The latest polymers are chameleon-like: they change colour on deformation. The transduction mechanism underpinning this effect could be used to make polymers that respond in many other ways to mechanical stress.
Imagine a polymer that could send a warning signal if stressed close to the point of mechanical failure. Or one that actually becomes stronger under load. Or even one that heals itself after being damaged. On page 68 of this issue, Davis et al.1 report a breakthrough that could enable all three possibilities: polymers in which the application of an external force activates preprogrammed chemical reactions, which in turn cause desired responses. The authors used this approach to make mechanochromic polymers — materials that change colour on deformation.
There is currently great interest in polymers whose properties change in response to stimuli such as chemicals, heat, light or electricity, because of their potential use in applications ranging from camouflage systems to artificial muscles to drug delivery2. Many such materials have been made, but comparably few polymers have been developed that respond in a useful way to mechanical stress3. These include materials that change their absorption or fluorescence colour on deformation, so providing visible warning signs before mechanical failure occurs4,5,6. The colour changes are caused by several mechanisms, including variations in the molecular interactions between dye molecules integrated into the polymers4, or in the molecular conformations of integrated dyes5. Alternatively, in photonic-bandgap materials (through which only light of a specific wavelength can propagate), deformation of the sample changes the distance between particles in the material's lattice structure, which in turn changes the wavelength of light that passes through. But these mechanisms are fundamentally different from those of mechanically responsive materials found in nature, essentially all of which translate macroscopic forces into chemical reactions7.
Davis et al.1 set out to emulate nature's approach by making artificial polymers in which mechanical stress provides the activation energy for specific chemical reactions. The mechanochemically reactive unit — the mechanophore — used by the authors is a 'spiropyran' dye (see Fig. 1a on page 69). Spiropyrans change colour when exposed to light or heat, sometimes converting from colourless to highly coloured states, and they are used in applications such as self-darkening spectacles. These optical changes occur because light or heat activates a reaction in which a weak bond in the spiropyran is broken, causing the molecules to rearrange into a coloured product. The colour changes can also be triggered by grinding the compounds8, demonstrating that such compounds have mechanochromic characteristics.
Seeking to make polymers that have mechano-chemical properties, Moore and Sottos (the lead authors of the paper by Davis et al.1) have previously incorporated several mechano-phores — including a spiropyran — into polymers9,10. Assuming that efficient force transfer between the macromolecules and the mechanophore would be essential, they developed several approaches for covalently connecting spiropyrans to polymers. One route involved attaching two chemical groups to a spiropyran core; the groups acted as initiation points from which polymer chains were grown. In this way, the authors prepared a polymer in which each molecule contains a spiropyran at its centre. They then showed that solutions of the polymer change from being colourless to pink on exposure to ultrasound10. This happens because ultrasound-generated shear forces are funnelled through the polymer chains to the spiropyran, which responds by reacting as described previously. Other research groups have also shown that mechanoreceptors in polymers can break if the mechanophores contain sufficiently weak bonds. This was demonstrated recently by the reported ultrasound-induced activation of certain catalysts11, for example.
So far, so good. But in most real-world situations, forces are not applied to polymer solutions by ultrasound; rather, they are applied to solids by stretching or compression. Furthermore, when solid polymers are exposed to excessive stress, the normal mechanochemical response is for the polymer molecules to break at random points along the chain12. A key question therefore remained: if solid spiropyran-containing polymers are placed under stress, does that stress break the mechanophores? Davis et al.1 now show that it does. When they subjected spiropyran-containing polymers to stress, either by stretching a rubbery poly(methyl acrylate) polymer or by compressing a glassy poly(methyl methacrylate) polymer, the colour and fluorescence of the materials changed, indicating that the spiropyrans had reacted (Fig. 1).
Using a combination of experimental and theoretical models, Davis et al. showed that the underlying mechanism of the colour change is undeniably stress-induced reaction of the spiropyran upon irreversible deformation of the material. They further showed that the connectivity between the mechanophore and the polymer is important: placing two polymer chains on opposite sides of the spiropyran allows the maximum transfer of force from the polymer chains, whereas attaching chains to the same side of the spiropyran, or using only one polymer chain, prevents or limits transfer to the desired breaking point. The authors thus report the first set of guiding principles for the design of solid mechanochemical polymers. Nevertheless, further studies are required to work out just how selective the mechanochemical transduction really is. Although the authors' computational studies suggest that chain scission occurs only at the spiropyran, the possibility that rupture also occurs in other parts of the polymer chains cannot be ruled out.
Davis and colleagues' approach to making mechanochromic polymers should be readily adaptable to other polymer systems. But the development of materials that use spiropyran mechanophores as built-in strain sensors will require several practical problems to be solved. For example, the colour change in the polymers described1 doesn't occur only in response to deformation; it also does so in response to heat and light. Furthermore, the ring-opening reaction of spiropyrans is reversed by exposure to light. The resulting loss of colour would clearly be a problem if the polymers are to be used as damage sensors.
The importance of Davis and colleagues' work goes far beyond the reported mechanochromic effect — it may become a milestone in polymer science, because the authors' general design concept can probably be adapted to make a plethora of materials that translate mechanical stress into all kinds of useful chemical reactions. The authors propose1 that polymerization and molecular-crosslinking reactions could be triggered upon deformation. This could be achieved by mechanochemical processes that release reaction initiators or catalysts, and might be used to create self-healing materials. The reverse effect — cutting across molecules at predetermined breaking points — could be used to create polymers that have stress-activated fuses.
Other applications include materials that have stress-controlled drug-release properties, which could be used in tissues that are naturally subjected to mechanical stress; and substrates for cell culture that contain dimples in which cells won't grow, because the properties of those regions were mechano-chemically altered when the dimples were stamped into the substrate. Scientists from many disciplines will be restricted only by their imaginations when it comes to finding ways of using mechanically responsive polymers for their specific needs. And studies of well-defined artificial systems might contribute to our often rather limited understanding of mechanochemical transduction in biological systems.
About this article
Nature Chemistry (2009)