A. Bernes

Combined dielectric spectroscopy and thermally stimulated currents studies of the secondary relaxation process in amorphous poly(ethylene terephthalate)

The dielectric secondary β relaxation process of poly(ethylene terephthalate) has been investigated by dielectric spectroscopy (DS) and thermally stimulated currents (TSC). To analyze the broad and asymmetric DS spectra, without previous assumptions on the shape and the number of overlapped loss components constituting the β relaxation, a description in terms of distribution of relaxation times (DRT) has been used. The Tikhonov regularization method has been used to extract the DRT from DS data. At lower frequency, the TSC technique revealed two components in the β relaxation process. The TSC spectra are experimentally resolved into elementary Debye processes by using the fractional polarization method. A set of elementary relaxation times with relaxation strength Δe i as statistical weight of TSC-DRT have been calculated. The temperature shifts of the DRT obtained from both dielectric techniques are analyzed. Whereas the maximum and the lower time wing of the DRT obtained from DS correspond to the β 2 process resolved by TSC and associated to the noncooperative motions of carbonyl groups, the upper time wing of DRT extracted from DS is in accordance with the β 1 process resolved by TSC and assigned to the cooperative motions of phenyl rings.

Thermally stimulated current study of relaxation in glassy polycarbonate

The transitions/relaxations of polycarbonate around its glass transition temperature, Tg, have been investigated by differential scanning calorimetry and thermally stimulated current spectroscopy. A sub-glass transition/relaxation has been observed with higher intensity in the quenched sample. This precursor of Tg is characterized by relaxation times following a Fulcher-Vogel equation. It might be explained by the diffusion of conformational defects all along the chain. The glass transition/relaxation is constituted of two components: (i) The lower-temperature component is associated with a distribution of relaxation times obeying a compensation law. It corresponds to cooperative movements of sequences of various length in the ‘true’ amorphous regions. (ii) The upper-temperature component is enhanced by annealing above Tg. It has been attributed to the ‘constrained’ amorphous regions. Note that such domains with high local order have been previously observed by electron microscopy and X-ray diffraction.

Annealing or Storage Influence on Pyroelectricity of Ferroelectric PVDF and P(VDF-TrFE) Copolymer

Abstract The aim of this work is to study the influence of thermal annealing on pyroelectric properties of PVDF and P(VDF-TrFE) 75–25 copolymer. A strong dependence of pyroelectricity decay upon ageing temperature and time is observed. There is no decay for ageing temperatures lower than +50°C for both polymers. A slower decay of P(VDF-TrFE) 75–25 copolymer pyroelectric properties are is observed for ageing temeperatures T a ≤ +110°C, i.e. lower than the Curie transition point (+128°C). The strong decay of the pyroelectric coefficient observed in PVDF is mainly due to shrinkage effects occuring at the α c -transition. The TSC study performed highlighted that changes of crystallinity occur only for high ageing temperatures (> 110°C).

DSC and TSC study of a VDF/TrFE copolymer

Abstract The semi-crystalline 75 25 mol.% P(VDF/TrFE) copolymer has been studied by thermostimulated current (TSC) spectroscopy. The analysis of the fine structure of the complex TSC spectrum has revealed that the relaxation mode associated with the glass transition has two components: the lower temperature component is characterized by relaxation times obeying a compensation law with Tc ≈ Tg + 25°C and has been associated with the glass transition of the true amorphous phase of the copolymer; the upper temperature component has been attributed to the constrained amorphous phase. The mode observed at highest temperature is characterized by a compensation phenomenon with a compensation temperature corresponding to the Curie transition temperature. Therefore, it has been associated with the ferro/paraelectric transition of the crystalline phase.

Thermal behavior of ferroelectric polyamide 11 in relation to pyroelectric properties

The pyroelectric properties of oriented thin films of ferroelectric Polyamide 11 have been studied in the temperature range of −100°C up to +140°C. The temperature dependence of the experimental pyroelectric coefficient has been analyzed. Three changes of slope of the pyroelectric coefficient are observed at −20, +50, and +100°C. The origin of the lower temperature event has not yet been defined. The upper transition is attributed to chain movements in crystalline regions, and more precisely, to a crystalline phase transition. The intermediate event is close to the glass transition temperature Tg observed by DSC. It is attributed to the manifestation of the glass transition. Below Tg, the variations of the pyroelectric coefficient are very small. For higher temperatures, it increases rapidly, attesting to a major contribution of secondary pyroelectricity and dimensional effects above Tg. The breaking of hydrogen bonds occurring at the glass transition temperature observed on DSC thermograms does not affect pyroelectric properties. Pyroelectric properties are mildly reduced after annealing at temperatures up to +140°C. A comparative study of oriented ferroelectric films prepared by quenching from the melt and nonoriented slowly cooled samples has been carried out by means of DSC.

TSC study of length ester side group effect on sub-vitreous relaxations in poly(n-alkyl methacrylates)

Abstract A comparative study by thermo-stimulated currents (TSC) of three poly(n-alkyl methacrylates) is performed. TSC complex spectra exhibit two dipolar relaxations considered in the following: the glass relaxation and a secondary relaxation related to motion of ester side groups. For each sample, the fractional polarisations protocol is applied to describe the fine structure of these complex spectra. It allows us to define the evolution of activation parameters with temperature. The influence of side chain architecture is then considered. It emphasises that the β mode is independent of the ester group length. On the contrary, the α mode is highly sensitive to structural change. These results are finally explained in terms of molecular mobility.

Dielectric relaxation spectra in ferroelectric P(VDF-TrFE) copolymers

Abstract Dielectric spectroscopy (DS) and Thermally Stimulated Current (TSC) are employed to study the dynamics of Poly(VinyliDene Fluoride-TriFluoroEthylene) copolymers. Dielectric spectra reveal the existence of three relaxation processes: the β relaxation or the glass transition relaxation, the secondary γ relaxation present in the glassy phase and the Curie transition well characterized by a thermal hysteresis. TSC spectra are in good agreement with DS results and resolve the well known in semi-crystalline polymers αc relaxation process, as well as observed by Differential Scanning Calorimetry. The experimental DS data are analyzed using empirical Havriliak Negami function while TSC spectra are described in terms of compensation law and Hoffmann Williams Passaglia model of relaxation by means of fractional polarization technique. Curie - Weiss temperature To = 26 °C is determined by DS spectroscopy and the Curie transition is well described by a compensation law at Tc = 121 °C.

Nature of Molecular Mobility Through the Glass Transition in Poly(n‐Alkyl Methacrylates): A Study by Dielectric Spectroscopies

Abstract For improving the understanding of molecular mobility through the glass transition, we compare results obtained by two dielectric techniques, dynamic dielectric spectroscopy (DDS) and thermo‐stimulated currents (TSC), for a series of amorphous model polymers, the poly(n‐alkyl methacrylates). This permits us to determine the effect of chain structure on relaxations appearing in the glass transition temperature range. This study confirms that the secondary relaxation is not influenced by the alkyl chain length. Contrarily, with increasing alkyl group length, the glass relaxation is subject to internal plasticization and its activation enthalpy decreases. The liquid–liquid relaxation is also shifted to lower temperatures, and we note a crossover of its behavior law. From these results, we then propose a phenomenological model describing the nature of molecular mobility through the glass transition.

Thermally stimulated current study of the microstructure of peek

Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning calorimetry (DSC) have been applied to the characterization of the microstructure of Poly (Ether Ether Ketone)/PEEK. the dielectric relaxation spectra show two modes, dependent upon crystallinity:* for the mode stuated in the vicinity of the glass transition temperature, two components have been distinguished and attributed to the molecular mobility in the ‘true amorphous phase’ and in the ‘rigid amorphous region’.* below 0°C, two sub-modes appear, situated around-110°C and-75°C, due to the two different crystal entities, beads and laths.ZusammenfassungZur Beschreibung der Mikrostruktur von Poly(Ether-Ether-Keton)/PEEK wurden TSC und DSC angewendet. In Abhängigkeit von der Kristallinität zeigen die Dipolrelaxationsspektren zwei Arten:* in der Nähe der Glasumwandlungstemperatur konnten zwei Komponenten unterschieden und der Molekülbeweglichkeit in der ‘echten amorphen Phase’ und in der ‘starren amorphen Region’ zugeordnet werden.* unter 0°C erscheinen wegen der zwei Kristallgebilde Perlen und Zäpfchen bei-110° und-75°C zwei Unterarten

Influence of molecular orientation on polarization phenomena in polyethylene terephthalate films

The influence of uniaxial and biaxial orientation on polarization phenomena of polyethylene terephthalate (PET) films has been investigated. Thermally stimulated current (TSC) spectroscopy has been applied to the study of the kinetics of polarization phenomena in unoriented and in uniaxially and biaxially oriented PET films. Around 50 degrees C, a relaxation mode that seems to be a precursor of the glass transition is observed. This sub-T/sub g/ mode has been attributed to free volume diffusion. Around the glass transition temperature (T/sub g/) a relaxation mode is observed in all samples: in unoriented PET, it is located at 82 degrees C, while in uniaxially oriented PET, is is shifted to 85 degrees C, and in biaxially oriented PET, it reaches 100 degrees C. These complex relaxation modes have been analyzed by using the technique of fractional polarizations, and the kinetics of the elementary depolarizations has been characterized by activation enthalpies and entropies. Around the crystallization temperature, a spontaneous TSC peak due to pyrocurrent is observed in unoriented and uniaxially oriented PET. It has been associated with the dielectric manifestation of the crystallization. >