Polystyrene-glass bead composites: Maxwell-Wagner-sillars relaxations and percolation

Dielectric spectrometry experiments are performed on a series of polystyrene–glass bead composites with volume filler content from 0 to 50% and with three particle diameters (5 μm, 20 μm, and 90 μm) in order to study the Maxwell–Wagner–Sillars (MWS) relaxations and the percolation phenomena. In the high-temperature region (130 to 220°C), the experimental data give evidence of MWS relaxations for all the composite systems, whatever the bead size and the filler content are. A good agreement is found between the experimental values of the maximum loss factor frequency and the theoretical ones drawn from the van Beek formula, especially for low contents. A percolation phenomenon is shown in the low-temperature region (40 to 120°C) for high-content/low-size composites. The percolation threshold, determined by considering the critical interparticle distance, is below 15.0% for the 5μm glass bead composites and above 47.3% for the 90 μm composites; it lies between 20.5 and 28.6% for the 20 μm composites. Two schematic models, based on a distribution of the sizes and on a random dispersion of the beads, are developed to show how MWS and percolation phenomena can both be observed for the high-content/low-size composites.