Thermal decomposition of biodegradable polyesters .2. Poly(lactic acid)

The thermal decomposition of the biologically degradable polymer poly(lactic acid) (PLA) was investigated by means of several thermoanalytical techniques: thermogravimetry, differential scanning calorimetry, time resolved pyrolysis-MS and pyrolysis-GC/MS. The results mainly confirm reaction mechanisms proposed in the literature. The dominant reaction pathway is an intramolecular transesterification for pure PLA (T-max = 360 degrees C), giving rise to the formation of cyclic oligomers. In addition, acrylic acid from cis-elimination as well as carbon oxides and acetaldehyde from fragmentation reactions were detected. PLA samples contaminated with residual Sn from the polymerization process show a preceding selective depolymerization step (T-max = 300 degrees C) which produces lactide exclusively. The GC analysis of the oligomers gives insight into the stereochemistry of the original polymer chain with respect to the configuration of the asymmetric C atoms, as well as into the stereochemistry of decomposition reactions. Other experimental findings, which do not fit the proposed reaction mechanisms, are also discussed. (C) 1996 Elsevier Science Limited.