(673a) Atomistic Investigation of Polymer-MOF Interfaces in Mixed Matrix Membranes

Membrane processes are promising alternatives to conventional amine absorption process for CO₂ separation from gas mixtures. While polymers have been preferred materials due to their processability, their separation performance needs to be enchanced in order to make them viable for industrial applications. This has motivated development of Mixed Matrix Membranes (MMM’s) that combines polymer processibility with high separation performance of fillers. Metal Organic Frameworks (MOF’s) are one of the best candidates as filler materials with their excellent gas adsorption properties.

While separation performances of individual components are main concern in MMM’s, it has been proven that compatibility of materials used in MMM’s are as important. Incompatibility between composite materials can result in interfacial voids leading to deviation from ideal separation performance. An interface with molecular scale or sub-molecular scale extra free volume between segments can also occur, and this produces a small decrease in selectivity below that of the pure polymer while still demonstrating an increase in permeability.

Another type of nonideality is matrix rigidification, which results in reduced free volume near the sieve surface. Rigidified matrix surrounding the sieves have lower permeability than that of bulk polymer, resulting in decreased membrane permeability.

In this study, in order to contribute to further understanding of MMM’s in molecular level, a full atomistic study of Matrimid 5218/ZIF-8 MMM was studied. CO₂ adsorption up to 30 bar was performed to observe the effect of CO2 induced swelling on the interface of polymer and filler and to understand the behavior of polymer-MOF interactions with increasing CO₂ concentration.

Quantum-level charge calculations were incorporated with all-atom force fields in order to model polymer and MOF behavior accurately. Subsequent Monte Carlo and Molecular Dynamics Simulations were performed using LAMMPS software. GCMC simulations combined with NPT and NVT ensemble MD simulations were used for simulating sorption behavior of the polymers. Polymers swell with increasing CO₂ concentration and this swelling increases the sorption capacity. To account this swelling, NPT and NVT MD runs after each GCMC run was added for simulation process. Sorption simulations were done at 35 °C in line with most experimental data in literature. A CO₂ specific Free Volume approach was used in order to estimate plasticization behavior of MMM. Interfacial Dynamics of both unplasticized and plasticized structure was investigated for further understanding of compatibility issues in MMM’s.