Dynamic reversible bonds enable external stress-free two-way shape memory effect of a polymer network and the interrelated intrinsic self-healability of wider crack and recyclability

In this paper, we propose a new method to prepare an external stress free two-way shape memory polymer. The core advance lies in the introduction of alkoxyamine moieties into the main chains of crystalline polycaprolactone (PCL) based crosslinked polyurethanes (PUs). By using the synchronous bond fission/radical recombination habit of C-ON bonds in alkoxyamine in a controlled manner, the stretched PU network undergoes uneven partial stress relaxation depending on the triggering status of C-ON bonds in the specific sites. Accordingly, the greatly relaxed portions tend to maintain the extended status, while the nearly unrelaxed portions tend to recover to their original undeformed shape. These two opposite tendencies successfully create an internal tensional force, which cooperates with the bidirectional deformation induced by melting/recrystallization of the crystalline phase and leads to two-way shape memory effect. Additionally, the dynamic reversibility of C-ON bonds further enables interrelated reprogramming, intrinsic self-healing of wider crack and recycling of the crosslinked PU. Such a diversity of smart stimuli-responsive properties is not available in the case of irreversible polymers.