Stress Quantification in a Composite Matrix via Mechanophores*

Polymer matrix composites (PMCs) combine the toughness of thermoset polymer with a stiff reinforcement phase which produces a material that is both stronger and tougher. Due to non-uniform straining during mechanical deformation, significant stress concentrations develop within the matrix near the interface. To understand failure mechanisms, developing methods to measure this stress concentration is crucial. Finite element analysis (FEA) is a computational modeling technique that can predict the stresses in PMCs for simple systems but becomes computationally expensive as the model becomes complicated. Mechanophores are a type of stimuli responsive molecules which have a labile bond in the structure, allowing for a change in color, fluorescence, or catalytic abilities after breaking. Nitro spiropyran (SPN) is a common mechanophore that is relatively easy to synthesize. By embedding SPN into a polydimethylsiloxane (PDMS) matrix, we obtain force responsive polymers. Using this SPN PDMS, we fabricated model PMCs using silane functionalized spherical silica particles. By correlating fluorescence intensity data from our micrographs to the maximum principal stress values calculated by FEA, we created a calibration curve that allows us to measure the stresses in a PMC based only on fluorescence intensity. This system was validated, and the versatility of this technique was demonstrated by testing PMCs with differing geometries and complexities which are difficult to model in FEA.