Preparation And Mechanism Of Functional Polymer Microspheres Based On Pickering Emulsion Polymerization

The polymer microsphere materials have special size, morphology and function. They have been widely used in the field of electronics, optoelectronics, catalysis, drug delivery, coatings and additives. Currently, the main methods for the preparation of polymer microspheres are emulsion polymerization and suspension polymerization. In the emulsion polymerization, surface active emulsifier is needed which may cause environmental pollution and is not easy to be removed from the system. This may very harmful for some applications. The suspension polymerization is actually a smaller size of the bulk polymerization. The main purpose of this method is to make a fine size product and to introduce a heat transfer medium. The molecular structure and surface structure of the polymer microspheres is difficult to control delicately when using this method.In the beginning of the last century, it was discovered that nanoparticles can be assembled at the oil-water interface to form a stable emulsion, named Pickering emulsion. Pickering emulsions have the advantages of cheap, low toxicity, excellent stability and less foam formation. As the polymer science developed quickly since the middle of the last century, polymerization using nanoparticles (which is named Pickering polymerization) instead of traditional surfactants as stabilizers developed rapidly. The polymerization mechanisms also mainly include two kinds:Pickering emulsion polymerization and Pickering suspension polymerization. In the Pickering polymerization system, no surfactant is needed and nanoparticles are introduced in the polymer microspheres natively. So the Pickering polymerization has attracted wide attention in both academic and industry since it’s born. It has been reported the use of silica, titanium dioxide, ferric oxide, clay sheet layer or even polymer microspheres themselves as the stabilizer to prepare polymer microsphere materials.As the most potential photocatalytic materials, titanium dioxide nanoparticle has the advantages of high refractive index, UV shielding, high catalytic activity, chemical stablility, non-toxicity, abundant supply as well as low price. So it has been widely used in waste water treatment, air purification, solar cells, and also made some progress in photocatalytic polymerization area. The recent-discovered carbon materials (carbon nanotubes, graphene) have attracted widespread attention since their excellent electrical, mechanical, thermal and optical properties. They have been widely applied for transistors, sensors, hydrogen storage materials.In this paper, nano-sized titanium dioxide powder with anatase crystalline is prepared with controlled non-hydrolysis sol-gel process. The powder with positive charge on the surface can be well dispersed in water to form aqueous sol which has good photocatalytic activity. The obtained Titania hydrosol is then used as stabilizer and photocatalyst, initiating the polymerization of styrene and divinyl benzene to form crosslinked polystyrene microspheres. The mechanism is proven to be Pickering emulsion polymerization. It is found that the adding ions of negatively charged copolymerizable monomer, vinyl benzene sulfonate (SSS), will significantly change the size and morphology of the polymer microspheres, and lead to a trace sulfonate-modified surface. PS microspheres with trace surface modification can be well dispersed in the surfactant-containing non-polar solvent, and can electrophoresis in electric field. The effect of SSS concentration on the morphologies and surface properties is studied carefully. The impact of surface properties on the dispersing performance in a non-polar solvent of the microspheres is also investigated. Considering the poor dispersing performance of microspheres prepared by the soap-free emulsion polymerization in surfactant-containing non-polar solvent, a hypothesis is proposed:for the microspheres modified with ionizable groups and with no polymer chains on the surface, uniform but trace amount of surface modification will lead the best performance for dispersing in surfactant-containing nonpolar solvent. By adding oleic acid as the pre-flocculants, nucleation process of Pickering emulsion polymerization is controlled and crosslinked polystyrene microspheres with close-to-optimum structure are obtained. The microspheres can well dispersed in surfactant-containing non-polar solvents for more than 6 months. Such kind of polymer microspheres is very suitable for the preparation of electronic ink.This thesis also systematically studies the characteristics of nanoparticles with different dimensions as the stabilizer in Pickering emulsion polymerization. Two-dimensional nanoparticles, graphene oxide (GO), are used as the stabilizer and polystyrene (PS) microspheres are prepared using Pickering emulsion polymerization method. The strong interaction between the GO and polystyrene is proved by FT-IR, UV, XRD and TGA characterization. This interaction is considered to play an important role in nucleation process of the Pickering emulsion polymerization. The pH value of aqueous phase also has an important effect on the morphology of the product. When the pH decreases, GO has increasing ability to stabilize Pickering emulsion, while the product is less stable in water. When the pH increases to a certain extent, the conversion rate of the polymerization will be very low. The mechanism of GO stabilized Pickering emulsion polymerization is proposed and the characteristics of Pickering.emulsion polymerization stabilized by two-dimensional sheet nanoparticles are analyzed.One-dimensional nanoparticles, oxidized carbon nanotubes (CNTO), is used as the stabilizer, polystyrene (PS) microspheres with chain-like structure are prepared by using Pickering emulsion polymerization method. The strong interaction between the CNTO and polystyrene is proved by TGA, DSC, Raman, and is considered to play an important role in nucleation process of the Pickering emulsion polymerization. The pH value and ion strength of aqueous phase also have an important effect on the morphology of the product. It is found that CNTO does not exist on the surface of the microspheres, but is embedded in the microspheres. At higher pH, the stability of the microspheres is mainly relied on the surface charge. At lower pH, the morphology of the product remains a spherical structure even when the product precipitates in water. The network structure formed by one-dimensional CNTO plays a crucial role in the stability of the microspheres. Mechanism of CNTO stabilized Pickering emulsion polymerization is proposed and the characteristics of Pickering emulsion polymerization stabilizedby one-dimensional nanoparticles are analyzed. Composite microspheres with chain-like morphology were also prepared using unoxidized carbon nanotubes (CNT) as the stabilizer. This may provide a new method to fabricate CNT-polymer hybrid materials with good dispersity.