Core-shell particles having silica cores and pH-responsive poly(vinylpyridine) shells

A new method has been developed for the preparation of core-shell particles with thick, pH-responsive, polymer shells. In the first step, poly(vinylpyridine) (PVP) microgel particles are adsorbed onto oppositely-charged, similar to 5 pm diameter silica spheres, to form "raspberry-like" structures. After separation, these "raspberries" are re-dispersed in water. A mixture of vinylpyridine and divinylbenzene (a cross-linking molecule), together with an oil-soluble initiator, are then added to the aqueous suspension. The added monomers and the initiator are absorbed into the sheath of PVP particles. Upon heating, the adsorbed PVP particles serve as the locus for further polymerisation, leading to seeded growth. The nature of the polymer shell formed is dependent on the level of surface coverage after the initial PVP particle adsorption step. A complete, irreversibly bound, polymer layer is only formed when the initial PVP particle surface coverage is high. The polymer shell possesses properties similar to those associated with microgel particles in solution, in that it was observed, using optical microscopy, to swell/de-swell in response to pH variation. Electron microscopy studies revealed that, in its de-swollen state, the polymer layer is thick and continuous. It is proposed that these types of core-shell particles could be of use for controlled release applications.