This shows the ordered structure of the high-quality COFs developed by the research team at Northwestern University. Image: Northwestern University.
This shows the ordered structure of the high-quality COFs developed by the research team at Northwestern University. Image: Northwestern University.

Synthetic polymers are ubiquitous – nylon, polyester, Teflon and epoxy, to name just a few – and all comprise long, linear structures that tangle into imprecise structures. Chemists have long dreamed of making alternative polymers with two-dimensional, grid-like structures, but this goal has proven challenging.

The first examples of such structures, now known as covalent organic frameworks (COFs), were discovered in 2005, but their quality has been poor and preparation methods are uncontrolled. Now a research team at Northwestern University has become the first to produce high-quality versions of these materials, demonstrate their superior properties and control their growth. The team reports its advance in a paper in Science.

The researchers developed a two-step synthesis process that produces organic polymers with crystalline, two-dimensional (2D) structures. The precision of the material's structure and the empty spaces its hexagonal pores provide will allow scientists to design new materials with desirable properties.

Even low-quality COFs have shown preliminary promise for use in applications such as water purification, electricity storage, body armor and other tough composite materials. Once developed further, higher-quality samples of these materials will allow these applications to be explored more fully.

"These covalent-organic frameworks fill a century-long gap in polymer science," said William Dichtel, an expert in organic and polymer chemistry at Northwestern University, who led the study. "Most plastics are long, linear structures that tangle up like spaghetti. We have made ordered two-dimensional polymers where the building blocks are arranged in a perfect grid of repeating hexagons. This gives us precise control of the structure and its properties."

The 2D COFs have permanent pores and an extremely high surface area, such that 2g of the material has the same surface area as a football field. Every little hole is the same size and shape and has exactly the same composition.

In the two-step synthesis process, the scientists first grow small particle ‘seeds’ to which they slowly add more of the building blocks, under carefully controlled conditions. The slow addition causes the building blocks to combine with existing seeds rather than create new seeds. The result is larger, high-quality particles made up of large, hexagonal sheets, rather than a bunch of aggregated crystals.

"This is primarily a synthesis paper, but we also measured properties that emerge only in these high-quality samples," Dichtel said. "For example, we show that energy can move throughout the structure after it absorbs light, which may be useful in solar energy conversion."

Once the 2D COFs were grown, fellow chemists Nathan Gianneschi and Lucas Parent carefully studied the particles using an electron microscope. They confirmed that the particles are individual and not aggregated, and are perfectly uniform throughout the entire structure. Next, chemists Lin Chen and Richard Schaller measured how one of the COFs interacts with light. Their studies showed that energy can move through these materials for much longer distances than possible with the previous, low-quality COFs.

"This study has been very gratifying – to successfully grow these materials and begin to see their promise," said Dichtel, who has been studying COFs for a decade. "We think this development will be enabling for the field of polymer science."

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.