  Understanding the transmission of nanoscale forces in the pico-to-nanonewton
range is important in polymer physics. While physical approaches have
limitations in analyzing the local force distribution in condensed
environments, chemical analysis using force probes is promising. However, there
are stringent requirements for probing the local forces generated before
structural damage. The magnitude of those forces corresponds to the range below
covalent bond scission (from 200 pN to several nN) and above thermal
fluctuation (several pN). Here, we report a conformationally flexible
dual-fluorescence force probe with a theoretically estimated threshold of
approximately 100 pN. This probe enables ratiometric analysis of the
distribution of local forces in a stretched polymer chain network. Without
changing the intrinsic properties of the polymer, the force distribution was
reversibly monitored in real time. Chemical control of the probe location
demonstrated that the local stress concentration is twice as biased at
crosslinkers than at main chains, particularly in a strain-hardening region.
Due to the high sensitivity, the percentage of stressed force probes was
estimated to be more than 1000 times higher than the activation rate of a
conventional mechanophore.
