Aglaia Vassilikou-Dova

Evaluation of the dielectric parameters from TSDC spectra: application to polymeric systems

Abstract The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analy-sis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (ΔW≈± 0.06 eV).

Dielectric characterization of poly(methyl methacrylate) geometrically confined into mesoporous SiO2 glasses

Abstract Characteristics of the dielectric relaxation mechanisms (γ,β,β′,α andρ) in poly(methyl methacrylate) (PMMA) and hybrids of PMMApolymerized into 5 nm SiO2 pore matrices were studied by means of the thermally stimulated depolarization currents (TSD) technique, applied in the range 10–460 K. Low frequency relaxations (10 µHz–10 mHz) were investigated by measuring the isothermal discharging current, with the loss factorɛ”(f) determined using a new development of the Hamon method. Compared to pure PMMA, the hybrids presented a 14 to 18 degrees reduction of the TSDβ-relaxation maximum (Tβ) and a drastic high temperature shift of the prominent syndiotacticα-peak. The average energy barrier for dipole (re)orientation (W) slightly decreases for both theα andβ relaxations. The latter observations, as well as the time evolution of the TSD spectra, are discussed in terms of the variation of the initiator (azobisisobutyronitrile) content and the effects of polymerization in spatial confinements (e.g. reduced monomer-to-polymer conversion at high initiator loadings and interaction effects). The shifts reflect the presence of several antagonistic mechanisms controlling the molecular dynamics of the polymeric phase. The extent of the polymer-SiO2 hydrogen bonding interaction induces an increase of the energy barrier for the activation of the motions of the carboxymethyl lateral groups (β-process) and parts of the main chain (α-process). On the other hand, the reduction of the chain entanglements (due to the pore-directed propagation of polymerization) loosens several steric hindrances on the rotational motion of the side group, explaining the accompanying decrease of W.

A tentative model for estimating the compressibility of multiphased mixed crystals

The growth of multiphased binary and ternary mixed crystals by the melt method using the miscible alkali halides, viz., NaBr and KCl, along with the experimental determination of their compressibility, have been recently reported. These materials are particularly useful for the construction of laser optical windows. Here, we suggest a theoretical model that enables the estimation of the compressibility of the multiphased mixed crystals in terms of the elastic data of the end members alone. The calculated values are in reasonable agreement with the experimental ones. The model makes use of an early suggestion that interconnects the Gibbs energy for the formation and migration of defects in solids, with bulk properties, and has been found of value in a large variety of solids.

Axially dependent dielectric relaxation response of natural hydroxyapatite single crystals

A thermally stimulated depolarization current (TSDC) study of single crystals of natural fluorine-rich hydroxyapatite, polarized with the electric field (Ep) applied parallel and perpendicular to the c axis, has revealed relaxation modes with axially dependent dielectric behavior. The (re)orientation of the OH− ions in the columns and the hydroxyls of the structural H2O molecules give rise to two prominent thermocurrent bands in the temperature range of ∼200–340 K. Several weak current bands, below ∼200 K, are related to the relaxation of impurity–Ca2+ vacancy (I–V) dipoles. The I–V mechanisms are particularly effective in the case where Ep⊥c axis and are considered to take place via cation jumps on planes perpendicular to the [0001] crystal axis, at regions near “distorted” Ca(II) triangles (i.e., triangles which include impurity ions and Ca2+ vacancies). The band attributed to rotational relaxation of the OH− ions in the columns shows particular strength when Ep⊥c axis, and an extensive distribution of ...

POLARIZING-FIELD ORIENTATION AND THERMAL TREATMENT EFFECTS ON THE DIELECTRIC BEHAVIOR OF FLUORAPATITE

A thermally stimulated depolarization currents (TSDC) study in natural fluorapatite single crystals has established different relaxation mechanisms for two polarization orientations (Ep parallel and perpendicular to the crystallographic c axis), which are discussed in relation to the defect chemistry and the specific columnar structure in apatite. The intensities of the thermostimulated current signals between the two poling field orientations demonstrate a difference of at least one order of magnitude, with the higher one recorded for the electric field parallel to the c axis. The TSDC thermogram appearing with the electric field parallel to c axis, in the 10–320 K range, consists of a broad and complex band (HT), with a maximum around 300 K. The relative intensity of associated current signals is indicative of extensive dipole-like ionic motions along c axis with a distribution in their activation energies ranging between 0.14 and 0.85 eV. The microdomain structure of fluorapatite along c axis permits t...

Effects of blending with fluorescing molecules on the dynamics of the β, α, and α' relaxations observed in poly(methyl methacrylate)

Perturbations in typical dielectric or calorimetric features of the molecular relaxation dynamics in amorphous poly(methyl methacrylate) and two blends with a neutral/apolar brominated perylene dye were analyzed to explore guest-host nanoscale interactions and their role on lasing properties. Aside from a gradual increase of the relaxation strength Δeβ (plasticization), the local β relaxation is nearly independent of the presence of dye. By contrast, segmental dynamics are moderately hampered: both the dielectric (Tα) and calorimetric (Tg) estimates of the glass transition temperature rise with increasing dye content, probably as a result of modifications in the free volume (antiplasticization). Analogous shifts are observed in the peak temperature of the space-charge relaxation (Tρ) and the α′ (liquid-liquid) transition (TLL), with Tρ∕Tα=1.04±0.01 and TLL∕Tα=1.10±0.01.

Thermal analysis studies of doping effects on the conformational motions of polymer chains in solid solutions with lasing molecules

The advancement of the solid-state dye laser performance largely depends on the systematic study of the dye-matrix interactions at the nanoscopic scale. The current work deals with blends of a comparatively inert dye host, poly(methyl methacrylate) (PMMA), with nonionic/apolar (substituted perylenes) and ionic/polar (rhodamine 6G, pyrromethene 567) dyes at ≈10−4 mol L−1 loading. Differential scanning calorimetry (DSC) and thermally stimulated currents (TSC) were used to explore the relative strength of inter- and intramolecular guest-host interactions by monitoring blending-induced modifications of the high-temperature signals: the segmental relaxation, the space-charge relaxation, and the liquid-liquid transition. Both techniques revealed the antiplasticizing role of the oligomeric organics on the relaxation dynamics of polymer segments, evidenced by clear glass-transition temperature upshifts. It becomes apparent that this effect is independent of the size, polarity, and ionicity of the dopant, signifyi...

Estimating the temperature dependence of the dielectric constant of multiphased mixed crystals

Measurements of the dielectric constant of multiphased binary and ternary mixed alkali halides, which have found a multitude of applications in optical and electronic devices, have been recently reported in the temperature range from room temperature to 423 K. It was found that the temperature variation in the binary mixed crystal (NaCl)0.587 (KBr)0.413 is drastically larger than in any other. We show that this variation can be quantitatively reproduced by means of a simple model, which does not contain any adjustable parameter and solely makes use of the properties of the end members, viz., NaCl and KBr.

Physical and chemical effects on the dynamics of the β-relaxation of PMMA in Rhodamine 6G+PMMA+SiO2 matrices

Abstract The Thermally Stimulated Depolarization (TSD) Current technique has been used to reveal confinement effects on the β-relaxation process of PMMA polymerized in-situ SiO2 in the presence of the laser dye Rhodamine 6G/Cl- (Rh6G). The TSD current band of the β process, associated with the rotational relaxation of the mobile carboxymethyl side groups of PMMA, presents a drastic low temperature shift with the introduction of Rh6G or the confinement of PMMA in the nanometric silica substrate. By using the fractional polarization technique we have decomposed this band into single current signals. Irrespective of the initiator type used in the polymerization process, the broad activation energy distribution associated with the β process is nearly identical in the Rh6G+PMMA+SiO2 materials and bulk PMMA. The increase of the dye concentration enlarges the band shift, without any significant modification in the TSD current spectrum (i.e. distinct bands from the polar chromophore). The changes in the molecular dynamics of the polar carboxymethyl side groups of PMMA are discussed in terms of the hydrogen-bond interactions between the silicic acid pore surface and the basic side-groups of PMMA and the decrease of the polymer chain entanglements due to pore directed propagation of polymerization. The relative importance of the various types of interactions between the two chemically identifiable phases in the presence of the dye is discussed.

Final thermally stimulated discharge current in nonpolar materials

The final thermally stimulated discharge current technique is used to study the molecular movements in nonpolar dielectrics. The main peak is an apparent peak being very sensitive to the experimental conditions. The experimental conditions can be chosen so that the thermal activation energy W, a characteristic parameter which has to be independent of the experimental conditions, can be determined. Five relaxations have been identified for Teflon FEP (a copolymer of tetrafluoroethylene with hexafluoropropylene) in the temperature range from 30to320K. W is in the range from 0.06to0.53eV.