Prepartion And Characterization Of Polymer/Silica Organic-inorganic Nanocomposite Spheres

In recent years, the strategy to fabricate nano-coating or shell on colloidal particles is of burgeoning interest, principally because such particles can display novel and enhanced properties (e.g. mechanical, chemical, electrical, rheological, magnetic and optical) by independently altering the composition, dimension and structure of the cores and shells. Also the fabrication of such organic-inorganic nanocomposite spheres has received great interest. In this study, polymer/SiO₂ organic-inorganic nanocomposite spheres with raspberry-like or core-shell morphology were obtained via surfactant-free methods. Also monodisperse hollow silica spheres based on the PS/ SiO₂ nanocomposite spheres were prepared via a one-step method. All the research contents and results are as follows:(1) We firstly used 1-VID as the auxiliary monomer and successfully synthesized a series of long-stand stable raspberrylike PMMA/SiO₂ nanocomposite particles via a "soap-free" heterophase polymerization in the presence of an ultrafine aqueous silica sol and water as the continuous phase. Since the surface hydroxyl group of silica particle is acidic, the amino group (basic) of 1-VID should promote the compatibility between polymeric phase and nanosilica phase. Some influencing parameters such as initial silica charge, reaction temperature, 1-VID charge, and pH value on the nanocomposite particles were investigated in detail, and a possible formation mechanism of raspberrylike nanocomposite particles was also discussed.(2) In the second chapter, cationic monomer MTC was used and electrostatic interaction of MTC with aqueous silica particles promote the formation of thenanocomposite particles. Only around 3 % MTC based on monomer mass was copolymerized with MMA in the presence of aqueous silica particles which were simultaneously electrostatically adsorbed onto the surfaces of the organic particles. Since the surface hydroxyl groups of silica particles are hydrophilic, they could act as "Pickering emulsifier" to stabilize the organic particles. The whole process neither required surface treatment for nanosilica particles nor addition of surfactant or stabilizer. It bore out that the electrostatic interaction between negatively charged silica and positively charged MTC was strong enough for the formation of long-stable hybrid microspheres with raspberry-like morphology.(3 ) In the third chapter, a modified stober method was proposed to prepare PS/SiO₂ hybrid spheres without surface modification or addition of surfactant (stabilizer). Firstly, positively charged PS colloids were prepared via surfactant-free emulsion polymerization of styrene by using azodiisobutyramidinedihydrochloride (AIBA, cationic) as initiator. Hydrolysis and condensation of tetraethoxysilane (TEOS) was carried out in acidic aqueous ethanol medium in the presence of surface positively charged PS colloids. Since silica sols were negatively charged and could be captured rapidly by the positively charged PS colloids, homogeneous nucleation of silica could be avoided and centrifugation/redispersion process of obtained dispersions was unnecessary. By using this method, core-shell PS/SiO₂ hybrid spheres with controllable thickness of silica shells could be obtained just by tuning the concentration of TEOS.(4) In the last chapter, we described a novel technique to fabricate PS/SiO₂ with core-shell morphology or hollow silica spheres via a one-step process. Firstly, monodisperse positively charged PS particles were prepared via dispersion polymerization of St with 2-(methacryloyl)ethyltrimemylammonium chloride (MTC) in ethanol/water medium. Then hydrolysis and condensation of TEOS was carried out in aqueous ammoniacal alcohol medium at 50℃, in which PS particles were "dissolved" subsequently even synchronously. Neither additional dissolution nor calcination process was needed. Under this condition, all silica formed as the shell on the core particle via the ammonia-catalyzed hydrolysis and condensation of TEOS, no free silica particles were found in the medium. Composite particles with PS as core and silica as shell or hollow silica spheres could be obtained by tinning theconcentration of ammonia in the system. Hollow silica spheres with the thickness of 0-100nm could be obtained by adjusting the concentration of TEOS in the formulation.