Silica-based inorganic/organic hybrid materials

• SiO2-basierte anorganisch-organische Hybridmaterialien

Zhao, Yongliang; Zhu, Xiaomin (Thesis advisor); Möller, Martin (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

This dissertation deals with two groups of silica-based composite materials, namely organic/silica composite particles and polymer nanocomposites containing ultrasmall silicate particles. For the synthesis of composite particles a silica precursor polymer - hyperbranched polyethoxysiloxane (PEOS) - is employed in combination with emulsion technique. PEOS is a highly hydrophobic liquid. Upon hydrolysis it becomes amphiphilic and interfacial active at the oil/water interface. In an oil-in-water Pickering emulsion stabilized by long-alkyl substituted silica nanoparticles PEOS acts as a glue to link the particles together at the interface to form microsized all-silica colloidosomes enclosing the oil phase. It is further demonstrated that oil-in-water miniemulsions are formed by dispersing oil/PEOS solutions in water without any additional particles due to high hydrophobicity and hydrolysis-induced interfacial activity of PEOS. In such a miniemulsion system methyl-functionalized silica nanoparticles, which are less interfacial active than the long-alkyl substituted ones, can catalyze the conversion of PEOS to a mechanically strong silica shell at the oil/water interface. Thus, the oil phase is encapsulated in monodispersed silica nanocapsules with almost 100% efficiency. This process depends strongly on pH of the aqueous phase, which controls the interfacial activity of both PEOS and silica nanoparticles as well as PEOS hydrolysis and condensation. The catalytic effect of the silica particles is the result of a delicate interplay between their interfacial activity that allows immersing catalytically active silanolate groups in the PEOS-containing oil phase and the repulsion between charged surfaces of these particles and the resulting silica nanocapsules that splits them apart. By combining PEOS with a hydrophobic monomer, a new type of surfactant-free miniemulsion polymerization has been developed. Thus, monodisperse polystyrene@SiO2 nanoparticles are obtained by emulsifying a PEOS/monomer solution in water and subsequent heating to initiate polymerization. As the polymerization proceeds, driven by osmotic pressure and incompatibility with polystyrene PEOS macromolecules migrate continuously towards the oil/water interface where sol-gel reaction takes place. As soon as the polymerization is completed, PEOS is fully expelled from the polymer phase and is converted to silica on the polystyrene surface. This method allows an easy control of silica shell thickness by varying the PEOS concentration. The particle size, on the other hand, can be regulated not only by the shearing force, but also by pH of the aqueous medium. In the case of a less hydrophobic monomer like methyl methacrylate, a more hydrophobic methylsilyl-substituted PEOS should be employed to obtain stable miniemulsions and subsequently composite nanoparticles with a narrow particle size distribution. The resulting particles exhibit a semi-interpenetrating network structure probably because of the compatibility of the polymer matrix with methylsilyl-substituted PEOS.Ultrasmall silicate particles used in this work are MQ resins having a core-shell structure with a SiO2-core and an organic shell. The synthesis is based on the acidic co-hydrolysis and condensation of monofunctional (M, e.g. 1,1,1,3,3,3-hexamethyldisiloxane) and tetrafunctional (Q, e.g. tetraethoxysilane) organosilicon compounds in an organic solvent, and the size of MQ resins increases with the decrease of M to Q ratio. The functionalized MQ resins are prepared by using 1,1,3,3-tetramethyldisiloxane as a comonomer and subsequent catalytic hydrosilylation with vinyl-containing compounds, and the degree of modification is defined by the ratio between 1,1,1,3,3,3-hexamethyldisiloxane and 1,1,3,3-tetramethyldisiloxane. As confirmed by dynamic light scattering and atomic force microscopy the MQ resin particles synthesized in this work have a size less than 10 nm and a narrow size distribution. MQ resins substituted with 1-vinyl-2-pyrollidinone and N,N-dimethylallylamine are blended with polyamide 6 via a melt extrusion process. It is shown that 2-pyrrolidinone-functionalized MQ resins are homogenously distributed in polyamide. By adding only a small amount of such particles (0.5 wt.-%), the viscosity of the polymer melt is significantly reduced, meanwhile the mechanical properties of the polymer remain almost unchanged. By comparing the interparticle half-gap and radius of gyration of the polymer chain it can be concluded that the change of entanglement density and release of topological constraint caused by MQ resin nanoparticles account for the viscosity reduction.