Ensemble-dependent diffusivity selectivity in amorphous polyethylene: a statistically validated molecular dynamics study of H2/CH4 transport
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2026-06-12
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American Chemical Society
Resumen
The integration of hydrogen into natural gas networks (H2NG) demands a rigorous understanding of relative gas transport properties in polymeric infrastructure to mitigate risks such as leakage and Rapid Gas Decompression (RGD). Polyethylene (PE) is a critical material for this transition due to its resistance to hydrogen embrittlement, yet its susceptibility to gas permeation remains a safety concern. In this work, we present a fully open-source, statistically validated Molecular Dynamics (MD) framework to quantify the diffusive transport of H2 and CH4 in amorphous polyethylene. Using the TraPPE-UA force field and a multistage high-pressure annealing protocol, we generated dense, well-equilibrated matrices (pure-polymer reference ρpure = 0.814 ± 0.001 g cm–3 and mix systems at ρMix,NPT = 0.803 ± 0.001 and ρMix,NVT = 0.819 ± 0.003 g cm–3) and characterized them using radial distribution functions, the full pore size distribution, and the standard porous-materials descriptors LCD (Largest Cavity Diameter) and PLD (Pore Limiting Diameter). Diffusion coefficients of H2 and CH4 were extracted from 48 independent production simulations (3 seeds × 2 systems × 2 ensembles × 4 temperatures) over the range 298–373 K under both isobaric (NPT, P = 1 atm) and isochoric (NVT) ensembles. Our central finding is an ensemble-dependent diffusivity-selectivity trade-off: thermal expansion in NPT conditions opens the free-volume network and reduces the H2/CH4diffusivity selectivity αD from ∼4.8 at 298 K to ∼1.9 at 373 K, whereas volumetric confinement (NVT) suppresses the free-volume dilation and yields a far more gradual decrease (αD ≈ 4.6 → 2.5). When the eight D(T) data points for each gas are pooled into a Cohen–Turnbull plot of ln D versus 1/FV0 (matrix void fraction), the NPT and NVT data collapse onto a single master line per gas with slopes whose ratio matches the squared kinetic-diameter ratio (σCH4/σH2)2 ≈ 1.73. This collapse provides direct quantitative evidence that the apparent ensemble dependence of selectivity is governed by free-volume modulation alone and that the activation parameter γv* scales with the cross-sectional area of the penetrant. Trajectory analysis further confirms an activated-hopping mechanism for H2 with a broad displacement distribution, while CH4 dynamics are dominated by cage rattling. We explicitly distinguish the diffusivity selectivity αD reported here from the experimental permeability selectivity αP = αD × αS, noting that for H2/CH4 in PE the solubility selectivity αS < 1 and therefore αP < αD. The combination of statistically validated diffusivities, density-resolved free-volume metrics, and the Cohen–Turnbull master curve provides a transferable framework for assessing how thermomechanical boundary conditions modulate the kinetic sieving behavior of polymer infrastructure exposed to hydrogen–methane mixtures.
Palabras clave
Diffusivity selectivity
Free volume
Hydrogen transport
Molecular dynamics
Polyethylene
TraPPE-UA
Free volume
Hydrogen transport
Molecular dynamics
Polyethylene
TraPPE-UA
Descripción
Materias
Cita
Delgado-Uriarte, D., & Lopez, A. (2026). Ensemble-dependent diffusivity selectivity in amorphous polyethylene: a statistically validated molecular dynamics study of H2/CH4 transport. ACS Applied Polymer Materials, 8(11), 8017-8031. https://doi.org/10.1021/ACSAPM.6C00287