Molecular Dynamics Study of Carbon Dioxide Sorption and Plasticization at the Interface of a Glassy Polymer Membrane

A series of large-scale molecular dynamics (MD) simulations of CO2 transport in a fully atomistic similar to 50000-atom fluorinated 6FDA-6FpDA polyimide membrane were carried out under six different conditions of applied external gas pressure in order to assess the uptake mechanisms of the penetrant at the glassy polymer interface. CO2-induced volume dilation was found to occur immediately with a progressive shift of the interface location toward larger values associated with a widening of the interface and a decrease in density. However, it only leads to limited structural relaxations of the matrices and as such, the corresponding increase in mobility is small. Void-space and excess chemical potential analyses show that the holes within the matrix are getting larger as sorption proceeds, but that they are immediately occupied by penetrant molecules. The only way to accommodate more penetrant is to further swell the polymer matrix. The concave behavior of the sorption curves is well reproduced by the models and compares favorably with experimental data, except for the highest-pressure system which is in a different regime and exhibits a quasi-supercritical behavior for CO2. In all cases, the penetrants first undergo a rapid adsorption at the polymer surface. This is followed by a slower uptake mode, with the time-dependent diffusion coefficient being only accessible for short time-intervals because of the moving boundaries. All trajectories display the oscillations in voids and occasional jumps mechanism, and no transitions to the paths characteristic of liquid-like diffusion were seen, even in the most swollen systems.