In the world of chemistry, getting polymers (long chains of large molecules) to form mechanical bonds inside their structures has proven extremely challenging. Unlike chemical bonds, which involve the sharing of electrons by atoms or the effects of electrostatic forces among them, mechanical bonds involve molecules physically threading through one another.

Now however, a breakthrough technique at Northwestern University (NU) has overcome the challenge. Researchers there made two-dimensional sheets out of X-shaped monomers, which are the building blocks of polymers. (In chemistry, two-dimensional objects are those consisting of just a single layer of atoms.) The monomers were made up of molecules holding four extended aromatic groups, which gave them their X shape.

Next, they layered these sheets in a crystalline structure and coaxed the ends of all the X's to attach to each other through the introduction of a chemical known as dialkyldichlorosilane. More layers caused more monomers to spread and connect through the lattice, resulting in a series of loops all threaded together in a super-strong web akin to the metal links in chainmail.

The researchers say that the new material has 100 trillion mechanical bonds per every one square centimeter, making it the substance with the highest density of these bonds ever created.

"We made a completely new polymer structure," said Northwestern's William Dichtel, the study's corresponding author. "It's similar to chainmail in that it cannot easily rip because each of the mechanical bonds has a bit of freedom to slide around. If you pull it, it can dissipate the applied force in multiple directions. And if you want to rip it apart, you would have to break it in many, many different places. We are continuing to explore its properties and will probably be studying it for years."