N. Suarez

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.

The study of water uptake in degradable polymers by thermally stimulated depolarization currents

Poly(DTH succinate) is a new, degradable polymer with potential applications as a medical implant material. This polymer can be classified as an alternating copolymer of succinic acid and desaminotyrosyl-tyrosine hexyl ester (DTH), a diphenolic monomer derived from the natural amino acid L-tyrosine. In this study, the effects of water uptake (hydration) on the secondary relaxations of poly(DTH succinate) were investigated using the technique of thermally stimulated depolarization currents (TDSC). Four relaxation peaks were precisely characterized by means of Gaussian activation energy distributions. Drying and rehydration treatments show that only small amounts of water, at most 0.5% (w/w), are necessary to hydrate two of the four polar moieties in poly(DTH succinate): the pendent chain ester carbonyls and the amide carbonyls in the polymer backbone. Water appeared to be more tightly bound to the amide carbonyl group and more loosely bound to the ester carbonyl group in the pendent chain. Even at a high state of hydration (1.2% w/w), TDSC indicated that no water was associated with the phenyl ester bond in the polymer backbone. This finding may explain the unexpectedly high stability of this polymer toward hydrolysis under physiological conditions. Polymer packing was also affected by hydration. In an intermediate hydration state (water content: 0.5% w/w) polymer packing was less dense than in the wet state (water content: 1.2% w/w). This study represents the first application of the TDSC technique to the study of hydration in a degradable biomedical polymer. The results obtained indicate that the TDSC technique may be useful to assist in the understanding of the mechanisms of hydration and subsequent hydrolytic degradation in degradable biomaterials.

Characterization of four hydrophilic sites in bituminous coal by ionic thermal current measurements

Abstract Ionic thermal current (ITC) techniques were used to study the behaviour of water in two low-rank coals of

The Molecular Interactions That Influence the Plasticizer Dependent Dissolution of Acrylic Polymer Films

Poly(methacrylic acid-methyl methacrylate, 1:2) (Eudragit S) is a commonly used pH-responsive polymer that can facilitate delivery to the ileo-colonic region of the gastrointestinal tract. Different plasticizers have been used to reduce the brittleness of Eudragit S films for the coating of solid dosage forms. To better correlate the dissolution rates of Eudragit S films, we have examined their dielectric response to understand the specific polymer-plasticizer interactions. Solvent cast Eudragit S films were prepared with one of four citrate plasticizers ranging from low to moderate aqueous solubility. Film dissolution was determined using a two-compartment permeation cell. Dielectric properties were measured by thermally stimulated depolarisation currents (TSDC). Secondary relaxations were deconvoluted and identified. The glass transition temperature (T(g)) was measured using TSDC, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Dissolution of the films was influenced by the solubility and structure of the plasticizers. While no correlation was found among the T(g)s obtained by TSDC, DSC, and DMA with dissolution time, the low temperature TSDC spectra showed a relationship of the total secondary relaxation area and relaxation of the carboxylic acid functional group with dissolution time. Dielectric secondary relaxations may be a good probe to predict plasticizer influence on dissolution of Eudragit S polymer films.

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.

Preliminary dielectric and rock magnetic results for a set of prehistoric Amerindian pottery samples from different Venezuelan Islands

Potsherds from 7 Venezuelan islands have been studied using a two-fold magneto/dielectric technique in order to identify clay sources and characterize different stages of pottery craftsmanship. This is the first study of archeological material using this technique. Petrographic analyses appear to agree with the clusters of data identified in scatter plots of initial magnetic susceptibility versus saturation isothermal remanent magnetization (SIRM), and natural remanent magnetization (NRM). Thus, these magnetic parameters appear to be suitable for describing clay source characteristics. Effective magnetic grain sizes, investigated via stability analyses of SIRM and anystheretic remanent magnetization (ARM) upon alternating field (AF) demagnetization, and SIRM acquisition and AF demagnetization crossover plots, seem to be related to different steps of pottery craftsmanship, namely clay preparation, finishing and firing. Thermomagnetic curves might also provide valuable information about original firing conditions. A scatter plot of SIRMs intersections versus maximum current depolarization temperatures and average activation energies, shows a coarse correlation due perhaps to the fact that these rock magnetic and dielectric data are both associated to pore-related features.

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.