Episode 19: Polymer grown on liquid metal nanodroplets

Prachi Patel of MRS Bulletin interviews Carmel Majidi of Carnegie Mellon University about utilizing atom transfer radical polymerization to create liquid metal–polymer hybrid materials with high stability, excellent dispersibility, and tunable mechanical and optical properties. Read the article in Nature Nanotechnology.

Transcript
PATEL: Rubbers and plastics have snuck into hundreds of products we use every day. They have excellent mechanical properties for these applications. But they are terrible at transporting heat and electricity. That’s a problem if you are an engineer who’s trying to make soft robots or artificial skin, like Carmel Majidi of Carnegie Mellon University. He is trying to create new kinds of composites by tailoring the electrical and thermal properties of polymers. He and his colleagues recently found a way to do this by filling polymers with nanoscale droplets made of liquid metals.  

MAJIDI: And the liquid metal itself is an alloy of gallium and indium. These are two metals that are solid at room temperature but when you mix them together they form this eutectic this liquid that has certain nice properties. It’s important that they’re liquid because that allows the droplets to deform with the surrounding rubber as the rubber stretches. 

PATEL: Up until now, researchers have tried to make such composites by using mixers that are a bit like kitchen blenders. You essentially throw in the liquid metal alloy along with the molten rubber or plastic. The metal breaks into tiny droplets that disperse through the polymer. But Majidi worked with chemistry professor Krzysztof Matyjaszewski to develop a new technique. They first break up the liquid metal into nanodroplets using high-frequency sound waves. But then, instead of adding the liquid metal to the polymer, they grow the polymer on the tiny metal droplets, like a coating. 

MAJIDI: You basically start with your nanoscale droplets of liquid metal and then pretty much atom by atom or monomer by monomer you grow these polymer chains from the surface of these droplets—almost like the rays of the sun kind of emanating out. We’re working with these you know gallium indium liquid metal alloys. It wasn’t really obvious whether his polymerization technique would apply to this you know very different and somewhat unique class of materials. So he gave it a shot and it turned out it worked you know pretty much the same way it’s worked for a lot of the other types of metal nanoparticles that he’s worked with. We were delighted that it did. 

PATEL: The technique, called atom transfer radical polymerization, or ATRP—gives very fine control on the length of the polymer chains so the droplets get evenly suspended in the composite. Plus, it enables the researchers to suspend these droplets in a much wider range of polymers and materials systems than was previously possible. Until now, the researchers could use commercially available materials like silicone rubbers and polyurethanes. 

MAJIDI: The ability to suspend these liquid metal nanodroplets in virtually any kind of polymer or kind of matrix material opens the door for kind of a wide range of functionalities. A lot of the materials we have been exploring and will continue to explore, they could be used in say wearable or even implantable applications. So we could work with polymers that are biocompatible. We’ve been looking at different types of polyacrylates. Those are materials that are generally kind of popular in engineering. Everything from adhesives to 3D printing. And again using ATRP we’ve been able to show you can suspend liquid metal droplets in those materials. I mean if you just think about just generally where are plastics used and where are rubbers used, there’s a huge spectrum out there.