Manipulating the properties of polyelectrolyte microcapsules by glutaraldehyde cross-linking

The stability of hollow microcapsules against environmental alterations such as pH, osmotic pressure, and temperature is a critical issue for practical applications. It is demonstrated here that multilayer capsules assembled from poly(allylamine hydrochloride) (PAH) and sodium poly(styrene sulfonate) (PSS) can be considerably stabilized by cross-linking of only the PAH component with glutaraldehyde (GA). Formation of a Schiff base between the aldehyde and the amine groups was evidenced by UV-vis spectroscopy. After cross-linking by 2% GA for 2 h, an apparently thicker capsule wall was obtained with higher folds, and no alteration of the macroscopic topology of the capsules was observed after incubation in 0.1 M NaOH for 24 h. The cross-linking significantly improved the mechanical strength of the capsules to resist osmotic pressure induced invagination. Consequently, both the critical pressure and the elasticity modulus (680 MPa) of the capsule wall were doubled compared with that of the control. The crosslinking also greatly lowered the permeability of the capsule wall, as evidenced by confocal laser scanning microscopy and fluorescence recovery after photobleaching. Quantitative analysis revealed that the permeation coefficient for dextran (Mw similar to 250 kD) was reduced by a factor of 3 after cross-linking.