Air Sorption and Separation by Polymer Films at the Molecular Level

Abstract

The molecular characteristics of air sorption and the subsequent separation of oxygen and nitrogen by dense glassy polymer membranes have been studied in detail by large-scale molecular dynamics (MD) simulations. Air was inserted at various pressures into explicit reservoirs on either side of an ∼50000-atom 6FDA-6FpDA polyimide thin film. The simultaneous sorption of oxygen and nitrogen and their transport through the dense polymer were followed for more than 300 ns. This very long time scale (with respect to current fully atomistic MD simulations) allowed for the concentration of both penetrants to reach equilibrium in the matrix. When exposed to air, the thin film undergoes a very limited amount of swelling. Oxygen is more soluble than nitrogen in the polyimide even if, following sorption, both penetrants tend to occupy the same range of favorable low-energy sites. These sites represent less than 2.5% of the matrix core volume for O2 and less than 2% for N2. Such subtle differences lead to a model solub...