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An important indicator of membrane performance are free volume elements(FVEs): microporous void spaces created by the inefficient packing of bulkygroups along the polymer chain. FVEs tend to degrade over time as polymerchains reorganize irreversibly. While it is widely accepted that polymerflexibility has an impact on membrane transport properties, the molecularnature of this impact is still not well-understood. By establishing acorrelation between local chain dynamics and the distribution of free volumeelements (FVEs), penetrant transport can be regulated more efficiently inamorphous polymer membranes. In this work, we implement all-atom moleculardynamics (MD) simulations to explore the relationship between chain dynamicsand free volume in three polymers with different levels of backboneflexibility: polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermallyrearranged polymer (TRP). We construct these polymers at different temperaturesand examine how temperature impacts the FVE distribution and segmentalmobility. Our analysis shows that chain segments near FVEs have higher mobilitycompared to the atoms in the bulk; the extent of this difference increases withchain flexibility. Increasing chain flexibility by increasing the temperatureresults in a broader FVE distribution. Rigid polymers such as TRP show the mostrobust FVE distribution and are not significantly affected by temperaturechange. To capture penetrant diffusion through the polymer matrix, hydrogen isinserted, and the diffusion is measured at different temperatures; hydrogenmobility is influenced by the FVE structure and overall mobility of polymerchains. At low temperatures, hydrogen mobility is influenced by voiddistribution, while at high temperatures, polymer dynamics dictates hydrogentransport.

Structure, Dynamics and Hydrogen Transport in Amorphous Polymers: An  Analysis of the Interplay Between Free Volume Element Distribution and Local  Segmental Dynamics from Molecular Dynamics Simulations