(749a) Polylactic Oligomers Degradation Kinetics
Polylactic acid (PLA) is a biodegradable and biocompatible, thermoplastic polymer produced from lactic acid (LA) as derived from renewable sources. Degradation studies have been reported in literature for both large scale applications, such as composting processes, and niche applications, such as in drug delivery systems. In the latter case, the degradation kinetics has a direct impact on the rate of drug release and system morphology and, therefore, on the final performance. The aim of the present work is to investigate experimentally the degradation kinetics of low molecular weight PLA as a function of selected key parameters, such as polymer chain length, pH, temperature and chain chiral composition.
PLA oligomers with different chiral composition were produced by polycondensation of DL-LA and L-LA and collected by semipreparative HPLC method. Hydrolysis experiments at different conditions, ranging from pH 1 to pH 7.4 and from 30 up to 120oC, were carried out in batch and characterized by HPLC analysis. Following the time evolution of the concentration of both initial oligomer species and all degradation products it was possible to identify different reaction mechanisms among the different explored conditions. Finally, a comprehensive kinetic scheme has been developed and the corresponding rate coefficients were estimated by minimizing the error between model predictions and experimental data. In particular, it was found that hydrolysis occurs preferentially through chain end-scission and backbiting mechanism at acid and basic pH, respectively, and that the overall degradation constant presents a minimum at the pKa of the carboxylic acid. Moreover, LL-oligomers were found to have larger reactivity compared to DL ones.