A thermoreversible conformational change in two poly(zwitterions) evidenced by thermally stimulated depolarization currents

A thermally induced conformational change of zwitterionic groups of two polymers has been observed by thermally stimulated depolarization currents. Different dipole moments of the two conformations is a prerequisite. The appearance of peaks of opposite sign confirms the process. Giant dielectric constants can be obtained at room temperature using high-temperature poling and rapid cooling.     

Hydrogen Bonds at Silica – Polyvinylpyrrolidone – Water Interfaces

The effect of the adsorption of polyvinylpyrrolidone on the surface of highly dispersed silica on the state of interfacial water was studied by ¹H NMR spectroscopy and thermally stimulated depolarization with freezing out of the bulk water.     

The influence of extended conjugation on electroactive properties of copper phthalocyanine macromolecules

A monomer and polymer of a copper(II) phthalocyanine have been synthesized in order to develop structure-property relationships. Various electroactive properties such as dielectric constant, resistivity and thermally stimulated depolarization effects were investigated in order to examine the influence of extended conjugation.     

Photo-oxidative degradation of polyvinyl alcohol and measurement of thermally stimulated current

An investigation was made of the correlation between changes in the chemical structure of polyvinyl alcohol during photo-oxidation and a thermally stimulated current. Photo-oxidation was initiated by dehydration and greatly accelerated as a result of chain scission, which resulted from the formation of carbonyl groups of ester, aldehyde, ketone and carboxylic acid types. Two main peaks of thermally stimulated current were observed at 220 and 250 K in photo-oxidized polyvinyl alcohol. It became clear that these two peaks, resulting from depolarization of dipoles, correspond to an increase in the content of carbonyl groups and a decrease in the content of hydroxyl group, respectively.     

Influence of γ-irradiation in the molten state on the dynamic mechanical β relaxation and on the thermally stimulated depolarization currents of polyethylene

Dynamic mechanical and thermally stimulated depolarization current data have been obtained on polyethylene, γ-irradiated in the molten state, as a function of irradiation dose. The β peak which appears in mechanical experiments has been attributed to motions of the chain backbone. The branches, like the crosslinks coming from γ-irradiation, are fixed tie points for the chains.     

A thermally stimulated discharge currents study of the molecular motions in two polysiloxane side-chain liquid crystalline polymers

The technique of thermally stimulated depolarization currents was used to study the dipolar relaxation mechanisms in two side-chain liquid-crystalline polysiloxanes. The studied polymers have the same mesogenic side-group, but they differ in the number (or density) of side groups attached to the main chain. In both cases three different relaxations were observed: one in the vitreous state, one in the liquid crystalline phase, and the glass transition relaxation. The features of those thermally stimulated discharges are discussed and some suggestions are made concerning the attribution of those discharges at the molecular level. © 1995 John Wiley & Sons, Inc.     

Piezoelectricity of uniaxially stretched poly(vinylidene fluoride) corona poled at elevated temperature

X-ray diffraction, thermally stimulated depolarization, and piezoelectric current measurements have been carried out on uniaxially stretched poly(vinylidene fluoride) film, corona poled both negatively and positively at room temperature and at 90°C. Corona poling at elevated temperature considerably enhances the piezoelectric constant and is a more efficient way of charging the film than conventional thermal poling. Advantages over the latter are discussed.     

Studies of thermally stimulated current in polypropylene/calcium carbonate/surfactant systems

Thermally stimulated polarization (TSP) and depolarization (TSD) experiments have been performed with two grades of polypropylene and some respective CaCO₃-filled composites containing small amounts of nonionic surfactant (0–2 wt%). The effects of electrode blocking, electrode materials on the thermally stimulated currents, and reproducibility of the measurements have been studied. The effect of water vapor adsorbed from the ambient air on the AC dielectric properties and on the thermally stimulated polarization behavior has been determined.The addition of either CaCO₃ or surfactant to PP decreases the intensity of the _c depolarization current peak in the pre-melting region, while the presence of both components increases the current. Partial discharges are present in poly propylene/CaCO₃ composites under high voltages if neither water vapor nor a coherent surfactant layer is present at the matrix/filler interface.A short literature survey is presented on the TSC studies of polyolefins and their composites, and various mechanisms responsible for the observed changes are discussed, including interfacial polarization, trap redistribution through nucleation, and oriented adsorption.     

Densification effect on physical properties of atactic polystyrene

Atactic polystyrene has been densified under hydrostatic pressure, up to 2 kbar, at about 180°C. Dielectric measurements were made on densified and nondensified polystyrene samples by using the thermally stimulated depolarization current (TSDC) and the absorption and resorption current methods. The dielectric loss is modified by the densification. This effect can be explained in terms of the free-volume reduction and the internal stress induced in the material during densification.     

Thermally stimulated and isothermal depolarization currents in low-density polyethylene

Thermally stimulated and isothermal depolarization currents of three low-density polyethylenes are investigated in the temperature range 80–320°K. The thermally stimulated current spectra can be analyzed in terms of a continuous relaxation time spectrum. This spectrum can be resolved into three Gaussian distributions of activation energies centered near 140°K (γ-relaxation), 205°K, and 245°K (β-relaxation) in agreement with electrical and mechanical loss factor measurements. The experimental observation are explained in terms of reorienting dipoles, coupling the molecular motion to the electrical field.     

An Electrical Model of Summation of Thermally Stimulated Depolarization Current by Means of a Ps-Pice Software

In this paper we present an electrical model of simulation. This model, used to simulate thermally stimulated depolarization current (TSDC) measurements, yields the same behaviour for its time constant as that observed for amorphous materials in a large temperature interval. This model, whose main device is a current source controlled by the exponential of a voltage with linear temperature dependence could be considered as a generalization of the classical model generally used in TSDC interpretation attempts. Its response is very close to that observed in various TSDC measurements carried out on different polymers.     

Assessment of the Nature of Depolarization Current in Thermally Treated Kapton Polyimide

Pyrolysis of normally insulating aromatic polyimide is known to impart electrical conductivity to the polymer due to the formation of carbonized regions in an insulating matrix with a concomitant change in the polymer’s structural arrangement. The wholly pyrolyzed polyimide is potentially useful for specific applications in certain types of semiconductor devices because of the polyimide’s insulator/conductor transition which creates a barrier type conduction. Pyrolysis, however, degrades the required mechanical integrity of the polyimide for construction of such devices. In order to evaluate the fundamental aspects of barrier conduction by high voltage electron transfer from metal contact that can still produce measurable current in thermally treated non-pyrolyzed polyimide, the nature of depolarization in Kapton was assessed by the thermally stimulated depolarization current (TSDC) technique. The results show that thermal treatment of polyimide without pyrolysis and therefore without loss of mechanical integrity offers a viable means of steady electron conduction for semiconductor operation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Characterization of Polyurethane Acrylate Resins

Polyurethane acrylate resins cured by two different ways, a thermal way and a photochemical way, are investigated by means of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). Even if both curing methods lead to the same material from a chemical point of view, we show that important differences exist between the thermocured resin and the photocured resin in terms of molecular relaxation behaviour.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermally Stimulated Depolarization Current Measurements

A law is proposed to link the evolution of the relaxation time τ with the temperature T in thermally stimulated depolarization current (TSDC) experiments (complex spectrum and elementary spectra). This model is equivalent to the classical one (compensation law) around T _c. Its response, obtained by simulation of the complex and elementary spectra, is very close to the real depolarization current curves throughout the whole range of temperatures. Experimental results were obtained on an amorphous copolymer: PETN8 (92% of poly (ethylene terephthalate) (PET); 8% poly(ethylene naphthalene) (PEN).     

Dielectric behaviour of a side-chain-bearing liquid-crystalline polysiloxane

The relaxation of electric field-induced polar orientation in a side-chain-bearing liquid-crystalline polysiloxane was measured by means of thermally stimulated depolarization currents. Different relaxation mechanisms were identified and characterized: the glass transition cooperative relaxation exhibits compensation behaviour. On the other hand, lowerT _g and upperT _g discharges were observed and their molecular nature is discussed.     

Molecular motion and relaxation studies of aliphatic polyureas by thermal windowing thermally stimulated depolarization current technique

Molecular motion and relaxation studies using a thermal windowing thermally stimulated depolarization current (TW‐TSDC) were performed for aliphatic polyureas 7 and 9. Global thermally stimulated depolarization current gave three characteristic major peaks corresponding to the α, β, and γ relaxation modes at 78.5, −44, and −136°C for polyurea 7 and at 80, −50, and −134°C for polyurea 9, respectively. The α relaxation is related to the large‐scale molecular motion due to micro‐Brownian motion of long‐range segments. This relaxation is significantly related to the glass‐transition temperature. The β relaxation is caused by the local thermal motion of long‐chain segments. The γ relaxation is caused by the limited local motion of hydrocarbon sections. Temperature dependence of relaxation times was expressed well using Vogel–Tammann–Fulcher (VTF) expression. 3‐D simulation of dielectric constants of dielectric strength and loss factor were performed in the frequency range from 10⁻⁶ to 10⁴ Hz and temperature range from −150 to 250°C, using the relaxation parameters obtained from the TW‐TSDC method. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 88–94, 2000     

Thermally stimulated depolarization current spectra of cross-linked polyethylene and the influence of cross-linking byproducts

Cross-linked polyethylene (XLPE) is notable for its use as power cable insulation. Its longevity is limited by space charge buildup linked to impurities such as the byproducts left behind by the cross-linking agent dicumyl peroxide (DCP). The goal of this work is to determine the impacts of these byproducts on charge trapping and detrapping in XLPE using the thermally stimulated depolarization current technique. XLPE with byproducts has one source of trapped charge, which originates from the byproducts. XLPE that was thermally treated via degassing exhibits two other sources of trapped charge, which are charge injection and dipolar relaxations. Oxidation from degassing was shown to control the trapping from these sources, which is useful knowledge for processing this material prior to its use. Reintroducing acetophenone, one of the major byproducts of DCP, suppresses those two peaks once more, showing that it controls the overall space charge buildup characteristics in XLPE.     

Ferroelectric behavior and structural change of poly(trimethylene terephthalate) induced by external electric field

The ferroelectric behavior and structural change induced by the external electric field were observed for poly(trimethylene terephthalate) (PTT), which has nonpolar crystalline structure with carbonyl groups aligned antiparallel to each other. The displacement‐electric field hysteresis curves of PTT films showed hysteresis above 140 °C, which indicated the polarization reversal of PTT. The pyroelectric constant and thermally stimulated depolarization current suggest the depolarization of the polar structure of PTT induced by the external electric field above 150 °C. An endothermic peak caused by the depolarization was also observed in the differential scanning calorimetry for the PTT film poled at 30 MV/m. The change of the crystalline structure and the orientation of carbonyl groups induced by the electric filed were investigated by X‐ray diffraction patterns and infrared spectra. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1991–1996, 2009     

Piezoelectricity and pyroelectricity of polyvinylidene fluoride corona-poled at elevated temperature

X-ray diffraction, thermally stimulated depolarization, and piezoelectric and pyroelectric current measurements have been carried out on biaxially oriented polyvinylidene fluoride film, corona-poled both negatively and positively, at elevated temperature. Larger piezoelectric and pyroelectric coefficients are obtained at lower values of the poling field, when corona charging takes place at elevated temperature rather than room temperature. Negative charging is more effective than positive. The polarization mechanism is dipole alignment, and a conversion of Form-II crystallites to Form I. The large space-charge injected by the corona, and trapped in the material, acts through creation of high internal fields, and shows little, if any, piezoelectric and pyroelectric effects of its own.     

Analysis of Thermally Stimulated Depolarization Currents (TSDC) Measured on Exchanged Clays

This work shows that hydration of clays can be studied by means of a new interpretation of thermally stimulated depolarization currents technique. These measurements have been performed on four exchanged natural clays: Na-montmorillonite, Ca-montmorillonite, kaolinite, and illite. The results are analyzed using both the recently developed analysis of relaxation times distribution and the electronegativity equalization method. They provide evidence of the surface heterogeneity of clays. It is established that sites, identical from a crystallographic point of view, are different when the energy of the phenomenon is considered. The main interest of this work is to give for the first time a value of the hydration energy of cation onto clay surfaces. Copyright 2000 Academic Press.