Structure-dependent oxidative pyrolysis mechanisms of PE, PP, and PS: Integrating TGA and ReaxFF MD

Abstract

Oxidative pyrolysis offers a promising strategy to overcome heat transfer limitations and reduce energy consumption in polymer recycling. However, the structure-dependent kinetic behaviors and atomistic mechanisms across different plastics remain poorly understood. Here, the conventional and oxidative pyrolysis of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were systematically investigated by combining thermogravimetric analysis (TGA) with reactive force field molecular dynamics (ReaxFF MD) simulations. Macroscopic kinetics evaluated via the Kissinger-Akahira-Sunose (KAS) method were compared across scales with atomistic simulations and trajectory tracking using visual analysis of reactive molecular dynamics (VARx MD). Introducing 5% oxygen lowers the degradation onset for all plastics, but the promotional effect strongly depends on polymer structure. PE exhibits the largest increase in conversion rate and net main-chain scission, followed by PP, driven by oxidation-generated radicals that accelerate chain transfer and scission. Conversely, oxygen provides limited main-chain scission enhancement for PS. Furthermore, varying oxygen concentration in PS pyrolysis confirms that oxygen alters the competitive balance among depolymerization, oxidation, and radical recombination rather than simply accelerating cracking. These multi-scale insights demonstrate the necessity of optimizing oxygen concentration based on polymer structure and target products.