Coarse-Grained Modeling of a Metal-Organic Framework/Polymer Composite and Its Gas Adsorption at the Nanoparticle...

Simulations have acted as a cornerstone to understand the MOF/polymer interface structure; however, no molecular-level simulation has yet been performed at the nanoparticle scale. In this work, a hybrid MARTINI/force matching (FM) force field was developed and successfully implemented to model the ZIF-8/PVDF composite at a coarse-grained resolution. Interphase interactions were modeled using FM potentials, which strive to reasonably reproduce the forces from an atomistic benchmark model, while intraphase interactions were modeled using the general-purpose MARTINI potentials. Systems made of a ZIF-8 nanoparticle embedded into a PVDF matrix were considered to evaluate the effect of nanoparticle size and morphology in the polymer structuration and in the CO₂ adsorption. Results show that simulations at the nanoparticle level are crucial for depicting polymer penetration. Notably, the smallest nanoparticle exhibited the least extent of polymer penetration, while the cubic nanoparticle exhibited the highest amount. Polymer conformation and local density values change following the same trend in all ZIF-8/PVDF systems depending on whether the polymer lies inside or outside the nanoparticle domain. All composite models present more significant CO₂ adsorption in the nanoparticle domain than in the PVDF phase, in agreement with experiments. More remarkably, the small rhombic dodecahedron ZIF-8/PVDF system presents a larger equilibrium amount of gas adsorbed at ambient conditions compared to the other two systems, in alignment with the observed polymer penetration trend. On the other hand, comparison of CO₂ adsorption capacities in cubic and rhombic dodecahedron ZIF-8 nanoparticles of similar sizes reveals that the former is more advantageous for CO₂ adsorption.