Mechanochromism and Strain-Induced Crystallization in Thiol-yne-derived Stereoelastomers.

Most elastomers undergo strain-induced crystallization (SIC) under tension; as individual chains are held rigidly in a fixed position by an applied strain, their alignment along the strain field results in a shift from strain-hardening (SH) to SIC. A similar degree of stretching is associated with the tension necessary to accelerate mechanically-coupled, covalent chemical responses of mechanophores in overstretched chains, raising the possibility of an interplay between the macroscopic response of SIC, and the molecular response of mechanophore activation. Here we report thiol-yne-derived stereoelastomers, doped covalently with a dipropiolate-derivatized spiropyran (SP) mechanophore (0.25-0.38 mol%). Material properties of SP-containing films are consistent with undoped controls, indicating that the SP behaves as a reporter of the mechanical state of the polymer film. Uniaxial tensile tests reveal correlations between mechanochromism and SIC, which are strain rate-dependent. Potential consequences of the correlations between mechanochromism and SIC include the possibility that mechanochromophores may provide a pathway to bridge these phenomena, since molecular-level mechanochemical activation results in a macroscopic color change in the bulk material. When mechanochromic films are stretched slowly to the point of mechanophore activation, the covalently tethered mechanophore remains trapped in a force-activated state, even after the applied stress is removed. Additionally, the kinetics of mechanophore reversion correlate with the applied strain rate, resulting in highly tunable decoloration rates. Because these polymers are not covalently cross-linked, they can be recycled by melt-pressing into new films, which increases their potential range of strain-sensing, morphology-sensing, and shape-memory applications. This article is protected by copyright. All rights reserved.