(272e) Probing Polymer Relaxation and Plasticization Dynamics Using Molecular Dynamics and Enhanced Sampling Techniques: Insights from Well-Tempered Metadynamics

Understanding the dynamics of polymer relaxation is crucial for addressing phenomena such as physical aging and plasticization in glassy polymers. Free volume within these materials can change due to polymer relaxation, chain rearrangement, and structural evolution towards equilibrium. Classical molecular dynamics simulations offer powerful means to analyze these dynamics. However, the associated timescales for physical aging can be prohibitively long. Traditional molecular dynamics often falls short in capturing slow dynamics related to physical aging due to these extended timescales. Enhanced sampling techniques can accelerate exploration of configuration space, improving the sampling of rare and slow events. This study utilizes well-tempered metadynamics, an enhanced sampling method, to probe the molecular mechanisms underlying polymer relaxation during physical aging and plasticization. Metadynamics employs a bias potential constructed from a sum of Gaussian functions deposited along the trajectory of a chosen collective variable (CV).

Our investigation focuses on polymers with intrinsic microporosity (PIM), characterized by rigid backbones and contortion sites that enhance free volume and microporosity. PIMs exhibit faster aging and swelling compared to other glassy membrane materials. We examine three different PIM structures (PIM-1, PIM-py, PIM-COOH). Polymers are parameterized using GAFF2 force field and constructed via the Polymatic simulated polymerization algorithm. Free volume of these polymers is analyzed using geometric (Zeo++) and energetic (VACUUMMS) approaches. Segmental relaxation, a key aspect of aging, is employed as collective variables (CVs) in our enhanced sampling. Specifically, we utilize various dihedral angles as CVs to construct free energy landscapes and visualize the energy surfaces along these CVs. Dihedrals angles around the spiro carbon shows a higher flexibility and lower torsion energy compared to dihedral angle around ether oxygen. Toluene is inserted as a plasticizer, and metadynamics incorporates CVs that capture toluene-polymer interactions. This approach elucidates how toluene influences chain dynamics and segmental mobility, ultimately leading to plasticization. All enhanced sampling calculations are performed using PLUMED package integrated with LAMMPS. Our findings shed light into relaxation and plasticization mechanisms into these state of the art polymers, and demonstrate the use of enhanced sampling to provide insight into the molecular origin of complex processes like physical aging and plasticization. This can pave the way for rational design strategies to develop more durable membranes for gas and solvent separation.