Preparation Of Composite Microspheres With Multi-layer Core-shell Structures By Microgel Template Method

The novel core-shell structures composites especially metal materials using ascore components have distinctive advantages over those materials just containing singlemetal because of surface coating of nanoparticles with different materials could haveunique properties different from originally presented in either core or shell materials.Furthermore, the enwrapping of inorganic shell can not only improve the stabilization ofmetal particles but avoid metal conglomeration by tuning thickness of inorganic shell.In recent years, the preparation studies on a variety of composites with core-shellstructures have made remarkable achievements, especially for those compositescontaining metal materials as the core components, including metal-polymer,metal-oxides, metal-metal composites, etc. Template method is a very effectivepreparation technique for core-shell composites among the different methods. Polymermicrogels are a group of typical soft-solid micro-particle materials with 3D networkstructures, which have the advantages of simple synthesis and adjustable chemicalfunctionality; furthermore, their sizes varied from nano- to microscale by choosing theappropriate polymerization technique. Therefore, polymer microgels have beenbecoming a kind of ideal template materials for preparing organic-inorganic composites.In recent years, our group proposed a novel route for preparing inorganic-polymercomposite microspheres with patterned surface structures by a polymer microgeltemplate. Based upon previous studies, the application of the polymeric microgeltemplate method was extended for the preparation composite microspheres withmulti-layer core-shell structures, taking tungsten powders as inner-shell materials, andpolymer, oxides, metal coated as exterior-shell components, respectively. The thesisincludes three parts as follows:(1) Acrylamide (AM) and methacrylic acid (MAA) copolymer microgels wereprepared by a reverse suspension polymerization technique. The microgels used astemplate for the preparation of poly(acrylamide-co-methacrylic acid)-tungsten(P(AM-co-MAA)-W) composite microspheres by centrifugal deposition. The compositemicrospheres swelled by the solution containing formaldehyde and urea were dispersed into reverse phase suspension, and then poly(acrylamide-co-methacrylicacid)-tungsten-urea formaldehyde resin (P(AM-co-MAA)-W-UF) were prepared. Themorphologies of composite microspheres and the crystal state were characterized byscanning electron microscopy (SEM) and X-ray diffraction (XRD). It has beendemonstrated that the surface morphology and quantity of urea-formaldehyde resin ofP(AM-co-MAA)-W-UF composite microspheres could be modulated by varying the pHvalue, the molar ratio of formaldehyde to urea and swelling time by formaldehyde andurea solution.(2) Application of P(AM-co-MAA)-W composite microspheres as template for thepreparation.of Poly(acrylamide-co-methacrylic acid)-tungsten-silica (P(AM-co-MAA)-W-SiO₂) microspheres with three-layer core-shell structures were prepared respectivelyby reverse phase suspension method and silica seed swelling method. The resultsdemonstrated that the modification of polyvinylpyrrolidone (PVP) onto the surface ofP(AM-co-MAA)-W composites could effectively improve functional group structureand change the surface properties of composite microspheres. The surface morphologyand quantity of silica of P(AM-co-MAA)-W-SiO₂ composite microspheres could becontrolled to some extent due to the enhancing of interaction between compositemicrospheres surface and silica particles.(3) The P(AM-co-MAA)-W composite microspheres modified with PVP wereused as template for the preparation of P(AM-co-MAA)-W-Ag composite microsphereswith core-shell structures by different three methods including reverse phase suspension,gas phase and direct reaction technique, respectively. The results showed that there weredistinct difference in the surface morphologies and quantity of silver ofP(AM-co-MAA)-W-Ag composite microspheres prepared by three different preparationmethods. Moreover, the surface morphologies and quantity of silver ofP(AM-co-MAA)-W-Ag composite microspheres could be modulated by the variationthe PVP and AgNO₃ concentration during the reverse phase suspension preparationprocess. Based upon the experiment results, the potential mechanism of silver ionreduction reaction was proposed. Furthermore, it was confirmed that tungsten coatedonto P(AM-co-MAA)-W composite microspheres could accelerate follow-up silverdeposition.