A. Bello

Influence of aging and crystallinity on the molecular motions in bisphenol-A polycarbonate

Thermally Stimulated Depolarization Current technique, Differential Scanning Calorimetry, and Dynamic Mechanical Analysis have been applied to amorphous and semicrystalline bisphenol-A polycarbonate with crystallinity degrees up to 21.8%, in a temperature interval covering the α and β relaxations. The secondary β transition is found to be the sum of three components whose variations in aged and annealed specimens have shown the cooperative character of the β1 and β2 modes, contrary to the localized nature of the β3 component. A Tg decrease was observed by both TSDC and DSC as a function of Xc and has been related to the possible confinement of the mobile amorphous phase in regions whose sizes are smaller than the correlation lengths of the cooperative movements that characterize the motions occurring at Tg. The α relaxation intensity variations with crystallinity show the existence of an abundant rigid amorphous phase in the semicrystalline material. The relaxation parameters deduced from the Direct Signal Analysis of the α relaxation for the mobile amorphous phase do not show significant deviations from those found for the amorphous material. The existence of the rigid amorphous phase has been associated to the ductile-to-brittle transition experienced by the material at low crystallinity levels.

α, β and γ relaxations of functionalized HD polyethylene: a TSDC and a mechanical study

The relaxation behavior of high density polyethylene functionalized with diethylmaleate has been determined by dynamic mechanical method and thermally stimulated depolarization technique. The transitions observed have been related to the structural parameters as well as the α-olefin comonomer content and degree of grafting, and compared with previous TSDC results on LLPE functionalized with the same polar group. The lowest relaxation is composed of a sub-γ and a γ mode assigned to localized motions of the polar groups and to the glass transition (at 137 K), respectively. The β-relaxation intensity grows with the grafting degree in agreement with its assignment to motions taking place in the interfacial regions. The behavior of the α-mode with different content of diethylmaleate has confirmed its dependence with the crystallite size.

Correlation between dipolar TSDC and AC dielectric spectroscopy at the PVDF glass transition

The αa-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC αa peak. This distribution has been used to predict the variation of the dielectric loss, e″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated e′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (102 ≤ f ≤ 105 Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed.

Molecular relaxation mechanisms of tyrosine-derived polycarbonates by thermally stimulated depolarization currents

A series of tyrosine-derived polycarbonates with different lengths (2 ≤ n ≤ 8) for the alkyl ester pendent chain were studied by measuring thermally stimulated depolarization currents (TSDC). The observed spectra could be separated into three regions: the low-temperature zone with a broad, complex β band (80-240 K), the intermediate zone (250-300 K), and the high-temperature zone (300-400 K) with a sharp a peak. The application of direct signal analysis ( DSA) to decompose the complex peaks into elementary processes led to the determination of the relaxation time distribution and temperature dependence of each process. The variation of the relaxation parameters as a function of the pendent chain length facilitated the tentative identification of the relaxation mechanisms responsible for the observed current peaks. It is proposed that as the temperature increases one observes, first, the individual motion of each polar group, then the concerted motion of the entire pendent chain, and, last, the movement of the polymer backbone.

Clustering of Y3+ in BaF2 using Gd3+ as paramagnetic probes

Abstract Ionic thermocurrent (ITC) and electron paramagnetic resonance (EPR) experiments have been performed on solid solutions of the type Ba1−xYxF2+x, 0.0001≤x≤0.05, additionally doped with a small fixed molar concentration, equal to 10−4 , of Gd3+ acting as paramagnetic probes. It is shown that the more abundant polarizable defect is the next nearest neighbor (nnn) dipole, Y3+−Fi−, whose molar concentration reaches a maximum for x∼0.005. Also, the EPR experiments show the coexistence of cubic and trigonal Gd3+ centers for the whole x range studied here. The variation of the population of these two paramagnetic centers peaks around x=0.0002 and the fraction of their relative abundances deduced from the observed EPR intensity is almost constant. The existence of several weaker lines evidences the presence of Gd3+ ions in additional sites to the cubic and trigonal ones. The comparison of the results obtained from the ITC and EPR techniques indicates the presence of Y3+ in larger clusters than the simple nnn dipole. To explain the variation of the abundance of the trigonal center at low Y3+ concentrations, a mechanism is proposed for the creation of the clusters formed by the trapping of an extra Fi− by either the Y3+ or the Gd3+ nnn dipoles. The existence of clustering among the trivalent ions in this system is supported by the variation of the linewidths of the EPR lines as x increases. These linewidths do not increase linearly with x but rather present a saturation effect at high x values. This indicates that a high doping levels the yttrium ions are not mainly dispersed in the lattice as nnn dipoles, but rather they are forming higher clusters whose interaction with the Gd3+ trigonal centers is small. The ITC results show that most of the clusters are either not polarizable or present a very small dipolar moment. As all the evidence presented here indicates that clustering is important in these crystals, one can conclude that the behavior of the high temperature ITC peak is best understood if it is attributed to an interfacial polarization around the dislocations of the crystal and not to a space charge effect.

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 To 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, Tg = 136.5 K, which has been discussed extensively in the literature. The β relaxation, Tgβ = 237 K, has also the expected characteristic of a glass transition; the existence of two Tgs in polyethylene could explain our results.

Dielectric relaxations and phase transitions in thermotropic polymer liquid crystals: poly(N-ethylene oxide terephthaloyl bis(4-oxybenzoate))s

Abstract The relaxations occuring in poly(triethylene oxide terephthaloyl bis(4-oxybenzoate)), PTrETOB, were studied by thermally stimulated depolarization currents technique (TSDC), and dynamic mechanical analysis (d.m.a.), and the results compared with those obtained by differential scanning calorimetry, thermo-optical analysis and wide angle X-ray diffraction. The low temperature relaxation observed by TSDC and d.m.a. is a complex band and as the temperature increases there are three very sharp and intense peaks which are attributed, by comparing the results of the various techniques used here, to the primary glass transition of the amorphous zones of the material, to an additional crystallization process and to the crystal-liquid crystal transition. Our results show the existence of a phase transition from this smectic mesophase to a nematic one at 210°C and a transition to the isotropic melt at 250°C. The direct signal analysis was applied to the low temperature spectrum in order to decompose the complex band in elementary curves characterized by Arrhenius relaxation times. It was found that the low temperature band covers a wide energy range from 5.1 to 15.9 kcal mol −1 and that the pre-exponential factor is almost constant in this range. Similar results were obtained for the polyester with four oxymethylenic units in the spacer. These results can be understood if the mechanisms responsible for this low temperature relaxation are very localized motions of the COO groups of the mesogenic unit, whose parameters are not affected by changes in the length of the spacer.

Moisture effects on dielectric relaxations of poly (ɛ-caprolactone)/starch biodegradable blends: Local, interfacial and segmental.

Poly(ɛ-caprolactone)/starch biocomposites in a wide composition range were studied by thermally stimulated depolarization currents, TSDC, during the drying process following the effect on the dielectric relaxations of moisture loss and blend composition. PCLs cooperative chain mobility is unaffected by the presence of water which is mainly located in the starch inclusions as its segmental relaxation is fixed at 207K. The heterogeneities found in the amorphous phase of neat starch are also observed here as a bimodal α relaxation which shifts to higher temperatures as it dries. The blend plasticization by water molecules also affects the blend secondary relaxations showing how the local motions are hindered by the loss of water which is lower in the blend than in pure starch. The interfacial peak which is intermediate between the α relaxations of the two components is affected by the inclusions change from liquid to glassy state as the blend dries.

On the origin of high temperature relaxation processes in rare-earth doped BaF2 crystals and its relation to dislocations

Abstract Ionic thermal current experiments have been performed on barium fluoride crystals doped with different trivalent rare-earth ions (La 3+ , Ce 3+ , Gd 3+ and Tm 3+ ) in a wide concentration range. The samples were either untreated, plastically deformed, or thermally treated, in order to study the effect of these treatments on the behavior of the high Temperature peak, and to elucidate the role of dislocations in this polarization process. The variation of the position, T M , and of the height of the HT peak, H HT , together with the dislocation density of the crystals were followed as a function of the molar concentration of impurities, of the amount of deformation, and of the thermal treatment of the samples. The results were interpreted in the scope of the Maxwell-Wagner-Sillars model for a heterogeneous dielectric by assuming that the inclusions are the zones of the crystal around the dislocation lines and their surrounding defect atmospheres which are easy diffusion paths. The role of the trivalent ions, acting as pinning points along the dislocation lines, explains the differences in the plastic deformation experiments observed for the different cations used here as dopants. The determination of the dislocation density after each of these treatments was performed in order to verify the predictions of the model by making simple assumptions on the geometrical parameters which describe the inclusions. The model accounts for all the results reported here. The role of dislocations in the relaxation process from which the HT ITC peak originates is thus favored against the idea of a space-charge polarization.

Dielectric Relaxations in Structurally Disordered Materials

Chain mobility in multiphase polymeric materials is studied by thermally stimulated depolarization currents and dielectric spectroscopy in poly(styrene)-b-poly(butadiene)-b-poly(c-caprolactone) triblock copolymers and poly(carbonate)/poly(c-caprolactone) blends. The variation in the relaxation time distribution of the dielectrically active components and in the number of orientable dipoles in the amorphous phases of these materials is interpreted as the result of the existence of a rigid amorphous phase constrained by the crystalline regions or in the case of the blends by a phase segregation which takes place when the crystallization process of the blend components advances. The mean relaxation times for the secondary and segmental motions are not affected when the phases are segregated.