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Polyurethane hot-melt adhesives featured with high adhesion strength and environmental friendliness possess broad applications in many fields. However, their practical application is hindered by existing drawbacks such as low adhesion strength and poor recyclability for thermoplastic polyurethane and reactive polyurethane hot-melt adhesives, respectively. Achieving high adhesion strength and efficient recycling for hot-melt adhesives simultaneously is challenging. Herein, reversible covalent bonds (oxime− carbamate) were introduced to construct a three-dimensional dynamically cross-linked polyurethane system which has the merits of favorably reworkable thermoplastic polyurethane hot-melt adhesives and high adhesive reactive polyurethane hot-melt adhesives to accomplish efficient bonding−debonding conversion on demand. Such a structure provided a high cross-linked covalent network and a fast reversible adhesive property, and hence improved the overall performance of holt-melt adhesives. As a result, the adhesive exhibited an impressive initial and ultimate adhesion strength of 5.96 ± 0.61 MPa (5 min curing) and 9.02 ± 1.36 MPa (10 days curing), respectively. More remarkably, favorable repeated adhesive capability was realized even after it was completely broken four times, while still retaining all above 4.40 ± 0.92 MPa. In addition, this adhesive also possessed excellent solvent resistance and good durability. Our work emphasized the importance of molecular design and a general method for strong and efficient adhesive systems.

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In this work, the layer-by-layer self-assembly technology was used to modify aramid fibers (AFs) to improve the interfacial adhesion to epoxy matrix. By virtue of the facile layer-by-layer self-assembly technique, poly(l-3,4-Dihydroxyphenylalanine) (l-PDOPA) was successfully coated on the surface of AFs, leading to the formation of AFs with controllable layers (nL-AF). Then, a hydroxyl functionalized silane coupling agent (KH550) was grafted on the surface of l-PDOPA coated AFs. The properties such as microstructure and surface morphology of AFs before and after modification were characterized by FTIR, XPS and FE-SEM. The results confirmed that l-PDOPA and KH550 were successfully introduced into the surface of AFs by electrostatic adsorption. The interfacial properties of AFs reinforced epoxy resin composites before and after coating were characterized by interfacial shear strength (IFSS), interlaminar shear strength (ILSS) and FE-SEM, and the results show that the interfacial adhesion properties of the modified fiber/epoxy resin composites were greatly improved.