A mechanically robust but readily repairable polymer

A mechanically robust polymer that can be repaired simply by manually pressing damaged portions together has been developed by researchers at RIKEN and the University of Tokyo¹. This finding demonstrates that healing is not limited just to soft materials, and it may lead to display materials that can be fixed rather than replaced.

Using durable polymers is important for realizing a sustainable society, but it is typically difficult to repair such mechanically robust polymers when they sustain damage. That is because they are usually made of long, entangled polymer chains, which do not readily diffuse to repair fractures unless the polymer is heated or even melted.

Shorter chains that are held together by many hydrogen bonds have been devised, but hydrogen-bonded structures tend to organize into ordered arrays, promoting crystallization and rendering the material brittle.

Now, Takuzo Aida from the RIKEN Center for Emergent Matter Science and his PhD student Yu Yanagisawa at the University of Tokyo, together with their co-workers, have hit on a way to circumvent crystallization by using a polymer made of thiourea units (–NH(C)SNH–), which engage in hydrogen bonding, and ether linkers.

The researchers stumbled on the concept serendipitously while studying another system. “We were synthesizing poly(ether thiourea) as a precursor for a biomolecular glue that is water soluble and adheres tightly to biomacromolecules such as proteins and nucleic acids,” explains Aida.

The team noticed that, in the presence of a solvent, this precursor strongly adhered to lab gloves and other surfaces. “After removing the solvent, we found that fractured surfaces of the dried polymer merged on manual compression, even though it was rigid and not tacky,” explains Aida.

The polymer is mechanically robust and yet can be mended in hours simply by compressing fractured fragments together at room temperature.

The team attributes this unusual combination of properties to the presence of both thiourea and ether moieties. Unlike most hydrogen-bond-forming units, which form linear arrays that lead to problematic crystallization, the thioureas are arranged in less ordered arrays (Fig. 1). At the same time, the ether groups are mixable with the thioureas, so that gentle compression at fractured sites allows hydrogen-bonded polymer chains to slip and interpenetrate, generating the cross-links necessary for repair.

“We hope this study will alter the preconception that only soft materials such as rubber and elastomeric gels are eligible for healing,” says Aida.

The team next intends to work on enhancing the physical properties of their polymer.