Synthesis of magnetic iron and iron oxide micrometre-sized composite particles of narrow size distribution by annealingiron salts entrapped within uniform porous poly(divinylbenzene) microspheres

Abstract

This article describes a new simple approach to produce poly(divinylbenzene) and carbon micrometre-sized magnetic composite particles of narrow size distribution. For this purpose, uniform porous poly(divinylbenzene) microspheres were prepared by a single-step swelling of uniform polystyrene microspheres with divinylbenzene and benzoyl peroxide, followed by polymerization of the divinylbenzene within the swollen polystyrene particles. Uniform porous poly(divinylbenzene) microspheres were then formed by dissolution of the polystyrene part of the polystyrene–poly(divinylbenzene) composite particles. Iron salts, e.g., FeCl₂·4H₂O, dissolved in an organic continuous phase were then entrapped by vacuum within the pores of the cross-linked poly(divinylbenzene) microspheres. The obtained microspheres containing the iron salts were then annealed at 250 °C under ambient atmosphere. The formed magnetic microspheres were then annealed at 500 and 800 °C, respectively, under inert atmosphere. The annealing temperature allowed for control of the composition, crystallinity, size and size distribution and the magnetic properties of the obtained microspheres. Magnetic PDVB–α-Fe₂O₃ microspheres were obtained at 250 °C, while annealing of these microspheres at 500 and 800 °C under inert atmosphere leads to carbonization of the particles, and thereby the formation of C–Fe₃O₄ and air stable C–Fe microspheres, respectively. The magnetic moment of the C–Fe microspheres is notably high, 106.5 emu g⁻¹. HR-TEM combined with STEM for elemental Fe mapping and imaging illustrated that by increasing the annealing temperature the size of the composite particles significantly decreased, while at the same time the size of the magnetic grains entrapped within the particles increased. Characterization of the various microspheres was also accomplished by other routine methods such as XRD, EDS, TGA, DSC, SQUID and Mössbauer spectroscopy.