Study Of Titania-Stabilized Pickering Polymerization Mechanism

As functional polymer materials, polymer/inorganic composite microspheres have been widely used in chemical analysis, electronic information, biochemical and other fields. Up to now, some conventional methods are used to prepare polymer microspheres. However, a certain amount of small molecule surfactants or dispersants have to be added so as to ensure the stability of the latices and the uniformity of their size, which may brings a lot of later difficulties in removing them and lowing the cost. As we know, the research on solid-stabilized emulsion polymerization has attracted more and more interests and effectively overcome the above problems. Silica, zinc oxide, graphene oxide, etc. have been widely used as Pickering stabilizer to prepare mono-dispersed composite microspheres with core-shell structure. Titania nanoparticles have attracted more attention for its high refractive index, nontoxicity, low cost, etc. and are considered as the most promising photocatalyst. Therefore, it is also widely used in Pickering emulsion polymerization to prepare polymer/titania composite microspheres.In this thesis, a novel controlled nonhydrolytic sol-gel method is used to prepare nano sized anatase titania, which can be used as Pickering stabilizer in preparation of polystyrene solid and hollow microspheres via photocatalytic polymerization. The influence of monomer, crosslink agent, titania and hexadecane on the structure and morphology of polymer microspheres has been studied in detail. The results show that the stabilizing effect and photocatalysis of titania are both important to form the microspheres. The size of polymer microspheres is affected by the amount of monomer and crosslink agent. During the fabrication of hollow spheres, the mass ratio of titania to oil phase can greatly affect the structure and size of the final hollow spheres. The addition of hexadecane and employee of high shear force are crucial for the formation of hollow structure.As a crucial component in Pickering emulsion, solid particles play an important role mainly on its surface properties, such as charge, size, shape and interaction between particles. They influence the stability of Pickering emulsion, as well as the morphology and structure of polymer microspheres. However, not all the nanoparticles can be employed to stabilize emulsions. Therefore, several methods, for example chemical grafting, ion exchange and surfactants addition have been reported to modify the particles. The synergetic stabilization of surfactants and particles has been proved to be an effective modification route for the stabilization of emulsions.In the thesis, oleic acid (OA) and sodium oleate (NaOA) are used to modified titania, and then modified titania is used as Pickering stabilizer to prepare polystyrene/titania composite microspheres with different structures. Unmodified titania is too hydrophilic to stabilize initial droplets, which induces homogeneous nucleation and the formation of solid spheres. The stabilization effect of titania works step by step during polymerization. The NaOA-modified titania can stabilize emulsions effectively and induce droplet nucleation, forming hollow spheres. The stabilization effect of titania starts before the polymerization. However, two kinds of different microspheres are obtained when OA-modified titania is used, we ascribe to the ununiform modification of titania. Therefore, we provide a novel route to control the morphology and structure of final polymer microspheres.The stabilization mechanism of solid particles and surfactants is still unclear although the modification method is easy and convenient. The research on chemical modification of solid particles is relatively sophisticated. The dispersion, adsorption, wettability and other properties will be changed after modification, which can affect stability of Pickering emulsion and morphology and structure of final polymer.A small amount of polystyrene was grafted onto the surface of titania to improve its hydrophobicity for the special surface initiation via photocatalytic polymerization. Then the chemically modified titania is used as Pickering stabilizer to prepare polystyrene/titania composite microspheres. Three modification methods are evaluated by exploring the amount and uniformity of grafted polymer on the surface and stability for droplets during polymerization. The results show the addition of AA or NaSS can improve the grafting rate of PS chains, which results in uniform flocculation. They has been proved to be a good Pickering stabilizer for emulsions and can form hollow microspheres armored by 100-200nm solid spheres with high content of titania in the composite spheres. However, the modification of pure titania is not so effective and uniform. A novel and easy modification method is proposed to improve the surface properties of solid particles.