Credit: © 2009 ACS

Biopolymers can combine various mechanical properties — for example the protein titin, found in skeletal muscles, exhibits strength, toughness and elasticity. Such combinations, however, have proved particularly difficult to reproduce in synthetic polymers because different properties often originate from different molecular mechanisms. Inspired by the modular structure of titin, Zhibin Guan and colleagues at the University of California, Irvine, have investigated ureido-pyrimidone (UPy) dimers as modules, which have been used in previous studies to form polymers that reversibly unfold and refold.

By incorporating the UPy modules into a linear polymer, the researchers prepared1 a stiff, tough material that also exhibited adaptive properties. Strong hydrogen bonds hold the UPy molecules together within a module and alkyl chains also link them together — reformation of the original dimers is therefore favoured. The polymer backbone is a simple hydrocarbon so as to avoid it interacting with UPy.

The stress–strain characteristics showed that the material obtained remained uniform and transparent under deformation. The polymer slowly returned to its original dimensions and properties when stress was released. This self-healing property — slow over time and rapid under heat — was accompanied by shape-memory behaviour. A similar polymer having protecting groups to prevent hydrogen bonding was found to form only a brittle material, suggesting that the reversible forming and reforming of the dimer contributes to the mechanical properties.