Polymeric stabilization of colloidal magnetite magnetic fluids

Stable colloidal suspensions of ~4.3-nm mean diameter magnetic particles have been prepared employing polystyrene-block-poly(ethylene/propylene) copolymer (M_n ~100000) for steric stabilization. The potential of the copolymers as both dispersants and inhibitors of long-term oxygen degradation of the magnetic properties of the particles has been assessed. Magnetic changes due to oxidation, aggregation, and sedimentation consequent to prolonged exposure to atmosphere and gravitational and magnetic field gradients have been studied employing transmission electron microscopy, vibrating sample magnetometry, and scanning column magnetometry techniques. The flow properties in magnetic fields have also been investigated. Stable fluids of typically 5 J^-1T^-1kg^-1 have been obtained. It appears that the polymer competes with oxygen at the particle surface to decrease colloidal sensitivity to oxidation