Thermally driven polymer disentanglement: An overlooked Key pathway in PBAT-microplastic aging during composting

The aging of microplastics (MPs) during composting is traditionally attributed to microbially driven biodegradation. However, the direct role of heat on MPs in composting has not been fundamentally understood. Here, we reveal for the first time that the composting heat drives MPs aging by directly disentangling polymer chains. We conducted a comparative experiment between hyperthermophilic composting (HTC) and thermophilic composting (TC), and combined it with molecular dynamics simulations and the Boruta machine-learning algorithm. During the early stage of HTC (90°C), van der Waals forces between PBAT molecular chains decreased to -2616.36 kcal/mol, the free volume fraction increased to 14.38 %, and the hydroxyl radical diffusion coefficient increased to 5.38 × 10^-3 Ų/ps. In addition to molecular chain disentanglement, composting aging resulted in significant changes in surface physicochemical properties of PBAT-MPs. Elemental analysis showed an increased surface oxidation degree, with the C/O ratio decreasing from 2.67 to 1.78 in HTC (compared to 2.02 in TC). After the molecular chain disentanglement, HTC further facilitated the development of a plastisphere core microbiota (Oceanobacillus and unclassified_f_Bacillaceae reached relative abundances of 9.19 % and 20.31 %), which boosted microbial degradation efficiency during late stage. We confirm that the direct disentangling effect induced by high temperature is pivotal for the aging process within the high-molecular-weight fractions of PBAT-MPs. These results revise the microorganism-dominant aging paradigm in composting, demonstrating that thermal energy directly disentangles polymeric molecular chains to drive MPs aging. This study provided new insights into the underlying mechanisms governing MPs aging in composting environments.