Molecular simulations of gas transport in hydrogenated nitrile butadiene rubber and ethylene-propylene-diene rubber

Diffusion and sorption of five gases (H-2, N-2, O-2, CO2, CH4) in hydrogenated nitrile butadiene rubber (HNBR) and ethylene-propylene-diene rubber (EPDM) have been investigated by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The diffusion coefficients of gas molecules in HNBR and EPDM are well correlated with the effective penetrant diameter except for CO2. CO2 shows a lower diffusion coefficient due to its linear shape. Additionally, the favorable interaction between CO2 and HNBR is another factor for its lower diffusion coefficient in HNBR. HNBR shows lower diffusion coefficients than EPDM. This is because the polar -CN groups in HNBR chains increase interchain cohesion and result in tight intermolecular packing, low free volume and poor chain mobility, which decreases the diffusion coefficients of HNBR. The solubility coefficients of CH4, O-2, N-2 and H-2 in HNBR are lower than those in EPDM, which is a result of the weak HNBR-penetrant interactions and low free volume of HNBR. However, the solubility coefficient of CO2 in HNBR is higher than in EPDM. This is attributed to the strong interaction between CO2 and HNBR. H-2, O-2, N-2 and CH4 show lower permeability coefficients in HNBR than in EPDM, while CO2 has higher permeability coefficients in HNBR. These molecular details provide critical information for the understanding of structures and gas transport between HNBR and EPDM.