Preparation Of Fe₃O₄/SiO₂/P(AA-MMA-St) magnetic composite microspheres by controlled radical polymerization

A novel method for preparation of magnetic composite microspheres was developed by emulsifier-free emulsion polymerization controlled by 1, 1-diphenylethylene (DPE) as free radical polymerization control agent. This paper reported the synthesis of clean composite microspheres with high magnetite content and carboxylic surface by this method. The synthetic strategy is a two-step polymerization. First, some amount of acrylic acid (AA), methyl methacrylate (MMA) and DPE reacted for a few minutes in the presence of persulfate potassium (KPS) as initiator. Subsequently, Fe₃O ₄/SiO₂ magnetic fluid prepared by surface modification of Fe₃O₄ nanoparticles with ethyl orthosilicate was added, and the mixture was further polymerized for another time period. Then the system was cooled down to room temperature. The reaction stopped and a precursor polymer which possessed amphiphilic nature and DPE-capped chains was obtained. One end of the precursor polymer chain was linked to the surface of magnetic fluid through chemical adsorption. This not only improved the hydrophobicity of magnetic fluid surface, which stabilized inorganic particles in oil monomers, but also immobilized the potential active species on the surface of nanoparticles. Secondly, the system was heated to a desired temperature and styrene (St) was added to the mixture. The DPE-containing precursor polymer was activated and initiated the monomer polymerization on the surface of magnetic nanoparticles. After a desired reaction time, some amount of acrylic acid was introduced to the system and polymerized for additional hours. At last Fe ₃O₄/SiO₂/P (AA-MMA-St) magnetic composite microspheres were obtained. The chemical structure of magnetic fluid and composite microspheres were characterized by Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectra (XPS). The carboxyl content on the surface of composite microspheres was determined by conductometric titration. The thermoanalysis of composite microspheres was conducted by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The morphologies of magnetic fluid and composite microspheres were observed under a transmission electron microscope (TEM). Particle size determination of composite microspheres was conducted on a laser particle analyzer. The magnetic properties of all magnetic particles obtained were evaluated by using a vibrating-sample magnetometer (VSM). The results of FTIR and XPS indicated the hydrolysate of ethyl orthosilicate formed SiO₂ layers on the surface of Fe₃O₄ nanoparticles. The SiO₂ layer made the saturation magnetization of Fe₃O₄/SiO₂ fall by 3.7% in contrast with Fe₃O₄, but it prevented Fe ₃O₄ from reacting with - COOH group, and improved the magnetic property of Fe₃O₄/SiO₂/P(AA-MMA-St) by 28% compared with Fe₃O₄/P(AA-MMA-St) obtained at the same experimental conditions. The TEM observation suggested the composite microspheres possessed core-shell structure. The particle size measurements disclosed the average diameter of composite microspheres was 422 nm and the particle size distribution was narrow, which indicated all the monomers polymerization on the surface of magnetic nanoparticles were controlled effectively by DPE. The TGA analysis revealed the content of inorganic particle (Fe₃O₄/SiO₂) was about 40 % in composite microspheres. The magnetic property evaluation indicated the saturation magnetization of Fe₃O₄/P(AA-MMA-St) was 34.850 emu/g. The conductometric titration result demonstrated the content of carboxyl group on composite microspheres surface was 0.176 mmol/g.DSC analysis showed the glass transition temperature of Fe₃O₄/SiO₂/P(AA-MMA- St) increased by 8.9°C compared with the copolymer of P(AA-MMA-St), which confirmed there was interaction between the core of magnetic nanoparticles and the shell of copolymer.