Unveiling Force Transmission Pathways in Polymer Networks: Backbone vs Cross-Linker Probed by Spiropyran Molecular Force Sensor

Understanding how mechanical force propagates through polymer networks is critical for enhancing their mechanical resilience and enabling advanced material functions. Here, we report the first direct experimental evidence of how applied mechanical force travels through polymer networks, uncovered through the use of spiropyran (SP) as a molecular-level force probe. Two structurally equivalent lightly cross-linked polymers are synthesized via atom transfer radical polymerization, differing only in the placement of the SP mechanophore─either in the backbone (SPiBB) or the cross-linker (SPiCL)─allowing us to precisely decouple the force pathways within the polymer. Under uniaxial tension, in situ full-field fluorescence imaging reveals initially uniform force distributions, followed by a pronounced divergence at the onset of strain hardening due to chain alignment, with force increasingly funneled through the backbone. These findings uncover a fundamental principle linking macroscopic mechanics to molecular-level force pathways, guiding the design of next-generation adaptive and responsive polymer systems.