Photo-Sensitive Urethane-Acrylate-Based Covalently Adaptable Networks with Enhanced Mechanical Properties

Abstract Photosensitive thermosets are used extensively in applications such as coatings and 3D printing because of their simple processing and superior properties emerging from their cross-linked molecular network structure. Limitations of thermosets are associated with their intrinsic brittleness and lack of reprocessability. While brittleness can be mitigated by tuning the chemical composition and introducing heterogeneity, the lack of reprocessability can only be addressed by incorporating dynamic covalent bonds into their structures. Such bonds may undergo dynamic bond exchange above the topology freezing temperature, Tv, which provides self-healing, reprocessability, and recyclability to these materials. While various bond exchange mechanisms have been explored so far, to our knowledge, UV-curable acrylate-based covalently adaptable networks (CANs) undergoing bond exchanges via urethane linkage chemistry have not been discussed in the literature. In this study, we develop photosensitive CANs which activate self-healing through both associative transcarbamoylation and dissociative urethane exchange. Associative transcarbamoylation requires the presence of OH groups and a Brønsted acid catalyst, while the urethane exchange mechanism could be triggered in the absence of catalyst and hydroxyl groups. For the compositions investigated, strength ranges from 7 to 66 MPa, Young’s modulus ranges from 80 MPa to 2.3 GPa with failure strains between 30% and 7%, and toughness in the range 1 to 3.24 MJ/m3, while Tg ranges from 21 to 87 °C. Cross-link density governs the mechanical behavior and the stress relaxation kinetics associated with dynamic bond exchange, with slower relaxation rate and improved mechanical properties as the cross-link density increases. This study extends the scope of photosensitive acrylates to urethane-based CANs with tunable mechanical properties.