Mechanoresponsive Molecules to Visualize the Stress Field in the Matrix of a Single Fiber Reinforced Polymer Composite

Stress accumulation in the matrix during fracture of fiber-reinforced composite materials is a critical phenomenon that substantially impacts the composite's overall performance. However, the instantaneous nature of the fracture event makes it very difficult to obtain experimental data of the damaged region. Moreover, there is a lack of experimental tools for sensing and quantifying real-time stress distribution within the matrix during fracture. Therefore, a molecular force probe (mechanophore) was employed within the polymer matrix (polydimethylsiloxane) for visualizing the stress field distribution during fracture. The spiropyran mechanophore transitions from a fluorescent inactive state to an active state (merocyanine) via isomerization under the application of force and strain. To simplify the composite model, one single glass fiber was used with the matrix for their single fiber fragmentation test (SFFT). SFFT demonstrates the fundamental failure modes prevalent in traditional fiber-reinforced composites during uniaxial tensile testing along the fiber direction. At the interface, the tensile load transfer from matrix to fiber via shear stress causing fiber fragmentation. Confocal microscopy was used to visualize mechanophore activation and quantifying the fluorescence intensity. As in previous work, the fluorescence intensity was correlated to hydrostatic stress distribution in the matrix. Uniaxial tensile test of dogbone samples showed conical stress fields in the matrix around fragmented fiber. The results indicated that these mechanoresponsive molecules could be a promising tool for visualizing real-time stress distribution and designing high-performance composites.