Quantitative Analysis of Spiropyran Mechanophore Activation in Multiple Network Elastomers

Multiple network elastomers (MNE) have been used as model systems to characterize the crack tip behavior in soft tough materials. These materials contain reinforcing swelled network chains that are largely responsible for the high fracture toughness of MNEs. By incorporating a spiropyran mechanophore as a chromatic force sensor in the swelled network, stress and energy density localization around a crack tip can be optically detected and quantified. Also, regions with a history of loading and unloading around a crack tip can be identified based on the reversible changes in absorbance of merocyanine isomers (spiropyran in its activated form) in the loading and unloading states. This allows for a more accurate identification of the onset of crack propagation, which occurs before reaching the peak stress. Based on the experimental results, a perspective can be constructed on how statistical molecular information, provided by the mechano-activation of spiropyran, can inform macroscopic modeling of soft tough materials.