Grzegorz W. Bak

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.

Influence of polycrystalline structure on dielectric properties of some organic molecular crystals

The results of investigations on the influence of the polycrystalline structure on the low-frequency dielectric properties of aromatic hydrocarbons are presented. It is suggested that the dielectric losses in most polycrystalline structures of p-terphenyl and tetrabenzofulvalene below about 100 Hz are strongly influenced by scattering of charge carriers at inter-grain regions. The activation energy of the scattering is equal to about 0.2-0.3 eV and is one order of magnitude greater than the activation energy of the hopping transport inside grains. The spectral shape of the G/ omega curve in the frequency region in which the scattering takes place is well described by a power law with the exponent -0.5.

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.

Electron drift mobility in some aromatic hydrocarbons

Results of measurements of electron drift mobility in polycrystalline structures of some simple aromatic hydrocarbons are presented. The mobilities are in the range of (1.1-3.3)-10-5 cm2/Vs for the measurements in vacuum and (1.7-6.7)10-4 cm2/Vs for the measurements in the air. The activation energies are of the order of a few hundredth of electronvolt. The results are interpreted in terms of hopping transport in a narrow band of localized states.

Dielectric relaxation in charge-carrier-dominated systems

The dielectric response in some organic low-molecular weight compounds is defined by the injection of charge from electrodes. THe dielectric response of a few organic compounds shows a dielectric loss peak due to the relaxation of charge among traps. We can estimate the depth of traps provided that the concentration of traps is high enough. If the concentration of the injected charge is much greater than the concentration of the intrinsic charge than the depolarization consists not only on the change of the spatial distribution of charge but on the outflow of the excess charge as well. If we assume that the depolarization current is proportional to the concentration of the free charge carriers than the depolarization process may be described by the appropriate equations. The numerical solutions of the equations show that in the case of a single trapping level of the depth Et the depolarization due to the outflow of excess charge is of Debye type. The relaxation time proves to be dependent on the concentration of traps. For the concentration smaller than 1021 m-3 the relaxation time is described by equation, for the greater concentrations the relaxation time increases twice as long as for the smaller trap concentrations.

Charge transport in hopping systems under open-circuit conditions: computer simulation

Discharge under open-circuit conditions may be often used to find the drift mobility of charge carriers in thin films of low-mobility materials. The existing theories of the discharge assume domination of charge transport in the conduction band. In order to find whether the theories may be used for interpretation of experimental results in hopping systems a computer simulation was carried out. It has been shown that direct usage of the existing theories may lead to wrong values of drift mobility found from the discharge. The method of estimation of real value of mobility in the temperature range 100-500 K for various values of the decay parameter of the electron localized wavefunction and for various standard deviations of the energetic distribution of the localized states is presented in this paper.

Detection of structural inhomogeneity in graded-index polymer materials by space charge measurements: numerical solutions

Introducing by diffusion guest molecules into a host polymer we can obtain very interesting materials with gradually changing refractive index. Such graded index materials may be used for many optical purposes. It has been shown previously that the gradient profile of guest molecules may be not consistent with the Ficks law. This means that it might be useful to find an independent non-destructive method of determination of gradient profile of guest molecules. If the guest polar molecules are diffused into a host non-polar matrix then the guest molecules may form charge traps. Detailed numerical analysis of space charge limited currents shows that it is possible to estimate the gradient profile of the charge traps at the injecting electrode if the concentration of the traps at the very surface is known.

Dielectric properities of polyethylene modified by monoesters of maleic acid

Dielectric properties of low-density polyethylene (LDPE) modified by monoesters of maleic acid have been investigated. The coagent molecules can interact with molecules of LDPE giving rise to cross-linking and changes of degree of crystallinity. Depending on the modifier used, the dielectric losses of modified LDPE at the lower frequencies may change significantly. The dielectric losses of LDPE modified with dodecyl maleate increases by over one order of magnitude.