Atomistic modeling framework for a cyclobutane-based mechanophore-embedded nanocomposite for damage precursor detection

Abstract

An atomistic modeling framework is developed to simulate mechanophore activation and evaluate the sensitivity of cyclobutane-based mechanophores. Mechanophores are force-responsive functional units, which when embedded in an epoxy-based thermoset polymer matrix provide self-sensing capability in polymeric composites. In the presence of damage or damage precursors, covalent bond dissociation of the mechanophore generates fluorescence, which is referred to as the mechanophore activation. A Tris-(Cinnamoyloxymethyl)-Ethane (TCE) monomer is used to synthesize the cyclobutane structure through ultra-violet (UV) dimerization; the synthesized cyclobutanes are incorporated into the thermoset polymer matrix. A hybrid molecular dynamics (MD) simulation framework is developed by integrating two force-fields: a classical force-field, Merck Molecular Force Field (MMFF) and a bond-order based force-field, Reactive Force Field (ReaxFF). The hybrid MD methodology enables construction of the molecular model of the cyclobutane-based mechanophore embedded nanocomposite and simulation of the mechanophore activation. The synthesis of epoxy network and cyclobutane structure is numerically simulated by a covalent bond generation method employing MMFF. Through this numerical synthesis, the physical entanglement between the epoxy network and cyclobutane chain is also captured, which determines the local force distribution within the novel nanocomposite. Covalent bond dissociation due to the applied local force on the mechanophore is simulated using ReaxFF and results from the virtual deformation tests show successful mechanophore activation. A local work analysis method is developed to evaluate the sensitivity of mechanophore. Results from the simulation framework show increment in the number of activated cyclobutanes during the deformation test. Good agreement is observed with experimental results: the intensity of fluorescence was found to be directly proportional to the deformation.