Self-Healing Mechanism of Microcracks on Waterborne Polyurethane with Tunable Disulfide Bond Contents

Self-healing mechanism of microcracks on waterborne polyurethane (WPU) with varied disulfide bond contents was investigated thoroughly in this paper. The self-healing process could be observed by an optical microscope, which was divided into two steps. The first step was the cohesive healing movement, closely related with microphase separations and further characterized by the Fourier transform infrared peak fitting, differential scanning calorimetry, and dynamic mechanical thermal analysis. The second step of the exchange reactions of disulfide bonds could be confirmed by the in situ Raman and tensile mechanics. In addition, the cohesive healing behavior was quantitatively described by the kinetic method, and the exchange reaction active energies of disulfide bonds were also calculated to be 20.42 kJ/mol by gel permeation chromatography tests. The best self-healing performance of SHWPUs was WPU-8, and it could be healed at 75 °C in 15 min with 100% self-healing efficiency. Combining the self-healing process observation with the kinetic analysis can provide deep insights into the self-healing mechanism of microcracks.