Estrella Laredo

Moisture effect on the low- and high-temperature dielectric relaxations in nylon-6

The effect of water sorption on the dielectric relaxation processes of nylon-6 samples with water concentrations ranging from the dry to the water-saturated polymer has been studied by thermally stimulated depolarization currents experiments in a broad temperature range, from 77 to 365 K. The strengths of the low-temperature modes, y- and β-peaks, are affected in opposite ways by the water concentration, h, as the first one shows a decrease in intensity and the second one grows as h increases. The precise determination of the relaxation parameters is made by the decomposition in elementary Debye processes and best fitting to the experimental profile of the complex peak. For h < 3%, the reorienting energies are almost independent of the water content, and the most significant intensity variations occur. The firmly bound water is held responsible for these effects. As for the higher temperature zone besides the α-peak, which is the dielectric manifestation of the glass transition, intermediate temperatures modes are observed at high h values and are originated by the loosely bound water, while the highest temperature peak is attributed to a Maxwell-Wagner interfacial polarization. The characteristic parameters of the α-mode are determined and related to the plasticization effect of water.

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.

Study of dielectric relaxation modes in poly(ε-caprolactone): Molecular weight, water sorption, and merging effects

The behavior of the various relaxation modes in poly(e-caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3×10−3 to 1.8×109 Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, e″(ω), is analyzed by best fitting a sum of Cole–Cole distributions corresponding to the γ and β local modes and to the α relaxation which is the dielectric manifestation of the dynamic glass transition. The kinetic parameters of the three predominant relaxations are determined and relaxation plots describing the temperature dependencies of the relaxation times are given as a function of 1/T. These relaxation plots are insensitive within experimental errors, either to the molecular weight or to the water concentration. The hydration level (<1%) only affects the intensities of the local processes and no plasticization effect is observed. At temperatures higher than those recorded for the α mode a fourth intense process, α′ is ob...

New comb-like poly(n-alkyl itaconate)s with crystalizable side chains

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n ¼ 12; 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state 13 C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices. q 2003 Elsevier Science Ltd. All rights reserved.

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.

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

Characterization of esterified cassava starch with long alkyl side chains and different substitution degrees.

The present work describes the characterization and thermal properties of hydrophobic starch obtained by the esterification of cassava starch with acyl imidazoles, acid chlorides and methyl ester derivatives of fatty acids with n-alkyl chains with 12-22 carbon atoms, in order to compare the dependence of their properties as a function of the length of the side chain and the methodology used for their synthesis. The n-acyl starches presented degrees of substitution (DS) between 0.06 and 1.2. Most of the derivatives obtained with acyl imidazoles were found to be stable at temperatures up to 300°C, whereas those synthesized with acid chlorides or methyl ester decomposed below. Finally, when the n-acyl starches were substituted with n-alkyl side chains of 16 or more carbon atoms, they were capable to crystallize in separate paraffinic phases independent of the starch backbone.

Relaxation time distribution from time and frequency domain dielectric spectroscopy in poly(aryl ether ether ketone)

A new application of the simulated annealing Monte Carlo procedure is presented and applied to the extraction of the relaxation time distribution from dielectric spectroscopy either in time or frequency domain. This decomposition method named simulated annealing direct signal analysis (SADSA), is applied to computer generated curves, e(t), e′(ω), and e″(ω), by using the most widely accepted empirical distributions. The discretized distribution fits exactly the analytical expression which can be evaluated in these cases for the set of parameters used in the simulation. Also, both distribution functions are found to be identical which proves that the method is certainly converging to the right solution in both cases. Experimental results on amorphous poly(aryl ether ether ketone) for e(t), e′(ω), and e″(ω) are analyzed with SADSA and the obtained relaxation time distribution is used to go from time to frequency domain and reciprocally. The results are compared to those obtained by assuming a Havriliak–Negam...

Superstructure observations in NaCl: Cd2+ by ITC experiments

Abstract A new ITC peak in highly doped NaCl crystals was detected at high temperature (318 K). By studying the behaviour of this peak after adequate thermal treatments, we are able to show that it is due to the precipitates of the Suzuki phase present in the crystal.