New Material Is Stronger Than Steel and As Light as Plastic
By ANNE TRAFTON, MASSACHUSETTS
INSTITUTE OF TECHNOLOGY
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| Stock video to illustrate the concept of a super strong cell phone. |
The new substance is the result of a feat thought to be
impossible: polymerizing a material in two dimensions.
Using a novel polymerization process, MIT chemical
engineers have created a new material that is stronger than steel and as light
as plastic, and can be easily manufactured in large quantities.
The new material is a two-dimensional polymer that self-assembles
into sheets, unlike all other polymers, which form one-dimensional,
spaghetti-like chains. Until now, scientists had believed it was impossible to
induce polymers to form 2D sheets.
Such a material could be used as a lightweight, durable coating
for car parts or cell phones, or as a building material for bridges or other
structures, says Michael Strano, the Carbon P. Dubbs Professor of Chemical
Engineering at MIT and the senior author of the new study.
“We don’t usually think of plastics as being something that you could use to support a building, but with this material, you can enable new things,” he says. “It has very unusual properties and we’re very excited about that.”
The researchers have filed for two patents on the process they
used to generate the material, which they describe in a paper published
in Nature on February 2, 2022. MIT postdoc Yuwen Zeng
is the lead author of the study.
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| The new material is a two-dimensional polymer that self-assembles into sheets and could be used as a lightweight, durable coating for car parts or cell phones, or as a building material for bridges or other structures. Credit: polymer film courtesy of the researchers; Christine Daniloff, MIT |
Two dimensions
Polymers, which include all plastics, consist of chains of
building blocks called monomers. These chains grow by adding new molecules onto
their ends. Once formed, polymers can be shaped into three-dimensional objects,
such as water bottles, using injection molding.
Polymer scientists have long hypothesized that if polymers could be induced to grow into a two-dimensional sheet, they should form extremely strong, lightweight materials.
However, many decades of work in this field led
to the conclusion that it was impossible to create such sheets. One reason for
this was that if just one monomer rotates up or down, out of the plane of the
growing sheet, the material will begin expanding in three dimensions and the
sheet-like structure will be lost.
However, in the new study, Strano and his colleagues came up with a new polymerization process that allows them to generate a two-dimensional sheet called a polyaramide. For the monomer building blocks, they use a compound called melamine, which contains a ring of carbon and nitrogen atoms.
Under the right conditions, these monomers can grow in two dimensions, forming
disks. These disks stack on top of each other, held together by hydrogen bonds
between the layers, which make the structure very stable and strong.
“Instead of making a spaghetti-like molecule, we can make a
sheet-like molecular plane, where we get molecules to hook themselves together
in two dimensions,” Strano says. “This mechanism happens spontaneously in
solution, and after we synthesize the material, we can easily spin-coat thin
films that are extraordinarily strong.”
Because the material self-assembles in solution, it can be made in
large quantities by simply increasing the quantity of the starting materials.
The researchers showed that they could coat surfaces with films of the
material, which they call 2DPA-1.
“With this advance, we have planar molecules that are going to be
much easier to fashion into a very strong, but extremely thin material,” Strano
says.
Light
but strong
The researchers found that the new material’s elastic modulus — a
measure of how much force it takes to deform a material — is between four and
six times greater than that of bulletproof glass. They also found that its
yield strength, or how much force it takes to break the material, is twice that
of steel, even though the material has only about one-sixth the density of
steel.
Matthew Tirrell, dean of the Pritzker School of Molecular
Engineering at the University of Chicago, says that the
new technique “embodies some very creative chemistry to make these bonded 2D
polymers.”
“An important aspect of these new polymers is that they are
readily processable in solution, which will facilitate numerous new
applications where high strength to weight ratio is important, such as new
composite or diffusion barrier materials,” says Tirrell, who was not involved
in the study.
Another key feature of 2DPA-1 is that it is impermeable to gases.
While other polymers are made from coiled chains with gaps that allow gases to
seep through, the new material is made from monomers that lock together like
LEGOs, and molecules cannot get between them.
“This could allow us to create ultrathin coatings that can
completely prevent water or gases from getting through,” Strano says. “This
kind of barrier coating could be used to protect metal in cars and other
vehicles, or steel structures.”
Strano and his students are now studying in more detail how this
particular polymer is able to form 2D sheets, and they are experimenting with
changing its molecular makeup to create other types of novel materials.
Reference: “Irreversible synthesis of an ultrastrong
two-dimensional polymeric material” by Yuwen Zeng, Pavlo Gordiichuk, Takeo
Ichihara, Ge Zhang, Emil Sandoz-Rosado, Eric D. Wetzel, Jason Tresback, Jing
Yang, Daichi Kozawa, Zhongyue Yang, Matthias Kuehne, Michelle Quien, Zhe Yuan,
Xun Gong, Guangwei He, Daniel James Lundberg, Pingwei Liu, Albert Tianxiang
Liu, Jing Fan Yang, Heather J. Kulik and Michael S. Strano, 2 February 2022,
Nature.
DOI: 10.1038/s41586-021-04296-3
The research was funded by the Center for Enhanced Nanofluidic
Transport (CENT) an Energy Frontier Research Center sponsored by the U.S.
Department of Energy Office of Science, and the Army Research Laboratory.
