Role of Mechanophore Orientation in Mechanochemical Reactions

The orientation of force-sensitive chemical species (mechanophores) in bulk polymers was measured via the anisotropy of fluorescence polarization. Orientation measurements were utilized to investigate the role of mechanophore alignment on mechanically driven chemical reactions. The mechanophore, spiropyran (SP), was covalently bonded into the backbone of poly(methyl acrylate) (PMA) and poly(methyl methacrylate) (PMMA) polymers. Under UV light or tensile force, SP reacts to a merocyanine (MC) form, which exhibits a strong fluorescence, polarized roughly across the long axis of the MC subspecies. An order parameter was calculated, based on the anisotropy of fluorescence polarization, to characterize the orientation of the MC subspecies relative to tensile force. For UV-activated SP-linked PMA samples, the order parameter increased with applied strain, up to an order parameter of approximately 0.5. Significantly higher order parameters were obtained for mechanically activated SP-linked PMA samples, indicating preferential mechanochemical activation of species oriented in the tensile direction. The anisotropy of fluorescence polarization in SP-linked PMMA also provided insight on polymer drawing and polymer relaxation at failure.