Flexible composite heals itself while staying tough

Scientists at Rice University have developed an adaptive material that exhibits self-healing and reversible self-stiffening properties.

The material, dubbed SAC (self-adaptive composite), consists of sticky, micron-scale rubber balls that form a solid matrix. The researchers say they made SAC by mixing two polymers and a solvent that evaporates when heated, leaving a porous mass of gooey spheres. When cracked, the matrix quickly heals, over and over, returning to its original form after compression.

The labs of Rice materials scientists Pulickel Ajayan and Jun Lou led the study. They suggested SAC may 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 healing contents when cracked. The Rice scientists said they wanted more flexibility and stability.

In SAC, tiny 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, as their thin shells deform easily. Their liquid contents are claimed to 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 also have the freedom to 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."

Ajayan said making SAC is simple, and the process can be tuned - a little more liquid or a little more solid - 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, the scientists said. With the addition of a solvent and controlled heating, the PDMS is claimed to stabilise 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 - a size-independent parameter used to characterise self-stiffening behaviour. This is much larger than that reported for solid composites and other materials, they said.

Tom Austin-Morgan

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