Transport in Epdm Elastomer: Experimental Study and Molecular Simulation

Abstract

Ethylene-propylene-diene rubber (EPDM) is an important synthetic elastomer used in a variety of barrier material applications. However, accurate first principle methods for predicting gas diffusion rates and solubilities in EPDM are not currently available. This project aims to create a molecular model for predicting the diffusion rates and solubilities of He, H₂, Ne, N₂, O₂, Ar, Kr, CO₂, and CH₄ in Royalene®580 EPDM. The model is generated in Accelrys Materials Studio Software and incorporates the COMPASS forcefield. Self-diffusivities are estimated using molecular dynamics simulations (MDS) with subsequent determination of root mean square displacement of diffusing molecules in the material. Sorption isotherms are created using Grand Canonical Monte Carlo (GCMC) simulations at a variety of fixed pressures. Simulation results are in good agreement with experimental data, which are gathered by a traditional time lag diffusion method at room temperature and in the pressure range of 400-1000 torr. The experimental diffusivities were assessed using predictive correlations based on critical volume, molecular diameter and kinetic diameter of the diffusing molecules, while the experimental solubilities were assessed using predictive correlations based on critical temperature, and Lennard-Jones well-depth parameter. These results were compared to that of the MDS, which was observed to generally agree with the correlations. The molecular model and associated COMPASS forcefield were shown to capture the fundamental principles of molecular movement and interaction in EPDM elastomers.