Self-Assembled SiO₂and Fe₃O₄@SiO₂Magnetic Composite Particles From Electrospun Nanofibers

Electrospinning is a polymer processing technique that produces one-dimensional nanoscale fibers whose secondary structures can be controlled. Electrospun fibers consist of a polymer filament with functional components dispersed throughout this matrix. The fibers are generally collected intertwined together in a mat. They possess favourable properties, such as ultrafine diameters, large surface areas, and the mats have high porosity. These attributes facilitate rapid dissolution, permeation and diffusion of solvent and solute. In addition, electrospun nanofibers can act as templates to control the transport and contact of molecular building blocks via secondary interactions:this can result in tightly controlled molecular self-assembly. In previous work, we had prepared nanoparticles through self-assembly driven by electrospun nanofibers templates. It was also proved possible to control the size of the self-assembled nanoparticles generated.In this thesis, we prepared two novel electrospun fibers:PVP/TEOS and PVP/Fe3O4composite fibers. Then SiO2and Fe3O4@SiO2magnetic composite particles were prepared by self-assembling from the two composite fibers, respectively. A possible mechanism for the formation of templated SiO2particles and Fe3O4@SiO2nanoparticles (NPs) was proposed. The main research could be summarized as follows:1. Electrospun fibers containing the hydrophilic polymer polyvinylpyrrolidone K90(PVP K90) with cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) were prepared. The effect of various factors on the morphology of electrospun fibers had been studied systematically. During spinning, the flow rate of the solutions was fixed at1.5mL/h～3.5mL/h, the concentration of PVP at6%～10%(w/v) and the voltage at12kV, uniform fibers could be obtained. And it was found that the diameters of the resulting fibers increased as the PVP concentration and the flow rate of the solutions were increased, while decreased with the increase of voltage. The concentration of CTAB could affect the electro-spinnability of polymer-surfactant solutions. When CTAB concentration was up to2%(w/v), all the fiber samples were not connected by "junctions" between multiple neighboring fibers. And the diameters of the resulting fibers decreased as the CTAB concentration (0.05%～2%) was increased. So, uniform PVP/TEOS composite fibers could be obtained at a constant condition, which would lay a foundation for practical applications.2. Silica particles were self-assembled from PVP/TEOS composite fibers. Spherical silica particles could be prepared on demand when the fibers were added to80%aqueous ethanol at pH9.0. The surface of the silica spheres was not smooth, which would increase their surface area to some extent. A possible mechanism for the formation of templated silica particles was proposed. We suggested that the synthesis of self-assembled SiO2particles was essentially a micelle formation and TEOS hydrolysis-condensation process. The fibers acted as templates, and the interactions among the various components in the nanofibers and water played an important role in the self-assembly process. In addition, the effect of various factors on the formation of self-assembled SiO2particles had been studied systematically. It was found that SiO2particles with different sizes could be obtained by manipulating various factors. The diameters of the SiO2particles increased as the TEOS concentration was increased, while decreased with the increase of reaction temprature. CTAB not only acted as templates for the formation of SiO2particles, but also prompted the hydrolysis of TEOS.3. Electrospun fibers containing the hydrophilic polymer polyvinylpyrrolidone K90(PVP K90) with cetyltrimethylammonium bromide (CTAB), tetraethylorthosilicate (TEOS) and Fe3O4were prepared. The effect of various factors on the morphology of electrospun fibers had been studied systematically. During spinning, the concentration of PVP was fixed at6%～10%(w/v), the flow rate of the solutions at1.5mL/h～3.5mL/h and the voltage at12kV～14kV, uniform fibers could be obtained. And it was found that the diameters of the resulting fibers increased as the PVP concentration and the flow rate of the solutions were increased, while decreased with the increase of voltage. When the Fe3O4concentration range from0%to2%, the diameters of the resulting fibers increased as the Fe3O4concentration was increased. So, uniform PVP/Fe3O4composite fibers could be obtained at a constant condition, which would lay a foundation for practical applications.4. Fe3O4@SiO2NPs could be prepared on demand when the fibers were added to80%aqueous ethanol at pH9.0. The product was characterized by HRTEM, XRD, VSM, FESEM and TEM. The results suggested that the main phase of core-shell Fe3O4@SiO2nanoparticles were amporphous SiO2and crystalline Fe3O4. Its saturation magnetizations was up to45.7emu/g. The sizes of the Fe3O4@SiO2particles could be manipulated from30nm to83nm by varying the Fe3O4content in the fibers. The number of magnetic cores inside the silica shells appeared to be largely invariant with the concentration of Fe3O4in the fibers. Hence, when the Fe3O4content was low, a smaller number of NPs form, with a large silica shell containing the magnetic cores. As the Fe3O4concentration was increased, a larger number of smaller NPs with reduced size silica shells formed. We suggested that the synthesis of self-assembled Fe3O4@SiO2NPs was essentially a micelle formation and TEOS hydrolysis-condensation process. And the synthesis of Fe3O4@SiO2NPs was co-driven by PVP molecules and Fe3O4. The electrospun fibers provided a template for the formation of CTAB micelles, while the Fe3O4particles were thought to initiate the nucleation of Fe3O4@SiO2. In addition, the templated nature of the self assembly process was also likely to facilitate the fabrication of more complex tertiary or quaternary core/shell nanoparticles. Hence, core-shell Fe3O4@SiO2nanoparticles with required magnetization and size could be prepared from PVP/Fe3O4composite fibers, which would also facilitate the fabrication of more complex tertiary or quaternary core/shell nanoparticles, resulting in practical applications of Fe3O4@SiO2nanoparticles in a varity of fields.