Thermal decomposition of biodegradable polyesters—II. Poly(lactic acid)

Abstract 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 ° 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 ° 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.

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