(350g) The Importance of Pore Connectivity in Gas Adsorption Atomistic Simulations for Porous Polymers

Authors: 
Anstine, D., University of Florida
Colina, C. M., University of Florida
Haranczyk, M., Lawrence Berkeley National Laboratory
Porous membranes are essential to many industrial applications; for instance, separation of hazardous volatile organic compounds used in chemical processing or carbon sequestration from the burning of fossil fuels. The ability to design porous polymeric membranes with enhanced properties (selectivity, permeability, total loading) has the potential to help combat adverse environmental effects that many industrial processes contribute to. To aid in the design of these membrane materials Monte Carlo (MC) and molecular dynamics (MD) simulations have been proven to be efficient tools for prescreening and predicting membrane performance for a range of porous materials and gasses with reasonable fidelity. However, a few challenges still exist to increasing the predictive capabilities of these molecular simulations for amorphous polymeric membranes, one such challenge being accounting for pore accessibility during MC simulations. This work seeks to address the issue of pore accessibility through combining the analysis available in Zeo++, an open source software capable of characterizing porous material structure and connectivity, with pysimm, an open source software package that can facilitate a combined MC/MD approach to predict gas adsorption for porous materials.

We present a simulation study aimed at improved predictions of the adsorption behavior of a polymer of intrinsic microporosity (PIM-1). We first perform Voronoi decomposition of the PIM-1 structures with Zeo++ and the pore accessibility is characterized from the connectivity in the Voronoi network. The porous space within in the network that is characterized as inaccessible is then restricted from MC steps. Pysimm is then utilized to facilitate a MC simulation run followed by a MD simulation in order to account for sorption induced plasticization effects. The rearranged framework is then recharacterized with Zeo++ to assess if new pores have become accessible, and the process repeats iteratively. The effects of incorporating pore connectivity on the predictions from simulation is illuminated by comparing adsorption isotherms against MC/MD simulations without accounting for pore accessibility and standard GCMC simulations.