Atomic force microscopy (AFM) is an excellent method for characterizing the topography of polymer films and artifacts, as well as many other features. In this aspect, AFM offers significant benefits over electron microscopy. For example, because most polymers are insulators, they would need to be coated for SEM examination, perhaps affecting their texture. AFM, unlike TEM, does not require a thin sample.
There are several experiments and settings suitable for polymer materials. This article addresses two forms of AFM measurements of polymer materials.
Phase imaging
Because polymer composites and copolymers are made up of mixed materials, modes like phase imaging are quite useful. The distribution of the phases is generally crucial for their characteristics.
Phase imaging is sensitive to sample viscoelastic characteristics as well as tip-sample adhesion. This means that phase imaging can distinguish numerous materials. Because of its ability to differentiate many materials, phase imaging has been used on a wide range of samples, including semiconductor film differentiation, nanoparticle characterisation and counting, observation of spherulites in polymer crystallisation, polymer blend and composite composition, protein adsorption to biomaterials, self assembled monolayers, and many more.
Phase imaging is very effective for detecting features in polymer films with low height contrast. As shown in the figure below.
Images of a polymer blend's topography (left) and phase (right). While the topographical image indicates the presence of many regions in the mix, only the phase image clearly distinguishes them.AFM meets these criteria; z-axis (height) measurements in AFM can be precise to 0.1 nm. AFM provides numerous additional advantages for characterizing nanoparticles:
Hardness measurements of polymers
Polymers have received special attention in AFM nanoindentation research. One explanation for this is the presence of nanoscale domains in many composite polymeric materials. Polymers incorporating fillers or other particle elements, as well as block copolymers, are examples. The stiffness of such domains may be measured to better understand their contributions to the overall mechanical characteristics of bulk materials. In certain circumstances, materials are explicitly added to a polymer to modify its mechanical characteristics, such as adding rigidity or increasing elasticity. The contact between the reinforcing material and the continuous polymer matrix can have a significant impact on the mechanical characteristics of the materials. The mechanical investigations of nanoscale phases and their interfaces are suited for AFM-based nanoindentation. An example of this is illustrated in the figure below.
The outcome of nanoindentation on a polymer composite. Raw force curves (at the top), height picture (at the bottom left), and indentation map (below right)Another benefit over light scattering methods is the ability to characterize both form and size.
In this example, the substance under investigation was a commercial silicone paint containing big calcium carbonate filler particles. A force-curve mapping procedure was used to perform AFM-based nanoindentation measurements on the polymer surface. When curves over elevated structures on the surface were measured, they were found to be much stiffer than the surrounding polymer matrix, as indicated in the image of indentation distance. Surprisingly, a softer zone seems to surround each hard particle, indicating that there are some issues at the matrix-filler interface. This shows one of the advantages of AFM-based nanoindentation for polymer investigation, namely the direct assessment of mechanical property change at the nanoscale.
Conclusions
AFM is also commonly used in polymer research to study crystallisation, heat-dependent characteristics, and frictional qualities. In polymer research, AFM is a very valuable and adaptable instrument.
This article was based in part on Eaton and West's Atomic Force Microscopy.
References
Atomic Force Microscopy, Eaton and West, Oxford, 2010, Chapter 7.
Scanning Probe microscopy in Industrial Applications, ed. D. Yablon, Wiley, 2014, Chapter 9.
Vanlandingham, M. R. et al., Nanoscale indentation of polymer systems using the atomic force microscope. Journal of Adhesion 1997, 64, (1-4), 31-59
Eaton, P., et al., Combined Nanoindention and Adhesion Force Mapping Using the Atomic Force Microscope: Investigations of a Filled Polysiloxane Coating, Langmuir, 18(25) 10011-10015 (2002).