ABSTRACT
The basic principles behind the interfacial electrodynamics of membranes and polymer films in the Nernst-Planck-Maxwell approximation are stated. Formulae for the complex permittivity and the impedance of heterogeneous biphase systems are derived and discussed in relation to measurements on membranes and on polymer films. The classical Maxwell-Wagner-Sillars (MWS) approach is only valid as a limiting case of high frequencies (or high concentrations at not too low frequencies) where the local current density is everywhere proportional to the local electric field and where the layers of polarization are restricted to infinitely thin boundary layers. This leads to the "principle of generalized conductivity" and formulae in which complex conductivities may be replaced by complex permittivities and vice versa. In other cases, it is necessary to account for a nonequilibrium electric double layer of the order of the Debye length, the electrodynamics of which layer can be understood (to a good approximation) by a combination of the Maxwell equations in the quasi-static limit (the equation of Poisson) with the electrodiffusion equations of Nernst, Planck and Smoluchowski. This dynamic electric double layer leads to an additional impedance, the excess impedance, over and above the MWS impedance. The problem of how to separate, in dielectric measurements of ion conducting amorphous polymer films, the contribution at low frequencies of the electric interfacial polarization from other low frequency dielectric contributions stemming from glass transition relaxations or even more slow "reptation modes" is discussed in relation to specific examples of dielectric studies of various polymers. Performing such a separation, the temperature dependence of the ionic diffusion coefficients in the polymers may also be found, throwing light on the "free volume" structure of the amorphous polymers under study.
