Water sorption and thermally stimulated depolarization currents in nylon-6

The influence of water on the low temperature dielectric relaxations of nylon-6 has been investigated in the range from dry to water-saturated polymer by means of sorption experiments and thermally stimulated depolarization currents (TSDC). Three different ranges of water sorption are observed and their reflection in the TSDC spectra is examined. It is found that position and intensity of the TSDC maxima exhibit characteristic and different behavior in each of these sorption ranges. The critical concentrations are 1.5 ± 0.5% and 6.0 ± 0.5% water content. An interpretation of the experimental results is given with respect to the molecular origins of the low-temperature dielectric relaxations and the mechanisms of water sorption in nylons. The most important idea is that water exists as firmly bound, loosely bound, and clustered water, depending on its concentration. An attempt is made to determine relations between these different states of water in nylon-6 and some characteristic features of the low-temperature TSDC spectrum. © 1996 John Wiley & Sons, Inc.     

The glass transition in linear low density polyethylene determined by thermally stimulated depolarization currents

New thermally stimulated depolarization currents (TSDC) results on LLD polyethylene functionalized with diethylmaleate polar groups are precisely computer fitted with the direct signal analysis technique. It is shown that the TSDC spectrum consists, with increasing temperatures, of a sub-γ peak, a sharp γ peak, and a β and an α relaxation. The first peak is analyzed in terms of Arrhenius relaxation times, whereas the γ and β transitions could only be fitted by using Vogel-Fulcher temperature dependence for the relaxation times. The best value for T_o obtained from both fittings is 69.7 K. This is a quantitative proof for the identification of the γ transition as one of the dielectric manifestations of the glass-rubber transition for polyethylenes, T_g = 136.5 K, which has been discussed extensively in the literature. The β relaxation, T_gβ = 237 K, has also the expected characteristic of a glass transition; the existence of two T_gs in polyethylene could explain our results. © 1996 John Wiley & Sons, Inc.     

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.     

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.     

Effect of physical aging on the Johari-Goldstein and alpha relaxations of D-sorbitol: a study by thermally stimulated depolarization currents

The relaxations in amorphous D-sorbitol have been studied by thermally stimulated depolarization currents during annealing at 255 K, which is 17 K below its calorimetric glass transition temperature Tg=272 K. As the glass structurally relaxes on aging, the features of the alpha relaxation and of the Johari-Goldstein (JG) relaxation change with time. For the alpha relaxation (i) the dielectric strength decreases; (ii) the activation energy decreases; and (iii) the relaxation time increases. For the JG relaxation the dielectric strength also decreases but with a different time dependence, and there is no evidence for any modification of the kinetic features of the mobility. The amplitude of response to aging is higher for the higher temperature motional components of the Johari-Goldstein relaxation compared with the lower temperature ones.     

Use of blocked electrodes in thermally stimulated depolarization current measurements

The use of blocked electrodes in thermally stimulated depolarization current (TSDC) measurements has been studied on phenol–formaldehyde novolac resin blocked by one or two Teflon foils. The measurements were performed after polarization at different temperatures T_p separately for the Teflon and the resin. Blocking decreases the effectiveness of a nominal polarizing field and suppresses the background current. It is explained here for the first time that the charge released from the charged Teflon interferes with the conductivity relaxation peak of the resin. The influence of the blocking is very dependent upon T_p, and this dependence is specific in regard to the type of current peak. The application of blocking at different T_p is recommended for the determination of the origin of peaks in TSDC measurements.     

Simultaneous thermally stimulated luminescence and depolarization current in low density polyethylene

Thermally stimulated luminescence (TSL) and thermally stimulated depolarization current (TSD) measurements were made simultaneously on low density polyethylene samples 0.175 mm thick, carrying evaporated gold electrodes. The samples were dc-polarized at room temperature at a field strength of 90 kV/cm, cooled to –190°C and X-irradiated with the field still applied, and then heated in short-circuit at 3 deg/min. Comparing the TSL and TSD data, particularly the effects of oxidizing the samples and immersing them in fuming nitric acid, it was shown that (1) the space-charge transport component of the TSD current between –190 and 30°C is negligible, (2) charge injection begins around –10°C (for evaporated gold electrodes) and increases rapidly with increasing temperature, and (3) all the injected charge is transported through the sample to the opposite electrode. Such transport takes place only through the amorphous regions of the sample.

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Zusammenfassung Es wurden simultane TSL- und TSD-Messungen an 0.175 mm starken, über aufgedampfte Goldelektroden verfügende Polyethylenproben geringer Dichte durchgeführt. Die Proben wurden bei Raumtemperatur und einer Feldstärke von 90 kV/cm gleichstrompolarisiert, auf –190°C abgekühlt, bei noch anliegendem Feld röntgenbestrahlt und dann unter Kurzschluß mit 3 Grad/min erhitzt. Durch einen Vergleich der TSL- und TSD-Daten, insbesondere der Wirkung der Oxydation der Proben bzw. des Eintauchens in rauchende Salpetersäure, zeigten, daß (1) die Raumladungstransport-Komponente des TSD-Stromes zwischen –190 und 30°C vernachlässigbar ist, (2) die Ladungsinjektion bei etwa — 10°C beginnt (für aufgedampfte Goldelektroden) und mit steigender Temperatur schnell anwächst und (3) die gesamte injezierte Ladung durch die Probe hindurch an die gegenüberliegende Elektrode transportiert wird. Ein derartiger Transport erfolgt nur in den amorphen Regionen der Probe.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday     

Hydration of a Na(+)-montmorillonite studied by thermally stimulated depolarization current

Thermally stimulated depolarization current (TSDC) technique is a powerful tool for probing dipole re-orientational motions in condensed matter. In the case of cation-exchangeable aluminosilicates, it allows the assessment of the potential barrier related to the hopping mechanism of cations and, consequently, the measurement of its evolution when molecules, i.e. water, are adsorbed and interact with the cations embedded in the solid framework. Then, using suitable models based on thermodynamics, the analysis of TSDC signals obtained at various hydration states provides insights about the surface properties of the studied solid and the mechanism of adsorption at the cationic site. In this work, TSDC is used to study the first stage, i.e. when the number of adsorbed molecules is below the occurrence of the water monolayer, of water adsorption in a Na(+)-montmorillonite from Mostaganem (Algeria). It is shown that the hydration process follows two stages. Using the "chemical force" concept it can then be concluded that when the number of adsorbed water molecules per cation is lower than 2, cation-water interaction dominates the energetics of adsorption, whereas at higher water loading the water "chemical force" is also involved into water-water and/or water-clay framework interactions. The number of water molecules for the monohydrated state is found to be about 7.     

Chemical origin of thermally stimulated discharge currents in polyacrylonitrile

Thermally stimulated discharge has been used in the past to diagnose and estimate the magnitude of electrical polarization in polymers. Nevertheless, molecular characterization of operative phenomena by this technique alone is often a difficult task. In the present work, infrared attenuated total reflection spectroscopy has been used to investigate the origin of thermally stimulated discharge currents near 200°C in externally unpolarized films of polyacrylonitrile (PAN). Spectroscopic analysis of thermally degraded films reveals some unsaturation of the PAN backbone and possibly the generation of cyanide ions. Opposite surfaces in a solvent-cast film give different spectra, indicating a gradient in chemical degradation products across the film thickness. Data suggest that nonuniform generation of charged species and unsaturated bonds gives rise to internal potentials in PAN. The origin of thermally stimulated currents in PAN near 200°C is thus believed to be associated with the onset of chemical degradation.     

Dipolar motions and phase transitions in a side‐chain polysiloxane liquid crystal. A study by thermally stimulated depolarization currents

The relaxation mechanisms present in a side‐chain liquid crystalline polymer have been studied by Thermally Stimulated Depolarization Currents (t.s.d.c.), in a wide temperature range covering the glassy state, the glass transition region, and the liquid crystalline phase. The thermal sampling procedure was used to decompose the complex relaxations into its narrowly distributed components. Three relaxation mechanisms were observed in this polymer: a relaxation below the glass transition temperature that is broad and extends from −150°C up to −110°C, the glass transition relaxation whose maximum intensity appears at ∼20°C, and a relaxation above the glass transition temperature, in the liquid crystalline phase. The attribution of these relaxations at the molecular level is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 227–235, 1999     

Investigations on electric properties of polymers—II. Dielectric relaxation in atactic polystyrene determined by thermally stimulated depolarization currents

The thermally stimulated depolarization currents (TSDC) from atactic polystyrene and d.c. conductivity of the same material have been studied over the temperature ranges 200–395 K and 363–393 K respectively. TSDC spectra are complex and consist of four peaks, β₁, β₂, β₃ (β₃ of reduced intensity) and α in order of increasing temperature. The first three peaks appeared below the glass transition temperature T_g of the polymer. The β₁ peak seems to arise from a single dipolar relaxation process. The β₂ peak and probably β₃ arise from a distribution in activation energy of dipolar relaxation processes. On the basis of the molecular origin of the β relaxation process, it was suggested that β₁, β₂ and β₃ peaks involve motions of backbone chain fragments of various lengths. The last peak α appeared at T_g and could be considered as a result of dipole relaxation and electric conductivity. TSDC peaks and d.c. conductivity have been related to the second order transition in the polymer.     

Thermally stimulated depolarization currents of tactic polymethyl methacrylates and their mixtures

Summary Thermally stimulated depolarization currents (TSDC) of atactic (a), syndiotactic (s) and isotactic (i) PMMA and mixtures ofs- andi-PMMA were measured. In pure tactic PMMA, TSDC peak was obtained at a temperature corresponding to the glass transition temperature of each PMMA. TSDC of mixtures isolated as precipitates from 1/1 and 1/2 (i/sweight ratio) mixed solutions showed a peak which can be attributed to the glass transition temperature of the stereocomplex. TSDC of original l/1 mixture showed two peaks which are ascribed to the glass transition temperature of residual i-PMMA and the stereocomplex. The results suggests that stereocomplex formation occurs ini/s weight ratio of 1/2 independent of the way of preparation.With 9 figures and 1 table     

Thermally stimulated currents depolarization study of gamma ray irradiated PMMA

We 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 T_c which decreases versus the radiation dose.     

Characterization of the amorphous phase of polyacetylene by thermally stimulated currents

The Thermally Stimulated Current (TSC) Spectroscopy has shown that cis-polyacetylene can be considered as a semi-crystalline polymer. Indeed a relaxation mode obeying a compensation law has been observed: it is the dielectric manifestation of the glass transition. Upon oxidation, two TSC peaks have been observed. They have been associated with two diffusion processes revealing the existence of two amorphous phases: - the intrafibrillar amorphous phase located between crystallites inside the fibrils, - the interfibrillar amorphous phase situated between the fibrils and so much more exposed to oxygen.     

Cooperative relaxations in semicrystalline fluoropolymers studied by thermally stimulated currents and ac dielectric

Semicrystalline fluoropolymers including poly(tetrafluoroethylene) (PTFE), a 8 mol % hexafluoropropylene (HFP)/92% TFE random copolymer (FEP), and poly(vinyl fluoride) (PVF) were studied using thermally stimulated current depolarization (TSC), ac dielectric, and other thermal analysis techniques. The TSC thermal sampling (TS) technique is emphasized here for the detection of broad and weak “cooperative” relaxations with all three of the polymers studied exhibiting two cooperative (i.e., relatively high apparent activation energy) transitions. The well-studied low-temperature γ relaxation in PTFE at ca. −100°C is characterized by this method as well as the γ relaxation in the less crystalline FEP sample. Higher temperature cooperative glass transitions, associated with constrained noncrystalline regions, are found at ca. 100°C in PTFE and ca. 80°C in FEP at TSC frequencies. Comparisons with relaxation studies of linear polyethylene are made, and the effects of crystallinity on the various transitions are discussed. The unique characterization by the TSC-TS technique in the detection of multiple “cooperative” relaxations, even in the case of overlapping transitions, is emphasized here. An example is the low-temperature relaxation in FEP. Two cooperative transitions were detected in PVF. The higher temperature one at ca. 45°C is the glass transition, as is well known in the literature. More information is needed to confirm the molecular origin and the effects of crystallinity and chemical structure on the low-temperature cooperative transition in PVF. © 1996 John Wiley & Sons, Inc.     

Thermally stimulated depolarization currents of crosslinked polyethylene relaxations in the fusion range of temperatures

With thermally stimulated depolarization currents, we researched the relaxations of crosslinked polyethylene as it is used in medium‐voltage cable insulation. Through conventional polarization two heteropolar peaks stand up in the spectra, at 80 and 105 °C. As the sample is annealed, a homopolar peak is developed at about 99 °C. With window polarization, our results indicated that the 80 °C peak is a structured peak related to polar crosslinking subproducts and impurities. The 105 and 99 °C peaks are fitted to the general kinetic‐order model because the 105 °C peak is related to free‐charge detrapping at the crystalline phase, in the bulk and maybe at the amorphous‐crystal interphases, and the peak that is observed at 99 °C is due to injected charge. Annealing at high temperatures promotes the creation of traps in the material. Charge trapping at T < 70 °C seems to be related to the increased insulator resistivity with annealing time. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1412–1421, 2003     

Evidence for clustering in N-butyl methacrylate ionomers as shown by thermally stimulated currents

The relaxation behavior of n-butyl methacrylate/methacrylic acid (MAA) copolymers (12 mol % MAA) and their alkaline salts (K, Li) with various degrees of neutralization was investigated from 77 to 400 K by means of thermally stimulated depolarization and polarization methods. For degrees of neutralization higher than 50%, the experimental results were found to be consistent with the existence of complex heterophase structures involving significant widening of the distribution function of relaxation times in the glass transition region, splitting of the side-chain transition in several discrete relaxation components, and formation of ionic cluster giving rise to their own relaxation process. By systematically studying the influence of polarization conditions, it was also shown that the appearance and stability of such clusters are markedly dependent on the thermal and electrical history of the samples and that the clustering relaxation is closely related to the ionic conductivity via space-change formation.     

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     

Anionic organic guests incorporated in zeolites: adsorption and reactivity of a Meisenheimer complex in faujasites

Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provided in support of an adsorption mechanism in which a neutral ion pair (alkali metal ion + Meisenheimer anion) is the actual species being adsorbed. In fact it appears that there is remarkable increase in the association constant for the ion-pair complex within the zeolite cavities as compared to DMF solution. Although this mechanism of adsorption as an ion-pair complex has precedents in the adsorption of some inorganic salts, what is novel is the notable increase in the stability and persistence of the Meisenheimer anion (a anionic reaction intermediate) as a result of zeolite inclusion. Adsorbed Meisenheimer complex exhibits much lower reactivity towards electron acceptors, oxygen, and water. Cyclic voltammetry of zeolite-modified electrodes reveals for the Meisenheimer complex adsorbed on LiY a reversible redox peak that is not observed in solution and has been interpreted as arising from site isolation and stabilisation of the electrochemically generated species.     

Vapor-induced depolarization currents in electrets

The charge decay of polymer electrets in the presence of solvent vapor is analyzed. In order to account for this phenomenon, several hypotheses are considered and tested experimentally. It is shown that the penetration of the solvent is responsible for the observed charge decay, and that it is the rate limiting factor. Moreover, the phenomenon is not due to the screening of the injected charges, but to the motion of these charges, which are liberated by the penetration of the solvent. Finally, the possible applications of this phenomenon in investigations on both the microscopic and macroscopic scale are outlined.