Nanoscratch-healing of single and bilayer thin films of hindered urea bond-based self-healable copolymers

Abstract

In this study, the effects of the surface properties of single and bilayer thin films of hindered urea bond (HUB)-based self-healable copolymers on their scratch-wound formation and self-healing were investigated. Initially, three prepolymers with bulky amine groups, PtB (control), PFtB (hydrophobic), and PMtB (hydrophilic), were synthesised by free-radical polymerisation. Subsequently, they were used to fabricate self-healing copolymers by crosslinking with HDI. The three crosslinked polymers (PtB-H, PFtB-H, and PMtB-H) showed contact angles of 80°, 93°, and 73°, respectively. Moreover, all the single-layer samples showed complete scratch healing within 2 h at ambient temperature, with PFtB-H and PMtB-H showing an initial elastic recovery immediately after scratch formation, unlike PtB-H. The formation of bilayer samples by stepwise coating was confirmed by X-ray photoelectron spectroscopy (XPS). Owing to HUB networks formed on their surface similar to the single-layer, bilayer coatings showed excellent scratch-healing performance at room temperature for nano-scratches with 40–50 nm depth, as indicated by AFM. PMtB-H@PtB-H showed a better scratch-healing performance than PFtB-H@PtB-H, and the scratch-healing ability of PMtB-H@PtB-H was improved by moisture. Their nano-scale self-healing ability was verified by micro-scale scratch tests; scratches with 4–12 µm depth healed completely with ∼100% SHE% at 60 °C.