Degradation of double-walled polymer microspheres of PLLA and P(CPP : SA)20 : 80. I. In vitro degradation

Double-walled (DW) polymer microspheres with a core of poly(1,3-bis-(p-carboxyphenoxy propane)-co-(sebacic anhydride)20 : 80 (P(CPP : SA)20 : 80) and an external coat of poly(L-lactic acid) (PLLA) were degraded for up to 6 months in vitro. The effects of hydrolytic degradation on the polymers were studied by Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and scanning electron microscopy (SEM). The inner core of the polyanhydride copolymer (P(CPP : SA)20 : 80) degraded first, breaking down into oligomers during the first week and then into monomers which were trapped by the outer shell of PLLA, crystallized, and remained in the core for the duration of the study. As expected, the PLLA coat degraded at a slower rate due to the ester bonds between lactic acid monomers being more hydrolytically resistant than the anhydride bonds. The PLLA in the DW microspheres decreased from its original molecular weight (Mw) of 24,000 g mol-1 to approximately 5,000 g mol-1 over the 6 month study. The melting temperature of the PLLA decreased more than 30°C during the study, but no changes were observed in the FTIR spectra. The 6 month samples were very brittle, showing the concomitant drop in mechanical strength with the decrease in Mw.