Environmental effects on mechanochemical activation of spiropyran in linear PMMA

Mechanophore-linked linear poly(methyl methacrylate) (PMMA) was synthesized using bisfunctionalized spiropyran (SP) as an atom transfer radical polymerization initiator. The resulting polymer had a T_g of 127 °C and molecular weight greater than 250 kDa. SP-cleavage experiments confirmed the incorporation of SP molecules approximately into the center of the polymer chain. Force-induced reaction of the SP species into a colored, fluorescent merocyanine (MC) form was investigated over a range of environmental conditions by monitoring color change or full field fluorescence during tensile loading. Activation of this glassy polymer was observed in a temperature range of 90-105 °C. At higher temperatures, approaching T_g, deformation was dominated by viscous flow at very low applied stress with no activation. At lower temperatures, brittle failure preceded activation unless the polymer was plasticized by exposure to methanol. Mechanically induced activation of SP in plasticized PMMA at room temperature (22 °C) was achieved at a critical strain (ca. 10%) substantially below that required in previous systems.