J. J. del Val

Study of the α and β relaxations on a commercial poly(vinyl chloride) by thermally stimulated creep and depolarization current techniques

Thermally stimulated creep and depolarization current measurements have been carried out in a temperature range between −180 and 120 °C, on a pure commercial poly(vinyl chloride). Mechanical and dielectric relaxation times involved in α and β relaxations are obtained by fractional stresses/polarizations procedure. The distribution of the Arrhenius‐like kinetic parameters, deduced from the relaxation times, is interpreted in terms of compensation laws. From the obtained results, a same molecular origin can be assumed for the mechanical and dielectric behavior in both α and β relaxations. Moreover, the compensation law analysis leads to two different relaxations modes, likely associated to two kinds of thermally activated molecular motions, in β relaxation. In the α region, the values of found kinetic parameters point out a high degree of cooperativeness of the involved molecular motions. Depolarization curve in this region shows a high temperature peak, not found in the mechanical one, whose evolution with...

A study of plasticization effects in poly(vinyl chloride)

Internally and externally plasticized poly(vinyl chloride) (PVC) samples have been investigated by means of different techniques concerning volumetric, elastic and dynamic properties and the results have been compared to those obtained from pure PVC. In all the cases, the glass transition temperature of PVC is markedly lowered. However, this decrease is more important for the external plasticization. This is confirmed by the position of the dielectric loss peaks of the α relaxation. Independently from the changes in temperature, the shape of the relaxation function is modified only in the case of internally plasticized PVC. We suggest that this change in the relaxation function could be related to the small variations produced in the wide angle diffaction patterns of the samples. We can observe that the plasticization effect is linked to a decreasing of the intensity of the dielectric β relaxation process but no important changes are produced in the activation energy of this process.

Optimization of the giant magnetoimpedance effect of Finemet-type microwires through the nanocrystallization

We studied correlation of magnetic properties, giant magnetoimpedance (GMI) effect and structure of Finemet-type glass-coated microwires obtained by the Taylor-Ulitovski technique. We observed considerable magnetic softening and increasing of the GMI ratio, ΔZ/Z, (from 3% up to 100%) after annealing of studied microwires. On the other hand, even in as-prepared Fe73.8Cu1Nb3.1Si13B9.1 microwire, we observed existence of α-Fe nanocrystallites with average grain size about 12 nm and considerable GMI effect (ΔZ/Z up to 50%).

Thermally stimulated depolarization current (TSDC) study of molecular motions in the glass-transition region of polyarylate (PAr)

Dipolar relaxation processes in the glass-transition region of a commercial polyarylate (PAr) have been studied by means of the thermally stimulated depolarization current (TSDC) technique. Complementary differential scanning calorimetry (d.s.c.) and thermogravimetry (t.g.) measurements have also been carried out for comparison. The TSDC global spectrum obtained is resolved into two thermocurrent peaks. The elementary processes involved in these broad peaks were separated by application of the fractional polarization procedure. The Arrhenius-like kinetic parameters corresponding to the different elementary processes have been obtained fitting each elementary TSDC peak by a Debye-like expression. Empirical distribution function of activation energies has been also deduced. The origin of the different studied dipolar relaxation processes is discussed in relation to the molecular motions in the glass-transition region.

Effect of Nanocrystallization on Magnetic Properties and GMI Effect of Fe-rich Microwires

We studied the giant magnetoimpedance (GMI) effect and magnetic properties of Finemet-type FeCuNbSiB microwires. We observed that the GMI effect and magnetic softness of glass-coated microwires produced by the Taylor–Ulitovski technique can be tailored by controlling the magnetoelastic anisotropy of as-prepared FeCuNbSiB microwires, and can also be considerably improved either by heat treatment and/or choosing the suitable fabrication conditions. We observed a considerable magnetic softening of the microwires after the appropriate annealing. This magnetic softening correlates with the devitrification of amorphous samples. Amorphous Fe-rich microwires exhibited a low GMI effect (GMI ratio below 5%). A considerable enhancement of the GMI effect (GMI ratio up to 100%) has been observed in heat-treated microwires with nanocrystalline structure.

A thermostimulated creep study of blends of poly(vinyl chloride) and chlorinated polyethylene

Thermally stimulated creep measurements have been carried out on polyvinylchloride and on blends of polyvinylchloride with chlorinated polyethylenes of various chlorine distributions. Global spectra show that the two polymers are immiscible. However, a fine structure study of the spectra shows that some degree of compatibility exists in samples blended with randomly and commercially chlorinated polyethylenes. The same analysis leads to the conclusion that the sample blended with blocky chlorinated polyethylene is segregated into incompatible chlorinated polyethylene and polyvinylchloride phases.

Dielectric properties of polyarylate (PAr) around the glass transition

Abstract Dielectric characterization of the α-relaxation in polyarylate (PAr) has been carried out by means of a dielectric spectroscopy technique in a frequency range of 10–30 kHz. Complementary thermally stimulated depolarization currents ( TSDC ) and differential scanning calorimetry d.s.c.) measurements have also been performed. The results are interpreted in terms of the standard Cole-Cole plot and Havriliak-Negami distribution for the dielectric relaxation times. Information about the temperature and frequency dependence of a.c. conductivity is also obtained from the experimental curves. However, the behaviour of the main dielectric relaxation time is deduced from the experimental data in a wide range of temperature around the glass transition. This behaviour results in close agreement with the theoretical predictions of a free-volume approach for the dielectric α-relaxation recently proposed by the authors.

Effect of stress and/or field annealing on the magnetic behavior of the (Co77Si13.5B9.5)90Fe7Nb3 amorphous alloy

Variations of coercive field, induced magnetic anisotropy, and saturation magnetostriction constant in (Co77Si13.5B9.5)90Fe7Nb3 amorphous ribbons submitted to stress and/or axial magnetic-field annealing are reported. The annealing was carried out by using the Joule-heating effect (average temperature values of the sample corresponding to the intensity of the electrical current were 273, 378, 409, and 445°C) and the applied stress and axial magnetic field during the thermal treatments were 500MPa and 750A∕m, respectively. As a result of these treatments, a uniaxial in-plane magnetic anisotropy, which affects drastically the soft magnetic character of the samples, was developed.

Structural study of glass coated Cu-based microwires

The microstructural characteristics of Cu based glass coated microwires are studied considering the different factors determined by their fabrication (composition and geometry). Two different sets of binary systems (CuxFe100� x and CuxCo100� x) and Cu63Fe36P and Cu80Co19Ni samples have been studied. The parameter that describes the geometry is the ratio r between the diameter of the metallic nucleus and the external one corresponding to the layer of the Pyrex glass. It straightforwardly relates to the fabrication process of the samples owing to the strong internal stresses in the samples because of the different thermal expansion coefficients of the glass coating and the metallic nucleus. The structural study, carried out by means of wide angle X-ray scattering (WAXS) and a subsequent identification of the crystalline peaks, reveals that Fea and Co phases coexist with a Cu phase in each system. In the case of the samples containing P or Ni atoms, a third phase, Cu3 Po r Cu 3.8Ni, respectively, appears. An evaluation of the relative content of each phase shows in each system a sort of a master curve with a non-linear r-dependence with a critical maximum at around r=0.25 for the Fea and Co phases in the two systems. The calculation of the grain size shows that nanocrystals around 40 nm in size are formed in all the cases by Fea, Co and Cu phases. Magnetic measurements are nowin progress in order to complement these structural results. # 2001 Elsevier Science B.V. All rights reserved.

X-ray diffraction study of the influence of temperature on the structural correlations of poly(2-hydroxypropyl ether of bisphenol A)

The X-ray diffraction pattern of glassy poly(2-hydroxypropyl ether of bisphenol A) is studied at room temperature on oriented samples in order to associate its different peaks to different structural correlations. On the other hand, X-ray diffraction patterns have been obtained at different temperatures from Tg − 50 K up to Tg + 50 K for the above-mentioned polymer. Attention has been paid to the evolution with temperature of the position of the wide diffraction maximum corresponding to interchain correlations in the polymer. The temperature evolution of this parameter shows a marked discontinuity just at the glass transition temperature.