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T0027 - Relaxations in amorphous and semi-crystalline polyesters. A study by thermally stimulated depolarization currents
Thermally stimulated depolarization currents and differential scanning calorimetry are performed on thermoplastic polyesters to characterize both and relaxations. The influence on the different relaxations phenomena of the chemical structure (size of the naphthalene groups, presence of cyclohexane, length of the aliphatic group, ) as well as the influence of the crystallinity are discussed. The three phases model with a crystalline part, a rigid amorphous part unable to relax and an amorphous phase able to relax at various temperatures depending on the distribution of the relaxation times is used to explain the evolution of the main relaxation while the standard two-phases model is sufficient to explain the variations of the relaxation mode. Elementary analysis of both and relaxations show that the relaxation characterized by a continuous variation of activation energies as a function of temperature follows the activated state equation with a zero activation entropy while the cooperative a relaxation exhibits a prominent maximum of the activation energies at the glass transition temperature.
M. Kattan, E. Dargent, J. Grenet, Journal of Thermal Analysis and Calorimetry 76 (2004) 379-394
T0026 - Thermally stimulated measurements on polycrystalline Pbl2 layers
Thermally stimulated current measurements have been performed on polycrystalline layers of lead iodide grown from solution. Several TSC peaks or bands have been observed at temperatures of 100 K, 195-220 K, 230-240 K and 270-300 K. Comparison with PbI 2 single crystal shows that the polycrystalline structure of the material introduces additional defects. In contrast the defect currently found in single crystal and attributed to the lead vacancy has not been clearly evidenced in the polycrystalline layers. The position and relative intensity of the peaks can be modified by Ag doping and thermal annealing. On the other hand, only small differences resulting from the anisotropy of the material have been observed depending on the orientation of the electric field with regards to the grains orientation.
J.P. Ponpon and M. Amann, Eur. Phys. J. Applied Physics 18 (2002) 25-31
T0025 - Thermally stimulated current in SiO2
Thermally stimulated current (TSC) techniques provide information about oxide-trap charge densities and energy distributions in MOS (metal-oxide-semiconductor) capacitors exposed to ionizing radiation or high-field stress that is difficult or impossible to obtain via standard capacitance-voltage or current-voltage techniques. The precision and reproducibility of measurements through repeated irradiation/TSC cycles on a single capacitor is demonstrated with a radiation-hardened oxide, and small sample-to-sample variations are observed. A small increase in Edelta' center density may occur in some non-radiation-hardened oxides during repeated irradiation/TSC measurement cycles. The importance of choosing an appropriate bias to obtain accurate measurements of trapped charge densities and energy distributions is emphasized. A 10 nm deposited oxide with no subsequent annealing above 400°C shows a different trapped-hole energy distribution than thermally grown oxides, but a similar distribution to thermal oxides is found for deposited oxides annealed at higher temperatures. Charge neutralization during switched-bias irradiation is found to occur both because of hole-electron annihilation and increased electron trapping in the near-interfacial SiO2. Limitations in applying TSC to oxides thinner than 5 nm are discussed.
D.M. Fleetwood, R.A. Reber Jr, L.C. Riewe and P.S. Winokur, Microelectronics Reliability 39 (1999) 1323-1336
T0024 - Thermally-stimulated current and dielectric loss measurement of polypropylene and teflon-FEP films immersed in diarylethane
Some properties of oil/PP (biaxially stretched polypropylene) and oil/FEP (Teflon FEP) composite insulators have been investigated with TSC (thermally stimulated current) techniques. The oil/PP system showed three TSC peaks originating from carriers captured in the swollen surface region of the PP. The TSC spectra depended strongly on the polarity of the poling voltage and on the impregnating temperature. Their analysis yielded information on the carrier traps existing near the PP surface in the oil/PP interface region. On the other hand, the TSC spectrum of the oil/FEP system has a small impregnating temperature dependence and a small effect of the poling voltage polarity. The difference in TSC between oil/PP and oil/FEP systems is closely related to the difference in the oil-polymer interaction. The TSC is a useful method for investigating carrier traps in the surface region and their change due to the oil-polymer interaction. To investigate further the relation between the carrier traps and tan delta, collecting bias TSC was measured on a specimen to which an ac voltage was applied. The results indicate that the decrease in tan(delta) during the ac voltage application depends on the amount of trapped carriers near the polymer's surface or, the decrease in carriers in the oil
S. Ochiai, H. Iwasaki, M. Ieda and T. Mizutani, IEEE Transactions on Dielectrics and Electrical Insulation 1 (1994) 487-495
T0023 - Simulation of thermally stimulated curents in dielectric : effects of thermal expansion
The influence of thermal expansion on thermally stimulated currents has been studied by means of model calculations based on the bistable model of Fröhlich, considering especially the case of materials characterized by expansion coefficients markedly different above and below the relaxation range. A good qualitative and quantitative agreement has been obtained between theory and experiment in elastomeric materials (styrene-isoprene-styrene and isoprene-styrene-isoprene block copolymers) showing that thermal expansion is the main factor responsible for the appearance of current reversals in thermally stimulated polarization processes
J. Vanderschueren, M. Ladang, J. Niezette and M. Corapci, Journal of Applied Physics 58 (1985) 4654-4657
T0022 - Thermally Stimulated Currents from corona-charged polypropylene films : a thermal effect of vacuum deposition of metallic electrodes
It is demonstrated that the thermally stimulated currents (TSC) from positively or negatively corona-charged polypropylene strongly depend on the order of the following two processes: a process of vacuum deposition of Al electrodes on the sample polymer and a process of heat-treatment of the polymer. Observed results are explained by a thermal effect which is introduced during the vacuum deposition of metallic electrodes. This thermal effect of the vacuum deposition of metallic electrodes is the largest for Al and the smallest for Bi among Al, Au, Ag, and Bi. Observed TSC spectra have three peaks at about 68, 142, and above 147°C for positively charged samples and four peaks at about 48, 90, 142, and above 147°C for negatively charged samples, respectively. Origins of these TSC peaks are discussed in some detail.
A. Baba and K. Ikezaki, Journal of Applied Physics 57 (1985) 359-365
T0021 - Dielectric relaxation phenomena in PEEK
Molecular movements in poly(ether-ether-ketone) have been investigated by thermally stimulated currents (TSC). The TSC spectra have been analyzed as a function of crystallinity. Around -80°C, two sub-modes are observed: the lower temperature sub-mode has been attributed to localized cooperative movements in the crystallizable amorphous phase; and the upper temperature sub-mode has been located in the crystalline phase. The magnitude of the corresponding compensation line increases with the ratio of crystallinity. Around the glass transition temperature (145°C), the existence of two sub-modes has been shown: the lower temperature sub-mode has been attributed to the dielectric manifestation of the glass transition and the cooperative movements have been assigned to the "true amorphous region" of PEEK; the upper temperature sub-mode is dependant upon crystallinity. Regardless of the preceding sub-mode, the activation enthalpies of the constituting elementary processes are practically constant. This sub-mode has been attributed to the "rigid amorphous region", constrained by the crystallites.
M. Mourgues-Martin, A. Bernès and C. Lacabanne, Thermochimica Acta 226 (1993) 7-14
T0020 - Influence of the crystalline phase on the molecular mobility of PVDF
Thermostimulated current (TSC) spectroscopy was applied to the characterization of dielectric relaxations associated with the glass transition of a series of P(VDF/TrFE) copolymers. The maximum temperature of the -mode increases slightly as the TrFE unit content is increased, in the same manner as the glass transition temperature. By using the technique of fractional polarizations to the resolution of this relaxation mode, we have isolated, for all the polymers investigated, a series of elementary relaxation times that obey a compensation law. This behavior is characteristic of the free amorphous phase of polymers. The mean activation energy of this mode increases as the TrFE unit content is increased, due to a stiffening of molecular chains. Annealing of the copolymers above their ferroelectric-to-paraelectric transition induces a strong crystallinity increase of the materials. As a matter of fact, the amorphous phase is squeezed in to dimensions lower than the characteristic length scale associated with the glass transition
G. Teyssèdre, A. Bernes and C. Lacabanne, Journal of Polymer Science 31 (1993) 2027-2034
T0019 - Low-loss behavior of alpha-PVDF
The exact nature of the polarization processes in low-loss materials is not sufficiently understood and good experimental material is relatively scarce. Dielectric response of alpha-PVDF covering 8.5 decades of frequency between 10^(-2) and 3x10^6 Hz and a range of temperatures between 103 and 138 K were analyzed to reveal the nature of the dominant polarization process. Correcting for a series resistance and a Debye-like loss process at 50 kHz, the 'universal' fractional power law of dielectric response is well maintained, the residual tan delta falls between 0.002 and 0.02 depending on temperature and frequency, with the residual loss process presumed to be due to hopping charge carriers having an activation energy of 0.035 eV. The universal process itself has a relatively high loss but its amplitude is low and this accounts for the low tan delta
A.K. Jonscher and J.Menegotto, IEEE Transactions on Dielectrics and Electrical Insulation 7 (2000) 303-305
T0018 - Dielectric relaxation properties of filled ethylene propylene rubber
Thermally stimulated discharge currents and time domain dielectric spectroscopy were employed to characterize the behavior of clay-filled ethylene propylene rubber. Measurements were made on samples with different clay concentrations and particle sizes. The main effect of the clay filler on the electrical properties is on interfacial polarization which occurs at the clay polymer interface. The experimental results are sensitive to the shape of the clay particles. A few results indicate that other mechanisms also affect the electrical behavior of this material
A.M. Jeffery and D.H. Damon, IEEE Transactions on Dielectrics and Electrical Insulation 2 (1995) 394-408
T0017 - Origins of Thermally Stimulated Current in polyethersulfone
In order to understand the dominant carrier species in electrical conduction in polyethersulfone (PES), thermally stimulated current (TSC) measurements were carried out under various conditions. It was found that PES has two TSC peaks in the temperature range from 20 to 220°C, consisting of an alpha peak at ~210°C and a beta peak whose peak temperature moves towards a higher temperature with an increasing poling temperature. Even when the sample was not poled, PES shows a spontaneous current with its peak at ~210°C. Both the spontaneous current peak and the alpha peak were found to disappear when the sample had been heated to 230°C, keeping the external circuit closed. From such similarity of appearance and disappearance between the two peaks, it is concluded that they have the same origin. From the change in the spatial distribution of space charges inside the sample measured simultaneously with the TSC measurements, positive charges, probably due to K+ ions, existed nonuniformly in the sample from the beginning, and are thought to be responsible for the two peaks. From a similar study, the beta peak is considered to be due to polarization of ionic space charges during the poling process
E.J. Kim, T. Takeda and Y. Ohki, IEEE Transactions on Dielectrics and Electrical Insulation 3 (1996) 386-391
T0016 - Analysis of the experimental distribution of relaxation times around the liquid-glass transition of poly(vinylidene fluoride)
The dielectric relaxation mode associated with the liquid-glass transition of a semicrystalline polymer, polyvinylidene fluoride, has been analyzed by the technique of fractional polarizations. A discrete distribution of relaxation times following an Arrhenius law has been obtained experimentally. The activation energy of the "single'' relaxation processes are distributed according to a Gaussian function. Moreover, these relaxation times follow a compensation law. These results allowed us to introduce a continuous distribution of relaxation times whose parameters are temperature dependent. The validity of the method was verified for the thermally stimulated current spectra. Use is made of this distribution to compute the temperature and frequency dependence of the complex dielectric permittivity.
G. Teyssèdre, P. Demond and C. Lacabanne, Journal of Applied Physics 79 (1996) 9258-9267
T0015 - Comparison of chemical treatments on the chain dynamics and thermal stability of bovine pericardium collagen
A new approach for the replacement of heart valves consists of obtaining an acellular matrix from animal aortic valves that performs mechanically, is nonantigenic, and is free from calcification and fibroblast proliferation. Novel biochemical treatments must be developed for this purpose. In this work, we focus on the characterization of collagen in acellular bovine cardiovascular tissues, fresh or glutaraldehyde treated, and stored in different solutions [phosphate-buffered saline (PBS), ethanol, octanol, and glutaraldehyde], to determine whether the resulting fibrous material is structurally preserved. The preservation of the triple helical structure of collagen is checked by differential scanning calorimetry (DSC), which is a well suited technique to analyze thermal transitions in proteins, such as denaturation. To get insight into the molecular dynamics of collagen in the nanometric range, we used thermally stimulated currents, a dielectric technique running at low frequency, that measure the dipolar reorientations in proteins submitted to a static electrical field. The combined use of these two techniques allowed us to evaluate the physical structure and conformation of collagen after the different chemical treatments. We have found that the glutaraldehyde treatment followed by octanol storage preserves the triple helical conformation of the polypeptidic chains of collagen, contrary to the ethanol and PBS storage that induce drastic changes in the thermal and dielectric behavior of the protein. Moreover, this particular chemical treatment stabilizes the collagen structure (shift toward high temperature of the collagen denaturation and stiffening of the chains by a cross-linking action) when compared to the control sample, and so could provide interesting fibrous material for the conception of bioprosthetic heart valve.
V. Samouillan, J. Dandurand, C. Lacabanne, R.J. Thoma, A. Adams, M. Moore, Journal of biomedical materials research 64 (2003) 330-338
T0014 - Molecular stability of elastin : effect of molecular architecture
The thermal and dielectric properties of elastin and two soluble derivatives ( k-elastin and derived elastin peptides from enzymatic elastolysis) were investigated in the freeze-dried state in a wide temperature range (from -180 to +220°C). The glass transition of these amorphous proteins was studied by differential scanning calorimetry (DSC). The dielectric relaxations of both proteins were followed by thermally stimulated currents (TSC), an isochronal dielectric spectrometry running at variable temperature, analogous to a lowfrequency spectroscopy (10^(-3) - 10^(-2) Hz) and by dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10^(-2) to 3 x 10^6 Hz. The combination of TSC and DDS experiments and the determination of the activation parameters of the relaxation times inform about the molecular mobility of the proteins, both in the glassy state and in the liquid state. Major differences between the relaxation behavior of elastin and its soluble derivatives have been discussed and correlated with the molecular architecture of the proteins.
V. Samouillan, J. Dandurand, C. Lacabanne and W. Hornebeck, Biomacromolecules 3 (2002) 531-537
T0013 - Study by thermostimulated currents of dielectric relaxations through the glass transition in an amorphous polymer : poly(n-butyl methacylate)
The study of poly(n-butyl methacrylate) (PnBMA) by thermostimulated currents has been performed in order to give a better definition of the molecular mobility when crossing the glass transition. It reveals the existence of two dipolar relaxation modes: alpha, ascribed to the glass-rubber relaxation, and alpha', which might be the dielectric manifestation of the liquid-liquid transition. The distribution of relaxation times and the evolution of the activation enthalpies when increasing the temperature have been studied by the fractional polarizations technique. It appears that the alpha and alpha' modes behave differently, showing the crossover from an Arrhenian to a Vogelian behavior. Furthermore, up to T, the cooperativity of molecular mobility is highlighted by the existence of a compensation law in agreement with Starkweather's criterion, and above T, results might be explained by the existence of an intermediate state neither glassy nor completely liquid.
E. Dudognon, A. Bernès and C. Lacabanne, Macromolecules 34 (2001) 3988-3992
T0012 - Dielectric study of the molecular mobility and the isothermal crystallization kinetics of an amorphous pharmaceutical drug substance
During the development of new pharmaceutical products based on drug substances in their amorphous form, the molecular mobility of an amorphous active ingredient was characterized in detail within a very broad time-temperature range. The relation between the isothermal crystallization kinetics and the dynamics of this amorphous substance was investigated. First, dynamic dielectric spectroscopy (DDS) and the thermostimulated current (TSC) techniques were used to analyze the molecular mobility of the amorphous drug substance over a wide frequency and temperature range (the drug substance is referred to as SSR in this text and was chosen as a model glassforming system). Two relaxation processes, corresponding to different molecular motions, were identified. The beta(a)-relaxation process, associated with intramolecular oscillation of small dipolar groups, followed Arrhenius temperature behavior over the entire time-temperature domain that was studied. However, the main alpha(a)-relaxation process, assigned to the dielectric manifestation of the dynamic glass transition of the amorphous phase, was described by Vogel-Fulcher-Tammann (VFT) and Arrhenius behavior above and below the glass transition temperature (Tg) respectively. The physical meaning of these complex dynamics is explained in the context of the Adam and Gibbs (AG) model, by the temperature dependence of the size of cooperatively rearranging regions (CRR) that govern the time scale of delocalized molecular motions. The distinction between the molecular mobility and the structural relaxation of amorphous systems below Tg is discussed. This work shows that the complementary nature of bothDDSandTSCtechniques is essential to directly analyze the intramolecular and molecular motions of disordered phases over a wide time-temperature range above and below the Tg. Second, real-time dielectric measurements were carried out to determine the isothermal crystallization kinetics of the SSR amorphous drug. Whatever the crystalline form obtained over time in the crystallization process, the decrease of the dielectric response of amorphous phase, which is characteristic of the isothermal crystallization, was studied to monitor the time dependence of the degree of crystallinity. The characteristic crystallization time, derived from Kohlrausch-Williams-Watt (KWW)- Avrami analyses performed at different temperatures, followed an Arrhenius temperature dependence. Behaviors specific to the molecular mobility of the amorphous drug substance were compared with the characteristic crystallization time. It was concluded that the crystal growth process of the SSR drug seems to be controlled by the intramolecular motions involving the beta(a)-relaxation mode and not by the molecular motions responsible for the alpha(a)-relaxation mode in the range of temperatures >Tg. Subsequent studies will focus on the crystallization process of the SSR drug in the glassy state (T<Tg).
J. Alie, J. Menegotto, P. Cardon, H. Duplaa, A. Caron, C. Lacabanne, M. Bauer, Journal of Pharmaceutical Science 93 (2004) 218-233
T0010 - Thermally stimulated current method applied to highly irradiated silicon diodes
We propose an improved method for the analysis of Thermally Stimulated Currents (TSC) measured on highly irradiated silicon diodes. The proposed TSC formula for the evaluation of a set of TSC spectra obtained with different reverse biases leads not only to the concentration of electron and hole traps visible in the spectra but also gives an estimation for the concentration of defects which not give rise to a peak in the 30-220K TSC temperature range (very shallow or very deep levels). The method is applied to a diode irradiated with a neutron fluence of ?n = 1.82 x 10^13 n/cm2.
I. Pintilie, C. Tivarus, L. Pintilie, M. Moll, E. Fretwurst, G. Lindstroem, Nuclear Instruments and Methods in Physics Research A 476 (2002) 652-657
T0009 - The beta-alpha branching in D-sorbitol as studied by thermally stimulated depolarization currents (TSDC)
The molecular motions in D-sorbitol (D-glucitol) have been studied by thermally stimulated depolarization currents (TSDC) in the temperature region between -160 and 10°C. The relaxation appears as a broad global peak between -160 and -50°C and its features were compared with those of the relaxation of maltitol (a D-glucitol derivative). A study of the relaxation of sorbitol, which shows a maximum intensity at approximately -1°C, is also presented, and from the obtained data, the fragility index of this glass former is calculated. The and relaxations are observed to merge in the frequency window of the TSDC technique, and it is underlined that this merging is a consequence of the overlap of the tails of these distributions in this frequency window. In this context, the merging observed by TSDC images the branching of the most probable times of the two distributions predicted by dielectric relaxation spectroscopy at the branching temperature T.
N.T. Correia, C. Alvarez, J.J. Moura Ramos and M. Descamps, J. Phys. Chem. B 105 (2001) 5663-5669
T0008 - Comments on the compensation effect observed in thermally stimulated depolarization current analysis
Compensation has been reported for the relaxation parameters (the activation energy W and the pre-exponential factor t0) determined by using the thermal sampling (TS) technique. For the peaks obtained by the thermally stimulated depolarization current (TSDC) measurement, there is a relationship between W, t0 and the temperature of maximum intensity of the peak Tm that can be employed to deduce a general relationship between W, Tm, the compensation temperature Tc and the compensation time tc. This relationship can be used for a basic analysis of the compensation effect. By numerical simulations, and using parameters similar to those reported for concrete measurements, we show that it is possible to observe a compensation point only if the activation energy is a monotonically increasing power function of temperature of power coefficient between 1 and 2, more precisely, if Wincreases with temperature stronger than linearly but weaker than quadratically. The actual values of the compensation parameters are determined by the relationship between the activation energy and the temperature. The experimental uncertainties affecting the compensation temperature and the compensation time are significant, and consequently, it is not possible to have a precisely defined compensation point.
E.R. Neagu, R. Neagu, Thermochimica Acta 395 (2003) 183-189
T0007 - Coherent vibrations in "polar / non-polar"-like complex polymeric mixtures. A dielectric approach to percolation phenomena
The analysis of thermostimulated currents by the fractional polarization procedure has been used to establish the existence of a coherent vibration in Linseed oil/mastic varnish mixtures. This excitation is seen in the oil component when its proportion in the system is inferior to its percolation threshold. The observed phenomenon complies with compensation laws and is interpreted in the framework of the formalism developed by Fröhlich to understand collective phenomena in biological systems.
O. Pagès, A. Lamure, J. Dandurand, A. Zaoui and C. Lacabanne, Journal of Thermal Analysis and Calorimetry 57 (1999) 401-407
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