Maximizing the symbiosis of static and dynamic bonds in self-healing boronic ester networks

Abstract

Networks that contain boronic ester crosslinks undergo dynamic bond exchange that enables self-healing behavior and reprocessing. However, networks crosslinked exclusively by rapidly exchanging bonds are also susceptible to creep and stress relaxation, which limits many potential materials applications. In this report, we studied the effect of combining various ratios of dynamic boronic ester crosslinks and static (i.e., irreversible) crosslinks in bulk polymeric materials. We also considered different mechanisms of bond exchange by preparing networks that contained free diols to enable crosslink exchange of boronic esters by transesterification. The networks were evaluated in terms of responsiveness to moisture, proclivity towards deformation, and ability to self-heal. The networks containing free diols could be remolded and healed upon heating. By controlling the humidity and temperature of the environment, the dominant boronic ester exchange mechanism could be shifted from hydrolysis/re-esterification to transesterification. Incorporating a fraction of permanent crosslinks yielded networks that maintained their structural integrity yet still underwent good healing and reproducible repair after multiple cut/heal cycles. When both free diols and irreversible crosslinks were incorporated into a single network, shape stability was enhanced, and improved healing was observed when compared to networks that contained either free diol or permanent crosslinks independently.