Effect of crystalline structure on the diffusion of volatile organic compounds (VOCs) in recycled polyethylene: Insights from molecular dynamics simulations

The migration of volatile organic compounds (VOCs) from recycled polyethylene (rPE) matrices poses significant safety concerns for food packaging applications primarily due to potential consumer exposure risks. In this work, united-atom (UA) models with VOCs (1-octene, myrcene, linalool, octanal, and tetradecane) of rPE by treating multiple atoms as a single bead were developed. We performed UA molecular dynamics (MD) simulations to reveal the mechanism of VOCs diffusion within rPE under the influence of temperature, crystallinity, and the deformation of fluid-induced crystallization (FIC). The results show that the diffusion capability of VOCs is stronger in the amorphous regions of rPE, and weaker in the crystalline regions. The crystallinity of the rPE model exhibited a negative correlation with the mean squared displacement (MSD) of VOCs, attributable to the reduction of amorphous regions and free volume at higher segment orientation. With increasing temperatures, a significant enhancement in the diffusion of VOCs was observed, exemplified by the diffusion coefficient of 1-octene, which rose from 1.69 ± 0.60 × 10^-6 to 6.49 ± 0.94 × 10^-6 cm²·s^-1 at 293-353 K. Concurrently, the elevated temperatures intensify the free volume expansion in rPE. The significantly increased orientation of rPE induced by fluid flow correlates well with the restricted diffusion behavior of VOCs, further confirming the decisive role of the crystallinity of rPE in the retention and removal of VOCs. Finally, the degree of VOCs diffusion trajectories in rPE space can be indirectly predicted from their MSD values and potential energy values obtained through molecular simulations.