Analysing dielectric interphases in composites containing nano- and micro-particles

We have investigated a molecular relaxation process in a solid polymer filled with dispersed magnetite particles (Fe3O4 in epoxy resin). In particular, we compare systems containing nano-particles with diameters between 20 and 30 nm and micro-particles with diameters between 0.5 and 5 mu m. Temperature-dependent broadband dielectric spectroscopy in a frequency range between 50 Hz and 1 GHz reveals that the presence of nano- or micro-particles does not affect the molecular dynamics, i.e. frequency, shape and thermal activation of the relaxation process. However, there is a marked difference in the polymer's relaxation strength, reflecting both the polarizability and the number of relaxing units. This quantity is evaluated from the measured effective data using the spectral representation, i.e. in spite of the complex microstructure we are able to separate unambiguously the contribution of the polarized conductive particles. While in the micro-composites the polymer matrix behaves bulk-like, its relaxation strength increases in the nano-composites, the deviation from the bulk value being proportional to the volume fraction of particles. We discuss the results in terms of interphases of thickness delta around particles and agglomerates, the volume fraction of which increases with increasing particle concentration and decreasing particle size.