A Methodology for Calibrating Mechanophore Activation Intensity to Applied Stress

Mechanophores (MP) are an emerging technology for self-reporting damage sensing applications in polymeric materials in the aeronautical, energy generation, and automotive industries. For some types of MP molecules, a mechanical stimulus leads to isomerization that induces or “activates” a fluorescence response. To date, semi-quantitative studies have shown that the fluorescent intensity increases with applied force, but systematic calibration of the MP response to local stresses remains a current challenge. Here, a methodology is presented to calibrate experimentally determined MP fluorescent activation intensities (I) with local hydrostatic stress (σ) determined through implementing finite element analysis (FEA) using an axisymmetric model composite comprised of a rigid spherical inclusion embedded in a MP-functionalized elastomeric matrix. The experimental approach employs a glass particle in a polydimethylsiloxane (PDMS) matrix with spiropyran (SPN) attached into the PDMS backbone. By modifying the cohesive zone elements and employing a constitutive hyperelastic model, FEA modeled the various stress states for different adhesion levels. The local σ of the PDMS/SPN system calculated through FEA were then compared directly to I to determine the relationship between and I and σ.