In vitro degradation of nanospheres from poly(D,L-lactides) of different molecular weights and polydispersities

The aim of this work was to study the stability of nanospheres from poly(D,L-lactide) in aqueous media. The influence of the poly(D,L-lactide) molecular weight and polydispersity, pH of the medium and temperature, was investigated. The degree of DL PLA degradation was estimated by monitoring the total acidity of the medium, the lactic acid production, the molecular weight of the polymer, the mean diameter of the nanospheres and the amount of polymer remaining insoluble. Poly(D,L-lactide) degradation rate depended on polymer molecular weight and polydispersity. The data suggested that it resulted from a hydrolysis which is much more catalysed at acidic and alkaline pH than at neutral pH. Two main hydrolysis mechanisms can be proposed: a random scission at acidic pH and a sequential cleavage from the chain end in alkaline medium. Higher temperature accelerated the degradation process. The best storage conditions for an aqueous dispersion of poly(D,L-lactide) nanospheres were deduced(temperature: 4 degrees C, pH close to neutrality).