Revealing the Filler–Matrix Interfacial Interactions: Real-Time Observation with Mechano-Responsive Spiropyran Microbeads

Interfacial interactions between polymers and fillers play a crucial role in determining the performance of composite materials. In this study, mechano-responsive spiropyran (SP) beads, which exhibit fluorescence changes under stress, serve as force probes to evaluate force transfer efficiency across two types of interfaces: noncovalent and covalent. These interfaces are engineered by respectively employing physical blending and grafting polymerization to integrate hydroxyl SP beads with a polyurethane (PU) matrix. A custom-built in situ opto-mechanical setup quantitatively assesses force transfer by monitoring changes in fluorescence intensity and peak wavelength during specimen stretching. The analysis reveals that the covalent interface significantly outperforms the noncovalent interface, demonstrating a 100% improvement in force magnitude and transfer rate from the PU matrix to the SP beads. Direct observation of SP beads within the PU matrix during tension unveils that enhanced force transfer efficiency is closely linked to changes in the SP beads' aspect ratio. Fluorescence changes in SP beads are solely a function of aspect ratio, making them effective independent force probes.