Predicting how dual gyroid networks form — ScienceDaily

Polymer scientists from the University of Massachusetts Amherst recently announced in the journal Nature Communication that they have solved a long-standing mystery surrounding a nanoscale structure, formed by collections of molecules, called a double gyroid. This shape is one of the most desirable for materials scientists and has a wide range of applications; but, until now, a predictable understanding of how these shapes form has eluded researchers.

“There’s a nice interplay between pure math and materials science,” says Greg Grason, lead author of the paper and professor of polymer science and engineering at UMass Amherst. “Our work investigates how materials self-assemble into natural forms.”

These forms can take several forms. They can be simple, like a layer, a cylinder or a sphere. “Kind of like soap films,” adds Michael Dimitriyev, postdoctoral researcher in polymer science and engineering at UMass Amherst, and one of the paper’s co-authors. “There is an intuitive understanding of the shapes that molecules, such as those in soap, can create. What we’ve done is reveal the hidden geometry that allows polymers to take the shape of a double gyroid.”

What does a double gyroid look like? It’s not intuitive. “They’re something between a layer and a cylinder,” says Abhiram Reddy, a postdoctoral researcher at Northwestern who completed this research as part of his graduate studies at UMass Amherst and lead author of the paper. In other words, imagine a flat piece of mosquito netting – a diaper – and then twist it into a saddle-shaped diaper that fits into a cubic box so that its surface area remains as small as possible. It’s a gyroid. A double gyroid occurs when a second material, also twisted into a gyroid, fills in the gaps of the first gyroid. Each gyroidal material forms a network of interpenetrating tubes. Together, they form an extremely complex material that is both symmetrical on all sides, like many crystals, but traversed by labyrinthine channels, each formed of different molecular units. Because this material is a hybrid of two gyroids, it can be engineered to have conflicting properties.

These double gyroids exist in nature and have been observed for a long time, but until now no one has fully understood how chain molecules, called block copolymers, know how to form double gyroids. Reddy and his co-authors built on a previous theoretical model, adding a heavy dose of thermodynamics and a new approach to thinking about the problem of packaging – or how best to fill a finished container with a material – borrowed to computational geometry and known as the medial map. Since copolymers must stretch to occupy every part of the self-assembled structure, understanding this formation requires knowing how molecules “meet the middle” of shapes, like gyroids, which are far more complex than spheres and the cylinders. The team’s updated theoretical model not only explains the puzzling formation of double gyroids, but holds promise for understanding how the packing problem works in a much wider range of self-assembled superstructures, such as double diamonds and primitive doubles, or even structures that are yet to be discovered.

The researchers, who were funded by the US Department of Energy, then plan to collaborate with synthetic chemists to begin refining their theory with experimental data. The end goal is to be able to design a wide variety of materials, which take advantage of the dual gyroid structure and can help advance a wide range of technologies, from rechargeable batteries to light-reflecting coatings.

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Material provided by University of Massachusetts at Amherst. Note: Content may be edited for style and length.

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