控制自由基聚合制备Fe3O4/SiO2/P(AA-MMA-St)磁性复合微球

用原硅酸乙酯对Fe3O4纳米粒子进行表面改性得到Fe3O4/SiO2磁流体.在Fe3O4/SiO2磁流体存在下,以1,1-二苯基乙烯(DPE)为自由基聚合控制剂,利用乳液聚合法制备了Fe3O4/SiO2/P(AA-MMA-St)核-壳磁性复合微球.用红外光谱(FTIR)、振动样品磁强计(VSM)、透射电镜(TEM)、X光电子能谱(XPS)、热重分析(TGA)、示差扫描量热仪(DSC)对所制备的磁流体、磁性高分子复合微球的结构、形态、性能进行了表征.研究发现,原硅酸乙酯水解后能在Fe3O4表面形成硅膜保护层从而避免Fe3O4的酸蚀,使Fe3O4/SiO2/P(AA-MMA-St)复合微球的比饱和磁化强度比同样条件下制备的Fe3O4/P(AA-MMA-St)微球提高了28%;DPE能有效控制自由基在Fe3O4/SiO2磁流体表面均匀地引发单体聚合,得到平均粒径为422 nm,无机粒子含量为40%,比饱和磁化强度为34.850 emu/g,表面羧基含量为0.176 mmol/g的磁性复合微球.

PREPARATION OF Fe3O4/SiO2/P(AA-MMA-St) MAGNETICCOMPOSITE 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,Fe_3O_4/SiO_2 magnetic fluid prepared by surface modification of Fe_3O_4 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_3O_4/SiO_2/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 SiO_2 layers on the surface of Fe_3O_4 nanoparticles.The SiO_2 layer made the saturation magnetization of Fe_3O_4/SiO_2 fall by 3.7% in contrast with Fe_3O_4,but it prevented Fe_3O_4 from reacting with —COOH group,and improved the magnetic property of Fe_3O_4/SiO_2/P(AA-MMA-St) by 28% compared with Fe_3O_4/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(Fe_3O_4/SiO_2) was about 40% in composite microspheres.The magnetic property evaluation indicated the saturation magnetization of Fe_3O_4/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 Fe_3O_4/SiO_2/P(AA-MMA-St) increased by 8.9℃ 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.