Biodegradation and thermal-oxidative degradation of propanediol based poly (lactic acid) oligomers

This work investigated the biodegradation and thermal degradation, of PLA oligomers with specific molecular weights, i.e., 800, 4000, and 10000 g/mol, coded as PLA800, PLA4000 and PLA10000, synthesized through ring-opening polymerization of lactide using defined amounts of 1,2-propanediol. From the biodegradation in soil, lower molecular weight oligomers degraded faster. The chemical changes followed through FTIR evidenced increase in OH bands, a shoulder around 1635 cm-1 due to amides bonds (N-H bending) which may be sign of enzymes and biofilm attached on the specimen surface, signs of ester bonds cleavage were also verified. Thermal degradations were investigated under nitrogen, synthetic air, and oxygen atmospheres. In general, higher heating rates provided higher thermal stability and higher degradation rates, with lower molecular weight PLA degrading faster. Distinct trends were verified under oxygen atmosphere due to complex reactions during thermal-oxidative degradation that headed to different molecular weight dependence and activation energy, which was computed using Friedman and Flynn-Ozawa-Wall (OFW) models. Friedman based activation energy is higher as it employs instantaneous rate values, and it is assumed as more realistic model, once activation energy based on OFW is derived assuming constant character along with the whole degradation and thus providing lower accuracy.