Researchers Use Electron-Based Imaging Technique to Capture Atomic-Scale Images of Polymers

Many contemporary materials, ranging from food containers and water bottles to tubing and toys, are manufactured from plastics. Every year, approximately 110 million tons of synthetic polymers like polypropylene and polyethylene are produced for these plastic products. Despite these developments, mysteries still surround these polymers at the atomic scale.

Since it is very difficult to capture the images of these materials at very small scales, images of separate atoms in polymers have only been achieved in computer illustrations and simulations, for instance.

Now, researchers headed by Nitash Balsara, a senior faculty scientist in the Materials Sciences Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and professor of chemical and biomolecular engineering at UC Berkeley, have modified a powerful electron-based imaging method to acquire a detailed image of atomic-scale structure in an artificial polymer. The team included scientists from UC Berkeley and Berkeley Lab.

This breakthrough study could eventually inform polymer fabrication techniques and result in the development of novel designs for devices and materials that integrate polymers.

The study has been reported in the American Chemical Society’s Macromolecules journal, in which the team has described the development of a cryogenic electron microscopy imaging method, helped by sorting techniques and computerized simulations, that detected as much as 35 arrangements of crystal structures in a sample of peptoid polymer. The synthetically created molecules—peptoids—imitate biological molecules, such as chains of amino acids called peptides.

At Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility for nanoscience research, the peptoid polymer sample was robotically produced. The scientists formed sheets of crystallized polymers, which when dispersed in water, measured around 5 nm (billionths of a meter) in thickness.

“We conducted our experiments on the most perfect polymer molecules we could make,” stated Balsara. Compared to standard synthetic polymers, the peptoid samples considered in the analysis were extremely pure.

The researchers produced small flakes of peptoid nanosheets and froze them to maintain their structure, and then, with the help of an electron beam, imaged the peptoid nanosheets. When imaging materials with a soft structure, for example, polymers, one inherent challenge is that the samples also get damaged when the electron beam is used for capturing the images.

The direct cryogenic electron microscopy images, achieved with the help of very few electrons to reduce beam damage, are rather blurred to expose individual atoms. The scientists were able to obtain resolution of about 2 angstroms, which is roughly twice the diameter of a hydrogen atom, or two-tenths of nanometer (billionth of a meter).

To achieve this, the researchers took more than 500,000 blurred images, sorted different motifs into different “bins,” and then averaged the images in each bin. The sorting techniques employed by the team were based on algorithms created by the structural biology community for imaging the proteins’ atomic structure.

We took advantage of technology that the protein-imaging folks had developed and extended it to human-made, soft materials. Only when we sorted them and averaged them did that blurriness become clear.

Nitash Balsara, Senior Faculty Scientist, Berkeley Lab; Professor of Chemical and Biomolecular Engineering, UC Berkeley.

Balsara added that before these high-resolution images, the variation and arrangement of the various types of crystal structures were not known.

We knew that there were many motifs, but they are all different from each other in ways we didn’t know. In fact, even the dominant motif in the peptoid sheet was a surprise.

Nitash Balsara, Senior Faculty Scientist, Berkeley Lab; Professor of Chemical and Biomolecular Engineering, UC Berkeley.

Balsara informed that Ken Downing, a senior scientist in Molecular Biophysics and Integrated Bioimaging Division of Berkeley Lab who passed away in August, and Xi Jiang, a project scientist in the Materials Sciences Division, were instrumental in capturing the high-quality images that were integral to the study and for creating the algorithms required to obtain atomic resolution in the polymer imaging.

Downing’s and Jiang’s know-how in cryogenic electron microscopy was complemented by David Prendergast’s knowledge of molecular dynamics simulations required to infer the images, Ron Zuckermann’s ability to produce model peptoids, Balsara’s knowledge in the field of polymer science, and Andrew Minor’s know-how in imaging metals at the atomic scale.

At the Molecular Foundry, Prendergast heads the Theory facility, Zuckermann heads the Biological Nanostructures facility, and Minor heads the National Center for Electron Microscopy and is also a professor of materials science and engineering at UC Berkeley. Most of the cryo-electron imaging was performed at Krios microscopy facility of UC Berkeley.

Balsara stated that his own study focused on utilizing polymers for electrochemical devices, including batteries, could gain from the study, because observing the position of polymer atoms could considerably help in designing materials for these devices. For instance, more advanced, automated filtering mechanisms that depend on machine learning may be required by atomic-scale images of polymers that are used in day-to-day life.

We should be able to determine the atomic-scale structure of a wide variety of synthetic polymers such as commercial polyethylene and polypropylene, leveraging rapid developments in areas such as artificial intelligence, using this approach.

Nitash Balsara, Senior Faculty Scientist, Berkeley Lab; Professor of Chemical and Biomolecular Engineering, UC Berkeley.

The determination of crystal structures may offer key information for other kinds of applications, such as drug development, because different crystal motifs may create relatively different therapeutic effects and bindingproperties, for instance.

The study was carried out within the Soft Matter Electron Microscopy Program at Berkeley Lab, which is supported by the U.S. Department of Energy’s Office of Science; and by the Bay Area Cryo-EM Consortium.