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Force-induced activation of covalent bonds in mechanoresponsive polymeric materials

Abstract

Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone1,2,3,4,5,6. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure7. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed8,9,10,11,12,13,14; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups—so-called mechanophores—that the directional nature of mechanical forces can selectively break and re-form covalent bonds15,16. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.

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Figure 1: Chemical structures and bulk polymeric samples.
Figure 2: First-principles dynamics and constrained optimization models of mechanical activation.
Figure 3: Mechanochromic response of mechanophore-linked PMA elastomer under tensile loading.
Figure 4: Mechanochromic response of glassy mechanophore cross-linked PMMA beads under diametral compression.

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Acknowledgements

We acknowledge the support of the ARO MURI programme (grant number W911NF-07-1-0409) for this research.

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Correspondence to Nancy R. Sottos.

Supplementary information

Supplementary Information

This file contains Supplementary Methods, Supplementary Figures S1-S5 with Legends and Supplementary Tables S1-S2; a Supplementary Discussion, Supplementary Figures S6-S21 with Legends and Supplementary References. (PDF 5761 kb)

Supplementary Video S1

Video S1 shows mechanophore-linked PMA-1-PMA dog bone specimen loaded in uniaxial tension to failure. (MOV 498 kb)

Supplementary Video S2

Video S2 shows SMD simulations of 1t showing C-O spiro bond rupture at a force of 2.0 nN. (MPG 1313 kb)

Supplementary Video S3

Video S3 shows SMD simulations of 3t showing no bond rupture at a force of 2.0 nN. (MPG 1243 kb)

Supplementary Video S4

Video S4 shows SMD simulations of 1e showing spiro C-O bond rupture at a force of 3.0 nN. (MPG 1671 kb)

Supplementary Video S5

Video S5 shows SMD simulations of 1e showing C-O ester bond rupture at a force of 3.0 nN. (MPG 2836 kb)

Supplementary Video S6

Video S6 shows SMD simulations of 3e showing C-C ester bond rupture at a force of 3.0 nN. (MPG 9689 kb)

Supplementary Video S7

Video S7 shows active PMMA-4 beads under compressive loading. (MOV 8587 kb)

Supplementary Video S8

Video S8 shows control PMMA-5 beads under compressive loading. (MOV 5804 kb)

Supplementary Data

This zipped data file shows XYZ coordinate data for all molecules used in molecular modelling. (ZIP 14 kb)

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Davis, D., Hamilton, A., Yang, J. et al. Force-induced activation of covalent bonds in mechanoresponsive polymeric materials. Nature 459, 68–72 (2009). https://doi.org/10.1038/nature07970

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