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.
The temperature dependence of resistivity, structure, and thermal expansion of composites based on polyethylene/polyoxymethylene (PE/POM) blends filled with dispersed iron (Fe) have been studied. The dependence of conductivity on filler content shows percolation behavior, with the values of the percolation thresholds equal to 21 vol% for PE‐Fe, 24 vol% for POM‐Fe, and 9 vol% for the filled blend PE/POM‐Fe, with two‐step character of the conductivity curve. The evolution of structure of the composite PE/POM‐Fe demonstrates transition from polymer matrix POM‐Fe, with inclusions of PE through cocontinuous phases of both POM‐Fe and PE, to PE matrix, with inclusions of POM‐Fe. Such a structure occurs due to localization of the filler only in one of the polymer phases, namely in POM. Measurements of the coefficient of thermal expansion α show the presence of two values of α: higher value (equal to α of PE) and lower value (equal to α of POM‐Fe), the transition between them corresponding to the structure with cocontinuous phases. The PE/POM‐Fe composites demonstrate double‐positive temperature coefficient (PTC) effect, with the presence of two transitions and a plateau between them. In such a system, two contributions to the PTC effect coexist: the first contribution originates from the thermal expansion of the nonconductive PE phase with higher value of the coefficient α, which leads to the break of the continuity of the conductive POM‐Fe phase; the second contribution originates from the break of the conductive structure of the filler inside the POM‐Fe phase because of the morphological changes in the vicinity of the melting temperature of POM. The double PTC effect is explained in terms of these two processes (thermal expansion and melting). POLYM. ENG. SCI., 47:34–42, 2007. © 2006 Society of Plastics Engineers
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