L. Apekis

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.

Electrical properties of carbon black-filled polymer composites

This work deals with the dielectric properties of conductive composite materials, which consist of thermoplastic polypropylene (PP) matrix filled with carbon black (CB). The CB concentration was systematically varied in a wide range. Our main interest is focused on the investigation of electrical conductivity mechanism and related percolation phenomena in these materials. To study the electrical and dielectric properties of composites we used broadband ac dielectric relaxation spectroscopy (DRS) techniques in a wide temperature range. By measurements of complex dielectric permittivity, e * , the dependence of ac conductivity, σ ac , and dc conductivity, σ dc , on the frequency, the temperature and the concentration of the conductive filler was investigated. The behavior of this system is described by means of percolation theory. The percolation threshold, Pc, value was calculated to be 6.2 wt. % CB. Both, dielectric constant and dc conductivity follow power-law behavior, yielding values for the critical exponents, which are in good agreement with the theoretical ones. Indications for tunneling effect in the charge carriers transport through the composites are presented. The temperature dependence of dc conductivity gives evidence for the presence of positive temperature coefficient (PTC) effect.

Broadband dielectric relaxation spectroscopy in interpenetrating polymer networks of polyurethane-copolymer of butyl methacrylate and dimethacrylate triethylene glycol

Abstract The electrical and dielectric properties of interpenetrating polymer networks (IPNs) based on crosslinked polyurethane (PUR) and a copolymer of butyl methacrylate and dimethacrylate triethylene glycol were studied by means of broadband AC dielectric relaxation spectroscopy to obtain information on the morphology and phase separation in these IPNs. Three relaxation processes were observed: a secondary β relaxation due to the copolymer, the α relaxation due to the glass–rubber transition of the PUR phase and a conductivity current relaxation due to charge carriers trapped at the interfaces in the microheterogeneous samples. The α relaxation due to the glass–rubber transition of the copolymer is masked by conductivity effects and is revealed after a fitting treatment. The dipolar α and β mechanisms and the AC conductivity mechanism were studied in detail at several IPN compositions, by analysing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism and power law forms. For the copolymer network, the dielectric loss e ″( ω ) shows a strong secondary β relaxation which becomes faster in the IPNs. The PUR network displays a relatively broad α -relaxation process which becomes slower in the IPNs. The energy and shape parameters of the response were determined for both mechanisms at several temperatures. From AC conductivity measurements we extracted information about the morphology and local structure of the IPNs. It is concluded that the IPNs studied are two-phase systems, but phase separation is incomplete in these materials.

Dielectric Study of the Hydration Process in Biological Materials

The sorption of water vapour by biological macromolecules is generally assumed to involve the binding of H2O molecules to specific hydrophilic sites at lower relative humidities, followed by condensation of multimolecular adsorption as the humidity increases. Several methods have been applied for the investigation and detailed study of the structure, mobility, extent and modes of binding of water molecules in various systems. Among them the most commonly used are IR and Raman spectroscopy (Luck, 1985), differential scanning calorimetry (Berlin et al., 1970), NMR spectroscopy (Kuntz and Kautzmann, 1974, Mathur de Vre, 1979), neutron scattering (Lehmann, 1984), sorption and desorption methods (Pethig, 1979) and dielectric methods (Bone and Pethig, 1982, Pethig and Kell, 1987, Grant et al. 1978, Kent and Meyer 1984). All of them yield some insight into the problem. One common feature observed in nearly all cases is that the relaxation times for reorientation and the diffusion constants of water molecules sorbed in various biological systems are much lower than the values observed for free water, while the enthalpy of vaporisation of the water sorbed is by about 100 cal g−1 higher than the value of liquid water (Berlin et al., 1970, Pethig, 1979, Grant et al., 1978). This behaviour suggests that the water molecules contributing to the first hydration layer exhibit restricted motion due to a significant decrease in the translational and rotational modes of motion caused by macromolecular-water interaction. Moreover, the dynamics of the material itself (relaxation and conductivity mechanisms) is strongly influenced by the presence of sorbed water.

Thermal Transitions of Polypropylene in Blends and Composites with Polypyrrole and Polypyrrole/Montmorillonite

Thermal transitions of the semi-crystalline isotactic polypropylene, in polypropylene/polypyrrole blends and polypropylene/polypyrrole/montmorillonite composites, processed by two different ways, were investigated by differential scanning calorimetry. Glass transition temperature of polypropylene was found to remain unchanged at 269 K, whereas the crystallization rate was found to be higher in the blends and composites, which is explained in terms of increased concentration of extrinsic crystallization nuclei. The effect is larger in the materials processed by mixing and subsequent compression molding as compared to the materials prepared directly by compression molding. The degree of crystallinity of polypropylene did not show any systematic variation with composition, whereas it is slightly higher for the samples prepared by direct compression molding, being in the range of 50–59%. The polypropylene in the blends and the composites crystallizes in the stable α form, whereas metastable crystallites of the β form were observed as a minor component, depending on the thermal history of the samples. The results are discussed on the basis of the picture emerging from morphological studies.

Dielectric Behaviour of Ice Microcrystals Dispersed within Suspensions: A DTC Study

Abstract The dielectric properties of water-in-oil (W/O) suspensions have been studied by means of the depolarization thermocurrent (DTC) method in the temperature range of 85-250 K. Two predominant peaks have been observed at about 140 and 225 K. Evidence has been obtained that the peak at 140 K and the dielectric absorption observed by many investigators at sub-zero temperatures in the kHz frequency range are due to the same relaxation mechanism.