Nature459, 68-72 (7 May 2009) | doi:10.1038/nature07970; Received 12 November 2008; Accepted 5 March 2009
Force-induced activation of covalent bonds in mechanoresponsive polymeric materials
Douglas A. Davis1, Andrew Hamilton2, Jinglei Yang3,6, Lee D. Cremar1, Dara Van Gough4, Stephanie L. Potisek1, Mitchell T. Ong1, Paul V. Braun1,3,4, Todd J. Martínez1,3,6, Scott R. White3,5, Jeffrey S. Moore1,3,4 & Nancy R. Sottos3,4
Department of Chemistry,
Department of Mechanical Science and Engineering,
The Beckman Institute,
Department of Materials Science and Engineering,
Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Illinois 61801, USA
Present addresses: School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore (J.Y.); Department of Chemistry, Stanford University, Stanford, California, USA (T.J.M.).
Correspondence to: Nancy R. Sottos3,4 Correspondence and requests for materials should be addressed to N.R.S. (Email: firstname.lastname@example.org).
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.