Magnetic-field-sensitive polymer composite materials

Polymer composite material, containing conducting filler particles (e.g. graphite) at concentrations above the percolation threshold should, as a conductor, exhibit certain magnetic-field-sensitive properties, such as the Hall effect or magnetoresistance. These effects not only allow them to be classified as smart materials, but also help in establishing the mechanism of conduction in such complex disordered systems. This paper reports the results of investigations on magnetoresistance in composite materials, prepared by direct polymerization of propylene on graphite particles. The method ensures grafting of polymer to the filler surface, imparting the properties to the composite. For composites with graphite, the magnetoresistance is positive for a filler volume fraction above the percolation threshold. Below the threshold the samples show weak negative magnetoresistance, characteristic of conduction through localized states in disordered systems. This result correlates with our model of double percolation over filler particles, surrounded by thin layers of injected charge. New magnetic effects have been found, such as oscillations of dark conductivity in magnetic field and resonance magnetic-spin effects.