Regulation of mechanical properties and self-healing performance of polyurethane nanocomposites by tuning the contents of free and associated hydrogen bonds

Hydrogen bonds facilitate self-healing of polymers. However, free and associated hydrogen bonds have different effects on mechanical properties and self-healing performance. Here, both unmodified nano-silica (nano-SiO2) and nano-SiO2 modified by 2-ureido-4[1H]-pyrimidinone (UPy) and KH-550 (UK-SiO2) were used to fill polyurethane (PU). Mechanical properties and self-healing properties of the PU nanocomposites can be regulated by changing the ratio of the two kinds of the nano-SiO2. In the PU containing 2 wt.% unmodified nano-SiO2 and 1 wt.% UK-SiO2, the UK-SiO2 particles hinder the agglomeration of the unmodified nano-SiO2 particles, and facilitate the formation of hydrogen bonds between PU chains and the nanofillers. More associated hydrogen bonds exist, resulting in higher mechanical strength but weaker self-healing ability. In the PU containing more UK-SiO2, the formation of hydrogen bonds between the unmodified nano-SiO2 particles and between the unmodified nano-SiO2 particles and PU chains would be restricted by the UK-SiO2 particles, leading to lower mechanical strength but stronger self-healing ability.