This work systematically investigates an exemplary mixed-matrix membrane (MMM) system composed of a UiO-66-NH2 metal-organic framework (MOF) and a 6FDA-Durene polyimide. The UiO-66-NH2 MOF is post-synthetically modified with 6FDA-Durene oligomers through either chemical or thermal imidization routes. The thermal(T)-PSM approach does not require reactive small molecules for synthesis, thereby allowing the cross-linking reaction between the MOF and the polymer to occur only on the MOF surface. Conversely, the chemical(C)-PSM approach requires reactive small molecules for synthesis, thereby altering internal amine functionality of UiO-66-NH2. Upon embedding these PSM-MOFs in high molecular weight 6FDA-Durene polymers, CO2 and CH4 pure-gas permeation tests reveal that the T-PSM-MOF has synergistic improvements in both permeability and selectivity due to enhanced interfacial compatibility without significant modification to the internal MOF framework Conversely, MMMs formed with the C-PSM-MOF experienced a significant decrease in permeability due to reactions that occurred inside of the MOF pore structure. These findings suggest that there is a critical need to consider reaction conditions and their corresponding effect on MOF structures when pursuing a PSM approach to form MMMs with desired performance enhancements.