Self-healing material has role as defect-tolerant structural component

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Materials scientists have developed an adaptive material that combines self-healing and reversible self-stiffening properties.

Dubbed SAC (self-adaptive composite), the material from Rice University is made by mixing two polymers and a solvent that evaporates when heated, which leaves a porous mass of sticky spheres. When cracked, the matrix repeatedly heals and returns to its original form after compression.

The labs of Rice materials scientists Pulickel Ajayan and Jun Lou led the study that appears in ACS Applied Materials and Interfaces. They suggested SAC might be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.

Other self-healing materials encapsulate liquid in solid shells that leak their contents when cracked.

“Those are very cool, but we wanted to introduce more flexibility,” said Pei Dong, a postdoctoral researcher who co-led the study with Rice graduate student Alin Cristian Chipara. “We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of flowing everywhere.”

In SAC, spheres of polyvinylidene fluoride (PVDF) encapsulate much of the liquid. The viscous polydimethylsiloxane (PDMS) further coats the entire surface. The spheres are extremely resilient, Lou said in a statement, as their thin shells deform easily. Their liquid contents enhance their viscoelasticity, a measure of their ability to absorb the strain and return to their original state, while the coatings keep the spheres together. The spheres can also slide past each other when compressed, but remain attached.

“The sample doesn’t give you the impression that it contains any liquid,” Lou said. “That’s very different from a gel. This is not really squishy; it’s more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers.”

Ajayan said making SAC is simple, and the process can be tuned to regulate the product’s mechanical behaviour.

“Gels have lots of liquid encapsulated in solids, but they’re too much on the very soft side,” he said. “We wanted something that was mechanically robust as well. What we ended up with is probably an extreme gel in which the liquid phase is only 50% or so.”

The polymer components begin as powder and viscous liquid, said Dong. With the addition of a solvent and controlled heating, the PDMS stabilises into solid spheres that provide the reconfigurable internal structure. In tests, Rice scientists found a maximum of 683% increase in the material’s storage modulus. This is much larger than that reported for solid composites and other materials, they said.

Dong said sample sizes of the putty-like material are limited only by the container they’re made in. “Right now, we’re making it in a 150-milliliter beaker, but it can be scaled up. We have a design for that.”