Molecular Mimicry: Plastic, Steel Line Up Like Kin

A marshmallow and coffee stirrer model a crystal structure

hide captionTasty Science: One of the first models produced by a research team to model the molecular structure was built out of simple marshmallows and coffee stirrers. The method, researcher Frank Bates says, is "very low tech as far as the visualization goes."

Courtesy of Frank Bates

Frank Bates didn't start out expecting to find a fundamental property of nature, but that's what he may have done.

Bates is a chemical engineer and materials scientist at the University of Minnesota. He makes block copolymers — a kind of plastic. Copolymer means they're made from two different plastics, the end product having the best properties of both.

"We had synthesized these molecules and wanted to know how they were organized down at the molecule scale," says Bates.

So he and his students went to the Argonne National Laboratory in Illinois, where there is a particularly powerful X-ray source, and shined a beam of X-rays on the block copolymers.

"We found something that was simply inexplicable," he says. They had a shape that no one had ever seen in a plastic before. Part of the explanation seems to be related to what the plastic molecule is made from.

"There's one part of the molecule that doesn't like the other part," Bates says. So the one part of the molecule folds up into a sphere, forcing the other part to the outside, where it makes little tendrils. It resembles a hairy sphere.

The X-ray pictures of the particular block copolymers Bates was studying — one polymer containing isoprene and lactide; and another polymer containing styrene, isoprene and ethylene oxide — suggested something unusual was going on.

"There's some new way of arranging our spherical, our hairy spherical particles," Bates says.

Model of a hairy spherical block copolymer particle i i

hide captionBates and his research team used pingpong balls and red pipe cleaners to model the "hairy spherical block copolymer particles." The actual spheres are roughly 1 million times smaller, and each contains about 200 polymer molecules, represented by the red pipe cleaners.

Courtesy of Frank Bates
Model of a hairy spherical block copolymer particle

Bates and his research team used pingpong balls and red pipe cleaners to model the "hairy spherical block copolymer particles." The actual spheres are roughly 1 million times smaller, and each contains about 200 polymer molecules, represented by the red pipe cleaners.

Courtesy of Frank Bates

To make models of how these spherical hair particles fit together, Bates used some surprising tools: pingpong balls.

"Pingpong balls with the pipe cleaners are festooned around the pingpong ball, which create this image of a soft, squishy, hairy sphere," he says. "Our other favorite tools are marshmallows and toothpicks and coffee stirrers. Very low tech as far as the visualization goes."

And it turns out, as the researchers report in the journal Science, the way Bates' spheres fit together resembled nothing so much as something called the Frank-Kasper sigma phase. That's the name for the way the spherical balls pack together. The Frank-Kasper sigma phase has never been seen in plastics, but it has been seen in some metal alloys like stainless steel and certain forms of uranium.

So how is it that a plastic resembles uranium? Bates has an answer.

"I think what this work does is that it makes contact with something that is more universal than just plastics. It has the capacity to teach us how nature works," he says.

And understanding how nature works is something all scientists strive for.

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