Subdiffusive High-Pressure Hydrogen Gas Dynamics in Elastomers

Abstract

Elastomeric rubber materials serve a vital role as sealing materials in the hydrogen storage and transport infrastructure. With applications including O-rings and hose liners, these components are exposed to pressurized hydrogen at a range of temperatures, cycling rates, and pressure extremes. High-pressure exposure and subsequent rapid decompression often lead to cavitation and stress-induced damage of the elastomer due to localization of the hydrogen gas. Here, we use all-atom classical molecular dynamics simulations to assess the impact of compositional variations on gas diffusion within the commonly used elastomer ethylene–propylene–diene monomer (EPDM). With the aim to build a predictive understanding of precursors to cavitation and to motivate material formulations that are less sensitive to hydrogen-induced failure, we perform systematic simulations of gas dynamics in EPDM as a function of temperature, gas concentration, and cross-link density. Our simulations reveal anomalous, subdiffusive hydrogen motion at pressure and intermediate times. We identify two groups of gas with different mobilities: one group exhibiting high mobility and one group exhibiting low mobility due to their motion being impeded by the polymer. With decreasing temperatures, the low-mobility group shows increased gas localization, the necessary precursor for cavitation damage in these materials. At lower temperatures, increasing cross-link density led to greater hydrogen gas mobility and a lower fraction of caged hydrogen, indicating that increasing cross-link density may reduce precursors to cavitation. Finally, we use a two-state kinetic model to determine the energetics associated with transitions between these two mobility states.

Structure, Dynamics, and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations

“…To investigate this further, we calculate the mean-squared displacement of all hydrogen atoms in the three systems at different temperatures (Figure ). Across each system, there is an increase in hydrogen diffusion as the temperature increases, in line with prior work . However, comparing the hydrogen MSD of the three systems at each temperature reveals interesting trends.…”

“…For each system, we find that hydrogen diffusion increases as the temperature increases, as expected. Prior work by Brownell et al has found Arrhenius dependence between hydrogen diffusion and temperature in amorphous polymers . At a temperature of 100 K, hydrogen molecules are almost completely confined in PMP and PS, with only modest mobility observed in TRP.…”

“…Prior work by Brownell et al has found Arrhenius dependence between hydrogen diffusion and temperature in amorphous polymers. 43 At a temperature of 100 K, hydrogen molecules are almost completely confined in PMP and PS, with only modest mobility observed in TRP. This is due to the high microporosity in TRP compared with the other systems at 100 K, which facilitates the transport of hydrogen.…”

“…Hydrogen molecules are modeled using the same OPLS-AA force field as the polymers. A short equilibration of 1 ns followed by a 20 ns production run was performed in NVT, similar to the method outlined in prior work. , In experiments, hydrogen is introduced on the feed side of the membrane and slowly saturates the membrane upon reaching steady state. The insertion protocol that we employ bypasses the long time scales needed to achieve a penetrant-saturated configuration.…”

“…Over the years, molecular simulations have successfully captured penetrant adsorption and transport through different amorphous polymers. Specifically, the Colina group has studied the adsorption of penetrants through polymers of intrinsic microporosity (PIMs) using a combination of grand canonical Monte Carlo (GCMC) and MD simulations, revealing complex links between matrix swelling, free volume element rearrangement, and sorption-induced adsorption of penetrants at different pressures. − Kumar et al have used coarse-grained models to study the relationship between penetrant size and polymer matrix motion, as well as the mechanism of gas diffusion in polymer-grafted nanoparticle membranes. − Additionally, several other modeling studies have elucidated the molecular nature of gas and liquid transport in amorphous polymers. ,− Overall, the relaxation of amorphous polymers like polystyrene and poly­(1-trimethylsilyl-1-propyne) has been studied using different molecular modeling techniques, including coarse-grained and atomistic methods. − However, limited work has been done correlating the free volume elements and segmental dynamics in state-of-the-art rigid polymers like thermally rearranged polymers that have shown promise as hydrocarbon separation membranes.…”