Direct signal analysis applied to the determination of the relaxation parameters from TSDC spectra of polymers

Local charge transport at different interfaces in epoxy composites

Charge transport in insulating composites is fundamental to designing high performance in electrical breakdown strength processes. A fundamental understanding of the charge transport at nanoscale in insulating composites remains elusive. Herein, we fabricate two types of interfaces in epoxy (EP) composites (Al₂O₃/EP and bubble/EP, respectively). Then the local dynamic charge mobility behavior and charge density are explored usingin situKelvin probe force microscopy. After the external voltage in the horizontal direction is applied, significant differences are demonstrated in the evolution of charge transport for epoxy matrix, filler/bubble, and their interface, respectively. The interface between Al₂O₃and epoxy is easier to accumulate the negative charges and introduce shallow traps. Lots of positive charges are located around a bubble where deeper traps are present and could prevent charge migration. Thus, this work offers extended experimental support to understanding the mechanism of charge transport in dielectric composites.

Effects of nanoscale dispersion in the dielectric properties of poly(vinyl alcohol)-bentonite nanocomposites

We investigate the effects of clay proportion and nanoscale dispersion in the dielectric response of poly(vinyl alcohol)-bentonite nanocomposites. The dielectric study was performed using the thermally stimulated depolarization current technique, covering the temperature range of the secondary and high-temperature relaxation processes. Important changes in the secondary relaxations are observed at low clay contents in comparison with neat poly(vinyl alcohol) (PVA). The high-temperature processes show a complex peak, which is a combination of the glass-rubber transition and the space-charge relaxations. The analysis of these processes shows the existence of two segmental relaxations for the nanocomposites. Dielectric results were complemented by calorimetric experiments using differential scanning calorimetry. Morphologic characterization was performed by x-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM and XRD results show a mixture of intercalated and exfoliated clay dispersion in a trend that promotes the exfoliated phase as the bentonite content diminishes. Dielectric and morphological results indicate the existence of polymer-clay interactions through the formation of hydrogen bounds and promoted by the exfoliated dispersion of the clay. These interactions affect not only the segmental dynamics, but also the secondary local dynamics of PVA.

TSDC spectroscopy of relaxational and interfacial phenomena

Applications of thermally stimulated depolarisation current (TSDC) technique to a variety of systems with different dispersion phases such as disperse and porous metal oxides, polymers, liquid crystals, amorphous and crystalline solids, composites, solid solutions, biomacromolecules, cells, tissues, etc. in gaseous or liquid dispersion media are analysed. The effects of dipolar, direct current (dc) and space charge relaxations are linked to the temperature dependent mobility of molecules, their fragments, protons, anions, and electrons and depend on thermal treatment, temperature and field intensity of polarisation, heating rate on depolarisation or cooling rate on polarisation. Features of the relaxation mechanisms are affected not only by the mentioned factors but also by morphological, structural and chemical characteristics of materials. The interfacial phenomena, especially the role of interfacial water, received significant attention on analysis of the TSDC data. Comparison of the data of TSDC and dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC), 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial water, adsorption/desorption of nitrogen, water and dissolved organics demonstrates high sensitivity and information content of the TSDC technique, allowing a deeper understanding of interfacial phenomena.

Local and segmental dynamics in homopolymer and triblock copolymers with one semicrystalline block

Thermally stimulated depolarization currents, TSDC, experiments have been performed on a series of poly(styrene)-b-poly(butadiene)-b-poly(epsilon-caprolactone) triblock copolymers SBC with different proportions of the poly(epsilon-caprolactone) crystallizable block, PCL. The morphology of the segregated microphases varies with the PCL content and has been observed by transmission electron microscopy. The crystallinity of the PCL block is estimated by wide angle x-ray scattering, WAXS. The relaxation times distribution is extracted by a numerical decomposition of the TSDC spectra and it is shown that this distribution is not significantly changed on going from the homopolymer to the triblock copolymer with 16 wt % to 77 wt % of PCL in the original samples. Better segregation of the mesophase structure is reached when the samples are annealed at 413 K and important variations in the TSDC and WAXS spectra are observed as a result of the thermal treatment. For the S09B14C77 triblock copolymer the results obtained can be explained by postulating the existence of a rigid amorphous phase in the PCL block. Such rigid amorphous phase is located between the core-shell cylinders formed by the other blocks [with poly(styrene)(PS) as core and poly(butadiene)(PB) as shell] and is constrained by undulated lamellae of crystalline PCL material. In the case of S35B15C50 triblock copolymer, an important amount of diffuse PS-PCL interphase where the homopolymers are mixed must be present before annealing. The results for the material with the less abundant PCL block are explained as a result of the confinement in nanotubes of PCL surrounded by PB embedded in a vitreous PS matrix. Broadband dielectric experiments on these same materials confirm the results obtained by TSDC spectroscopy.