Mechanism of hydrolytic degradation of poly(L-lactide) microcapsules: effects of pH, ionic strength and buffer concentration

The hydrolytic degradation rate of poly(L-lactide) molecules constituting the microcapsule membrane was estimated at different pH, ionic strength and buffer concentration. Poly(L-lactide) microcapsules were observed to be hydrolytically degraded rapidly in a strongly alkaline solution to lactic acid as the final product. The degradation was accelerated when the poly(L-lactide) microcapsules were immersed in solutions of high ionic strength. The effect of pH and ionic strength of the bulk solution is interpreted in terms of the electric potential distribution in the membrane. It is suggested that the concentration of OH- in the membrane has an important role in the hydrolysis of poly(L-lactide) microcapsules, when the microcapsules are dispersed in solutions where the zeta potential of the microcapsules is negative. On the other hand, when the zeta potential is positive, the concentration of H+ in the membrane has a predominant effect on the degradation. The degradation was also found to be affected by the salt concentration in buffered solutions, suggesting that the cleavage reaction of the polymer ester bonds is accelerated by conversion of the acidic degradation products into neutral salts.