Transport Diffusion of Light Gases in Polyethylene Using Atomistic Simulations.

We explore the temperature dependence of the self-, corrected-, and transport-diffusivities of CO, CH, and N in a polyethylene (PE) polymer membrane through equilibrium molecular dynamics simulations. We also investigate the morphology of the polymer membrane based on the intermolecular radial distribution function, free volume, and pore size distribution analysis. The results indicate the existence of 1.5-3 Å diameter pores in the PE membrane, and with the increase in the temperature, the polymer swells linearly with changing slope at 450 K in the absence of gas and exponentially in the presence of gas. The gas adsorption isotherms extracted via a two-step methodology, considering the dynamics and structural transitions in the polymer matrix upon gas adsorption, were fitted using a "two-mode sorption" model. Our results suggest that CO adsorbs strongly, whereas N shows weak adsorption in PE. The results demonstrate that CO is more soluble, whereas N is least soluble. Further, it is found that an increase in the temperature negatively impacts the solubility of CO and CH but positively for N; this reverse solubility behavior is due to increased availability of pores accessible to N, which are kinetically closed at the lowest temperatures. The reported self-diffusivities of the gases from our simulations are on the order of 10 cm/s, consistent with the experimental evidence, whereas transport-diffusivities are 2 orders of magnitude higher than self-diffusivities. Furthermore, the temperature dependence of the self-diffusivity follows Arrhenius behavior, whereas the transport-diffusivity follows non-Arrhenius behavior having different activation energies in low and high temperature regions. Also, it is seen that loading has little effect on the self- and corrected-diffusion coefficients of all gases in the PE membrane.