The characteristic frequencies of electrode polarization and of interfacial polarization effects in dielectric spectra of ionic liquids and of polymer bi-layers are determined and systematically analyzed, based on dielectric measurements by means of broadband dielectric spectroscopy, numerical simulations, and analytical calculations. It is shown that, to a large extent, identical scaling laws can be derived for these two dielectric phenomena taking place at external and internal interfaces. Surprisingly, a fundamentally different behavior concerning the interrelation between the characteristic frequencies is found. This brings direct evidence that different manifestations of the phenomenon of electrical polarization can be discriminated by examining the inter-relation governing their characteristic frequencies, which can be of significant importance in disseminating the nature of different contributions appearing in the dielectric spectra of complex materials. Based on our analysis, we derive a new formula, valid for both electrode polarization and interfacial polarization effects, that allows one to determine the conductivity value from the frequency position of the Maxwell-Wagner-Sillars peak. An excellent agreement between experiment and calculations is obtained. The formula can be used, furthermore, to estimate the thickness of the interfacial layers formed due to electrode polarization effects. Values in the order of several nanometers, increasing with decreasing temperature, are reported.
An efficient approach to obtain polymeric materials with high permittivity values and low dielectric losses is presented in the current study. For this purpose, dielectric measurements by means of broadband dielectric spectroscopy, numerical simulations, and analytical calculations have been carried out for bilayer structures consisting in an insulating and a conductive polymer layer. Polyethyleneterephtalate and polytetrafluoroethylene have been used as insulating layers while, as conductive materials, blends of polyvinyl acetate with an ionic liquid, 1‐butyl‐3‐methylimidazolium tetrafluoroborate. The dielectric properties of the samples have been investigated in a broad frequency (from 10−1 to 107 Hz) and temperature range in order to determine, through the analysis of the scaling laws governing the interfacial polarization effects, the characteristic frequency ranges and the amplitude of the enhanced permittivity. An excellent agreement is found between the experimental results, the numerical simulations, and the analytical calculations. Finally, we show that bilayer polymeric materials with permittivity values as high as ε′ = 556 and with low dielectric losses (tan(δ) = 0.001) can be readily obtained by the current approach. This could have multiple applications, especially in the field of organic electronics. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47551.
An experimental and theoretical investigation of the scaling laws governing the phenomenon of Maxwell-Wagner-Sillars interfacial polarization in composite materials in dependence on morphology, volume fraction, orientation of fillers, form factor and the presence of interphases is presented in the current study. By considering the complex dielectric function of the matrix and of the fillers, the dielectric spectra are calculated in the frequency range from 107 Hz to 10-2 Hz and compared to dielectric measurements by Broadband Dielectric Spectroscopy, carried out in the frequency range from 107 Hz to 0.5Hz and between -90oC and 150oC. The characteristic frequencies of the global dielectric response are reported to strongly vary with the conductivity value of the conductive phase, while a much weaker dependence is observed upon varying the volume fraction, the form factor and the orientation of fillers. The value of permittivity at low frequency does not change with the conductivity value, whereas a significant variation is observed in dependence on the composite morphology, form factor, orientation of fillers and presence of interfaces with different gradients of properties. Two possible applications of our analysis are reported: (i) measuring the conductivity of materials without employing a direct electrical contact between the electrodes and the sample and (ii) discriminating different phenomena of electrical polarization in complex materials by analyzing the scaling laws. Our study delivers thus a useful and necessary analysis of the dielectric behavior of composite materials, where interfacial polarization effects play a major role.
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