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Narrowly size-distributed thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with a hydrophilic core composed of an aqueous solution of cobalt tetrafluoroborate (CoTFB) were synthesized via a one-pot polymerization of N-isopropylacrylamide (NIPAM) and the cross-linking agent N,N′-methylene bis(acrylamide) (MBA) in inverse miniemulsion. CoTFB not only worked as template and lipophobe, but also promoted the dissolution of NIPAM in water by the formation of a complex between NIPAM and CoTFB. The formation of PNIPAM nanocapsules was verified by transmission electron microscopy (TEM) and atomic force microscopy (AFM). This is the first report to synthesize nanocapsules in inverse miniemulsion by following a phase separation mechanism as proposed by Tiarks et al. in direct miniemulsion [ Tiarks F. Landfester K. Antonietti M. Tiarks F. Landfester K. Antonietti M. Langmuir200117908918]. The particle size distribution was narrowed by increasing the weight content of CoTFB, MBA, and water. The particle size of the nanocapsules could be conveniently tuned by varying the weight content of CoTFB and MBA. The as-prepared PNIPAM nanocapsules showed reversible thermosensitive properties. An ordered monolayer of PNIPAM nanocapsules could form by controlling the evaporation of apolar solvent, and moreover the size and thus interparticle distance of PNIPAM nanocapsules in the ordered arrays could be tuned by drying the sample under different temperatures below and above the lower critical solution temperature (LCST) of PNIPAM.

Cobalt-containing hybrid particles have been prepared through the encapsulation of cobalt tetrafluoroborate hexahydrate (CoTFB) via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). We systematically varied the amount and type of cosolvent (water, methanol, ethanol, ethylene glycol), apolar continuous phase (cyclohexane, isooctane, isopar M, hexadecane), amount of cobalt salt, and molecular weight of the polymeric surfactant. The influence of those parameters on the particle size, size distribution, and particle morphology were investigated. Narrowly size-distributed hybrid particles with good colloidal stability could be obtained in a wide range of cobalt content between 5.7 and 22.6 wt % salt relative to the monomer. The addition of a cosolvent such as water not only promotes the loading of metal salt but also has a positive influence on narrowing the particle size distribution. We assume that generally narrowly size-distributed particles can be obtained for a large variety of combinations of polar/apolar phase by adjusting the balance between osmotic and Laplace pressure via the solubility of the metal salt in the continuous phase and lowering the interfacial tension by adjusting the hydrophilic-lipophilic balance (HLB) value of the surfactant. The results show a significant advantage of the inverse miniemulsion over the direct system with respect to the variability and total amount of metal salt without losing the narrow particle size distribution and colloidal stability.

Inverse Pickering emulsions with droplet diameters between 180 and 450 nm, a narrow droplet size distribution, and an outstanding stability were prepared using a miniemulsion technique. Commercially available hydrophilic silica nanoparticles were used to stabilize the emulsions. They were hydrophobized in situ by the adsorption of various neutral polymeric surfactants. The influence of different parameters, such as kind and amount of surfactant as hydrophobizing agent, size and charge of the silica particles, and amount of water in the dispersed phase, as well as the kind of osmotic agent (sodium chloride and phosphate-buffered saline), on the emulsion characteristics was investigated. The systems were characterized by dynamic light scattering, transmission electron microscopy, cryo-scanning electron microscopy (cryo-SEM), thermogravimetric analysis, and semiquantitative attenuated total reflection infrared spectroscopy. Cryo-SEM shows that some silica particles are obviously rendered hydrophilic and form a three-dimensional network inside the droplets.

Raspberry‐like hybrid nanocapsules with a hydrophobic liquid core were successfully prepared via the copolymerization of styrene, divinylbenzene (DVB), and 4‐vinyl pyridine (4‐VP) in Pickering‐stabilized miniemulsions by using silica particles as the sole emulsifier and hexadecane (HD) as liquid template. When compared with conventional Pickering miniemulsions and Pickering suspensions, the colloidal stability of the current systems is much more sensitive to the variation of reaction parameters such as pH, size, amount of silica particles, and content of 4‐VP. The systems without coagulum were only obtained in a narrow pH range at around 9.5 and by using 12 nm silica particles as emulsifier. The formation of well‐defined raspberry‐like capsules was confirmed by transmission electron microscopy (TEM) and high‐resolution scanning electron microscopy (HRSEM). The stable attachment of silica particles on the surface of hybrid particles was verified by centrifugation and subsequent characterizations, such as Fourier transform infrared spectroscopy, TEM, and HRSEM. The influence of pH and weight content of HD, DVB, and 4‐VP on the particle morphology was extensively investigated. Interestingly, the particle morphology strongly depends on the particle size. When compared with the organic surface‐active surfactant, the formation of capsule morphology could be promoted by the application of silica particles taking advantage of their surface inactivity. The formation mechanisms of capsules/solid particles are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011