Paul van Gerven
16 February

The elusive two-dimensional polymer proves to be every bit as strong as everyone thought it would be.

By putting a new spin on the polymerization process, MIT scientists have created a plastic that by some measures is twice as strong as steel at about 1/6th the density. And, like all plastics, it’s easy to make large quantities of it.

The material’s unique combination of characteristics is the result of polymerizing into two dimensions, creating discs instead of strands – pizzas instead of spaghetti, if you will. Since the discs hold on to each other pretty tightly, a surprisingly strong plastic is created.

Such a material could be used as a durable coating for car parts or cell phones but also for structural elements of buildings and public works. “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,” says Michael Strano, senior author of the study published in Nature.

Prepositioning

Chemists have long suspected that two-dimensional polymer sheets would make for a very strong material. Nobody has been able to put this to the test, however, since these materials proved extremely hard to make. After various approaches had been tested over the years, many were ready to call it: 2D polymers are impossible to synthesize.

There’s a good reason why that should be. Chemical simulations show that the number of sites from which a polymer keeps growing always scales unfavorably in 2D compared to 3D. As long as growth occurs in-plane, everything is fine, but as soon as a single molecule decides to rotate out-of-plane, the material will start expanding in three dimensions faster than in two and the sheet-like structure is lost. And nowhere is Murphy’s law more true than on a molecular scale: plenty of molecules will decide to rotate.

Unless you pick the right molecules, Strano and co-workers figured. They picked two complementary repeating units – so-called monomers – that are intrinsically flat, which are obviously conducive to forming discs. Next, the chemical bonds that are formed during polymerization are ‘stabilized’ in-plane through an intramolecular energy effect. And finally, the monomers in solution tend to stack on top of each other thanks to weak interactions called hydrogen bonding and Van der Waals attractions. This encourages the 2D growth pathway by stabilizing the disc structures being formed as well as absorbing monomers from solution and prepositioning them for in-plane growth.

“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.

Makeup

The MIT scientists 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. Another interesting feature of 2DPA-1 is that it’s impermeable to gases, thanks to the intermolecular interactions that keep the discs tightly stacked. This property could prove useful to protect steel and delicate electronic materials from water and other atmospheric gasses.

Strano’s group is now studying in more detail how this particular polymer is able to form 2D sheets. They’re also experimenting with changing its molecular makeup to create other types of novel materials.