Environmental Stress Cracking of Polyethylene: A Molecular-Scale Fatigue Process?

In a previous study, we used molecular dynamics (MD) simulations to show that environmental stress cracking (ESC) of polyethylene (PE) likely initiates with the coalescence of free volumes at the PE–ESC agent interface, modeled using nonyl ethoxylate (NE). In this work, we expand on those findings by applying tensile and compressive stresses to PE systems. The simulations reveal that tensile stress enhances free volume coalescence significantly more than compressive stress, consistent with experimental evidence that tensile stress promotes ESC, whereas compressive stress does not. We also observe that free volume coalescence occurs cyclically, and that PE chains near NE molecules experience more cyclic force fluctuations than chains in systems without ESC agents. This behavior is reminiscent of macroscopic fatigue, a brittle failure mode caused by repeated loading well below a material’s tensile strength. Since ESC is also brittle, we propose that ESC in PE, a ductile polymer, may be understood as a form of molecular-scale fatigue. The proposed mechanism involves the sustained coalescence of free volumes driven by inhomogeneous, cyclic, nonbonded force fluctuations at the NE–PE interface. These arise from the incompatibility between NE’s hydrophilic head and the hydrophobic PE chains. Notably, these force fluctuations are substantially weaker in the absence of NE or under compressive stress, conditions under which ESC does not occur, supporting the connection between cyclic molecular forces and fatigue-like failure.