Mechanochromic Polycarbonate: Seeing Plasticity with Color

We use force-driven chromism to detect plasticity in polycarbonate. We create this functionality by embedding spiropyran, a mechanochromic compound, in the polymer backbone. While there are many examples of force-driven functionality in elastomers, there are few for glassy polymers. In prior experiments, the activation of spiropyran in glassy polymers was limited to tests near Tg, or with an added plasticizer. With glassy polycarbonate, we demonstrate tensile activation at room temperature. Spiropyran activation begins after yield and increases with hardening during plastic flow. For monotonic tensile tests, the activation is inversely related to the strain rate, while the stress is comparable. These results suggest that for glassy polymers, the activation of mechano-responsive compounds is coupled with stress, plastic flow, and time. Understanding how these factors contribute to activation will enable using mechano-responsive polymers for quantitative sensing.