Understanding the interactions between PMMA polymer and common industrial gasses: a computational xTB, DFT, SAPT and MD study

In this paper, extended tight binding (xTB), density functional theory (DFT), symmetry-adapted perturbation theory (SAPT) and molecular dynamics (MD) simulations were used to understand the interaction between PMMA polymer and industrial gases (H₂, O₂, N₂ and H₂O). Geometrical pre-optimizations of all structures have been performed using different methods, starting with GFN2-xTB method, followed by re-optimisation with the B3LYP-D3, M06-2X and r²SCAN-3c methods. Binding energies were calculated using a semiempirical GFN2-xTB method, density functional methods B3LYP-D3 and M06-2X and composite methods r²SCAN-3c. These calculations revealed that the binding of H₂O molecules to the PMMA chain is undoubtedly the strongest compared to the other investigated gases in this paper. The strengths of noncovalent interactions explained the obtained trends in binding energies. The SAPT2 calculations have provided insights into the interaction energy and charge transfer components. The PMMA and H₂O systems show the strongest attractive interaction and H₂ and N₂ molecules show much less charge transfer, which leads to weaker interactions. O₂ shows little charge transfer, leading to a moderate interaction strength with PMMA. Using MD simulations, we investigated the behaviour of gas molecules in the presence of the examined polymer, which is important for studying adsorption and solubility within the polymer matrix.