A team of chemical engineers at MIT have used a novel polymerisation process to create a new material that’s stronger than steel and as light as plastic, and can be easily manufactured in large quantities.
According to Michael Strano, the Carbon P Dubbs professor of chemical engineering, the new material could potentially be used as a lightweight, durable coating for car parts or mobile phones, or as a building material in bridges and other structures.
‘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.’
Polymers, which include all plastics, consist of one-dimensional, spaghetti-like chains of building blocks called monomers. The chains grow by adding new molecules onto their ends. Once formed, the polymers can be shaped into three-dimensional objects, such as water bottles, using injection moulding.
Polymer scientists have long hypothesised that if polymers could be induced to grow into a two-dimensional sheet, the resulting material would be lightweight but extremely strong; however, after decades of work, the general conclusion was that it was impossible to create such sheets.One potential stumbling block was the fact that if just one monomer rotates up or down, out of the plane of the growing sheet, the material will begin to expand in three dimensions and the sheet-like structure will be lost.
In the new study, Strano and his colleagues developed a new polymerisation process that enables the generation of a two-dimensional sheet called a polyaramide. They used a compound called melamine, which contains a ring of carbon and nitrogen atoms, for the monomer building blocks. Under the right conditions, these monomers can grow in two dimensions, forming disks that stack on top of each other. The disks are held together by hydrogen bonds, 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,’ said Strano. ‘This mechanism happens spontaneously in solution, and after we synthesise 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.
The researchers found that the new material’s yield strength, a measure of the amount of force it takes to break, is twice that of steel, despite having only about a sixth steel’s density. Its elastic modulus, a measure of how much force it takes to deform the material, is between four and six times greater than that of bulletproof glass.
Another key feature of 2DPA-1 is that, unlike other polymers, which are made from coiled chains with gaps that allow gases to seep through, the new material is made from monomers that lock together like Lego brinks, making it impermeable to gases.
‘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 experimenting with changing the material’s molecular makeup in the hope of creating other types of novel materials.
The research has been published in Nature.