Polymers are attractive membrane
materials owing to their
mechanical
robustness and relatively inexpensive fabrication. An important indicator
of membrane performance are free volume elements (FVE): 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 the establishment
of 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 with the atoms in the bulk; the extent of this difference
increases with chain flexibility. Increasing the 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 the 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
dictate hydrogen transport.