Strong and durable adhesive-free PET–PET bonding assisted by interfacial amide bond formation

Adhesive-free bonding of identical polymer materials remains a key challenge in developing recyclable monomaterial plastics. Poly(ethylene terephthalate) (PET), despite its widespread use and established recycling infrastructure, is typically joined using adhesives or welding processes that compromise recyclability or impose thermal and geometric constraints. Here, we report a simple and robust strategy for strong, durable adhesive-free PET–PET bonding based on interfacial chemical interactions, reproducibly achieving uniform bonding even in a solid–solid configuration. Glycol-modified PET (PETG) substrates were activated by low-pressure plasma to introduce oxygen-containing functional groups, including carboxyl groups, and poly(allylamine) (PAA) was introduced at the interface as a molecular crosslinker. After contacting the PAA-treated and plasma-treated substrates with a small amount of water and applying mild thermal treatment below the heat distortion temperature, strong interfacial bonding was achieved without coupling reagents or bulk adhesives. Lap-shear strengths exceeding 10 MPa were obtained, often causing fracture of the PETG substrates rather than interfacial failure. Reliable bonding was achieved over a small area (0.2 × 0.5 cm2) with good reproducibility. The bonded interfaces showed excellent durability, maintaining high strength after one year at room temperature and after immersion in aqueous sodium dodecyl sulfate solution. Spectroscopic and microscopic analyses suggest that a small fraction of interfacial amide bonds forms, contributing to mechanical robustness and chemical durability, while the interfacial layer remains ultrathin and distinct from bulk adhesive layers. This minimalistic, reagent-free process provides a practical pathway toward recyclable monomaterial PET assemblies and provides a practical framework for extending interfacial covalent bonding strategies to other polymer systems.