Low-temperature synthesis of water-dispersible magnetic composite particles with high monodispersity

Abstract

Highly monodisperse, water-dispersible, magnetoresponsive composite particles with submicron sizes were prepared by heterocoagulation between negatively charged polymer particles and positively charged magnetite nanoparticles in combination with silica coating of the heterocoagulates using a modified sol-gel method. Monodisperse polymethylmethacylate (PMMA) particles with three different sizes of 47, 106, and 217 nm were prepared as the negatively charged polymer particles in soap-free emulsion polymerization where potassium persulfate was used as an anionic initiator in the presence of 3-methacyloxypropyltrimethoxy silane (MPTMS). The suspension of negatively charged PMMA particles was mixed with suspension of the positively charged nanoparticles for the electrostatic heterocoagulation. Silica coating of the heterocoagulates was undertaken by hydrolysis of a mixed alkoxysilane of tetraethyl orthosilicate and MPTMS to improve the dispersibility of the heterocoagulates in a mixed solvent of ethanol and water. The silica coating in the mixed solvent successfully prepared monodisperse silica-coated heterocoagulates with average sizes of 63, 112 and 226 nm. Although the saturation magnetization of silica-coated heterocoagulates was increased with a decrease in the size of negatively charged polymer particles due to high surface/volume ratio suitable for the heterocoagulation, the highest magnetoresponsivity was attained for middle-sized heterocoagulates with thin silica layer. These comparisons indicated that not only the saturation magnetization but also the number of magnetic nanoparticles incorporated into a support particle is crucial to build up an efficient system to recycle the magnetoresponsive composite particles.