Ac Dielectric Relaxation Spectroscopy Research Articles

Dielectric Relaxation Spectroscopy of Poly (Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl Acrylate)/Poly (Acrylonitrile-Butadiene-Styrene) Polymer Blend

The dielectric techniques used to investigate the relaxation behavior of poly (vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH), poly (acrylonitrile-butadiene-styrene) (ABS), and its polyblends include broadband AC dielectric relaxation spectroscopy (DRS) in the frequency range from 10−2 to 105 Hz, and thermally stimulated depolarization current (TSDC) technique in the temperature range from 300 K to 413 K. It was observed that PVVH is characterized by a dipolar relaxation peak around 347 K and a space charge peak in the temperature range 353–383 K, whereas pure ABS is characterized by a dipolar relaxation peak at 389 K. On the other hand, polyblend samples are found to be characterized by two different relaxation peaks at 359 K and 387 K, respectively. The dielectric properties of pure materials and their polyblend are investigated. All samples are characterized by high dielectric constant (ϵ′) at very low and high temperature. The dielectric loss shows a single peak for pure materials, whereas polyblends show a broad peak at low frequency which could be attributed to the Maxwell-interfacial polarization (MWS) and another peak at high frequency which could be attributed to the dipolar relaxation. The temperature dependence of AC conductivity was investigated for all samples. The values of the exponent n suggest that the hopping mechanism dominates at lower temperatures.

Dielectric Relaxation Behavior Studies in Amorphous Terpolymer of Poly(Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl acrylate) Using Dielectric Spectroscopy and Thermally Stimulated Current Techniques

Dielectric studies on random terpolymer of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH) have been carried out. The dielectric techniques used include broadband ac dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) techniques. TSDC technique enables us to observe the primary α-relaxation, which is associated with the glass transition temperatutre, and the space charge relaxation. The peak parameters value of α-relaxation, namely, the activation energy E a and the preexponential factor τ 0, have been evaluated and found to be 1 eV and 3 × 10−11 s, respectively. The variation of the dielectric properties with frequencies and temperatures has been related mainly to two processes: (i) the α-relaxation and (ii) the conductivity relaxation. Electric modulus representation shows two peaks, namely, the α-relaxation and conductivity relaxation. The dielectric data were fitted to Havriliak–Negami (HN) relaxation model, and the shape parameter values suggest a broad and an asymmetric distribution of relaxation time. The activation energy value (˜1 eV) of the α-relaxation, obtained from electric modulus representation, was found to be the same as that obtained from TSDC measurements.

Electrical and dielectric behavior in blends of polyurethane-based ionomers

In the present work, the electrical and dielectric behaviors in ionomer blends of an anion-containing polyurethane (PU 1) and polyaminourethane (PU 2) have been investigated by using ac Dielectric Relaxation Spectroscopy (DRS), Differential Scanning Calorimetry (DSC) and Thermally Stimulated Depolarization Currents (TSDC) methods. The ionomer blends are characterized from microphase separation of soft-rich and hard microregions. Two conductivity mechanisms contribute to the dc conductivity of the ionomer blends. That of the shorter relaxation time is correlated to the soft-rich microregions and the other with the longer relaxation time is correlated to the hard microregions. From the comparison between ionomers of different composition, it is found that a faster conductivity relaxation mechanism of the soft-rich microregions implies a faster conductivity relaxation mechanism of the hard microregions. This behavior can be understood in terms of concept of the dynamic energy barriers. From the comparison between the ionomer blends, a smaller temperature difference, Δ T 1= T MWS− T α, between the temperatures of the current maximum of the Maxwell–Wagner–Sillars (MWS) and α-relaxation mechanisms, corresponds to a greater dc conductivity. The formalisms of the dielectric function ε*, electric modulus M*, and complex impedance Z* of the ac dielectric spectroscopy reveal the existence, with different weights, of the various mechanisms of dipolar and conductivity relaxation. The combined use of these formalisms, and especially their imaginary parts, gives the possibility to extract conclusions about the origin and the characteristics of the various relaxation mechanisms, as well as about their correlation with the physical processes which take place in the bulk of the materials.

Electrical properties of carbon black-filled polymer composites

This work deals with the dielectric properties of conductive composite materials, which consist of thermoplastic polypropylene (PP) matrix filled with carbon black (CB). The CB concentration was systematically varied in a wide range. Our main interest is focused on the investigation of electrical conductivity mechanism and related percolation phenomena in these materials. To study the electrical and dielectric properties of composites we used broadband ac dielectric relaxation spectroscopy (DRS) techniques in a wide temperature range. By measurements of complex dielectric permittivity, ϵ*, the dependence of ac conductivity, σac, and dc conductivity, σdc, on the frequency, the temperature and the concentration of the conductive filler was investigated. The behavior of this system is described by means of percolation theory. The percolation threshold, PC, value was calculated to be 6.2 wt.% CB. Both, dielectric constant and dc conductivity follow power-law behavior, yielding values for the critical exponents, which are in good agreement with the theoretical ones. Indications for tunneling effect in the charge carriers transport through the composites are presented. The temperature dependence of dc conductivity gives evidence for the presence of positive temperature coefficient (PTC) effect.

Molecular mobility in polyurethane/styrene–acrylonitrile blends studied by dielectric techniques

We report in this work the results of dielectric studies on microphase separation in blends of thermoplastic polyurethanes (TPUs) and styrene–acrylonitrile (SAN) copolymers with 0, 25, 50, 90 and 100% SAN (w/w). The dielectric techniques used include broadband ac dielectric relaxation spectroscopy (DRS), in the frequency range of 10 −2 to 10 7 Hz and a temperature range of 20–200°C, and thermally stimulated depolarization currents (TSDC) techniques in the temperature range of −170–30°C. These techniques enable us to observe the primary α relaxations and the secondary γ and β relaxations of TPU and SAN in the pure components and in the blends and to follow the dependence of their characteristic parameters on composition. DRS proves to be very sensitive in monitoring the melting of the crystalline soft segments of TPU in the blends. In addition the investigation of dc conductivity effects in the blends provides information on the morphology of the blends at mesoscopic level. The results suggest that the extent of partial miscibility of the two components is very limited, whereas the addition of SAN in TPU promotes the separation of hard and soft segments in TPU.

Thermodynamic state, temperature transitions, and broadband dielectric relaxation behavior in gradient interpenetrating polymer networks

Traditional and gradient interpenetrating polymer networks (IPNs) of various composition have been prepared on the basis of crosslinked siliceous polyurethane and a copolymer of butylmethacrylate and dimethacrylate triethylene glycol. For various layers of gradient IPNs cut from the surface to the center of the sample detailed investigations by methods of volume dilatometry, vapor sorption and broadband ac dielectric relaxation spectroscopy were carried out. From data of benzene vapor sorption by various layers of gradient IPN the free energy of mixing of the IPN components has been calculated. For all the layers the free energies of mixing are positive and depend on the distance from the sample surface. Each layer consists of two phases with their own transition temperatures. From these data the composition of each phase for various layers has been calculated. The dipolar α mechanism of the polyurethane-rich phase and the ac conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity and the modulus formalism. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:161–171, 1998

Broadband dielectric relaxation spectroscopy in interpenetrating polymer networks of polyurethane-copolymer of butyl methacrylate and dimethacrylate triethylene glycol

Abstract The electrical and dielectric properties of interpenetrating polymer networks (IPNs) based on crosslinked polyurethane (PUR) and a copolymer of butyl methacrylate and dimethacrylate triethylene glycol were studied by means of broadband AC dielectric relaxation spectroscopy to obtain information on the morphology and phase separation in these IPNs. Three relaxation processes were observed: a secondary β relaxation due to the copolymer, the α relaxation due to the glass–rubber transition of the PUR phase and a conductivity current relaxation due to charge carriers trapped at the interfaces in the microheterogeneous samples. The α relaxation due to the glass–rubber transition of the copolymer is masked by conductivity effects and is revealed after a fitting treatment. The dipolar α and β mechanisms and the AC conductivity mechanism were studied in detail at several IPN compositions, by analysing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism and power law forms. For the copolymer network, the dielectric loss e ″( ω ) shows a strong secondary β relaxation which becomes faster in the IPNs. The PUR network displays a relatively broad α -relaxation process which becomes slower in the IPNs. The energy and shape parameters of the response were determined for both mechanisms at several temperatures. From AC conductivity measurements we extracted information about the morphology and local structure of the IPNs. It is concluded that the IPNs studied are two-phase systems, but phase separation is incomplete in these materials.

Temperature- and hydration-dependence of molecular mobility in seeds

Broad-band ac dielectric relaxation spectroscopy (DRS) and various techniques of thermally stimulated currents (TSC) have been used to investigate molecular mobility in cereal and legume seeds, over wide ranges of water content and temperature. We focused our interest on the detailed study of the interactions between water and seed constituents. The results are quantitatively discussed, using various concepts dictated by the experimental techniques employed and in relation to the protein and carbohydrate contents of the seeds. In addition, the glass transition in the seeds, freezing and melting of water, and the protonic conduction process have been studied in some detail.