Novel Molecular-Level Insight into the Self-Healing Behavior and Mechanism of Polyurethane-Urea Elastomer Based on a Noncovalent Strategy

Abstract

A larger number of studies successfully prepared various polymer materials with excellent self-healing properties, but the study on the underlying self-healing mechanism remains comparably backward and still unclear. In this study, we prepared a self-healing polyurethane-urea (PUU) elastomer based on noncovalent bonds. Then, a coarse-grained model of PUU was successfully constructed using the iteration Boltzmann inversion (IBI) method. Microphase separation and mechanical properties of PUU were reproduced using this model by coarse-grained molecular dynamics (MD) simulation. The three-stage healing mechanism comprised the following: (1) movement of the material to close the gap, (2) interdiffusion of the polymer, and (3) bond exchange. The mechanism was revealed by determining the effects of hard segment content on the microstructure (chain entanglement, interactions of soft and hard segments, chain motility) and healing capacity over healing time. In the initial stage of healing, the polymer chains were disentangled, and the degree of entanglement of the healed samples decreased. A novel experimental strategy confirmed the transition of hydrogen bonds from disorder to order during the healing process. The motility of the cut polymer chains (low molecular weight), especially the cut soft segment, and the disordered hydrogen bonds played a key role in the healing capacity. The increased content of the ordered hydrogen bonds led to the formation of a hard segment network, which was not conducive to healing. Finally, the promoting mechanism of external factors, such as heating and trace amount of solvent, on the healing of PUU was explained. Our work systematically and profoundly reveals the self-healing behavior and mechanism of microphase-separated PUU at the molecular level.