(688b) Stress-Sensing Thermoset Polymer Networks Via Grafted Cinnamoyl Mechanophores in Epoxy

The employment of mechanophores and mechanochemistry in materials has enabled the development of novel force-responsive materials. The term ‘mechanophroe’ denotes a class of moieties which undergo a chemical reaction in response to external stress stimuli. These response signals can vary greatly depending on the specific moiety. Some examples of signals include heat generation, conductivity, magnetism, and fluorescence. Embedding these stress responsive mechanophores into polymer composites opens pathways for damage monitoring in early damage detection. The fluorescent cinnamoyl moiety is a prime candidate due to its mechanophore capabilities and chemical reactivity. Studies exploring the force sensing capabilities of the UV-dimerized cinnamoyl moiety have shown that after severing its cyclobutane bond under application of an external force, the moiety will revert to its initial fluorescent state. This has been experimentally demonstrated in literature through embedding the cinnamoyl group into an epoxy thermoset matrix. However, the covalent grafting of these mechanophores into the backbone of a thermoset is noticeably missing from mechanophore literature. Two specific approaches for bonding the cinnamoyl group have been explored. These include bonding the cinnamoyl group to the epoxide within the resin component, as well as a bonding to an amine-based hardener molecule. The goal of this work is to improve upon the fluorescent response of the cinnamoyl moiety as well as mitigate property loss from mechanophore incorporation over previous particulate approaches. It was found that mechanophore response was indeed capable of stress response after grafting to an epoxy resin molecule. However, mechanophore sensitivity did not significantly improve over previous particle embedding approaches, with detection occurring very close to the yield point of the material. Additionally, the mechanical property loss was still present. The second approach, grafting of the cinnamoyl group to the amine-based hardener component, proved to significantly increase sensitivity of the mechanophore as well as offer room for improvement in mechanical properties. Further modifying of this hardener-based grafting approach shows lowering of the bond association energy for the force-responsive cyclobutane, as well as significant improvement in glass transition temperature, Young’s modulus, and yield strain. Mechanophore activation for these materials occurred much lower than the yield strain, indicating that early damage detection is a possible in cinnamoyl-grafted thermoset epoxies. Overall, these results show that chemical modification and grafting of mechanophores can offer large advantages in sensing and mechanical properties over traditional stress responsive composites through particle embedding. Additionally, this work aims to provide a fundamental understanding of mechanochemistry as a whole.