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  1. Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s–1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications. 
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    Free, publicly-accessible full text available April 17, 2025
  2. The mechanical properties of a polymer network reflect the collective behavior of all of the constituent strands within the network. These strands comprise a distribution of states, and a central question is how the deformation and tension experienced by a strand is influenced by strand length. Here, we address this question through the use of mechanophore force probes with discrete molecular weights. Probe strands, each bearing a mechanochromic spiropyran (SP), were prepared through an iterative synthetic strategy, providing uniform PDMS-functionalized SP force probes with molecular weights of 578, 1170, and 2356 g/mol. The probes were each doped (9 mM) into the same silicone elastomer matrix. Upon stretching, the materials change color, consistent with the expected conversion of SP to merocyanine (MC). The critical strain at which measurable mechanochromism is observed is correlated with the strain hardening of the matrix, but it is independent of the molecular length of the probe strand. When a network with activated strands is relaxed, the color dissipates, and the rate of decoloration varies as a function of the relaxing strain ((ε_r ) ̅); faster decoloration occurs at lower (ε_r ) ̅. The dependence of decoloration rate on (ε_r ) ̅ is taken to reflect the effect of residual tension in the once-activated strands on the reversion reaction of MC to SP, and the effect of that residual tension is indistinguishable across the three molecular lengths examined. The combination of discrete strand synthesis and mechanochromism provides a foundation to further test and develop molecular-based theories of elasticity and fracture in polymer networks. 
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  3. The utility and lifetime of materials made from polymer networks, including hydrogels, depend on their capacity to stretch and resist tearing. In gels and elastomers, those mechanical properties are often limited by the covalent chemical structure of the polymer strands between cross-links, which is typically fixed during the material synthesis. We report polymer networks in which the constituent strands lengthen through force-coupled reactions that are triggered as the strands reach their nominal breaking point. In comparison with networks made from analogous control strands, reactive strand extensions of up to 40% lead to hydrogels that stretch 40 to 50% further and exhibit tear energies that are twice as large. The enhancements are synergistic with those provided by double-network architectures and complement other existing toughening strategies. 
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  4. null (Ed.)