Degradation of poly(L-lactide) microcapsule membranes.

Abstract

Poly (L-lactide) microcapsules were observed to be hydrolytically degraded rapidly in a strongly alkaline solution into lactic acid as the final product. The degradation was accelerated when poly (L-lactide) microcapsules were immersed in solutions of high ionic strengths. The effects of pH and ionic strength of the bulk solution were interpreted in terms of the electric potential distribution in the membrane.The charge distribution in the microcapsule membrane changes during degradation. In the intact poly (L-lactide) microcapsule membrane, the charged groups are considered to be distributed uniformly in it. In the early stage of the degradation process, the zeta potential became more negative with the time elapsed. Analysis of the ionic strength dependence of the zeta potential on the basis of a simple model shows that hydrolytic scission of ester bonds of the polymer chains takes place preferentially at the microcapsule membrane surface to create negative charges localized at the membrane/solution interface. In the later stages of the degradation process, the zeta potential again became less negative. This suggests that liberation of degraded segments takes place in the later stages. It was concluded, therefore, that both the cleavage of the ester bonds and the liberation of degraded segments of poly (L-lactide) molecules start from the surface of poly (L-lactide) microcapsules.