Stress-sensing thermoset polymer networks: Via grafted cinnamoyl/cyclobutane mechanophore units in epoxy

The incorporation of mechanophores into networked thermoset polymers, such as epoxy, is notably missing from the mechanochemistry literature, which focuses more on traditional thermoplastic and elastomeric polymers. In this work, we develop novel approaches for direct covalent grafting of photoactive mechanophore units into an epoxy matrix (a two-part network polymer), to create a self-sensing thermoset network nanocomposite, linked by both epoxide and mechanophore bonds. Two routes of grafting mechanophore units into an epoxy system to form a self-sensing nanocomposite were explored, including grafting of the mechanophore precursor molecule cinnamamide to the epoxy resin, with subsequent hardener addition and ultraviolet curing to form the mechanically sensitive cyclobutane rings, and the separate grafting of the solution-made mechanophore di-cinnamamide to the epoxy resin to allow for maximum cyclobutane concentration in the formed nanocomposites. Under a compressive force, the cyclobutane rings in the mechanophore units break, increasing the overall fluorescence, which can then be correlated with the applied stress. The goals of this work included detecting early damage by fluorescence spectroscopy, environmental robustness, and retention of the mechanical and thermal properties of the composite. Overall, there was successful formation of self-sensing nanocomposites and achievement of the early damage detection functionality. This systematic work additionally aims to provide further fundamental understanding of mechanochemistry as a whole.