Damage-Detectable and Self-Healable Photoluminescent Dual Dynamic Supramolecular Networks

The development of self-healing polymers with the ability to identify the location of damage and to nondestructively detect whether the damage is healed is essential for the application of self-healing materials in actual products. Such an ability would allow the healing process to be carried out only on the damaged area and to be monitored in real time, ultimately enhancing the reliability of the self-healing properties of the products. Herein, we synthesized ureidopyrimidinone end-functionalized poly(ε-caprolactone) with pendant pyrene groups (Py-UPCL) and investigated its photoluminescent, mechanical, and rheological properties, in addition to its damage detection and self-healing behaviors. Nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, capillary viscometry, and fluorescence spectroscopy revealed that Py-UPCL possesses a dual dynamic supramolecular network structure derived from UPy quadruple hydrogen bonding and pyrene excimerization. Py-UPCL exhibited superior mechanical properties compared to ureidopyrimidinone end-functionalized poly(ε-caprolactone) without pendant pyrene groups (UPCL) and compared to a simple mixture of UPCL with pyrene (Py/UPCL). This was attributed to the cross-linking structure of Py-UPCL formed through pyrene excimerization. In addition, the Py-UPCL film exhibited a healing efficiency of ∼98.7% under the healing condition of 100 °C for 20 min, and the healed status was maintained without any change in shape. Importantly, upon ultraviolet irradiation, the photoluminescent properties of Py-UPCL allowed the direct detection of microcracks that are not normally visible to the naked eye. Rheometry and fluorescence experiments showed that the UPy quadruple hydrogen bonds and pyrene excimers dissociated at 130 and 140 °C, respectively, thereby accounting for the stable healing behavior of Py-UPCL.