Stable microcapsules assembled stepwise from weak polyelectrolytes followed by thermal crosslinking

Stable weak polyelectrolyte (PE) microcapsules were fabricated by applying thermal treatment on poly(styrene sulfonate) (PSS) doped CaCO3 microparticles coated alternatively with poly(allylamine hydrochloride) (PAH) and sodium poly(acrylic acid) (PAA) in a layer-by-layer (LBL) assembly manner, followed by core dissolution with disodium ethylenediaminetetraacetate. Confocal laser scanning microscopy (CLSM) observations revealed the hollow nature and good dispersion of the resultant capsules. Energy dispersive X-ray spectroscopy (EDX) found that sodium PSS, which was used to modify morphology of the CaCO3 particles during the preparation process, contributed approximately 37% of the total mass of the capsules. Confocal Raman spectra verified that part of the PSS originating from the cores had been adsorbed or intertwined in the PAH/PAA multilayers, and part had been released into the capsule interiors. Stability of the capsules after thermal treatment was largely improved as evidenced by incubation of the capsules in an alkaline solution with a pH value of 12, or 0.1 M HCl for 3 weeks. By contrast, the untreated PAH/PAA microcapsules completely disappeared after incubation in the same alkaline solution. Spontaneous deposition of rhodamine 6G into the crosslinked PAH/PAA capsules was also observed. It is demonstrated that this simple thermal treatment method can be successfully applied to covalently crosslink the weak PE layers and stabilize the microcapsules. The encapsulated PSS molecules can attract diverse positively charged substances. Thus the structure-stabilized capsules can be good candidates as drug delivery vehicles and microcontainers over a wide pH range. Copyright (c) 2005 John Wiley & Sons, Ltd.