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

TL;DR

This study uses molecular dynamics simulations to analyze how local chain dynamics and free volume element distribution influence hydrogen transport in amorphous polymers with varying flexibility.

Contribution

It establishes a correlation between chain segment mobility and free volume distribution, highlighting the impact of polymer flexibility and temperature on transport properties.

Findings

Higher chain mobility near FVEs compared to bulk

Broader FVE distribution with increased temperature in flexible polymers

Rigid polymers maintain stable FVE distribution regardless of temperature

Abstract

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.