Environmental Stress Cracking of Polyethylene: A Molecular-Scale Fatigue Process?
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".