Water bottles, food containers, and many other items are made of plastics, but there are still mysteries about the polymers, the repeating molecular units used to make plastics. Scientists haven’t captured images of polymers at the atomic scale. Why? It’s difficult to capture images of polymers at such a small scale. Now, a team of Molecular Foundry users and staff has developed a new technique to get atomic-scale images of synthetic polymers. Capturing such images could ultimately inform fabrication methods and lead to new designs for certain materials and devices.
For the first time, scientists imaged a polymer with 2-angstrom resolution. For reference, a single hydrogen atom has a diameter of 1 angstrom. The new technique will inform polymer fabrication methods. It will also lead to new designs for materials and devices that incorporate polymers.
From water bottles and food containers to toys and tubing, many modern materials are made of plastics. And while we produce about 110 million tons per year of synthetic polymers like polyethylene and polypropylene worldwide for these plastic products, there are still mysteries about polymers at the atomic scale. Capturing images of these materials at tiny scales is difficult. Using electron beams in imaging materials with a soft structure, such as polymers, provides challenges because the beam used to capture images also damages the samples. Scientists have only realized images of individual atoms in polymers in computer simulations and illustrations.
Now, a team of Molecular Foundry staff and users has adapted a powerful electron-based technique to obtain an image of the atomic-scale structure in a synthetic polymer. The team created tiny flakes of peptoid nanosheets, froze them to preserve their structure, and then imaged them using an electron beam.
Researchers achieved a record resolution of about 2 angstroms for soft materials, which is two-tenths of a nanometer (billionth of a meter), or about double the diameter of a hydrogen atom. However, these images were too blurry to reveal individual atoms. So they took over 500,000 blurry images and sorted different motifs into different “bins,” and averaged the images in each bin. The sorting methods they used were based on algorithms developed by the structural biology community to image the atomic structure of proteins.
Using this approach, the team identified 35 arrangements of crystal structures in a peptoid polymer sample. Determining crystal structures can provide vital information for polymers and other applications, such as the development of drugs, as different crystal motifs could produce quite different binding properties and therapeutic effects, for example.
Molecular Foundry and Berkeley Lab
Lawrence Berkeley National Laboratory
Funding for this work was provided by the Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy. The electron microscope used in this study is located at the Krios microscopy facility, University of California at Berkeley, supported by the Bay Area Cryo-EM Consortium and the Howard Hughes Medical Institute. Synthesis was performed at the Molecular Foundry, a Department of Energy, Office of Science user facility.
X. Jiang, D.R. Greer, J. Kundu, et al., “Imaging unstained synthetic polymer crystals and defects on atomic length scales using cryogenic electron microscopy.” Macromolecules 51(19), 7794 (2018). [DOI: 10.1021/acs.macromol.8b01508]
Berkeley Lab news release: Scientists Bring Polymers into Atomic-Scale Focus