Thermal decomposition of biodegradable polyesters—I: Poly(β-hydroxybutyric acid)

The thermal decomposition of the biologically degradable polymer poly(β-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 ( Tmax = 235 °C) than the purified polymer ( Tmax = 290 °C). An apparent activation energy of E Λ = 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.