In new research, Texas A&M University scientists have for the first time
revealed a single microscopic defect called a "twin" in a soft-block copolymer
using an advanced electron microscopy technique. This defect may be exploited
in the future to create materials with novel acoustic and photonic properties.
"This defect is like a black swan—something special going on that isn't
typical," said Dr. Edwin Thomas, 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 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."
The results of the study are detailed in the Proceedings of the National
Academy of Sciences (PNAS).
Materials can be broadly classified as hard or soft matter. Hard materials,
like metal alloys and ceramics, generally have a very regular and symmetric
arrangement of atoms. Further, in hard matter, ordered groups of atoms
arrange themselves into nanoscopic building blocks, called unit cells.
Typically, these unit cells are comprised 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 at a much larger scale from assemblies of large molecules.
In particular, for an A-B diblock copolymer, a type of soft matter, the
periodic molecular motif comprises of two linked chains: One chain of A
units and one chain of B units. Each chain, called a block, has thousands of
units linked together and a soft crystal forms by selective aggregation of
the A units into domains and B units into domains that form huge unit cells
compared to hard matter.
Another notable difference between soft and hard crystals is that structural
defects have been much more extensively studied in hard matter. These
imperfections can occur at a single atomic location within 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 exquisite
"canary" yellow diamond. In addition, imperfections in crystals can be
elongated as a line defect or spread across an area as a surface defect.
By and large, defects within hard materials have been extensively investigated
using advanced electron 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-and-view scanning electron
microscopy. This method allowed the researchers to use a fine ion beam to trim
off a very thin slice of the soft material, then they used an electron beam to
image the surface below the slice, then slice again, image again, over and
over. These slices were then digitally stacked together to get a 3D view.
For their analysis, they investigated a diblock copolymer made of a
polystyrene block and a polydimethylsiloxane block. At the microscopic
level, a unit cell of this material exhibits a spatial pattern of the
so-called "double gyroid" shape, a complex, periodic structure consisting of
two intertwined molecular networks of which one has a left-handed rotation
and the other, a right-handed rotation.
While the researchers were not actively looking for any particular defect in
the material, the advanced imaging technique uncovered a surface defect,
called a twin boundary. At either side of the twin juncture, the molecular
networks abruptly transformed their handedness.
"I like to call this defect a topological mirror, and it's a really neat
effect," said Thomas. "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."
The researcher added that the consequences of having a twin boundary in a
periodic structure that does not by itself have any inherent mirror symmetry
could induce novel optical and acoustic properties that open new doors in
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," said Thomas. "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."
Reference:
Xueyan Feng et al, Visualizing the double-gyroid twin, Proceedings of the
National Academy of Sciences (2021). DOI:
10.1073/pnas.2018977118