P. Pissis

Electrical and thermal conductivity of polymers filled with metal powders

Abstract The electrical and thermal conductivity of systems based on epoxy resin (ER) and poly(vinyl chloride) (PVC) filled with metal powders have been studied. Copper and nickel powders having different particle shapes were used as fillers. The composite preparation conditions allow the formation of a random distribution of metallic particles in the polymer matrix volume for the systems ER–Cu, ER–Ni, PVC–Cu and to create ordered shell structure in the PVC–Ni system. A model is proposed to describe the shell structure electric conductivity. The percolation theory equation σ ∼( ϕ − ϕ c ) t with t =2.4–3.2 (exceeding the universal t =1.7 value) holds true for the systems with dispersed filler random distribution, but not for the PVC–Ni system. The percolation threshold ϕ c depends on both particle shape and type of spatial distribution (random or ordered). In contrast to the electrical conductivity, the concentration dependence of thermal conductivity shows no jump in the percolation threshold region. For the description of the concentration dependence of the electrical and thermal conductivity, the key parameter is the packing factor F . F takes into account the influence of conductive phase topology and particle shape on the electrical and thermal conductivity.

Role of oxidative stress and DNA damage in human carcinogenesis.

Cells in tissues and organs are continuously subjected to oxidative stress and free radicals on a daily basis. This free radical attack has exogenous or endogenous (intracellular) origin. The cells withstand and counteract this occurrence by the use of several and different defense mechanisms ranging from free radical scavengers like glutathione (GSH), vitamins C and E and antioxidant enzymes like catalase, superoxide dismutase and various peroxidases to sophisticated and elaborate DNA repair mechanisms. The outcome of this dynamic equilibrium is usually the induction of oxidatively induced DNA damage and a variety of lesions of small to high importance and dangerous for the cell i.e. isolated base lesions or single strand breaks (SSBs) to complex lesions like double strand breaks (DSBs) and other non-DSB oxidatively generated clustered DNA lesions (OCDLs). The accumulation of DNA damage through misrepair or incomplete repair may lead to mutagenesis and consequently transformation particularly if combined with a deficient apoptotic pathway. In this review, we present the current status of knowledge and evidence on the mechanisms and involvement of intracellular oxidative stress and DNA damage in human malignancy evolution and possible use of these parameters as cancer biomarkers. At the same time, we discuss controversies related to potential artifacts inherent to specific methodologies used for the measurement of oxidatively induced DNA lesions in human cells or tissues.

Water sorption and diffusion studies in an epoxy resin system

The kinetics of water absorption in epoxide materials was studied by the aid of a diglycidyl ether of bisphenol-A–triethylenetetramine (DGEBA–TETA) epoxy–resin system containing various amounts of the plasticizer THIOCOL (0–40 phr). The presence of plasticizer permits the formation of products with different crosslinking densities and hydrophilic characters. Dynamic water absorption experiments were carried out at 15, 40, and 70°C. For the fitting of the experimental results, a new model was used, based on a model proposed earlier by Jacobs and Jones. This model considers epoxide product as a two-phase system consisting of a master phase (where the major part of the water is absorbed), which is homogeneous and nonpolar (phase 1), and of a second phase with different density and/or hydrophilic character (phase 2). By making the assumption that water diffusion can take place independently in the different phases of the material in accordance with Ficks second law, we can calculate the diffusion coefficient D and the water content at saturation M∞ for each phase separately. Equilibrium water sorption measurements were performed at 40°C, and the data were analyzed and discussed based on the Guggenheim–Anderson–de Boer (GAB) equation, the results being in support of the two-phase model used in the analysis of absorption kinetics. The linear expansion coefficient and the glass transition temperature of the materials, employed in the discussion of the results, were measured by thermomechanical analysis.

Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films

The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel - Tamman - Fulcher equation at temperatures higher than the glass transition temperature, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

Water effects in polyurethane block copolymers

Equilibrium and dynamic sorption isotherm measurements, differential scanning calorimetry (DSC) measurements, and, mainly, dielectric relaxation spectroscopy (DRS) measurements by means of the thermally stimulated depolarization currents (TSDC) method were used to investigate the hydration properties of linear segmented polyurethane copolymers. Three types of samples were investigated with various fractions of hard and soft block segments. They were based on polyethylene adipate (PEA), 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO). At 20°C the water content h of the samples at various values of relative humidity rh increases in proportion to the weight fraction of soft block segments phase. At saturation (rh = 100%) the ratio of sorbed water molecules to polar carbonyl polyester groups is 0.13. At saturation at 20°C there is no fraction of freezable water. The glass transition temperature, Tg, measured by DSC and by TSDC, shifts to lower temperature with increasing h by about 8–10 K at saturation at 20°C. A dielectric relaxation mechanism related to interfacial polarization in the phase-separated morphology is also plasticized by water in a way similar to that observed for the main (α) relaxation.

Dielectric studies of molecular mobility and phase morphology in polymer–layered silicate nanocomposites

Abstract Epoxy resin(ER)–smectite clay nanocomposites of exfoliated structure were prepared by dispersing the organically modified smectite clay in the ER and subsequent curing at 75 and 130 °C. Molecular mobility in the nanocomposites was investigated in relation to micromorphology by broadband dielectric relaxation spectroscopy. The large-scale heterogeneity, characteristic of the ER matrix, is suppressed in the nanocomposites and replaced by small-scale heterogeneity related to the presence of the silicate layers. The overall molecular mobility is found to decrease, in general, in the nanocomposites as compared to the ER matrix.

Dielectric effects of water in water-containing systems

Abstract The dielectric properties of several water-containing systems (synthetic polymers, saccharides, proteins, plant tissues) were systematically studied by means of the thermally stimulated depolarization currents (TSDC) method, following recently developed concepts for using the TSDC method to investigate the structures of water in different systems. The TSDC spectra recorded between 77 and 300 K exhibit several distinct relaxations which can be classified into three groups: relaxations due to the reorientation of water molecules themselves; dipolar relaxations, which are also present in the dry samples and which are influenced by water; and conductivity relaxations related with the water-dependent dc conductivity due to ions, mostly protons. In most cases, the results are compatible with a three-phase model: molecularly dispersed water tightly (non-rotationally) bound at primary hydration sites, loosely bound water in layers or clusters around primary hydration sites and free water at higher water contents.

Influence of chain extenders and chain end groups on properties of segmented polyurethanes. II. Dielectric study

Abstract Thermally stimulated depolarization currents (TSDC) measurements (temperature range 77–300 K) and broadband a.c. dielectric relaxation spectroscopy (frequency range 10 mHz–2 GHz) were employed to investigate molecular mobility and microphase separation in model segmented polyurethanes (SPUs) from oligotetramethylene glycol 1000, 4,4′-diphenylmethane diisocyanate and different chain extenders. The magnitude of the interfacial Maxwell-Wagner-Sillars (MWS) polarization TSDC peak and of d.c. conductivity have proved to sensitively reflect changes of the degree of microphase separation (DMS). The dielectric strength of both the primary and the secondary transition of the soft segments rich microphase are highest for the SPU with the highest DMS, whereas frequency (temperature) position and shape of the response are not significantly affected by DMS.

Porous poly(2-hydroxyethyl acrylate) hydrogels

Abstract Porous hydrogels were prepared by copolymerisation of 2-hydroxyethyl acrylate and ethyleneglycol dimethacrylate (as crosslinking agent) in solution using water or ethanol as solvents. Macroscopic pores are formed due to the segregation of the solvent from the polymer network during the polymerisation process. In the dry state the polymer network had nearly the same density as the poly(2-hydroxyethyl acrylate) polymerised in bulk thus showing that the pores collapse during the drying process. When the dry samples were swollen in water the pores opened and the volume fraction of pores could be determined by weighing. The pore morphology was observed by scanning electron microscopy. The dependence of the pore size on the solvent used and on the monomer/solvent ratio in the polymerisation process is shown. The elastic modulus and loss tangent were measured as a function of temperature in the region of the main (or α) dynamic-mechanical relaxation process. These spectra were correlated with the morphology of the samples.

Electrical conductivity studies in nylon 11

The results of electrical and dielectric studies in semicrystalline nylon 11 (degree of crystallinity 62%) by means of dielectric relaxation spectroscopy (DRS) in the frequency range 10−2–106 Hz at temperatures between the glass transition temperature of about 40 °C and the melting temperature of about 190 °C are reported. Emphasis is put on the investigation of electrical conductivity and conductivity effects. Additional information on dc conductivity is obtained from triangular voltage measurements. The results of DRS were analyzed within the formalism of permittivity (ac conductivity), impedance, and electric modulus. At temperatures higher than about 100 °C the spectra are dominated by dc conductivity and conductivity current relaxation. Despite high values of dc conductivity (σdc=10−6–10−5 S/m at 170 °C), electrode effects and space charge polarization, giving rise to high values of e′ at low frequencies, are negligible in the whole temperature range of measurements. This feature, as well as the rath...