Assessing the Martini Force Field for Modeling Polyolefin Nanoplastics near Lipid Membranes

We present an improved Martini-type coarse-grained (CG) model for polyethylene (PE) nanoplastics and benchmark its performance against three existing Martini PE models from the Martini 2 and Martini 3 force fields. While current Martini models reproduce conformational statistics for molten PE, the PE chains do not crystallize below experimental melting temperatures. With improved bonded interactions, our CG PE chains exhibit melt properties and semicrystalline morphologies consistent with all-atom (AA) simulations and experimental data. Using our improved model, we generate semicrystalline PE nanoplastics (NPLs) at body temperature (310 K) in agreement with the AA reference, whereas NPLs from current Martini PE models remain amorphous. We further investigate the interaction of semicrystalline PE NPLs with a POPC lipid membrane in the Martini framework. The membrane exhibits unphysical behavior similar to that observed for amorphous NPLs, bending toward and mixing with the nanoplastic. When embedded within the membrane core, the semicrystalline Martini-type NPL spreads laterally and dissolves, losing its crystalline domains, whereas the all-atom NPL remains intact, with alkane-membrane interactions promoting further crystallization. These results indicate that improving the PE model alone is insufficient and that refining the membrane model is also required to accurately describe polyolefin nanoplastic-membrane interactions in the Martini force field.