  An important indicator of membrane performance are free volume elements
(FVEs): microporous void spaces created by the inefficient packing of bulky
groups along the polymer chain. FVEs tend to degrade over time as polymer
chains reorganize irreversibly. While it is widely accepted that polymer
flexibility has an impact on membrane transport properties, the molecular
nature of this impact is still not well-understood. By establishing a
correlation between local chain dynamics and the distribution of free volume
elements (FVEs), penetrant transport can be regulated more efficiently in
amorphous polymer membranes. In this work, we implement all-atom molecular
dynamics (MD) simulations to explore the relationship between chain dynamics
and free volume in three polymers with different levels of backbone
flexibility: polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermally
rearranged polymer (TRP). We construct these polymers at different temperatures
and examine how temperature impacts the FVE distribution and segmental
mobility. Our analysis shows that chain segments near FVEs have higher mobility
compared to the atoms in the bulk; the extent of this difference increases with
chain flexibility. Increasing chain flexibility by increasing the temperature
results in a broader FVE distribution. Rigid polymers such as TRP show the most
robust FVE distribution and are not significantly affected by temperature
change. To capture penetrant diffusion through the polymer matrix, hydrogen is
inserted, and the diffusion is measured at different temperatures; hydrogen
mobility is influenced by the FVE structure and overall mobility of polymer
chains. At low temperatures, hydrogen mobility is influenced by void
distribution, while at high temperatures, polymer dynamics dictates hydrogen
transport.
