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Probing theStructural Evolution and Stability ofAged Polyethylene Nanoparticles through Coupled Atomistic-MesoscaleSimulations
Summary
Scientists used computer simulations to study how tiny plastic particles (like those found in polyethylene, a common plastic) break down over time as they're exposed to oxygen and other environmental factors. They found that as these nanoparticles age, they can become unstable, break into smaller pieces, and even dissolve in water—meaning old, weathered microplastics might release more fragments into water and food than fresh plastic, which matters since we're still learning how microplastics affect our bodies once they get inside us.
Understanding the aging process of microplastics is crucial for controlling the lifespan of polymer-based materials and evaluating their potential impact on biological systems. Studying the aging process requires developing accurate characterization methods to explore underlying molecular mechanisms. To this end, we propose mutually complementary simulation models that enable the study of polyethylene nanoparticles (NPs) structural evolution at different spatiotemporal scales within the framework of all-atom molecular dynamics (AMD) and two mesoscopic methods: Langevin dynamics and dissipative particle dynamics. We use the desorption time of oligomer chains from an aged NP as a basic characteristic to verify all models. This characteristic correlates with the fraction of oxygen-containing monomer units and the length of oligomer chains in the nanoparticle, which are directly related to aging processes. For systems with a low content of oxygen-containing monomer units, AMD simulations require a 1 μs time scale to ensure accurate determination of the desorption time. Using mesoscale models after fitting interparticle interactions based on AMD results allowed us to study the evolution of aged NPs of different molecular weights over a wide range of parameters. Our results indicate that aged nanoparticles become unstable due to a progressive accumulation of polymer chain scissions and oxygen-containing monomers. This transformation renders the particles water-soluble and capable of releasing oligomer chains into the environment. Conversely, cross-linking, which can occur during aging, may greatly alter the degradation pattern of polyethylene nanoparticles. As a result, the fate of nanoplastics is governed by the quantity of oxygen-containing monomers, chain scissions, and their cross-linking.