Hop, Skip, and Jump: Hydrogen Molecular Transport through Amorphous Polyethylene Matrices Studied via Molecular Dynamics Simulations

In the pursuit of advancing and diversifying energy technologies for a more sustainable future, the possibilities of hydrogen (H-2) usage will broaden, as will our understanding of its containment materials. Polyethylene (PE) has a vast assortment of uses and applications, which are growing with demands for alternative energy possibilities. One use of PE liner is as a prime candidate for nonmetallic piping and pressurized type IV storage devices. Such applications require PE to effectively prevent H-2 transport through containment systems. To study the molecular transport mechanism of hydrogen through polymeric barriers, a system containing hydrogen molecules absorbed within amorphous PE is modeled here using molecular dynamics simulations. The simulations are conducted within a range of temperatures that span the glass transition temperature of amorphous PE. The simulated PE displays bulk density, radius of gyration, and self-diffusion coefficient that are consistent with experimental data. The simulated trajectories are interrogated to study the movement of the guest gas molecules. The results show that the diffusion coefficients increase with temperature, as expected. However, the mobility of the PE chains is found to affect the mobility of absorbed H-2 molecules to a much lower extent than it affects that of CH4 molecules because of the much smaller size of the former than of the latter guest. From a molecular perspective, a "hopping" mechanism is observed, according to which H-2 molecules hop between one vacant free volume space to another within the polymer matrix, in combination with longer, straight, undisturbed "jumps" or "skips" along directions aligned with regions of ordered PE chains. This suggests that the orientation of the crystallites within the semicrystalline PE matrix affects the H-2 containment. Implications of these findings toward PE usage as containment material are discussed.