Effect of diisocyanate structure on steric restructuring of hindered urea bonds for self-healable coating

Abstract

Hindered-urea bonds (HUBs), made of bulky amine and isocyanate, are reversible and the reversibility generally comes from the steric hindrance of the bulky substituents on the amine. In this study, the steric effects of diisocyantes (DIs) on the reversibility of HUBs and HUB-bearing polymers were studied. An acrylic prepolymer bearing tertiary-butyl amine (PtB) was prepared and crosslinked with five different DIs. The reversibility of the HUBs as well as the thermodynamic and self-healing behaviours of the HUB-bearing polymer networks (PtB-DIs) were investigated with respect to the structure of alkyl substituents on DIs. Proton nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and oscillatory rheometry were performed, and it was confirmed that the reversibility of the HUB increases as the order of isocyanate group increases (1° > 2° > 3°). Self-healing behaviours were examined by single scratch tests, and the self-healing efficiency increased in the same order. Blending of two isocyanates, 1,4-bis(isocyanatomethyl)benzene (XDI) and tetramethylxylene diisocyanate (TMXDI), expanded the choices for the mechanical and self-healing properties of the polymers. It was remarkable that the mechanical properties of the HUB-bearing polymers at ambient temperature were more significantly affected by the spacer structure of diisocyanate than the steric hindrance.