Comparison of degradation kinetics derived from heat-flow and mass-loss signals: a combined TGA-DSC-FTIR study of PLLA and PHBV

The thermal degradation of biodegradable polyesters was investigated using a combined thermoanalytical and spectroscopic approach in order to compare degradation kinetics derived from different experimental signals. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(L-lactic acid) (PLLA) were studied using simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), complemented by thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy (TGA-FTIR). The extent of conversion and the effective activation energy ( E α ) were determined using an advanced isoconversional method from both the mass-loss signal (TGA) and the heat-flow signal (DSC). Both polymers exhibited a single dominant degradation step under inert atmosphere, with PHBV decomposing between 260-290°C and PLLA between 330-360°C. However, the apparent activation energies obtained from TGA and DSC signals showed strong variations attesting a multi-step degradation pathway, and comparable trends during the main degradation stage, indicating that the heat-flow events detected by DSC correspond to the chemical reactions responsible for the mass loss measured by TGA. However, slight but systematic deviations were observed at low and high conversion levels, particularly for PHBV, reflecting a partial decoupling between chemical reactions and volatilization processes. TGA-FTIR analysis revealed the evolution of crotonic acid derivatives during PHBV degradation and lactide during PLLA decomposition, confirming distinct degradation pathways. These results demonstrate that DSC-derived kinetic parameters can provide information consistent with TGA kinetics when degradation reactions and volatile formation occur simultaneously, highlighting the complementarity of thermoanalytical techniques for investigating polymer degradation mechanisms.