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Local variations in filler particle concentration and/or shape and orientation in static filler/polymer composites are modelled as distributions of percolation thresholds. The concentration variations can be due to insufficient mixing, formation of semicrystalline voids during cooling from the melt, shrinkage during polymer curing, flow during physical compression or the like. Irregular filler shapes, especially elongated shapes, reduce the percolation threshold; thus, natural variations in the shapes and orientations of filler particle aggregates lead to locally varying percolation thresholds. A distribution of percolation thresholds leads to an apparent average percolation threshold based on the conductivity below the mean percolation threshold. For filler concentrations above the apparent percolation threshold, the dielectric constant continues to increase before reaching a lowered peak value at the mean percolation threshold and then decreasing. This can explain some ‘anomalous’ published experimental results concerning the dielectric constant just above the percolation threshold. In the frequency plane, the percolation threshold distribution can lead to a slight reduction of the apparent critical exponents x and y of the frequency dependencies of the conductivity and relative dielectric constant, respectively. Our experimental results on ethylene butylacrylate copolymer/carbon black composites support the theory.

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The changes in the frequency dependences of the AC conductivity and complex permittivity during the pressure deformation of silicone rubber/polypyrrole composites have been examined. The experiments revealed that the pressure loading of the composite with particle concentration in the vicinity of the upper part of the percolation region causes disconnection of particle contacts and the transition of the semiconducting particle chain structure to a non-conducting composite structure corresponding to mutually insulated particles below the percolation threshold. A close relation between the frequency dependences of the conductivity and permittivity of the composites at various pressure loadings has been confirmed.