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Adsorption and Diffusion of Methane and Carbon Dioxide in Amorphous Regions of Cross-Linked Polyethylene: A Molecular Simulation Study

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journal contribution
posted on 16.04.2019, 00:00 by Yafan Yang, Arun Kumar Narayanan Nair, Shuyu Sun
We perform Monte Carlo (MC) and molecular dynamics (MD) simulations to study the adsorption and diffusion properties of methane and CO2 in cross-linked polyethylene in the temperature range 300–600 K. A hybrid MC/MD approach was used to incorporate the effects of framework flexibility and polymer swelling on the gas adsorption. The polymers show negligible swelling at the studied conditions. A nonmonotonic behavior of gas adsorption as a function of the cross-linking degree was obtained. Notably, a similar behavior was observed for the void fraction and pore diameters. This shows a direct correlation between gas adsorption and the pore characteristics of the cross-linked polymer network. Mobility of methane and carbon dioxide in the polymer matrix increases with temperature. Also, gas mobility decreases with increasing cross-linking degree, consistent with experiments. These results can be explained by the fact that the waiting time for a gas molecule in a cavity before the jump increases with decreasing temperature and increasing cross-linking degree. Interestingly, the activation energy for gas diffusion generally decreases with increasing cross-linking. This is possibly due to the fact that increasing the cross-linking degree leads to smaller pore sizes especially at high temperatures. Such a molecular-level understanding of adsorption and diffusion of gases in cross-linked polyethylene is important in improving the performance of polymer networks for potential applications in gas separation, barrier technology, and food packaging.