Solubility of Gases and Free Volume Evolution in R-BAPB Polyimide: Molecular Dynamics Simulations and Analytical Theory Insights into Cooling Velocity Effect

In the present work, we investigate the solubility of various gases (CH4, CO2, O2, N2, and He) in a thermoplastic polyimide R-BAPB at room temperature by means of extensive molecular dynamics (MD) simulations, focusing on the effects of thermal prehistory during cooling of polymer melts with different cooling velocities. We show that the cooling velocity employed in MD simulations is an important factor affecting the solubility of gases. Lowering the cooling velocity by 4 orders of magnitude (from 1000 to 0.25 K/ns) reduces the simulated gas solubilities by a factor of 2, with no indication of saturation to steady solubility values. In order to extrapolate the dependence of the gas solubility values to experimental cooling velocities, we use an analytical phenomenological approach based on Struik’s theory of isothermal free volume evolution in polymer aging. As a result, we are able to estimate gas solubility values in MD-simulated polymer samples prepared at experimentally relevant cooling velocities. We verify our approach by comparing the direct MD solubility data, the theory-based extrapolation, and the limited experimental data available for R-BAPB polyimide. For additional verification, we extend our approach to ULTEM polyimide, which is much better studied experimentally. We show that the theory-based extrapolation considerably improves the correspondence between modeling and experiment in the case of ULTEM. By combining extensive all-atom MD simulations with an analytical theory, we capture the effects of thermal history on the observed physical properties (specific volume and solubility of gases) and provide the means to bridge a huge gap between the experimental and computational time domains.