A first-principles DFT study and molecular dynamics simulation of the mechanism of water adsorption in polyamide-6

Abstract

A computational study combining Molecular Dynamics (MD) and Density Functional Theory (DFT) explored water interactions with polyamide-6 (PA-6), focusing on hydrogen bonding and water aggregation due to the polymer’s amphiphilic nature. Water adsorption affects PA-6’s physical and mechanical properties. MD simulations show strong affinity at low water content, supported by radial distribution functions and hydrogen bond analysis. Free energy and interaction energy peak near 5 wt% water, driven by the electrostatics and bonding with amide C–O groups. At higher water concentrations, a decline in adhesion work is observed, correlating with the enhanced formation of water aggregates in a solvent-saturated environment. The DFT results support these findings. Furthermore, theoretical infrared (IR) spectra confirm that the driving force behind water-PA-6 interactions is the formation of strong intermolecular hydrogen bonds. The comparison of the Raman spectra for water and nitrogen (IR inactive) moieties, confirms significant differences between the two molecules in the adsorption process.