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In this guest editorial written by Park Systems senior application scientist Vladimir Korolkov, learn about AFM as an important tool for studying the molecular structure of polymers.
The study of the structure of polymers is crucial, because the applications of polymer materials range from micellar drug carriers to bulletproof vests. Their refined molecular structure provides a variety of unique properties for polymers. Therefore, the ability to image this molecular structure in real space is crucial. The only technique that can perform this kind of imaging is atomic force microscopy. (atomic force microscope).
AFM has been developed for more than three years1. However, it takes a long time to obtain a high-resolution image of a polymer using an atomic force microscope. We can certainly mention Hobbs and Mullin2,3 to observe the development of torsional tapping of a single polyethylene molecule, the bimodal tapping of Proksch4 and the higher eigenmode imaging of Korolkov5. The latter technology is different from other technologies and does not require any special cantilever or custom modified AFM components. In this work, we implemented this technology on a commercial AFM (Park Systems NX20) to achieve molecular resolution on real-world Teflon samples.
Polytetrafluoroethylene (PTFE), commonly known as Teflon, is a fluorocarbon solid with one of the lowest coefficients of friction. Therefore, it is widely used as a low-adhesion material or as an inert coating. Despite its chemical simplicity, PTFE exists in four different crystalline phases, which have been studied by electron diffraction techniques6,7. Interestingly, so far, no high-resolution AFM data of Teflon has been published.
As we all know, Teflon is a semi-crystalline polymer7. Figure 1 shows a set of large-scale images of Teflon surfaces. A 100 µm x 100 µm image (Figure 1a and b) already shows two different areas: a large area of 20 µm and a rope-like area connecting them. A closer look at the domain area will reveal their high directivity. The high-resolution phase image (Figure 1d) shows mainly crystalline areas separated by smaller amorphous areas on the polymer surface. As shown in Figures 1c and d, these crystalline domains exhibit flat terraces.
In the height and phase images below (Figure 2), we have examined these flat terraces more closely. The image at a height of 100 nm x 100 nm (Figure 2a) shows steps of about 5 Å with sharp edges. The corresponding phase images (Figures 2b and c) reveal the true molecular nature of these flat steps—we can clearly distinguish individual molecules with a period of 5.6Å. The cross section (Figure 2d) gives a clear periodic structure of the terraces. From this cross section, we can also measure the full width at half maximum of a single line as 3.5 Å. This determines the maximum resolution achieved on this sample. When comparing the observed 5.6Å period with the reported diffraction data for the PTFE unit cell a = 5.66Å7, we can notice a significant agreement.
All in all, we have demonstrated a direct and practical high-resolution imaging method that uses the higher intrinsic mode of the standard cantilever on the commercial large-scale NX20 Park Systems AFM to achieve the molecular resolution of the PTFE sample in the tap mode. AFM is a surface-sensitive technology that is always limited to the outermost/outermost layer, which can accurately reproduce the results obtained from the volume average technology. Therefore, it is proved that AFM is an important tool for studying the molecular structure of polymers. In fact, AFM can provide more structural information by irradiating light to the amorphous area of Teflon-which is not easily achieved by diffraction technology. For example, the phase image (Figure 2b) shows that PTFE molecules extend from the ordered crystalline regions to the amorphous regions of the polymer. The highly localized and high-resolution characteristics of AFM put it in a unique position for studying polymer structures in real space.
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