Electron microscopy techniques have enabled us to study the physical world at an exponentially smaller scale than our naked eyes or even classic optical microscopes allow. Techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) enable materials scientists to study individual atoms in polymers, observe dynamic surface phenomena on the nanoscale, and explore intricate patterns in polymer samples’ nanostructured surfaces.
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Electron Microscopy and Polymer Research
Optical microscopes work by manipulating light (waves and particles of photons) with lenses to create a magnified image. However, they are limited by the physical length of light’s energetic wavelength.
To overcome this diffraction limit in optical microscopy, researchers use energy that radiates at a higher frequency (shorter wavelength) than light: electricity.
Electrons interact with matter just like all forms of energy including light, and they are affected by these interactions. Electron microscopy uses special detectors to sense electrons returning from interactions with the sample, and computers read that data and interpret it to produce imagery at extremely high levels of magnification.
Electron microscopy has played a vital role in polymer characterization and analysis. With extremely high magnification levels, it can provide researchers with important information, including new materials’ morphology and composition. It can also help identify contaminant materials that are only present in polymers in trace amounts, and analyze plastic component failures to identify future design improvements.
The adoption of electron microscopy for polymer analysis has enabled the polymers industry to continue to meet the wider economy’s need for materials. As well as helping with the development of new materials, it is also now employed to help the industry move toward more sustainable practices by analyzing recyclability in waste polymers, or by ensuring that new polymer parts are as durable and long-lasting as possible.
SEM and TEM
The two most common types of electron microscopy are both used for polymer analysis. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are broadly similar, but they do have subtle differences which makes them suitable for different polymer analysis applications.
SEM scans an electron beam across the sample and records electrons as they bounce back toward the microscope. The properties of the returning electrons provide the device with data about the sample’s surface that is computationally processed to create extremely high-resolution imagery.
This scanning technique enables us to image the surface of almost any kind of sample material, including polymers. SEM is also quick, relatively unrestrictive, and can be performed with minimal sample preparation. However, when scientists need to conduct polymer analysis at exceedingly high levels of detail, they often turn to TEM instead.
Instead of scanning an electron probe over a sample’s surface and recording electrons as they bounce back from the sample, TEM transmits electrons through ultrathin sample slices and detects them on the other side. TEM is used to observe tiny details in samples as small as individual atoms, and provides high levels of detail about materials’ complex nanostructures.
Because TEM transmits electrons through the sample, it can provide information about the material’s internal structure in three dimensions. While SEM is faster, it cannot achieve this.
Producing and handling sample material at the necessary thickness for TEM is a challenge as it relies on passing electrons through a thin slice of sample material (such as a projector passing photons through a thin slice of film in a slideshow or cinema), restricting its practical applications.
For polymer analysis, SEM can be used to image a relatively large area of the sample’s surface at nanoscale resolution, while TEM can be used to image the three-dimensional structure of a smaller area at resolutions showing individual atoms and quantum particles.
Polymer Sample Preparation for SEM and TEM
Both SEM and TEM are suitable inspection methods for polymers, although some sample preparation is required for both methods.
As SEMs typically have a larger specimen chamber, sample preparation for this technique can be relatively “light-touch.” However, non-conductive polymers and even weakly conductive polymers must be coated with a conductive element such as carbon or gold. The coating is ultra-thin and enables the sample to interact with the SEM’s electron beam. Conductive coatings can be applied in bulk, and do not otherwise limit the higher throughput capacity of the SEM technique.
TEM, on the other hand, requires a fair bit more sample preparation for polymer analysis.
Ultrathin slices of the polymer material need to be obtained first. This can be done subtractively by extracting sections of a larger piece of material using a focused ion beam, for example. Depending on the material, polymer films can also be manufactured additively by depositing extruded material in an ultrathin layer or, in the case of some advanced polymers, epitaxially growing polymer crystals on substrate material.
Once suitably sized samples are extracted, they are placed into a TEM matrix where they may also need to be coated or doped with a conductive material.
Which is Better, SEM or TEM?
SEM and TEM both play different roles in polymer analysis and are not comparable like-for-like. SEM is more restricted in terms of its ultimate magnification limit, but TEM requires more sample preparation and takes longer.
In other words, the question of which is better depends – like most questions about inspection techniques – on what the desired information output is.
References and Further Reading
Drummy, L.F., J. Yang, D.C. Martin, et al (2004). Low-voltage electron microscopy of polymer and organic molecular thin films. Ultramicroscopy. Available at: https://doi.org/10.1016/j.ultramic.2004.01.011.
Ilitchev, A. (2019). Transmission (TEM) vs. Scanning (SEM) Electron Microscopes: What’s the Difference? Thermofisher.com. [Online] Available at: https://www.thermofisher.com/blog/microscopy/tem-vs-sem-whats-the-difference/.
Musselman, I., N. Panapitiya, D. Bushdiecker, et al (2014). SEM, TEM, and AFM Analyses of Phase-Separated Polymer Blend Membranes for Gas Separations. Microscopy and Microanalysis. Available at: https://doi.org/10.1017/S1431927614012057.
Open University. SEM and TEM sample preparation of materials. Open University. [Online] Available at: http://www9.open.ac.uk/emsuite/services/sem-and-tem-sample-preparation-materials.
Takaku, Y., H. Suzuki, I. Ohta, et al (2015). A ‘NanoSuit’ surface shield successfully protects organisms in high vacuum: observations on living organisms in an FE-SEM. Proceedings of the Royal Society B. Available at: https://doi.org/10.1098/rspb.2014.2857.