Suspension polymerization based on inverse Pickering emulsion droplets for thermo-sensitive hybrid microcapsules with tunable supracolloidal structures

Polymerization using Pickering emulsion droplets as reaction vessels is being developed to become a powerful tool for fabrication of hybrid polymer particles with supracolloidal structures. In this paper, two kinds of thermo-sensitive hybrid poly(N-isopropylacrylamide) (PNIPAm) microcapsules with supracolloidal structures were successfully prepared from suspension polymerization stabilized by SiO2 nanoparticles based on inverse Pickering emulsion droplets. SiO2 nanoparticles could self-assemble at liquid-liquid interfaces to form stable water-in-oil inverse Pickering emulsion. NIPAm monomers dissolving in suspended aqueous droplets were subsequently polymerized at different temperatures. The hollow microcapsules with SiO2/PNIPAm nanocomposite shells were obtained when the reaction temperature was above the lower critical solution temperature (LCST) of PNIPAm. While the core-shell microcapsules with SiO2 nanoparticles' shells and PNIPAm gel cores were produced when the polymerization was conducted at the temperature lower than LCST using UV light radiation. The supracolloidal structures with different cores could be tuned by simply changing reaction temperature, which was confirmed by confocal laser scanning microscopy and scanning electron microscopy. The interesting properties of both microcapsules were their ability of reversibly swelling during drying/wetting cycles and responsive to temperature stimulus. Such functional microcapsules may find applications in double control release system due to the presence of the supracolloidal structures and thermo-sensitivity.