Thermally Stimulated Discharge Current Spectra Research Articles

Effect of γ-irradiation on thermally stimulated depolarization current spectra of polyethylene-grafted-poly(Acrylic acid)

The graft copolymerization process of polyethylene (PE) with acrylic acid is performed using a 60Co-gamma irradiator, and the samples are irradiated at different doses, 30, 50, 70 and 100 kGy with a dose rate of 0.55 Gy/s. Global thermally stimulated discharge current (TSDC) spectrum of irradiated and non-irradiated PE-g-AAc has been investigated from RT to ∼ 440K. The TSDC spectrum of non-irradiated PE-g-AAc exhibited two relaxation processes. The first process is located at ∼ 329 K, and the second one is at ∼390 K. These processes are called dipolar and space charge relaxation, i.e. α- and ρ-relaxation, respectively. On the other hand, the position of these processes is shifted towards a higher temperature for irradiated samples. The activation energy for α-relaxation process of non-irradiated samples is ∼ 0.21 eV, while for irradiated samples, it is 0.34 eV. The thermal sampling technique (TS) has been applied to decompose the global TSDC spectrum of all samples into its elementary peaks. The molecular parameters, such as activation energy (Ea ), pre-exponential factor (τ 0), enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG), are estimated for all samples. The compensation phenomenon has been verified for all samples in different ways, such as plotting ln τ 0 against Ea or entropy versus enthalpy.

Evaluations of dielectric property and drug release profile of 5-FU patches based on plasma charged electrets

In the present study, the electret 5-fluorouracil patch was developed, the effective surface potential, piezoelectric coefficient d33, open-circuit thermally stimulated discharge (TSD) current spectra and shear adhesion of the patch were measured. The drug release profile of the patch was determined by using high performance liquid chromatography method. A stable potential difference which was positively dependent on the surface potential of the electret was generated on two sides of the patch. The measurements of d33 coefficient, TSD current spectra and adhesion performance showed that the electrostatic field of the electret could cause polarization and cohesive strength decreasing of the matrix molecules, change the distribution and interaction of the drug molecules in patch, therefore to increase the release of drug from the transdermal patch.

Dielectric relaxation behavior of C5+ (70 MeV) swift heavy ion irradiated polyetheretherketone (PEEK) using TSDC technique

The dielectric relaxation characteristics of polyetheretherketone (PEEK) irradiated with C5+ (70 MeV) ion, have been investigated in the temperature range 60–230° as a function of poling temperature TP (50, 100, 150, and 200°C), poling field EP (200, 300, 400, and 500 kV/cm), and storage time ts (2, 24, 48, and 120 h), using thermally stimulated discharge current (TSDC) technique. The TSDC spectra show a prominent maximum around glass transition temperature (Tg ∼ 143°C) named as α-peak. This peak is attributed to the movement of ketone dipoles linked with the main chain. It is observed that the magnitude of α-peak increases with the increase in poling temperature and poling field. The peak current and area under the α-peak are found to be diminished with the increase of storage time ts for electrets. The β-peak (space charge peak) is absent in irradiated PEEK samples as compared to pristine PEEK samples. The results obtained, in the present studies, are compared with the results on pristine samples. The activation energies and pre-exponential factor for PEEK samples determined using Bucci plot method.

Dielectric behavior of polyetheretherketone (PEEK) using TSDC technique

The dielectric relaxation behavior of Polyetheretherketone (PEEK) has been investigated by using thermally stimulated discharge current (TSDC) technique. The dependence of TSDC characteristics of PEEK on poling temperature (TP) [50–200 °C], poling field (EP) [200–500 kV/cm], storage time (tS) [2–120 hrs] and various thicknesses 25 μm, 50 μm and 75 μm have been investigated in the temperature range [60–230 °C]. The TSDC spectra shows a prominent maxima around glass transition temperature (Tg) i.e. at 143 °C named as α-peak and the other peak is observed around 200 °C named as β-peak. The α-dipolar relaxation is taking place because of the movement of ketone (>C = 0) dipoles linked to the main chain. The β-peak is attributed to the space charges. It is observed that the magnitude of α-peak increases with the increase in poling field. The peak current and area under the α-peaks are found to be diminished with the increase of storage time (ts) for electrets. The amplitude of α-peak decreases with the increase in thickness. The activation energies for PEEK sample at different conditions in the present work are found to be 0.38 eV–1.70 eV. The values of activation energy (U) and pre-exponential factor (τo) for α- relaxation are determined using Bucci plot method and support the nature of the relaxations.

Chemical surface treatment and charge stability of polypropylene cellular electret film

The charge stability has been systematically studied by measuring open-circuit thermally stimulated discharge (TSD) current spectra, in situ TSD charge spectra and isothermal charge decay for the polypropylene (PP) cellular films after chemical treatments ( extraction, oxidation and HF solution treatment). The results showed that a very strong new current peak, which the virginal film does not have, appeared at about 184℃ in the TSD current spectrum of the PP cellular film properly oxidized and then HF-treated at room temperature (RT), and that its charge storage thermal stability is significantly improved compared with that of the virginal PP film, and is enhanced further by high temperature charging method. The reason that charge stability of the HF-treated PP films is improved has been clarified by attenuated total reflection infrared (ATR-IR) spectroscopy. The charge stability of treated films can be improved by appropriately prolonging either HF treatment time at RT or oxidation time. The theory of in situ TSD charge indicates that an in situ TSD charge spectrum contains combined information on the changes of the mean charge depth of the deposited charge layer and the deposited charge value. Combining the TSD current spectrum and the initial mean charge depth of a PP cellular film at RT, the change of the mean charge depth can be in situ investigated in heating process.

Dielectric, Thermally Stimulated Discharge Current, Pyroelectric and Surface Morphology of Pmma Thin Films Prepared by Isothermal Immersion

Metal–Insulator–Metal structure was used to investigate the dielectric, thermally stimulated discharge current(TSDC)and pyroelectric behavior of poly methyl methacrylate(PMMA)thin films prepared by isothermal immersion method. The surface morphology and the structure of the above said films were studied by using atomic force microscopy(AFM)and X-ray diffraction(XRD)respectively. The XRD spectrum of as grown and films annealed at 513 K indicated the amorphous nature. No pits and dendritic features were observed from the AFM spectrum. The only important topographic features observed is the hillocks of about 10–12 nm large with a peak to valley vertical distance of about 0.5–1 nm. Both as grown and films annealed at 513 K showed very smooth surface and amorphous nature. From the TSDC spectrum activation energy(E), capture cross-section(σ), escape frequency(ν)and relaxation time(τ)were calculated. The observed amorphous phase, low loss, dielectric and thermal behavior imply the feasibility of utilizing PMMA thin films as gate dielectric insulator layer in organic thin film transistors(OTFTs), which can find application in flat panel display.

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.

Comparative dielectric and TSDC studies of molecular mobility in liquid-crystalline side-chain poly(methacrylate)

The molecular mobility of a LC side-chain polymethacrylate (PM6) was carried out by dielectric spectroscopy and by thermally stimulated discharge current (TSDC) methods. Six relaxation processes γ2, γ1, β, β1, α, and δ at successively increasing temperatures were observed in PM-6. For each of them, a molecular interpretation was proposed. There is a good correlation between the peak temperature positions obtained from the global TSDC spectrum at an equivalent frequency and those obtained from the dielectric method. The TSDC thermal windowing experiments for PM-6 show the high ability to resolve the overlapping processes at low temperatures. Dielectric and TSDC methods giving different details of the observed relaxation processes, allow us to create the overall picture of molecular mobility in the system under investigation. These two methods revealing the differences and similarities of the obtained results could be considered as complementary to each other.

Study of Dielectric Relaxation Processes in Poly(p-phenylene sulfide) Using a Thermally Stimulated Discharge Current Technique

The various dielectric relaxations in poly(p-phenylene sulfide) have been investigated using a thermally stimulated discharge current (TSDC) technique. The TSD current spectra in the temperature range 30 to 250°C was analyzed as a function of poling temperature (100–250°C), poling field (5–18 kV/cm), poling time (1–4 h), specimen thickness (0.8–1.6 mm), heating rate (1–4°C/min.) and storage time (2–720 h). From the variation of the properties of peaks appearing in the TSDC spectra, it is concluded that the peak appearing at around 110°C (β-relaxation) is a dipolar transition due to C–S linkage. Samples poled at higher (Tp > Tg) show a peak at around 170°C (β ′-relaxation), which has been associated with the formation of cross-linked ether linkages. The β ′-relaxation gives all the character of dipolar polarization. The peak at around 210°C (α-peak) has been ascribed to a space charge transition. A rigid amorphous phase (RAP) and unsaturated phenylene groups provide the deep charge trapping centers. The activation energy obtained from the Bucci plot method confirms the respective nature of these peaks. The observed dependence of peak temperature on poling temperature and poling time indicates the presence of continuous distributed relaxation.

Effect of high energy [formula omitted] ion-irradiation on the dielectric relaxation behaviour of Kapton-H polyimide film. Thermally stimulated depolarization current study

The effect of high-energy 238 U ions (11.6 MeV/n, flux 10 6 ions/cm 2, GSI facility, DARMSTADT) on the various dielectric relaxation processes in Kapton-H polyimide using thermally stimulated discharge (TSD) current technique has been investigated. The irradiated Kapton-H samples (25 μm thickness, area 1.13 cm 2) were polarized at different temperatures (363, 393, 423 and 473 K) under various biasing fields (140–600 kV/cm). TSD currents were obtained at the heating rate of 2 K/min. The peak appearing at low temperature and associated with dipolar polarization (β-transition) is largely affected in irradiated samples. This is mainly due to the demerization of the carbonyl groups. In the vicinity of β-transition, small multiple peaks in TSD current spectra are observed. These peaks have been associated with the cross-linking of imidic groups as a result of high-energy ion-irradiation. The same has been confirmed by the activation energy values of these peaks and also through the TSD current behaviour of repoled samples. Increase in activation energy of high-temperature peak (α-transition) associated with space charge relaxation is observed in irradiated samples. This is mainly due to the creation of new deep energy centers resulting from the formation of triple bonds and free radicals. The activation energy has been calculated using Bucci-plot method.

Evaluation of the dielectric parameters from TSDC spectra: application to polymeric systems

The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analysis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (ΔW ≈ ± 0.06 eV).

Bioelectret state in amylase: a study by the thermally stimulated discharge current technique

The bioelectret state has been proposed to be a universal property of enzymes and to play an important role for the catalytic action of enzymes. In the present investigation the bioelectret state in an enzyme amylase (EC 3.2.2.1) has been studied using the thermally stimulated discharge current (TSDC) technique. The enzyme has been found to be able to manifest the bioelectret state; the TSDC spectrum is characterized by four peaks at temperatures around 170 K, 220 K, 265 K and 300 K. The 265 K peak is attributed to rotation of some polar amino acid residues, whereas bound water molecules are identified as the source of polarization storage for all other peaks. The 170 K peak is ascribed to bound water molecules directly attached to the polypeptide backbone; the 220 K peak is ascribed to a second layer of multiple hydrogen-bonded water molecules, proposed to be a chain-like structure around the polypeptide backbone, and the 300 K peak is attributed to the bound water molecules adsorbed on the surface of the enzyme matrix. The existence of several relaxation polarizations supports the Fröhlich model of longitudinal polarization waves that are proposed to play an important role for the catalytic action of the enzyme. © 2000 Society of Chemical Industry

The effects on polyetheretherketone and polyethersulfone of electron and γ irradiation

In this study, the effects of ionizing radiation on the aromatic polymers polyethersulfone (PES) and polyetheretherketone (PEEK) were investigated. UV light, /spl gamma/ rays and a high-energy electron beam were used as radiation sources. A range of techniques has been utilized to investigate changes in PES and PEEK as a result of exposure to these sources. Electron spin resonance (ESR) measurements showed the creation of radicals in PES by UV or /spl gamma/ radiation. These radicals decayed, but some were still present 24 h after irradiation at room temperature. In PEEK, radicals were formed as a result of UV at room temperature and by UV or /spl gamma/ irradiation at 77 K. After irradiation at room temperature, these radicals decayed totally within 24 h. UV-visible absorption measurements of PES samples showed a dose related change at the absorption edge; no such change was observed for PEEK. Phosphorescence after /spl gamma/ irradiation increased in intensity for PES at low temperature but decreased at higher temperatures. For PEEK, no such changes were observed after irradiation. The differential scanning calorimeter (DSC) measurements showed changes in the crystallization, glass transition and melt-point temperatures of both PES and PEEK after electron irradiation. Electrical changes induced by irradiation were investigated using thermally stimulated discharge current (TSDC) and low frequency dielectric responses, derived from transient current (TC) measurements. Dielectric changes were observed for PES exposed to either /spl gamma/ rays or an electron beam. For PEEK only minor changes in the TSDC spectrum were observed after exposure to /spl gamma/ radiation. More significant changes were observed after exposure to the electron beam. Correlations were found between the production of radicals and the changes in the structural and electrical properties of the materials. Since significant changes were observed in the dielectric spectra of PEEK and PES at low frequencies, dielectric spectroscopy may provide a measure of aging in both insulation systems.

Electret behaviour of ion irradiated polycarbonate and kapton-H polyimide films

The electret state of a polymer arises from the induced polarization owing to the frozen in phenomenon of electric charges. The study of electret nature using thermally stimulated discharge current (TSDC) technique reveals the nature of various relaxation processes in a polymer. When a polymer is irradiated with high energy ions their dielectric relaxations get affected which can be investigated using above mentioned technique. Reported here are the results of TSD current spectra of films of polycarbonate (PC) (irradiated with 100 MeV Ag +, flux; 1 × 10 11 and 1 × 10 9 ions/cm 2) and kapton-H polyimide (irradiated with 100 MeV O ++, flux; 1 × 10 5 ions/cm 2). The as-received and irradiated samples of PC were polarised at 120 °C using a 200 kV/cm electric field while those of kapton-H were polarised at 90 °C, field; 133 kV/cm and 180 °C, field; 266 kV/cm. With PC it is observed that the irradiation does not affect the location of TSDC peak occurring at 144 °C (activation energy; 0.6 eV); however, the peak magnitude changes with irradiation dose. The peak being associated with dipolar relaxation the ion-irradiation seems to have changed the number of dipoles. The high temperature TSDC spectra is however largely affected by irradiation giving evidence of the presence of new traps. With kapton-H, it is observed that TSD current changes its direction (anomalous TSDC) in irradiated samples in the high temperature region. Such behaviour has not been observed in the thermoelectrets of as-received samples. Only the corona charged electrets were observed to give such anomalous TSDC behaviour. This suggests the creation of new energy traps from irradiation. The bulk nature of this relaxation is confirmed by the TSDC of repoled samples. The irradiation induced shallow energy traps along with dipolar relaxation are also observed from the TSDC spectra in the low temperature region.

A relation between the space charge distribution and TSDC spectrum of corona charged PTFE thin films

A space charge density distribution measuring system based on a PPP (piezoelectrically induced pressure pulse wave) method was constructed. The pressure pulse wave is generated by a pulse-driven piezoelectric PVDF (polyvinylidene fluoride) thin film used as a piezoelectric pulse generator. A thin aluminum plate is used as a shield electrode to electrically separate the pressure pulse generator and the sample holder. This thin shield electrode reduces attenuation and dispersion of the pressure pulse wave compared to the former system. The spatial resolution of the system is estimated to be about 7 μm, based on the PPP response to a sheet charge that is artificially placed on a dielectric thin film. Using space charge distribution measurements and TSDC (thermally stimulated discharge current) measurements, the charge behavior of the corona-charged PTFE Teflon TM (polytetrafluoroethylene) thin films was studied.

Thermally Stimulated Discharge Current Analysis of Polymeric Solid-State Ionic Conductors

Thermally Stimulated Discharge Current (TSDC) analysis was used to study the electrical properties of two systems of polymeric solid-state ionic conductors. One system consisted of LiClO4 doped polyethylene glycol polyurethaneureas (PEUU) characterized by bi-ionic conduction and the other system was based on a sulfonated polyethylene oxide polyurethane ionomer (SPUI) characterized by a single-ion transport mechanism. TSDC spectra of the PEUU as well as SPUI matrices showed four peaks (γ, β, α, and ρ). These peaks are found to be related to the local motion of polyether soft segments, glass transition, Maxwell-Wagner-Sillars interfacial polarization and transport of charged ionic species, respectively. The discharge currents without polarization varying with temperature for both, the PEUU–LiClO4 complex and SPUI, are also measured. These curves of discharge current show only one peak which is attributed to transport of charged ionic species. The temperature range in which the discharge current exhibits a steep increase is consistent with the temperature range in which a turning point appears on the plot of lg σi versus 1000/T obtained by ac impedance analysis. The experimental results demonstrate that the TSDC analysis is a valuable tool for studying polymeric solid-state ionic conductors.

Relaxation of Polar Order in Poled Polymer Systems: A Comparison between an Isothermal and a Thermally Stimulated Experiment

The relaxation of polar order in the poled polymer [OPh(R)OCOPh(Pr) 2 CO] m [OPh(R)OCOPh(OBz) 2 CO] n (R= (CH )6 OPHCH=CHPhNO 2 ) system which had been designed in the context of nonlinear optical (NLO) materials was investigated. The isothermal relaxation of the Pockels effect as measured by the modulation of waveguide modes for different temperatures are best described by a symmetric Havriliak-Nagami relaxation time distribution. The width of the distribution increases with decreasing temperature. The temperature dependence of the mean relaxation time followed a modified version of the Vogel-Tamann-Fulcher equation. Thermally stimulated discharge current (TSDC) experiments were carried out in order to verify the proposed time-temperature behavior. The modified Vogel-Fulcher description of relaxation times describes the TSDC spectra well using the same relaxation time distributions as found for the isothermal relaxation

The influence of ions on the thermally stimulated discharge current spectra of water-treed additive-free low-density polyethylene

The effects of water treeing on steady state dc currents and thermally stimulated discharge current spectra (TSDC) of additive-free low density polyethylene (LDPE) were studied using heavily water-treed material in solutions of LiCl, LiCl plus HCl, MnCl/sub 2/ and FeCl/sub 3/. Significant changes in the dc conductivity of treed samples are attributed to the effects of ions. For such samples, graphs of the logarithm of current versus inverse temperature show three distinct linear regions. The transition temperatures demarcating these regions appear to correlate with those reported for thermal relaxations. TSDC spectra of water-treed samples show a dependence on both the ionic solutions used and on the poling conditions. For LiCl treed samples, twin peaks in the TSDC spectra were produced only under specific poling conditions. These peaks may be correlated with thermal relaxations observed using differential scanning calorimetry (DSC). The melting point temperature and the melting peak area derived from DSC spectra were reduced by treeing. >

Silicondioxide electret films prepared by the sol-gel process

Sol-gel silica electret films were coated on silicon substrates by a spin coating technique. The process of preparing the films is described and the electret behavior of the films is discussed. The films were corona charged at room temperature. Then, isothermal decay of the surface potential at room temperature and open-circuit thermally stimulated discharge (TSD) experiments were carried out. The results showed that the main peak of the TSD current spectra was situated at /spl sim/250/spl deg/C and charge deposited in the film indicated good stability for the negative charged sample. After storage at room temperature for 50 days, the surface potential of the sample still retained at /spl sim/90% of the initial value. The authors found activation energies of 1.3 and 0.7 eV for negative and positive corona charged samples. Experimental results suggest that the sol-gel process could be useful in electret and electronic technique applications. >

Thermally stimulated discharge current studies on styrene-methyl methacrylate copolymer films

Thermally stimulated discharge current (TSDC) studies on thin films of styrene-methyl methacrylate copolymer in the ratio 45:55 are reported. Films of thickness 7.36 μm were grown by the isothermal immersion technique. Only one current maximum was observed in the TSDC spectrum for polarizing fields < 20 × 10 5 V/m and two current maxima were observed at higher fields for all polarizing temperatures. Relaxation times and activation energies were evaluated using existing theories. The low-temperature peak was attributed to dipolar orientation and the high-temperature peak to glass transition relaxation.