The effect of polymer chain alignment and relaxation on force-induced chemical reactions in an elastomer

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. Simultaneous measurements of mechanical properties, birefringence and fluorescence images characterize the relationship between stress, polymer chain alignment, and activation of a force-sensitive chemical species centrally bonded into a polymer backbone.