Unexpected damage of ‘Black Swan’ discovered in soft matter for the first time

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In new research, scientists at the University of Texas at A&M first revealed a single microscopic defect called a “twin” in a soft-block copolymer using an advanced electron microscopy technique. This defect can be exploited in the future to create materials with new acoustic and photonic properties.

“This defect is similar to a black swan – something special happens that is not typical,” said Dr. Edwin Thomas, a professor in the Department of Materials Science and Engineering. “Although we chose a certain polymer for our study, I think the twin defect will be fairly universal across a multitude of similar soft matter systems, such as oils, surfactants, biological materials, and natural polymers. Therefore, our findings will be valuable for a variety of research. across the field of soft matter. “

The results of the study are detailed in the study Proceedings of the National Academy of Sciences (PNAS).

Materials can be broadly classified as hard or soft matter. Hard materials, such as metal alloys and ceramics, generally have a very regular and symmetrical arrangement of atoms. Furthermore, in hard matter, ordered groups of atoms arrange themselves into nanoscopic building blocks, called unit cells. Typically, these unit cells consist of only a few atoms and stack together to form the periodic crystal. Soft matter can also form crystals consisting of unit cells, but now the periodic pattern is not at the atomic level; it occurs on a much larger scale of sets of large molecules.

In particular, for an AB blockchain polymer, a type of soft matter, the periodic molecular motif consists of two linked chains: One chain of A units and one chain of B units. Each chain, called a block, has thousands of units bound together and soft crystal is formed by selective aggregation of the A units into domains and B units into domains that form huge first cells compared to hard matter.

Another notable difference between soft and hard crystals is that structural defects have been much more widely studied in hard matter. These imperfections can occur at a single atomic location within a material, called a point defect. For example, point defects in the periodic arrangement of carbon atoms in a diamond due to nitrogen impurities create the delicate “canary” yellow diamond. In addition, imperfections in crystals can be elongated as a linear defect or spread through an area as a surface defect.

In general defects in hard materials have been extensively investigated by advanced electronic imaging techniques. But in order to be able to locate and identify defects in their block copolymer soft crystals, Thomas and his colleagues used a new technique called slice-scanning electron microscopy. This method allowed the researchers to use a fine ion beam to cut a very thin slice out of the soft material, then they used an electron beam to visualize the surface under the slice, then cut again, visualize again, again. These slices were then digitally stacked together to get a 3D view.

For their analysis, they investigated diblock copolymer made of polystyrene block and polydimethylsiloxane block. At the microscopic level, a unit cell of this material shows a spatial pattern of the so-called “double thyroid,” a complex periodic structure consisting of two intertwined molecular networks, one of which has left-handed rotation and the other right-handed rotation.

While the researchers were not actively looking for any particular defect in the material, the advanced imaging technique discovered a surface defect, called a twin boundary. On either side of the twin junction, the molecular networks suddenly transformed their hand.

“I like to call this defect a topological mirror, and it’s a really neat effect,” Thomas said. “When you have a twin border, it’s like looking at a mirror image in a mirror, because every network crosses the border, the networks exchange a hand, the right turns left and vice versa.”

The researcher added that the consequences of a twin boundary on a periodic structure that by itself does not have any of its own mirror symmetry could induce new optical and acoustic properties that open new doors in material engineering and technology.

“In biology, we know that even one defect in DNA, a mutation, can cause disease or some other observable change in an organism. In our study, we show one twin defect in double thyroid material,” Thomas said. “Future research will investigate to see if there is anything special about the presence of an isolated mirror plane in a structure that otherwise lacks mirror symmetry.”

Conductive nature in crystal structures revealed at an increase of 10 million times

Additional information:
Xueyan Feng et al, Displaying the double-thyroid twin, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073 / pnas.2018977118

Granted by University of Texas A&M

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