(337al) Molecular Dynamics Study of Penetrant Diffusion in Dense Crosslinked Poly-n-Butyl-Acrylate Networks

I am interested in developing models to help understand the mechanism of physical systems and to aid the design of materials for different applications. In my Ph.D. research, I used Molecular Dynamics simulation to investigate molecular transport in tight crosslinked polymer networks. By studying how penetrant transport is regulated by crosslink density, temperature, and penetrant geometry, we aim to provide insight into the selection of materials for membrane separations. I am also interested in combining molecular simulation with finite element analysis and data-driven approaches to accelerate product development and manufacturing.

Research Summary:

Passive membranes are being developed to substitute energy intensive separation processes such as distillation and extraction. Organic liquid separations, for example, aromatic and aliphatic hydrocarbons, are particularly difficult due to small differences in molecular weight and boiling points. There exists a real challenge of improving membrane separations for molecules with similar physical and chemical properties. We propose that tight rubber polymer networks operated near glass transition temperatures (Tg) can improve separation efficiency because the sub-nm mesh sizes can discriminate differences in geometries of two molecules and strong coupling between penetrant and matrix motion. To clarify how penetrant transport is regulated in tight networks, we performed Molecular Dynamics simulation with the standard bead-spring model parametrized for poly-n-butyl-acrylate (PnBA) networks synthesized by experimental collaborators. We first probe Tg of networks with different crosslink densities by quantifying alpha relaxation time at various temperatures and both Tg and alpha relaxation time show good agreement with experiment and theory. Diffusion coefficients and penetrant alpha time for spherical penetrants with different radius were calculated. By comparing our results to theoretical expressions, we can determine the relative importance of entropic mesh effect and coupling between segmental dynamics of penetrant and surrounding matrix. We concluded that for the systems studied by our experimental collaborators, crosslinks primarily affect penetrant transport through segmental motion of the polymer networks and the entropic mesh effect is more apparent for large penetrants at high temperatures. We also extend the model to study penetrant diffusion in vitrimers, crosslinked polymer networks with associative dynamic covalent bonds. We aim to understand how the rearrangement of network topology and the rate of bond exchange affect molecular transport.

References:

1. B. Mei*, T.-W. Lin*, G. Sheridan, C.M. Evans, C.E. Sing, K.S. Schweizer, Structural Relaxation and Vitrification in Dense Crosslinked Polymer Networks: Simulation, Theory and Experiment", Macromolecules, 2022, 55, 4159-4173.
2. B. Mei, T.-W. Lin, G. Sheridan, C.M. Evans, C.E. Sing, K.S. Schweizer, How Segmental Dynamics and Mesh Confinement Determine the Selective Diffusivity of Molecules in Dense Crosslinked Polymer Networks", ACS Cent. Sci., 2023, 9, 3, 508-518
3. T.-W. Lin, B. Mei, K.S. Schweizer, C.E. Sing, Simulation Study of the Effects of Polymer Network Dynamics and Mesh Confinement on the Diffusion and Structural Relaxation of Molecular Penetrants", J. Chem. Phys. 2023. In Press.