Polymer-metal oxide interfaces in XHNBR/PA6 blends: computational insights toward sustainable crosslinking

Polyamide 6 (PA6) is a high-performance thermoplastic widely used in engineering applications, while carboxylated hydrogenated nitrile rubber (XHNBR) provides viscoelastic damping and reactive carboxyl groups for efficient ionic crosslinking with metal oxides. Their combination enables thermoplastic elastomer systems with improved toughness and vibration attenuation. In this work, the interfacial interactions between PA6, XHNBR, and representative metal oxides (ZnO, MgO, CaO, and MgO₂) were investigated using density functional theory (DFT) combined with a conformational clustering approach. Cluster models of the oxides, (MO)₁₂ and (MO₂)₆, were employed to reproduce local coordination environments and enable detailed electronic characterization through natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses. The results reveal distinct binding preferences among polymer functional groups: carboxyl moieties exhibit the strongest and most complex interactions, involving proton transfer and metal-oxygen coordination, while amine and amide groups form weaker, primarily electrostatic contacts. Among the oxides, CaO produces the most exothermic and predominantly ionic interactions, making it a promising, less toxic alternative to ZnO for crosslinking applications. To account for conformational flexibility, a clustering-based sampling strategy was applied to PA6-XHNBR dimers, allowing exploration of the configurational landscape and evaluation of Boltzmann-weighted interaction energies. The analysis demonstrates that conformers with significant population weights govern the effective interfacial stabilization, underscoring the importance of conformational diversity in accurately describing polymer-oxide and polymer-polymer interfaces. These findings provide molecular-level guidelines for designing greener damping materials for automotive and industrial applications.