Sub-Tg Self-Healing in Glassy Hyperbranched Polyurethane Governed by Mobile Peripheral Sub-Segment Units

To meet environmental challenges and advance sustainable development, it has become imperative to develop engineering glassy polymers with autonomous and room-temperature self-healing capability. Although several strategies for achieving self-healing in glassy polymers have been reported, the underlying mechanism dominating the self-healing process below glass transition temperature (Tg) remains elusive. Herein, we synthesized hyperbranched polyurethanes (HPUs) to leverage their abundant hydroxyl end groups for constructing a dense hydrogen-bonding network. The HPUs exhibit good self-healing ability below Tg (the self-healing efficiency can reach ~50% and ~90% after 0.5 h and 48 h of self-healing, respectively). Both experimental and simulation results revealed that a large number of end groups and branching units in the HPUs undergo secondary relaxation, and can move locally at temperatures below Tg, which is beneficial for the sub-Tg self-healing. In addition, the HPUs containing lower hydrogen bond contents exhibit stronger sub-segment mobility and higher self-healing efficiency, indicating that the mobility of sub-segmental units plays a more important role in promoting the self-healing of glassy hyperbranched polymers rather than the recombination of hydrogen bond content.