Molecular Dynamics Investigation of the Effect of Shear during Extrusion on the Permeation Behavior of CH4 in High-Density Polyethylene

Abstract

Deep-water flexible composite pipes have been widely employed in the domain of deep-water oil and gas transportation, and high-density polyethylene (HDPE) is used to seal the inner sheath of internal oil and gas media containing H2S and CH4, due to its favorable barrier properties and mechanical properties. The morphological evolution of HDPE during the extrusion process exerts a direct impact on the material’s barrier properties. The grand canonical Monte Carlo (GCMC) approach and the molecular dynamics (MD) method were coupled in this study to examine the morphological evolution of HDPE under various shear rates as well as the penetration of methane (CH4) in HDPE under various shear rates. The results indicate that with an increase in shear rate, the HDPE undergoes decoupling, leading to the formation of a densely arranged, rigidly oriented structure. Gas solubility and diffusion coefficients exhibit an initial increase followed by a subsequent reduction as the shear rate increases, which corresponds to the evolution of microscopic morphology. The current simulation can effectively forecast the microscopic morphology and material permeability coefficient and provide valuable insights for enhancing the barrier effectiveness of the inner sheath.