Polymer–inorganic hybrid microparticles with hierarchical structures formed by interfacial instabilities of emulsion droplets

Abstract

We introduce a facile, yet robust route for fabricating polymer–inorganic hybrid microparticles with hierarchically-structured morphologies by taking advantage of the interfacial instabilities of emulsion droplets and the sol–gel chemistry of silica precursors. In general, chloroform-in-water emulsion droplets containing an amphiphilic diblock copolymer [polystyrene-b-poly (ethylene oxide) (PS–PEO)] and metal oxide precursors [tetraethyl orthosilicate (TEOS) or mixtures of TEOS and tetra-tert-butyl orthotitanate (TBOT)] were prepared by shaking. Upon solvent evaporation, interfacial instabilities occurred while the inorganic precursors started to hydrolyze and condense when brought in contact with the aqueous solution, ultimately leading to the formation of a mineralized shell at the oil–water interface and hybrid microparticles. By varying the rate of solvent evaporation, qualitatively different mechanisms of the instabilities (e.g., the budding of vesicles or a spines-to-vesicles transition) were observed. The effect of the synthesis conditions, such as the amount of TEOS, the solvent evaporation rate, and the pH value on the self-assembly of the copolymer, the interfacial behavior of the solvent/water, and the morphology of the microparticles were investigated. Hierarchical microparticles with different morphologies, ranging from cage-like, honeycomb-like, dendritic, to budded microparticles, were prepared by combining the sol–gel process with the self-assembly of the polymers in the emulsion droplets. Moreover, the intermediate structures of the instabilities were kinetically trapped by the mineralization process. Our study indicated that the hybrid microparticles formed by interfacial instabilities of emulsion droplets can significantly expand the range of accessible morphologies, which provides the potential for advanced applications in drug storage, coatings, controlled release, and catalysis.