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

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 alpha show the presence of two values of alpha: higher value (equal to et of PE) and lower value (equal to alpha 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 alpha, 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). (c) 2006 Society of Plastics Engineers