J. Sellarès

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.

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.

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.

TSDC study of the glass transition: correlation with calorimetric data

The glass transition in amorphous poly(ethylene terephthalate) is studied by thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The ability of TSDC to decompose a distributed relaxation, as the glass transition, into its elementary components is demonstrated. Two fractional polarization techniques, windows polarization (WP) and non-isothermal windows polarization (NIW) are employed to assess the influence of thermal history in the results. The Tool–Narayanaswami–Moynihan model has been used to fit the TSDC spectra. The most important contributions to the relaxation comes from modes with a value of the non–linearity parameter (x) around 0.7. Activation energies yield by this model are located around 1 eV (96 kJ mol−1) for polarization temperature (Tp) below 50 °C and they rise up to values higher than 8 eV (771 kJ mol−1) as Tp increases (up to 80 °C). There are few differences between results obtained with WP and NIW but, nonetheless, these are discussed. The obtained kinetic parameters are tested against DSC results in several conditions. Calculated DSC curves at several cooling and heating rates can reproduce qualitatively experimental DSC results. These results also demonstrate that modelling of the non–equilibrium kinetics involved in TSDC spectroscopy is a useful experimental tool for glass transition studies in polar polymers.

Dielectric study of the glass transition of PET/PEN blends

An analysis of the glass transition of four materials with similar chemical structures is performed: PET, PEN and two PET/PEN blends (90/10 and 70/30 w/w). During the melt processing of the blends transesterification reactions yield block and random PET/PEN copolymers that act as compatibilizers. The blends obtained in this way have been characterized by 1H-NMR and differential scan calorimetry (DSC). A degree of randomness of 0.38 and 0.26 has been found for the 90/10 and 70/30 copolymers. It is shown by DSC that this copolimerization is enough to compatibilize the blends. The α relaxation, the dielectric manifestation of the glass transition, has been studied by thermally stimulated depolarization currents. The relaxation has been analysed into its elementary modes by means of a relaxation map analysis. The activation energies of the modes of the glass transition do not change significantly between the four materials: in all cases the modes with a larger contribution have around 3 eV and modes with less than 1 eV are not detected. The change in the pre-exponential factor accounts entirely for the relaxation time change from material to material, that is larger as the PEN content increases. The compensation law is fulfilled and compensation plots converge for high-frequency modes. The polarizability decreases as the PEN content increases due to the increased stiffness of the polymer backbone. An analysis of the cooperativity shows that the central modes of the distribution are the most cooperative while high-frequency modes tend to behave more as Arrhenius. The low-frequency modes are difficult to study due to the asymmetry of the distribution of relaxation times. PEN turns out to be the less cooperative material. It is demonstrated how the parameters obtained from the dielectric study are able to reproduce calorimetric data from DSC scans and are, therefore, a valid description of the glass transition.

A study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

The glass transition of poly(ethylene terephthalate) (PET) crystallized for 4 h at temperatures between 413 and 453 K was studied. Secondary crystallization processes were monitored by differential scanning calorimetry and the glass transition of the remaining interlamellar amorphous phase was studied by thermally stimulated depolarization currents measurements. Non-isothermal window polarization is employed to resolve the relaxation in modes with a well-defined relaxation time that is subsequently adjusted to several standard models. An analysis of experimental results reveals that cooperativity can be disregarded in the modelization of data. The evolution of modes during secondary crystallization, once primary crystallization has been completed, gives more weight to lower energy modes. As a consequence, secondary crystallization tends to lower the glass transition temperature of the amorphous interlamellar phase, although remaining noticeably higher than in amorphous samples. The evolution of calorimetric scans of the glass transition is simulated from the obtained results and shows the same behaviour. Regarding the glass transition temperature of the material, it can be concluded that primary and secondary crystallization act in opposite directions even though the effect of secondary crystallization is much smaller. The interpretation of these results in terms of current views about secondary crystallization is discussed.

Sublinear dispersive conductivity in polyetherimides by the electric modulus formalism

Two commercially available polyetherimides, Ultem 1000 and Ultem 5000, have been studied by means of Dynamic Electrical Analysis. Results show that at temperatures above the glass transition dielectric response is highly influenced by space charge. Obtained data is analyzed using the electric modulus formalism. The real part of the conductivity is conveniently described by a sublinear power law dependency (ωn with n<;1), as Argand plots reveal, associated with correlated hopping of carriers. The imaginary part of the electric modulus shows a peak at low frequencies related to conduction processes. The modelisation of this peak allows us to obtain the dependence of the conductivity (σ0), the fractional exponent (n) and the crossover frequency (ωp) on the temperature, among other parameters. The α relaxation, which appears at higher frequencies, has also to be modeled since it overlaps the conductivity relaxation. The study of the parameters in terms of the temperature allows us to identify the ones that are thermally activated. The difference between the conductivity relaxation time and the Maxwell relaxation time indicates the presence of deep traps. The coupling model points out that the correlation of the ionic motion diminishes with temperature, probably due to increasing disorder associated with thermal agitation.

Method to distinguish between space-charge and dipolar relaxation in the TSDC spectra of polyethylene electrical insulation

Medium-voltage cross-linked polyethylene (MV-XLPE) cables have an important role in the electrical power distribution system. For this reason, the study of XLPE insulation may lead to improve cable features and lifetime. Although relaxational analysis yield a lot of information about XLPE properties, sometimes their results are difficult to interpret. To overcome this handicap, we have used a combination of thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. In order to discard spurious effects from the semiconductor interfaces, preliminary measurements have been done on specially prepared cables. TSDC have been performed using conventional poling between 140 °C and 40 °C. IDC measurements also have been carried out at temperatures between 90 °C and 110 °C in 2 °C steps. The TSDC spectra are dominated by a broad peak of uncertain origin. On the other hand, IDC show a combination of power and exponential currents. Exponential currents are fitted to a KWW model. The relaxation times obtained from the model present an Arrhenius behavior with Ea = 1.32 eV and τ0 = 3.29×10−16 s. The KWW parameter obtained is β = 0.8. The calculated depolarization current given by the exponential relaxation matches the predominant peak of TSDC spectra. Therefore, we conclude that in the MV cables studied the most visible peak of the TSDC spectrum has a dipolar origin.

Space charge studies on mid-voltage cable by thermally stimulated depolarization currents in the melting temperature range

In the present work, a XLPE mid-voltage cable from General Cable co. has been studied by thermally stimulated depolarization currents. Systematic measurements have been carried out in order to compare the conductive processes in this cable with previous results. Depolarization current as a function of thermal annealing, thermal history, polarizing field and polarizing time and temperature has been obtained. The results show the presence of a broad and complex heteropolar process between 60 and 120 /spl deg/C as expected. Annealing of the sample at temperatures above 80 /spl deg/C develops an homopolar contribution associated to chemical components diffused from the cable semiconducting layers into the XLPE bulk. For annealing times of 60 min at 140 /spl deg/C and 2 days at 90 /spl deg/C, the homopolar current intensity reaches a maximum, decreasing and recovering the heteropolar sign with further annealing. Experiments performed with different polarizing times and temperatures show as well the presence of an homopolar contribution, overlapped to the heteropolar behavior, that increases continuously with polarizing time. These results indicate that conductive processes within the XLPE are probably responsible of homopolar charge injection.