Revealing the Dependence of Molecular-Level Force Transfer and Distribution on Polymer Cross-Link Density via Mechanophores.

The correlation between polymer architecture and molecular-level forces has long been a challenging research subject. Herein, spiropyran, a mechanophore that exhibits fluorescence change under force, was incorporated as a cross-linker between PMMA backbone segments. Using an in situ opto-mechanical setup to probe the molecular-level forces, the mechano-response of SP-linked PMMA as a function of the cross-link density was monitored during deformation. The dependence of the molecular-level force on cross-link density was quantitatively examined and revealed. First, a higher cross-link density shifted the fluorescence onset, that is, the onset of the spiropyran-to-merocyanine transition, to lower strains, eventually shifting the onset long before yield, without requiring sufficient chain mobility, owing to the higher efficiency of the force transfer. Under the same energy, the increase in cross-link density allowed for faster force transfer, but only to a certain level. Finally, the overall amount of spiropyran-to-merocyanine conversion linearly decreased with increasing cross-link density.