Design, Synthesis, And Morphology Control Of Inorganic/Polymer Composite Microspheres

In recent years, inorganic/polymer nanocomposites (microspheres) have attracted extensive academic and industrial attention because of their applications in coating materials, fire-retardant materials, photoresist materials, catalytic materials, optical devices, etc. In the inorganic/polymer nanocomposites, polymer component acts as an ideal carrier which is easy access to active substances and favors the recovery and reultilization of materials. The introduction of inorganic component not only improves the mechanical properties of polymer composites, but also imparts the functionality to composite materials. The morphology of composite particles plays an important role on their functionality. For example, monodisperse microspheres are the most studied and best established example as the self-assembling building blocks to successfully fabricate three dimensionally (3D) ordered colloidal crystals, which can be used as the removable templates to fabricate 3D macroporous materials. However, the self-assembly method need to be further developed and the colloidal crystals film need to be strengthened. Hollow nanostructural microspheres have giant commercial applications including encapsulation and controlled release of sensitive materials, such as drugs, cosmetics, DNA, energy storage and conversion, and catalysis. However, the feasibility of preparation and control of hollow microspheres deserve further investigation. The stacking types of nonspherical particles are different from those of spherical particles so that nonspherical particles can be used to improve the optical properties of materials, act as self-assembly blocks for biological structural materials, control the rheological properties of suspensions, and design composite materials. Although the synthesis of nonspherical inorganic nanoparticles has been widely studied, the preparation, formation mechanism, and morphology control of nonspherical polymer microspheres still deserve further study.A series of research work on the preparation and microsphere morphology control of the above mentioned inorganic/polymer composite materials have been carried out. We studied the synthesis of inorganic/polymer hollow microspheres templated from multiple Pickering emulsions; the preparation, morphology control, and properties of SiO2/polymer composite microspheres via double in situ miniemulsion polymerization; the design and synthesis of Janus inorganic/polymer composite microspheres via double in situ miniemulsion polymerization; the synthesis and properties of nano silver particles/polymer composite microspheres by a two-step dispersion polymerization; and the synthesis and self assembly of monodisperse polymer microspheres to prepare colloidal crystal and further porous materials. The main results on the seven aspects of the research work are as follows:(1) Stable W/O/W multiple emulsions were successfully prepared by using the in situ formed amphiphilic silica particles as the Pickering emulsifier. Inorganic/polymer hollow spheres were fabricated after the polymerization of interlayer oil monomers. The water-in-oil (W/O) emulsions were first prepared by using the mixture of styrene, tetraethoxysilane (TEOS), hexadecane, and y-(trimethoxysilyl) propyl methacrylate (MPS) as the oil phase, and aqueous triethylamine solution as the inner water phase, respectively. By the hydrolysis-condensation of TEOS under basic conditions, silica particles were formed and modified by MPS at the O/W interface. W/O/W emulsions were fabricated by adding water as the outer phase. Finally, hybrid hollow microspheres were obtained after the further polymerization of the interlayer phase. The results showed that the time of surface functionalization had great effects on the size of silica particles and then the wall thickness of hollow microspheres. The modification level of silica nanoparticles were influenced by the content of MPS and volume ration of W/O phase of the primary W/O emulsions.(2) The SiO2/polymer seed microspheres were fabricated via in situ miniemulsion polymerization of MMA using MPS as functional monomer and in situ hydrolysis-condensation reaction of TEOS under basic conditions, and non-spherical polymer particles were obtained after styrene monomer was added dropwise and initiated to polymerize. The results showed that the addition of crosslinking agent divinylbenzene (DVB) and the contents of SDS and MPS in the formulation have great effects on the morphology of microspheres. The hybrid microspheres have a core-shell structure without the use of MPS and turn into a plum-like structure in the presence of MPS. If both DVB and MPS were added, the microspheres showed a peanut-shaped morphology.(3) SiO2/polymer nanocomposite microspheres were prepared by double in situ miniemulsion polymerization in the presence of methyl methacrylate, butyl acrylate, MPS, and TEOS. By taking full advantage of phase separation between the growing polymer particles and TEOS, inorganic/polymer microspheres were fabricated successfully in a one-step process. The formation of SiO2 particles and the polymerization of organic monomers took place simultaneously. The results showed that nanocomposite microspheres had a raspberry-like structure with silica nanoparticles on the shells of polymer microspheres. The silica particles of about 20 nm were highly dispersed within the nanocomposite films without aggregations. The transmittance of nanocomposite film was comparable to that of the copolymer film at around 70-80% from 400 to 800 nm. The mechanical properties and the fire retardant behavior of the polymer matrix were improved by the incorporation of silica nanoparticles.(4) Inorganic/polymer Janus microspheres were fabricated successfully in a one-step process by miniemulsion polymerization. Our method involved the in situ miniemulsion polymerization of styrene and the in situ hydrolysis-condensation of TEOS under basic conditions. The use of water-soluble initiator plays a key role on the formation of Janus-type microspheres. Thus, the primary radicals form in the water medium, spread to the interface of miniemulsion droplets, and initiate the polymerization of monomer at the interface. With the phase separation between the growing polystyrene particles and TEOS, Janus microspheres consisting of a silica hemisphere and a polystyrene hemisphere were obtained. The Janus microspheres showed the amphiphilicity due to the different chemical composition of each hemisphere.(5) A facile and effective approach has been developed to prepare hybrid hollow/bowl-type SiO2/PMMA microspheres via the double in situ miniemulsion polymerization by taking full advantage of phase separation. The results showed that the content of emulsifiers had great effect on the stability of polymerization and then the morphology of the as-prepared microspheres. At a given content of SDS, MPS content had a great impact on the morphologies of hybrid microspheres. The hybrid particles had a "bowl-type" structure without MPS. With more and more MPS in the formulation, hybrid microspheres changed into a collapsed hollow structure. Silica nanoparticles with size of about 20 nm were on the outer shells of polymer microspheres.(6) A facile method for the preparation of silver/polystyrene (Ag/PS) composite microspheres was discovered. PS microspheres with carboxyl and nitrile groups on the surfaces were synthesized via a two-step dispersion copolymerization of styrene, itaconic acid, and acrylonitrile in ethanol/water media. Ag/PS composite microspheres were prepared successively by the addition of AgNO3 aqueous solution to the dispersion since Ag+ions were absorbed on the modified surfaces of PS microspheres, and then reduced to silver nanoparticles by aqueous hydrazine hydrate. The results showed that Ag nanoparticles with size of about 50 nm located on the shell of PS microspheres due to the interaction between the carboxyl and nitrile groups of PS microspheres and the in situ formed silver nanoparticles. The as-prepared Ag/PS microspheres showed good catalytic properties.(7) A rapid and facile method of preparing free-standing colloidal crystals from monodisperse charged polystyrene (PS) microspheres was proposed. Mixed solvents (ethanol/water) were used as the dispersion medium in the self-assembly process of colloidal crystals. By a simple "floating self-assembly" method, PS microspheres floated on the surface of liquid and self-assembled into large area of three-dimensional (3D) ordered colloidal crystals within 15 min. Then epichlorohydrin was added in as a cross-linking agent to strengthen the colloidal-crystal film. The obtained colloidal-crystal film was free-standing and could be easily transferred to other substrates. Using tetrabutyl titanate as a titania precursor,3D porous TiO2 materials with rodlike skeletal structure were fabricated from the prepared free-standing colloidal crystal.