Credit: © 2009 NPG

Applying mechanical stress to most polymeric materials can lead to unselective breaking of covalent bonds, resulting in damage and potential failure of the material in its application. Mechanosensitive polymers, in contrast, have properties that are favourably altered in response to mechanical stimuli. A few such polymers have been developed, but they respond to mechanical force by non-covalent processes, which may limit the long-term stability of these materials and the extent to which their properties can be modified.

Building on previous work, Nancy Sottos and colleagues at the University of Illinois at Urbana–Champaign have developed1 elastomeric and glassy polymers in which covalent bonds are activated by mechanical force. A mechanically activated chemical group — a mechanophore — was introduced into the polymer chain. Under tensile stress, covalent bond scission in this group resulted in an electrocyclic ring-opening. The colourless spiropyran, used as a mechanophore, undergoes a well-characterized colour change on ring-opening, which allows the reaction to be visualized as plastic deformation occurs.

The position of mechanical stress is therefore visible, so these materials act as molecular force sensors and have potential applications in damage sensing. This strategy could lead to the development of polymers with other desirable properties such as self-healing materials.