Atomic Force Microscopy (AFM) for Polymer Characterization
AFM is a powerful method for imaging polymers, polymer blends, and polymer composites with nanometer lateral resolution. For polymer applications, the AFM now resides alongside optical microscopy and electron microscopy (SEM – scanning electron microscopy and TEM – transmission electron microscopy) as essential tools for characterization. However, atomic force microscopy provides specific advantages over other microscopies because it provides mechanical interaction between the tip and sample. This mechanical source contrast often provides contrast in situations where electron or photon based microscopies struggle or even fail.
The predominant mode for imaging polymers is phase imaging, which is associated with vibrating mode. In phase imaging, the AFM provides excellent contrast, sensitivity, and discrimination based on various material properties of the polymers.
AFM is very useful to image polymers:
- Suitable for polymers, polymer blends, polymer composites
- Used to establish structure-property relationships
- Lateral resolution is ~10nm
Key features on using AFM to characterize polymer materials:
- Information obtained includes morphology, dispersion, domain size, internal structure
- Sample preparation – cryomicrotoming - often needed to remove skin effects from molding, other processing methods
- Phase imaging is excellent method for contrast where contrast is based on material/mechanical properties such as stiffness and adhesion
- No post processing needed
AFM imaging requires flat surfaces for imaging. However, polymer samples may require additional sample preparation beyond this. If there is a sample with a "skin", or a sample that has been processed and only the inner bulk material needs to be imaged, it will need to be cryomicrotomed for AFM imaging. Cryomicrotoming is a process by which a very smooth surface is cut and prepared at cold temperatures on a cryomicrotome. Many samples such as thin films or spin-coated films can be imaged as is without this preparation.
Aside from cryomicrotoming, no further sample preparation is required – i.e. no staining is required as is the case for electron microscopy based methods such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Topography of Polymer Film with Oriented Fibers
See below a 10µm x 10µm topographic image of an oriented polypropylene showing a network of fibers.
Phase Imaging Contrast
The predominant mode for imaging polymers is phase imaging, which is associated with vibrating mode. In phase imaging, the AFM provides excellent contrast, sensitivity, and discrimination based on various material properties of the polymers including stiffness and adhesion.
Explanation of Phase Imaging
Phase imaging is a channel collected in vibrating mode and requires no post processing. Phase imaging collects the information on the phase shift (Φ) induced in the cantilever vibration motion. The cantilever is driven at a resonant frequency and interacts with the sample at a given oscillation amplitude set by the user. The phase shift (Φ) is then induced by interacting with the sample and is mapped as a channel simultaneously with the topography channel while the tip raster scans over the surface. A variety of material properties can affect the tip-sample interaction and induce a phase shift including: stiffness, adhesion, viscoelasticity, and capillary forces.
Imaging a Polymer Blend
A phase image over a 5µm x 5µm area of a blend of thermoplastics and rubbers is shown.
The phase imaging differentiates components. The thermoplastic matrix is in the background with brown/orange contrast. The bright white rubber domains are clearly differentiated from the matrix. The rubbers appear in a variety of diameters from a few hundred nm to several microns. Note the diverse shapes of the rubber domains with some inclusions being more round while others are more oval and even irregularly shaped. Additionally, inside the rubbers are small orange type inclusions, further showing the material structure in this sample.
Block Copolymer Sample
The extraordinary contrast provided by phase imaging is demonstrated nicely in block copolymer samples as well. Below is a 1µm x 1µm phase image of SEBS (styrene/ethylene/butylene polymer). Compare the phase image on the left with the corresponding topography image on the right. Although the existence of the two domains is observed in the topography, the different blocks are more clearly differentiated in the phase image on the left, enabling a clearer understanding of the morphology, dispersion, and size of the various block domains.
In this high resolution 500nm x 500nm phase image below, the smallest domains of ~10nm in diameter are easily visible.
Upcoming AFM and Polymer Characterization Workshop
If you would like to learn more about using AFMs to characterize polymers, polymer blends, and polymer composites with nanometer lateral resolution, you may be interested in attending AFMWorkshop's May 10-11, 2016 Polymer Characterization Workshop, led by Dalia Yablon, Ph.D.
Our AFM Users Recent Polymer Reference Publications
AFMWorkshop's TT-AFM provides all the major Atomic Force Microscopy (AFM) modes needed to characterize polymers, including vibrating mode for topography/morphology and force distance curves for mechanical properties such as adhesion and stiffness. A number of researchers have published their work on polymers using the TT-AFM, characterizing important properties such as size, shape, and dispersion. Click here to read our highlights of a few recent articles where researchers are utilizing the TT-AFM for work on polymers in biomedical and electronic device applications.