Thermal decomposition of biodegradable polyesters .1. Poly(beta-hydroxybutyric acid)

The thermal decomposition of the biologically degradable polymer poly(beta-hydroxybutyric acid) (PHB) was investigated by means of several thermoanalytical techniques: thermogravimetry, differential scanning calorimetry, time resolved pyrolysis-MS and pyrolysis-GC/MS. The results conform with reaction mechanisms proposed in the literature. The dominant reaction pathway is a stereoselective cis-elimination giving rise to the formation of trans-crotonic acid and its oligomers. Crude PHB embedded in the original biomass is significantly less stable (T-max = 235 degrees C) than the purified polymer (T-max = 290 degrees C). An apparent activation energy of E(A) = 235 kJ . mol(-1) was derived from DSC measurements. The data from dynamic thermogravimetric and pyrolysis-MS experiments are not consistent with first order random cleavage kinetics, as would be expected from the reaction mechanism. Thermogravimetry offers a convenient tool for analyzing quantitatively the PHB content of biomasses. (C) 1996 Elsevier Science Limited