Shock Wave-Induced Degradation of Polyethylene and Polystyrene: A Reactive Molecular Dynamics Study on Nanoplastic Transformation in Aqueous Environments

Degradation of polyethylene and polystyrene was studied theoretically using reactive molecular dynamics based on the ReaxFF force field. The degradation reactions were carried out on nanoparticles (approximately 2 nm in diameter) composed of ideal low-density polyethylene and polystyrene in the presence of water. The reactions leading to degradation were triggered by applying a shock wave through the simulation box. This approach allowed the energy to be transferred to the sample in a controllable manner and initiated the reactions. The state of the nanoparticles after the shock wave passage was investigated in detail, focusing on the type and quantities of new surface functional groups and new chemical connections in the bulk samples. It was found that polyethylene predominantly reveals surface hydroxyl groups (some of which can be protonated) and has the ability to release linear polyhydroxy alcohols. Other surface functional groups with significant presence are ether groups. The degradation of polystyrene proceeds through the addition of hydroxyl groups primarily to the benzene rings, causing their dearomatization. The number of hydroxyl groups in a single ring increases with the degree of degradation, and some hydroxyl groups are also protonated. Polystyrene is also susceptible to crosslink formation, mainly between aromatic rings, leading to branched and dearomatized forms that are chemically distinct from styrene.