Microporous glassy polymers currently in use for gas capture and separations can undergo plasticization and swelling through a high uptake of adsorbates. In this work, we evaluated their effects on the structural and adsorption properties of a polymer of intrinsic microporosity (PIM-1) through atomistic molecular simulations. The sorption-relaxation behavior in PIM-1 induced by CO2, CH4, H-2, N-2, and O-2 gases and binary mixtures have been studied using a combined approach of Monte Carlo and molecular dynamics. Through accounting for polymer relaxation, we showed that structural properties were significantly altered, especially by CO2 and CH4. For adsorption properties, significant differences in loading and configuration of gases in PIM-1 were observed when comparing between fixed and flexible frameworks. Our MC/MD simulations revealed a threshold value around 3 mmol/g at 300 K at which gas loading initiate plasticization and swelling within the polymer matrix. Moreover, working capacity and regenerability were improved upon sorption-relaxation in several cases indicating that the phenomena do not always correlate with a reduction in performance as usually assumed in the literature.