MIT researchers have created a two-dimensional polymer that self-assembles into sheets with a yield strength twice that of steel, even though the material only has about one-sixth of its density. Applications include lightweight, durable coatings for car parts or cell phones, or as a building material for bridges or other structures.
Until this breakthrough, scientists believed it was impossible to induce polymers to form 2D sheets, reports Anne Trafton from the MIT News Office. Polymers typically form one-dimensional, spaghetti-like chains, and after decades of research, the scientific consensus was that it would be impossible to maintain a 2D sheet-like structure. One reason is that if just one monomer were to rotate up or down, out of the plane of the growing sheet, the material would begin expanding in three dimensions and the sheet-like structure would be lost, writes Trafton.
In this new study, Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT, and his colleagues invented a polymerization process that uses a melamine compound containing 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.
A molecular hook-up
“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 told Trafton. “This mechanism happens spontaneously in solution, and after we synthesize the material, we can easily spin-coat thin films that are extraordinarily strong.” The researchers demonstrated that they could coat surfaces with films of the material, which they call 2DPA-1.
The new material’s elastic modulus is between four and six times greater than that of bulletproof glass, according to the researchers, and its yield strength is twice that of steel, even though the material has only about one-sixth the density of steel.
“An important aspect of these new polymers is that they are readily processable in solution, which will facilitate numerous new applications where a high strength-to-weight ratio is important, such as new composite or diffusion barrier materials,” commented Matthew Tirrell, Dean of the Pritzker School of Molecular Engineering at the University of Chicago. He was not involved in the study.
Superior barrier properties
Because it is composed of monomers that lock together like Legos, leaving no gaps, 2DPA-1 is impermeable to gases. “This could allow us to create ultrathin coatings that can completely prevent water or gases from getting through,” said Strano. “This kind of barrier coating could be used to protect metal in cars and other vehicles, or steel structures.”
A paper describing the research was published in Nature on Feb. 2. Strano is the senior author and MIT postdoc Yuwen Zeng is the lead author.
The researchers have filed for two patents on the process used to generate the material.
They are now studying how this particular polymer is able to form 2D sheets, and experimenting with changing its molecular makeup to create other types of novel materials, according to the article in the MIT newsroom.
The research was funded by the Center for Enhanced Nanofluidic Transport (CENT), an Energy Frontier Research Center sponsored by the US Department of Energy Office of Science, and the Army Research Laboratory.