M. J. Sanchis

On electromechanical stability of dielectric elastomers

This paper studies the competition between electric and mechanical force fields simultaneously applied to a polar elastomer that can lead to electric breakdown. The analysis of the system, performed assuming that the free energy of the elastomer is simply the addition of polarizing and stretching energies leads to the classical “thermodynamic” (in this case “electromechanical”) stability.

Comparative study of mechanical and electrical relaxations in poly(etherimide). Part 1

Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations γ, β and α in the temperature range -140 to 250°C. The first one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show: (1) a peak (p) at which is attributed to space charge temperatures above the α relaxation, (2) indications of structure in the relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the fine structure of the β relaxation, which is formed by three subrelaxations. In this work, the activation energies calculated by the different techniques are compared, and a molecular origin for each relaxation is proposed.

Comparative study of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) by TSDC, DEA, DMA and DSC

Abstract A comparative study of the relaxational behavior of amorphous and partially crystalline poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), has been carried out by thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic electric analysis (DEA). As received, PEN (partially crystalline) shows, in the temperature range from −150 to 200°C, four relaxations located, in increasing temperature order, around −70°C (β), 60°C (β∗), 130°C (α) and 170°C (ρ). Amorphous PEN has been crystallized thermally heating up to different temperatures between 170 and 200°C. The DSC measurements of these samples show a small endothermic premelting peak, once the crystallization of the sample is completed. This peak increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the presence of this endothermic peak, the ρ relaxation passes through a maximum and presents a sharp decrease as it is further thermally treated. The α-relaxation, detected by dynamic mechanical as well as dielectric (ac) measurements, shifts to higher temperatures and broadens as the crystallinity degree increases. The kinetic parameters of the observed relaxations have been determined fitting experimental data to standard models.

Space charge relaxation in polyetherimides by the electric modulus formalism

Dynamic electrical analysis shows that at high temperatures (above the glass transition temperature), the electrical properties of polyetherimide are strongly influenced by space charge. In this article we have studied the relaxation of space charge in two commercial varieties of polyetherimide: Ultem 1000 and Ultem 5000. Their conductive properties were determined by dynamic electrical analysis, using the electric modulus formalism. The complex part of the electric modulus was fitted to Coelho’s model which considers ohmic conductivity and diffusion as the prevailing charge transport mechanisms. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with space charge. A good agreement between experimental and calculated data is observed after the fitting process to Coelho’s model. Differences in the electrode behavior were required: blocking electrode conditions in the case of Ultem 5000 and partially blocking electrode for Ultem 1000. In both cases the...

Study of space charge relaxation in PMMA at high temperatures by dynamic electrical analysis

Abstract Dynamic electrical analysis shows that at high temperatures (above the glass transition temperature), the electrical properties of polymethyl methacrylate are strongly influenced by space charge. In this paper we present an study of space charge in this material and its conductive properties by dynamic electrical analysis, using the electric modulus formalism. The complex part of the electric modulus was fitted to Coelhos model, which considers ohmic conductivity and diffusion as the prevailing mechanisms of charge transport. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with space charge and a good agreement between experimental and calculated data is observed after the fitting process to the Coelhos model. The data obtained indicate that the electrode is partially blocked. The conductivity determined is thermally activated and it increases with the temperature due to an increasing mobility, that is also thermally activated.

Effect of an electric field on the bifurcation of a biaxially stretched incompressible slab rubber

This paper describes the effect of an electrical field on the bifurcation phenomenon that appears in a biaxially stretched slab of Mooney-Rivlin material (M. Mooney, J. Appl. Phys. 11, 582 (1940)) subjected to equal dead loads. The main conclusion of the analysis is that the stretch ratio at which the bifurcation phenomenon appears crucially depends on the configuration of the system rubber slab-electrodes. The theoretical foundations of the present study are based on a recent formulation on this subject carried out by Dorfmann and Ogden (A. Dorfmann, R.W. Ogden, Acta Mech. 174, 167 (2005); J. Elasticity 82, 99 (2006)).

Physical ageing studies in polyetherimide ULTEM 1000

Thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC) have been used to study the effect of physical ageing in polyetherimide (PEI: ULTEM 1000®). Ageing temperatures (T a ) between 48 and 8°C below the glass transition temperature (T g ) measured by DSC at 10°C min -1 heating rate (T g =218°C), and ageing times (t a ) between 0 and 8640min, were analysed. Calorimetric results show the presence of an endothermic peak, associated with the enthalpy recovery at the glass transition. This peak shifts towards higher temperatures and its height and area increase as the annealing time increases for a given T,. The maximum enthalpy relaxation is achieved at T a = T g - 18°C (T a = 200°C) for every annealing time, indicating that this is the optimum temperature to age this material. TSDC results show for each T a a shift of the α peak towards higher temperatures, and a considerable decrease in maximum current and area as t a increases. The activation energies (E a ), calculated from the TSDC curves for the α relaxation, are mainly independent of the annealing temperature and time. The pre-exponential factor for the relaxation time (τ 0 ), evaluated from the same curves, shows a slight decrease when the annealing time is increased at a given temperature.

Comparative study of the relaxation behavior at very low frequencies of acrylate polymers with pendant 1,3-dioxane rings in their structure

The dielectric activity of poly {5-[(acryloxy)methyl]-5-methyl-1,3-dioxacyclohexane} (PAMMD), poly{5-[(acryloxy)methyl]-5-ethyl-1,3-dioxacyclohexane} (PAMED) and poly{5-[(acryloxy) methyl]-2-phenyl-5-ethyl-1,3-dioxacyclohexane} (PAEDP) is studied by thermostimulated discharge current (TSDC) techniques. The global TSDC curves corresponding to PAMMD, PAMED, and PAEDP exhibit subglass absorptions centered in the vicinity of −105, −100, and −120 °C, respectively. The dielectric activity of PAEDP in the glassy region is negligible in comparison with that of both PAMMD and PAEDP which have nearly similar relaxation strengths. Prominent peaks associated with the glass–rubber transition are located at 41, 38, and 57 °C for PAMMD, PAMED, and PAEDP, respectively. The real and loss components of the complex dielectric permittivity in the frequency domain (10−10–10−2 Hz) were calculated at several temperatures from elementary peaks obtained by partial TSDC curves in the glassy region. The differences observed in the ...

Monitoring molecular dynamics of bacterial cellulose composites reinforced with graphene oxide by carboxymethyl cellulose addition

Broadband Dielectric Relaxation Spectroscopy was performed to study the molecular dynamics of dried Bacterial Cellulose/Carboxymethyl Cellulose-Graphene Oxide (BC/CMC-GO) composites as a function of the concentration of CMC in the culture media. At low temperature the dielectric spectra are dominated by a dipolar process labelled as a β-relaxation, whereas electrode polarization and the contribution of dc-conductivity dominate the spectra at high temperatures and low frequency. The CMC concentration affects the morphological structure of cellulose and subsequently alters its physical properties. X-ray diffractometry measurements show that increasing the concentration of CMC promotes a decrease of the Iα/Iβ ratio. This structural change in BC, that involves a variation in inter- and intramolecular interactions (hydrogen-bonding interactions), affects steeply their molecular dynamics. So, an increase of CMC concentration produces a significantly decrease of the β-relaxation strength and an increase of the dc-conductivity.

Biparabolic model to represent dielectric relaxation data

The expression of a biparabolic model to represent the dielectric relaxation data of polymers is combined with a statistical method (the LEVM6 program) to provide a reasonable estimation of the parameters of this model. The graphical and multi-response methods are compared by using data for poly(2,6-dimethylphenyl methacrylate) (PDMP). The statistical techniques lead to a much quicker, objective estimation of parameters and also permits a sensitive analysis of the residuals.