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Metrology & Instrumentation: Polymer Imaging


Scope of AFM Applications

Atomic Force Microscopy of Polymers:
How is it Useful?

Initially introduced as an accessory to the Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM) (Binnig G, Quate C, Gerber Ch Phys Rev Lett 1996, 56, 930) has became a most advanced and developed scanning probe technique with broad applications in academic and industrial research. It is also applied for quality control in a number of industries. In general, atomic force microscopy (AFM) can be described as high-resolution profiling of surfaces with a sharp probe. In this function the technique can be applied for measurements of surface roughness of various samples and also for quantitative examination of shapes/profiles of technologically important surface structures, e.g. DVD and CD patterns. In the past decade, many researchers have recognized the importance of AFM as a characterization technique for macromolecules and polymer materials. Recent achievements and further developments in this field have been discussed at the 3d International Conference on Scanning Probe Microscopy on Polymers, which took place in the Netherlands on 15-19 July 2003.


High-resolution visualization of surfaces is the unique AFM capability that defines most of its applications. Small nanometer-scale features as individual macromolecules and their assemblies (e.g. lamellae) as well as larger-scale morphologies can be easily recognized in AFM images. Also the fact that stiffness of AFM probes is comparable with stiffness of polymer materials allows a discrimination of sample locations with different mechanical properties. In this way, various components of heterogeneous polymer materials can be identified in the images. For many years, examination of polymer samples with AFM was mostly performed at ambient conditions. At present, these studies are extended to different temperatures and various gas and liquid environments. In this way, AFM can be used for in-situ monitoring of structural changes induced by thermal transitions and for visualization of structure transformations caused by swelling and other effects. It is worth to note that any kind of samples can be studied with AFM, yet best results are obtained on relatively smooth sample surfaces with corrugations below 100nm. Therefore, in many cases sample preparation is a important part of the experiment in AFM of polymers. Such preparative procedures, as etching and ultramicrotomy, provide AFM access to bulk polymer structures. Once the sample is prepared and an environment for the experiment is chosen then imaging should be performed having in mind the following: the selection of appropriate AFM mode and probe, as well as optimization of imaging parameters. All these conditions will help one get the most valuable AFM images.


Main research areas, which represent atomic force microscopy of polymers, are:

  • Visualization of single macromolecules and their self-assemblies on different substrates
  • Studies of lamellar architecture and morphology of semicrystalline polymers
  • Compositional mapping of heterogeneous polymer systems: block copolymers, polymer blends and multicomponent polymer materials
  • Examination of local mechanical and thermal properties


    • These fields alone are reviewed separately but here it is important to point out that in these studies AFM complements a large number of characterization methods.


    As compared with optical microscopy, AFM provides an access to smaller structures and also allows quantitative measurements of objects in all three dimensions. Scanning electron microscopy (SEM) is assisted by AFM in visualization of small corrugations of relatively flat surfaces. Polymer materials, which are too sensitive to the electron beam, could not be studied with SEM and transmission electron microscopy (TEM). But this is not a problem for AFM examination. Composition imaging of polymer materials with TEM requires a selective staining of individual components that might be difficult to realize. In AFM, compositional imaging, which is based on differences of mechanical properties of components, does not need any staining. It is worth noting that, a TEM micrograph of a heterogeneous polymer system presents a 2D projection of electron beam transmission through thin but still 3D section of the polymer material. Therefore, quantitative ratio between components deduced from 2D TEM micrograph might be erroneous. The analysis of AFM images of heterogeneous polymer materials is free from this complication.


    Structural order of polymer materials is often examined by various diffraction techniques (light scattering, SAXS, neutron scattering, etc), which requires a construction of structural models for restoring real-space information from diffraction patterns. This task might be quite difficult and not necessary unambiguous. Visualization of polymer structures with AFM can definitely help to make a choice between structural models used in analysis of diffraction data. Yet diffraction data provides averaged information, whereas microscopic measurements have a local character of all microscopic studies and AFM is not an exception, leaves opened the question if a particular image is representative of the sample. Time-consuming imaging of large number of sample locations is the only statistically sound approach to check this.


    Local character of AFM allows probing of mechanical and thermal properties of materials at sub-micron scale. Examination of force curves in different sample locations, nanoindentation and scratch experiments are used for these purpose. In this function, atomic force microscope extends macroscopic testing of polymer properties to nanoscale, which is becoming important with current developments in nanotechnology.