Capsule clusters fabricated by polymerization based on capsule-in-water-in-oil Pickering emulsions

Novel capsule clusters with a capsule-in-capsule structure were facilely fabricated by polymerization based on capsule-in-water-in-oil Pickering emulsions. First, dibutyl phthalate (DBP)-loaded polyurea (PU) capsules were prepared via interfacial polymerization templated from oil-in-water Pickering emulsions stabilized by hydrophilic silica nanoparticles. Then, the aqueous dispersion of the DBP-loaded PU capsules was emulsified into toluene as a water-in-oil emulsion using hydrophobic silica nanoparticles as the emulsifier. Finally, the capsule clusters were obtained by polymerization of melamine and formaldehyde in the water phase. These capsule clusters consisted of poly(melamine formaldehyde) (PMF)-silica hybrid shells and DBP-loaded PU capsules as the cluster core. Both the PU capsules and the capsule clusters had a spherical shape. The size of the PU capsules would decrease with increasing the hydrophilic silica concentration, and the capsule cluster size would reduce with elevating the volume ratio of oil to water. SEM tests indicated that the PU capsules were composed of a core-shell structure and the capsule clusters possessed a capsule-in-capsule structure as expected. FTIR demonstrated that DBP and PU capsules were encapsulated into their target microcontainers, PU capsules and capsule clusters, respectively. From the TG analysis, it could be calculated that the DBP content in PU capsules was 75.2 wt%. At the same time, the DBP content, PU shell content and PMF content in capsule clusters were 70.0 wt%, 23.1 wt% and 6.9 wt%, respectively. Furthermore, the release properties of two dyes in the capsule clusters were investigated. Such novel capsule clusters are inherently permeable exhibiting excellent multidrug controlled release properties in polar organic solvents.