Percolation phenomena in polymers containing dispersed iron

Metal-polymer composites based on polyethylene (PE), polyoxymethylene (POM), polyamide (PA) and a PE/POM blend as matrix and dispersed iron (Fe) as Her have been prepared by extrusion of the appropriate mechanical mixtures, and their electrical conductivity, dielectric properties and thermal conductivity have been investigated. The filler spatial distribution is random in the PE-Fe, POM-Fe and PA-Fe composites. In the PE/POM-Fe composite the polymer matrix is two-phase and the filler is contained only in the POM phase, resulting in an ordered distribution of dispersed Fe in the volume of polymer blend. The transition through the percolation threshold (, is accompanied by a sharp increase of the values of conductivity a, dielectric constant epsilon' and dielectric loss tangent tan B. The critical indexes of the equations of the percolation theory are close to the theoretical ones in the PE-Fe and POM-Fe composites, whereas they take unusually high values in the PE/POMFe composite. Thus, t in the equation sigma similar to (phi - phi(c))(t) is 2.9-3.0 in the systems characterized by random distribution of dispersed filler and 8.0 in the PE/POM-Fe system. The percolation threshold (p, depends on the kind of polymer matrix, becoming 0.21, 0.24, 0.29 and 0.09 for the composites based on PE, POM, PA and PE/POM, respectively. Also the thermal parameters of the PE/POM-Fe composite are different from those of all other composites. A model explaining the unusual electrical characteristics of the composite based on the polymer blend (PE/POM-Fe) is proposed, in agreement with the results of optical microscopy.