(338k) Molecular Dynamics Simulations to Unravel the Correlation between Chain Local Dynamics, Relaxation of Free Volume Structure, and Transport Properties in Amorphous Polymers

Polymer materials have high tunability, superior mechanical properties, and low manufacturing cost, rendering them highly suitable for membrane fabrication. Polymer flexibility has a significant impact on membrane performance and lifetime. Polymer structures experience different levels of relaxation, which sometimes lead to a compromise in transport performance. Microporous spaces known as Free Volume Elements (FVEs) are formed by the inefficient packing of bulky groups in polymer chains and serve as transport pathways for penetrant molecules. FVE distribution has a transient nature in polymer materials, and the change in FVE structure usually undermines the membrane’s permeability or selectivity. This study examines the relationship between chain segmental dynamics and FVE distribution using all-atom molecular dynamics (MD) simulations. The study is conducted on three polymers with different levels of backbone flexibility: polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermally rearranged polymer (TRP). The three polymers are constructed and compared to determine the role of polymer chemistry on polymer mobility and how that mobility impacts the stability of FVE distribution. The three systems are constructed under a wide temperature range (100 K – 500 K) to compare trends at different relaxation timescales. Interestingly, the analysis indicates that surface chain segments near FVEs show higher mobility than the atoms in the bulk, and the extent of this difference increases with chain flexibility. The dynamics of surface segments directly correlate with FVE dynamics. Polymers with flexible backbones, such as PMP, typically exhibit a less stable FVE distribution. This transient nature becomes more evident as temperature increases. Conversely, rigid polymers, such as TRP, tend to exhibit a more durable FVE distribution that remains relatively unaffected by fluctuations in temperature. Hydrogen is inserted into the polymer matrix to measure penetrant diffusion at different temperatures. The mobility of penetrants is impacted by the chain rearrangement and dynamic nature of FVE structure, especially in flexible polymers.