No abstract

Spiropyran (SP) mechanophore was synthesized into the soft or hard phase of segmented polyurethanes (SPU) and used as a molecular probe of force and orientation. Upon either tensile stretching or irradiation with UV light the SP-linked segmented polyurethane (SP-SPU) adopt a deep purple coloration and are fluorescent, demonstrating the force and UV-induced formation of the open merocyanine (MC) form of the mechanophore. Order parameters calculated from the anisotropy of the fluorescence polarization of merocyanine (MC) were used to characterize the orientation in each phase.Exploiting the ability of SP to be force activated, the SP-SPUs were also mechanically activated to track the force and orientation in each domain of segmented polyurethane during uniaxial tensile loading.

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.

Simultaneous measurements of mechanical response, optical birefringence, and fluorescence signal are acquired in situ during tensile testing of a mechanophore‐linked elastomeric polymer. Mechanical stress, deformation, and polymer chain alignment are correlated with force‐induced chemical reaction of the mechanophore. The mechanochemically responsive polymer under investigation is spiropyran‐ (SP‐) linked poly(methyl acrylate) (PMA). Force‐driven conversion (activation) of SP to its merocyanine (MC) form is indicated by the emergence of a fluorescence signal with 532 nm light incident on the sample. Increasing rate of tensile deformation leads to an increase in both stress and SP‐to‐MC conversion, indicating a positive correlation between macroscopic stress and activation. Simultaneously collected birefringence measurements reveal that rapid mechanophore activation occurs when maximum polymer chain alignment is reached. It is found that SP‐to‐MC conversion in PMA requires both a sufficient level of stress and adequate orientation of the polymer chains in the direction of applied force.