Carbon Dioxide Solubility in Three Fluorinated Polyimides Studied by Molecular Dynamics Simulations

Fluorinated polyimides are interesting polymer materials for gas separation applications because of their good mechanical, thermal, and transport properties. We have performed molecular dynamics simulations (MD) of CO 2 sorption and desorption in three fluorinated polyimides: 6FDA-6FpDA, 6FDA-6FmDA, and 6FDA-DAM. These polyimides are known to vary significantly in their gas permeation properties. A stepwise procedure was used to insert CO 2 molecules into the previously prepared polymer matrices in order to mimick the experimental procedure of progressive loading and to avoid the necessity of preswelling the samples. An iterative technique was then used to estimate the vapor pressure of CO 2 that would have to be applied in order to obtain the imposed uptake. The resulting sorption isotherms are found to be in relatively good agreement with their respective experimental curves, and the trend in solubility was reproduced (6FDA-DAM > 6FDA-6FpDA > 6FDA-6FmDA). Desorption isotherms were also calculated starting from systems corresponding to an applied pressure of 60 bar. Hysteresis was evident even upon immediate unloading. Changes in volume, void space, potential energies, etc., have been characterized and compared to experimental data and to theories of gas sorption and plasticization in glassy polymers.