Multiscale Identification of Weak Links in Polyethylene Microplastics: Bridging Bond Dissociation Energy and Isoconversional Kinetics

Polyethylene (PE) microplastics are environmentally persistent contaminants whose progressive fragmentation suggests that degradation may not be governed by uniform backbone stability. In this work, density functional theory and non-isothermal thermogravimetric analysis were combined to evaluate PE degradation from complementary molecular and kinetic perspectives. A C90H182 polyethylene oligomer was optimized at the M06-2X/def2-TZVP level, and position-resolved C–H and C–C bond dissociation energies were calculated along the chain. The C–H bonds showed comparatively high and homogeneous stability, whereas the C–C backbone displayed lower dissociation energies and a localized energetic depression in the central region. The minimum C–C BDE was found at C44, with a value of 85.73 kcal·mol−1, identifying a model-specific low-BDE region within the finite all-trans-derived oligomer that may favor backbone scission under the evaluated computational conditions. Thermogravimetric analysis under nitrogen at 5, 10, and 20 °C min−1 showed a dominant degradation event, with DTG maxima shifting from 462.6 to 494.6 °C as the heating rate increased. Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose analyses revealed a progressive increase in apparent activation energy from approximately 170–175 kJ·mol−1 at low conversion to 280–285 kJ·mol−1 at high conversion. Although BDE and apparent activation energy are not directly equivalent, their combined interpretation supports a heterogeneous degradation model in which PE fragmentation preferentially initiates at localized low-BDE C–C environments before progressing toward regular backbone scission and secondary degradation reactions.