Tailoring dynamic mechanism of covalent adaptable polyurea networks by varying the category of isocyanates

Hindered urea bonds (HUBs), a type of dynamic covalent motifs, have attracted much attention due to their facile dynamic features. However, when varying HUBs are synthesized by altering isocyanate compounds the dynamic mechanism of HUBs is still puzzled. To systematically investigate the dynamic mechanism of HUBs, a series of model reactions was performed firstly. It was found that HUBs deriving from the aromatic isocyanate displayed a dissociated mechanism, whereas those synthesized by aliphatic isocyanate presented an associated pathway. Based on the difference in dynamic mechanisms, varying covalent adaptable polyurethane networks (CANs) were synthesized by tuning the type of isocyanate monomers. Owing to the dissociated mechanism, the CANs prepared by aromatic isocyanate exhibited a lower activated energy accompanying with a shorter stress relaxation time, compared with the CANs composed of aliphatic urea dynamic motifs. Furthermore, the CANs consist of aromatic urea moiety exhibited rapidly self-healing properties and temperature dependence of moduli in thermal measurement. The gradual decrease of modulus with temperature further verifies the dissociated mechanism of aromatic urea bonds, which accelerates the self-healing properties of CANs. Based on the dissociated mechanism of aromatic HUBs, PU-M composited of aromatic HUBs enabled be remodelled by 3D-printer and applied for the preparation of dynamic crosslinked elastomers. This work provides a systematic insight into the dynamic mechanism of HUBs and a novel application pathway in fibrous materials.