Comparing gas transport in three polymers via molecular dynamics simulation.

People use polymers as materials for preparing separation media or containers. It is necessary to have a molecular level profound knowledge of gas transport in e bulk and interface regimes of different polymers, but few researchers have done a molecular level study of the bulk and interface behavior of gases in several types of non-homologen polymers thoroughly or developed expressions to correlate gas transport properties with cavity size distribution and chain oscillation flexibility. Therefore, in this work, molecular dynamics (MD) simulation was employed to study the transport of methane and n-butane molecules in the bulk and interface region of polyethylene (PE), poly(4-methyl-2-pentyne) (PMP) and polydimethylsiloxane (PDMS). Penetrant diffusivity, solubility and permeability in the bulk were studied first. The subdiffusion behavior of gas molecules is explored to obtain the mechanisms behind penetrant transport. Both penetrants have much smaller diffusivities in PE than in PMP and PDMS, and they have larger diffusivities in PDMS than in PMP. PE has lower accessible cavity fraction (ACF) and average oscillation amplitudes (AOAs) of the chains than PDMS and PMP. PE also has much smaller solubilities and permeabilities of both penetrants than PDMS and PMP. Though the permeabilities of both penetrants in PDMS are higher than the corresponding values in PMP, PMP has a higher selectivity of n-butane over methane than PDMS. Nonequilibrium MD simulation was performed to study the interface property and gas transport in the interface region. Equations to predict penetrant diffusivity and permeability from the accessible cavity fraction (ACF) and average amplitude of chain oscillation were developed successfully. Penetrant diffusivity and permeability are proportional to the value of ACF to the power of one third and that of ACF to the power of four thirds, respectively.