In vitro degradation behavior of poly(lactide)-poly(ethylene glycol) block copolymer micelles in aqueous solution

Self-assembling micelles were prepared from polylactide-poly(ethylene glycol) (PLA-PEG) block copolymer by using two different methods: direct dissolution and dialysis. The in vitro degradation properties of the micelles were investigated at 37°C and monitored by using various techniques. During the investigated degradation time, the size of micelles by dialysis remains stable, while that of micelles by direct dissolution first increases, followed by a collapse of micellar structure. The composition of PLA-PEG copolymers greatly affects the degradation of micelles. Micelles with longer hydrophobic PLA blocks exhibit less size changes due to more compact structure. On the other hand, the structural integrity of L/D mixed micelles is preserved for longer time than that of single micelles, in agreement with the stereocomplexation effect between L-PLA and D-PLA blocks. As degradation proceeds, the average molar mass of copolymer decreases and the distribution becomes wider, especially for micelles by dialysis and L/D mixed micelles with a more compact structure. Additionally, the PEG content in the copolymer chains increases during degradation, leading to a decrease of glass transition and crystallization temperature of the copolymers. However, the residual LA fragments produced by degradation disfavors the crystallization of PEG blocks, thus resulting in the decrease of melting temperature and melting enthalpy.