Building Nanoplastic Models for Molecular Calculations

In exploring the toxicity of micro- and nanoplastics, molecular simulations of plastic nanoparticles have been gaining traction recently. Modeling of nanoplastics involves the folding of multiple polymeric chains into an entangled particle, which is a challenging process, necessitating thorough optimization of the folding procedure. In this contribution, we use the simulated annealing procedure in a systematic workflow for preparing stable nanoplastic structures for the first time, based on the CHARMM36 force field. On the structures prepared with the fine-tuned protocol, we carry out quantum chemical geometry optimizations with the GFN2-xTB method, followed by benchmarking single point calculations with GGA and hybrid DFT functionals. We further demonstrate the applicability of this approach through four plastic systems, including polyethylene, polypropylene, polystyrene, and nylon 66. Remarkably, the geometry of the resulting most stable assemblies show similarities to features observed earlier theoretically and experimentally for such systems. For polyethylene, a highly ordered, crystalline structure is obtained, in which the polymer chains possess long sections with all C-C-C-C units in trans configuration. For polypropylene and polystyrene, helical structures are observed, formed by alternating gauche and trans configurations along the backbone. For nylon 66, the structure-directing effect of the hydrogen bonds between amide moieties complicates the folding, resulting in parallelly arranged hydrogen bonding chains throughout the particle. Especially, we make the whole set of optimized structures available for the community in an online repository, with hopes of advancing simulation studies in the field, even making ensemble simulations accessible.