Regiochemical Effects on Mechanophore Activation in Bulk Materials

Mechanochromic force probes, including spiropyran derivatives, have proven to be useful in visualizing the stress/strain distribution and fracture behavior in polymeric materials. Here, we report the macroscopic response of silicone elastomers including cross-links made up of three spiropyran (SP) regioisomers. The SP derivatives SP(o), SP(m), and SP(p) are connected to the network through an identical attachment point on the indoline fragment and regioisomeric attachments ortho, meta, and para to the spirocyclic C-O bond on the benzaldehyde fragment, respectively. The relative colorimetric response of these regioisomers under quasi-static uniaxial tensile load is SP(o) > SP(m) > SP(p), consistent with the expected mechanical sensitivity of the regioisomers obtained from molecular modeling. The extrapolated strain onset for detectable activation of all three regioisomers, however, is indistinguishable and occurs at similar to 90% uniaxial strain. Finally, the ratiometric response of the three isomers is constant across the strains investigated (90-135% uniaxial strain), in contrast to expectations based on simulations of strained intact polymer networks.