Stress-Driven Nanostructural Evolution and Its Impact on Hydrogen Diffusion in PE and PA6

Polyethylene (PE) and polyamide 6 (PA6) are critical liner materials for high-pressure hydrogen storage, yet the coupling between triaxial stress and hydrogen (H2) diffusion and permeability remains poorly quantified. Using molecular dynamics simulations, we investigate microstructural evolution and H2 transport kinetics of amorphous PE and PA6 under anisotropic triaxial loading. Our results reveal a fundamental divergence in material response: under applied stress, PE develops strongly aligned-chain domains, characterized by pronounced backbone orientation and local densification. In contrast, the rigid H-bonding network of PA6 suppresses chain reorganization, maintaining a stable amorphous state. We find that H2 diffusion in PE is inversely correlated with the degree of chain alignment, with highly aligned-chain domains acting as effective barriers that substantially reduce the diffusion coefficient. The primary contribution of this work is the identification of a dynamic tortuous path inhibition, where local phase transitions under service-level stresses actively modulate H2 permeability. These findings provide a theoretical framework for designing smart polymer liners that utilize operational mechanical loads to enhance their own barrier performance.