title: Transport Diffusion of Light Gases in Polyethylene Using Atomistic Simulations.
abstract: We explore the temperature dependence of the self-, corrected-, and transport-diffusivities of CO2, CH4, and N2in 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 CO2adsorbs strongly, whereas N2shows weak adsorption in PE. The results demonstrate that CO2is more soluble, whereas N2is least soluble. Further, it is found that an increase in the temperature negatively impacts the solubility of CO2and CH4but positively for N2; this reverse solubility behavior is due to increased availability of pores accessible to N2, which are kinetically closed at the lowest temperatures. The reported self-diffusivities of the gases from our simulations are on the order of 10-6cm2/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.
authors.affiliation: School of Chemical Engineering, The University of Queensland , Brisbane, Queensland 4072, Australia.
authors.forename: Ravi C
authors.initials: RC
authors.lastname: Dutta
authors.affiliation: School of Chemical Engineering, The University of Queensland , Brisbane, Queensland 4072, Australia.
authors.forename: Suresh K
authors.identifier: 0000-0001-9716-0112
authors.initials: SK
authors.lastname: Bhatia