(192b) Effect of Chain Dynamics on the Free Volume Elements of Glassy Polymers from Atomistic Molecular Dynamics Simulations

Membranes offer a promising alternative to energy intensive methods that are currently used in gas separation applications such as olefin/paraffin separation, carbon capture, and natural gas production. Polymers are attractive membrane materials due to their scalable and inexpensive fabrication. Below a specific temperature (glass transition temperature), amorphous polymers exist in a rigid and relatively brittle state known as glassy state, and they move to a viscous rubbery phase if heated above this temperature. Glassy polymers form microporous structures due to their rigidity and the inefficient packing of bulky groups on their backbones. The distribution of void spaces thus created is known as free volume elements (FVE) and plays a significant role in determining membrane selectivity and overall performance. Despite many advantages, the use of polymer membranes in gas separation applications has been hindered by plasticization, the irreversible reorganization of polymer chains that alters FVE distribution and compromises membrane selectivity. Plasticization occurs at high pressures when gas molecules dissolve into the polymer matrix making the chains flexible and more susceptible to deformation. FVEs are transient in polymer membranes, which means that their size and distribution depends on the translational and vibration motion of the surrounding polymer chains. This transient nature as well as the small time and length scales at which plasticization reshape FVEs makes it challenging to understand the interplay between FVE and chain dynamics.

We utilize molecular dynamics (MD) simulations to correlate membrane FVE to chain dynamics and flexibility. We perform all-atom simulations using GROMOS and OPLS-AA forcefields on three different glassy structures with different backbone rigidities – polymethylpentene, polystyrene, and HAB-6FDA TR. The structures are validated by comparing density, glass transition temperature, structure factor, and persistence length with experiments. MD simulations allow us to access a length and time-scale small enough to capture the correlation between FVEs and chain dynamics. First, we calculate the distribution of cavities in the membrane and compare it across the three systems. The chain dynamics are examined by looking at the mean squared displacement and the bond vector autocorrelation function. A correlation is established between FVE distribution and the relaxation of the polymer segments near the voids. A mixture of ethane/ethylene is introduced into the polymer systems at high pressure to generate a plasticized polymer. The effect of plasticization is evaluated by comparing the dynamics and FVE distribution in both the initial and plasticized (swollen) structures. Understanding the effect of chain dynamics on FVE will provide crucial insights that guides the design and synthesis of highly permeable plasticization-resistant polymer membranes.