Preparation And Characterization Of Composite Microspheres With High Magnetic Susceptibility

Due to the rapid development in the field of biomedicine and their demands for new materials,people pay more attentions to the multifunctional magnetic microspheres with stimuli-responsive to the environments.For the requirements of biomedicine to the high-performance magnetic microspheres,it is not only necessary to explore the synthesis of the high-performance magnetite,but also to fabricate the multifunctional high-performance magnetic microspheres,such as preparation of polymer and silica composite microspheres,in which the magnetite particles serve as a magnetic block.Based on the foregoing reasons,the research interest of this thesis is focused on the preparation and characterization of high-performance magnetic composite microspheres with a polymeric or silica matrix.The interaction between magnetite particles and matrix in the fabrication of the composite microspheres was investigated.Several kinds of magnetic polymeric or silica microspheres with different hierarchical structures were prepared.The main conclusions in this thesis are listed as follows:(1) For the preparation of a magnetic unit of high-performance magnetic microspheres,different kinds of magnetite particles were synthesized with various diameter size,morphology and surface properties.Based on the recent references,we developed several strategies to synthesize the nano,sub-micro or micro magnetite particles with specific sizes,shapes and surface properties.The reaction parameters were investigated on the as-prepared iron oxide particles'nature,such as the particle size,the size dispersity,surface and magnetic property.At first,iron oxide nanoparticles with a 15 nm diameter and a monolayer oleic acid capping surface by chemical co-precipitation process were synthesized.Then mono-disperse iron oxide nanoparticles with 4nm,6nm,9nm,12nm and 14nm diameters were prepared by high-temperature decomposition and seeded growth method.Subsequently,micro and sub-micro iron oxide particles with different diameters and morphology were prepared by the oxidized ferrous hydroxide gels. Through control the ferrous concentration in the reaction solution and the atmosphere, the size and the morphology of magnetite particles were well manipulated.Finally, near monodispersed magnetite particles with sub-micro size were prepared by solvent-thermo process.(2) High-performance polymeric microspheres with high magnetic content were prepared using the monolayer oleic acid capping iron oxide nanoparticles as magnetic unit.The magnetic polymeric microspheres with different hierarchical structures were synthesized by emulsion polymerization based on the principle of selective effect on the magnetic nanoparticles in the polymer.At first,Fe₃O₄/PMMA microspheres with a high loading of monolayer oleic acid capping iron oxide nanoparticles were prepared.Changing the concentration of the surfactant,monomer,and magnetic micelles in the polymerization,a serious of Fe₃O₄/PMMA microspheres with various contents of magnetic nanoparticles was prepared,and the highest loading of magnetic nanoparticlese was near 60%in the composite microspheres.Then,the principle of selective effect on magnetic nanoparticles in the polymer matrix was studied.Controlling the kind of monomers and the feeding order of monomers in the polymerization,a serious of magnetic microspheres was prepared with different hierarchical structures,such as PMMA/ Fe₃O₄,PSt@Fe₃O₄,PMMA/Fe₃O₄@PSt,PSt@PMMA/Fe₃O₄.The principle of selective effect on magnetic nanoparticles in polymer matrix was discussed based on the thermodynamic theory.Finally,based on the foregoing work of preparation different kinds of hierarchical composite microspheres,Janus magnetic microspheres were synthesized,and the Janus magnetic microspheres could self-assemble in one dimension.The self-assembly phenomena of Janus magnetic microspheres can be explained by magnetic properties of composite microspheres.(3) Several synthesis strategies were developed to prepare the silica composite microspheres with high magnetic content,in which magnetite particles served as a magnetic unit.According to the Stober method and the emulsion technology,different kinds of magnetic silica microspheres were synthesized.In the preparation,we found that the position of magnetic particles in magnetic silica microspheres was determined by the concentration of ammonia.Using the multiple emulsion method,magnetic hollow microspheres with a Fe₃O₄/SiO₂ magnetic silica shell were synthesized by the hydrolyzation and condensation of TEOS.In this preparation,the TEOS was used as oil phase,in which magnetic nanoparticles with a monolayer oleic acid were dispersed,and a W/O/W multiple(water-in-oil-in-water) emulsion was prepared with control of the emulsion parameters and the feeding speed of the water in the phase inversion process.Furthermore,controlling the initial concentration of reaction agents, kinds of magnetic composite microsperes were prepared,such as egg-plant,line and hollow structures.In this preparation,Na₂SiO₃ is as silica precursor,ethyl acetate or butyl acetate as pH controller and CTAB as surfactant.The influence parameters,such as the kind of ester and the feeding way of CTAB,were investigated.Finally, high-performance magnetic silica microspheres were prepared,in which the near mono-disperse sub-micro iron oxide particles synthesized by the solvent-thermo process were used.Before encapsulation of the composite microspheres,double bond was grafted on the silica surface by a modification of MPS.The precipitation polymerization of NIPAM was initialized by KPS in water,and the concentration of the initiator and the monomer was studied,then dual-responsive high-performance magnetic microspheres were prepared with a Fe₃O₄@SiO₂@PNIPAM structure.