Soft, stretchable composite features enhanced thermal stability and conductivity

Image of nanoscopic droplets of liquid metal dispersed in polymer from Carnegie Mellon University
A single liquid metal nanodroplet
grafted with polymer chains. Image courtesy Carnegie Mellon University.

A new technique that uniformly disperses eutectic gallium indium (EGaIn), a metal alloy that is liquid at ambient temperatures, in an elastomer, has resulted in a soft, stretchable, multi-functional composite with a high level of thermal stability and electrical conductivity. The polymer composite has potential applications in soft robotics, self-healing electronics and medical devices, according to a team of polymer chemists and engineers at Carnegie Mellon University (CMU; Pittsburgh).

Conducting research into soft materials for biomedical and other applications, Carnegie Mellon’s Carmel Majidi developed rubber composites seeded with nanoscopic droplets of liquid metal, writes Jocelyn Duffy on the CMU website. The material showed potential, according to Majidi, professor of mechanical engineering at CMU and director of the Soft Machines Lab, but the mixing technique yielded materials with inconsistent results and, thus, varying properties.

Majidi turned to CMU polymer chemist and J.C. Warner University Professor of Natural Sciences Krzysztof Matyjaszewski, who developed atom transfer radical polymerization (ATRP) in 1994. ATRP is described as the first and most robust method of controlled polymerization, allowing monomers to be individually strung together to produce highly tailored polymers with specific properties. The researchers used ATRP to attach monomer brushes to the surface of EGaln droplets. The brushes were able to link together, forming strong bonds to the droplets. As a result, the liquid metal uniformly dispersed throughout the elastomer, resulting in a material with high elasticity and thermal conductivity, reports Duffy.

Matyjaszewski also noted that after polymer grafting, the crystallization temperature of EGaIn was suppressed from 15° to –80°C, extending the droplet’s liquid phase—and its liquid properties—down to very low temperatures.

“We can now suspend liquid metal in virtually any polymer or copolymer in order to tailor their material properties and enhance their performance,” Majidi told Duffy. “This has not been done before. It opens the door to future materials discovery.”

The process could be used to combine different polymers with liquid metals while controlling the properties of the new materials, according to the researchers. With the help of artificial intelligence, they anticipate potentially designing “made-to-order” elastomer composites with tailored properties. Soft robotics, artitificial skin and biocompatible medical devices are among the more immediate applications envisaged by the researchers.

The research was published in the May 20 issue of Nature Nanotechnology. It was funded by the National Science Foundation and the U.S. Air Force Office of Scientific Research.

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