M. Mudarra

Glass transition studies in physically aged partially crystalline poly(ethylene terephthalate) by TSC

Abstract Physical ageing in poly(ethylene terephthalate) (PET) was studied by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and thermally stimulated polarization currents (TSPC) as a function of the material crystallinity degree. Calorimetric measurements confirm the presence of two separated glass transitions in partially crystallized samples (x c =27%), associated with the presence of two different amorphous regions: the interspherulitic region and the interlamellar region within the spherulites. The α peak obtained by TSDC shows the progressive decrease of the chain segments mobility in the amorphous fraction as ageing progresses. The increase in the crystallinity degree of the material also produces a decrease of the mobility in the amorphous fraction. In this case the decrease observed is associated with the interlamellar conformation of these regions. However, TSDC measurements do not show evidences of a double glass transition. TSPC measurements on aged samples show a current peak, which is directly related to the polarization of the amorphous regions when mobility is recovered above the glass transition. The study of these peaks in aged partially crystallized samples shows evidences of a double glass transition associated with theinterspherulitic and interlamellar amorphous regions.

Windowing polarization: considerations for the study of the space charge relaxation in poly(methyl methacrylate) by thermally stimulated depolarization currents

The windowing polarization technique is an electret formation method which has been widely used to study the structure of polar relaxations in polymers. In this paper we have studied the effect of some polarization parameters to extend the applicability of this method to the explanation of space charge relaxation in poly(methyl methacrylate).

Comparative study of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) by TSDC, DEA, DMA and DSC

Abstract A comparative study of the relaxational behavior of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), has been carried out by thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic electric analysis (DEA). As received, PEN (partially crystalline) shows, in the temperature range from −150 to 200°C, four relaxations located, in increasing temperature order, around −70°C (β), 60°C (β∗), 130°C (α) and 170°C (ρ). Amorphous PEN has been crystallized thermally heating up to different temperatures between 170 and 200°C. The DSC measurements of these samples show a small endothermic premelting peak, once the crystallization of the sample is completed. This peak increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the presence of this endothermic peak, the ρ relaxation passes through a maximum and presents a sharp decrease as it is further thermally treated. The α-relaxation, detected by dynamic mechanical as well as dielectric (ac) measurements, shifts to higher temperatures and broadens as the crystallinity degree increases. The kinetic parameters of the observed relaxations have been determined fitting experimental data to standard models.

Study of space charge relaxation in PMMA at high temperatures by dynamic electrical analysis

Abstract Dynamic electrical analysis shows that at high temperatures (above the glass transition temperature), the electrical properties of polymethyl methacrylate are strongly influenced by space charge. In this paper we present an study of space charge in this material and its conductive properties by dynamic electrical analysis, using the electric modulus formalism. The complex part of the electric modulus was fitted to Coelhos model, which considers ohmic conductivity and diffusion as the prevailing mechanisms of charge transport. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with space charge and a good agreement between experimental and calculated data is observed after the fitting process to the Coelhos model. The data obtained indicate that the electrode is partially blocked. The conductivity determined is thermally activated and it increases with the temperature due to an increasing mobility, that is also thermally activated.

Comparative TSPC, TSDC and DSC physical ageing studies on PET-a

Abstract In the present work a comparative study of physical ageing of poly(ethylene terephthalate) (PET) by thermally stimulated polarization currents (TSPC), thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC), is presented. In all cases, physical ageing was carried out by annealing the sample at 60°C for several ageing times (from t a = 0 to 264 h). Structural relaxation has been studied by DSC through the analysis of the endothermic peak superposed on the glass transition, which increases and shifts towards higher temperatures as the annealing time increases. Measured current spectra of annealed samples show the appearance of a relaxation peak which we named α a . The evolution of this peak with the annealing time is presented, and it has been related directly to the endothermic peak obtained by DSC, and therefore directly associated with the physical ageing of PET. Experiments carried out show the dipolar origin of the α a peak, related to the polarization of chains within the material when mobility is recovered above T g .

Polarization time effect on PMMA space‐charge relaxation by TSDC

The effect of the polarization time in the formation of the PMMA space charge relaxation has been studied. The study has been carried out by thermally stimulated depolarization currents of electrets formed by windowing polarization. The results obtained have been fitted to the general kinetic order model, and they are consistent with the existence of a space-charge polarization mechanism with the kinetic order of 1.14. The intensity of the peak maxima results in being a good indicator of the trapped carrier number evolution. For high temperatures and high electrical fields the saturation of this mechanism is achieved faster, which is attributed to a carrier mobility increase with these parameters. Finally, a compensation law has been found for space-charge-relaxation in PMMA.

Cold crystallization effects in free charge relaxation in PET and PEN

Abstract A comparative study of free charge relaxation in amorphous and partially crystallized poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly(ethylene terephthalate) (PET) has been carried out by thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC), and X-ray diffraction. Amorphous films have been crystallized thermally at temperatures between 170 and 200°C (PEN); 100 and 150°C (PET) by the thermal stimulation by steps method. The windowing polarization (WP) technique has been applied to form PET and PEN thermoelectrets. TSDC of these electrets polarized at 86°C (PET) and 130°C (PEN) show only one peak which is attributed to space charge relaxation (ρ peak). The evolution of this peak has been fitted to the general kinetic order model. DSC measurements of these samples show the appearance of a small endothermic prefusion peak once the crystallization of the sample is completed. This peak increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the appearance of this endothermic peak, the ρ relaxation passes through a maximum with a sharp decrease with further heat temperature. The X-ray diffraction measurements of these samples show that the decrease in the ρ peak is associated with the improvement of the amorphous–crystal interphases.

Annealing effect on the conductivity of XLPE insulation in power cable

Conductivity (Σ) in XLPE insulation of power cables annealed at 90°C at temperatures between 50 and 97°C has been measured. In all cases there is an initial increase in conductivity that develops a maximum and finally decreases for long annealing times. This maximum appears in the sample annealed 20 days when conductivity is measured at 50°C and shifts gradually to higher annealing times up to 40 days when the measurement is performed at 97°C. A linear behavior of ln(Σ) versus T-1/4 is observed, which implies that the transport mechanism is basically via thermally assisted hopping conduction. Infrared spectroscopy indicates that, during annealing, some chemical species diffuse from the semiconducting shields (SC) into the XLPE. Thermally stimulated depolarization currents technique (TSDC) and intensity-current measurements (I-V) point out as well the presence of this diffusion process that becomes less significant after long annealing times. The initial increase in Σ is explained in terms of the increase in traps density due to the diffusion process from the SC shields. Long term decrease in Σ is justified by the observed decrease of diffusion rate for long annealing times.

TSDC study of XLPE recrystallization effects in the melting range of temperatures

The electrical properties of crosslinked polyethylene (XLPE), employed in mid-voltage cable insulation are studied using thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC) and x-ray diffraction. A complex heteropolar peak appears by TSDC between 50 and 110 °C, with a maximum at 105 °C. These measurements reveal that there is an optimal polarization temperature (Tpo) around 90 °C. For this polarization temperature, the measured discharge peak area is maximum. Although the presence of a Tpo is common in the study of relaxations by TSDC, in this case one would expect a monotonic decrease in the TSDC response with increasing polarization temperatures due to the decrease in the total crystalline fraction. In this paper, TSDC curves obtained under several conditions are interpreted in terms of recrystallization processes in XLPE during the polarization stage, if the sample is polarized in the melting temperature range. In this case, the recrystallization of a fraction of the material molten at this temperature promotes the formation of more stable and defect-free crystals. The presence of recrystallization processes is detected by DSC and confirmed by x-ray diffractometry. TSDC measurements have been performed with samples polarized at several temperatures (Tp) cooling from the melt or heating from room temperature. Also, TSDC results are obtained with previous annealing or with several cooling rates. These results allow us to infer that crystalline material grown from recrystallization processes that take place in the polarization stage attains a particularly stable polarization. Possible microscopical causes of this effect are discussed.

Behaviour of amorphous poly(ethylene terephthalate) annealed at T<Tg by thermally stimulated currents

Thermally stimulated current (TSC) discharges in open circuit of amorphous poly(ethylene terephthalate) (PET) corona-charged electrets show a heteropolar relaxation at 87 °C, ρ1 between α and ρ peaks. This relaxation tends to become homopolar when the sample is annealed at temperatures below the glass transition temperature. This is due to the formation of a trapped charge density on the surface of the material that originates, during the TSC discharge, a current that counteracts the one that results in ρ1 This trapping effect, which initially is null, increases with annealing due to the rise in resistivity. On the other hand, TSC discharges in short-circuited annealed samples result in a heteropolar peak, ρ*, that corresponds to ρ1 The area of ρ* increases with the annealing time in a bounded way. This peak is related to the formation of thermal nuclei (embryos) in the bulk of the material that act as heterogeneities. This suggests that ρ1 is associated with a barrier-type polarization. If the annealed sample is heated to temperatures above the glass transition temperature, the tendency to the inversion of ρ1 vanishes and ρ* disappears, whereas α and ρ are modified. This suggests that ρ relaxation is related to a Maxwell-Wagner-Sillars effect.