The correlation between polymer architecture and mechanics has long been an interesting subject for both fundamental and applied aspects. Even though macroscopic load-bearing capacity according to polymer structure has been extensively studied, force in a molecular level within the polymer has been limited in research, which hinders fundamental understanding of polymer physics and mechanics. Recently, a molecular level sensor called mechanophore has been introduced, which undergoes a specific chemical transformation in response to applied force. Thus, it will be beneficial for in-depth understanding of polymer mechanics to track the molecular-level force within the polymer through the incorporation of appropriate mechanophore. In this study, mechanophore spiropyran (SP) was covalently incorporated as crosslinks between PMMA backbone segments, and in-situ opto-mechanical set-up was developed to monitor the dependence of mechano-response, i.e. molecular-level force on polymer architecture during mechanical deformation. SP is a mechanophore that undergoes color and fluorescent change, converting into merocyanine (MC) form upon mechanical force. To the best of our knowledge, this is the first systematic in-situ study of tracking molecular-level force depending on crosslinking density using mechanophore-linked polymer. Consequently, SP mechano-response shows three characteristic regimes of initiation, activation, and saturation step, each of which was quantitatively analyzed as summarized in three aspects: (a) as the degree of crosslinking increases, the pathways for force transfer are increased, leading to the occurrence of SP → MC conversion well before yield without requiring sufficient mobility; (b) under the same energy basis, the rate of force transfer shows a linear correlation followed by gradual saturation with increasing crosslinking, suggesting there exists a critical crosslinking density that can no longer improve the rate of force transfer; (c) level of saturation intensity, i.e. the overall amount of spiropyran-to-merocyanine conversion linearly decreased with increasing cross-link density.

Study of molecular-level force transfer and distribution depending on polymer cross-link density via the spiropyran mechanophore

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