Scientists have created a plastic equivalent of steel – with all its strength, but not with its weight. Plastics, which chemists sometimes call polymers, are a class of long-chain molecules consist of short repeating units called monomers. Unlike previous polymers with the same strength, this new material comes exclusively in the form of a membrane. It is also 50 times more airtight than the most impermeable plastic on the market. Another notable aspect of this polymer is that its synthesis is simple. For the process, which can be carried out indoors, requires only inexpensive materials, the polymer can be mass-produced in large sheets with a thickness of only nanometers. The researchers reported their findings in a journal Nature February 2.
The material in question is called polyamide, a threaded network of amide molecular units (amides are chemical groups of nitrogen attached to an oxygen-related carbon atom). This class of polymers includes Kevlar fiber, which is used to make body armor, and refractory fabric Nomex. Like Kevlar, the polyamide molecules in this new material bind to each other through hydrogen bonds along the entire length of their chains, thereby enhancing the overall strength of the material.
“They stick together like Velcro,” says lead author Michael Strano, a chemical engineer at the Massachusetts Institute of Technology. The rupture of the material requires not only the separation of individual molecular filaments, but also the overcoming of broad intermolecular hydrogen bonds that penetrate the entire polymer beam.
Moreover, a new polymer can automatically form into sheets. This makes the material easy to handle, as it can be used to make membranes or applied in the form of thin-film coatings on the surface. Traditional polymers tend to grow in linear chains or branch out and bind repeatedly in three dimensions indiscriminately in either direction. But the polymer Strano grows uniquely in 2D, forming nanolayers.
“Is it possible to polymerize in a leaf? It turns out that in most circumstances you could not come to our work, ”Strana said. “So we found a new mechanism.” In this recent work, his team overcame obstacles to make this two-dimensional polymerization possible.
The reason for the planar structure of polyamide is that polymer synthesis involves a mechanism called the autocatalytic pattern: as polymers elongate and stick together on monomer building blocks, a growing polymer network causes the next monomer to bond only in proper orientation – to provide common two – dimensional structure. The researchers demonstrated that they could easily apply this polymer in solution to the plates to create a laminate an inch wide less than four nanometers thick. This is almost a millionth of the thickness of ordinary office paper.
To quantify the mechanical properties of the polymeric material, the researchers measured the required force to punch holes in the suspended sheets of material with a fine needle. Polyamide has indeed been tougher than conventional polymers such as nylon, the fabric used to make parachutes. It is noteworthy that to break this high-strength polyamide requires twice as much force than to break steel of the same thickness. According to Strano, this substance can be used as a protective coating for metal surfaces, for example, for finishing cars, or as filters for water purification. In the latter function the ideal filter membrane should be thin but strong enough to withstand high pressure without letting small annoying contaminants into our final deliveries – well suited for this polyamide material.
In the future, Strano wants to extend the polymerization method to other polymers besides this Kevlar analogue. “Polymers around us,” he says. “They do everything.” Imagine transforming many different types of polymers, even exotic ones that can conduct electricity or emit light, into thin membranes that can cover a wide variety of surfaces, he adds. “Perhaps because of this new mechanism, other types of polymers are now available,” says Stano.
In a world plagued by plastic, society has reason to worry about another new polymer whose mechanical properties are not at all ordinary, says Strana. The fact that this polyaramide is so durable means that we can potentially replace everyday plastic, from coatings to food packaging and packaging, with less material, which is also more durable. In terms of sustainability, adds Strano, this heavy-duty two-dimensional polymer is a step in the right direction to wean the world off plastics.