New lightweight material is stronger than steel
CAMBRIDGE, MA – Using a new polymerization process, chemical engineers at MIT have created a new material which 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 believed that 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, Carbon P. Dubbs professor of chemical engineering at MIT and lead author of the new study.
“We don’t usually think of plastics as something 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 are very excited about it.”
The researchers have filed two patents on the process they used to generate the material, which they describe in an article that appeared today in Nature. MIT post-doctoral fellow Yuwen Zeng is the lead author of the study.
Polymers, which include all plastics, are made up of chains of building blocks called monomers. These chains grow by adding new molecules to 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 made to expand into a two-dimensional sheet, they should form extremely strong and lightweight materials. However, many decades of work in this area have led to the conclusion that it is impossible to create such sheets. One reason for this is that if a single monomer rotates up or down, out of the plane of the growing sheet, the material will begin to expand three-dimensionally and the sheet-like structure will be lost.
However, in the new study, Strano and his colleagues proposed a new polymerization process that allows them to generate a two-dimensional sheet called polyaramid. For monomeric 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 discs. These disks stack on top of each other, held together by hydrogen bonds between the layers, making the structure very stable and strong.
“Instead of creating a spaghetti-shaped molecule, we can create a sheet-like molecular plane, where we get the molecules to hook together in two dimensions,” Strano explains. “This mechanism occurs spontaneously in solution, and after synthesizing the material, we can easily spin coat extraordinarily strong thin films.”
Because the material self-assembles in solution, it can be made in large quantities by simply increasing the amount of raw materials. The researchers showed they could coat surfaces with films of the material, which they call 2DPA-1.
“With this breakthrough, we have planar molecules that will be much easier to shape into a very strong but extremely thin material,” Strano says.
Light but strong
The researchers found that the new material’s modulus of elasticity – a measure of the force required to deform a material – is between four and six times greater than that of bulletproof glass. They also discovered that its yield strength, or the force needed to break the material, is twice that of steel, even though the material has only about one-sixth the density of steel.
Another key feature of 2DPA-1 is that it is gas impermeable. 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 can’t come between them. .
“This could allow us to create ultra-thin coatings that can completely block water or gases from passing through,” Strano says. “This type 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 altering its molecular composition to create other types of new materials.
– This press release was originally posted on the Massachusetts Institute of Technology website