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 thermoanalytic al techniques: thermogravimetry, differential scanning calorimetry, ti me resolved pyrolysis-MS and pyrolysis-GC/MS. The results mainly confi rm reaction mechanisms proposed in the literature. The dominant reacti on pathway is an intramolecular transesterification for pure PLA (T-ma x = 360 degrees C), giving rise to the formation of cyclic oligomers. In addition, acrylic acid from cis-elimination as well as carbon oxide s and acetaldehyde from fragmentation reactions were detected. PLA sam ples contaminated with residual Sn from the polymerization process sho w a preceding selective depolymerization step (T-max = 300 degrees C) which produces lactide exclusively. The GC analysis of the oligomers g ives insight into the stereochemistry of the original polymer chain wi th respect to the configuration of the asymmetric C atoms, as well as into the stereochemistry of decomposition reactions. Other experimenta l findings, which do not fit the proposed reaction mechanisms, are als o discussed. (C) 1996 Elsevier Science Limited.