Abstract
Using a novel polymerization process, chemical engineers at the Massachusetts Institute of Technology (MIT) have invented a plastic that is twice as strong as steel, and that they believe could one day be used as a building material.
The new material, named 2DPA-1, is light and mouldable like plastic but it has a strength and resistance that the researchers have likened to steel and bulletproof glass.
This new material is a two-dimensional polymer that self-assembles into sheets, unlike all other polymers, which form one-dimensional ‘spaghetti’ like chains. Before now, it had been believed that it was impossible to induce polymers to form 2D sheets.
To understand why scientists thought this, we first need to understand how polymers are formed. Polymers 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 using injection moulding.
For years Polymer scientists have theorised that if polymers could be induced to grow into a two-dimensional sheet, they should form extremely strong, lightweight materials. However, many decades of research 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 this new study, the researchers 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. And It’s this two-dimensional molecular structure that gives the polymer its super-strength. By stacking these disks on top of each other and using hydrogen bonds between the layers to stick them together, the structure becomes very strong and very stable.
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. Because of the material's closed molecular structure compared to other plastics, it is impermeable to water and gases, so it offers an extremely high degree of protection from oxidation, rust, or rot.
Another promising quality of this new material is that it can be easily made in large quantities. The MIT researchers made their 2DPA-1 in beakers, but to make the plastic in large quantities, you would simply need to increase the amount of starting materials, meaning that it should be easy to scale up manufacturing outside of the laboratory. And like other plastics, its manufactured at room temperature, so it doesn't require vast amounts of heat.
The engineers also theorise that it should be recyclable. If it’s used as fibres, it should be able to be re-spun or pulped as with Kevlar fibres, and in other forms it may be able to be chemically recycled as with nylon.
The team of engineers envision the new material being used as a coating to enhance the durability of objects, such as car parts or mobile phones but also believe that due to its strength, it will eventually be used as a structural material.
The team of researchers 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.
