Polymers react to stress

摘要

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. rnImagine 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. 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 mechano-chromic polymers - materials that change colour on deformation. rnThere 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 delivery. Many such materials have been made, but comparably few polymers have been developed that respond in a useful way to mechanical stress. These include materials that change their absorption or fluorescence colour on deformation, so providing visible warning signs before mechanical failure occurs. The colour changes are caused by several mechanisms, including variations in the molecular interactions between dye molecules integrated into the polymers, or in the molecular conformations of integrated dyes. 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.