Molecular Models of Nanoplastics from Semi-Crystalline Polyethylene

Molecular models for nanosized polar crystalline fragments generated during the degradation of semicrystalline polyethylene (PE), suitable for hybrid-particle field molecular dynamics, are proposed. These models provide detailed thermodynamic and structural characterization of the agglomeration of fragments constituting nanoplastics in aqueous media. Effects of aging and environmental conditions in terms of surface chemistry, pH, and counterion valence of the models are studied. In particular, the colloidal stability of fragment pairs in water has been investigated by calculating free energy as a function of interfragment distance using the thermodynamic integration technique. The behaviors obtained are in line with experimental literature. Large-scale MD simulations of multifragment systems in water show spontaneous self-assembly for all considered fragment models. Due to their molecular resolution and computational efficiency, the proposed models are able to include detailed representations of environmentally relevant systems, paving the way to a molecular understanding of the behavior of nanoplastics from semicrystalline polymers in the environment.