CAREER: Illuminating Interfacial Mechanics: Utilizing Mechanophores to Visualize Mechanical Performance at Soft Matter Interfaces

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The overall performance of polymer matrix composites is directly tied to the strength of the interface. Filled elastomers like the synthetic rubber in car tires rely on the strength of the interface to transfer stress from the softer rubber matrix into the stiffer carbon black particles to enhance their toughness and reduce wear. In fiber reinforced polymer matrix composites (e.g. glass fiber in epoxy), the toughness of the interface again determines the overall mechanical properties of the system. The directly and actively monitored interfacial stress transfer mechanism leads to a greater understanding of the impact of interfacial strength, contact geometry, and materials properties on composite failure. The reduction in failure and potential for stronger composites are critical to US prosperity and continued economic growth. Mechanically responsive fluorescent molecules, termed mechanophores, will be installed at composite interfaces to experimentally observe interfacial stress transfer. Fluorescence from this marker molecule during loading is a direct indicator of the local interfacial state. This Faculty Early Career Development (CAREER) award's research provides a fundamental understanding of the activation of interfacial mechanophores and will inform design principles towards the development of a new class of molecular sensors that can report real time feedback on interfacial damage in polymer matrix composites. When this technology is deployed in the field, service life of wind turbine blades and aerospace components could be extended and catastrophic failure prevented through early detection of interfacial defects and stress concentrations. Mechanophores are a new class of stimuli-responsive materials that undergo a molecular isomerization or structural rearrangement in response to a mechanical stimulus. Here, dimerized anthracene will be covalently bound at the interface of silica and poly(dimethyl siloxane). As interfacial separation occurs, the dimers will separate into two fluorescent anthracene molecules on both sides of the advancing crack. Contact mechanics adhesion experiments coupled with laser scanning confocal microscopy enables in situ monitoring of the interfacial toughness (quantified through vertical load, displacement, and contact area) simultaneously with the fluorescent activation response of the mechanophore. Finite element analysis that utilizes nonlinear cohesive zone models will provide insight into the complex stress state at the silica/siloxane interface. A fundamental understanding of the materials properties and geometric parameters that influence the activation of interfacial damage sensors will be achieved through this research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.