Effect of Mechanical Stress on Spiropyran-Merocyanine Reaction Kinetics in a Thermoplastic Polymer

Mechanical force alters the potential energy surface of a mechanophore reaction by modifying the activation energy for conversion. The effects of force on the rate constants and activation energies are not well characterized for mechanophores in bulk polymers. In this work, spiropyran-linked polyurethanes are synthesized and the kinetics of the spiropyran-merocyanine transition in the bulk polymer measured under different values of a macroscopic tensile stress. Above a critical threshold stress, the forward rate constant (spiropyran to merocyanine transition) increases, while the reverse rate constant (merocyanine to spiropyran transition) decreases with applied stress. A tensile stress of 50 MPa enhances the forward rate constant by 110% and lowers the forward activation energy by 1.8 kJ/mol compared to the unstressed condition. Also, this same amount of stress reduces the reverse rate constant by 65% and increases the reverse activation energy by 2.5 kJ/mol.