Enhancement of Mechano-Sensitivity for Spiropyran-Linked Poly(dimethylsiloxane) via Solvent Swelling

Abstract

Spiropyran (SP) is a mechanophore that undergoes changes in chemical structure and thus in optical character when subjected to a force. Incorporation of SP into polymer chains enables optical detection of stress/strain and damage in the material under load. However, the mechano-sensitivity of SP-linked polymers is in general insufficient for many applications. Attempts have been made to enhance the mechano-sensitivity of SP-linked polymers via modification of the chemical structure of the SP mechanophore or the mesostructure of the polymer matrix. In the present study, we explore how the mechano-sensitivity of SP incorporated in a poly(dimethylsiloxane) (PDMS) network is influenced by the pre-swelling of PDMS with an organic solvent (xylene). The effect of PDMS swelling on the optical property of the mechanophore was investigated by measuring the fluorescence intensity from the SP-linked PDMS in situ during uniaxial deformation using a custom-built opto-mechanical measurement setup. The results suggest that a longer swelling time causes a decrease in the initial fluorescence, a decrease in the activation onset strain, and an increase in the activation slope, resulting in enhanced apparent mechano-sensitivity. For instance, the 15 min pre-swollen SP-linked PDMS showed an 8.3-fold increase in mechano-sensitivity relative to the unswollen reference. Such effect is not limited to uniaxial extension but also observed in other modes of deformation, such as compression and bending. When corrected against the variation in sample thickness during uniaxial extension, and pre-strain and initial fluorescence intensity due to solvent swelling, the SP fluorescence intensity vs true strain (or chain entropy) curves obtained at various degrees of initial swelling collapse into a single curve, which suggests that the swellingenhanced mechano-sensitivity is due to the pre-strain of the network and a decrease in initial fluorescence intensity caused by the pre-swelling.