Percolation phenomena in polymers containing dispersed iron

Abstract

Metal‐polymer composites based on polyethylene (PE), polyoxymethylene (POM), polyamide (PA) and a PE/POM blend as matrix and dispersed iron (Fe) as filler 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 ϕc is accompanied by a sharp increase of the values of conductivity σ, dielectric constant ε′ and dielectric loss tangent tan δ. 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 σ ∼ (φ – φc)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 φc 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.

PTC effect and structure of polymer composites based on polyethylene/polyoxymethylene blend filled with dispersed iron

“…2 (similar to those found in Ref. 25. The values of j c for PE-Fe and POM-Fe are close to each other due to similar spatial distribution of the conductive filler (Fig.…”

“…We have found [25] that, for the PE-Fe and POM-Fe systems, Eq. 2 is in a good agreement with the experimental data with values of t ¼ 2.9 and 3.0, respectively.…”

“…2 is in a good agreement with the experimental data with values of t ¼ 2.9 and 3.0, respectively. For the composite PE/POM-Fe, the value of t ¼ 8 was found, which was explained in terms of the complicated structure of the composite with ordered (nonstatistical) distribution of conductive filler [25]. By using Eq.…”

“…This effect leads to an anomalous low value of the percolation threshold and to particular dielectric properties [25].…”

“…In a previous research [25], it was found that the filled polymer blend polyethylene/polyoxymethylene (PE/POM) possesses such a structure where the filler (dispersed iron) is involved in the phase of POM only and creates an ordered conductive network at a defined ratio of the components. This effect leads to an anomalous low value of the percolation threshold and to particular dielectric properties [25].…”