Influence of molecular weight on the thermal degradation of poly(lactic acid): A crystallization perspective

In this study, the thermal degradation behavior of poly(lactic acid) (PLA) with different molecular weights (S1, S2 and S3 are 40.58 × 10³, 49.06 × 10³ and 67.49 × 10³ g/mol, respectively), was systematically investigated using thermogravimetric analysis (TGA). Through deconvolution of differential thermogravimetric (DTG) curves using the Fraser-Suzuki function, two distinct degradation components were identified: Pseudo 1 (associated with ester exchange reactions) and Pseudo 2 (associated with free radical reactions). A comprehensive kinetic analysis was performed using four model-free methods to elucidate the thermal degradation mechanism. The results revealed a significant molecular weight dependence of activation energies (E_α). The E_α range had a decreasing trend with the increase of molecular weight, which were 152∼247, 144–232, and 111–197 kJ/mol for S1, S2 and S3, respectively. This inverse correlation between molecular weight and E_α indicates that lower molecular weight PLA exhibits slower thermal degradation kinetics and enhanced thermal stability. Complementary analysis of non-isothermal cold crystallization kinetics using Liu-Mo and Kissinger methods demonstrated that decreasing molecular weight resulted in higher F(T) values. The calculated activation energies (ΔE) for cold crystallization were 66.17 kJ/mol, 70.92 kJ/mol, and 85.76 kJ/mol for S1, S2 and S3, respectively, which accelerated crystallization kinetics in lower molecular weight PLA. From a crystallization perspective, lower molecular weight PLA exhibited superior non-isothermal cold crystallization capability, forming more abundant lamellar crystals. This crystalline architecture preserves the integrity of long polymer chains during thermal degradation, thereby decelerating degradation rates and improving overall thermal stability. The study establishes a fundamental relationship between molecular weight, crystallization, and thermal degradation mechanisms in PLA.