The impact of chemical composition on the degradation kinetics of poly(lactic-co-glycolic) acid copolymers cast films in phosphate buffer solution

We have investigated the in vitro degradation of poly(lactic-co-glycolic) acid copolymer with different lactic to glycolic ratio: 50:50, 65:35, 75:25, 95:05 and 100:00 (mol. %). The degradation studies were performed on solvent cast films of controlled thickness and shape. The samples were incubated at 37 °C in phosphate buffered saline solution. The degradation was followed using potentiometry, light microscopy, gravimetry, size exclusion chromatography and differential scanning calorimetry. The same degradation process, as discussed in detail in our previous article for PLGA 50:50 (E. Vey et al., J. of Polym. Deg. and Stab. 2008, 93, 1896-1876), was observed for all the samples investigated, however the time scale over which the different events/degradation steps were observed increased with increasing lactic content of the polymer. The glass transition temperatures of the films increase with lactic content and are thought to have a significant impact on the rate of diffusion of water into the films - the higher the glass transition the slower the diffusion of water - and therefore on the degradation dynamics of the films. Kinetic parameters were extracted from the acid release, molecular weight and mass loss data. In each case linear correlations between the rate constants extracted and the lactic content of the polymer were found. The overall degradation rate of the films was found to decrease with increasing lactic content.