An official website of the United States government
Abstract Most elastomers undergo strain‐induced crystallization (SIC) under tension; as individual chains are held rigidly in a fixed position by an applied strain, their alignment along the strain field results in a shift from strain‐hardening (SH) to SIC. A similar degree of stretching is associated with the tension necessary to accelerate mechanically coupled, covalent chemical responses of mechanophores in overstretched chains, raising the possibility of an interplay between the macroscopic response of SIC and the molecular response of mechanophore activation. Here, thiol‐yne‐derived stereoelastomers doped covalently with a dipropiolate‐derivatized spiropyran (SP) mechanophore (0.25–0.38 mol%) are reported. The material properties of SP‐containing films are consistent with undoped controls, indicating that the SP is a reporter of the mechanical state of the polymer. Uniaxial tensile tests reveal correlations between mechanochromism and SIC, which are strain‐rate‐dependent. When mechanochromic films are stretched slowly to the point of mechanophore activation, the covalently tethered mechanophore remains trapped in a force‐activated state, even after the applied stress is removed. Mechanophore reversion kinetics correlate with the applied strain rate, resulting in highly tunable decoloration rates. Because these polymers are not covalently crosslinked, they are recyclable by melt‐pressing into new films, increasing their potential range of strain‐sensing, morphology‐sensing, and shape‐memory applications.
more »
« less
This content will become publicly available on September 29, 2026
Strain-dependent multicolor mechanochromism of 3H-bis-naphthopyran in solid polymeric materials
Multimodal mechanophores are important targets for the design of complex stress-sensing materials due to their multicolor mechanochromic properties, which potentially enable discrete visual outputs under varying levels of stress and/or strain. We have developed a novel 3H-bis-naphthopyran mechanophore that imbues solid polymeric materials with force-dependent colorimetric sensing capabilities. Polydimethylsiloxane (PDMS) elastomers incorporating a 3H-bis-naphthopyran crosslinker were synthesized and deformed under uniaxial tension. The relative distribution of two distinctly colored merocyanine dyes is systematically biased under varying levels of applied stress and/or strain, resulting in the appearance of distinct coloration, which is characterized by pronounced changes in visible absorption spectra. This work demonstrates that judiciously designed bis-naphthopyran mechanophores can function as force sensors that visually report on the magnitude of applied force in elastomeric materials.
more »
« less
- Award ID(s):
- 2145791
- PAR ID:
- 10640508
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Chemical Science
- ISSN:
- 2041-6520
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Multimodal mechanophores that react under mechanical force to produce discrete product states with uniquely coupled absorption properties are interesting targets for the design of force-sensing polymers. Herein, we investigate the reactivity of a 2H-bis-naphthopyran mechanophore that generates thermally persistent mono-merocyanine and bis-merocyanine products upon mechanical activation in solution using ultrasonication, distinct from the thermally reversible products generated photochemically. We demonstrate that a force-mediated ester C(O)–O bond scission reaction following ring opening establishes an intramolecular hydrogen bond, locking one merocyanine subunit in the open form. Model compound studies suggest that this locked subunit confers remarkable thermal stability to bis-merocyanine isomers possessing a trans exocyclic alkene on the other subunit, implicating the formation of an unusual trans merocyanine isomer as the product of mechanochemical activation. Density functional theory calculations unexpectedly predict a thermally reversible retro-cyclization reaction of the bis-merocyanine species that could explain the mechanochemical generation of the unusual trans merocyanine isomer.more » « less
-
Mechanophores (MPs) undergo chemical reactions to become fluorescent in response to a mechanical stimulus that reflects the magnitude and distribution of applied stress. MPs are an emerging technology for self‐reporting damage sensing applications in polymeric materials in the aeronautical, energy generation, and automotive industries. However, quantitative calibration of the MP response to local stresses remains an outstanding challenge. Herein, a method to calibrate the intensity of the MP fluorescent activation (I) with local hydrostatic stresses (σh) is presented. Uniaxial tension is applied to a simple composite comprised of a rigid sphere (silica) embedded in a MP‐functionalized elastomeric matrix (spiropyran (SPN) functionalized polydimethylsiloxane (PDMS)). By monitoring the fluorescence intensity with a confocal microscope while a quasi‐static deformation is applied, in situ observations of MP activation as a function of applied uniaxial strain are obtained. To calculate the associated stress fields, a finite element analysis (FEA) with cohesive zone elements is employed. By comparingσh, calculated through FEA with theIof the PDMS/SPN system, a linear relationship betweenIandσhis directly determined. The technique presented can be employed for many MP‐containing materials systems to calibrateItoσh.more » « less
-
Strain rate of stretch affects crossbridge detachment during relaxation of intact cardiac trabeculaeJohnson, Daniel M (Ed.)Mechanical Control of Relaxation refers to the dependence of myocardial relaxation on the strain rate just prior to relaxation, but the mechanisms of enhanced relaxation are not well characterized. This study aimed to characterize how crossbridge kinetics varied with strain rate and time-to-stretch as the myocardium relaxed in early diastole. Ramp-stretches of varying rates (amplitude = 1% muscle length) were applied to intact rat cardiac trabeculae following a load-clamp at 50% of the maximal developed twitch force, which provides a first-order estimate of ejection and coupling to an afterload. The resultant stress-response was calculated as the difference between the time-dependent stress profile between load-clamped twitches with and without a ramp-stretch. The stress-response exhibited features of the step-stretch response of activated, permeabilized myocardium, such as distortion-dependent peak stress, rapid force decay related to crossbridge detachment, and stress recovery related to crossbridge recruitment. The peak stress was strain rate dependent, but the minimum stress and the time-to-minimum stress values were not. The initial rapid change in the stress-response indicates enhanced crossbridge detachment at higher strain rates during relaxation in intact cardiac trabeculae. Physiologic considerations, such as time-varying calcium, are discussed as potential limitations to fitting these data with traditional distortion-recruitment models of crossbridge activity.more » « less
-
Abstract The field of polymer mechanochemistry has been revolutionized by implementing force-responsive functional groups—mechanophores. The rational design of mechanophores enables the controlled use of force to achieve constructive molecular reactivity and material responses. While a variety of mechanophores have been developed, this Mini Review focuses on cyclobutane, which has brought valuable insights into molecular reactivity and dynamics as well as innovations in materials. We discuss its reactivity and mechanism, dynamics and stereoselectivity, as well as impacts on material properties.more » « less
