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

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. • An integrated MD–DFT computational approach elucidates how hydration affects the structural and energetic properties of PA-6, offering insights into its performance in humid environments and paving the way for experimental verification. • A molecular-level description indicates that at higher water contents, hydrogen bonding is weakened, leading to decreased interaction energy and adhesion, and promoting the formation of water aggregates due to the amphiphilic nature of PA-6. This is confirmed by FTIR spectra simulations.