Unexpected “Black Swan” defect first discovered in soft matter

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In a new study, scientists at Texas A & M University used advanced electron microscopy technology to uncover for the first time a single microscopic defect called the “twin” of soft block copolymers. This defect can be exploited in the future to create materials with new acoustic and photonic properties.

“This flaw is like a black swan. This is something unusual and special,” said Dr. Edwin Thomas of the Faculty of Materials Science and Engineering. “We have selected specific polymers for our research, but twin crystal defects are fairly universal across a bunch of similar soft matter systems, such as oils, surfactants, biomaterials, and natural polymers. I think, therefore, our findings are valuable for diverse research throughout the soft matter field. “

The results of the study are Minutes of the National Academy of Sciences (((PNAS).

Materials can be roughly divided into hard matter and soft matter. Hard materials such as metal alloys and ceramics generally have a very regular and symmetrical atomic arrangement. Moreover, in a difficult problem, ordered groups of atoms are placed in nanoscale building blocks called unit cells. Normally, these unit cells consist of only a few atoms and stack together to form periodic crystals. Soft matter can also form crystals of unit cells, but the periodic pattern is not at the atomic level. It emanates on a much larger scale from a collection of large molecules.

In particular, in the case of AB diblock copolymer, which is a type of soft matter, the periodic molecular motif consists of two linked chains, one chain of A unit and one chain of B unit. Thousands of units are linked to each chain called a block, and soft crystals are formed by selectively aggregating A unit into a domain and B unit into a domain, which is a huge unit compared to a hard substance. Form a cell.

Another notable difference between soft and hard crystals is that structural defects have been studied much more extensively in hard matter. These defects can occur at a single atomic position in the material, called point defects. For example, point defects in the periodic placement of diamond carbon atoms due to nitrogen impurities create an exquisite “canary” yellow diamond. In addition, crystal defects can be stretched as line defects or spread throughout the region as surface defects.

In general, defects in hard materials have been extensively investigated using advanced electronic imaging techniques. However, to help find and identify defects in the soft crystals of block copolymers, Thomas and his colleagues used a new technique called slice-and-view scanning electron microscopy. In this way, researchers use a thin ion beam to cut very thin slices of soft material, then use an electron beam to image the underlying surface of the slice, then slice, and many times. I was able to image it again. We then digitally stacked these slices to get a 3D view.

For their analysis, they investigated diblock copolymers consisting of polystyrene blocks and polydimethylsiloxane blocks. At the microscopic level, the unit cells of this material exhibit a so-called “double gyroid” shaped spatial pattern. This is a complex periodic structure consisting of two intertwined molecular networks, one rotating counterclockwise and the other counterclockwise. Right rotation.

Researchers were not actively looking for specific defects in the material, but advanced imaging techniques revealed surface defects called twin boundaries. On either side of the twin junction, the molecular network suddenly changed their dominant hand.

“I like to call this flaw a topology mirror, and it’s a really nice effect,” said Thomas. “If there is a twin boundary, it’s like looking at the reflection on the mirror. When each network crosses the boundary, the network switches the dominant hand, the right is the left, and vice versa.”

Researchers have found that the result of having twin boundaries in a periodic structure that does not have its own mirror symmetry could induce new optical and acoustic properties that open new doors to materials engineering and technology. I added that there is.

“In biology, we know that even a single defect, mutation in DNA, can cause disease and other observable changes in an organism. In our study, it is double. It shows a single twin defect in the gyroid material, “said Thomas. “Future research will investigate whether there is anything special about the existence of isolated mirrors in the structure, otherwise there is no mirror symmetry.”

Conductivity of crystal structure revealed at 10 million times magnification

For more information:
Xueyan Feng et al, Double Gyroid Twin Visualization, Minutes of the National Academy of Sciences (2021). DOI: 10.1073 / pnas.2018977118

Courtesy of Texas A & M University

Quote: The first unexpected “black swan” defect discovered in soft matter (May 19, 2021) is https: // Obtained from May 19, 2021.

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Unexpected “Black Swan” defect first discovered in soft matter

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