Thermostimulated Depolarization Currents Research Articles

The slow molecular mobility in the amorphous solid and supercooled liquid phases of brucine, an anti-cancer drug. A study by differential scanning calorimetry (DSC), thermostimulated currents (TSC) and dielectric relaxation spectroscopy (DRS)

In this work, we explore in detail the molecular mobility in amorphous brucine, i.e. in the glassy state and in the supercooled liquid immediately above the glass transition, where viscosity is high and where a spatially heterogeneous dynamic is detected. Brucine is an alkaloid used in ancient times by Chinese medicine in the treatment of cancer, it can be obtained in the amorphous form by cooling from the melt and the high stability exhibited makes it a good candidate for being use in novel formulations.Combining thermostimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) different types of motions have been probed. In the glassy state, two secondary relaxations γ and β were characterized, the latter with features compatible with a Johari-Goldstein mobility. The cooperative α relaxation was clearly identified, and by combining also with results taken from differential scanning calorimetry (DSC), it was possible to determine parameters such as the glass transition temperature, and fragility index which indicate a moderately fragile glass former. In addition, the determination of the βJG relaxation times and activation energy, both above and below Tg, were a new result of the current investigation.Additionally, TSDC measurements, although with some uncertainty, reveal the existence of a mobility above Tg. On the other hand, in the supercooled liquid region, the analysis of the conductivity obtained by DRS, shows a breakdown of the Debye-Stokes-Einstein relation pointing to a heterogeneous dynamic that could be in the origin of the observed mobility by TSDC.

Molecular mobility in Soluplus, a polymer with extremely low dynamic fragility; A study by thermally stimulated depolarization currents

This is a thermostimulated depolarization currents (TSDC) study of the slow molecular mobility in Soluplus®, an amorphous polymer used in the production of amorphous solid dispersions for pharmaceutical purposes. We carried out this study due to the importance of these applications and the very specific characteristics of this polymer, namely an extremely low dynamic fragility. TSDC proved to be a suitable technique to clarify the nature of slow molecular mobility in Soluplus® and understand the suitability of this glass former for applications.

Thermodepolarization of Bi12SiO20 Crystals Doped with Fe

The polarization processes in Bi12SiO20 crystals doped with Fe (Bi12SiO20 : Fe) in the temperature interval 300–800 K are studied by the thermoactivation spectroscopy methods. The temperature dependences of the thermostimulated depolarization (TSD) currents and the dependences of the intensities of TSD current peaks on the polarization conditions are obtained. The mechanisms of space charge and quasidipole polarizations are revealed. The temperature dependences of the optical absorption of Bi12SiO20 : Fe crystals are also investigated.

Characterization of the secondary relaxations in glass-forming liquids. The contribution of the Starkweather line and of the Coupling Model to the rationalization of TSDC results

This work addresses the study by thermostimulated depolarization currents (TSDC) of Johari-Goldstein (JG) β-relaxations in glass forming systems. Here we analyze the differences between the β-relaxation and the fast secondary (γ, δ, ε, …) relaxations on the one hand, and the α-relaxation on the other.It is shown that TSDC is able to characterize JG although this mobility is often close to, or even partially overlapped with, the α-relaxation.It is also shown that the Starkweather line or zero entropy line (ZEL) can be used as a reference element to identify the secondary relaxations: non-compliance with the ZEL is an indicator of cooperativity.We finally showed that the non-cooperativeness of the Johari-Goldstein relaxation is highlighted by the joint consideration of the Coupling Model and the Starkweather line.

Dielectric relaxation study of poly (ether imide) by thermally stimulated depolarization currents

AbstractThis work presents a detailed study, by thermostimulated depolarization currents, of the molecular mobility of poly (ether imide) in a broad temperature range covering the amorphous solid state, the glass transition, and the rubber state. Taking advantage of the very wide distribution of motional modes of the PEI glass transition (the broadest of all known glass formers), we carried out an in‐depth study of physical aging involving different modes, which provided a more comprehensive view of the aging process. We also analyzed in detail the mobility below Tg where, in addition to the fast secondary relaxation, we identified a Johari–Goldstein. In the rubber state, on the other hand, we detected a mobility but showed that it corresponds to space charge motions rather to dipolar reorientations.

Molecular mobility in amorphous biobased copolyesters obtained with 2,5- and 2,4-furandicarboxylate acid

Poly(ethylene 2,5-furandicarboxylate) (2,5-PEF) is one of the most credible biobased alternative to poly (ethylene terephthalate) (PET). The Henkel disproportionation reaction that leads to furandicarboxylic acid (FDCA) provides three position isomers: 2,5-FDCA (obtained with the highest yield), 2,4-FDCA (so far considered as a by-product), and 3,4-FDCA (traces). The copolymerization of the two main isomers of FDCA with a diol, e.g. ethylene glycol (EG), is an interesting approach to obtain a family of furan-based biopolymers with adjusted physical properties. This work investigates the molecular mobility of three copolymers obtained with EG and ratios of 2,5-FDCA and 2,4-FDCA ensuring the complete disruption of crystallization (90:10, 85:15 and 50:50 mol % of 2,5:2,4 FDCA), as compared to the homopolymers 2,5-PEF and 2,4-PEF. The molecular mobility was investigated by cross-comparing the results obtained by Modulated-Temperature Differential Scanning Calorimetry (MT-DSC), Dielectric Relaxation Spectroscopy (DRS) and Thermo-Stimulated Depolarization Currents (TSDC), with the aim of evaluating the local and segmental molecular mobilities, their activation energies, as well as the temperature dependence of the relaxation time and of the cooperatively rearranging regions at the glass transition. The furan ring in 2,5-FDCA (2,5-PEF) has a rotation axis that is less linear compared to the benzene ring in terephthalic acid (PET), with consequences on the ring-flipping mechanisms. 2,5-FDCA and 2,4-FDCA differ by the position of the carbonyl groups on the furan ring, which adds asymmetry to non-linearity. The incorporation of 2,4-FDCA-based units into a polymer backbone mainly constituted of 2,5-FDCA-based repeating units is responsible for longer relaxation times associated with the local β relaxation processes, no striking effects on the kinetic fragility index m, no obvious effects on cooperativity (a slightly increase in the cooperativity length is observed in the liquid state), no effects on the activation energy for the segmental α relaxation in the liquid state, and a decrease in the activation energy in the glassy state.

Adsorption and dielectric properties of natural composite polymer wood

The state of water in a natural polymer, wood, was studied by its eff ect on the dielectric parameters and currents of thermostimulated depolarization (TSD) of the adsorption system. It has been shown that surface wood hydroxyls do not play a signifi cant role in the adsorption of water, which occurs mainly in pores of varying volume. Interpretation of the behavior of the TSD currents showed that at the initial stage the polarization linearly depends on the water adsorption value, then this linearity is violated, which we associate with the onset of migration polarization.

Second harmonic generation and charge relaxation of poled glasses

We measured thermal relaxation of the second order nonlinearity (SON) observed in a thermally-poled soda-lime silicate glass. It is shown that 50-min annealing of the glass at 400°C results in a complete SON degradation. Performed studies of thermo-stimulated depolarization current (TSDC) revealed that the poling-induced spatial electric charge relaxes above the glass transition temperature. This indicates that SON is not induced by this spatial charge.

The Effect of Density and Concentration of Trapping Centers on Charge Carrier Mobility in Polymer Dielectrics

The spectra of the currents of thermostimulated depolarization and the time dependences of the electret potential difference were measured in polymers irradiated with soft X-rays of electric gas discharge. The results of measurements are used to determining the mobility of electrons and holes in polymer cable and capacitor dielectrics having different values of density and concentration of electrically active trapping centers.

Thermopolarization Phenomena in the Temperature Range of Glass Transition of Amorphous Polydiethylsiloxane

The transition from the glass to the highly elastic state in polydiethylsiloxane (PDES) is not reflected on the temperature dependences of the relative permittivity, the dielectric loss tangent, and the specific volumetric electrical conductivity. But, the peak of the current of thermostimulated depolarization (TSD) is fixed in the temperature range of the transition to the highly elastic state. The peak of the TSD current at T ~ 130K indicates a continuous amorphous phase formation in the experimental conditions. The maximum value of the TSD current directly depends on the content of the amorphous phase in the polymer. The cooling of the polymer in an electric field reduces the magnitude of the peak. Exposure in the mesophase leads to an almost complete absence of thermopolarization effects near the glass transition temperature. This peak of the TSD current in the absence of preliminary polarization is characteristic, presumably, only for flexible-chain polymers where structural units have pyroelectric properties. Under certain conditions, PDES demonstrates itself as an active dielectric in the temperature range of 90 - 180K.

Thermally stimulated currents depolarization study of gamma ray irradiated PMMA

ABSTRACTWe will investigate the effect of γ-irradiation on the polymethyl methacrylate (PMMA) properties. The Fourier transform infrared spectroscopy technique in attenuated total reflexion mode (FTIR-ATR) results allowed us to show that the irradiated PMMA undergoes a scission in its lateral chain combined to an oxidation phenomenon. In fact, a decrease in the carbonyl index, versus the radiation dose, is observed. The differential scanning calorimetry and the thermostimulated depolarization currents revealed that the scission in the lateral chain provides a better flexibility to the PMMA chain and an increase in the free volume of the material. The consequences of these phenomena appear in the decrease of the activation energy of PMMA and the appearance of a local order in the material at a temperature Tc which decreases versus the radiation dose.

Thermostimulated electret depolarization currents: Methods for estimating and controlling the characteristic parameters of dipoles

The optimal number of method for estimating and controlling the characteristic parameters (energy E D and frequency ν) of dipole relaxation has been determined using the theory of thermostimulated depolarization currents. Along with the known method for estimating energy E D from the slope of the initial segment of the curve describing thermostimulated depolarization currents, these techniques include another method for determining frequency ν proposed in this study, as well as the method for estimating parameters E D and ν by fitting the experimental and calculated spectra of thermostimulated depolarization current, and the diagram of characteristic dipole parameters E D and ν (which is universal for the entire set of electrets) and the corresponding spectra of thermostimulated depolarization currents.

Analysis of relaxation processes by mathematical modeling

The mechanisms of charge accumulation and relaxation in electrically insulating films of cross-linked polyethylene prior to and after thermal aging are studied using the method of thermostimulated depolarization currents. It is demonstrated that the stability of the electretic state in samples of silane cross-linked polyethylene is higher than that in samples of peroxide cross-linked polyethylene. The spectra of currents are analyzed by mathematical modeling. The mechanisms of charge relaxation in films of cross-linked polyeth-ylene are determined.

Dissociating space charge processes from orientation polarization in poly(ethylene naphthalate) films

Thermo-stimulated depolarization current (TSDC) measurements and space charge measurements were performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic and polar polyester. The aim is to develop an understanding of the dipolar and conduction processes at play in this material and in particular to understand the effects of temperature. For the TSDC measurements, when polarizing at 130 and 170 °C, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170 °C, one more current peak, labelled ρ peak, is observed at temperatures above the glass transition. This peak is not only of dipolar origin and could be associated with charge detrapping in the material. To unravel the mechanisms behind this process, a TSDC was combined with space charge measurements using the pulsed electroacoustic method (PEA) and the partial heating method was used. It is shown that the ρ peak is predominantly associated with the release of the negative charge build-up in the material.

Vacancy Dipole Currents of Thermostimulated Depolarization in a Plastically Deformed KCl Crystal

A thermostimulated depolarization current of divacancies (υ − υ + ) created due to plastic deformation of a KCl crystal is detected. The thermostimulated depolarization current with a maximum at 205 K is correlated with the increasing degree of relative deformation of the KCl crystal. The mechanism of effective formation of (Cl 3 − ) aca centers is interpreted as a result of interaction of two mobile interstitial halogen atoms in the field of divacancies (υ − υ + ).

X-ray, Mössbauer, and dielectric studies of the Co1 − x Ni x Cr2O4 ceramic system

The temperature dependences of the dielectric constant, ɛ(T), dielectric loss tangent, tanδ(T), and thermostimulated depolarization currents of (Co1 − xNix)Cr2O4:y57Fe2O3 samples with 0.2 ≤ x ≤ 0.6 and 0.01 ≤ y ≤ 0.04 exhibit anomalies at temperatures of T1 ≈ 220 K and T2 ≈ 240 K that indicate a transition to an ordered ferroelectric state at temperatures below T2. Also observed at the same temperatures are abrupt changes in the isomer shift and quadrupole splitting for two Mossbauer spectral doublets of a sample with x = 0.2.

Relaxation of the electret charges of PET films after the action of aerosol particles

The electret properties of PET films before and after the action of aerosol particles (APs) are investigated. These properties are shown to depend not only on the nature and method of the polymer polarizations but also on the variations in their micro- and permolecular structure (PMS). The singularities of the differences in the processes of the relaxation of stabilized electret charges in the bulk and in the surface layers of PET films have been revealed based on the results of the analysis of the spectrum of the thermostimulated depolarization (TSD) current. The thus-obtained data on the variations in the relaxation properties of electret PET film electrets allow us to say that these films can be used for fabricating efficient aerosol catches for different purposes and aerosol-sensitive sensors operating based on the principle of decreasing the values of the electret charges.

Coronoelectrets based on polypropylene composites dispersed by a TlIn x Ce1 − x Se2 semiconductor filler

The electret properties in the heterogeneous polymer-semiconductor system have been investigated by the method of the electret-thermal analysis (ETA) of the PP composites with TlInSe2 and TlInxCe1 − xSe2 semiconductor fillers. It is shown that a small change in the filler structure causes a significant alteration of the spectrum of the thermostimulated depolarization currents. As the investigation shows, the TlInSe2 and TlInxCe1 − xSe2 fillers having the p-type conductivity play the role of the structure-forming agent in the polypropylene matrix changing the structure of the polymer and the near-boundary layer of the polymer and the filler.

Glass transition investigated by a combined protocol using thermostimulated depolarization currents and differential scanning calorimetry

Relaxation times of bisphenol A polycarbonate around the glass transition temperature are estimated using the combination of differential scanning calorimetry (DSC) and thermostimulated depolarization currents (TSDC). These measurements are performed using samples with different thermal histories below and above the vitrification transformation. This protocol enables the extension of the range of equilibrium relaxation times measured by dielectric spectroscopy. By this mean we may recalculate the values of the Kauzmann temperature and fragility index.

The mechanism of dielectric relaxation and proton conductivity in an α-LiIO3 nanostructure

Thermostimulated depolarization currents are studied and analyzed. As a result, a mechanism of dielectric relaxation is developed for α-LiIO3 crystals with the interatomic distance about 0.55 nm. This mechanism is due to formation of the Н3О+ and OН− defects resulting from the displacement of a proton along the valence bond in water molecules and its following migration through tunneling both inside the IO 3 − ions and between these ions in the neighboring layers. In addition, the НIO3 and НI3О8 impurities can serve as proton donors. The negative maximum of thermostimulated depolarization currents caused by false hysteresis loops due to a conductivity lag is explained.