]benzene was polymerised in solid-state with the diamine poly(1,4-butanediol)bis(4-aminobenzoate). The polymer obtained exhibited liquid crystal properties. The molecular dynamics and phase transitions including the liquid crystal phase of PAZ-PV were investigated by dielectric spectroscopy. The dielectric constant decreases with a decrease in the temperature, except in the nematic phase region, where a slight increase was observed. The temperatures of the phase transitions were well seen and were compared with differential scanning calorimetry (DSC) and polarised optical microscopy experiments. All values determined from dielectric data were a few degrees higher than those obtained in previous measurements (DSC).

Synthesis and phase transitions of mesogenic compounds with functional groups in the tail

The properties of two pairs of recently synthesised compounds with mesogenic unit were compared. The four compounds consist of a central segment (based on biphenyl or phenylbenzoate) with symmetric carbon chains ended with vinyl or epoxy groups. Typical measurement techniques (including microscopic observations, differential scanning calorimetry and wide-angle X-ray scattering and dielectric studies) were used to examine phase transitions in the studied materials. The phase transition sequences were determined for all materials. A liquid crystalline phase appeared in some cases. It was observed that replacing the functional group in the tails visibly changes the temperatures and sequences of phase transitions.

Comparing Liquid Crystalline Properties of Two Epoxy Compounds Based on the Same Azoxy Group

Two recently synthesised diepoxy compounds based on the same rigid central segment with carbon chains of different lengths were compared. Microscopic observations, differential scanning calorimetry (DSC) and dielectric measurements confirmed the existence of a mesophase in both compounds and enabled determination of the temperatures of phase transitions. Wide-angle X-ray scattering (WAXS) measurements and optical observations enabled identification of mesophase types. Nematic phase appeared in both compounds; in the material with longer chains smectic A phase was also detected. Lengths of the molecules and average nearest neighbor distances were estimated using the wide-angle X-ray scattering results. Lengths of the molecules were also computed by quantum mechanical methods resulting in very good agreement with the experimental data.

Liquid crystal epoxy resins based on biphenyl group cured with aromatic amines - studied by dielectric spectroscopy

In this paper we investigate the dielectric response of cured epoxy resins based on biphenyl group. We chose two monomers, which were earlier synthesised, containing the same mesogen but with long and short symmetric tails. They were cured with standard amines (DDM, DMAP). Firstly we optimised conditions of the curing process by dielectric and optical observation in situ. Small differences in the dynamics of the curing process were observed in different mixtures. For the four selected products we made dielectric measurements in a wide range of frequency and temperature. Although, generally, dielectric response is similar in all the cured materials, differences can be seen in the details. At lower temperatures, quite strong relaxation process is seen in all products, its low activation energy points towards a β-process, which appears in many polymers. The Havriliak-Negami formula was fitted to the data and characteristic parameters of the observed relaxation processes were compared in detail. In some cases the α-process also appears. We conclude how the longer spacer and the type of curing process influence the electrical properties of the resin.

Dielectric relaxations in new liquid crystalline diepoxy monomer

A series of new liquid crystalline divinyl and diepoxy monomers of different length of stiff segments, were synthesized. Relaxation phenomena in one of these compounds were investigated at different temperatures using Solartron 1260 Impedance Analyzer with Chelsea Dielectric Interface in the frequency range 10-2 Hz - 1 MHz. The liquid crystalline material was introduced between two parallel glass plates covered with conducting ITO layer and polyimide thin film with anti-parallel rubbing. The microscope observations have revealed a planar texture in this kind of sample. In order to find the influence of the polyimide thin film on the texture the investigations of a sample with out the polyimide film were also carried out. In this case the homeotropic structure was detected at zero external electric field. Three loss peaks were detected in the investigated frequency range. The dielectric response is sensitive to the phase transitions observed in this material. The temperatures of the phase transitions determined from the dielectric study are in a reasonable agreement with those obtained from thermo-optical analysis.

Electrical properties of some plasma polymers obtained by remote microwave plasma chemical vapor deposition

Some electrical properties of amorphous hydrogenated silicon-carbon films produced by remote microwave hydrogen plasma chemical vapor deposition technique are discussed. The DC conduction of Al/a-Si:C:H/Au and Al/a-Si:C:H/n-Si structures has been measured. Dielectric response of these structures has also been examined. The films are weakly conducting materials with the activation energy of the conductivity equal to 0.28 eV and 0.03 eV and 0.03 eV. MIS structures containing the amorphous silicon-carbon films as an insulator show the influence of trapping phenomena at the insulator-silicon interface.

Dielectric spectroscopy of liquid crystalline unsymmetrical azomethines with one imine bond: influence of rod length and type of terminal chains

In this work we have studied the dielectric properties of five unsymmetrical azomethine (A1−A5) liquid crystals prepared from different aldehydes or obtained from various amines. The effects of rod length and type of terminal chains on the thermal and mesomorphic behaviours were investigated by dielectric spectroscopy. The wide frequency range dielectric spectroscopy confirmed the phase transition detected in an earlier study. Moreover, a relaxation behaviour was detected in three compounds in various phases, which was connected to a particular dipolar group present in these materials, in contrast to two other compounds having a similar molecular structure but lacking strong dipolar groups. The relaxation processes changed their character or disappeared during the phase transition.