Given that the application of polymer materials is so versatile, and ranges from micellar drug carriers to bulletproof vests, structural studies of polymers are essential. Polymers have a variety of unique properties thanks to their truly exquisite molecular architecture. It is therefore essential to be able to image this unique molecular structure in real-space, the only existing technique for which is called Atomic Force Microscopy (known as AFM).
The process of AFM was created over a period of more than three decades1. Despite its obvious benefits, it took a long time for the scientific community to recognize the potential of using AFM to obtain these high-resolution images of polymers. Other valuable techniques involved which merit a mention include the development of torsional tapping to observe single polyethylene molecules, created by Hobbs and Mullin2,3, Proksch’s4 bimodal tapping technique, and the higher eigenmode imaging technique implemented by Korolkov5. Unlike others, the latter technique does not need any custom-modified AFM components or special cantilevers. This technique has now been implemented on a commercial AFM –namely Park Systems NX20 – in order to obtain molecular resolution performed successfully on a real-world sample of Teflon.
Teflon, more commonly and widely known by the name Polytetrafluoroethylene (PTFE), a fluorocarbon solid, Teflon has one of the lowest coefficients of friction, which is why it is widely used as a low-friction material with low adhesion or as an inert coating. PTFE exists in four different crystalline phases that have been studied with electron diffraction techniques, despite its chemical simplicity 6,7. To date, however, no high resolution AFM data of Teflon has been published.
Figure 1. Large-scale height and phase tapping mode images of a Teflon surface. a – height image, 512 x 512 px, scan rate 0.5 Hz. b – phase image acquired simultaneously with image A. c – phase image, 512 x 512 px, scan rate 4 Hz. d – high resolution phase image showing both crystalline and amorphous regions, 512 x 512 px, scan rate 4 Hz. Captured by Park NX20 Large Sample AFM. Image Credit: Park Systems Europe
It is known that Teflon is a semi-crystalline polymer 7. Figure 1 depicts a set of large scale images of a surface of Teflon. Figure 1 a and b depict a 100 µm x 100 µm image with two distinctive areas: with large 20 µm domains and rope-like areas linking them. Their highly directional nature is revealed with a closer look at domain areas. Figure 1d, a high resolution phase image, reveals predominantly crystalline regions that are separated by smaller amorphous regions on the polymer’s surface. Flat terraces are exhibited by these crystalline domains, as observed in Figure 1c and d.
Figure 2. High resolution AFM images of a Teflon surface showing single PTFE molecules. a – height image, 512 x 512 px, scan rate 2 Hz. b – phase image, 512 512 px, scan rate 4 Hz. c – phase image, 512 x 512 px, scan rate 6 Hz. All images were acquired using a 3rd eigenmode of Multi75Al-G cantilever at 1.1 Mhz and set point of 1.15 nm. d – a cross-section illustrates the periodicity of the terraced surface. Captured by Park NX20 Large Sample AFM. Image Credit: Park Systems Europe
These flat terraces are examined further on the following height and phase images (Figure 2), Fig. 2a (a 100 nm x 100 nm height image) reveals ~5Å steps with sharp edges. The true molecular nature of these flat steps are revealed in the corresponding phase images (Fig. 2b and c), in which we can clearly resolve single molecules with a period of 5.6Å.
Fig. 2d is a cross-section which gives a visible periodic structure of the terrace with some clarity. The width of a single line at full width at half maximum being 3.5Å can also be measured from this cross-section, therefore determining the maximum resolution achieved on this sample. A remarkable agreement can be noted when comparing the observed period of 5.6Å to the reported diffraction data of PTFE unit cell a = 5.66Å7.
In conclusion, a straightforward practical approach for high-resolution imaging has been demonstrated, using higher eigenmodes of a standard cantilever in tapping mode on a commercial large-scale NX20 Park Systems AFM achieving a Teflon sample’s molecular resolution. This surface-sensitive technique which is always confined to the utmost/outermost surface layer, AFM, is, therefore, able to correctly reproduce results that had been obtained from volume average techniques.
This, therefore, reveals that AFM is a vital aspect when investigating the molecular structure of polymers. Furthermore, it is clear that AFM can offer more structural information by revealing Teflon’s amorphous regions, that cannot be easily achieved by diffraction techniques.
Fig. 2b, for instance, is a phase image depicting the fact that PTFE molecules extend from well-ordered crystalline regions even further into the amorphous region of the polymer. AFM, therefore, enjoys a highly localized and high-resolution nature which places the technique in a unique position to investigate the structure of polymers in real space.
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References and Further Reading
- Binnig, G., Quate, C. F. & Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 56, 930–933 (1986).
- Mullin, N. & Hobbs, J. Direct Imaging of Polyethylene Films at Single-Chain Resolution with Torsional Tapping Atomic Force Microscopy. Physical Review Letters vol. 107 (2011).
- Mullin, N. et al. “Torsional tapping” atomic force microscopy using T-shaped cantilevers. Appl. Phys. Lett. 94, 173109 (2009).
- Kocun, M., Labuda, A., Meinhold, W., Revenko, I. & Proksch, R. Fast, High Resolution, and Wide Modulus Range Nanomechanical Mapping with Bimodal Tapping Mode. ACS Nano 11, 10097–10105 (2017).
- Korolkov, V. et. al. Ultra-high resolution imaging of thin films and single strands of polythiophene using atomic force microscopy. Nat. Commun. 10, 1537 (2019).
- Clark, E. S. The Crystal Structure of Polytetrafluoroethylene, Forms I and IV. J. Macromol. Sci. Part B 45, 201–213 (2006).
- Brown, E. N., Clausen, B. & Brown, D. W. In situ measurement of crystalline lattice strains in phase IV polytetrafluoroethylene. J. Neutron Res. 15, 139–146 (2007).
Produced from materials originally authored by Vladimir Korolkov from Park Systems UK Ltd.
This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.
For more information on this source, please visit Park Systems Europe.