Crystallinity-Controlled Reaction Zones and Adhesion at Thermoset/Semicrystalline Polymer Interfaces

Abstract Adhesion between thermosetting and semicrystalline thermoplastic polymers depends not only on interfacial chemistry but also on the spatial distribution of reaction products across the interface, yet how polymer crystallinity controls this distribution remains unclear. Here, we investigate how the crystallinity of polyamide 6 (PA6) controls the development of interfacial reaction zones and adhesion strength at epoxy resin (ER)/PA6 interfaces. PA6 films with different crystallinities were prepared, and their buried interfaces with ER were analyzed using atomic force microscopy, ­infrared spectroscopy, and gas cluster ion beam-assisted X-ray photoelectron spectroscopy. While comparable relative amounts of covalent bonds were detected at the nominal interface, PA6 with low crystallinity exhibited a markedly broader reaction zone due to enhanced penetration of reactive epoxy species into amorphous regions. In contrast, PA6 with high crystallinity confined covalent ester or urethane formation to a narrow interfacial region. Wedge tests revealed substantially higher fracture energy for laminates containing PA6 with low crystallinity, demonstrating that the spatial extent and continuity of the interfacial reaction zone, rather than its interfacial density alone, determine adhesion performance. These findings provide a molecular-level insight into crystallinity-controlled adhesion at thermoset/semicrystalline thermoplastic interfaces.