Advanced Microscopy Reveals Twin Boundary Defect in Soft-Block Copolymer

Using a sophisticated electron microscopy technique, researchers from Texas A&M University have uncovered a single microscopic defect, known as a “twin” in a soft-block copolymer, for the first time.

This microscopic defect could be manipulated in the days to come to produce materials that have unique photonic and acoustic characteristics.

This defect is like a black swan—something special going on that isn’t typical. Although we chose a certain polymer for our study, I think the twin defect will be fairly universal across a bunch of similar soft matter systems, like oils, surfactants, biological materials, and natural polymers. Therefore, our findings will be valuable to diverse research across the soft matter field.

Edwin Thomas, Professor, Department of Materials Science and Engineering, Texas A&M University

The study results have been described in the Proceedings of the National Academy of Sciences (PNAS).

Materials can be widely divided into soft or hard matter. Hard materials, such as ceramics and metal alloys, usually have a highly regular and symmetrical arrangement of atoms. Besides this, ordered groups of atoms in hard matter organize themselves into nanoscopic building blocks, known as unit cells.

These unit cells usually contain just a few atoms and form the periodic crystal by stacking together. Even soft matter can form crystals containing unit cells, but the periodic pattern no longer remains at the atomic level and instead occurs at a relatively bigger scale from large molecular assemblies.

Specifically, in the case of an A-B diblock copolymer—a kind of soft matter—the periodic molecular motif contains a couple of linked chains: that is, a single chain of A units and a single chain of B units.

Each chain, known as a block, has scores of units joined together and a soft crystal simultaneously forms through a selective accumulation of the A units into domains and B units into domains that create large unit cells when compared to hard matter.

Another prominent difference between hard and soft crystals is that structural flaws have been much more elaborately investigated in hard matter. Such flaws can take place at a single atomic region inside the material, known as a point defect.

For instance, point defects seen in the periodic arrangement of carbon atoms in a diamond caused by nitrogen impurities produced the magnificent “canary” yellow diamond. Moreover, crystal imperfections can be distributed across an area as a surface defect or extended as a line defect.

Generally, flaws within hard materials have been broadly studied with the help of sophisticated electron imaging methods. However, to detect and identify flaws in their block copolymer soft crystals, Thomas and his collaborators employed a novel method known as slice-and-view scanning electron microscopy.

Through this technique, the team was able to apply a fine ion beam to trim off an extremely thin slice of the soft material, and subsequently applied an electron beam to image the surface underneath the slice. The researchers repeated this process several times by slicing and imaging again. Such slices were later digitally arranged together to obtain a 3D view.

In their study, the researchers studied a diblock copolymer composed of a polydimethylsiloxane block and a polystyrene block. At the microscopic level, a unit cell of this copolymer has a spatial pattern of the supposed “double gyroid” shape—a complex and periodic structure containing a couple of entwined molecular networks of which one has a right-handed rotation and the other has a left-handed rotation.

Although the team was not intensely searching for any specific flaw in the material, the sophisticated imaging method unexpectedly revealed a surface defect, known as a twin boundary. At both sides of the twin juncture, the intertwined molecular networks suddenly changed their handedness.

I like to call this defect a topological mirror, and it’s a really neat effect. When you have a twin boundary, it’s like looking at a reflection into a mirror, as each network crosses the boundary, the networks switch handedness, right becomes left and vice versa.

Edwin Thomas, Professor, Department of Materials Science and Engineering, Texas A&M University

Professor Thomas further added that the outcomes of having a twin boundary in a periodic structure that itself lacks any intrinsic mirror symmetry could induce new acoustic and optical properties that pave the way to materials engineering and technology.

In biology, we know that even a single defect in DNA, a mutation, can cause a disease or some other observable change in an organism. In our study, we show a single twin defect in a double gyroid material. Future research will explore to see whether there’s something special about the presence of an isolated mirror plane in a structure, which otherwise has no mirror symmetry.

Edwin Thomas, Professor, Department of Materials Science and Engineering, Texas A&M University

Others who contributed to this study include Xueyan Feng from the materials sciences and engineering department and also Mujin Zhuo and Hua Guo from Rice University.

The study was funded by a grant from the National Science Foundation.

Video Credit: Texas A&M University.

Journal Reference:

Feng, X., et al. (2021) Visualizing the double-gyroid twin. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2018977118.