Molecular origins of interfacial interaction in immiscible polymer blends: Interdiffusion and association motifs from all-atom molecular dynamics

Interfaces among polypropylene (PP), polystyrene (PS), styrene–acrylonitrile copolymer (SAN), and polyamide 6 (PA6) underpin many industrial blends and advanced nanocomposites, yet a molecular-level description of the interactions governing the phase morphology remains incomplete. In this study, all-atom molecular dynamics was used to resolve interfacial energetics, nanoscale interdiffusion, and atomistic association motifs across five phase-separated binary interfaces constructed from these resins. For each interface, we computed the interfacial interaction energy ( E inter ) and interfacial tension ( γ sim ), quantified the thickness of the compatibilized region ( W inter ), and analyzed cross-radial distribution functions to identify dominant atom-type pairings. γ sim was consistent with experimentally reported trends in interfacial tension and phase-separated morphologies, supporting the qualitative validity of the simulations. W inter correlated positively with E inter , demonstrating the role of mutual chain penetration, while the strength of this correlation varied among systems depending on the presence or absence of local interaction motifs. Cross-radial-distribution-function analysis revealed frequent polar–polar associations at PA6/SAN, particularly between amide and cyano groups (H am –N cy , C am –N cy , O am –C cy ), which rationalize its strong interfacial affinity. SAN/PP exhibits moderate associations between SAN cyano/aromatic sites and PP methyl groups, yielding intermediate adhesion. PS/PP shows nonpolar, non-directional contacts without pronounced localization, yet attains relatively high E inter through efficient mutual interdiffusion rather than specific binding. PA6/PP and PA6/PS display neither pronounced motifs nor extensive interdiffusion, leading to the weakest interfacial affinities. This study presents an atomistic map linking specific interfacial motifs to interfacial energetics, providing molecular-level guidance for designing compatibilization strategies that target functional groups or mobility to control blend morphology and enhance macroscopic performance. • Atomistic molecular dynamics quantifies interfacial energies and structures inaccessible experimentally. • Local association motifs and interdiffusion jointly strengthen polymer–polymer interfaces. • Atomic-level insights enable precise tuning of interfaces to control phase morphology and improve properties.